Calculation of Dose Rates at the Surface of Storage Containers for High‐Level Radioactive Waste
Abstract In several countries, the high‐level radioactive waste that will be disposed of in deep geological formations has to be retrievable for a certain time. Since 2010, retrievability is required for the operation phase of a repository also in Germany. Depending on the effort and the feasibility of remote handling, a certain exposure of the involved employees to ionising radiation is caused. The estimation of the exposure requires the knowledge of the inventory of radionuclides and the radiation field around the storage containers. This paper focuses on German concepts for final storage casks for the drift emplacement in rock salt for both spent fuel rods and high‐active waste (HAW) coquilles. Calculations of dose rates at the surfaces of the casks are presented. The calculations show that the gamma radiation of fission and activation products can be efficiently shielded by materials like cast iron or low‐alloyed steel. Ductile cast iron, however, has a positive influence on the neutron moderation because of the high carbon content. The shielding of the neutron radiation strongly depends on the quantity and position of the polyethylene (PE) rods which are used as neutron moderator. PE has unfavourable features at elevated temperatures that can be reached in a repository for high‐level waste. The simulations show that TiH2 is a promising alternative material for the shielding of neutron radiation. The current assumption that a final storage cask for HAW coquilles has the same outer dimensions as the cask for spent fuel rods will lead to unacceptable dose rates at the side wall. An increase of the diameter is necessary to provide sufficient shielding. Graphite insets in the cask interior lead to a considerably lowered dose rate.
Radionuclide Inventory of Vitrified Waste After Spent Nuclear Fuel Reprocessing at La Hague
The primary aim of the present work was to determine the inventories of the radionuclides and stable elements in vitrified high-level waste produced at La Hague and delivered to Germany, which are of importance for long-term safety assessment of final repositories for radioactive wastes. For a subset of these radionuclides and stable elements, the inventories were determined — either by direct measurements or by involving established correlations — and reported by AREVA. This allowed verification of the validity of application of a model approach utilizing the data of burnup and activation calculations and auxiliary information on the reprocessing and vitrification process operated at La Hague.Having proved that such a model approach can be applied for prediction of inventories of actinides, fission and activation products in vitrified waste, the present work estimated the minimum, average and maximum inventories of the radionuclides, which are of importance for long-term safety assessment of final repositories for radioactive waste but were not reported by AREVA for delivered CSD-V canisters. The average and maximum inventories in individual CSD-V canisters predicted in the present approach were compared to the inventories predicted by Nagra for canisters with vitrified waste delivered from La Hague to Switzerland [1]. This comparison revealed a number of differences between these inventories despite the fact that the canisters delivered to Switzerland were produced in essentially the same way and from the common reprocessing waste stock as CSD-V canisters delivered to Germany.Therefore, a further work is required in order to identify the reason for the discrepancy in the present estimation versus the Nagra estimation [1]. Such a work should also address the recommendation by the international peer review of the Safety Report of the Project Opalinus Clay to obtain estimates of the inventories of long-lived mobile radionuclides (such as 14C, 36Cl, 79Se, and 129I), which contribute most to the dose estimates in the radiological safety assessments, if possible, in agreement with other countries with similar waste streams in order for a coordinated set of data to be generated [2]. Since vitrified waste from reprocessing of spent nuclear fuel at La Hague was delivered to several countries — Belgium, France, Germany, Japan, Netherlands, and Switzerland — an international effort can be recommended.Copyright © 2013 by ASME
A quantitative measure of nitrifying bacterial growth.
Nitrifying bacteria convert ammonia (NH3) to nitrate (NO3-) in a nitrification reaction. Methods to quantitatively separate the growth rate of these important bacterial populations from that of the dominant heterotrophic bacteria are important to our understanding of the nitrification process. The changing concentration of ammonia is often used as an indirect measure of nitrification but ammonification processes generate ammonia and confound this approach while heterotrophs remove nitrate via denitrification. Molecular probe methods can tell us what proportion of the microbial community is nitrifying bacteria but not their growth rate. The technique proposed here was able to quantify the growth rate of the nitrifying bacterial populations amidst complex ecological processes. The method incubates [methyl-3H] thymidine with water samples in the presence and absence of an inhibitor of nitrification-thiourea. The radioactively labeled DNA in the growing bacteria was extracted. The rate of incorporation of the label into the dividing bacterial DNA was used to determine bacterial growth rate. Total bacterial community growth rates in full-scale and pilot-scale fixed-film nitrifying reactors and an activated sludge reactor were 2.1 x 10(8), 4.1 x 10(8) and 0.4 x 10(8)cell ml(-1)d(-1), respectively; the growth rate of autotrophic-nitrifying bacteria was 0.7 x 10(8), 2.6 x 10(8) and 0.01 x 10(8)cell ml(-1)d(-1), respectively. Autotrophic-nitrifying bacteria contributed 30% and 60% of the total bacterial community growth rate in the nitrifying reactors whereas only 2% was observed in the activated sludge reactor that was not designed to nitrify. The rates of ammonia loss from the nitrifying reactors corresponded to the rate of growth of the nitrifying bacteria. This method has the potential to more often identify factors that enhance or limit nitrifying processes in both engineered and natural aquatic environments.