Study On Physical Characteristics Of The New MOX Fuel Assembly In Supercritical Water Cooled Reactor

With the rapid development of nuclear power, the fourth generation advanced nuclear reactor technology is under actively developing in the world. As the only water cooled reactor of the fourth generation reactor, the Supercritical Water Cooled reactor(SCWR) has the advantage of simple system, high thermal efficiency and security, which makes it become an alternative reactor for large-scale advanced pressurized water reactor. And with the increase of nuclear power proportion, more and more spent fuel produced. Therefore, recovery of U and Pu in spent fuel has gradually got the attention of people. Plutonium was usually mixed with Uranium to fabricate Mixed Oxide fuel(MOX). Then MOX fuel is used in SCWR which combines the fourth generation of advanced nuclear reactor technology and the U-Pu fuel cycle technology. However, physical characteristic of MOX fuel is very distinct with that of UO2 fuel, so it is necessary to study physical characteristics of the new SCWR assembly.First of all, total neutron reaction, absorption, fission, scattering cross section of MOX fuel were calculated by MCNP software. And they were compared with that of UO2 fuel. The differences were obtained. MOX fuel has a higher total neutron reaction, absorption and fission cross section, but their neutron scattering cross section was approximately the same. Neutron characteristics of each nuclide in MOX fuel are also derived, and the differences of their characteristics are analysed between MOX and UO2 fuels. The changes of capture to fission ratio of both fissile isotopes in MOX fuel were obtained in thermal and fast neutron energy ranges. Effects of PuO2 content in MOX fuel assembly on its physical characteristics were further discussed.It is necessary to redesign the fuel assembly when using MOX fuel instead of UO2. The influences of the fuel diameter(D) and the pitch to diameter ratio(P/D) on fuel assembly physical properties were calculated. The fission rate, capture rate, Keff, assembly power, relative radial power peak and the energy spectrum have been studied with the variation of fuel diameter and pitch to diameter ratio. Results show that physical properties of the assembly can be improved by resizing the fuel diameter and the pitch to diameter ratio.Furthermore, coolant temperature reactivity coefficient(CTRC) and moderator temperature reactivity coefficient(MTRC) were investigated. It is found that coolant temperature reactivity coefficient reached its minimum near the pseudo-critical temperature, and PuO2 content in the MOX fuel affects the coefficient. MOX fuel has a large negative reactivity temperature coefficient which can be affected by the PuO2 content and the enrichment of 235 U. Its inherent safety can be ensured through adjusting the content of PuO2 and 235 U enrichment.Finally, effects of control rod and soluble boron control were analyzed. Comparisons of single control rod worth with different enrichments of 10 B and different diameters were carried out. Effects of soluble boron concentration on the Keff?MTRC and relative radial peak power were also studied. Together with effects investigation of PuO2 content on the boron worth. It is found that insertion of control rod does not affect uniformity of radial power distribution, and the higher concentration of soluble boron, the more uniform power distribution will be obtained. The control effect can be improved by increasing the enrichment of 10 B, control rod diameter and soluble boron concentration in the new MOX fuel assembly.