Transmutation von Transuranen in einem gasgekühlten beschleunigergetriebenen System

The peaceful usage of nuclear energy by light and boiling water reactors is connected with a buildup of long-lived high-level radioactive waste. Compared to the direct disposal, partitioning and transmutation (P&T) is considered as an effective way to reduce this waste in its quantity by converting it into short-lived radio nuclides. By that the long term radiotoxicity is reduced compared to direct disposal. Subcritical systems, which are powered by spallation processes for free neutron production to maintain the nuclear chain reaction, allow a target-oriented transmutation. As a subcritical system a gas-cooled accelerator driven system (ADS) for transmutation of transuranic elements has been modeled in this thesis to evaluate the reduction of the radio toxicity by P&T. The simulation of neutron-physical processes is based on the Monte Carlo computer program MCNPX. The development of an equilibrium core made it possible to study the transmutation and operating behavior for several fuel variations in a magnesiumoxid matrix and develop a simplified burnup method. Americium as part of the fuel has a stabilizing effect on the neutron multiplication due to its conversion into plutonium during the operation. Thorium was investigated as an alternative matrix for the fuel in order to replicate the stabilizing effect of americium by the conversion of thorium in $^{233}$U. By that a consistent operating cycle in the later P&T-process is ensured. Calculation of the nuclide composition at the end of a P&T-process leads to an expansion of the mathematical description of the mass reduction (transmutation efficiency) by the material located in the reactor. The achieved transmutation efficiency with the investigated ADS is 98.8 %. The transmutation time was examined with different operating strategies regarding the number, size and thermal power of use of transmutation facilities to determine the effort for the P&T-process depending on efficiency. It turns out that a transmutation time of more than 338 years, even at low reprocessing losses of 0.1 %, is necessary for transmutation efficiency of 98.8 %. One reason for this duration is due to a reprocessing time of 10 years. The development of repository-relevant quantities radiotoxicity, decay heat and activity for transuranic elements on repository periods up to 1 million years is presented for direct disposal, after the P&T-process and for not transmutable assumed waste. The waste is divided into already vitrified heat-generating waste, uranium and fission products. Efficiencies for decay heat and radiotoxicity are deduced from the vitrified waste, which limits the additional contribution of transmuted transuranic elements to this level. The efficiencies differ with 96 % and 96.6 %, only slightly from each other and lead to transmutation durations of 232 and 245 years. With the consideration of the initial hazard potential, the simulation of the operation of a transmutation system, the calculation of the final storage masses after the P&T-phase, the development of the resulting repository-relevant variables, and the consideration of the transmutation time, this thesis provides an in-depth picture of the transmutation process

Effect of Target Configuration on the Neutronic Performance of the Gas-Cooled ADS

With the utilization of nuclear energy transuranic elements like Pu, Am and Cm are produced causing high, long term radioactivity and radio toxicity, respectively. To reduce the radiological impact on the environment and to the repository Partitioning and Transmutation is considered as an efficient way. In this respect comprehensive research works are performed at different research institutes worldwide. The results show that the transmutation of TRU is achieved with fast neutrons due to the higher fission probability. Based on Accelerator Driven Systems (ADS) those neutrons are used in a particular system, in which mainly liquid metal eutectic (lead bismuth) is used as coolant. The neutronic performance of an ADS system based on gas cooling was studied in this work by using the simulation tool MCNPX. The usage of the Monte-Carlo method in MCNPX allows the simulation of the physical processes in a 3D-model of the core. In dependence of the spallation target material and design several parameters like the multiplication factors were investigated, in order to analyze the neutronic performance