RADIOLOGICAL PROTECTION OF WORKERS

The protection of workers exposed to ionizing radiation during nuclear reactor dismantling operations is a major challenge for nuclear operators. A predictive dose assessment is conducted prior to the start of dismantling operations so as to fine-tune the technical protection and organizational measures adopted for each worksite based on the radiological protection objectives considered. Significant deviations are often observed between predicted dose values estimated prior to the start of work (before and after the radiological protection optimization study) and effectively measured dose values, which are usually well below predicted values. These deviations may lead to poor resource allocation in the choice of radiological protection actions or dismantling methods and techniques adopted (decontamination method, pipe cutting technique, etc.). In addition, during actual dismantling activities, such overestimates of predicted dose values could compromise the rapid identification of a drift in individual or collective dose exposure, thereby preventing timely management of contingencies.

These deviations are mainly due to the difficulty of fine-tuning the data needed to calculate predicted dose values prior to the start of dismantling operations, i.e. source term and associated dose rates, workload dose exposure, number of workers and workstations, and exposure coefficient associated with each workstation. This data is usually estimated based on expert opinion, as opposed to being derived from experience feedback obtained through similar activities (e.g. maintenance work, modification of existing reactor units). The duration of dismantling projects (often several years) also contributes to the difficulty of obtaining reliable predicted dose values, due to the evolution of radiological conditions (implementation of biological shielding, waste evacuation, radioactive decay, etc.).

Existing radiological maps are often unsuitable for the calculation of predicted dose values at different workstations. If possible, it is advantageous to provide more detailed radiological maps, better suited for worksite-specific workstations. In cases where radiological mapping is not technically feasible, dose rate estimates are generally obtained from models. The superposition of hypotheses not well- suited for radiological protection during modelling calculations may lead to significantly overestimated dose rate values. In cases where uncertainties remain, it is important to assess the degree of uncertainty and to establish a dose rate interval on the basis of a critical analysis of radiological maps or a sensitivity analysis of selected hypotheses. The reliability of the radiation measurements obtained with regard to the low dose rates often encountered in dismantling worksites (less than 1 Sv/h) poses a problem. The associated measurement uncertainties may be quite large, resulting in significant deviations between predicted and effectively measured dose values. A sufficiently sensitive radiation meter must be used. Actual radiological conditions must be verified at the start of work so as to update predicted dose values.

Workload dose exposure is also subject to uncertainties. These uncertainties are increased by the long duration of dismantling projects and the implementation of improved work methods during project execution. A reassessment is required during project execution. There is not enough experience feedback available to define parameters such as exposure coefficients, which could be used to fine-tune predicted dose values by taking into account the actual locations of dismantling personnel with respect to the dose rates considered for their workstations. A coefficient of 0.7 based on experience feedback from work performed in NPPs currently in opera ration is often used, but it does not seem well suited for certain dismantling projects. Experience feedback from dismantling projects previously completed could be used to fine-tune the value of this coefficient based on the exposure conditions present at different workstations.

Changes in dose values and radiological context must be monitored throughout the project. This is indispensable given the significant uncertainties associated with the assessments conducted prior to the start of dismantling operations. Hold points for regular reassessment of estimated dose values and planned protection actions (based on actual radiological conditions effectively encountered) must be defined prior to the start of dismantling operations.