Generation IV reactors are expected to exhibit significant safety improvements compared to current ones. In sodium-cooled fast reactors (SFRs), fuel melting during transient over power (TOP) should be avoided as this is identified as a relatively frequent accident. Among these TOP accidents, a control rod withdrawal (CRW) accident is the most likely to happen and its impact depends on the magnitude of the inserted reactivity. This paper presents the required excess reactivity for different core designs and the way to reduce the reactivity inserted during a CRW transient through the use of burnable poisons (BPs).

After evaluating various candidate materials, it appears that a low-enrichment boron carbide combined with a zirconium hydride moderator is the most promising BP for use in sodium fast spectrum reactors. Burnable poisons are located in pins of particular assemblies, which are in fixed positions in the core over the entire fuel cycle.

Four core designs with different loading schemes and BPs are investigated. Core designs with BPs display low reactivity loss over the fuel cycle and thus limit the required initial excess reactivity of the core to compensate with control rods.

Another constraint comes from the core power distribution, which should remain almost stable through the fuel cycle. This core power distribution can be modified by a suitable loading of BP assemblies. However, as their positions are fixed over the fuel cycle, they can compensate only part of the local flux tilt. These BP core designs slightly improve the reactivity feedback coefficients as they contain light materials slowing down neutrons. It is finally shown that a CRW transient with BPs reduces significantly the maximal fuel centerline temperature compared to a design without BPs and that a fuel melting during a CRW transient is avoided in the large SFR core.