Analysis Of Hypothetical Core Disruptive Accident In Lead-based Research Reactor

Lead-based reactor is an important candidate reactor for the Generation-IV nuclear energy systems and the Accelerator Diven subcritical Systems.Like other fast reactors,lead-based reactor is not designed in the most reactive configuration.The reactivity would increase due to the molten material relocation,which may result in a large amount of mechanical energy released.To ensure that the lead-based reactor can contain the radioactive materials which may threaten public and environment under severe accident,it is nessary to analyze Hypothetical Core Disruptive/Disassembly Accident(HCDA).However,few studies relating to the HCDA in lead-based reactor are currently available so that the role and safety goal of HCDA in lead-based reactor accident analysis is unclear,and the level of understanding of the physical phenomena involved and the accident evolution process are also very limited.In this paper,based on the design of China lead-based research reactor,HCDA studies were performed from the macroscopic evaluation approach to the micro mechanism of accident phenomenon aiming at the existing problems.The main research contents including:(1)Based on the latest nuclear safety philosophy in post-fukushima era and the experience accumulated in sodium-cooled reactor,an evaluation approach for lead-based reactor was established,which treated HCDA as design extention condition,proposed the deterministic and probabilitic requirements for re-criticality free.This approach first clarified the role of HCDA in design and licensing of lead-based reactor,the safety goal for HCDA should be achieved.Then it was further pointed out that hybrid methods including mechanistic phenomenological and probabilistic method should be employed,which provides a methodology for subsequent chapters.(2)Based on the phenomenological method,numerial simulation study was performed for the key HCDA involved physical phenomena in lead-based research reactor by using NTC-2D code.The unique thermohydraulic phenomena were identified:the molten corium can sweep out of the active core,which has pontential to avoid the re-criticality.But the cladding melt would freeze resulting in the blockage of flow channel and the material stratification would happen in the molten pool resulting in the concentration of fuel melt.These two phenomena that might lead to severe consequence should be paid attention.According to the results,the experimental needs for HCDA of lead-based reactor were also proposed.(3)Based on the mechanistic method,lead-based research reactor accidental analysis models were established with NTC-2D code and the whole core transient and the corresponding influence factors under HCDA initiated by two type accidents(unprotected transient overpower and total instantaneous blockage)were investigated.It found that HCDA induced by unprotected transient overpower was less likely to happen in lead-based subcritial reactor than in lead-based critical reactor and the fuel porosity is a significant factor in both reactors under HCDA induced by total instantaneous blockage.Molten corium can be cool in both reactors through selecting key parameters including fuel porosity,coolant driving force and so on,rather than additional engineering measures.The research results not only provide an important basis for re-criticality free of the lead-based research reactor,but also improve the phenomenological knowledge of severe accident in lead-based reactor,which have important reference meaning for the future commercial lead-based reactor.