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Criticality Warning System (CWS) Omission

Underpinning the justification of CWS Omission

In the UK nuclear industry, the default position is that all facilities handling and storing fissile material should have a Criticality Warning System (CWS) provided, unless its omission can be justified.

What is CWS Omission?

A CWS is designed as a safeguard to protect plant operators and other persons against any “unforeseen” events that might result in a criticality excursion with potential to emit large radiation doses causing serious deterministic effects. The rapid detection of criticality, together with an effective alarm system and prompt evacuation, has the potential to reduce the doses received.

Two possible approaches can be adopted in justifying omission of a CWS: one based on the dose consequences of a criticality incident and one based on the frequency of a criticality incident.

For dose based CWS Omission justification, an assessment may be made of the potential for an operator in the vicinity to receive a radiation dose in excess of a pre-determined detection criterion. Plant specific conditions and information may be used to determine the source type and fission yield.

For frequency based CWS omission cases, an assessment may be made of the plant in which all criticality controls that rely on human agency or on mechanical or electrical arrangements had failed. If it may be judged that a criticality accident would still not be reasonably expected in the natural order of events, for the anticipated normal operation conditions, then CWS omission is justified. For the CWS omission case, the k-effective criterion is that of the critical state, a sub critical margin is not applied.

Why is CWS Omission 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.

Adequate protection and warning systems are therefore often a requirement.

Where omission of a CWS may be justified, as well as there being significant initial and ongoing maintenance cost savings to be made, there are significant dose savings to be made.

Where a CWS is installed on plant, there are occasions when it may be unavailable due to planned maintenance or system failure. Therefore, it is appropriate to consider which operations may safely continue during the outage and which may not. The advice from such an assessment can be formalised into the working arrangements in advance, i.e. to suspend specific operations; or, in the case of system failure, advise on the appropriate actions where operations are in-train.

TÜV SÜD's Services

TÜV SÜD Nuclear Technologies Division offer a complete range of CWS Omission services:

  • CWS Omission against the dose criterion – determination of the plant specific source type and maximum credible fission yield coupled with determination of the potential dose to personnel
  • Dose Calculations – determine the potential dose to personnel using appropriate radiation transport modelling techniques (MCNP, Attila, FLUKA, analytical techniques)
  • CWS Omission against frequency criterion – assessment as to whether criticality would reasonably be expected in the event of the failure of any or all of those criticality controls which rely on human agency or on mechanical or electrical devices
  • Calculation of k-effective - simulations using neutronics computer codes such as MCNP to model the materials and their geometries in the system to determine k-effective
  • CWS Outage Arrangements – determination of appropriate actions during CWS outages


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