MODELING THE PERFORMANCE OF HIGH BURNUP THORIA AND URANIA PWR FUEL

Abstract

Fuel performance models have been developed to assess the performance of ThO[subscript 2]-UO[subscript 2] fuels that can be operated to a high burnup up to 80-100MWd/kgHM in current and future Light Water Reactors (LWRs). Among the various issues raised in high burnup fuel applications, the pellet rim effect, fission gas release (FGR), and response to reactivity initiated accidents (RIA) were of special interest in this work. These phenomena were modeled by modifying the NRC licensing codes FRAPCON-3 for normal operation and FRAP-T6 for transient conditions. These models were verified and compared to the results of previous thorium fuel studies and high burnup uranium fuel evaluations. The buildup of plutonium in the outer rim of LWR UO[subscript 2] pellets has been observed to create a region of high fuel burnup, fission gas buildup and high porosity at the fuel rim. The power distribution of the thoria and urania fuel was calculated using a neutronics code MOCUP. Due to the lower build-up of Pu-239 (less U-238 in ThO[subscript 2]-UO[subscript 2] fuel) and flatter distribution of U-233 (less resonance capture in Th-232), thoria fuel experiences a much flatter power distribution and thus has a less severe rim effect than UO[subscript 2] fuel. To model this effect properly, a new model, THUPS (Thoria-Urania Power Shape), was developed, benchmarked with MOCUP and adapted into FRAPCON-3. Additionally a porosity model for the rim region was introduced at high burnup to account for the larger fuel swelling and degradation of the thermal conductivity. The mechanisms of fission gas release in ThO[subscript 2]-UO[subscript 2] fuel have been found similar to those of UO[subscript 2] fuel. Therefore, the general formulations of the existing fission gas release models in FRAPCON-3 were retained. However, the gas diffusion coefficient in thoria was adjusted to a lower level to account for the smaller observed gas release fraction in the thoria-based fuel. To model accelerated fission gas release at high burnup properly, a new athermal fission gas release model was developed. Other modifications include the thoria fuel properties, fission gas production rate, and the corrosion model to treat advanced cladding materials. The modified version of FRAPCON-3 was calibrated using the measured fission gas release data from the Light Water Breeder Reactor (LWBR) program. Using the new model to calculate the gas release in typical PWR hot pins gives data that indicate that the ThO[subscript 2]-UO[subscript 2] fuel will have considerably lower fission gas release beyond a burnup of 50 MWd/kgHM. Investigation of the fuel response to an RIA included: (1) reviewing industry simulation tests to understand the mechanisms involved, (2) modifying FRAP-T6 code to simulate the RIA tests and investigate the key contributors to fuel failure (thermal expansion, gaseous swelling, cladding burst stress), and (3) assessing thoria and urania performance during RIA event in typical LWR situations. ThO[subscript 2]-UO[subscript 2] fuel has been found to have better performance than UO[subscript 2] fuel under RIA event conditions due to its lower thermal expansion and a flatter power distribution in the fuel pellet (less power and less fission gas in the rim region). Overall, thoria has been found to have better performance than urania in both normal and off-normal conditions. However, calculations using the modified FRAPCON-3 showed that the internal pressure and cladding corrosion at the required high burnup of 80- 100MWd/kgHM are not acceptable with the current fuel design. Therefore, advanced fuel designs (including larger gas plenum, larger fuel grains, advanced cladding materials), and carefully designed operating strategy (i.e. decreasing power history) were assessed and the results showed that the targeted high burnup can be achieved. Further investigation of burnup issues is needed, such as the distribution of hydrogen in the cladding for heterogeneous fuels, and response of high pressure fuel pins to a loss of coolant accident, in order to assure satisfactory high burnup behavior.