DOE OSTI2011
High Temperature Gas Reactor (HTGR), widely recognized as one of the top candidates for the Next Generation Nuclear Power-plant (NGNP) fleet, has received ever-increasing interest due to its superior safety and many other desirable features. TRISO fuel particles are the basic form of the fuel used in both pebble-bed and prismatic reactors. Since they are a key component of the HTGR core design, the success of the reactor depends on the safety and quality of the TRISO particles. During operation, a TRISO particle undergoes complex thermo-mechanical processes. Fission gases accumulate inside the kernel and the buffer layer, and lead to buildup of internal pressure. Uneven thermal conditions lead to asymmetric thermal expansion. The pyrolytic carbon, in both the IPyC and OPyC layers, is known to experience dimensional change (shrinkage or swelling) under fast neutron irradiation. In the mean time, both the IPyC and OPyC layers creep as a function of stress and fast neutron fluence. Macro cracks in the IPyC layer, corrosion in the SiC layer due to chemical attack, and debonding at the interface between IPyC and SiC, have been observed in post-irradiation examinations. All these processes further complicate the stress states in a TRISO particle, and might lead to failure of a single coating layer or the particle as a whole. It is very difficult to duplicate the actual environment that a particle is exposed to in experiments. It is even more difficult to isolate one process from another, thus making it nearly impossible to evaluate the impacts that a particular process has on the fuel particle by conducting experiments. The problem can however at least partially be addressed by modeling and simulation. Hence, multi-dimensional, multi-physics and comprehensive models are greatly needed to simulate and evaluate the performance of a TRISO particle under normal and accidental conditions. Existing models are inadequate as they are usually one-dimensional models that over-simplified these processes or do not include all the main physics. The goal of this thesis is to develop a comprehensive, multi-dimensional and multi-physics model to simulate and evaluate TRISO fuel performance. It includes all the main processes taking place in the fuel particle. Three equally important components of this modeling exercise are: 1) 3D heat transfer and temperature distribution analysis; 2) 3D neutronics, heat generation, and fission gas generation and release analysis; 3) 3D stress analysis and failure prediction of the TRISO particle. The 3D thermal model consists of the solution of the heat conduction equation using the finite difference technique. The temperature calculated using this model is compared to analytical solutions and to the results reported in literature. Good agreements are observed. Thermal models are developed to calculate temperature distribution in a pebble under different power and asymmetric convective boundary conditions caused by, for example, contacts with other pebbles. Three-dimensional temperature distribution in a TRISO particle is also calculated under various conditions such as in the presence of an off-center gas bubble. Three-dimensional neutronics models are developed using MCNP5/X. Burnup calculations are performed to track the number densities of a few important isotopes and to calculate gas production. The fractional gas release on both intra- and inter-granular level are calculated using the White and Tucker’s model. The gas pressure inside the buffer layer is also quantified. Finally, 3D axisymmetric stress models are developed using an open-source finite element code, FEAP. Modifications are introduced to FEAP to include PyC shrinkage and creep, etc. The modified code is benchmarked against Miller’s results, and good agreements are observed. A number of variables are studied to evaluate their impacts on stresses and failure probabilities. These variables include creep constant, creep Poisson’s ratio, irradiation temperature, BAF, layer thickness, etc. Some transient thermal conditions such as power surge, and defective cases such as a radial crack in IPyC, debonding and corrosion to SiC, are also studied. Among all, a few variables/conditions have significant impacts on stresses and failure probabilities, namely: shrinkage strain, creep constant, creep Poisson’s ratio, particle size, irradiation temperature, BAF and cracked IPyC. Order of magnitude of stresses resulting from temperature changes, pressure buildup due to gas generation, and due to PyC shrinkage/creep show that in the case of TRISO particle (when using specific empirical constants or existing dataset found in literature to calculate shrinkage/creep strains), stresses due to temperature changes and internal gas pressure are orders of magnitude lower than those resulting from shrinkage/creep. Failure probabilities of both IPyC and SiC are also calculated. The failure probability of IPyC under most conditions borderlines with the design requirement (1E-4) except under low irradiation temperature. The failure probability of SiC under normal conditions is minimal because it is always under compression; except when there is a radial crack in IPyC, in which case the stress in part of SiC becomes tensile and the failure probability is higher than 1E-4, and increases at higher burnup. The comprehensive fuel performance model developed in this work and implemented in a suite of computer codes simulates the thermo-mechanical behavior of a TRISO fuel particle under irradiation. This work leads to a better understanding of TRISO fuel particle evolution over its lifetime, and provides insights in how to improve the safety and reliability of TRISO fuel. The models developed here can be useful for other purposes related to TRISO fuel or for fuel performance evaluation of other forms of nuclear fuels. (author)
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗