COLD SPRAY ALTERNATIVE SOLUTIONS FOR NUCLEAR COMPONENT LIFE EXTENSION

Abstract

This paper presents an overview of a feasibility study completed to assess the viability of Metallic Cold Spray (CS) for use in the mitigation / repair of cracking in the Calandria Relief Ducts (CRDs) at the Bruce Nuclear Generating Station B (BNGS) Unit 7. The Unit 7 CRDs are constructed from 304L stainless steel. During plant operation, the CRDs are exposed to high levels of gamma and neutron radiation, humidity, and low levels of chlorides. It is believed that metal used in fabricating the Unit 7 CRDs was contaminated with iron during original construction. As a result, the CRDs have suffered degradation from chloride induced transgranular stress corrosion cracking (TGSCC). Among other functions, the CRDs act as an essential barrier between the heavy water moderator in the CANDU calandria and light water in the shield tank surrounding the calandria. Accordingly, demonstrating the ability to prevent and arrest postulated leakage through the pre-existing flaws in the CRDs was a key objective of the cold spray feasibility study.

Cold spray uses high-pressure gas to accelerate metallic powders through a convergent-divergent nozzle above the critical velocity for the particles to mechanically adhere to the target substrate. The feasibility study considered numerous coupon configurations simulating representative spray scenarios in the degraded CRDs and evaluated variations in several essential parameters and cold spray materials (e.g., Titanium, and Alloy 625) with the goal of identifying appropriate spray conditions for reliably and efficiently sealing actively leaking flaws with suitable spray materials while supporting post-spray NDE inspectability of the underlying substrate.

The completion of this work presented opportunities for future work applications in leak prevention and corrosion protection to manage aging conventional and nuclear systems.