THIBO experiments — Thermohydraulically induced fuel pin oscillations in Na-cooled reactors

Abstract Unusual wear marks and damage to pins and spacer grids have been found in KNK II fuel element. For instance, there was a pronounced interaction not only between the spacers and the cladding, but also among the fuel pins outside the spacer grids. There is a theory postulating that the causes are not hydraulically induced vibrations, but powerful low-frequency pin oscillations created by special thermohydraulic and geometric conditions. This implies that the pin power, the sodium mass flow and velocity, the pin clearance within the spacer grid system, and the pin structure play the decisive roles. This phenomenon is so interesting that we are performing out-of-pile experiments in order to contribute to the verification of the theory and to learn something about the mechanism and the limiting conditions. The test section simulates a single fuel pin in a KNK II subassembly. There is an electric heater with dimensions of Mark II type pins. All spacers can be adjusted very precisely by micrometer screws. A large number of thermocouples indicate the sodium temperatures around the heater. An X-ray system allows part of the heater to be made visible. In 1988, we succeeded in making the fuel pin simulator oscillate with different parameters. The accompanying azimuthal temperature oscillations grew to more than 100 K and a period of more than 10 s. Maximum pin bending, as determined by X-raying, was nearly one millimeter. The reproducibility of the oscillations was remarkably good.