Improving the Safety of Minor Actinide Burning in Fast Reactors

In most of the European Union's 27 member states, nuclear power is considered to be a risk rather than an advantage. The positive aspects of nuclear power, such as greater energy security and lower greenhouse gas emissions, are cancelled out by fears of terrorist attacks against nuclear facilities, the potential misuse of nuclear material and concerns about the safety of nuclear waste disposal and nuclear installations. To achieve effective synergy between breeding and minor actinide (MA) waste burning in a fuel cycle, fast reactors should be designed to recycle MAs from several (2-3) LWRs/CANDUs. For homogeneous recycling this in turn means that the MA fraction in the fuel should be about 5%, which, in comparison to MA-free cores, nearly halves the negative Doppler reactivity feedback, however, and increases the positive coolant temperature reactivity coefficient. In the present study, we compare the effectiveness of different moderating materials to improve Doppler fuel temperature reactivity feedback and the coolant temperature reactivity coefficient in lead-cooled and sodium-cooled fast reactors (LFRs and SFRs) homogeneously recycling MAs. The materials investigated were hydrides (CaH_2), metallic beryllium, and enriched boron carbide (11^B_4C). The calculations showed that, in homogeneous mode, both LFR and SFR systems could burn about 80 kg of MAs per year, with the burn-up reactivity swing remaining below 2.5$/yr. Hydride moderating pins appeared to be the most effective in improving reactivity coefficients and a Doppler coefficient twice as large as coolant temperature reactivity feedback was obtained in both an LFR and SFR when ~5% of the fuel pins in each sub-assembly were replaced with CaH_2 moderating pins. The hydrides also proved to have the least influence on core breeding performance, but susceptibility to decomposition at relatively low temperatures (~1100 K) might pose safety problems. The moderating power of beryllium and 11^B_4C is significantly smaller than that of hydrides, which means that the number of moderating pins in a sub-assembly has to be higher. This, however, has a negative influence on breeding and further exacerbates the burn-up reactivity swing. The LFR self-breeder shows a number of advantages over an SFR counterpart regarding its behaviour in unprotected loss-of-flow and unprotected loss-of-heat sink accidents due to superior natural coolant circulation and the larger heat capacity of lead.

TUCEK Kamil;  WIDER Hartmut; 

2008-07-30

American Nuclear Society

JRC43305