Safety Improvement of Advanced Nuclear Systems: the Role of Core Sub-criticality

jeudi 2 déc. 2004, 00:00 → 00:00 Europe/Paris

Salle 9 (LPSC)

Pavel Bokov (CEA Saclay) Core sub-criticality can play an important role if safety enhancement of a nuclear system is necessary, in particular, when minor actinides submitted for transmutation cause an essential degradation of the reactivity feedback effects or/and a significant reduction of the delayed neutron fraction. Our studies show that a core sub-criticality together with a thermo-hydraulics optimization is able to compensate the possible degradation of the Doppler-effect and the reduction of delayed neutron fraction. The particular dependence of the spallation neutron yield allows creating a supplementary negative feedback effect in the case of accelerator coupled hybrid systems. A number of quantitative examples are provided in this context. Namely, an inter-comparative analysis of the deterministic safety potential of innovative molten salt systems with critical and sub-critical cores is presented for both fast and thermal neutron spectra. The analysis included the study of multiple unprotected transients which have been simulated on the basis of one-point kinetics and simple thermo-hydraulic schemes considering realistic feedback effects. Different levels of sub-criticality were tried to improve safety of the nuclear system and, at the same time, to define the intensity of the external neutron source. It is shown that a considerable expansion of the time interval of core vitality can be achieved even if a very low sub-criticality level (350-700 pcm) is applied. Consequently, the requirements for the external neutron source intensity are considerably decreased when compared to deeply sub-critical systems. Moreover, as it was demonstrated, core sub-criticality offers some new opportunities with respect to safety enhancement.