Simulation Study Of LSPBR Using Monte Carlo Method

Liquid fluoride salt cooled pebble bed reactor (LSPBR) is one of fluoride salt cooled high temperature reactor (FHRs), belongs to the Gen-IV reactors category. It has a good economy, safety, sustainability and prevention of nuclear proliferation. The use of liquid fluoride salt as coolant and the use of coated particle fuel are two key features of LSPBR.In this paper, reliabilities of three Monte Carlo simulation programs (SCALE5.1, MCNPX and MOCBurnup) were firstly tested using a single cell model benchmark, a single component model benchmark and two whole core model Benchmarks. The results show that, relative deviation among three Monte Carlo simulation program SCALE5.1, MCNPX and MOCBurnup is not to exceed0.5%under the static verification. The average relative deviation is less than1%under the dynamic validation. In the preliminary design of the reactor, this deviation can be accepted.Then, SCALE5.1code was used to simulating the neutron transport in LSPBR under the different fuel mixture. Some important parameters like keff, Effective Full Power Days (EFPD), fuel burnup and neutron spectrum were investigated and compared. The following results are presented.1) When using the same fissile material, due to the thermal neutron absorption of232Th is smaller than238U, it will be a higher initial keff when starting with232Th, and also the initial spectrum will be a little soft.2) Using233U or235U for starting, the initial spectrum is very hot, due to233Pa has a larger thermal neutron absorption cross section reaction, therefore, the keff of using a combination of232Th falls quickly in the late, while using a combination of238U has a relatively stable Keff decrease rate, and ultimately, the actual amount of fissile material conversion of232Th is better than238U which will consume more fissile nuclear fuel.3) The use of239Pu for starting,239Pu thermal neutron absorption resulted in a harder neutron spectrum. In this case,232Th showed good conversion performance, the reactor can be maintained longer, but because of epithermal neutron be substantial absorbed by238U, reactors using238U cannot be self-sustaining. From saving fissile nuclear fuel and prolong the lifetime of the reactor core comprehensive survey, the conclusion is:without online refueling and reprocessing,232Th is underperformed in thermal reactor compared with U, however, it is the opposite in epithermal neutron reactor.Finally, a sub-region simulation on the LSPBR under233U-232Th fuel combination was carried out to optimize the fuel layout structure and the neutron utilization. The following results are presented.1) The neutron flux distribution decreases gradually in the radial direction, so the burnup is also substantially decreases radially, as the reactor operation, the power of five zones will tend to flatten.2) After radial zoning fuel arrangement, although in a relatively short time power tends to flatten, but to improve the core cycle length had no significant effect, the security implications of different areas of power flattening greater than the physical meaning.3) The fuel partition layout helps to flatten the power, but fuel enrichment span districts should not be too large, the degree of enrichment span of about1%(less than2%) is appropriate.4) From the neutron balance analysis, it’s concluded that this type of reactor is not yet suitable for thorium fuel proliferation, and require further careful optimization.