CONTROL ASSEMBLY EJECTION MODELING FOR VVER-440 REACTOR BY PARCS-RELAP COUPLED CODE

Control Assembly ejection in VVER-440 initiated by the loss of integrity of the Control Assemblies drive housing has been analyzed. This event causes a very rapid reactivity insertion to the core and cmall break LOCA which potentially could lead to rapid power increase and redistribution of heat release in the core resulting in a fuel, cladding and coolant temperature rise; primary pressure increase, radiological consequences due to loss of primary coolant and potential loss of cladding integrity and fuel disintegration (if applicable). Methodology of the analysis is based on conservative assumptions as well as on deterministic approach for selection of functioning logic of systems and equipments to maximize reactor core power and minimize power decreasing reactivity feedback. Computational analyses were performed by 3D kinetics PARCS-RELAP coupled code. VVER-440 fuel cross-section libraries, diffusion coefficients and kinetics parameters were calculated by HELIOS code. Several Control Assembly ejection modeling approaches have been used in PARCS to address Control Assembly's fuel part movement problem in VVER-440. The most conservative one has been developed.