Passive Systems and Inherent Safety

11.1. INTRODUCTION/OBJECTIVES

The purpose of this chapter is to focus on how the design basis for evolutionary water reactors is being extended. The approach continues to be based on the defence-in-depth but a major difference is to attempt to include more severe (core melt) accidents within the design basis. This is achieved in evolutionary designs by the adoption of new technical features, not only to protect against present design basis events affecting the core and primary circuit, e. g. loss of cooling accidents (LOCA), steam line break (SLB) and steam generator tube rupture (SGTR) but also ultimately to protect against early and late containment failure.

Many evolutionary plant designs incorporate passive safety systems in place of active systems but in other respects do not vary substantially from current generation designs. In this chapter, the focus is again on water reactor technology for power generation since these reactors are such an important class of interest. Reviews of advanced light water reactor designs are given in IAEA-TECDOC-968 (1996), covering evolutionary medium — and large-size reactor designs for power generation. Further review of evolutionary designs including strategic issues and economic viability is given in IAEA-TEC- DOC-1117 (1999). A common feature is that decay heat is removed from the primary circuit to large tanks or pools via natural circulation. There are some new phenomena associated with decay heat removal in advanced designs with such components that are not found in present generation reactors. These are discussed in Relevant thermal-hydraulic aspects of advanced reactor design (1996). An issue here for the plant designer is to ensure that such systems have sufficient heat capacity and also initiate as intended. In addition, reactor coolant inventory is maintained using passive injection rather than active pump injection.

Different containment designs have been proposed, utilising steel, concrete or composites. Heat removal may need to be via natural circulation cooling of the containment wall in the case of steel or enhanced in concrete based containments using passive heat exchangers. These and other passive systems are covered in this chapter.

The design basis for the containment has traditionally been that it must survive the peak pressure arising from a double-ended guillotine break of the largest primary or secondary pipes. The design basis for more advanced plants will have to cover a broader selection of accident sequences, perhaps including significant core melting. This selection will be based on a combination of probabilistic and deterministic analyses. The lowest probability high consequence sequences will still need to be covered by engineering judgement or other means. There will be a tendency for deterministic analyses to be carried out by best estimate rather than conservative methodologies.

Advanced evolutionary water containments include other measures to ensure they survive under severe accident loads. Measures (IAEA-TECDOC-752,1994) are introduced to prevent fuel coolant interactions (FCIs) to prevent direct containment heating (DCH) and to control hydrogen. They are also designed to reduce the source term by improving leak tightness. This is achieved via inherent safety features in the design, utilising passive heat removal systems in many cases. In addition to internal events, external events such as aircraft crashes and seismic events are also receiving special attention.

A number of more revolutionary designs of water reactor have been put forward as ‘inherently safe’ designs. These eliminate almost entirely active systems, e. g. relying on reactivity control via careful management of boron concentration. Some of these approaches are also summarised briefly although these are unlikely to be developed further at the present time.

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