Justifying maintenance in operation at Kozloduy

RiskSpectrum - article - Justifying maintenance in operation 663x320

RiskSpectrum Magazine asked Mr Emil Kichev, head of the PSA team at KNPP, to tell us a bit about the reasons for investing in a risk monitor and how and for what purposes they use it at the plant.

Risk monitoring was developed within the framework of the project for maintenance optimisation of Unit 5 and 6 (WWER-1000) at Kozloduy NPP. The project was one of many steps taken by the Kozloduy NPP management to achieve a high safety level and competitive operation in a deregulated electricity market in Bulgaria. The deregulated electricity market was planned for introduction in 2007.

The main goal of the project was to justify a reduced duration of the units’ outage without compromising safety, by means of optimisation of maintenance, an in-service inspection programme (ISI), test intervals and equipment Allowed Outage Time (AOT).

Since 2005, the KNPP PSA Level 1 has been updated twice. In the RiskWatcher implementation project, the PSA Level 1 model including full power and low power operation and shutdown mode for internal events, internal floods, internal fires and seismic activity was adapted for use in RiskWatcher. The Bulgarian Nuclear Regulatory Agency (NRA) requires that a PSA Level 1 and PSA Level 2 are developed and maintained for continued licence to operate the units. There are also NRA guidelines describing the areas of application of PSA. One of the tools for ensuring safe operation of the units is risk monitoring but the development of a risk monitor is not obligatory. The decision to invest in a risk monitor is one taken by the KNPP management on its own.

RiskWatcher is used for calculating the risk profile, and this information is used for justification of equipment maintenance scheduled in the unit’s outage. RiskWatcher is also used for supporting decisions to take equipment out of service in limiting conditions of operations and calculating the relevant AOT. One example is maintenance of the service water system (support safety system, called QF – three trains, six pumps). It is included in each unit’s annual outage in the original scheme (shutdown mode). However, for a couple of years, maintenance of QF has been done at power (before the annual outage). This takes between five to 15 days, depending on the type of maintenance. The decision to do this was supported using the results of the probabilistic assessment too, including calculation of risk profile at power with and without one train of QF available.

There are also some nice features in RiskWatcher that we did not initially foresee. The cumulative risk calculations for comparing two or more risk profiles are useful when planning outages. The combination of quantitative and qualitative risk calculations (defence-in-depth) for making risk-informed decisions has proven to be very useful for us as well.

This article appeared in RiskSpectrum magazine, June 2014.