Circumferential drypatch spreading on a simulated CANDU fuel string with non-uniform axial heat flux

Abstract

In the past decade, a series of experiments was carried out at the Chalk River Nuclear Laboratories of Atomic Energy of Canada to obtain detailed thermalhydraulic information on CANDU fuel channels. The experiments were performed with simulated 37-element fuel strings of CANDU geometry.

CANDU bundles consist of 37 elements arranged in three rings around the central pin. Each element is 50 cm long and is kept in place between two endplates. Spacers at midplane prevent individual elements from bowing into contact with another element or the pressure tube. During the experimental program several strings simulating 12 CANDU bundles placed end to end were subjected to a detailed experimental investigation using water as the test-fluid. All strings were electrically-heated and simulated the radial heat flux depression observed in natural UO₂ fuel.

The last string to be tested was built with elements of continuously varying wall thickness. The elements were assembled such that the axial flux distribution had the shape of a cosine, skewed towards the outlet end. Every element in the string was supplied with sophisticated measurement devices. One-half of the elements were equipped with Resistance Temperature Devices for CHF-detection. The other half was instrumented with thermocouples. These thermocouples could he positioned at any axial and radial position inside the element by a remotely controlled thermocouple drive system.

The emphasis of this paper is on the effectiveness of the coolant to keep the fuel from overheating at heat fluxes beyond critical.

Detailed temperature maps were constructed for all elements that indicated high surface temperatures and drypatch spreading. Figures are shown with temperature maps for different conditions of flow, inlet subcooling and heat flux. Drypatch spreading with increasing heat flux is evident from these figures.

Even at very high fluxes, the temperature of a significant portion of the surface remained near saturation. The temperature of other areas indicated that the efficient nucleate boiling heat transfer had somewhat deteriorated. Only a fraction of the surface reached the temperature levels expected in a film boiling mode of heat transfer.