Verification of neutronics and thermal-hydraulics coupling method for FLUENT code using the MSRE pump startup, trip data

Abstract

The major objective of the present study is to verify a method for the FLUENT code applying the user defined function (UDF) which enables the coupling simulation of neutronics and thermal-hydraulics of a molten salt fast reactor based on the one-point kinetics. The UDF with the added logic to control the control rod drive (CRD) is linked to the FLUENT code to analyze the reactivity change when the fuel pump of the molten salt reactor MSRE was started and tripped. These experiments were carried out under zero power conditions. The core of the MSRE is modeled on a cylinder with the same flow channel area as MSRE, and the control rod in the simulation model can compensate for the reactivity changes that accompany with the flow rate changes. When the pump is started, the response of the control rod overshoots because the reactivity change is in a form close to the stepwise change. This behavior has been reproduced almost correctly using a simple proportional-integral-derivative (PID) controller program. The response when the control constant is changed has been also investigated. In the fuel pump trip experiment, the transient change is relatively moderate. This behavior is also reproduced almost correctly with the same control logic. These simulations show that the UDF has been programmed correctly, and the discretized point kinetic model with prompt jump assumption can be applied to the molten salt reactor in combination with the code-to-code benchmarks already performed.