The RSC experience has spanned in all aspects of PRA analysis and no area in PRA analysis has been outside the scope of RSC projects. Projects have been as small as a single day to those lasting several years. In all cases, RSC has assembled a team that is familiar with the assessment being performed through training and experience and has defined a client-specific solution that would meet the needs of the client in an efficient and cost effective manner. RSC also provides follow up support when needed to address regulatory comments or to provide updates/upgrades to existing assessments. A sampling of projects are provided below.
Integrated Leak Rate Testing Extensions
RSC was in the front of the risk-based assessment of ILRT and in supporting utility requests for testing extensions. RSC has experience over the range of ILRT extension approaches as they have evolved and has provided clients with a dependable tool in support of extension applications. RSC has also supported client RAI resolution when required and in some cases has examined seismic and flooding aspects on risk.
RSC’s TIFA software application provides a platform that extends beyond current industry tools for internal flooding assessment by examining individual breaks and flow rates to define the important flooding scenarios and associated component and structural fragilities. Timing is critical when evaluating operator actions and TIFA provides the ability to track rate of water rise in zones, propagation between multiple zones and different failure heights to provide a detailed timing estimate in support of the human reliability analysis. TIFA also provides inputs and flood frequencies that can be used to integrate the results directly into a plant model developed in standard tools such as CAFTA. RSC has used this tool in the development of several PWR and BWR PRAs with positive results. The detailed database allows for future updates and sensitivities studies to be integrated quickly into the model as needed to account for plant modifications that may impact frequency, flooding rates or how a particular component may be impacted by a specific flood.
The use of segmented hazard curves can result in significant overestimation of CDF when performing a seismic PRA. It is also important to address structural impacts on equipment without overestimation of the core damage frequency. RSC has performed seismic risk analyses for several clients to assess impact of service water structural fault, underground piping failure, masonry block wall failure, overall risk and issues such as stack failure. In the risk estimation, the RSC-developed SIP code is used to provide an integrated result for the analysis and to eliminate the need for overestimation. The cut set
generator can be a typical tool such as CAFTA. The output is then transformed into input into SIP and the integration performed. A third order approximation is utilized to avoid overestimation. The results are presented as plant level fragility, plant level core damage frequency/distribution and individual cut set results.
Significance Determination and Backfit Support
RSC has performed specific assessment to address issues such as the need to install a main generator breaker. In this study RSC developed the risk profile for both cases with and without the break and then estimated the overall reduction in risk in terms of value added. The risk averted was then compared to the installation cost to demonstrate that the change was not beneficial. The result was that the change was not required.
For another client RSC examined the impact of high containment temperature on success criteria within the containment and how elevated temperature would impact recirculation conditions. Utilizing this assessment and other deterministic assessments the client was able to gain an exception to continue operation.
Completion of a Full-Scope Shutdown Risk Study
For another client, RSC developed a full scope shutdown study that examined all non-power operating states and included consideration for off-normal status for specific areas such as maintenance of switchyards. The analysis included an assessment of initiating events both consistent with full power (loss of heat removal) and those specific to the accident phase (transition failure). The model also included consideration for a range of external events such as internal fire, flooding and seismic. Shutdown specific walkdowns were performed in order to supplement the existing at power walkdown and to define new events that may occur due to shutdown operations. As a direct result of the study, the utility made substantial changes to the maintenance practices at shutdown and reduced shutdown risk.