Demonstration of Helium Measurement Capability to Support Repair of Irradiated Components

As many nuclear power plants enter license renewal operating periods, there is increased probability that repairs will be needed on components that have been exposed to significant neutron fluence. The neutron-driven transmutation of nickel and tramp boron in austenitic materials commonly used in reactor internals can lead to the generation of trapped helium and the associated risk of helium-induced cracking (HeIC) during weld repairs. In the weld heat affected zone, where temperatures are insufficient to allow the helium to diffuse out of the material, the helium can remain trapped. Upon cooling, the residual stresses, combined with weakened grain boundaries due to helium coalescence, can lead to cracking.

The current ASME limit for helium content for Code repairs is 0.1 appm [1]. Prior work [2,3] has demonstrated a strong inverse correlation between helium content and permissible weld heat input for avoidance of HeIC. The helium concentration in the material to be repaired is thus a critical input to the development of weld repair processes to be applied to these materials. The reliable measurement of helium in irradiated materials at concentrations relevant for the evaluation of HeIC risk is a specialized process. It is important to demonstrate that the capability is available and can be practically leveraged to support emergent repairs.

This paper presents on the execution and results of a multi-laboratory test program aimed at demonstrating the industry capability of acquiring accurate, repeatable, and timely measurements of relatively low concentrations of helium (≤ ~20 appm) within austenitic materials commonly used in reactor internals. Participating laboratories were supplied with equivalent specimens extracted from boron-doped coupons that were irradiated to drive the boron-to-helium transmutation reaction. The results and lessons learned from the program are expected to support the development of industry guidance for the acquisition of similar measurements supporting nuclear component repairs.