Toward a Lean Analysis for Waste to Support the Decommissioning Operation
Résumé
All over the world, several nuclear facilities have been shut down few years ago and are now under dismantling or will be dismantled in the future.Dismantling project takes place according to three major phasesStudy and definition of the dismantling scenarioDismantling site operationsDecommissioning and final state characterization.Each step is lead in agreement with safety, security and waste management. This requires analytical tools appropriate to the radiological environment and aims. For instance, for safety, fissile products, explosives... will be researched and quantified while security requires quantification of radiological, carcinogenic, mutagenic and reprotoxic elements such as As, Se, Cr(IV) ... or even asbestos in order to follow more and more strengthened rules. For waste management, radiological and chemical quantification are necessary but must be completed by determining physical and chemical properties such as reactivity, stability or dissolution of the waste...Required level of knowledge, analytical deadline, uncertainties, limits of detection and matrix variety depend on the decommissioning phase.Setting up a dismantling scenario requires the most possible accurate knowledge of the initial source term present in the facility, in order to establish waste management flow-sheets, define dismantling intervention modes, and prepare the associated safety demonstrations. As history is often not sufficient, in-situ investigations and off-site laboratories are necessary to assign an initial waste and radiological inventory and propose some strategy operations.When a first scenario is chosen, more detailed samplings and analyses are required to fully prepare estimates of waste volumes and categories, and define and validate the extraction waste system would be used during the dismantling operation. As the definition of the dismantling plan takes shape, data must be more and more accurate and reliable in order to define the waste outlet. The scaling factors for the waste must be defined in order to assure the refunding of the waste. This includes complete characterizations of various matrixes, with high precision, low detection limit in a limited time of few months, as sketchy data could lead to fix inappropriate plan.Once site operations begin, it is necessary to control the safety operational conditions and waste production. Measurements requirement is under the safety threshold. Matrixes are often limited to filter. The most important factor is deadline, because operations delay is costly.Finally at the end of dismantling operations, analyses must be performed to demonstrate the successful clean-up of the work site. Analytical methods with low detection limit are required.Thus, characterization appears as a graded and iterative process and is a key issue for decommissioning and dismantling operations in term of efficiency and cost. In order to face this huge analytical demand, a strategy for sampling and characterization must be developed regarding the real needs. On site measurements are usually performed with existing hand-held instruments. A basic first level of knowledge is obtained without delay, feeding the dismantling scenario. On these bases, a sampling strategy permits to select some representative specimens to be more precise, and accurate analysis is performed on off-site laboratories introducing an additional difficulty the sampling on the site and sub-sampling in the laboratory itself. Whatever the level of sampling, aliquots are small considering the dimension of the dismantling area.This step is crucial. Sampling strategy must be optimized according to objectives (scaling factors measurement, waste level). But in the particular case of high radioactive level waste, due to security manipulation constraints, the number and mass of samples collected are limited. At the opposite for low level waste, large number of samples is collected meaning that large analytical method capacity is necessary.Optimization of the analytical plan is critical. The analytical strategy (including control of subsampling, analysis sequence according to analytical capability and performances, especially for pure beta emitters) developed for real case will be detailed.
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