Sadaf Waqar, PhD, Department of Physics & Applied Mathematics, PIEAS, June 2008." Static and Dynamic Simulation of HEU and LEU Cores of Research Reactors using Multi- group and Coupled Space-Time Thermal Hydraulic Approach”; Supervisor: Dr. Nasir M. Mirza; Co-Supervisor: Dr. Sikander M. Mirza; Department of Physics & Applied Mathematics, PIEAS, Nilore 45650, Islamabad. A comparative study has been performed for neutronic analysis of HEU and potential LEU cores for the Pakistan Research Reactor-2 (PARR-2) taken as a typical MNSR system. The group constant generation has been carried out using transport theory code WIMS-D4 and a detailed five-group RZ-model has been used in the CITATION code for multigroup diffusion theory analysis. The neutronic analysis of the 90% HEU (reference fuel) and potential LEU alternative fuels: UO2, U3Si2 and U9Mo, has been carried out yielding 11%, 20.7% and 14.25% enrichments with corresponding values of excess reactivity: 4.33, 4.30 and 4.07 mk. These results have been found in good agreement with recently reported Monte Carlo based transport theory calculations. The diffusion theory based calculated values of thermal flux profiles for axial as well as for radial directions have been found to agree well with the corresponding experimental measurements. The UO2 based LEU core has been found having flux spectrum closest to the reference core while U9Mo core has significantly harder flux spectrum at irradiation site. Fuel burn-up study and buildup of actinides and fission products for potential LEU fuels (UO2 and U9Mo) with existing HEU fuel (UAl4-Al, 90% enriched) for a typical Miniature Neutron Source Reactor (MNSR) has been carried out using the WIMSD4 computer program. For the complete burnup, the UAl4-Al, UO2 and U9Mo based systems show a total consumption of 6.89, 6.83 and 6.88 g of Relative to 0.042 g 235 U respectively. 239 Pu produced in case of UAl4-Al HEU core, UO2 and U9Mo based cores have been found to yield 0.793 and 0.799 g respectively indicating much larger values of conversion-ratios and correspondingly high values of fuel utilization factor. The end-of-cycle activity of the HEU core has been found to be 2284 Ci which agrees well with the value found by Khattab [48], where as for UO2 based and U9Mo based LEU cores show 1.8% and 4.8% increase with values of 2326 and 2394 Ci respectively. A two-group, three dimensional diffusion theory based methodology coupled with one-dimensional single-phase heat transfer calculations has been developed for the transient analysis of typical material test reactors (MTRs). This methodology has been implemented in a Fortran based computed program MTRAP3. It uses the CITATION computer program for static neutronic calculations while the group constant generation is performed by employing the WIMS-D/4 code. The MTRAP3 program uses Cranck- Nicolson (CN) based numerical scheme for solution of time dependent neutron diffusion calculations while time-implicit strategy is employed for detailed heat-transfer calculations. The CN-scheme has been found to remain stable for much larger time steps (∆t~10-5 s) as compared with the time-explicit scheme which remains stable for very small time steps only (∆t~10-10 s). For step as well as for ramp reactivity induced transients, the predicted values of core integrated reactor power and core average temperatures has been found to agree well with the corresponding values found by using the PARET computer program. The assembly-wise power profile as found by the MTRAP3 program has been found consistent with the corresponding experimental measurements.