Sharp reduction in maximum fuel temperatures during loss of coolant accidents in a PBMR DPP-400 core, by means of optimised placement of neutron poisons

Using thorium to reduce the maximum fuel temperatures during depressurized loss of coolant accidents in a once-through-then-out (OTTO) PBMR DPP-400 core

“…While this method delivered good results, it has a drawback in the sense that the poison distribution cannot be manipulated in detail, for the sake of optimizing the performance: once the fuel spheres containing the poison has been loaded into the top of the core, the poison concentration will decrease with burn-up as the fuel flows down, without the designer being able to control specific poison concentrations at specific positions in the core. As has been discussed above and in Section 5.3.1, Serfontein [6] showed that the ideal axial power profile is not a flat one and that it is therefore advantageous to tailor the vertical poison distribution in detail, for the sake of optimizing the axial power profile. Furthermore, that it is advantageous to place all the poison in the central reflector or in the inner fuel layers and none in the outer regions of the core, so that the peak in the radial power profile can be pushed outwards from the central towards the external reflector.…”

“…Several attempts have been made to reduce the maximum DLOFC temperature. Serfontein [6] suppressed the standard peak in the axial power profile and pushed the radial power outwards towards the external reflector by placing an optimized distribution of 10 B neutron poison in the central reflector of the PBMR-400 DPP. This resulted in a large reduction of the maximum DLOFC temperature from 1577 to 1297°C.…”

“…Simulation parameters for the standard 9g LEU fuel spheres were retained [6]. For the LEU/Th mixtures the addition of large amounts of Th was facilitated by increasing the heavy metal (HM) content from 9 g/sphere (i.e.…”

“…Therefore, the burn-up level of the fuel increases quickly with distance from the top, causing fast continuous depletion of the fissile enrichment and buildup of fission product poisons with increasing burn-up [5]. The continuous increase of the helium coolant temperature and thus of the fuel temperature with distance from the top increases the Doppler effect and thus the radiative capture of neutrons in the capture resonances of 238 U [6]. The combined effect is that the reactivity of the fuel decreases quickly with distance from the top.…”

“…The absorption by these control rods thus strongly increases the neutron leakage in the top part of the external reflector, which suppresses fission in the top part of the outer fuel flow channels. Secondly, the cylindrical effect focuses the neutron flux towards the center of any cylindrical core, which results in higher neutron flux in the inner fuel flow channels [6]. While in reality fuel flow is continuous, the VSOP 99/05 diffusion code model subdivides the fuel core into five discrete fuel flow channels, which are each again subdivided into discrete fuel layers ( Figure 1) in order to simulate the fact that the fuel spheres flow slower in the regions directly adjacent to the reflectors, due to friction against these reflectors.…”