Application of the scoping tool and AMBER model in radiological safety assessment of the barrier systems of the borehole disposal system for the disposal of disused sources in Ghana

Radiological safety assessment was carried out to assess the integrity and longevity of the barrier systems of the borehole disposal system (BDS) for the disposal of disused sources in Ghana. The design of the BDS incorporates both the engineered and natural barriers into its safety concept. The safety of the BDS requires confidence in the ability of the engineered barriers on the host environmental conditions to provide containment for the disused sources for a sufficient length of time. The results obtained from the scoping tool indicated that the dose limit was exceeded which showed that the containment provided by the engineered barriers was not sufficient to ensure safety of the disposal system for the specified environmental conditions and radionuclides inventory. Detailed modelling was performed with an AMBER model to evaluate the containment provided by the engineered barriers and the geosphere with some identified scenarios. The AMBER model results showed that the calculated peak dose from any of the disposed radionuclides for the identified scenarios was below the dose constraint of 0.3 mSv/y. The peak doses occurred around 2000–1000000 years. The peak doses were seen to be mainly derived from Ra-266 and Am-241 and/or their daughters. The disused sources could be disposed of safely in the disposal system either in an oxidizing or reducing fractured/porous flow environment without posing any significant radiological threat to humans and the environment.