No abstract

Hull and end-piece wastes generated from reprocessing plant operations are expected to be disposed of in a deep underground repository as Group 2 TRU wastes under the Japanese classification system. The activated metals that compose the spent fuel assemblies such as Zircaloy claddings and stainless steel nozzles are mixed and compressed after fuel dissolution, and then stuffed into stainless steel canisters. Carbon 14 is a typical activated product in the hulls and end-pieces and is mainly generated by the 14N(n,p)14C reaction. In the previous safety assessment of the TRU waste in Japan, the radionuclides inventory was calculated by ORIGEN-2 code. Some conservative assumptions and preliminary estimates were used in this calculation. For example, total radionuclides generated from a single type of fuel assembly (45 GWd/tU for a PWR unit), and the thickness of the Zircaloy oxide film on the hulls (80 μm) were both overestimated. The second assumption in particular has a large effect on exposure dose evaluation. Therefore, it is essential to have a realistic source term evaluation regarding such items as the C-14 inventory and its distribution to waste parts. In the present study, a C-14 inventory of the hull and end-piece wastes from the operation of a commercial reprocessing plant in Japan corresponding to 32,000 tU (16,000 tU in each BWR and PWR) was calculated. Analysis using individual irradiation conditions and fuel characteristics was conducted on 6 types of fuel assemblies for BWRs and 12 types for PWRs (4 pile types × 3 burnup limits). The oxide film thickness data for each fuel type cladding were obtained from the published literature. Activation calculations were performed by using ORIGEN-2 code. For the amount of spent assembly and other waste characteristics, representative values were assumed based on the published literature. As a preliminary experiment, C-14 in irradiated BWR claddings was measured and found to be consistent with the calculated activation. The total C-14 inventory was estimated as 4.46×1014 Bq, consisting of 2.58×1014 Bq for BWRs and 1.87×1014 Bq for PWRs, and is consistent with the safety assessment of 4.4×1014 Bq. However, the distribution of the C-14 inventory to hull oxide, which was estimated under the assumption of instantaneous radionuclide release in the safety assessment, decreased from 5.72×1013 Bq (13% of the total) in the previous assessment to 1.30×1013 Bq (2.9% of the total; consisting of 1.48×1012 for BWRs and 1.15×1013 for PWRs). In other words, the exposure dose peak is reduced to approximate 25% of its previous value due to the use of detailed oxide film data that the BWR cladding has a thin oxide film. Other instantaneous release components for C-14 such as the fuel residual were negligible.

Characterization of C-14 in PWR Radioactive Wastes has been researched and formation mechanisms of C-14 have been discussed. It was found from the research results that the chemical form of C-14 existed in primary coolant was organic and was low molecule compounds which are soluble in water． On the other hand, most of C-14 components existed in condensed liquid waste and existed on solid waste were insoluble in water and chemically stable. The insoluble C-14 component was considered to be produced by activation reaction between neutron and substances with nitrogen. Those were thought to be decomposition substances escaped from high molecular organic materials, such as ion exchange resin, diaphragm seal, etc.

The formation mechanism and chemical form of insoluble C-14 found in PWR need to be examined in order to predict its environmental behavior after disposal. This study investigates the alteration of ion-exchange resin by heating and irradiation, because past studies indicated the ion-exchange resin may be the origin of insoluble C-14.Resin was heated at 300 °C in solution with low oxygen content to simulate the environment of PWR coolant. The sulfo group was found to detach within 8 h, and structures similar to polystyrene were remained. This is followed by detachment of H from the alkyl group, condensation reaction, and the formation of amorphous carbon-like structure. After heating for 24 and 96 h, the resin was irradiated by 60Co γ-rays in the solution. The FT-IR and TG measurements after irradiation suggested that OH and COOH groups were formed on the surface of the resin. These functional groups may be involved in reactions that finally form the amorphous carbon.In addition, the characteristics of heated and irradiated resin were compared to real insoluble-C (CRUD) sample in PWR (in Appendix).

In the safety assessment of a radioactive waste disposal, it is extremely important to assess the migration behavior of long-half-life radionuclides in the disposal environment. The migration behavior in the disposal environment for a radionuclide varies with the chemical form of the nuclide. In particular, C-14 has various chemical forms, and its migration behavior in the disposal environment substantially varies with the chemical forms. It has been reported that hardly soluble C-14 is generated in PWRs. However, the chemical form of this hardly soluble C-14 is little known. In this study, the thermal decomposition behavior of particles containing C-14 and the mass spectra of gases released through thermal decomposition were analyzed in order to examine the chemical form of C-14 generated in PWRs. In this study, the gases released during the thermogravimetric (TG) analysis were partially oxidized to CO2, trapped in an alkaline solution and analysed for C-14. Another part of the gas was analysed directly by mass spectrometry (MS). The residues obtained after TG analysis were also analysed for C-14 by oxidizing the residue to CO2, trapping the CO2in alkaline solutions and analysing C-14 by LSC. During TG-analysis in inert gas (He) atmosphere, about 90% of C-14 was found in residue, while when air was used during TG-analysis, no C-14 could be detected in residue. From the MS-analysis of species released during TG-analysis in inert gas, fragments regarded as originating from ion-exchange resins were detected in released gases. Based on this result, it was found that while substances originating from ion-exchange resin were present in radioactive particles generated in a PWR, the main part of C-14 was contained in the residue after heating in the form of thermally stable substances, easy to be oxidized by air at high temperature. It was not possible to determine their exact chemical composition in this work but also, In addition, sorption of insoluble C-14 to cementitious materials was preliminarily examined. As a results, the concentration of insoluble C-14 decreased greatly in 7 days. That means insoluble C-14 tended not to stay in water. Elucidation of the sorption mechanism in the disposal environment of these C-14 is also a future task.