Inspecting for Cracks in Tight Places

• Detect and characterize cracking in an inaccessible and limited space.

CRDM penetration nozzles that penetrate into the reactor vessel head are subject to various stresses, which can trigger axially oriented Inner-Diameter (ID) cracking. These flaws are typically surface breaking, 1.0–1.5 mm (0.039–0.059 in) deep, 5.0–5.2 mm (0.197–0.205 in) long, although some stress-corrosion and subsurface cracking may also occur in all orientations.

The only way this area can be inspected is from within the reactor vessel so, for obvious reasons, the inspection method must be robotized.

The penetration nozzle’s geometry poses a unique challenge that the current inspection method could not tackle. At the mouth of the nozzle, there is an interstice approximately 12 mm (0.472 in) wide between the nozzle’s ID and the thermal sleeve. However, the nozzle tapers around the sleeve, reducing this gap to a minimum of 2 mm (0.079 in). The current probe, while able to inspect the wider diameter, cannot fit into a 2 mm gap. The potential solution would have to adapt to this geometry variation. Reactor engineers must nevertheless inspect the penetration point’s ID for cracking.

The reactor vessel’s head and thermal sleeve are both made of austenitic alloys, which are corrosion and oxidation resistant, but the magnetic properties of these components pose yet another unique challenge for electromagnetic inspection. Because while cracks at the ID must be detected, the unwanted interference from the thermal sleeve must also be eliminated to leave only indications from at the ID.

• A unique, ultra-thin shielded ECT probe capable of adapting to geometry changes.

The solution to this challenge is both simple and elegant. It finds its inspiration in nature: the flying squirrel.

As illustrated below, the probe’s head can collapse and expand around the thermal sleeve (like a flying squirrel’s wings) in such a way that the probe can fit in a space of 2 mm (0.079 in).

To achieve this while maintaining enough pressure on the probe’s head to keep contact with the nozzle’s ID constant, the probe’s head is made of finely machined technical thermoplastic. This material is extremely durable and yet flexible, which enable it to reshape itself and avoid using moving parts.

Inside the probe’s head, the ECT coils are shielded from the thermal sleeve’s interference by a highly conductive alloy that effectively produces clear signals from the nozzle’s ID.