Reactivity Insertion Accidents Analysis Based On RELAP5-3D Code

The Reactivity insertion accidents(RIA)analyses care the changes of reactor power,fuel temperature,coolant temperature and other parameters during the accidents happen.Not only the transient changes in the fluid,but also the transient changes in the reactor power distribution and the feedback influence on the reactor power during the accidents happen should be considered.Therefore,there is important significance in the study of three-dimensional transient thermal physics coupling procedure applied in reactivity insertion accidents.In this paper,three-dimensional transient thermal-physics coupled program RELAP5-3D was used to analysis the reactivity insertion accidents,including control rod enjection accident and control rods withdraw accident.Two geometry type reactors were selected as the research objects,which are the quadrilateral Qinshan Phase II nuclear reactor and the hexagonal WWER-1000 type reactors.The reactivity insertion accident analysis of Qinshan Phase II nuclear power plant using two modeling schemes: the pressure vessel modeling scheme and the whole circuit modeling scheme.According to the results of the Qinshan accident analysis,1000 MW hexagonal reactor modeling scheme was the pressure vessel modeling scheme merely.Two scenarios,both of the hot zero power scenario and the full power scenario,was simulated in reactivity insertion accident analysis of the two reactors.However,only the hot zero power scenario was simulated in control rods withdraw accident.The results show that taking the pressure vessel modeling scheme or the whole circuit modeling scheme shows no significant differences in the control rod enjection accident analysis results when using the RELAP5-3D code.Therefore,only taking the the pressure vessel modeling scheme in the control rod enjection accident analysis is accepped to saving computing time and memory.In addition,the reactor power distribution will changes greatly distorted during the control rod ejection accident happens,so it is necessary to consider the three-dimensional transient effects for getting a more accurate reactor power distribution.The control rods withdraw accident analyses show that,the RELAP5-3D program can not handle the control rod cusping effect,there will be a differential controlrod worth discontinuity phenomenon,leading a reactor power fluctuation when control rod moving.In the case of strict compliance with the WWER-1000 type reactor control rods procedures,when the control rods withdraw accidents occurring at full power,the consequences of the accident are not serious,that can be a good description of the reactor control rods procedures can limit the consequences of the control rod withdraw accident.