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Simulation and Validation of a Full Flow and Mini Flow Verification Test in NEK with Best Estimate HPIS Model in APROS

A model of High Pressure Injection System (HPIS) of the Krško NPP (NEK) was built using the APROS – Advanced PROcess Simulation environment. APROS is a multifunctional software for modelling and dynamic simulation of various physical processes of different types of nuclear power plants, including PWRs. APROS allows us to build complete models of whole plants, with a complete thermal-hydraulic, electrical and regulation systems including reactor kinetics. Such models could be used for various purposes like as the for new NPP design verification, possible optimization of NPP systems, for training purposes as an engineering simulation tool, for verification and validation of plant abnormal (AOPs) and emergency (EOPs) operating procedures and also as a model for full scope simulator. The data used to describe the properties of the HPIS subsystem modelled in APROS, were the data describing Krško NPP and its 23rd cycle. Basis for data collections, nodalization, structure and simplifications was NEK RELAP5\MOD3.3 Engineering Handbook [1]. The goal was to create a detailed simulation model a HPIS in APROS code and validate it for simulating the safety injection system response during design basis accident. To reach the goal few plant surveillance tests were simulated. Upon completion of modelling/nodalisation and documenting of the model in the APROS, the model validation has been made in accordance with surveillance NEK procedure. The full and mini flow surveillance tests during refuelling shutdown were simulated by APROS. The HPIS simulation model injects water from the Refuelling water storage tank (RWST) directly in to the reactor vessel and the cold legs. The two Safety Injection (SI) pumps taking suction from RWST and pumps water through subsystems lines via associated check and control valves. Valves are modelled with corresponding pressure drop as a function of the flow velocity, pipes with corresponding wall resistance and pumps with performance curve to achieving the right subsystem flow characteristics. The comparison with testing data obtained from Krško NPP surveillance tests and simulation results shows good agreements and flows are within the allowable limits. Such verified and validated HPIS model will be added to the full NEK APROS model for the next step of verification and validation full model itself.

APROS; High pressure injection system; Nuclear power plant Krško; Validation; Verification; Safety injection; thermal-hydraulics

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