Fuel Cycles and Waste Management

Abstract

For the HTR, different fuel cycles have been developed and used in the reactors with great success. Highly enriched Uranium with Thorium, low-enriched Uranium, medium-enriched fuel, and different types of BISO- and TRISO-coated particles have been applied. It was shown that these fuel elements can be operated with high burnup and high temperature and that the release of radioactive isotopes was very small. High conversion and near-breeding are possible if necessary in the future. The use of Thorium as fertile material opens a second large resource of fuel for the nuclear technology. It was shown that the Uranium demand of different fuel cycles applicable to HTR could be much smaller than that of today-used reactors. The fabrication of HTR fuel elements is state-of-the-art technology, and fuel with high quality was produced (1 million spherical fuel elements). Especially, the contamination of the graphite matrix with Uranium during the fabrication process was strongly reduced and the quality of the coating layers was improved. As a consequence, the helium circuits of the HTR plants were very clean with a characteristic contamination of around 1 Ci noble gases/MW_th. The intermediate storage for spent HTR fuel elements is done in a very safe form in thick-walled iron vessels, which are cooled from the outside just by conduction, radiation, and free convection. This is the principle of self-acting decay heat removal without any need for water and electricity supply. A compact storage of spent fuel in a water pool, as necessary for LWR fuel in the first years, is not necessary for HTR fuel elements. The retention of radioactivity in the dry air storage vessels for HTR fuel is even given in case of very extreme impacts from outside. The fuel elements can stay for many decades in this intermediate storage. For the final storage of spent HTR fuel elements, solutions have been worked out and the main aspects have been tested. Estimation in different countries shows that the possible release of radioactive substances from a final storage can just cause very small changes of the radioactive burden in the neighborhood of a geological deposit. Mainly, the radiotoxicity of Plutonium and minor actinides has to be considered in this connection. Fuel cycles, in which these isotopes are reduced and used for energy production, will have advantages related to this question. Reprocessing the spent fuel in a far future and by this way to reduce the content of Plutonium and minor actinides even stronger stays an open option if a long time of intermediate storage is foreseen. For the assumed accidents in a final storage, the retention capability of the coated particles has great importance. Requirements of nonproliferation and safeguard aspects, which are important for all fuel cycles, are fulfilled by use of LEU fuel with less than 10% enrichment. Established concepts of control and supervision are available from the development work done until now.