Development of a methodology for dose assessment and estimate of amount of NORM residues disposable in a conventional landfill

Abstract

The final disposal of NORM wastes in conventional landfill generally determines problems of acceptance by the landfill operators, since their willingness to accept Naturally Occurring Radioactive Material (NORM) is often limited due to their concern about the radiological risks and reluctance of the local community to have at local landfills material that despite being cleared is still perceived as ‘radioactive’. In order to raise awareness among landfill operators, and also among other stakeholders on the actual radiological risk of exempted or cleared NORM wastes, it is of interest to estimate the mass of annual wastes containing NORM that can be disposed of in a landfill for conventional waste complying with the annual dose criterion of 1 mSv.

A methodology was developed considering a hypothetical homogeneous large landfill and assuming that NORM wastes are delivered with an initial activity concentration of 1 kBq kg⁻¹. The methodology uses exposure scenarios and assumptions from Radiation Protection 122 Part 2, the codes RESRAD-ONSITE and OFFSITE, and the most recent dose coefficients from ICRP. The exposure scenarios considered are the “worker handling NORM at an active landfill” and “members of the public living near an active landfill”, For these scenarios, screening levels (called Operational Levels in this work) in terms of activity concentration fulfilling the annual dose criterion of 1 mSv, for members of the public (including all ICRP age groups) and workers at a landfill, have been calculated for each decay chain segments using a generic and conservative approach. The Operational Levels evaluated in this work are almost independent from the landfill size and are relevant to the landfills complying the EU directive 1999/31 requirements.

The obtained Operational Levels (OL) are all higher than 1 kBq kg⁻¹, for U-238 and Th-232 series radionuclides and 10 Bq g⁻¹ for K-40, which are the general clearance levels adopted in the European Union. The estimated OLs range between 2 kBq kg⁻¹ (for Th-232sec) and about 150 kBq kg⁻¹ (for Po-210). For the U-238 decay chain, the segments with the smallest OLs values are U-238sec (OL = 2.5 kBq kg⁻¹) and Ra-226+ (OL = 3.7 kBq kg⁻¹). For Th-232 chain, the most critical segments are Th-232sec and Th-228+ (OL = 3.8 kBq kg⁻¹). As regards the mass of annual NORM wastes that can be disposed of in a conventional landfill, the levels obtained for Po-210, Pb-210+ and K-40 indicate that from the radiological point of view limitations to the annual admissible mass may occur only for large activity concentrations (i.e. 40 kBq kg⁻¹ and above). For the other segments, instead, limitations can occur for activity concentration levels in residues above 2 kBq kg⁻¹.