Diagnosis and cure of Dhruva fuel vibration

Abstract

Dhruva is a high flux research reactor with a nominal thermal power of 100 MW. The fuel for the reactor is in the form of seven-pin cluster of metallic natural uranium clad with aluminium. The optimisation from the physics and thermal hydraulic considerations has resulted in this design of small diameter, long pins arranged hexagonally ensuring a minimum specified clearance between the pins. The clearance is maintained throughout the length by a number of spacers located at regular intervals. This seven pin cluster is assembled inside an aluminium flow tube and the assembly goes into coolant channels made of zircaloy. The fuel assembly is constrained radially (i.e. in the horizontal plane) by the bulges at the two ends of the flow tube. The fuel was endurance tested in an out-of-pile flow test facility for many thousands of hours without any visible damage. However, on loading them in the reactor, many of the fuel pins failed due to fretting wear at the spacer locations. The maximum wear- was on the outer pins near the mid-length of the fuel assembly. The paper gives the details of the measurement and analysis carried out to understand the causes. The solution adopted was to make the supporting bulges flexible - the bottom one by cutting axial slits to obtain a collet type fixture and the top by a sleeve with slits to obtain leaf spring type support. With these design changes, the fuel performs satisfactorily.