The Use of Risk Based Methods for Establishing ISI Priorities for Piping Components at Oskarshamn 1 Nuclear Power Station
B. Brickstad, SAQ Kontroll AB Technology
ABSTRACT
In Sweden, a qualitative risk-informed system for choosing pressurized components in Nuclear Power Plants for inspections has been in operation since 1987. Since then there has been a strong development in the plant specific PSA studies in Sweden. It is now common to directly include pipe breaks into the PSA and not only malfunctioning pumps and valves etc. Also, there has been a worldwide development in estimating failure probabilities, both from historical data and by using probabilistic fracture mechanics. In Sweden, where the dominating damage mechanism in the BWR primary piping is IGSCC, efficient tools to estimate leak- and rupture probabilities for IGSCC have been developed. Therefore, it has been a natural step to take advantage of these developments to make quantitative estimates of the risk for core damage and use this information to check and develop the existing risk based ISI program in Sweden. In the paper, a recently completed pilot study of the BWR-plant Oskarsha mn, unit 1 is reported where the quantitative risk evaluations are compared both with the current Swedish system for ISI-selection and the procedures defined by ASME/WOG and EPRI.
Some of the main conclusions of the pilot study are listed here:
The results of the pilot study have shown that:
it is important to include a model for leak rate detection in order to obtain realistic risk evaluations.
the highest risk contributions are those which have IGSCC and vibration fatigue as damage mechanisms at the same time.
if no qualified inspection has been done before, it is important for an effective risk reduction to perform a qualified inspection as soon as possible. This is because there may be undetected stress corrosion cracks present for which non-qualified inspections have been unable to detect.
The current ISI-selection procedure in Sweden for nuclear piping components:
is efficient to select all high risk locations
also selects many low risk locations.
is a very efficient starting point to make quantitative RBI-studies. This is because it gives information of which damage mechanisms that are present and where in the plant they occur.
It is possible to apply both the ASME/WOG- and the EPRI-procedure for RBI for Swedish nuclear power plants.
With a quantitative RBI-analysis, it is possible to combine a reduced number of inspection sites with a reduction of overall risk for core damage
Publication Source: 2nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, May 24-26, 2000, New Orleans, Louisiana USA. Publisher: EPRI - [Homepage]
