Numerical simulation of operational transients in sodium-cooled fast reactors

Abstract

Reactor startup and shutdown are planned plant transients that are expected numerous times in the life of a nuclear reactor. These transients are desired to be carried out quickly for economic benefits and at the same time safely, which demands slower execution. The various plant design and safety constraints to be met during the execution of these transients in a typical medium-sized sodium-cooled fast reactor (SFR) were discussed. Operational procedures for the execution of these transients were reported in detail. A “wait and raise” mode for the startup and a “wait and lower” mode for the shutdown were adopted. During these procedures, primary and secondary loop flow rates were decided to be maintained at a nominal 100 % flow to avoid thermal stratification issues in sodium pools. Feedwater flow will be controlled based on sodium temperature at the steam generator (SG) outlet. The dynamics analysis for a reference SFR design was then carried out using plant dynamics code DYANA-P. The modeling details and approximations for simulating the startup and shutdown transients using the DYANA-P code were discussed. Two operational parameters, namely, control rod movement distance and waiting time between control rod movements, were optimized as 2.3 mm and 2.75 min, respectively, for the reactor's beginning of equilibrium cycle (BOEC) core configuration. The startup and shutdown durations were both estimated as ∼20.5 h each. The hot pool temperature change rate was estimated as ∼19 °C/h, which is less than the allowable limit of 20 °C/h. The SCRAM parameters, namely, reactor period and reactivity, were less than their alarm limits, thus, not interfering with the planned operations. The design and operational constraints of various plant systems were also satisfied. Parametric studies were also carried out for a few parameters: the fuel-clad interaction and the core burnup conditions. The assumptions of fuel being stuck to the clad (in feedback reactivity calculation) and the BOEC core configuration gives a higher power raising rate for a given control rod stroke and waiting time.