Sunday, March 13, 2011

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 T
ōkai Facility

This nuclear power station is in T
ōkai 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 Honsh
ū. 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.

Friday, March 11, 2011

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.