Site reclamation will be carried out over 30 years by dividing the work into four stages (see Table 1).
The total amount of solid radioactive waste from the decommissioning will be 26,700 tons. This includes waste which is not required to be handled as radioactive material (because it is below the clearance level), but does not include sodium. The total amount of sodium in the primary and secondary systems amounts to around 1,700 tons.
The decommissioning costs are assessed as 150 billion yen. According to media reports, an additional 225 billion yen is needed for maintenance and management of facilities. Moreover, additional costs, not included in the above, must be added on to meet the new regulatory standards. It is said that buildings, etc. are to be demolished and removed, but this demolition refers to buildings, etc., “other than underground buildings that have been confirmed not to be contaminated with radioactive substances, underground structures and building foundations.” If these are included, costs and waste volumes would further increase. This will also influence how the vacated site is eventually used.
It is thought that the reason why the plan has not been firmly drawn up from the second stage onward is that it has not yet been decided how the spent fuel and sodium will be handled. Fukui Prefecture is demanding that all the fuel, sodium and waste material be removed from the prefecture.
Stored at Monju are the spent fuel assemblies, fresh fuel assemblies, both fresh and used blanket fuel assemblies as well as experimental fuel assemblies (all referred to as “fuel assemblies” below). The amounts of these and the storage locations are summarized in Table 2.
When removing the fuel assemblies from the core, “mock” (empty) assemblies will be substituted for those removed. As the Monju fuel assemblies are fixed at the lower end, but not at the upper end, if mock assemblies are not inserted there is a possibility that some fuel assemblies will not maintain their correct position as removal progresses, making their removal impossible. Originally, mock assemblies were to be substituted for all the fuel and it was stated that the manufacture of new mock assemblies would be needed for this. However, this plan was later amended and the number of assemblies to be substituted has been reduced. This reduction is said to be based on an assessment which was carried out to determine how many fuel assembly replacements would be necessary to ensure that assemblies would not shift out of their correct position during the removal process.
Further, it was also said that to prevent the fuel coming into direct contact with water, the assemblies would be inserted into canisters for storage in the fuel pools, but this plan has been basically abandoned. However, since the number of fuel racks is limited, racks for canister use will be employed. Some of the assemblies will therefore be inserted into canisters for storage.
Fuel assemblies removed from liquid sodium will have sodium attached to them. This must be removed before the assemblies are placed in the fuel pool. This is achieved by using steam and water.
JAEA says that the liquid sodium level in the reactor core will be maintained until fuel removal is completed. Therefore, as the fuel assemblies are removed, sodium will have to be added to take up the volume that is not filled by mock assemblies. In this situation, it is possible that the remaining fuel assemblies may shift, in an earthquake, for example. In a seismic backcheck conducted in 2010, assessment of the vertical movement of the assemblies due to an earthquake showed that there was an extremely small safety margin (38mm movement for an allowable 40mm). The premise of this assessment was that the core contained a full load of fuel assemblies, but in the case that some of the assemblies are missing, there is a fear that the fuel assemblies will shift or topple over, resulting in inability to remove them. The reason is that the positions of fuel assemblies is controlled by computer. It can be said that the failure to conduct such an assessment is a serious deficiency.
Further, because an accident of this nature could occur, an assessment regarding criticality safety is also necessary. There is a fear that a severe accident may occur, in which case public exposure to radiation is inevitable. If fuel assemblies cannot be removed, it is thought that it may become necessary to conduct a fundamental review of the decommissioning schedule plan, e.g. prioritizing the removal of the primary system sodium.
JAEA has stated that the policy of the government when it made the decision to decommission Monju was to either recycle or sell the sodium, but the application says that the handling and disposal of the sodium will be “finally determined by the government” before the time when removal of the fresh fuel assemblies has been completed. At the same time, spent fuel “will be reprocessed in Japan or in a country that has signed an agreement for cooperation on the peaceful uses of atomic energy,” the specific plan and method being finally determined by the government by the end of stage one. The policy for fresh fuel is to transfer it to domestic dealers. This will be reflected specifically in the application when the government’s final decision is made.
However, as there is no reactor core in Japan that uses precisely the same fuel assembly specification, it is thought unlikely that a buyer for the fresh fuel will appear. Furthermore, there is no facility for reprocessing the spent fuel in Japan, and as Monju was developed using Japanese technology, it is uncertain whether the spent fuel can be reprocessed as it is overseas. While there is no prospect for the reprocessing of the spent fuel, making it a matter of policy to reprocess the spent fuel is tantamount to requiring that it be handled in accordance with Japan’s business-as-usual ‘everything must be reprocessed’ policy. Reprocessing should be abandoned and the spent fuel reassessed as radioactive waste.