(Host) The safety of nuclear power hinges on the concept of “redundancy.” How much redundancy is one of the questions raised by the proposal at the Vermont Yankee plant in Vernon to boost power by 20 percent. Critics say that at least two layers of safety systems are needed to control radiation releases. But they charge that the increase in power could trigger a situation in which a key safety system could fail.
VPR’s John Dillon reports in the second of two parts on Vermont’s only nuclear power plant.
(Dillon) Everyone’s worst nightmare at a nuclear plant is what’s known as a “loss of coolant” accident. In an emergency, a nuclear reactor must be quickly cooled by water. If the pumps that provide the water fail, or if the water rapidly boils off into steam, a meltdown could result.
(Sherman) “With power uprate, power uprate results in hotter temperatures of the water because there’s more fuel. There’s 20 percent more fuel in the reactor.”
(Dillon) William Sherman is the state nuclear engineer. He’s concerned that federal regulators have weakened an essential safety standard in the case of Vermont Yankee. The problem has to do with boiling water and the pumps that are needed to cool the reactor in an emergency.
In a loss of coolant accident, these massive pumps are supposed to quickly flood the reactor. The danger arises if boiling hot water from the reactor causes bubbles of steam to form, which could make the pumps fail.
As Sherman, the state nuclear engineer, points out, after the 20 percent power upgrade the emergency water inside Yankee will be heated up to 194 degrees. That makes the pumps less effective.
(Sherman) “So these hotter temperatures mean that the water, the hotter water, is less dense. It means that the pressure from the column of water in the suction line is less.”
(Dillon) Now think of a pressure cooker. Water under pressure boils at a higher temperature. So if there’s enough pressure inside the reactor containment area, the pumps should still work, even at the higher temperatures after the power upgrade. Yankee is counting on this pressure cooker effect to keep the safety system operating in an emergency. Here’s Sherman explaining how it’s supposed to work:
(Sherman) “What happens in accidents is that, it is postulated that steam leaks into the containment and the pressure in containment increases. So I believe they have stated that they want to take credit for that increased pressure as a safety function.”
(Dillon) But the Nuclear Regulatory Commission has in the past been very reluctant to allow nuclear operators to rely on pressure inside the containment – “take credit” it’s called – as a way to guarantee that the emergency pumps will work. Sherman told a recent meeting of the state nuclear advisory panel that the NRC has changed long standing safety policy.
(Sherman) “That’s a new thing and we’re pretty skeptical about that. We’ve asked the NRC a number of very specific questions about a change in policy that they appeared to have made only in the last year or so. We’ve asked them for the basis for their changing the policy. And we’ve asked for the safety implications of allowing credit for containment accident pressure.”
(Dillon) The issue is important because there are accident scenarios in which containment pressure could be lost due to equipment failure, operator error, even a terrorist attack. And if there’s no pressure, the emergency cooling water may not reach the reactor core in time.
(Paul Blanch) “If that pressure is lost through any one of many single failures that could occur -a valve fails to go shut, a valve opens by mistake – then what happens is you lose the containment integrity. Meaning that at the elevated pressure, we’ll have radioactive material leaking out.”
(Dillon) Paul Blanch is an engineer with 35 years of experience in the nuclear industry. He says he might be able to support the increase in power, if there was more review to show it’s safe. In the Vermont Yankee case, Blanch was a witness for the New England Coalition, the group opposed to the power upgrade.
According to Blanch, the NRC has in the past always required levels of redundancy in safety systems. The concept is called “defense and depth.” The idea is that no single failure of a key system should allow radiation to release or an accident to escalate. Blanch says that in the Yankee case, the NRC has violated the basics.
(Blanch) “It’s an extremely significant thing and it takes away all the defense and depth and it takes away the basic premise that nuclear power is built on. And that is defense and depth, and not tolerating a single failure.”
(Dillon) Yankee officials have argued there’s a wide margin of safety built in. In an accident, they believe the containment would stay pressurized for hours, thus assuring sufficient water and pressure to cool the reactor.
Plant spokesman Rob Williams says Yankee is not the only plant for which NRC has approved the pressure issue.
(Williams) “Twenty-five other reactors are allowed to take credit for that existence of pressure in the containment. So we have that issue now before the Nuclear Regulatory Commission, and they’re reviewing it.”
(Dillon) But if that’s the case, critics say, there’s reason to question the effectiveness of the NRC oversight. David Lochbaum, a nuclear engineer with the Union of Concerned Scientists in Washington, says Yankee is wrong about the other reactors in which the NRC has reviewed the containment issue.
(Lochbaum) “What’s unique about Vermont Yankee is that the amount of pressure is much higher than in the previous cases and the time period that the pressure has to be there is much longer than in the prior cases. So those two factors make it less likely that there’ll be success at the end. It’s a pretty big departure from precedent.”
(Dillon) According to Lochbaum, there’s a simple solution to the problem.
(Lochbaum) “The least expensive thing they could do is not go for 20 percent upgrade. Right now, the plant doesn’t take credit for containment over-pressure. There’s nothing that requires them to increase power pressure by 20 percent, other than greed. There ought to be some sanity involved, so that we don’t sacrifice safety just so the company can make a few more dollars.”
(Dillon) Yankee insists that the accident scenario that the critics foresee is extremely unlikely. Meanwhile, the NRC has not yet responded to the Vermont state nuclear engineer Bill Sherman, who formally asked in December about the safety implications of the containment pressure issue.
For Vermont Public Radio, I’m John Dillon.