Development Of A Steady State And Time-dependent Coupled Neutronics/Thermal-hydraulics Model And Safety Characteristics Research For The Molten Salt Reactor

The liquid fuel type Molten Salt Reactor?MSR?adopts the high temperature liquid salt as fuel,which dissolved fission materials.MSR has the advantages of inherent safety and excellent neutron economy,and with the promising capabilities of Th-U breeding and TRU transmutation.The research on MSR concept can date back to the 1950 s in the Oak Ridge National Laboratory?ORNL?.Due to the advantage mentioned above,in 2002,MSR was selected by Generation IV International Forum as one of the six advanced reactor concepts to be deployed in the future.In 2011,Chinese Academy of Sciences?CAS?launched the Thorium-based Molten Salt Reactor?TMSR?nuclear system project.It aims to build a series of molten salt reactors to utilize the thorium resource efficiently.The graphite-moderated channel type molten salt reactor is a typical design of the liquid-fuel MSR?named MSR hereinafter?.The reactor core of this type MSR consists of a large number of graphite stringers which form the fuel channels.And fission reaction takes place when the fuel salt flows through the fuel channel.The graphitemoderated channel type molten salt reactor is much different from these reactors using solid fuel in the thermal-hydraulics.First,the fission energy is released predominantly in the fuel salt and removed directly by the flowing fuel salt.Therefore,MSR is cooled by the fuel salt itself.Because the graphite moderator deposits a small part of energy due to gamma and fast neutron radiation,in the MSR,the moderator?graphite?is cooled by fuel salt.Second,as there is no transverse flow between different fuel channels,the thermal coupling of fuel salt in this type of MSR is achieved through heat conduction in the graphite moderator.The molten salt adopted in MSR has a high boiling point and low saturated vapor pressure,the molten salt maintains the liquid form under normal operating conditions,hence the thermal-hydraulic model of MSR need not consider the fuel salt in the vapour phase.As the delayed neutron precursors?DNP?have long lifetimes,in the MSR,DNP will drift with the flowing salt,which is quite different compared with the solid fueled reactor.Due to the liquidity of fuel salt,a part of delayed neutron precursors?DNP?will drift out of the core and emit delayed neutrons?DN?at the external loop,and the distribution and fraction of DN in the reactor will be changed.Therefore,the kinetic behavior of the MSR need consider the drift effect of DNP.Considering the above-mentioned different characteristics,the code developed for solid fuel reactors are not suitable for the liquid fueled MSR.The present work aims to develop a new type of three-dimensional thermal-hydraulic model,a coupled neutronics/thermal-hydraulics code,neutron kinetics models and dynamic analysis models for the graphite-moderated channel type molten salt reactor.The main research content of this paper includes:1)Development of a three-dimensional thermal-hydraulic model?3DTH?for the graphite-moderated channel type molten salt reactor and validation of its correctness.Based on COMSOL Multiphysics and MATLAB software,a threedimensional thermal-hydraulic model is developed for the graphite-moderated channel type molten salt reactor.The 3DTH couples a three-dimensional heat conduction model?for the solid region?and the one-dimensional single-phase flow model?the fuel salt in the fuel channel?,which overcomes the defects of the multichannel thermal hydraulic model.3DTH is able to consider the heat conduction between different assemblies and calculate the temperature field of the reflector.The result from the RELAP5 code is consistent with that from the 3DTH,which verify the correctness of 3DTH.2)Development of a coupled code?3DTH&SCALE?for steady-state analysis of the graphite-moderated channel type molten salt reactor,and applied it to analyze the steady characteristics of the 2MW thorium molten salt reactor?named 2MW-TMSR hereinafter?.3DTH&SCALE is developed by coupling the SCALE and the threedimensional thermal-hydraulic model 3DTH in the MATLAB platform.The coupling of SCALE and 3DTH is based on the exchange of power density distribution,temperature and density data.The mass flow distribution,temperature field,k_eff and power density distribution for a conceptual design of the 2 MW-TMSR are calculated and analyzed by the coupled code under both normal operating situation and the central fuel assembly partly blocked situation.Based on the converged power density distribution,the temperature filed and mass flow distribution of the reactor are analyzed when different fuel channels and outer salt layer are blocked.3)Development of two time-dependent models?3DTH&POINT and 3DTH&3N?for dynamic analysis of the graphite-moderated channel type molten salt reactor,and application to analyze the transient characteristics of the 2MW-TMSR.A point reactor model?POINT?and a three-dimensional time-dependent neutron model ?based on diffusion theory,named 3N hereinafter?considering the drift of delayed neutron precursors?DNP?are developed and coupled with 3DTH.The correctness of the POINT model and 3N model is validated by a series of benchmarks.The correctness of 3DTH&POINT is validated by the simulation of the benchmarks of MSRE.The results of 3DTH&3N and 3DTH&POINT are compared in several cases,which verify the correctness of 3DTH&3N.With the aid of 3DTH&POINT,unprotected step reactivity addition,change of mass flow,cold-slug transient and change of inlet temperature are modeled for the 2MW-TMSR,and the reactor power and temperature evolution are analyzed.3DTH&3N adopts three-dimensional model for both neutronics and thermal-hydraulics.3DTH&3N is suitable for the transient which has significant spatial effect.In this paper,3DTH&3N is used to analyze the transients which arise from different number of fuel channels blocked at different locations.In the present work,a new type of three-dimensional thermal-hydraulic model 3DTH has been developed for the graphite-moderated channel type molten salt reactor.Based on the thermal-hydraulic model 3DTH,a steady state coupled code?3DTH&SCALE?and two transient analysis models?3DTH&POINT and 3DTH&3N?have been built and validated.In the present study,we select the 2MW-TMSR as the research object.And the steady behavior and transient behavior of it has been analyzed by the 3DTH&SCALE,3DTH&POINT and 3DTH&3N.The analysis models built in this paper satisfy the basic requirements for steady-state and transient state analysis of the channel type molten salt reactor,which are quite useful in the engineering design of MSR.