Risk and Safety Management January 2016 ISSUE STATEMENT Probabilistic risk assessments (PRAs) often focus on nuclear power plant conditions associated with operation at essen- tially full power. Operating experience has, however, dem- onstrated that upset events during other modes of operation (that is, during shutdown or when the plant is in transition from one operating mode to another) can present challenges to plant safety as well. Unique considerations related to the plant configuration, availability of systems, nature of opera- tor guidance and response, and other factors affect the way in which PRAs evaluate the risk during low-power and shut- down (LPSD) operations. Improvements to the methods, data, and guidelines for using PRA are needed to enhance the ability to investigate conditions during LPSD operating modes. DRIVERS e manner in which low-power and shutdown PRAs are performed varies from country to country, as does the extent to which the results are employed for risk management. While low-power and shutdown PRAs are already con- ducted in some countries, in others (including the U.S.), limitations in the technology or lack of clarity in the poten- tial benefits have resulted in fewer plants performing such PRAs. Support for Effective Risk-Informed Decision Making e risks associated with events traditionally evaluated in detail (i.e., events projected to occur during power opera- tion) have decreased as a result of system modifications and improved operating practices, such as formal risk-manage- ment programs. is has increasingly led to the need to con- sider other possible hazards that might previously have received less attention. A thorough, integrated understand- ing of the relative risks posed by these hazards is essential so that resources are applied most effectively to manage these risks. Better Configuration Risk Management Many nuclear power plants use some form of risk assessment to manage risks for different plant configurations. at is, a PRA is used to examine the risk associated with removing specific pieces of equipment from service for scheduled test and maintenance activities to help ensure that the resulting plant configuration does not lead to an undesirable level of risk. ose plants that employ such practices typically use quantitative risk assessment during power operation, but more often perform qualitative assessments of the remaining defense-in-depth for configurations during LPSD condi- tions. is is largely the result of the many different configu- rations that may be entered during LPSD operations, and the relatively dynamic scheduling of activities during out- ages. While these qualitative approaches have proven ade- quate, formal PRAs would enable risks to be identified and managed in a more systematic and effective manner. Improvements to the methods and tools for applying PRAs are needed to allow these configuration-specific issues to be addressed in a timely and efficient manner. RESULTS IMPLEMENTATION Research results will be used to guide the development of effective risk-assessment and risk-management tools for LPSD conditions. ese tools will enhance the understand- ing of risks posed by conditions other than power operation and enable nuclear plant owners to make appropriate deci- sions regarding management of these risks. Utilities also will have the ability to perform more comprehensive and system- atic configuration risk management during all modes of operation. e primary users of this technology will be risk analysts, work-week planners, and those responsible for on- line and outage work management. PROJECT PLAN e development of more effective methods and tools to support low-power and shutdown PRA will involve the fol- lowing activities: • Collection and assessment of operating data. e con- tinuing collection of operational data provides both quali- tative and quantitative inputs to the risk-assessment pro- cess. e qualitative understanding of the nature of off-normal events and other challenges supports the development of more realistic and comprehensive risk models. e quantitative information is used to character- ize the frequencies of relevant challenges considered in the risk assessment. • Development of improved methods for low-power and shutdown PRA. Gaps or apparent weaknesses are per- ceived to exist in several aspects of low-power and shut- down PRA. ese include the ability to define and evalu- ate different plant operating states for configurations that may evolve rapidly during a shutdown and the application IN USE: METHODS FOR RISK ASSESSMENT FOR LOW-POWER, SHUTDOWN, AND TRANSITION CONDITIONS
Transcript
Risk and Safety Management January 2016
ISSUE STATEMENT
Probabilistic risk assessments (PRAs) often focus on nuclear power plant conditions associated with operation at essen-tially full power. Operating experience has, however, dem-onstrated that upset events during other modes of operation (that is, during shutdown or when the plant is in transition from one operating mode to another) can present challenges to plant safety as well. Unique considerations related to the plant configuration, availability of systems, nature of opera-tor guidance and response, and other factors affect the way in which PRAs evaluate the risk during low-power and shut-down (LPSD) operations. Improvements to the methods, data, and guidelines for using PRA are needed to enhance the ability to investigate conditions during LPSD operating modes.
DRIVERS
The manner in which low-power and shutdown PRAs are performed varies from country to country, as does the extent to which the results are employed for risk management. While low-power and shutdown PRAs are already con-ducted in some countries, in others (including the U.S.), limitations in the technology or lack of clarity in the poten-tial benefits have resulted in fewer plants performing such PRAs.
Support for Effective Risk-Informed Decision MakingThe risks associated with events traditionally evaluated in detail (i.e., events projected to occur during power opera-tion) have decreased as a result of system modifications and improved operating practices, such as formal risk-manage-ment programs. This has increasingly led to the need to con-sider other possible hazards that might previously have received less attention. A thorough, integrated understand-ing of the relative risks posed by these hazards is essential so that resources are applied most effectively to manage these risks.
Better Configuration Risk ManagementMany nuclear power plants use some form of risk assessment to manage risks for different plant configurations. That is, a PRA is used to examine the risk associated with removing specific pieces of equipment from service for scheduled test and maintenance activities to help ensure that the resulting plant configuration does not lead to an undesirable level of risk. Those plants that employ such practices typically use
quantitative risk assessment during power operation, but more often perform qualitative assessments of the remaining defense-in-depth for configurations during LPSD condi-tions. This is largely the result of the many different configu-rations that may be entered during LPSD operations, and the relatively dynamic scheduling of activities during out-ages. While these qualitative approaches have proven ade-quate, formal PRAs would enable risks to be identified and managed in a more systematic and effective manner. Improvements to the methods and tools for applying PRAs are needed to allow these configuration-specific issues to be addressed in a timely and efficient manner.
RESULTS IMPLEMENTATION
Research results will be used to guide the development of effective risk-assessment and risk-management tools for LPSD conditions. These tools will enhance the understand-ing of risks posed by conditions other than power operation and enable nuclear plant owners to make appropriate deci-sions regarding management of these risks. Utilities also will have the ability to perform more comprehensive and system-atic configuration risk management during all modes of operation. The primary users of this technology will be risk analysts, work-week planners, and those responsible for on-line and outage work management.
PROJECT PLAN
The development of more effective methods and tools to support low-power and shutdown PRA will involve the fol-lowing activities:• Collection and assessment of operating data. The con-
tinuing collection of operational data provides both quali-tative and quantitative inputs to the risk-assessment pro-cess. The qualitative understanding of the nature of off-normal events and other challenges supports the development of more realistic and comprehensive risk models. The quantitative information is used to character-ize the frequencies of relevant challenges considered in the risk assessment.
• Development of improved methods for low-power and shutdown PRA. Gaps or apparent weaknesses are per-ceived to exist in several aspects of low-power and shut-down PRA. These include the ability to define and evalu-ate different plant operating states for configurations that may evolve rapidly during a shutdown and the application
IN USE: METHODS FOR RISK ASSESSMENT FOR LOW-POWER, SHUTDOWN, AND TRANSITION CONDITIONS
EPRI | Nuclear Sector Roadmaps January 2016
of techniques for human reliability analysis to LPSD con-ditions. EPRI participation in a self-assessment for one plant revealed a need to better understand how at-power PRA data are adapted for use in LPSD PRAs.
• Refinement of methods and implementation of qualita-tive risk assessment techniques for LPSD conditions. There is substantial variability in the use of qualitative or defense-in-depth methods among operating plants. Best practices will be identified to provide utilities with an improved and more consistent approach.
• Support of industry standards. The American Nuclear Society and the American Society of Mechanical Engi-neers have approved a new standard for the conduct of low-power and shutdown PRA for trial use in pilot appli-cations (TUPA). This standard will need to be evaluated through trial applications. Based on experience with other PRA standards to date, further refinement is likely to be required.
RISKS
There is a risk that operating plants will continue to experi-ence challenges during LPSD operations that might have been avoided if a more comprehensive and effective risk assessment had been performed. The ability to perform such assessments will be greatly enhanced by this effort.
There is also a risk that the development envisioned for this effort could raise expectations on the part of regulators in those countries in which LPSD risks are not currently inves-tigated through detailed quantitative assessment. That is, regulators may expect more extensive risk analysis than has been performed previously when better tools and methods are available. It is likely, however, that regulatory pressure will increase to consider LPSD risks, whether or not this effort is undertaken.
RECORD OF REVISION
This record of revision will provide a high level summary of the major changes in the document and identify the Road-map Owner.
revision description of change
0 Original Issue: August 2011 Roadmap Owner: Doug Hance
1 Revision Issued: August 2012 Roadmap Owner: Doug Hance
Changes: Revised dates to reflect deferral in initiating major research activities and to reclassify the roadmap as “in-use” rather than draft.
2 Revision Issued: August 2013 Roadmap Owner: Doug Hance
Changes: Revised dates to reflect current status. LPSD PRA methods and guidance activities were re-characterized to reflect an ongoing LPSD PRA pilot. The improvement of qualitative risk methods now includes implementation in addition to risk methods in the roadmap activity workscope.
3 Revision Issued: August 2014 Roadmap Owner: Doug Hance
Changes: Revised dates to reflect current status and estimated completion dates.
4 Revision Issued: December 2014 Roadmap Owner: Doug Hance
Changes: Extended the duration of LPSD PRA pilot efforts and methods development/demonstration to the full trial use duration (late 2017) of the Trial Use in Pilot Application (TUPA) LPSD PRA Standard approved in November 2014. Added a new activity – to pilot the qualitative content of the TUPA LPSD PRA Standard and extended the end date of the demonstration of new guidance to late 2017.
Risk and Safety Management January 2016
revision description of change
5 Revision Issued: August 2015 Roadmap Owner: Doug Hance
Changes: Changed the top row from “Utilities” to “Utilities and Owners’ Groups. Moved the start date of Apply Advanced LPSD PRA Methods as Appropriate to late 2017. Added Completed Milestones for 1st LPSD PRA Standard Pilot Effort and Publication of Trial Use LPSD PRA Standard. Deleted undesignated milestone symbols (editorial change). Moved start date for “Methods Improvement and Development” to early 2017. Moved the start date for “New Methods Demonstration Large or Small Pilots” to late 2017. Under “Assess Qualitative Risk Assessment Methods and Implementation”, the end date for “Develop Improved Guidance is extended to late 2017 and the start date for Demonstration of New Guidance is moved to late 2017. In the row for the Standards Organizations, the “Completion of Trial Use in Pilot Applications Standard” box is extended into 2015 to correspond with the Completed Milestone for publication of the Standard, and the “Revise Standard for Final Publication” box is extended to the end of the roadmap.
6 Revision Issued: December 2015 Roadmap Owner: Doug Hance
Changes: Under “Assess Qualitative Risk Assessment Methods and Implementation”, change “Development of New or Improved Qualitative Risk Assessment Models (DID)” to “Pilot Qualitative Content in the Trial Use LPSD PRA Standard”. This more accurately identifies the RSM contribution to improvements in modeling.
