Study On Safety Analysis Technique For Spent Fuel Pool Based On System Analysis Code

As an interim storage to accommodate large quantities of spent fuels,the spent fuel pool(SFP)is designed to remove the substantial decay heat generated by spent fuel assemblies from the storage pool water,which also provides shielding from the radiation and precludes the possibility of critical accidents.The safety issue and potential risk for the SFPs raised concerns extensively after the SFP event at the Fukushima Daiichi unit 4,and the research involving SFP safety assessment and emergency mitigation becomes significant.Due to the shortage of dedicated code for SFP analysis and the limitation of code applicability,most of the available code for the SFP analysis does not possess the general capability to treat the full spectrum phenomena that could occur during emergency conditions in the SFP.In this paper,based on the demonstration of the code capability and reliability of analysis model,advanced SFP modeling techniques of system analysis code was successfully developed,to analyze both thermal hydraulic and severe accident.Besides,an extended refilled operating mode was proposed and designed for the decommissioning plant,and the assessment of the feasibility and safety for the special operating mode was performedThe work involved in the thesis is concluded as below:(1)According to the study on important phenomena and physical process during the thermal hydraulic response and severe accident,the applicability of the system code for SFP analysis was assessed.The significant phenomena and model involving in SFP analysis model includes natural circulation in SFP,evaporation or boiling,radiative heat transfer,hydraulic resistance model,oxidation model,emergency spray.The investigations on important phenomena and thermal hydraulic model are beneficial to the development of SFP analysis model.(2)The SFP modeling techniques was developed by RELAP5 code,with the general capability to treat the full spectrum phenomena that could occur during an accidental codition in the SFP.To develop the MELCOR SFP model,the whole spent fuel storage in the SFP is modeled as the entire core and a lower plenum,dependent on the general characteristic between SFP and reactor core.A comparison of a counterpart calculation of both steady state and loss-of-cooling accident was demonstrated,with both RELAP5 and MELCOR.The cross verification between the two codes is performed to assure SFP modeling without any major deficiency.The SFP model was further applied to evaluate SFP emergency operation procedure involving makeup and spray mitigation in the event of a loss of cooling or loss-of-coolant.(3)The severe accident progression and potential phenomenon after loss of cooling accident was demonstrated.The SFP MELCOR model was applied to calculate the severe accident progression both after loss of cooling and loss of coolant in the SFP.The essential results of the severe accident progression including fuel degradation,rack structure heat up and failure,oxidation reaction,air cooling,hydrogen generation was investigated.Moreover,the combined analysis utilizing RELAP5 and MELCOR was performed to study the severe accident progression after the loss of coolant accident.(4)An extended refilling operating mode is proposed and designed after the permanent shutdown of reactor.During the extended refilling operation,the reactor vessel,refilling cavity and the SFP are all connected and combined into a new extended spent fuel storage pool.Systematic evaluation was also performed to ensure the feasibility and the safety of the extended refilling mode operation,by means of development of the advanced modeling techniques for the extended storage pool.Moreover,an assessment of the effectiveness of existing SFP emergency mitigation was performed,and a suggestion of the emergency mitigation was provided for an effective cooling to the reactor core after loss of coolant accident.The study provides a methodology of development on modeling technique for SFP safety analysis based on system analysis code,and systematic analysis and evaluation was performed to verify the modeling technique for SFP safety analysis.An extended refilling operating mode is proposed and designed,accompanied by the evaluation of the feasibility and the safety of the special model operation.The work provides valuable information for the development and application in SFP safety analysis.Moreover,the proposal of the extended refilling operating mode offers the engineering significance for the decommissioning plant.