Repositioning of garter springs of neclear fuel assembly by electromagnetic technique

Abstract

The development of an electromagnetic technique for remote repositioning of metallic spacers called Garter Springs (G/S) has assumed great importance for nuclear reactors. The G/S act as spacers between the coolant tube (carrying uranium fuel bundles) and the outside calandria tube. The G/S are placed along the length to maintain co axiality between the tubes by avoiding sagging. The G/S, however, get dislocated during assembly and operation and are to be brought to their design location in order to improve the operating life of the reactor. As the G/S are not directly accessible, remote repositioning methods have to be employed. The electromagnetic method discussed here consists of discharging capacitor energy into a solenoidal coil, placed concentrically with the G/S, thus establishing magnetic coupling. The pulsed magnetic field induces currents in the G/S. The interaction of these currents with the magnetic field produces force on the G/S. The analysis of force and displacement becomes complex due to (i) the diffusion of magnetic field through the intervening coolant tube and (ii) the consideration of voltage and current induction, accounting for transient coupled circuit conditions. The approach followed for optimization of the coil consists of:

the computation of axisymmetric magnetic field and electromagnetic parameters; suitable modifications in the analysis procedure based on measurements; and experiments carried out by simulating diffusion conditions.

The experiments were carried out on a small scale facility. The experimental approach helped in developing good insight into the complex phenomena, and the parameters governing optimization were identified. This kind of approach has not been reported in the literature. Experiments were also carried out with an aluminium tube in place of a zircaloy coolant tube. This revealed many interesting facts regarding damping and diffusion time constant. The observations on diffusion aspects were thus supplemented. In conclusion, it can be said that a combined approach consisting of computation and experiments on diffusion aspects can lead to the optimum design of a displacement coil. However, the design procedure needs refinements, leaving good scope for further investigations.