Abstract
This paper investigates numerically a new phenomenon of power oscillations, which is found to take place after unprotected loss of flow (ULOF) in a large power European Sodium Fast Reactor (ESFR) with a sodium plenum above the core.The oscillations are induced by sodium boiling and re-flooding events and can trigger a power excursion, if the oscillation amplitude becomes stronger and the reactivity exceeds the prompt criticality during transients. Numerical results show that after the sodium boiling onset in and in the vicinity of the sodium plenum, a negative reactivity effect is introduced, that reduces the power, and the sodium boiling is stopped. When liquid sodium re-floods into the voided region, introducing a positive reactivity effect, the power increases, and then sodium boils again. This process repeats, i.e. power oscillations take place. A long numerical simulation shows that the oscillations may disappear as the cover gas pressure increases up to a certain value during the transient. This gives us an idea that an increased cover gas pressure could prevent the oscillation for the considered case, simply because the sodium boiling is suppressed. An additional transient simulation with a higher initial cover gas pressure confirms this idea.
Published Version
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