50 Source Term Assessments in PSA Level 2 for the Outage Period (PSAM-0168)

In a PSA Level 2 for an outage state the prerequisites are quite different compared to an analysis of the power operation state or of shut-down and start-up operation. From a level 2 and source term point of view some of the main differences are:

• The containment can not be assumed to be operational, except for a short time in the beginning and at the end of the outage period.

• Only few safety systems or severe accident mitigating systems are in operation during most of the outage time. As an example, the containment sprays, the containment venting system and the pressure suppression system are not working for most of the outage period.

• The types of fuel damage include not only overheated and melted fuel, but the fuel may also be damaged mechanically, by criticality events or by fire.

• The damaged fuel may not only be situated in the reactor vessel but also in the fuel pools.

In a level 2 PSA for power operation, the source term is often assessed by integral codes like MAAP, MELCOR or ASTEC. These codes are designed for accidents where the fuel is overheated and positioned in the reactor vessel. As a result, they are not easily adapted to accidents typical for outage analysis.

The number of accident sequences is considerable lower than in the PSA for power operation, but it is still advantageous from both an analytical and a pedagogical point of view to use the concept of release categories.

The Oskarshamn 1 Level 2 PSA for the outage period was finalized in late autumn 2005. The paper presents analysis assumptions, estimated source terms and a discussion on general results.