An experimental study of level swell in a partially water filled fuel cluster

As part of the UKAEA programme of research into the safety of pressurized water reactors (PWRs) a series of large scale experiments is being carried out at the Atomic Energy Establishment at Winfrith. The first of these was an exploratory study of the thermal/hydraulic behaviour of a partially water filled fuel cluster relevant to a small breach accident such as occurred at Three Mile Island. A 3.6m long cluster of highly instrumented fuel pin simulators was mounted in the thermal hydraulic emergency cooling test installation (THETIS) which provided the appropriate test environment covering a range of pressure from 2 to 40 bar 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 temperature in the steam cooled zone above. Some of the results are described, attention is drawn to the significant effect of pressure on level swell and the influence of grids on the rod temperatures in the uncovered region. (author)