Operations & Maintenance

Although no major problems resulted from a 5.8 scale earthquake and hurricane that struck three days apart, a renewed focus has been placed on safety at US nuclear power facilities.

By John R. Johnson, US correspondent

Preparing for unexplained events

More than two million people on both sides of the US-Mexico border were left without power in recent weeks caused by a major yet unexplained outage that extended from Arizona to southern California, including the nation’s eighth largest city, San Diego.
 
Mike Niggli, chief operating officer of San Diego Gas & Electric Co. ruled out terrorism, but said the cause was unclear. To his knowledge, it was the first time that they had lost an entire system.

But what hit closer to home for the nuclear power industry is that the unexplained power cut caused the San Onofre nuclear power plant operated by Edison International's Southern California Edison utility to automatically shut down.

The power plant's two units had enough off-site power to operate the plant's safety systems, but the event calls to the forefront the vital necessity for all plant operators to have clear cut back up plans and the power to back them up.

Natural disasters

Natural disasters and unforeseen power outages are placing safety protocols on the top of list of discussions at the US Nuclear Regulatory Commission. The concerns, once only felt from an armchair perspective in regards to the catastrophic earthquake and tsunami that occurred in Japan in March, are now very real for the Commission as their team of on-site inspectors have been put to the test on home soil.

Two reactors at the North Anna Power Station in Virginia remain shut down following the recent earthquake that centred around the Capital as inspectors from the NRC complete testing at the plant. However, Dominion Virginia Power has informed the NRC that detailed inspections found no significant damage to equipment. The North Anna plant is located 12 miles from the earthquake’s epicenter.

Dominion says that North Anna, like other U.S. nuclear stations, was hardened against earthquakes and other potential hazards in the 1990s. An analysis at that time demonstrated that it could successfully withstand, without significant damage to safety systems, a quake much larger than the one recorded in Central Virginia.

"We are seeing exactly what independent seismic experts have told us to expect – minor damage such as insulation that was shaken off some pipes, electrical bushings that will be replaced and some surface cracking on non-seismic qualified walls," says Eugene Grecheck, vice president-Nuclear Development.

"Still, we will not restart the units until we have demonstrated to ourselves and the NRC that it is safe to do so. Based on results to date, we believe all tests and repairs will be completed on Unit 1 by the latter part of September. Unit 2 is going into a planned refueling outage and its restart will be based on that schedule."

Hurricane Irene “still an open book”

“Hurricane Irene pretty much passed without much event,” says Dave McIntyre, a spokesman for the NRC. “But the earthquake is still an open book. We have our inspection team analyzing the North Anna plant, and it’ll be a few weeks before we have results to report.”

The plant reopening is not likely to happen until mid-October, when the NRC releases the results of its own report on the North Anna facility. Specifically, the NRC is examining whether the earthquake may have subjected the plant to more ground movement than specified in its original designs. Dominion reports while although vibrations very briefly exceeded the station's licensing design basis at certain frequencies, none exceeded the level that the station subsequently demonstrated it could withstand.

Older plants: risk assessment a priority

The investigation into the earthquake’s full impact dovetails with previous work initiated by the NRC. The government body is in the midst of receiving approval to request that US nuclear plants re-examine their sites’ seismic risk and report back to the NRC with new data. The move represents the next step in the NRC’s ongoing examination of updated seismic hazard information for eastern and central US. The initiative began long before the events at the Fukushima Dai-ichi nuclear plant in Japan and the earthquake in Virginia.

“We are asking nuclear licensees to re-examine seismic risks because the study of seismology has changed in the last 20 years since plants were built and our knowledge of seismic risks in country is more complete,” says McIntyre. “We are trying to determine what the current risk basis is for the older plants.”

According to the Washington, D.C.-based Nuclear Energy Institute, US nuclear facilities have a long history of safely responding to natural challenges. According to the NEI, nuclear power plants operate in 31 US states, including two large plants in the earthquake-prone state of California. The US receives enough energy from its nuclear facilities to power one of every five US homes and businesses.

“Nuclear power plants are designed to withstand natural occurrences greater than those encountered in the regions where they are located,” says Scott Peterson, senior vice president at the Nuclear Energy Institute. “They are built to withstand floods, earthquakes and high winds, and have numerous safety systems that will operate and safely shut the reactor down in the event of a loss of off-site power.”

Operational changes

Just the same, NEI has been looking into operational changes since the terrorist attacks occurred in the US in 2001, in the event an airplane were to crash into a nuclear plant or a similar terrorist event. The research can be applied to natural disasters as well. For example, NEI has examined the consequences of an accident making shutoff panels inaccessible, and has suggested that plants have repetitive shut off panels that can be accessed from various locations.

Other places where changes are occurring include collecting better information on station blackouts, venting, and fuel pool monitoring, the requirements for which have been updated since the Japan accident. Now, nuclear plant operators must have fully automated visibility into fuel pool levels to determine when they might reach unsafe levels due to an extended power loss. This requirement was used successfully when the North Anna facility initially lost power.

“It’s one of the lessons learned,” says Steve Kraft, senior director at the NEI. “The newer plants have this technology already, but it’s the older plants that we’re concerned with.”