Among the fundamental objectives assigned with the reactors of 4th generation, the improvement of the safety of the Fast reactors with sodium coolant is of major concern. The characteristic of LMFR's cores is such, that under normal operation, they are not in their most reactive configuration and so may be subject to prompt reactivity excursion caused either by sodium voiding, especially for large cores, or by re-compaction after a first core melting. The reduction of the sodium voiding reactivity effect is thus the top priority assigned to the innovating design of 4th generation LMFR’s cores. In complement to this objective, an useful indicator of a satisfactory core behaviour is identified as the ratio of the sodium void effect to the Doppler effect, calculated between the fuel nominal temperature and its melting point. A value of 1 is interesting as it tends to prevent, by design, any risk of generalized core fusion . Indeed the negative Doppler feedback tends to counterbalance the positive reactivity contribution caused by the sodium voiding. Obtaining a Doppler effect identical to that brought by the sodium voiding effect would thus make it possible not to exceed, at the time of such an accident, the melting point of fuel. In addition, achieving an economy of resources requires a positive breeding gain (BG). Furthermore, non-proliferation objectives result in avoiding the use of blankets which lead to design breeder cores without blankets. This is a real challenge as, for example, the fuel regions of the EFR(European Fast Reactor) core have a slightly negative breeding gain (BG = -0,20).
The study carried out there introduces innovating core designs able to reach objectives defined just before, with 3 successive phases: - parametric and sensitivity calculations of the main parameters of design of the core with respect to the defined objectives, - selection of the most promising options, - overall combination and performance evaluation At the end of the first phase, the most attractive options appear to be : - with respect to the sodium voiding effect, the reduction of its spectral component by decreasing the fraction of sodium and by introducing a moderating material into the core and the increase of its “leakage” component with geometrical forms as “pancake” type of the core (height over diameter core ratio close to 0,25), associated to the introduction of a “sodium plenum” above the core, - for the Breeding Gain as for the Doppler effect, maximizing the fuel volume fraction is the best option. In this respect, dense fuels such as carbide, nitride or metal present an attractive option.
The calculations carried out in the third phase of the study confirm the interest of the selected options. For a large size core such as European Fast Reactor(3600 MWth), one shows that it is possible with an oxide fuel to reduce by more than a factor 2 the effect of sodium voiding while reaching a positive BG. The use of a moderating material makes it possible to be close to 1 for the Voiding / Doppler ratio. The use of Carbide fuel makes it possible to improve further the performances while reducing the size of the core and the Pu inventory.
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