76 Risk Assessment of the Bohunice V1 NPP Units for the State after Termination of the Unit 1 Operation (PSAM-0476)

According to an agreement reached with the EU several years ago, the Bohunice V1 plant is to shut down prematurely. An agreement requires that Unit 1 will be shutdown in December 2006 and Unit 2 in December 2008. The current operating licenses for both units are valid substantially beyond the required shutdown dates.

During preparation of the Unit 1 shut down, the situation was investigated when only one unit will remain in operation. This was necessary because the Bohunice V1 plant was fundamentally designed as a twin unit plant with numerous interconnections between the units. These interconnections were explicitly considered in the original safety case. Additional safety improvements and modifications implemented over the last decade were also based on assumed operation of both units. For example, during the major upgrade of the V1 plant in the mid-1990's, some of the important safety upgrades (e.g., emergency feedwater system) were designed and constructed as shared systems serving both units.

In order to asses the site risk and its contributors under the condition when one of the units is shutdown prematurely, a risk study was performed to look into the consequences of single unit operation. This study concluded that the site risk is increased during the period of single unit operation. The risk increase was found to be mainly due to the plant design and the way the upgrades were implemented, but also due to specific elements of the plant's technical specifications and required operability of equipment. The study suggested that, from the risk perspective, operation of both units and their simultaneous shutdown is the „lowest risk“ option.

The objective of the paper is to describe the risk analysis of the Unit 2 of the Bohunice V1 plant after termination of the Unit 1 operation. Risk assessment is performed for full power operation of the reactor and for the refuelling outage. The risk is compared with the current risk level. Risk assessment is also performed for the Unit 1 in the time period of 2007–2008 and for final shutdown of both units after 2008.

For the risk assessment the integrated level 1 PSA models (full power and shutdown PSA models) of the units were used developed in the RISK SPECTRUM PSA code.