Thermal hydraulic analysis of a passively controlled DHR system

The safe operation of nuclear reactors cooled by liquid metal is an important issue to be addressed for the development of nuclear technology and presents specific aspects related to the properties of the coolant. While the main parameters influencing safety for water-cooled reactors are reasonably well known, DHR systems for reactors cooled by liquid metal present additional challenges related to coolant freezing, since solidification temperature is higher than the temperature of the final heat sink. If the primary coolant solidifies obstructions of the primary flow path can occur, inhibiting natural circulation and core cooling. This paper presents an innovative passive safety system for decay heat removal which passively delays the coolant freezing. The system adopts noncondensable gases to passively control the power removed from the primary system and to delay freezing in the long term while keeping primary system temperatures below an acceptable limit. The system is simulated by means of the Relap5-3D computer code for a loss of offsite power of the innovative lead-cooled reactor ALFRED. A sensitivity analysis has been carried out in order to study the effect of noncondensable gas pressure on the performance of the passive decay heat removal system, and in particular on the primary coolant temperatures.