Instability analysis on PRHRS of SMART-P using D-decomposition method

Passive Residual Heat Removal System (PRHRS) removes the core decay heat and sensible heat by natural circulation in case of an emergency such as unavailability of feedwater supply or loss of off-site power. The PRHRS consists of 4 independent trains with 50 % capacity each. Two trains are sufficient to remove the decay heat. The each train is composed of a Heat eXchanger (HX) submerged in the Refueling Water Tank (RWT), fail-open valves, and a Compensating Tank (CT). Figure 1 shows a schematic diagram of PRHRS. The CT with water and gas margin is connected to the water intermediate circuit (between SG and HX) by means of pipeline with orifice. The fluid circulation in the intermediate circuit is assured by weight difference of hydrostatic column in its riser and downcomer sections, which creates positive motive head of natural circulation. Availability of two compressible volumes in the intermediate circuit (steam one formed by steam regions of the intermediate circuit and gas one in the form of gas reservoir in the CT) and SG operation under low flow rate with relatively high hydraulic resistance of steam line may induce fluctuating circulation instability. Simple analytical model for the PRHRS instability was developed and impacts of design parameters on instability occurrence were studied in this paper