Probabilistic safety analysis (PSA) is a structured approach toward quantitative risk assessment and is under this name mostly (although not exclusively) related to nuclear power plants (NPPs). It aims at quantitative assessment of risk from condition that may exceed design bases, which is usually expressed in terms of core damage frequency (“Level 1 PSA”) and radioactivity release frequencies (“Level 2 PSA”). Quantitative risk assessment, PSA included, typically consists of three major steps: 1) identification and analysis of initiators / hazards; 2) analysis of facility’s vulnerability to identified initiators / hazards; and 3) building of risk model and risk quantification.

APOSS staff has large experience with:

  • Level 1 PSA:
    • Initiating events analysis; accident sequence logic modeling (event trees), with supporting analyses; system analyses; common cause failure (CCF) and dependency analysis; data collection and analysis; human reliability analysis (HRA); risk model integration and quantification; results interpretation;
  • Level 2 PSA:
    • Containment systems analysis / Bridge trees (interface to Level 1); accident progression event trees; supporting severe accident analyses; source term analyses; risk model integration and quantification; interpretation of results;
  • PSA for low power and shutdown modes:
    • Plant operating states (POS) evaluation; initiators analyses, event trees and systems analyses, data collection / analysis, HRA, risk model integration and quantification with results interpretation;
    • PSA for spent fuel pool;
  • Seismic PSA:
    • Integrating of seismic hazard curves and structures / systems / components fragility curves into seismic PSA model; particularly: seismically-induced initiators identification and characterization; seismic event trees and system fault trees; seismic HRA; seismic risk model integration and quantification, with interpretation of results;
  • Internal (e.g. fire and flood) and other external hazards:
    • Hazard screening and bounding risk evaluations; characterization of screened-in hazards; developing and integration of corresponding risk models and risk quantification, with results interpretation;
  • PSA standards, guidelines and norms;
  • All relevant PSA applications:
    • Optimization of Technical Specifications; optimization of in-service inspection programs; graded QA / SSC classification; risk monitoring; risk significance of events and issues; prioritization of corrective measures; development of EOPs / SAMGs; aging risk assessment, and others.

APOSS also offers reliability and risk assessments for other facilities and systems, including simplified techniques and methods.


APOSS - System Reliability and Safety Analyses Inc. for services and engineering

APOSS – System Reliability and Safety Analyses Inc. for services and engineering