Experiments Conducted in Support of Transport Flask Heat Transfer Assessments

Abstract

Water filled and gas filled flasks are used to transport irradiated LWR fuel. A programme of experimental work has been conducted to provide data for thermal modelling. Operational, regulatory and extra regulatory conditions have been covered.

In some wet flask designs the fuel is carried with an inner container separated from the flask body by a narrow water filled annulus. Experiments have been conducted to correlate the heat transfer across the annulus. In dry flask designs however a significant temperature drop occurs between the fuel array and its surroundings and a series of tests on a simulated fuel element in a gas environment has been carried out. Fins are generally fitted to the outside of flasks carrying high heat output fuel in order to improve the convectional heat transfer to ambient. Design data have been derived from tests on large diameter finned sections with various combinations of pitch and fin height. Other work has included an experimental survey of flask emissivity data and thermal trials on a full size flask.

During a fire, heat is transferred to the flask surface by radiation and convection from the flames. To provide for heat input data, tests have been carried out to determine the heat flux to a full size finned section of a flask surface when subjected to a hydrocarbon pool fire. Although not a regulatory requirement, BNFL have also investigated the thermal response of a wet flask in the extremely unlikely event of loss of water. Tests have been conducted on a wet flask operated dry, containing simulated fuel elements. Transient tests were carried out over a range of fuel powers, filling gases, internal pressures and partial refill with water.