Advanced fuels for thermal spectrum reactors

Sammanfattning: The advanced fuels investigated in this thesis comprise fuels non− conventional in their design/form (TRISO), their composition (high content of plutonium and minor actinides) or their use in a reactor type, in which they have not been used before (e.g. nitride fuel in BWR). These fuels come with a promise of improved characteristics such as safe, high temperature operation, spent fuel transmutation or fuel cycle extension, for which reasons their potentialis worth assessment and investigation. Their possible use also brings about various challenges, out of which some were addressed in this thesis. TRISO particle fuels with their superior retention abilities enable safe, high−temperature operation. Their combination with molten salt in the Advanced High Temperature Reactor (AHTR) concept moreover promises high operating temperature at low pressure, but it requires a careful selection of the cooling salt and the TRISO dimensions to achieve adequate safety characteristic, incl. a negative feedback to voiding. We show that an AHTR cooled with FLiBe may safely operate with both Pu oxide and enriched U oxide fuels. Pu and Minor Actinides (MA) bearing fuels may be used in BWR for transmutation through multirecycling; however, the allowable amounts of Pu and MA are limited due to the degraded feedback to voiding or low reactivity.We showed that the main positive contribution to the void effect in the fuelswith Pu and MA content of around 11 to 15% consist of the decreased thermalcapture probability in Pu-240, Pu-239 and Am-241 and increased fast and resonance fission probability of U-238, Pu239 and Pu-240. The total void worthmoreover increases during multirecycling, limiting the allowable amount ofMA to 2.45% in uranium−based fuels. An alternative, thorium−based fuel allows for 3.45% MA without entering the positive voiding regime at any point of the multirecycling. The increased alpha−heating associated with the use of transmutation fuels, is at level 24−31 W/kgFUEL in the uranium based fuels and 32−37 W/kgFUEL in the thorium−based configurations. The maximum value of the neutron emission, reached in the last cycle, is 1.7·106 n/s/g and 2·106 n/s/g for uranium and for thorium−based fuels, respectively. Replacing the standard UO2 fuel with higher−uranium density UN orUNZrO2 fuels in BWR shows potential for an increase of the in-core fuelresidence time by about 1.4 year. This implies 1.4% higher availability of the plant. With the nitride fuels, the total void worth increases and the efficiency of the control rods and burnable poison deteriorates, but no major neutronics issue has been identified. The use of nitride fuels in the BWR environment is conditioned by their stability in hot steam. Possible methods for stabilizing nitride fuels in water and steam at 300◦ C were suggested in a recent patentapplication.