E. F. Hicken, H. Jaegers

Institute for Safety Research and Reactor Technology, Forschungszentrum Julich Germany

Abstract. The decay heat in commercial Light Water Reactors is commonly removed by active and redundant safety systems supported by emergency power. For advanced power plant designs passive safety systems using a natural circulation mode are proposed; several designs are discussed. New experimental data gained with the NOKO and PANDA facilities as well as operational data from the Dodewaard Nuclear Power Plant are presented and compared with new calculations by different codes. In summary, the effectiveness of these passive decay heat removal systems have been demonstrated; original geometries and materials and for the NOKO facility and the Dodewaard Reactor typical thermal-hydraulic inlet and boundary conditions have been used. With several codes a good agreement between calculations and experimental data was achieved.

1. INTRODUCTION

The decay heat in commercial Light Water Power Reactors is commonly removed by active safety systems which require redundant systems as well as emergency power. This is expensive and requires time for maintenance and testing.

Therefore — when studying advanced power plant designs — the decay heat removal by passive safety systems was re-evaluated. In Fig. 1 some passive safety systems under consideration are shown.

For PWR the use of heat exchangers submerged in a large water pool (up to several thousand m3) is evident. The working principle is well known and can be calculated with well validated codes. However, to avoid heat losses during normal operation in the range of several per cent of the total thermal power valves have to be installed in the pipes to and/or from the heat exchanger. This definitely reduces the reliability of the decay heat removal as well as it results in additional costs. The expensive valves can be avoided if the principle used for the so-called
"Thermal Valve" is applied: the heat exchanger — now without valves in the high-pressure lines — is installed within a bell-shape volume. On signal by the reactor protection system or by manual operation a valve at top of the volume opens allowing heat removal by water circulation. This proposed design, however, has to be validated against experiments and related code calculations.

The principle already used in BWR’s and again proposed for advanced BWR designs is the Evaporation — Condensation mode: water in the core region is evaporated by decay heat and condensed within a heat exchanger placed in a water pool, the condensate returns to the core region.

This principle has been used in the Dodewaard Reactor and some other reactors already decommissioned. This principle has been proposed for the advanced BWR, the SBWR and the SWR 1000. Experimental results and comparison with code calculations will be given below.

The two new designs — for the SBWR and the SWR 1000- show some remarkable differences; some will be discussed below.

1) Both designs need a large water pool. Due to the fact that the heat exchanger for the SWBR can be placed above the Reactor Pressure Vessel (RPV) — thus allowing more flexibility in the design — valves are needed in the lines to and from the heat exchanger. Therefore, the heat exchanger can also be placed outside the containment. The heat exchanger and the water pool of the SWR 1000 have to be placed at the elevation of the core and within the containment.

2) The operation of both designs is quite different. When opening the valves in the connecting lines the full heat exchanger capacity is available from the beginning while the heat exchanger of the SWR 1000 will start slowly from zero to full capacity.

3) These passive heat exchangers are mainly designed to be used for a decay heat removal without a loss-of-coolant sequence; for some time they assist the heat removal in case of small breaks — for large break LOCA they are of no benefit.

4) The modelling of the phenomena and system behaviour with these designs is not always easy as it will be shown below, because the condensation behaviour inside the heat exchanger tubes — including the presence of non-condensables — as well as the heat transfer from these tubes to the water pool has to be considered.

5) Heat exchangers using condensation of steam are always sensitive to the accumulation of non-condensables. If no venting capability is installed the concentration has to be kept below a value, where the non-condensables can be dissolved in the condensate.

6) There exists some operating experience mainly in the Dodewaard Reactor.