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Criticality Safety Assessment

Ensuring the safe handling of fissile material

A criticality accident is an uncontrolled nuclear fission chain reaction capable of emitting large and potentially lethal radiation doses to people in the vicinity. All nuclear facilities that handle or store fissile material must be assessed and demonstrated to present as low a risk of accidental criticality as reasonably practicable.

What is Criticality Safety Assessment?

A Criticality Safety Assessment is a demonstration that a process or operation concerned with fissile material has adequate controls in place to prevent a criticality accident. The assessment is a systematic consideration of all operations in a facility or area, under both normal and potential fault conditions. This applies to all stages of the lifecycle of a facility handling fissile material, from design stages through to decommissioning.

Various hazard identification techniques may be employed to identify all potential failure modes. Once identified, various fault analysis techniques may be applied to assist in assessing the level of defence in depth. The assessment process may then determine which aspects of the operations require preventative safety measures and will designate safety measures necessary in the interests of maintaining criticality safety. The assessment must ultimately make a reasoned argument or judgement in relation to the adequacy of the overall level of defence in depth.

Safety designations are made up of engineered measures and operational measures. When determining appropriate safety measures, these should be identified in accordance with the fundamental safety hierarchy. All measures upon which that demonstration of safety is predicated must be clearly identified so that they may be discussed and agreed with facility operators and management prior to implementation and be suitably maintained during operations thereafter.

The assessment should determine the magnitude of the residual risk of criticality accident after account has been taken for the control measures put in place. This will inform the judgement that the overall risk presented by all faults in the safety case is ALARP / ALARA.

Why is Criticality Safety Assessment Important?

Health and safety regulatory bodies place a duty of care upon employers so that the health, safety and welfare of employees and members of the public is ensured, so far as is reasonably practicable or achievable.

By understanding all aspects of the process or operation concerned with fissile material, assessing the risk of criticality accident and documenting the means of control, the employer and delegated process owner is fulfilling that duty of care to manage the risk and protect people.

Nuclear operators must often conform to legal requirements which directly impinge upon criticality safety and therefore have direct relevance to the ongoing permission to carry out activities handling fissile material.

TÜV SÜD's Services

TÜV SÜD Nuclear Technologies Division offer a complete range of criticality safety assessment services:

  • Hazard Identification – structured analysis of the process to determine the potential failure modes of a process or operation that could lead to a criticality accident.
  • Fault Analysis – application of appropriate fault analysis techniques such as: Design Basis Accident Analysis; Probabilistic Safety Assessment; Double Contingency Principle; Barrier Analysis; Claims Arguments, Evidence Analysis.

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