CWS Placement and Coverage Optimisation Expertise

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.

What is CWS Placement Coverage and Optimisation?

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 doses received.

A CWS coverage assessment demonstrates that the distribution and locations of detectors within a facility can detect a criticality incident. This considers the potential criticality source locations, the distance between the source and detectors and the presence of attenuating media between the two. The type of criticality and inherent attenuation may also be factored into the assessment.

In the first instance, the criterion is for the detection of the “minimum incident of concern”, i.e. a minimum fission yield, which is dependent upon the system type and worker proximity.

Use of an alternative criterion may also be justified where the balance of risks lies with demonstration that a CWS is capable of detecting an incident that could cause deterministic harm, rather than incur additional dose, time and cost to enhance and/or maintain an existing system.

Why is CWS Placement Coverage and Optimisation 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.

Design, installation, substantiation, and the ongoing routine testing of detector heads, maintenance of detectors and alarm system over the lifetime of the plant are all costly, time consuming and potentially dose intensive activities. A CWS coverage assessment will determine the optimum number of detectors with optimum placement to ensure the most efficient system is designed and justified.

TÜV SÜD's Services

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

• Dose Calculations – determine the potential dose at the detectors or to personnel using appropriate radiation transport modelling techniques (MCNP, Attila, FLUKA, analytical techniques)
• Emergency Planning – Calculation of criticality dose contours in order to assist with emergency planning
• Selection of the appropriate criterion – determination of the plant specific source type and maximum credible fission yield coupled with determination of the potential dose to personnel
• CWS detector placement advice – determination of the optimum number and positioning of detector heads to provide adequate coverage in the most efficient manner
• CWS detector coverage assessment – analysis of existing CWS detector locations upon variation of operations to advise on acceptability and recommend modifications as necessary
• 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