Use of Passive Safety Features in Nuclear Power Plant Designs and their Safety Assessment

Passive safety features (i.e., those that take advantages of natural forces or phenomena such as gravity, pressure differences or natural heat convection) have been in use in nuclear power plants to accomplish safety functions without requiring an active power source for many decades. Traditionally however, the use of such passive features, for instance for reactor scram, were of limited extent, and safety systems heavily relied on active driving devices, e.g. electrical or Diesel motors.

More recently, however, new reactor designs are making a more extensive use of passive safety features for a variety of purposes, for instance for core cooling during transients, design basis accident or even severe accidents or for containment cooling, with the claim that passive systems are highly reliable and reduce the cost associated with the installation and maintenance of systems requiring multiple trains of equipment requiring expensive pumps, motors and other equipment as well as redundant safety class power supplies. However, the weak driving forces of many of such passive safety features based for instance on natural circulation and small pressure differences pose significant challenges to the design and safety demonstration of  passive system for a broad range of accident conditions and also additional loads that can be posed by internal or external hazards.

Nuclear regulators worldwide are facing also important challenges in the licensing of the new passive and innovative reactor designs. For demonstrating the reliability of passive systems or safety features also addressing the independence of the different levels of defence in depth, the requirements for design, methods and computational tools for analyses of the physical phenomena as well as experimental tests for validation are of capital importance.

Objective

This project will aim first at identifying the different uses of cooling passive systems for available advanced reactor designs and the different kinds of natural phenomena associated to them. Once a categorization of passive systems and natural phenomena is established, the project will address the methods and tools for analysing the different phenomena and factors that affect them. Ultimately, the project goal is to establish a compendium of passive cooling safety features in use by different reactors with their associated phenomena, the relevant design aspects of the safety features and how the safety demonstration analysis of such passive systems is performed resp. what the challenges are.