Responses to the Regulatory Standards for the Rokkasho Nuclear Fuel Reprocessing Facility Concerning Changes in the Fuel Conditions
by Kamisawa Chihiro, CNIC staff
More than one hundred meetings to discuss compliance of the Rokkasho Nuclear Fuel Reprocessing Facility with the new regulatory standards have now been held since they began on January 17, 2014. Japan Nuclear Fuel Limited (JNFL) has summarized and explained the points made thus far and the discussions are proceeding with the Nuclear Regulation Authority (NRA) indicating its questions and setting forth its demands for revisions.
Points of contention over safety in the court case
The lawsuit (The 10,000 Plaintiff Group Lawsuit to Stop the Nuclear Fuel Cycle) to cancel the business designation of the Rokkasho Nuclear Fuel Reprocessing Facility, now ongoing at the Aomori District Court, has taken up issues concerning safety, earthquakes and active faults, aircraft crashes and collisions, volcanic eruptions, criticality calculations and criticality accidents (criticism of reliance on transportability), and various accidents that have occurred at the Rokkasho Nuclear Fuel Reprocessing Facility. Regarding earthquakes and active faults, it was mentioned in issue 545 (November 2019) of CNIC’s Japanese newsletter that the continental shelf break fault and the Rokkasho fault had been effectively eliminated from the discussions. I will leave the description of the later results and assessment of JNFL’s additional survey to another opportunity.
Concerning aircraft crashes and collisions, a study meeting will be held soon (February 2020) by the lawyer Ito Yoshinori, who has taken charge of this issue over a long period. He is scheduled to speak on all aspects of this issue, including the ridiculous nature of the calculations of crash probabilities that are used to judge whether or not a building or structure requires a protective design, the inadequacy of limited protective design specifically for the F-16 fighter aircraft, and the issue regarding the fact that the no-fly zone has not been observed in air space over nuclear power facilities.
Accidents that might possibly occur at the Rokkasho Nuclear Fuel Reprocessing Facility
A reprocessing plant is a nuclear chemical plant that consists of several groups of buildings, vent stacks, cooling towers, and so on. After shearing spent nuclear fuel and dissolving it in concentrated nitric acid, uranium and plutonium are extracted, and high-level liquid waste is separated out and vitrified.
At each of these steps, it is possible to imagine several types of accidents that could lead to the release of large amounts of radioactive substances. These are inability to cool the spent nuclear fuel, criticality (dissolving process, plutonium refining process, etc.), boiling of the solution, generation and explosion of hydrogen due to radiation decomposition, organic solvent fires, generation and explosion of red oil (an intermixture of a reddish explosive substance that may appear when tributyl phosphate, organic solvents and concentrated nitric acid are heated: evaporation, enrichment and denitration processes)
Selected conditions of a severe accident
The following definition of a severe accident at a reprocessing plant, “an accident occurring under conditions more severe than those stipulated by the design,” is given in the “Regulations on Spent Fuel Reprocessing Businesses.”
(1) A criticality accident occurring in a cell (a room divided off by steel-reinforced concrete, etc.), (2) Evaporation/solidification of a radioactive solution originating from spent fuel, (3) A hydrogen explosion in a cell, (4) A fire or explosion due to organic solvents and other substances generated in a cell, (5) Significant damage to spent fuel during storage, and (6) Leakages of radioactive substances in cells or buildings.
The evaporation/solidification in (2) refers to an accident that might occur if a euthermic (heat generating) radioactive solution cannot be cooled. The temperature rises causing boiling, and as the moisture evaporates radioactive substances are released, this eventually leads to a dry state where the release of radioactive substances continues, possibly leading to destruction of the vessel by explosion.
In the review of nuclear power stations, severe accidents are defined as “significant damage to the nuclear core” and “significant damage to fuel assemblies stored in fuel assembly storage facilities or spent fuel.” In comparison with this, reprocessing plants leave a large amount of room for arbitrariness on the part of the business operator when making the condition selections for severe accidents.
Changes in fuel conditions
In the past, JNFL has applied for alterations in the cooling period after removal from the nuclear reactor of the spent fuel it handles at Rokkasho Nuclear Fuel Reprocessing Facility. Under the current permission, the cooling period is at least one year before acceptance by the reprocessing facility and at least four years before the start of shearing. Currently, the latest (March 8, 2019) written amendment in the application for altering cooling periods is as follows: At least four years before reprocessing facility acceptance, however for a storage volume of 3,000 tons, of this, at least 2,400 tons shall be cooled for at least 12 years, and when 600 tons or less is stored for between 4 to 12 years, the cooling period up to start of shearing shall be at least 15 years.
Due to this change in the fuel conditions, in relation to the consideration of a severe accident, the volume of radioactive ruthenium 106 (half life approximately 373.6 days) embodied in the high-level radioactive liquid waste will be greatly reduced, the amount of decay heat given off will be reduced and may be handled as a small value for analytical purposes. Because of this, the analytical hurdles to be cleared for evaporation/solidification (ruthenium is easily released) and hydrogen explosions become drastically lower.
The graphs shown here are plots of the impacts and speed of progression for severe accidents by accident type, the original plots being by JNFL. The horizontal axis shows radioactive substances released to the environment converted to cesium 137, the values increasing to the left. The vertical bold dashed line is the judgment criterion of 100 terabequerels. The vertical axis shows the time leading up to an accident, the time increasing toward the bottom of the graph.
Compared with four-year cooling, 15-year cooling shows a reduction from several thousand terabequerels to several terabequerels for maximum volume of released radioactive substances in evaporation/solidification (black squares). The progression time of hydrogen explosions (black circles) for four-year cooling is on the 0.1 day line, but the fastest for 15-year cooling stretches out to around one day. In particular, please note that while the release limit for severe accidents in the four-year cooling was exceeded in five accident cases, in the 15-year cooling this did not occur. This was a “realistic response” proposed by the NRA side simply so that they could clear the judgment criterion for a severe accident by changing the fuel conditions. It does not really indicate that the reprocessing plant is any “safer” in terms of the kinds of accidents that might occur, as mentioned above.
Due to this change in fuel conditions, much of the spent fuel from NPPs will remain onsite at the NPP for at least 12 years after it has been removed from the reactor. This attempt to alter cooling periods also shows clearly that the regulatory authorities are not adequately considering the kind of severe accident that we really must consider, one in which plutonium (plutonium 239 has a half-life of 24,000 years) is released.
Nuclear Regulation Authority, Handouts at meetings on the Rokkasho Nuclear Fuel Reprocessing Facility http://www.nsr.go.jp/disclosure/committee/yuushikisya/tekigousei/nuclear_facilities/REP/committee/index.html
Atomic Energy Society of Japan, Reprocessing and Recycling Section, 13th Section Seminar on Reprocessing and Recycling (March 9, 2018), Theme II: Safety Improvements for Reprocessing Facilities against Severe Accidents, Basic Policy for Dealing with Severe Accidents, etc.（http://www.aesj.or.jp/~recycle/rsemi13_tm2-1.pdf）