Probabilistic safety assesment of nuclear power plants


Mr. R.K. Saraf, Ex-Head, PSA Section RSD, Bhabha Atomic Research Centre

Mr. R.K. Saraf, Ex-Head, PSA Section RSD, Bhabha Atomic Research Centre, Trombay, Mumbai 400085
Currently holding the position of Chairman, PSA Committee of Atomic Energy Regulatory Board, Mumbai. Holds Bachelor’s Degree in Electronics & Telecommunication from Pune University and Post Graduate Diploma in Nuclear Engineering.Retired as Head, Probabilistic Safety Assessment Section, Scientific Officer –H, Reactor Safety Division, BARC.

Nuclear Power Plants is the fourth largest source of electricity in India. Nuclear Power Plant safety, occupational/public safety and environmental protection are major concerns in Nuclear Power plants Industry. Identification of hazards viz. radiological and conventional hazards associated with operation and control has been always on the top priority. This paper discuses the use of Probabilistic Safety Assesment procedures in Nuclear Power Plants.

KEYWORDS : Nuclear Safety, Probabilistic Safety Assesment, PSA objectives, Risk, Application of PSA


Safety as it relates to public protection from dieses has a history extending the earliest times. The discovery of nuclear fission made possible a far more concentrated energy source than was ever available before. Its hazards were recognized from the beginning. The first report on nuclear power plant accidents WASH 740 was issued in 1957.

The consequences predicted were unacceptable at that time. The importance of this report felt only after Three Mile Island NPP accident as that accident was predicted in this report. Now PSA study of Nuclear Power Plants are carried out in many countries and are mandatory requirement by their regulatory bodies.


Main goal of the nuclear safety is to keep the radiation exposure from nuclear facilities of the public and workers As Low As Reasonably Achievable (ALARA) during normal operation states and in the event of an accident.

Objectives of nuclear safety in regards to potential accidents are

  • Reduce the probability of accident
  • Minimize the release of radioactive Material if accidents happen
  • Minimize population exposure if radioactive materials are released

2.1 Nuclear Power Plant Safety is achieved by

  • Engineered safety in Design & Construction
  • Safety in Site Selection
  • Safety in operation & maintenance
  • Emergency planning and Preparedness

2.2 Requirement of Nuclear Power Plant Safety

  • Establish and maintain nuclear power plant in safe subcritical state
  • Remove residual heat from the reactor coolant system
  • Maintain the integrity of the reactor coolant system and adequate supply of reactor coolant


PSA is associated with models which predict offsite radiological releases resulting from potential accidents. It deals mainly with the identification of accident sequence applicable to the design of a reactor and further relegating the risk to various process and safety systems and down to the components and operator actions. In fact it is possible to rank the systems and components in terms of their risk significance.

In short PSA is an analytical technique for integrating diverse aspect of design and operation in order to assess the risk of a particular nuclear power plant and to develop an information base for analysing plant specific and generic issues.

3.1 Objective of PSA studies

  • Determining the core damage frequency (CDF) using a set of internal Initiating Events (IEs) and external IEs like loss of offsite power supply.
  • Identification and quantification of the dominating accident sequences, uncertainties and specific contributors to system failures.
  • Identifying design and operational weaknesses.
  • Supporting decision on safety issues.
  • Developing test and maintenance schedules and determining allowable outage times to assist in the establishment of criteria for technical specifications.
  • Correlating accident sequences to release categories.
  • Consequences modeling and risk estimation

3.2 Various Levels of PSA Study

  • Level 1 PSA: Identification of accident sequences and their quantification, estimation of overall CDF
  • Level 2 PSA: Definition of source term to the environment (Release categories) and release frequency estimation
  • Level 3 PSA: Consequence analysis in public domain

3.3 Input data for PSA

For estimating IE frequency, system unavailability, CDF the data required is important. The confidence in PSA results depend upon the input data. Data required are

  • Component failure data.
  • Component maintenance data.
  • Human error data.
  • Common cause failure data.

In addition, vast amount of design information about reactor process and safety system is also required.


Risk is defined as
Risk = Accident * consequence
Accident is cause and is expressed in frequency

Consequence: Result of accident such as radioactivity releases, property damage or human death.

Since consequences of NPP accident are high and carried over to next generation, one should control the probability of accident which is expressed as

Accident (frequency) = Frequency of occurrence of IE * unavailability of mitigating / safety system(s)

4.1 Risk reduction

Risk reduction is possible by

  • Minimizing the accident frequency.
  • Minimizing the consequence of accidents
  • Minimization of both total
  • accident and normal operation risk

4.2 Quantification of accident frequency

Identifying accident sequence and estimating frequency of accident which includes IE frequency estimation and Safety system unavailability estimation. For IE frequency estimation, either generic, plant specific data is used or fault tree analysis technique is used. For estimation of safety system unavailability, fault tree analysis technique is used.

4.3 Result of PSA

Level-1 PSA results are as:

  • Overall CDF of NPP is of the order of 1*10-6 / Reactor year.
  • Safety System unavailability is of the order of 1*10-3 year/ year.
  • IE frequency is of the order of 0.1/ year

4.4 Application of PSA

PSA results are very useful for

  • Equipment design improvement
  • Developing operating procedure
  • Technical specification evaluation
  • Accident management procedure
  • Maintenance procedure
  • Regulatory decision


PSA of nuclear power plant is an effective technique

  • To identify all probable accident scenarios and their quantification to estimate overall risk.
  • To minimize accident frequency and consequence by better design technique in power plant system and containment.
  • To develop emergency preparedness procedure.
  • To develop operational maintenance and accident management procedure.


Probabilistic Risk Assessment in the Nuclear Power Plant Industry – Fundamentals and applications, Ralph Fullwood & Robert E. Hall, Brookhaven National Laboratory, NewYork, USA, 1988, Pergamon Press.