Abstract
A correlation has been developed for rewetting quality in a fuel assembly under BWR (Boiling Water Reactor) transient conditions. The derivation is based on phenomenological considerations for the elementary processes in rewetting mechanism instead of empiricist approaches having been applied to previous correlations.
Two premises based on experimental observations are made: (1) The rewetting quality in steady state is in good agreement with the critical quality at the onset of boiling transition. (2) That in a transient state can be represented as the sum of a critical quality and a quality deviation corresponding to the time delay due to liquid film propagation.
As for the derivation, based on the above observations, the following hypotheses were made,
(1) Both dryout and rewetting are caused by the propagation/retrogradation of the liquid film front between two axially adjacent spacers.
(2) The maximum retrogradation length can be estimated by using mass and energy balance for liquid film.
(3) To evaluate delay in a transient, the propagation time required can basically be calculated with an empirical velociry correlation for the quench front.
The correlation has been qualified by comparisons with 30 transient post-BT tests for simulated 9×9 fuel assemblies, which leads to the applicability to 9×9 and its analogous assemblies.
