Comparison of the behaviour of two core designs for ASTRID in case of severe accidents

Autor: J.M. Seiler, G. Prulhière, Nathalie Marie, Frédéric Bertrand, J. Lecerf
Rok vydání: 2016
Předmět:
Zdroj: Nuclear Engineering and Design. 297:327-342
ISSN: 0029-5493
DOI: 10.1016/j.nucengdes.2015.04.020
Popis: The present paper is dedicated to the studies carried out during the first stage of the pre-conceptual design of the French demonstrator of fourth generation SFR reactors (ASTRID) in order to compare the behaviour of two envisaged core concepts under severe accident transients. Among the two studied core concepts, whose powers are 1500 MWth, the first one is a classical homogeneous core (called SFRv2) with large pin diameter whose the sodium overall voiding reactivity effect is 5 $. The second concept is an axially heterogeneous core (called CFV) whose global void reactivity effect is negative (−1.2 $ at the end of cycle at the equilibrium). The comparison of the cores relies on two typical accident families: a reactivity insertion (unprotected transient overpower, UTOP) and an overall loss of core cooling (unprotected loss of flow, ULOF). In the first part of the comparison, the primary phase of an UTOP is studied in order to assess typical features of the transient behaviour: power and reactivity evolutions, material heating and melting/vaporization and mechanical energy release due to fuel vapor expansion. The second part of the comparison deals with the calculation of the reactivity potential for degraded states (molten pools) representative of the secondary phase of a mild UTOP and of a strong UTOP (strong or mild qualifies the reactivity ramp inserted). According to the reactivity potential, the amount of fuel to extract from the core and the amount of absorber material to inject in the pool of molten fuel in order to compensate this potential are calculated. These amounts give an order of magnitude of the strength of the mitigation actions required for each type of core in order to limit the mechanical energy release due to secondary power excursions. The calculation results have shown that in case of UTOPs (ramp ranging from several $/s to 60 $/s), the mechanical energy released by the expansion of a fuel vapor bubble is about one order of magnitude lower for the CFV core than for the SFRv2 core. Moreover, in case of core melting due to an UTOP, both cores require approximately the same amount of absorber material to be inserted in order to reach a sub-critical state. For the same degraded state a zero reactivity could be reached with a lower amount of fuel extracted from the CFV core than from the SFRv2 core. Regarding ULOFs, the assessment of the reactivity that could be induced by molten cladding relocation has shown a largely better behaviour for the CFV core. More precisely, for this latter core design, a severe power excursion induced by cladding relocation is very unlikely. Finally, the times of boiling onset, of flow redistribution in the core and of material melting, indicate that mitigation actions (absorber insertion and fuel extraction) should start earlier for the SFRv2 core than for the CFV one. This result is a good point for the CFV core because it allows later mitigation actions which are more likely to succeed because time is required to initiate the fuel relocation through in-core dedicated ducts. The results presented in this paper have contributed to retain the CFV core for the studies currently carried-out for the ASTRID design.
Databáze: OpenAIRE
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