Projected external doses from an accidental release of ESS spallation-target products: time-dependence and radionuclide contribution.

The Swedish Radiation Safety Authority has presented a report on severe accident scenarios at the European Spallation Source (ESS) for dimensioning the emergency preparedness zones around the facility. The source-term in the scenario consisted of more than 80 tungsten-target spallation products with physical half-life (T½) exceeding 1 hour. The purpose of this study is to establish which of these radionuclides will become of highest importance in terms of the radiological consequences to residents in areas affected by an accident release. In analogy with accidents at nuclear power plants, where the fission product 137Cs is a key nuclide for estimating projected external doses to affected residents, a corresponding key nuclide for ESS is required for the emergency preparedness. Using existing accident source terms in combination with reported values on ecological half-times of the gamma emitter 137Cs, the external dose rates and cumulative doses per unit initial ground deposition of the suggested key-nuclide (182Ta) could be estimated. In terms of 50 y dose from a dry deposition of the released source-term, 172Lu (T½ = 6.7 d, supported by 172Hf with T½ = 1.87 y) contributes up to 50% of the 50 y dose, depending on the ecological half-times for the element. The isomer 178nHf (T½ = 31 y) is the second largest contributor to gamma-ray dose, followed by 182Ta (T½ = 115 d), that contribute with about 15% and 10% of the 50 y dose, respectively. The results thus suggest that 172Hf/172Lu may be more suitable for long-term follow-up of projected doses from accidental ESS releases than 182Ta.