In a nuclear power plant, vessels and pipes that contain steam and hot water are insulated against heat loss. If a pipe containing such steam and hot water inside the reactor building breaks, the outflowing water will collect in the so-called sump. The water collected here will then be pumped back into the cooling circuit and used again for the cooling of the reactor. In order to prevent any insulating material from the thermal insulation that may have been released due to the break or indeed any other impurities from the sump from entering the cooling circuit, the water will be filtered through so-called sump strainers. If the sump strainers get clogged, this may result in problems for cooling and hence also for the safety of the reactor. Detailed technical information on sump strainer clogging can be found in the paper "Reliability of Core Cooling and Cooling Systems in Case of a LOCA with Release of Insulation Material".
Event in the Swedish Barsebäck nuclear power plant
In 1992, a serious accident almost occurred in the Swedish Barsebäck nuclear power plant as a result of a clogged sump strainer. The event happened because a valve in the cooling system inside the containment had been wrongly reassembled during an inspection outage. All safety systems sprang into action. As required, the nuclear power plant was hermetically sealed off from the environment. Cooling was therefore exclusively carried out via the water inventories available inside the containment. Owing to a clogged sump strainer, there was a risk that it would not be possible to pump enough water into the cooling circuit. When the shift personnel became aware of the situation, they switched the system to backflushing mode. This resulted in the clogging being cleared and averted the loss of cooling and the risk of a core meltdown.
Activities at the GRS
GRS analysed the event and pointed out the safety relevance of clogged sump strainers in the form of an Information Notice. Comprehensive studies were carried out on this topic. GRS evaluated these studies on behalf of the German Federal Environment Ministry. The insights gained from these studies and the work of GRS subsequently led to major upgrades in German nuclear power plants.
OECD/NEA Task Group on Sump Clogging
In 1994, a year and a half after the event in the Swedish nuclear power plant, the OECD/NEA set up a first task group on sump strainer clogging. It was composed of experts from Canada, Finland, France, Germany, Japan, Korea, Spain, Sweden and the United States. Experts from GRS worked in this task group as German representatives. This task group published a first comprehensive State-of-the-Art report. It soon became clear that there still remained many unresolved issues. A following group is now engaged in focusing on subsequent effects and chemical influences. Amongst other things, particular attention is paid to
• the identification of open research issues,
• the assessment of new research results, and
• the updating of the state-of-the-art report "Knowledge Base for Emergency Core Cooling System Recirculation Reliability".