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.
Backfilling and sealing of underground repositories for radioactive waste in salt, Phase II (BAMBUS project)
The project "Backfill and Material Behaviour in Underground Salt Repositories, Phase II" (BAMBUS II) addresses the behaviour of the backfill and of the Excavation Disturbed Zone (EDZ) in a geological repository for heat generating radioactive waste in rock salt. Its objective is to increase the knowledge on important phenomena and processes in and around backfilled repository drifts and to strengthen the scientific basis required for repository design and performance assessments. The project consists of four major parts: in situ investigations, laboratory studies, modelling studies, and desk studies. The data obtained from the experiments are used to calibrate and refine the constitutive equations needed for predicting the mechanical and hydraulic behaviour of the backfill and the rock in the vicinity of emplacement drifts. By comparing calculation results with experimental data the predictive capability of the numerical models is assessed. The project is a joint undertaking of eight partners from four countries: France, Germany, Netherlands, Spain. It started on 1 August 2000 and will last 33 months until 30 April 2003. In this paper a brief outline is given of the project components and results are presented of the work performed during the first year of the project duration. In the first year, activities concentrated on preparation and performance of in situ and laboratory work. A master plan was developed for dismantling the test drift, backfill sampling, and recovery of measuring instruments and corrosion specimens. All these tasks have been completed. At present, the samples are under investigation at different laboratories and the instruments will be analysed for accuracy and reliability of the data generated during the about 8.5-years (100-months) -duration of the TSDE-experiment heating phase.
La Hague Continuous Improvement Program: Enhancement of the Vitrification Throughput
The vitrification of high-level liquid waste produced from nuclear fuel reprocessing has been carried out industrially for over 25 years by AREVA/COGEMA, with two main objectives: containment of the long lived fission products and reduction of the final volume of waste. At the 'La Hague' plant, in the 'R7' and 'T7' facilities, vitrified waste is obtained by first evaporating and calcining the nitric acid feed solution-containing fission products in calciners. The product-named calcinate- is then fed together with glass frit into induction-heated metallic melters to produce the so-called R7/T7 glass, well known for its excellent containment properties. Both facilities are equipped with three processing lines. In the near future the increase of the fuel burn-up will influence the amount of fission product solutions to be processed at R7/T7. As a consequence, in order to prepare these changes, it is necessary to feed the calciner at higher flow-rates. Consistent and medium-term R and D programs led by CEA (French Atomic Energy Commission, the AREVA/COGEMA's R and D and R and T provider), AREVA/COGEMA (Industrial Operator) and AREVA/SGN (AREVA/COGEMA's Engineering), and associated to the industrial feed back of AREVA/COGEMA operations, have allowed continuous improvement of the process since 1998: - The efficiencymore » and limitation of the equipment have been studied and solutions for technological improvements have been proposed whenever necessary, - The increase of the feeding flow-rate has been implemented on the improved CEA test rig (so called PEV, Evolutional Prototype of Vitrification) and adapted by AREVA/SGN for the La Hague plant using their modeling studies; the results obtained during this test confirmed the technological and industrial feasibility of the improvements achieved, - After all necessary improved equipments have been implemented in R7/T7 facilities, and a specific campaign has been performed on the R7 facility by AREVA/COGEMA. The flow-rate to the calciner was increased from 76 l/hr to 90 l/hr in three steps. Results of each step were followed by a team of experts from CEA, AREVA/COGEMA and AREVA/SGN analyzing the impact on the vitrification facility. The results obtained at the end of this testing campaign were a success: the target feeding flow-rate was reached, improving the line productivity without consequences in equipment availability and off gas treatment performances. This paper presents the results obtained for each step of this technological improvement and illustrates the La Hague continuous improvement program performed in order to enhance the vitrification throughput. (authors)« less