Due to the non-availability and growing proliferation concerns regarding the use of highly enriched uranium (HEU) fuel in nuclear research reactors, some HEU cores of research reactors around the world have already been converted into low enriched uranium (LEU) cores. However, a large number of HEU cores of research reactors could not be converted into LEU cores in the past due to the non-availability of suitable LEU fuel. Infact, the conversion of an HEU core into an LEU core without loss of performance requires several times higher density LEU fuel to replace the HEU fuel. The highest density LEU fuel developed, qualified and licensed up till now is based on U3Si2-AI with density 4.8 gU/cm3 and more higher density fuels are being developed for the conversion of existing HEU cores of research reactors around the world into LEU cores.
In 1992, HEU core of the reactor under study i.e. Pakistan Research Reactor-I (PARR-I), was also converted into an LEU (19.99 % 235U) core by utilizing silicide dispersion (U3Si2-AI) fuel of density 3.28 gU/cm3. In order to compensate the reduction in the neutron flux due to core conversion from HEU fuel to LEU fuel and to increase the neutron fluxes at the irradiation sites, power of the reactor was also increased from 5 to 10 MW at the time of core conversion.
High neutron flux per unit power is always desired in research reactors and can be achieved by compacting the core. The available and upcoming higher density LEU fuels can be utilized to make the existing LEU cores of research reactors more compact without loss of performance i.e. by increasing the fuel loading per unit volume.
The simulation study, presented in thesis, was conducted with the aim to assess the feasibility of utilizing various available and upcoming higher density fuels in PARR-1, i.e. for the enhancement of its performance, with and without requiring any modifications in the existing reactor system. Objectives of this study were achieved by designing relatively compact cores for PARR-1 i.e. by utilizing the available and upcoming higher density LEU fuels, without compromising on the reactor safety and economy. Standard computer codes such as a transport theory lattice simulation code WIMS-D/4, diffusion theory based global core calculations computer code CITATION, core thermal hydraulic simulation code PARET and a locally developed computer code Fuel Cycle Analysis Program (FCAP) were used to perform this study.
This study showed that although reactivity was added more rapidly with the addition of fuel in an optimally moderated core, slightly under moderated core of the size of optimally moderated core gave better performance in terms of fuel cycle length, fuel burnup, higher neutron fluxes at the out-core irradiation sites and core thermal hydraulics. It was also noted from the study that various performance parameters e.g., fuel cycle length, fuel burnup, neutron fluxes etc. are interrelated and any of these parameters can be improved at the cost of other parameters. This simulation study depicted that by utilizing the available highest density dispersion fuel of density 4.8 gU/cm3 in PARR-1, neutron fluxes could be increased at the irradiation sites of this reactor up to 47% without loss of performance in any other respect and without any modifications in any of the existing reactor systems. Moreover, if the water reflector around this new core was partially replaced by inserting beryllium elements in the unused grid positions of the reactor then the neutron flux in PARR-1 could be enhanced up to 82% and at the same time cost of producing neutron fluxes would reduce by 14% without loss of performance in any other respect.
The study presented in this thesis also revealed that upcoming higher density fuels could not be utilized effectively in PARR-1 under the constraint of not modifying the existing reactor system. However, by increasing the existing water channel width from 2.1 to 3.0 mm, partially replacing the water reflector around the core by beryllium and utilizing the upcoming monolithic fuel of density 15.3 gU/cm3, neutron fluxes can be increased in PARR-1 up to 89% and at the same time the cost of producing neutron fluxes can be reduced by 29%. However, the core providing 89% higher neutron flux requires 48% higher coolant flow rate for its safe operation at 10 MW.