Nuclear Criticality Safety Calculating For Dissolution And Pulsed Extraction Column

This thesis describes the necessity of spent fuel reprocessing,and introduces the main reprocessing plants in the world as well as the dissolver and pulse extraction column of reprocessing plants,and analyzes the problems existing in the dissolver and pulse extraction column of reprocessing plants at present.The critical safety calculation and analysis of the critical equipment in the post-treatment plant can effectively avoid the nuclear critical reaction during the operation of the equipment.The critical safety design principle and the critical control method of the post-treatment plant are also introduced.At present,the domestic reprocessing plant can only deal with the uranium dioxide core of fuel rod with an initial ²³⁵U concentration of 3.30%.To meet the need of reprocessing plants'design when the initial ²³⁵U concentration in UO₂ core of fuel rod is 4.45%or 5.00%,the MCNP program is used to calculate and investigate K_eff limited by nuclear criticality safety of dissolver and pulsed extraction column and to determine whether the dissolver and pulsed extraction column with set parameters can deal with the spent fuel with an initial ²³⁵U concentration of 4.45%or 5.00%in the uranium dioxide core of fuel rod.K_inf limited by three different ²³⁵U enrichments?3.30%,4.45%and 5.00%? are respectively calculated and analyzed according to the short rod model in the case of specular reflection of dissolver,and the best radius and slowing down model are found out.According to the best radius and the best slowing down model,the best grid model was established to calculate the nucleus critical safety of k_eff when the three ²³⁵U enrichment degrees?3.30%,4.45%and 5.00%?are treated by the dissolver.Through the analysis,it can be concluded that the dissolver with set parameters cannot deal with the spent fuel when ²³⁵U enrichment degrees is 4.45%or 5.00%.In order to solve this problem,some schemes are proposed,such as adding neutron poison,reducing the size of the basket of the dissolver and annular arrangement of multiple dissolvers,and the relevant critical control parameters are given.There are two systems in the pulse extraction column:organic phase continuous system and water phase continuous system.When the concentration of ²³⁵U is 3.30%,4.45%,5.00%and the concentration of ²³⁹Pu is 100%,the k_eff of total water reflection and vacuum reflection are calculated,respectively.The k_eff of the central column of the extraction column in case of accident was also calculated.The results show that when the concentration of ²³⁵U is 5.00%,k_eff in the whole water reflection,vacuum reflection and accidental feeding of materials into the central column of extraction column in the water phase continuous system are all beyond the critical safety acceptance criteria.Therefore,it is concluded that the large-scale annular baffled pulse extraction column cannot deal with the material with the ²³⁵U concentration of 5.00%.In order to solve this problem,the scheme of adding neutron poison,adding polyethylene in the central column,changing the size of equipment and so on are put forward,and the critical safety calculation of the scheme is carried out,and the relevant critical control parameters are given.