Performance of Passive Heat Removal System Under Accident Conditions: a Study Case

In many innovative LWR designs, passive safety systems are widely envisaged. In general, their behaviour involves natural and simple physical events as free convection, thus entailing an higher safety degree of the plant, inversely proportional to the necessity of human intervention or energized devices to prime the safety system. A thorough behaviour’s knowledge of this kind of systems re-quires experimental campaigns as well as numerical simulations: both the Regulatory Commissions and the Utilities are engaged to verify the effectiveness and reliability of these systems. From the simulation view point, a different choice in nodaliza-tion, heat transfer correlation and system dimensions could lead to different system performance during accident transients. This problem is pointed out in this paper: as test case, a Passive Heat Removal loop under Steam Line Break accident conditions is concerned. The results obtained show that thick circuit nodalization, con-sistent with a value of the Courant number close to unity, is a ne-cessity to find out the correct system’s behaviour. Besides they confirm the need of experimental facilities, in particular to inves-tigate the circuit’s activation, and of parametric studies for opti-mization design; for the last issue, computer codes that hold fast-running feature even with a large number of circuit nodes are very useful tools.