A new methodology for worker evacuation and dose assessment following a radiological release

A new methodology has been developed by Wood for nuclear power plant worker evacuation following on-site radiological incidents. In the event of a radiological release, it is important that workers in the vicinity are evacuated promptly to minimise their exposure to radiation. The evacuation methodology needs to take into account the radiation field within the facility in order to establish an evacuation route that minimises doses consistent with the principle of ALARA.

Application of the Eliminate, Reduce, Isolate, Control, Personal Protective Equipment, Discipline (ERIC-PD) hierarchy is a key part of achieving this objective and ensures that the plant design is developed with the aim of avoiding faults and reducing their consequences should they occur. Where faults resulting in a radiological hazard cannot be eliminated from the design, measures to minimise the exposure of workers include the provision of adequate alerts about increases in radiation levels and adequate routes for evacuation to a safe area, together with appropriate safety management arrangements to ensure that the evacuation is effective.

The fundamental steps in the process of defining radiological evacuation routes are shown in the figure below.

Figure 1: Illustration of Radiological Evacuation Methodology

 

For each potential radiological hazard identified, the following information needs be determined:

  • Location and type of radiation hazard;
  • Dose rate information for all worker locations at the time the fault occurs and at a sufficient number of locations along their potential evacuation routes (radiation map);
  • Information on the evolution of the radiological hazard with time. 

This information can be used to determine the level of radiation hazard to personnel and identify the most appropriate evacuation route for any given radiological hazard scenario.

The worker dose is determined in the following steps:

  • Definition and grouping of faults from the initiating faults schedule (FMEA)
    • Identification of bounding faults
    • Selection of representative faults for dose analysis
  • Identification of unmitigated events and the associated dose rates and evacuation times
    • Calculation of transient dose rates (radiation map) for each representative fault using Monte Carlo computer codes such as MCNP or MCBEND.
    • Determination of average dose along each evacuation route
  • The assessment and identification of the need for mitigation actions consistent with the need to minimise radiation exposure
    • Selection of evacuation route which minimises radiation exposure and taking other hazards such as fire into account.  Note that prompt evacuation as a form of mitigation is only acceptable if implemented as part of other control measures consistent with ERIC-PD.

The evacuation time for each of the evacuation routes is identified for each radiological hazard and for each expected worker location. The evacuation time model incorporates a full human factors assessment. The estimates for evacuation times include suitable conservatisms such as latency between radiological release and detection, latency in decision-making to evacuate and any requirement to consider continued occupation either to verify alarm authenticity or to ‘close out’ diversionary tasks.

The different components of the evacuation time assessed for each potential evacuation route is combined with dose rate information to determine the dose to an evacuating worker.

As well as evacuation of personnel, the radiation exposure of workers in the event of a radiological incident can be minimised by:

  • The design and management of radiation and contamination control zones;
  • The use of personal protective equipment;
  • The use of shielding (permanent or temporary) as a prior measure to reduce radiation exposure;
  • The use of appropriately designed ventilation and filtration systems.

 

Contact
Dr. Ahmed Aslam, Wood

Sarah Hunak, Wood