SAFETY ENHANCEMENT IN SHUTDOWN AND

Considering the results of both operability studies and SLP PSA studies and international operating experience for low power and shutdown, many plants world-wide embarked on a variety of activities aimed at enhancement of safety in those operating modes. While the actual background ( source ) of an improvement is not always easy to determine (if an improvement was decided upon the finding of a SPSA study or operational experience or vendor s advice) it is obvious that most of the improvements were implemented or proposed after the high risk contribution from shutdown operation was revealed by SPSA studies. 

The improvements/enhancements during shutdown and low power operation may be divided into three broad categories  

Administrative and Operational improvements/enhancements are sometimes overlapping and it is difficult to strictly define boundary between them. In many cases Operational practice depends on or/and is related to Administrative restrictions. 

Many similarities in identification of problems and issues were observed internationally. However, it is obvious that the solutions are highly dependent upon the reactor type and general operating practices in specific countries or even specific plant. An illustrative example is the typical German practice with PWR reactors, where the whole core is imloaded at the beginning of an outage, with which, the mid loop operation risk becomes irrelevant. Most of the other PWR reactors find mid loop operation the risk dominant issue. 

The following sections discusses some of the commonly identified problems and their solutions. While this review is not attempting to be an all-inclusive overview of all issues relevant for shutdown, it is believed that it provides a quite representative sample of major issues which are considered internationally. 

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