Sorry, you need to enable JavaScript to visit this website.

Electrical discharge machining - Strengthening nuclear operations post-Fukushima

Seven years on from the earthquake and tsunami that triggered the monumental meltdown and subsequent explosions at Japan’s Fukushima Daiichi nuclear power plant in Ōkuma, nuclear facilities around the globe have been undergoing intensive safety upgrades to protect against severe accidents.

The 2011 disaster, the worst since Chernobyl in 1986, crippled three reactors and released harmful radiation into the atmosphere. Today, while the Japanese government has announced that it is safe to return to at least part of the region, only a few of the 100,000 people evacuated have returned home.

Following global regulatory reviews of the accident, nuclear plant operators in most countries were required to install back-up sources of electric power and cooling water along with additional protection from earthquakes and flooding. In the US, the Nuclear Regulatory Commission ordered more than 100 nuclear plants to carry further, more technically challenging campaigns, to contain radiation and avoid catastrophic damage.

Such maintenance work in these inherently hazardous facilities raises the risk of material contamination that machining can create. As part of post-Fukushima upgrades, machining specialists, Hydratight was the first company to use electrical discharge machining (EDM) on a live nuclear reactor pipeline system for a major US-based energy company.

EDM is one of the most common methods for micro-hole fabrication and is often applied to diesel fuel injector spray holes as well as cooling vents for gas turbine blades. On a nuclear reactor pipeline system, Hydratight’s EDM process uses a heated solid electrode to accurately cut a hole within the reactor feed water line to create another way in which to send coolant into the main line if there is ever loss of power to the reactor. Coolants are also used to maintain manageable pressures within the core. Microscopic cuttings are then removed using back-flushing and vacuums before being filtered and removed, ensuring no risk of contamination exists.

This method prevents 99.5% of foreign material exclusion (FME) particles, such as drill cuttings and debris, from entering the reactor, which can cause contaminations and lead to unplanned outage. The use of EDM on a live reactor averted the need for a costly shutdown and took just 48 hours off the reactor outage schedule.

Typically deployed as an alternative to drilling and hot tapping during planned outages, the EDM line used at the US nuclear facility had a water pressure of 120 PSI and electrical conductance exceeding 3,500 micro-siemens. Hydratight’s specialty services and personnel developed customized tooling to negotiate a 15-inch-long, 1-inch wide pipe as an entry point to reach the area where the ¾-inch EDM penetration was to take place.

As well as preventing FME particles from entering process piping, highly accurate cuttings can maintain a tolerance of .002-inch when making the hole in the pipe. To mitigate tool pressure, the machining method is completely contactless.

Hydratight has been working with the nuclear power industry for more than three decades and boasts an exemplary history of performing these services with our own in-house EDM machines. This is a major milestone, not just for the company, but for the wider nuclear industry in its concerted efforts to carry out maintenance activity to meet enhanced safety regulations as safely, accurately, and with as little disruption as possible.

For the US, which gets about one-fifth of its electricity from nuclear energy and where treacherous hurricanes, storms and flooding are the norm on the east coast, lessons learned from Fukushima will ensure the industry remains vigilant rather than vulnerable.