Abstract
The long-term conditions and potential radiological consequences of the legacy radioactive waste stored in the RADON-type disposal facility outside Chisinau is of concern for the central government authorities of the Republic of Moldova. A radiological risk assessment of “zero alternative scenario” for the RADON-type of near-surface disposal facility has been conducted. The objective of this risk assessment is to assess the long-term safety conditions of the facility and its potential radiological impact on humans and the environment as well as to provide a basis for decision making regarding the decommissioning of the legacy radioactive waste.
Based on the current state of the art, the procedures defned by IAEA’s standards and best practices the ISAM methodology and IAEA’s BIOMASS methodology are adapted in this risk assessment. We use the site specifc information as much as possible to derive the parameter values used in the assessment. Instead of using a stylised biosphere object the relevant biosphere object and associated catchment areas were identified based on the site specific DEM (digital elevation model) using GIS tools. As to the relevant biosphere object we mean that the identified biosphere object is close to the disposal site boundary so as to avoid excessive spatial dilution and the size of the object is large enough to supply the dietary needs of at least a small family group.
The generation of scenarios has been conducted according to ISAM approach, which contains various state of the disposal and human behavior components for a generic RADON-type facility. A limited number of deterministic sensitivity analysis was performed to explore the model uncertainty and parameter uncertainty. The most important pessimistic assumptions and parameter values used in the assessment are as the following:
- no retardation of radionuclides in the waste material itself and the engineered barrier (concrete wall that is degraded at the initial sate)
- the shortest possible transport distances of releases from the disposal facility to a well or a stream
- the hydraulic gradient follows surface inclination
- no sorption to waste mass in the flooding scenario
With pessimistic assumptions, the estimated doses from the calculation cases of the design scenario, i.e. for the well case and the stream case are lower than the IAEA’s criteria. Estimated doses for the on-site residence scenario after institutional control are higher than IAEA’s criteria. The results show that human intrusion activities after the institutional control can lead to radiological exposure above the level of 1 mSv/a for up to 100,000 years. Long lived radionuclide Pu-239 dominates the doses for the on-site residence scenario. Of course, the very conservative assumptions used in the modelling of the on-site residence scenario can be discussed. Nevertheless, measures should be taken for this matter if the waste is at its present place of disposal.
The potential efects on non-human biota from exposure to released radionuclides were assessed. The stream case of the design scenario was considered. The maximum values of the radionuclide concentrations in fresh water and in soil were compared with Environmental Media Concentration Limits (EMCL). If the ratio between the maximum values and EMCL is less than one no further assessments are required. For most of radionuclide concentrations calculated from the stream case are below one except C-14 and Pu-239 in freshwater.
Scenarios with high calculational consequences are obviously of interest though not necessarily because they are a true expression of radiological hazard. This first iteration has the primary function of assessing potential radiological impacts thereby identifying where better local information might reduce conservatism and lead to a more realistic expression of the assessment the radiological impact.
The disposal facility is located on the upstream area of Chisinau, which might be not an optimal choice of the site for a radioactive waste disposal.
Contact person SSM: Viviana Sandberg