The thermal performance of a partially water filled fuel cluster. Part 1: an exploratory experimental study of level swell at pressures from 2 - 40 bar

An exploratory study of the thermal/hydraulic behaviour of a partially water filled fuel cluster relevant to a small breach PWR accident such as occurred at Three Mile Island has been carried out. A 3.6m long cluster of highly instrumented simulated fuel pin was mounted in the Thermal Hydraulic Emergency Cooling Test Installation (THETIS) which provides the appropriate test environment covering a range of pressure from 2 to 40 atmospheres and powers equivalent to reactor fission product decay heating. The cluster was partially filled with water and the power increased in steps while measurements were made of the level of the boiling two-phase mixture (level swell) and the rod temperatures in the steam cooled zone above. Results obtained are presented together with a simple hydraulic model which includes a voidage correlation expressed in terms of drift flux parameters. The significant effect of pressure on level swell and the influence of grids on the rod temperatures above the swell level is demonstrated. Also contained are recommendations for improvements in modelling and in experimental techniques to be incorporated into a second series of experiments. (U.K.)