Dose apportionment criteria and disposal rate limits for near surface radioactive waste disposal facilities

The dose apportionment for near surface disposal facilities (NSDFs) in India is regulated as 0.05 mSv/y uniformly for all the sites. However, the nuclear power capacity at different sites is different and the generation of low and intermediate level waste (LILW) at these sites depends on the nuclear power capacity. Therefore, the dose apportionment for the NSDFs should be site-specific and there is a need to review the adequacy of the existing dose apportionment. In this paper, an environmental exposure assessment model is used to estimate the spatial and temporal profile of the effective dose rates to members of the public due to the disposal facility through groundwater drinking pathway. The model considers continuous disposal of radioactive waste into the facility for 50 years and leaching of the radionuclides from the waste form into the groundwater flowing in the unconfined aquifer below. Uncertainty analysis of five parameters such as the distribution coefficient, fractional release rate, groundwater velocity, longitudinal dispersivity and thickness of the aquifer is also carried out using the stochastic response surface method to derive a safety factor. The dose apportionment and discharge rate limits for the NSDFs at Trombay, Rawatbhata, Narora, and Kaiga are estimated as an example. It is concluded from the study that the dose apportionment for the NSDFs satisfy the regulation of 0.05 mSv/y for these sites. It is to be noted that the LILW from nuclear power plants (NPPs) and fuel-reprocessing plants (FRPs) under operation are only considered for this review. This study underlines the importance of site-specificity of the dose apportionment for the NSDFs and reiterates the need for its periodic review whenever changes in the inventory of LILW are expected. (author)