Nuclear / PSA

Probabilistic Safety Assessment (PSA)

PSA has been used for safety analysis of nuclear power plants since the late 1970s. It allows integrated modelling and evaluation of complex systems and interactions, including system functions, manual interactions, as well as CCF events and other dependencies. The analyses are performed using RiskSpectrum PSA Professional.

PSA Level 1 - Core damage frequency assessment:

• Complete, detailed and structured event tree and fault tree model
• Full set of initiators (Pipe breaks (LOCA), Transients, Area Events, External Events, Common Cause Initiators (CCIs))
• Full set of operating states (Power operation, start-up/shurdown and cold shutdown/refuelling outage)
• Detailed modelling of component and system dependencies (CCF as well as functional and physical dependencies)
• Integration of dependence on operator actions and preventive maintenance.
• Calculation of core damage frequency.

PSA Level 2 - Release frequency assessment. A level 2 analysis builds on a level 1 analysis and is extended with:

Containment analysis (release paths and structural capacity)

• Investigation and assessment of phenomena during a severe accident
• Level 1 to level 2 interface
• Definition of plant damage states, containment event trees and release categories
• Release calculations (time and composition of releases)

LPSA, Living PSA – models and applications:

Living PSA (LPSA) models allow instantaneous risk calculations based on the actual plant status and configuration. Relcon Scandpower has the methods and procedures for the entire LPSA model development process. We also have the tools and procedures for efficient creation of LPSA models.

The LPSA model can be used in a number of LPSA applications:

• Risk Follow-up - calculation of the actual risk at each moment when looking back (all known component states are taken into account). Risk follow-up is used to identify situations that have had a large impact on the risk and to provide guidance on safety improvement measures.
• Risk Monitoring - calculation of the risk in real time, on-line. The risk monitor application (RiskSpectrum RiskWatcher) support operators, maintenance and planning staff.
• Surveillance Test Interval Optimisation - the instantaneous risk model is used to optimise surveillance test intervals with regard to:

• Maximum safety level with a given number of tests
• Minimum number of tests for a given safety level

• Allowed Outage Times - the LPSA model is used to optimise AOT's. The risk for remaining at power operation is assessed and compared to the risk of shutting down.
• Preventive Maintenance - the preventive maintenance analysis is carried out in order to find:

• Which measures, e.g. tests, are most effective for keeping a high safety level during preventive maintenance.
• Which actions should not be carried out during the preventive maintenance, e.g. shifting of pumps.