| Improving the safety of reactors under normal operation and accident transients is always one of the most important issues for the research field on reactor thermal hydraulics and safety.However,it is very difficult and inconvenient to significantly modify designs of current reactors,and thus performing fuels revolution will be an aggressive schedule to enhance the safety of reactors.After the Fukushima accident,the U.S.DOE initialed a program focused on developing accident tolerant fuels（ATFs）in light water reactors（LWRs）in 2012 for the purpose of radically enhancing the accident tolerance capacity under extreme accidents.Then,the topic of advanced fuels research was transferred from increasing fuel performances to enhance accident tolerance.This paper focuses on the numerical simulation analysis of thermal hydraulic characteristics of accident tolerant fuels and optimization designs of a core in LWRs.Six types of ATFs pellets and two types of ATFs claddings were selected,namely UO2+Be O,UN,U3Si5,U3Si2,UN+U3Si5,UN+U3Si2,Fe Cr Al（APMT）,Si Cf/Si Cm（SA3/Py C50-A）.Thermal hydraulic system analysis Relap5/Mod3.4 code,thermal hydraulic sub-channel analysis COBRA-EN code and Monte Carlo neutron physics calculation RMC code were used to perform numerical calculations.Three types of reactor models were built for thermal hydraulic analyses,including a complete 5×5 assembly model,an eighth of 17×17 assembly model and an eighth of reactor core model.In order to verify the accident tolerance of ATFs under accident transients,COBRA-EN code was used to analyze fuel behaviors of ATFs pellet-cladding systems under four basic accident transients and the critical powers of ATFs pellets and ATFs cladding were also assessed.After that,the reactor reactivity feedback mechanism was taken account into evaluating thermal hydraulic behaviors and the safety of ATFs in real LWRs.Relap5/Mod3.4 code and COBRA-EN code were carried out the process of complete loss of coolant flow accident（CLOFA）and reactivity initiated accident（RIA）,and then the reactivity feedback functions of UN and UN+U3Si2 were discussed respectively.Abovementioned works shows that there are obvious differences in thermal hydraulic performances between ATFs pellet-cladding systems and UO2-Zr system,and therefore it is essential to explore the compatibility between mainstream designs of reactor cores and ATFs pellet-cladding systems.Finally,sensitivity analysis of geometric size of coolant channel in a reactor core and sensitivity analysis of geometric size of fuel rod in a fuel assembly were individually conducted to implement optimized researches in core of LWRs.Results of thermal hydraulic analysis show that ATFs pellet with higher thermal diffusivity and higher thermal effusivity can gain a lower maximum fuel centerline temperature（MFCT）and a lower reactor power due to a larger negative reactivity feedback under accident transients,while usually accompanying with a higher peak cladding temperature（PCT）,a lower minimal departure from nuclear boiling ratio（MDNBR）and a lower critical power ratio.With regard to ATFs claddings of a lower thermal diffusivity,it can lightly improve the critical power ratio,and Fe Cr Al（APMT）can obtain a lower PCT.Results of core optimized design analyses in LWRs imply that when the lateral length of coolant channel is broadened 10%,the PCT of ATFs pellet-cladding systems can be considerably decreased under a RIA transient.In addition,the U3Si2-Si C design with a 9.0mm fuel rod in diameter can achieve reasonable fuel temperatures,a higher MDNBR and a better neutronic economy level. |
