Framatome implements cavitation peening at Millstone

Reactor vessel head nozzle penetrations, bottom mounted nozzle instrumentation penetrations, and reactor vessel primary nozzles - comprised of Alloy 600 welded with Alloy 82/182 - are highly susceptible to primary water stress corrosion cracking (PWSCC).

Framatome's UHP cavitation peening process uses submerged, ultra-high-pressure water jets to work the surface of reactor vessel components. The high-pressure water flow creates vapour bubbles. As these vapour bubbles collapse on the component's surface, shock waves travel into the material and create compressive residual stresses. Rather than allowing operational stresses to create multiple, random fractures on a component's surface, cavitation peening creates compressive stresses on the surface of the material in a controlled manner, preventing PWSCC initiation.

According to Framatome, UHP cavitation peening can extend the life of nuclear reactor primary components - including the hot leg primary nozzles - for up to an additional 40 years. It also says the process reduces outage time and saves money by eliminating the need to replace components or address indications with traditional repair methods.

To prepare for the work at Millstone, Framatome demonstrated the qualified reactor vessel primary nozzle cavitation peening technology on a full-scale mock-up at the company's technical training centre in Lynchburg, Virginia, in early 2019. The actual work was conducted during Millstone's spring 2019 maintenance outage.

Millstone is the only operating nuclear power plant in the state of Connecticut. Its two pressurised water reactors have a combined capacity of 2088 MWe, and generate about 45% of the state's electricity.

In 2014, Exelon selected Framatome's UHP cavitation peening technique for the surface mitigation at its two-unit Byron and Braidwood plants to extend the life of the reactor vessel closure heads by alleviating the effects of primary water stress corrosion cracking. Collectively, the application of this technique saved Exelon more than USD370 million, avoided more than 580 rem of dose radiation to workers, and reduced the plants' outage duration by more than 150 days.

"Cavitation peening is an industry game-changer that was recognised in 2017 as one of the Top Innovative Practices for work completed on the Byron and Braidwood reactor vessel closure heads," said Craig Ranson, senior vice president of Framatome's North America Installed Base Business Unit. "We are proud to work with Dominion to expand our proven capabilities and engineer a solution for this unique primary nozzle repair."