Easter Sunday

Easter is a holiday that is important to Christians because they believe that it is the day that Jesus Christ rose to heaven after being crucified by the Romans. However, as with most religious holidays different religions have been stealing each other's events and repackaging them for their own. In the case of Easter, it started off as a celebration of the vernal equinox as this signified the end of winter and the beginning of spring. It was not until the first Council of Nicaea in 325 C.E. that Easter became a floating Christian holiday.

I am am an atheist, but I still enjoy the trappings of Easter because of the fact that I like chocolate and Easter candy. I really like the different chocolate eggs that are made by the British chocolate company, Cadbury. The peanut butter-filled eggs are my favorite, but the cream-filled eggs are good as well. I bought some of those for myself today in addition to a chocolate rabbit. I also colored hard-boiled eggs on Friday simply because it is fun to do. For dinner, I made a roast leg of lamb and had an enjoyable evening. Although I do not believe in or follow the religious mythologies behind Easter, I think that the holiday is more about the renewal and the awakening of plants and animals during the spring after the monotony of winter.

The grass is finally starting to turn green where I live and leaf buds are appearing on the trees. Early spring is a rather depressing time of year because everything is drab and dirty after the snow melts and things do not begin to brighten up where I live until late March. Spring in most of the midwest is a rather short affair, and Illinois is no exception because the transition between winter and summer weather is rather abrupt here. The summer is my favorite time of year because that is when the weather is most conducive to swimming and other activities in addition to the fact that is usually when the local gardens and trees are in full bloom and the fragrance of their flowers hangs on the humid air.

I hope that today was an enjoyable day for my readers and I hope that their local weather is pleasant. At the moment, I am boiling a whole pot of lamb and beef bones that had been in my freezer for awhile as I am making stock for future use. I will see if I can get part 5 of my Nuclear Technology Basics series tomorrow as I want to start talking about different types of light water reactors.

Nuclear Technology Basics: Part 4 Reactor Components

Introduction

Part 1

Part 2

Part 3

The process of nuclear fission can be carried out by several different fissile isotopes, depending on the design of the nuclear reactor. Most reactors that have been built in America for energy generation are what are called light water reactors or simply "LWRs" as an abbreviation. Light water reactors are thermally-based reactors that use regular water for both coolant and as a neutron moderator as opposed to heavy water which has a high ratio of deuterium which is an isotope of hydrogen that has a greater atomic mass than normal hydrogen.

In future posts I will go over the different types of light water reactors, but in this case I will use the layout of a typical pressurized water reactor or "PWR". PWRs constitute the most common type of nuclear reactor in the world, including the US. Although many different types of reactor designs exist, the PWR remains the most common design because of both politics and technical familiarity with engineers in the nuclear field.

While I normally frown on Wikipedia as a source of accurate technical information, I did find a nice animated diagram of a PWR reactor layout there.



So, here are the basic components of our nuclear reactor:

1. The Reactor Vessel
The reactor vessel is the component within a nuclear reactor that contains the reactor core and where coolant circulates to prevent the core from overheating. Some designs lack a reactor vessel but the BWR and the PWR types of light water reactors both have this component in their similar designs. The basic layout is that of a cylindrical tube made out of a steel alloy containing manganese and molybdenum because of its durability, while the interior of the reactor itself lined with a layer of stainless steel to prevent corrosion from rust since it comes into contact with the coolant fluid, which is water in LWR designs. The top of the reactor vessel is designed to be removable to facilitate the replacement and insertion of fuel assemblies. Coolant is pumped in through the inlet nozzles where it flows around the fuel assembly; removing heat in the process. The heated coolant is then pumped out of the reactor vessel and into the steam generator.

2. The Pressurizer



The coolant circulation system and the steam that is produced within the steam generator is under constant pressure. Maintaining the degree of pressure within the coolant circulation system and the steam generator is an important task and this is carried out by the pressurizer. An increase in the temperature of the circulating coolant causes the density of the fluid to drop, and the volume of the liquid to expand in volume. This pushes the fluid into the pressurizer, causing the steam within the top of the component to become compressed, and pressure levels to increase. A drop in the temperature of the coolant increases the density of the water, causing it to contract. Fluid drops out of the pressurizer, reducing the degree of steam compression within the top of the pressurizer. Should the pressure increase too much or decrease below safe levels, the pressurizer will bring fluid pressure levels back to a safe equilibrium by either spraying cold water through the top of the pressurizer which would cause the compressed steam to cool down and turn back into water. Lower than normal pressure levels will cause the pressurizer to activate a series of electric heaters embedded within the walls of the component to raise the ambient temperature of the water within the pressurizer. Should pressure levels continue to fall, safety systems will cause the reactor to shut down automatically.

3. The Control Rods


The control rods regulate the speed and ratio of fission within a nuclear reactor. Each control rod is composed out of materials that can capture and absorb neutrons without undergoing fission themselves, such as indium, cadmium, and boron to name a few. When nuclear fission occurs within the nuclear fuel rods, control rods serve to prevent some of the neutrons from striking fissile atoms within the fuel assembly which slows down the nuclear reaction. The speed of fission can be increased by lifting the control rods further out of the reactor vessel, leaving more neutrons available to initiate fission. Lowering the control rods into the reactor vessel can decrease the rate of fission as they absorb more the neutrons that were being emitted by the fuel rods. In the event of an emergency, the control rods will be pushed into the reactor vessel at their maximum depth in order to slow down the rate of fission as much as possible.

4. The Neutron Moderator


A neutron moderator is a material that serves to slow down the speed of fast neutrons so that they will cause a fission reaction when they strike the nucleus of a fissionable atom. As reactor designs are grouped by their moderator type, light water reactors use ordinary water as their neutron moderator but moderators can be made out of many substances. In water moderated reactors, a bluish glow can be seen around the control rods as charged particles are moving faster than the speed of light within the medium. Since the electric field of the particles is unable to keep up with them as they travel, photons are produced in an optical equivalent of a "sonic boom", producing light towards the blue wavelengths of the color spectrum.

5. The Containment Structure



All nuclear reactors in the world are now constructed with a containment building over the reactor vessel to protect the reactor from damage and to physically prevent the release of radiation in the event of a core meltdown. The containment building is a solid concrete or steel shell that is several feet thick that is extremely durable and tests have shown their ability to withstand earthquakes or impacts with aircraft with minimal damage. The containment building is what prevented the release of any significant degree of radiation during the Three Mile Island incident in 1979 and what prevented the damaged reactors at Fukushima Daiichi in Japan from irradiating the populace after being subjected to a massive earthquake. If the infamous Chernobyl reactor had been built with a containment building over the reactor vessel, the effects of the meltdown on the surrounding area would have been negligible.

6. The Steam Generator



The PWR light water reactor design has a steam generator, the BWR design does not. Hot coolant flows from the reactor vessel into pipes that are surrounded by secondary coolant within the steam generator. The pipes containing the hot coolant cause the fluid surrounding them to heat up and begin to boil and generate steam. Heat energy is transferred from the hot pipes into the secondary coolant causing the primary coolant to cool down as it is pumped back into the reactor vessel. The steam generated by the boiling secondary coolant rises and is forced through a moisture separator into the turbine chamber.

7. The Reactor Turbine



Within the reactor turbine, steam is pushed through the center of the turbine which then turns the blades of the turbine assembly as it expands outward. Most turbine designs have a moisture separator after the first high-pressure turbine which separates the condensed water from the steam and forces the steam through a series of low pressure turbines. This is how pressurized steam is converted into mechanical energy.

8. The Generator



The generator coverts mechanical energy into electricity. A rod runs from the generator to the turbine that spins along with the motion of the turbine. Within the generator, the rod is wrapped with a piece of wire that is surrounded by magnets. Electrical current is generated within wires surrounding the magnets on the side of the generator shaft as it spins, which is then sent out of the nuclear power plant.

9. The Condenser



Steam passes into the condenser chamber after passing through the turbine where cool water circulated from a water box causes the steam to condense into water. The water is then circulated within the secondary coolant loop to the steam generator. Then it is used to generate steam from the heated pipes of the primary coolant loop.

This was just a brief summary on the major components of a "typical" nuclear reactor such as a pressurized water reactor, or PWR. On my next entry, I will be taking a look at how reactor designs differ, starting with the different types of light water reactors. The PWR is by no means the final word in nuclear reactor technology.

Follow Up on Fukushima Daiichi

The situation at the Fukushima Daiichi reactor is largely under control. Although I have been dismayed by the degree of exaggeration and outright falsehoods that were evident in the coverage on the status of the Fukushima Daiichi reactor, I should not have been surprised considering how ignorant the general public often is about how nuclear reactors work and what measures are in place to ensure the safety of the workers and the surrounding area when something goes wrong. The electricity provided to Fukushima Daiichi by emergency diesel-powered generators was not affected from the second earthquake on April 7th. A leak near Unit-2 has been sealed, preventing the further release of water contaminated with highly radioactive nuclides. The source of radioactivity is unknown at this time.

The spraying of water on the exposed fuel assemblies within Units -1 through -4 continues and nitrogen gas is being pumped into Unit-1 in order to prevent any further explosions involving hydrogen gas build up. Makeshift dams out of silt and steel plates are being installed in the ocean surrounding the reactor site in an attempt to contain some of the mildly contaminated water that was released offshore. While the risk to civilians and most personnel from radiation exposure has been minimal since the beginning of the incident, radiation levels surrounding the site continue to drop dramatically, and the Japanese government has lifted restrictions on consuming milk and produce from farms surrounding Fukushima Daiichi as of April 8th.

Here are some numbers detailing the total casualties that have resulted from Fukushima Daiichi since the first earthquake, courtesy of the Depleted Cranium blog:

Deaths: Two workers died from injuries resulting from the first earthquake, which was unrelated to the operation of Fukushiima Daiichi itself.

Injuries: Twenty three workers have been injured at Fukushima Daiichi. Eight of those people were involved in accidents involving the operation of non-nuclear equipment during the earthquake while fifteen people have received minor injuries during the hydrogen explosions shortly after the earthquake. Two people have received minor radiation burns that did not require further treatment after being evaluated at a hospital.

Radiation Exposure: Seventeen workers had to undergo radiation decontamination procedures on-site after minor radiation exposure, but not enough to warrant further decontamination measures off-site.

Prognosis For Workers Exposed to Minor Radiation: Excellent, possibly a slightly increased risk of developing cancer but this is statistically negligible when compared to the probability of developing cancer as a function of age for the average person in the general population.

Effects on the General Public: None

Injuries to the General Public: None

Casualties Resulting From the Earthquake and Tsunami: 30,000 and counting.

Radionuclides in the Water Table:

There has been some discussion about the safety of Japan's water supply in regards to contamination by iodine-131. There have been warnings issued about the levels of iodine-131 recorded in Japan's water supply on the twenty-third of March as they were above the 100 Bq/Kg (Becquerels per kilogram) limit set by Japan for infants, but still well below the 300 Bq/Kg limit for adults. Japan's guidelines for exposure to iodine-131 are also extremely conservative as the WHO's limits for iodine-131 are 3,000 Bq/kg. In any case, levels of iodine-131 in the water table have dropped dramatically since March 23rd because iodine-131 has a half-life of only eight days and the levels of iodine-131 have been below the Japanese 100 Bq/Kg limit for infants since the 24th of March.

Now, with all of this being said I hope that we can all put our fears of nuclear power to rest as many of them were unwarranted and there is also the fact that the incidents at Fukushima were relatively minor when one considers the fact that the structure was designed to survive earthquakes at a maximum of 8.0 on the Richter scale when the first earthquake that hit the island of Honshū was 8.9. If Fukushima had been any other sort of power-generating structure, the potential for casualties would have been much higher as natural gas and coal generating facilities are prone to explosions, and hydroelectric dams often break during earthquakes. The problems at Fukushima should serve to reinforce the lesson on how safe nuclear energy really is considering the intensity of the earthquake.

Special Post on the Nuclear Energy Situation in Japan

The earthquake that occurred in Japan was magnitude 8.9 on the Richter scale. Aside from the damage caused to Japan's infrastructure from the earthquake, a ten-meter high tsunami flooded the northeastern coastline of the island. As of today, 1,597 people are reported dead, 1,923 injured, and 1,481 people missing across sixteen regions but these estimates may soon rise in the next few days.

There has also been widespread panic over the state of Japan's nuclear power stations. Several nuclear facilities have been severely damaged in addition to the rest of Japan's northern industrial centers. Because of the shoddy reporting from the media as is typical with anything concerning nuclear energy, it is difficult to sort out fact from fiction. As people are already comparing what is happening in Japan with Chernobyl, I feel that it is important that I try and clear up any dangerous inaccuracies.

The Fukushima Daiichi facility

This station is in the town of Ōkuma, on the northeastern part of the island of Honshū. It is composed of six boiling water reactors, leading to a combined power output of 4.7 gigawatts. All six of the reactors have suffered damage but in varying degrees.

Unit-1 (SCRAMed) is the oldest reactor on-site. Because of its age, it was originally scheduled to be decommissioned in roughly two weeks after this blog post was written. Water used to cool the reactor came in contact with the superheated fuel rods, causing instant vaporization and separation of the water into hydrogen and oxygen gas. The hydrogen built up and ignited resulting in an explosion that has destroyed the concrete shell over the reactor vessel, but the vessel itself remains intact. The reactor also suffered from a loss of cooling after on-site generator failure from the impact of the tsunami. The reactor SCRAMed (Automatically shut down) during the coolant flow malfunction, but borated seawater has been used to cool the reactor because of the residual heat still coming from the core. The reactor itself is probably damaged beyond practical repair, but temperature and pressure levels remain under control, and the containment dome remains intact.

Unit-2 (SCRAMed) has switched to auxiliary cooling in the wake of the tsunami, and the reactor itself remains in good condition. However, the turbine, generator, and surrounding machinery have been badly damaged. It is likely that Unit-2 will be able to be restarted after it has been repaired.

12:02 AM (CST) Update: The auxiliary coolant system seems to have failed and an explosion has occurred. It is not yet known what the cause of the explosion was, but steps have been taken to cool the reactor core with borated seawater as with Unit-3 and Unit-1. The containment dome remains intact and undamaged.

Unit-3 (SCRAMed) suffered a temporary loss of cooling similar to that of Unit-2, but recent investigation has revealed that auxiliary cooling systems have taken over. The safety release valve has been opened by workers to relieve pressure. Borated water has been injected into the reactor vessel to reduce the residual heat of the reactor core.

1:55 PM (CST) Update: An explosion of hydrogen gas similar to the incident at Unit-1 has occurred at Unit-3 at 11:01 AM JST (9:01 PM CST). The reactor vessel is still thought to be intact, but it is not yet known what the overall status of Unit-3 is and how much radiation, if any has been released into the outside environment. The effects of the explosion are still being investigated.

11:57 PM (CST) Update: Unit-3 has been written off as a loss as the borated seawater will irreparably damage the reactor. However auxiliary cooling systems have been inadequate so operators are going to have to resort to such measures to reduce the heat of the reactor core. A partial meltdown did occur but the containment dome remains intact and no significant amount of radiation has escaped from the core.

Unit-4 has been shut down in order to allow for inspection. Coolant levels are adequate and there appear to be no signs of leakage and the containment vessel seems to be intact.

3-15-2011 12:02 AM (CST) Update: A small fire has been reported at Unit-4. It has been contained and extinguished without incident.

3-15-2011 12:05 PM (CST) Update: The fire appears to have resulted from the cladding of Unit-4 igniting after coolant levels covering the bundle of fuel rods has dropped, allowing heat to build up. As the reactor was only recently shut down, the temperature of the fuel rods was much higher than if the reactor had been shut down a few days ago. Some radioactive material might have been released with the evaporating water surrounding the core, but the quantity and concentration is still unknown and it is likely that the danger level is small to nonexistent.

Unit-5 has been shut down in order to allow for inspection. Coolant levels are adequate and there appear to be no signs of leakage and the containment vessel seems to be intact.

Unit-6 has been shut down in order to allow for inspection. Coolant levels are adequate and there appear to be no signs of leakage. The containment vessel seems to be intact.

The degree of radiation in the immediate vicinity of the plant has increased. Small amounts of radioisotopes were dissolved in the steam vented to relieve pressure inside the reactors at Fukushima Daiichi. However, this is not a cause for concern as the concentration of these isotopes within the steam is very low, and most of them are very weakly radioactive. Determining the actual levels of radiation released by the nuclear facility can be problematic, because elevated radiation levels can also be attributed to the naturally occurring radioisotopes found in ash from fires and other particulate matter that has been swept into the area.

To date, there have been no deaths at Fukushima Daiichi, but ten employees have received medical attention and two workers are reported missing. Three workers have been reported to have been exposed to abnormally high doses of radiation, but I have not found any reliable information yet concerning the details of how much radiation they were exposed to, and what the source was thought to be. Despite the alarmism, there have been no credible reports of the nearby populace being contaminated with radioactive fallout.

3:31 AM (CST) Update: There has been one death at Fukushima Daiichi in an accident during the operation of a crane, yet it is unrelated to the incidents at the reactors themselves.

The Onagawa Facility

The Onagawa nuclear station is located near the town of Onagawa on the island of Honshū. It consists of two 825 megawatt reactors and one 524 megawatt reactor. The earthquake damaged the generator and turbine systems, causing all three of the reactors to SCRAM even though the reactors themselves sustained minimal damage and cooling systems remain intact. A fire occurred in Onagawa-3 resulting from a malfunctioning turbine but it was immediately controlled and put out without incident. No casualties at the Onagawa nuclear power station have been reported.

The Higashidōri Facility

The nuclear power plant near the town of Higashidōri is located on the northern tip of Honshū. It consists of four units, with Higashidōri-1 and the planned Higashidōri-2 being controlled by the Tōhoku Electric company, while the other two reactors planned to be built on-site are run by Tōkyō Electric. Higashidōri has been shut down following the disaster to carry out inspection and maintenance. The extent of the damage, if any, at Higashidōri remains unclear but no reports have surfaced concerning the failure of any critical systems. However, the earthquake might have damaged the three reactors at Higashidōri that are still in the construction phase. No casualties at Higashidōri have been reported.

The Tōkai Facility

This nuclear power station is in Tōkai on the central-eastern coast of Honshū. Unit I had reached the end of its operating license and was decommissioned while Unit II remained operational. The reactor SCRAMed during the earthquake, but the auxiliary cooling system took over. No casualties at Tōkai have been reported.

The Rokkasho Reprocessing Center

While not a nuclear power generating facility in of itself, this is where Japan fabricates most of its fuel for its nuclear power stations and reprocesses spent material. Although it does not appear to have suffered any catastrophic damage, it is currently running on auxiliary power because of the loss of electricity to much of northern Honshū. Normal operation has been suspended until primary power comes online. No casualties have been reported.

This is currently what I have found out about in the aftermath of the earthquake and the following tsunami in Japan. I have also heard of an eruption of the Shinmoedake volcano but it is not yet clear if it was triggered by the earthquake in Sendai. I welcome any comments suggestions, or updates on the current status of Japan's nuclear infrastructure in the coming days.

Nuclear Technology Basics: Part 3 The Process of Reprocessing

Introduction

Part 1

Part 2

As stated previously, most of the nuclear energy plants in the world are based on light water reactors and their variants. One disadvantage with the light water reactor design is that it only utilizes a small percentage of the Uranium-235 that is available within a fuel rod. Over ninety percent of the volume of spent fuel is uranium-235 that can be reprocessed to produce more fuel for nuclear energy plants and greatly reduce the volume and radioactivity of material to be disposed of. While the practice of fuel reprocessing remains a politically sensitive issue in many parts of the world, the fears that it will somehow lead to the increased proliferation of nuclear arms are erroneous.

There are different types of reprocessing methods, and some are still in the experimental stage, or only exist on paper. The most common type of nuclear fuel reprocessing is the PUREX (Plutonium and Uranium Recovery by EXtraction) method. The PUREX process was originally invented by Herbert H. Anderson, and Larned B. Asprey during their work on the Manhattan project in 1947. However, the spent material from fuel-grade uranium is too high in plutonium-240 to allow for the production of nuclear warheads.

PUREX and Its Derivative Processes



Nitric acid is used to dissolve the spent fuel in the reprocessing center where insoluable debris is removed in order to prevent contamination of the of the solute. A Tributyl phosphate/kerosene mixture is then used to claim the available plutonium and uranium from the nitric acid solute, leaving the transuranic elements such as curium and americium behind. Ferrous sulphmate is used to separate the reclaimed uranium from the plutonium as it reduces plutonium's oxidation state to +3 allowing the plutonium nitrates to be separated from the uranium nitrates. This liquid extraction process must be repeated several times to get an acceptable amount of plutonium and uranium reclaimed from the spent fuel. Uranium and plutonium are typically converted to their oxide forms for ease of storage and fabrication into MOX (Mixed OXide) fuel.

The UREX (URanium EXtraction) technique is based on the PUREX method to prevent the separation and extraction of plutonium. The plutonium is reduced with acetohydroxamic acid before any metal extraction takes place. This greatly increases the difficulty of recovering neptunium and plutonium isotopes in the solute. UREX was designed to add an extra measure of proliferation resistance during fuel reprocessing, although this is largely unnecessary.

Transuranics are radionuclides that have atomic numbers greater than 92 in the periodic table. They are produced in nuclear reactors or as a result of nuclear chemistry experiments. These elements typically have half-lives that are greater than twenty years, and can produce moderate to high amounts of alpha radiation. The PUREX reprocessing method can be modified to carry out the TRUEX process which allows for the extraction of the transuranics, which reduces the radioactivity of the resulting MOX fuel. TRUEX is nearly identical to PUREX, except that octyl(phenyl)-N, N-dibutyl carbamoylmethyl phosphine oxide and tributylphosphate are added to the solution after the uranium and plutonium have been extracted to allow for the extraction of transuranics. Some of these transuranics such as americium, have industrial uses. The SANEX (Selective Actinide EXtraction) process has been proposed to allow for the extraction of specific radionuclides. Although the SANEX process is still theoretical, researchers are looking into the viability of using bis-triazinyl bipyridines or dithiophosphinic acids as reagents.

Another reprocessing method based on PUREX is the UNEX (UNiversal EXtraction) process. This was developed to facilitate the separation and extraction of the more toxic actinides left behind in spent fuel; such as cesium-137, strontium-90, and other minor actinides. The UNEX process is identical to the TRUEX process, except that the extracted actinides are diluted with polar aromatic compounds.

Other Methods

In the past, spent fuel was often processed by using solvation techniques involving reactions with a chemical reagent that served to increase the concentration of the solute within a solution to serve as a carrier for the desired elements. The bismuth phosphate, hexone, and butex techniques were phased on in favor of the current PUREX system because of the large amount of extra material to be disposed of that was created in these prior techniques.

Pyroprocessing



Pyroprocessing is the name given to reprocessing methods that involve the application of high temperatures and metallurgical properties of radioisotopes rather than water and organic solvents. Pyroprocessing methods for fuel fabrication from spent nuclear material would have several advantages over the current PUREX method. This is because pyroprocessing techniques would be more streamlined and would also allow for on-site reprocessing of spent nuclear material. In addition, some pyroprocessing methods would facilitate the extraction of several different radioisotopes at the same time without having to carry out further extraction steps. In addition, many pyroprocessing methods are more efficient at extracting useful fissile material as the remaining waste that would result would be smaller in volume as well has have a significantly shorter half-life.

The most promising pyroprocessing potential lies with the molten-salt reactor (MSR) designs of the Oak Ridge National Laboratory designs of the 1960s in the US. The liquid core of a molten salt reactor can be chemically separated into its different elements for reprocessing without having to suspend them in solution. In addition, the remaining radionuclides contained within the core would allow for a greater degree of burn-up resulting in less material to be disposed of. Finally, the reactor would not have to be shut down during fuel fabrication because the molten fuel could be continually pumped in and out of the reactor chamber.

Christmas Stasis

I have been extremely busy since last weekend with packing my stuff, graduation, and moving to a new location. I will also be spending Christmas at the home of one of my aunts and so I will be out of town until the twenty-sixth of this month. Because of this, my blog series on nuclear technology will be suspended until next week.

In addition, I will wish you all a merry Christmas in advance. Whether or not you identify as Christian, the holiday should be a joyful occasion. This is because to me as well as many other atheists, Christmas is an important cultural holiday and the values of goodwill, love, peace, and prosperity are just as relevant from a secular standpoint as they are from people who profess belief in supernatural deities.

Who among you does not like a tree full of beautiful lights and decorations? Who would turn his head from the wonderful Christmas dinners, candy and cookies, snacks? How many of us would freely part from the exchanging of gifts that we experience during Christmas? Our relatives and guests might be a bit loud and raucous at times after the wine, beer, and egg nog flows freely but Christmas has a miraculous way of preventing resentment from becoming full-scale arguments.

So, the nuclear reactor and the machines that run my blog will continue to operate all throughout Christmas as they do every day until my return on the 26th where we will pick up where we left off with the plutonium fuel cycle. For those of you who have to travel far, I do hope that the roads are safe and that the TSA does not become too taxing on your nerves with their fraudulent security theaters. In addition, I wish that everybody gets plenty to eat this Christmas and that they get all that they have asked for or wanted this year.

Finally, in order to keep up with the festivities, I have decided to decorate this facility for the Christmas season and play some fitting music. I hope you do not mind listening to Burl Ives, Bing Crosby, and Dean Martin for awhile. Enjoy!

Merry Christmas everybody!

Nuclear Technology Basics: Part 2 Thorium Fuel Cycles

Introduction

Part 1

Although most nuclear reactors in the world today use fuel cycles based on the element uranium, it is also possible to use thorium as a source of nuclear energy with some types of nuclear reactors. A thorium-based fuel cycle has several advantages over one that is based on uranium, making it an increasingly attractive option to invest in from an energy production standpoint.



Thorium is element 90 in the periodic table. Like uranium, thorium is a naturally occurring actinide metal that is slightly radioactive when found in nature. Although as many as 33 different isotopes of thorium are possible, the thorium that is found in nature is mostly thorium-232. Thorium ores can be found in abundance all across the world, particularly in India and the western steppes of the US. Since there is currently little use for thorium from a commercial standpoint, there has been little effort to exploit these resources.



By itself, thorium-232 is not fissile, but if a neutron source is provided such as uranium-235, it can "jumpstart" thorium-232 into a fission chain reaction by causing it to absorb a neutron and become thorium-233. Thorium-233 has a half-life of twenty two minutes at the end of which it emits an electron, causing it to decay into proactinum-233. After 27 days, proactinum releases a second electron and becomes uranium-233.

232Th (n,γ) 233Th (β−) 233Pa (β−) 233U (n,2n)

Uranium-233 has a higher neutron yield than Uranium-235 when it undergoes fission, and therefore releases more energy per neutron absorbed. The decay of Uranium-233 would lead to the creation of numerous isotopes that would be useful from a medical and industrial standpoint and would be able to breed more uranium-233 within the reactor from the neutron irradiation of thorium-232. From a weapons-proliferation standpoint, the thorium-fuel cycle would be very difficult to divert into making fissile warheads. This is because proactinium can also decay into Uranium-232 which emits hard gamma radiation which is a hazard to people who would tamper with the reactor material in addition to the fact that although it is fissile within a reactor, it interferes with fast fission reactions like those within a thermonuclear bomb.

232Th (n,γ) 233Th (β−) 233Pa (n,2n) 232Pa (β−) 232U

Thorium-based fuel cycles have much in the way of economic potential and could be utilized by using off-the-shelf technology. The US experimented with thorium-based reactors during the molten-salt reactor experiment at the Oak Ridge National Laboratory during the mid-1960s until the project was abandoned in favor of uranium-based light water reactors for political reasons. Russia, China, and India are currently looking into the viability of thorium for nuclear energy production, and India is currently using thorium in its pressurized heavy water reactors (PHWRs) and its liquid metal fast breeder reactors (LMFBRs). Ideally, the full potential of thorium could be utilized in a liquid fluoride thorium reactor (LFTR) but it remains to be seen if the LFTR concept gains enough political momentum to allow it to be commerically realized.

Nuclear Technology Basics Part 1: Uranium Fuel Cycles

Introduction

Most reactors in the world today utilize the uranium fuel cycle to sustain fission, but there are other fuel cycles as well such as ones based on thorium and plutonium. Light water reactors (LWRs) typically have a once-through fuel cycle in which results in various degrees of spent fuel to be disposed of. Breeder reactors and various reprocessing centers can greatly reduce the quantity and half-life of material to be discarded, but nuclear reprocessing is banned in some countries because of errant political concerns rather than for any technical reason such as is seen in the US.

Uranium is a common element that is found in many locations across the world, usually in the form of Uranium oxide. Uranium oxide is a yellowish-brown powder, and is often referred to as "yellowcake". Large deposits of uranium are found in Australia, Africa, Canada, Spain, Russia, and the US where it is mined and sent to an ore processing center. Uranium mines may be either open pit mines when the uranium is close to the surface, or in underground mining tunnels for deeply-buried deposits. Most uranium in the US and Australia is mined using in-situ leeching methods where the uranium oxide is dissolved from the surrounding rock in solution using water that is acidified by carbon dioxide. A LWR reactor requires around .2 metric tonnes of uranium oxide per megawatt produced for its continual operation.

The uranium isotope, U-235 is the primary isotope of interest for power generation. In chemistry and nuclear physics, an isotope of an element is an atom that has a different number of neutrons from the typical number of an atom from that type of element. Uranium has 33 different isotopes, and all of them are radioactive with varying degrees of radioactivity and half-lives. Only .7% of the atoms in naturally occurring uranium oxide are U-235 on average, while the most abundant isotope of uranium is U-238 which accounts for 99.28% of uranium atoms found in nature. Rarer still is the naturally occurring isotope of Uranium U-234 which is slightly more than half a percent of uranium found in deposits on Earth.

In order for mined uranium oxide to be viable for usage in a LWR it must be brought to a fuel fabrication facility where uranium oxide is converted into uranium hexafluoride where the percentage of U-235 is concentrated up to three percent. This is done either through the gaseous diffusion process or the centrifuge process. In either case, "tailings" are produced as a by-product of the process. Uranium "tailings" are largely devoid of the U-235 isotope and consist mostly of U-238. This "depleted" uranium is only weakly radioactive and has many commercial uses because of Uranium's density, ranging from aircraft counter-weights, radiation shielding, boat keels, and munitions. Although uranium itself has a toxicity comparable to lead from a chemological standpoint, uranium is not easily absorbed by living organisms if ingested. The greatest danger comes from the accidental inhalation of the material if it is finely ground into a powder, because the particles can become lodged in the lungs so respiratory protection should be worn when working with powdered uranium compounds. However this is true for many fine particulate substances and is not necessarily unique to uranium. The fears of "depleted uranium" are largely unfounded and baseless.

When the uranium hexafluoride has been enriched to the desired level, it is converted into uranium dioxide which is a fine powder. The uranium dioxide is mechanically pressed into small pellets for use as fuel within a nuclear reactor fuel assembly. The pellets are stacked within tubes made from a metallic alloy of zirconium and serve as fuel rods in the nuclear reactor vessel.

Within the reactor vessel, the Uranium-235 isotope undergoes nuclear fission. Uranium-235 captures and absorbs a stray neutron to become the unstable isotope, Uranium-236. U-236 commonly decays into isotopes of barium, tellurium, krypton, and zirconium and releases energy and two or three neutrons in the process.



These stray neutrons impact other nearby atoms, causing the process to be repeated. In addition, the decay of the daughter products of uranium can create isotopes of other elements as well. Three of the more common decay chains of Uranium U-235 are represented by these equations:

U-235 + n ===> Ba-144 + Kr-90 + 2n + energy

U-235 + n ===> Ba-141 + Kr-92 + 3n + 170 MeV

U-235 + n ===> Zr-94 + Te-139 + 3n + 197 MeV

Interestingly enough, the atomic masses of the isotopes created from the decay of uranium-236 are usually around the low 90s to the mid to upper 130s because of the law of Conservation of Mass in regards to matter. The total mass of the isotopes resulting from the decay of uranium-236 and the neutrons that are released equals a mass of 236, just like the uranium-236 that they decayed from.

After a year or so, 33% of the fuel rods within a nuclear reactor are removed and the reactor is refueled with new fuel to keep the fission reaction going. The spent fuel rods are submerged in a pool of water within the power plant so that they can cool down long enough for further processing and for some of the more radioactive, shorter-lived isotopes to decay. After a few years, the assemblies containing the spent fuel are taken out to be disposed of.

The fission products that were created during the nuclear fission process can be divided into short, intermediate, and long-lived half-life categories.The half-life of an element is the average amount of time for the atoms within a sample of material to have undergone radioactive decay into another element. Most of the fission products have short half-lives that are less than a year. Although many of these isotopes are highly radioactive, they undergo decay during their period in the spent fuel pool and do not present a problem from waste disposal standpoint. Isotopes with an intermediate half-life can be somewhat problematic as they can range anywhere from a year to a century or two and can emit moderately high levels of radiation such as with the case of strontium-90 and cesium-137. These elements can be transmuted into less dangerous isotopes through further neutron bombardment but it is much more cost effective to simply dilute them with inert compounds to the point to where their radioactivity no longer poses a problem. Isotopes with half-lives lasting longer than three centuries can make up to 20% of the spent fuel to be disposed of, but one must keep in mind the inverse relationship between half-life and radioactivity.

Although the half-life of some of these fission by-products can be up to several billion years, they are only weakly radioactive to the point of being barely above the background levels of radiation that all of us are exposed to in our daily lives. As a case in point, potassium-40 has a half-life of 1.3 billion years, and it can set off alarms from radiation detection equipment. However, it is quite abundant in foods with large amounts of potassium in them, such as bananas and it is also found in our bones. However, it is very weakly radioactive as a person only gets an exposure of a few picocuries per year. Eating one banana a day for each day in a year would increase your exposure to radiation by 3.6 milirems per year and the average person receives several hundred milirems per year from naturally occurring background sources with no ill-effects.

Exposure to Radon Per Year By County (Red means high levels of radon)



New Cases of Cancer Diagnosed Per Year By County (High rates are purple)



In countries such as France that use nuclear reprocessing the useful isotopes are separated from the spent fuel assemblies. Since over 90% isotopes within a spent fuel assembly consist of un-fissioned uranium-235, and fissionable plutonium-239 this greatly reduces the volume of the material to be disposed of. The material from a spent fuel assembly can be reduced through reprocessing to a piece of material the size of a cigarette lighter with a half-life of three centuries. The fuel created from this process is known as mixed-oxide fuel, or "MOX" fuel. There are many different types of fuel reprocessing. The most common type is the PUREX method, although research is being conducted into "pyroprocessing" techniques.

Unfortunately, it is often politics that drive policy not common sense and the US is no exception. Although the once-through spent fuel disposal method is wasteful from the standpoint of throwing away a source of useful nuclear fuel, there is not much of it at all. There are three categories of "nuclear waste"; low-level waste, intermediate waste, and high-level waste.

Low-level waste consists of anything from pens and pencils from the offices within a nuclear power plant to the gloves and protective gear worn by personnel. Low-level waste from a nuclear power plant is often very weakly radioactive if it is radioactive at all, and is typically burned or buried close to the surface of a special landfill. Intermediate waste includes things like the actual components of the reactor itself in addition to the materials used in the construction of a nuclear reactor. There is usually not much in the way of intermediate waste to be disposed of and it is often buried in a shallow repository. High-level waste consists of the spent fuel that is marked for disposal.

This material is a metallic solid that has been encased in glass, lined with concrete, and sealed into an extremely durable cask. Tests have demonstrated the ability of these casks to withstand impacts with freight trains. Doomsday scenarios featuring terrorists stealing spent fuel material in order to construct bombs leave out the fact that the concentration of Uranium-235 needs to be enriched up to at least 90% for it to be weapons-grade material. Spent fuel does contain plutonium-239 which can be used for a plutonium bomb, but it is also contaminated with plutonium-240 which is a poison for a nuclear bomb as it absorbs neutrons without fissioning, effectively stealing the neutrons that would be able to strike plutonium-239 that would cause the rapid fission reaction. Fission would occur, but not in the rapid fashion that you would need it to for a nuclear bomb. To make things worse for a terrorist, it would be very difficult to separate the plutonium-239 from the plutonium-240 and it would require highly specialized equipment. It would simply be cheaper and easier to build a special reactor dedicated to producing weapons-grade material like most nations do.

Finally, the amount of high level waste to be disposed of is quite small. All of the high-level waste ever produced in the US as a by-product of nuclear energy could easily fit into a room the size of a high school gymnasium, just two stories high. Compare this to the mountains of coal ash and carbon dioxide generated by the burning of fossil fuels which will be just as toxic millions of years from now as it was the day that it was created.

In part two, we will be taking a look at thorium-based fuel cycles and how nuclear reprocessing works in detail. I hope that this post was easy to read and understand and that it was not too long or boring. Stay tuned!

Nuclear Technology Basics: Introduction (Fun With Fission)

Well, I have just returned from Thanksgiving break. As promised, I will begin a series of posts concerning nuclear reactor technology and how different types of nuclear reactors differ from one another. In the near future, I will also include a glossary entry on my blog that people can reference at a later date in case they come across terminology that is not clear to them.

At the most basic level, all thermally-based power plants share the same mechanics of how they generate electrical energy. A heat source is used to generate steam, which causes a turbine to spin from the pressure provided by the steam which is being used as a working fluid. The action of the spinning turbine is connected to a generator which converts mechanical energy into electrical energy by the rotation of the turbine. Coal, oil, biomass and natural gas facilities use the combustion of these fuels to provide heat to create steam while solar thermal power stations use light from the sun and convert it into a source of heat. Geothermal energy is also thermally-based because it relies on heat from under the ground in geologically active regions in order to operate. Non-thermally based methods of electricity generation such as wind, hydroelectric, and wave energy turbines are directly spun by the movement of wind or water. Photoelectric solar stations generate electricity from solar cells using the photoelectric effect.

In the case of nuclear energy, heat is harnessed from a sustained nuclear chain reaction to drive a steam turbine. Nuclear reactions concern the interaction of an atom's nucleus with the nuclei of other atoms. Heat and subatomic particles are often produced as a result of a nuclear reaction, depending on what type of nuclear reaction it is and what elements are involved. A nuclear chain reaction is when the products of one nuclear reaction trigger additional nuclear reactions within a whole group of nearby atoms in a positive feedback loop. There are two main types of nuclear chain reactions, nuclear fission and nuclear fusion.

Nuclear fusion is when the nuclei of a pair or more of atoms become fused together. The fusion of the atoms releases large amounts of energy. Nuclear fusion is what powers stars in space and has also been achieved within a human laboratory. While nuclear fusion could hypothetically be used as a source of terrestrial power, this has proven to be quite difficult. Surrounding each atom is a positively charged field known as the electrostatic force that tends to repel other atoms away before a pair of atoms can become close enough for their nuclei to fuse. It requires massive amounts of energy to overcome the repulsion of the electrostatic forces between neighboring atoms. Although the development of a nuclear fusion reactor has been a high priority for many governments around the world for many decades, nuclear fusion reactions being carried out in a laboratory have yet to result in a sustainable fusion chain reaction as it seems to require more energy to cause atoms to fuse than what is actually released during the fusion process when attempted on Earth. Because of this, it is likely that a working fusion reactor is still many years away from being a reality.

Nuclear fission is the second type of nuclear chain reaction. It is basically the process of causing atomic nuclei to fragment by ramming them with subatomic particles, which in turn causes the subatomic particles that result from the fragmented nuclei to crash into the nuclei of other atoms and repeat the process. Fission reactions produce heat and other forms of radiation depending on what the products of the fission reaction are. Since the successful operation of the first fission reactor in 1942 at the University of Chicago, all reactors that have been built by humans have been fission-based. Interestingly enough, the existence of naturally occurring fission reactors has also been observed in nature such as the Oklo fossil reactors in Gabon, Africa where the isotopic ratio of uranium deposits within the area allowed nuclear fission to sustain itself. In addition, the georeactor theory in the geological field postulates that the Earth's magnetic field and the heat that is produced from its core might arise from the activities of a naturally occurring reactor in its interior similar to what has been seen at Oklo. However, the georeactor theory has little in the way of evidence that supports it at this time although this may change in the future.

That is enough for now, as I do not want to get too long-winded with each post. The next part of this series will be a look at the basics of nuclear fuel, reactor design, and the fuel cycle itself. Feel free to ask any questions that you might have.

Thanksgiving Day Post

Today is Thanksgiving, which is an important holiday in the American calendar. Although Thanksgiving is said to commemorate the feast that the Native American tribes had with the early puritan colonists, the actual holiday itself was not established until 1863 by president Lincoln during his Thanksgiving proclamation. Up until then, many states scheduled their own "thanksgiving" events as an irregular observance, often during years when there was an especially bountiful harvest.

A magazine editor by the name of Sarah Josepha Hale wrote a series of letters to president Lincoln urging him to declare Thanksgiving as an official holiday. America was being torn apart by civil war and Mrs. Hale felt that nationalizing the custom of thanksgiving would help restore a feeling of unity throughout the US. The holiday was vaguely based on the Puritan harvest festival at Plymouth Plantation during 1621, which lasted three days from late September to early October. However, the idea behind Thanksgiving as set forth by Mrs. Hale was more of a celebration of "home and hearth" before the dead of winter rather than a specific historical event.

Since the Thanksgiving proclamation, Thanksgiving has been a national holiday that is celebrated on the last Thursday in November. Although the Puritans were more than likely eating venison at Plymouth, turkey is traditionally eaten on the holiday because West Point students were customarily served turkey during Thanksgiving, which had generally been a northeastern culinary tradition until then. Because of this, many West Point troops had been exposed to turkey, which helped cement its place on the Thanksgiving dinner table. After the end of World War II the famous Norman Rockwell image of a roast turkey serving as the symbol for "Freedom From Want" made turkey the national icon for Thanksgiving.

I myself had an enjoyable Thanksgiving today. The turkey was roasted upside-down to make sure that the breast area does not become desiccated during the cooking process. The dog, myself, and everybody else had plenty to eat and everybody seemed to have a good time. My favorite cut of a turkey is the leg or "drumstick" as I prefer dark meat to white meat when it comes to poultry flesh. The "white meat" on birds consists of fast-twitch muscle fibers, which are used for short bursts of intense activity, while the "dark meat" contains slow-twitch muscle fibers, which are mainly meant for sustained physical activity. Slow-twitch muscle fibers contain more myoglobin, which is a protein in muscle tissue that gives it a darker color.

I wish everybody an enjoyable Thanksgiving and I hope that they make sure they get enough to eat and that the company of their dinner guests is not too trying on their patience. The best part of Thanksgiving is often the leftover food the day after, as it is just as good if not better than the day before. Finally, one must not underestimate the remaining turkey skeleton, which is highly valuable for making soup stock out of after the last of the meat has been picked off by ravenous humans.

Happy feasting everybody!

All Fission Reactors Are Not Created Equal

For the next few days or so, I will be taking a look at the different types of nuclear reactors that have existed or have only been theorized about on paper. The reason being is that there are so many different potential reactor designs that it is often confusing to people outside of the field of nuclear engineering to determine how reactor designs differ and what the pros and cons of each design are. To make matters worse, the names of these reactors are often abbreviated to different acronyms making it even more difficult for laypeople to understand what the different terms mean.

This will be a bit of an undertaking, as there are literally hundreds of different reactor designs. Some have only existed on paper, others were only experimental prototypes, while others have been built but have since been decommissioned, either from age, lack of economic viability, or from politics. Although some reactor types are highly impractical or dangerous and have rightfully been consigned to the dustbin of history, there are some designs that would have been quite impressive from an economic and commercial standpoint.

At the moment, I am wondering how to proceed in terms of how I will talk about this. I am leaning towards a series of posts, with each post concerning a different "family" of reactor types based on what they use as their moderator materials. However, I am open to ideas from anybody who might offer suggestions.

Nuclear Energy in Asia is Going Full Steam Ahead

India has started construction of a new nuclear facility in Gujarat. It will be a pair pressurized heavy water reactors (PHWRs) that will generate 700 megawatts each. Planning started in January of this year after the site for the reactor was excavated within a short time frame of four months. Construction began today, and the reactor is expected to be online by 2015.

If India can do this, then why should it be so difficult to build new reactors in Europe and the US?

Possible New Reactor in Green River, Utah

The Emery County corporation has started to push the limits of its current energy infrastructure in Utah. Because of this, there has been a serious effort to construct a new nuclear facility near the town of Green River. The planned design would be a power plant with a pair of reactors that would generate 1,500 megawatts each. The nuclear powerplant would be a boon to the nearby community as well as the state of Utah with the positions that it would be able to offer people in its construction as well as its operation.

However, the plant has met opposition as opponents have attempted to prevent the facility from getting rights to water from the Green River that would be used for cooling. Most of the water that is circulated within a light water reactor is then returned to its source, while only a small amount of water is actually evaporated during the cooling process.

If this planned reactor goes forward, it would mark the first time since the 1970s that the construction of a new nuclear reactor was completed. If this project is successful, it might help revive nuclear energy in the US by encouraging the construction of new reactors in other locations. In any case, this is a promising sign that the "nuclear renaissance" will be more than just words.

First New Uranium Mine in Years

Phase I of the South Texas Palanga uranium mining project has been completed by the UEC (Uranium Energy Corporation) under-budget and on schedule. Phases II and III are expected to be completed in 2011. This marks the first time in several years that uranium demand has allowed for the opening of a new mining facility. Mining operations will commence using in-situ leeching methods, where water that has been acidified with carbon dioxide gas will be pumped into the mining site. This is what allows the uranium to be extracted from the surrounding limestone as the uranium is dissolved in the water when it is pumped out again during mining operations.

The economic activities of the uranium mining industry have been depressed for years because of the lack of demand for nuclear energy in the US since the mid-1980s. In the early 2000s the price of uranium bottomed out and it has only been in the last three years that the uranium market has been showing signs of recovery. As the price of uranium has increased since then, there has been a renewed interest in re-opening old mines and prospecting for new sources of high-grade ore.

Many people raise fears that the world supply of uranium will peak in 80 years. One must keep in mind that this estimate is based on existing production rates of uranium ore and nuclear fuel fabrication. There are many mines across the world that have been forced to close either through political pressure, or because existing world uranium demand could be easily met by a smaller number of mines. The amount of uranium required by most reactor types is quite small, especially when compared to the fuel consumption rates of fossil-fuel generators like coal and natural gas. The fissile isotopes of uranium are extremely compact compared to other energy sources. A single fuel pellet like those used in a nuclear reactor is the equivalent of 1,780 pounds of coal from an energy standpoint. Although hundreds of these pellets are used to fabricate fuel rods in a light water reactor, the amount of uranium required to fuel a reactor is still a rather tiny amount.

In the event of a large build-out of new nuclear reactors, it would not be too difficult to increase the production of uranium ore to meet an increased demand since uranium is such a common element. However, up until now there has been little need to do so. In fact, should the easily recoverable sources of uranium ever run out like the most dire scenario erroneously predicts, existing stockpiles of spent fuel could easily be reprocessed for more fuel. Finally, uranium can be extracted from seawater. Although the cost of recovering uranium from using this method would be roughly ten times conventional mining methods, it would still be economically viable as the operational costs of nuclear electricity generation are relatively insensitive to price increases of fissile material.

N^4 Has Been Redesigned!

I thought that my layout for N^4 was getting a bit stale, so I overhauled it. It turned out better than I thought. The new options for Blogger also allowed me to get rid of that damn space between my blog articles and the sidebar that Firefox kept insisting on doing for some reason. As a bonus, I also put in a new header image.