|Mansion with No Toilet|
|The Journey of Spent Fuel Rods|
|Current State and Problems of Nuclear Plants|
A nuclear power station is sometimes referred to a “mansion with no toilet”. Why?
Oil, coal, and natural gas have been used as fuel for thermal power plants in Japan. When these fuels are burnt, they create carbon dioxide by joining their carbon with oxygen in the air (oxidation). What is created when uranium is “burnt” at a nuclear power plant?
“Burning uranium” does not mean “oxidizing uranium in the air” but means “fissioning uranium”. When an atomic nucleus of uranium 235 splits into two different atomic nuclei, a lot of heat is released.
Because of this, people call nuclear fission “burning uranium” and call a rod containing uranium pellets a “fuel rod”, although nuclear fission is a totally different reaction from oxidation. Fuel rods (spent fuel) taken out of a nuclear reactor are like cinder, except nuclear power generation began without a clear idea for how to treat this cinder. The expression “mansion with no toilet” is an aphorism for the nuclear policy. Spent fuel is different from cinder of burnt trees; it contains a large quantity of radioactive materials. The danger of radioactive materials is obvious; therefore, there has been strong opposition against building nuclear reactors for commercial power generation without establishing technology for disposal. Let’s have a look at detailed contents of spent fuel.
(Explanation for Fig.3)
A fuel rod is made of a metal sheath that contains oxidized and hardened uranium pellets containing 3-5% uranium 235 (U235). 50-80 fuel rods are assembled into a fuel assembly for use in a Boiling Water Reactor (BWR); 200-300 fuel rods are assembled into a fuel assembly for use in a Pressurized Water Reactor (PWR). BWR installs 400-800 fuel assemblies; PWR installs 100-200 fuel assemblies. (See Principle of Nuclear Power Generation)
(Explanation for Fig.4)
1% of uranium 235 (U235) and 95% of uranium 238 (U238) remain unchanged. 1% of plutonium is created when U238 absorbs neutrons. 3% of fission products are the general term for various nuclei that are created when uranium and plutonium fission.
Although it is omitted in Fig.4, approximately 0.1% of transuranic elements (other than plutonium) are created when uranium absorbs neutrons. Fission products and transuranic elements are often unstable nuclei; they keep changing and become different nucleuses while emitting radiation. Therefore, composition of spent fuel will always be changing. Spent fuel treatment is difficult: spent fuel emits not only radiation but also decay heat; spent fuel contains a mixture of nuclear species whose radioactive half-life and chemical nature vary. (As fuel, uranium oxide is used instead of uranium metal. In Fig.4, oxygen is omitted.)
Examples of fission products: strontium 90 (28 year radioactive half-life), cesium 137 (30 year radioactive half-life). Examples of transuranic elements: americium 241 (433 year radioactive half-life), curium 244 (18 year radioactive half-life).
Note: When fission products and transuranic elements are emitted to the environment, such as by atomic bombs or the Chernobyl nuclear power plant accident, they become a great threat for creatures. Radioactive fallout damages creatures by exposing them to radiation externally and internally. Cesium 137 released from the Chernobyl nuclear power plant accident has caused a grand-scale food contamination, and the damage is still continuing.
The course of spent fuel rods differs depending on the type of the nuclear reactor. Let’s follow the journey of the most common spent fuel rods used in Japanese nuclear reactors. (The majority of spent fuel rods are still stored in cooling pools; the first stage of its course.)
(a) Storage in cooling pools at nuclear power plants
Fission products and transuranic elements are often unstable nuclei; while emitting radiation, they keep changing to different nucleuses (=decay, or disintegration). While fission products and transuranic elements are changing, they emit a great amount of heat. Thus, fuel assemblies need to be cooled by water. The intermediate storage plant (under contemplation) will store spent fuel that nuclear power plants cannot store any more while cooling them.
The contents of fuel rods are taken out from the metal sheath and uranium and plutonium are separated chemically. The rest remains as waste liquid that contains fission products and transuranic elements. (See Fig.6. For more detail, go to Problems with reprocessing plants in "Can Nuclear Fuel be ‘Recycled’?")
Vitrification of waste liquid after reprocessing: waste liquid is mixed with molten glass and poured into stainless steel containers called canisters. It is cooled and stabilized. This vitrified waste (high-level radioactive waste) is planned to be 130 cm high, 43 cm in diameter, 500 kg gross weight.
(c) Storage at the High-level Radioactive Waste Storage Management Centre
Even after vitrification, disintegration of atomic nuclei in high-level radioactive waste continues and generates decay heat. This vitrified waste is stored and managed for 30-50 years at the storage management centre, to allow it to cool (air cooling). One year’s operation of a nuclear power plant (output 1,000,000 kw) generates spent fuel equivalent to approximately 30 rods of vitrified waste. The amount of spent fuel generated from Japanese nuclear power plants by the end of 2002 is estimated to be the equivalent to approximately 16,600 rods of vitrified waste. So far, spent fuel has been reprocessed and vitrified partially in Tokai, Japan, but mainly in Britain and France. By the end of 2003, 890 rods of vitrified high-level radioactive waste are stored in Japan (760 rods in Rokkasho, Aomori Prefecture, and 130 rods in Tokai, Ibaraki Prefecture).
(d) The geological disposal plans
The amount of vitrified waste converted from spent fuel generated from Japanese nuclear power plants by 2020 is estimated to be about 40,000 rods. These vitrified waste rods are planned to be buried at a stable stratum deeper than 300m. The plan is to store vitrified waste (43cm in diameter, 134cm high) in carbon steel overpacks (19cm thick) and wrap them with buffer material (bentonite, a kind of clay), and bury it at the disposal tunnel in bedrock. After burying vitrified waste over a couple of decades, the tunnels will be filled in.
The Nuclear Waste Management Organization of Japan (NUMO), established as the implementing body for disposal work, is aiming to commence final disposal in the 2030s. The cost is estimated to be 3,000,000,000,000 yen.
Spent fuel rods are leftover from nuclear power generation. Handling, even excluding reprocessing, this leftover waste is a huge procedure. Not for generating electricity, but just disposing of nuclear waste will need the intermediate storage plant, the high-level radioactive waste storage management centre, and the geological disposal site, where the environment and human health will always be at risk from radioactive contamination.
The intermediate storage plant is still at the planning stage. Until now, nuclear power plants have managed to store spent fuel rods by narrowing the space between each fuel rod, or building more storage pools in the power plant sites. Spent fuel rods are also being transported to the fuel pools of the reprocessing plant in Rokkasho, where reprocessing has not started yet. Even with these measures, a shortage of spent fuel storage is expected. A shortage of spent fuel storage will stop it being removed from nuclear reactors thus preventing the reactors’ operation and stopping electricity generation. It can be viewed as a sort of constipation.
The completion of the reprocessing plant in Rokkasho has been delayed due to defects in welding. The number of spent fuel rods in the storage pool will continue to grow until reprocessing begins. Even if reprocessing is put into operation, it is questionable that the plant would be able to reprocess all spent fuel generated in nuclear power plants. The storage capacity of the high-level radioactive waste storage management centre in Rokkasho is 1440 rods. Although there is a plan for building storage for another 1440 rods, the concern for a shortage of storage will still remain if vitrification of nuclear waste proceeds as planned.
Considering its risks (possibilities of explosion and radiation contamination) and economical value (the value of recovered uranium and plutonium), even those who are in nuclear industry insist that the operation of the reprocessing plant should be reconsidered. Some countries dispose of spent fuel without reprocessing (in this case, both uranium and plutonium are disposed of together). The Japanese government’s policy is to reprocess all spent fuel.
Technology of neither reprocessing nor geological disposal has been established yet; thus, the cost for research and development alone would be enormous. Nuclear power requires a great amount of natural and human resources, energy, land and money for nuclear waste disposal. (Waste generated at nuclear power plants is not only high-level radioactive waste; but discussion about other waste is not discussed here.)
Before use, radioactivity in fuel is emitted from uranium only; however, in spent fuel more radioactive materials are found, such as fission products and transuranic elements. The radioactive level of fuel rods just after being taken out of the reactor is 100,000,000 times higher than before use, as radioactivity in fission products is enormous. Although the radioactive level declines as time passes, when fuel rods are reprocessed, their level of radioactivity is still 100,000 times or more than the level in fuel rods before use. Even after waste is vitrified, disintegration of fission products and transuranic elements continues until their atoms become stable. Humans cannot stop this disintegration. Compared to radioactive materials that exist in the natural world, vitrified waste emits prodigious amounts of radioactivity, a phenomenon which has never existed in human history before. The only way to protect the human body from this radioactivity is to isolate vitrified waste. It is believed that it will take at least 1 million years for these radioactive materials to reach the radioactive level of a fuel rod before use.
While vitrified waste is stored at the high-level radioactive waste storage management centre for a certain period of time, it is possible that canisters containing vitrified waste would corrode and radioactive materials would leak out. There is no way to make this material safe other than either repackaging vitrified waste or dumping it in a remote place (e.g. deep underground) so that humans won’t be affected even if the canisters corrode. The government’s policy is to bury vitrified waste deep underground (geological disposal). (See Spent Fuel Treatment and Disposal) Against the government’s geological disposal plan, those who suspect its safety issues insist that the method of waste treatment should be reviewed. Instead of sticking to the fixed policy, a search for minimising affects of the negative legacy the nuclear power leaves behind society is needed.
Note 1: The radioactive level at the surface of a vitrified waste rod once it is made is 14,000 sieverts/h (the lethal dose is approximately 7.8 sieverts, which is a 2 second exposure). After 50 years, the radioactive level declines to 0.01 sieverts/h (0.001 sieverts, which is a 6 minute exposure, is equivalent to the annual radiation exposure limit for an ordinary person).
Note 2: Repackaging vitrified waste rods has never been done. Also, the storage plant won’t last forever as it will be damaged by radiation.
Note 3: The OMEGA plan – the aim of the plan is to reduce the half-life of all radioactive materials to be less than 10 years (conversion/extinction of nuclear species) by partitioning high-level radioactive waste liquid and, nuclear reaction in a fast reactor or a high-energy accelerator. It is expected that this method would reduce the burden for geological disposal. It is still at the research stage. It will require a huge amount of energy to change the structure of atomic nuclei.
Fig.2 Nuclear fission
Fig.3 Fuel rod
Fig.4 Changes inside of the fuel rod
Fig.5 The Journey of spent fuel rods
Fig.6 Reprocessing process, radioactive waste and environment contamination