What’s So Hard(ened) About Vents?

The idea of “containment venting” has been front and center in discussions about the Fukushima Dai-ichi nuclear accident and what the NRC wants plants to do to improve their vents. But to most people outside the nuclear industry vents are the things in our houses that hot or cold air flows through. So here’s a little background.

The accident in Japan involved what’s called a Mark I boiling-water reactor. Mark I designs have a relatively small structure, or “containment,” to hold in steam and radioactive material if an accident occurs. If pressure inside the containment gets too high during an accident, the reactor’s safety systems will have trouble pumping water into the core to keep it cool – which will make the accident much worse and possibly lead to high levels of radiation escaping into the environment. Part of this accident scenario also involves hydrogen gas building up inside containment. As we saw at Fukushima, if hydrogen is not allowed to escape, it can explode and damage the reactor building, which also could lead to radiation leaking into the environment.

This is where vents come in. They can be used to reduce pressure in containment so that water can still be pumped through to cool the fuel rods. The vents can also safely release built-up hydrogen to prevent explosions.

Decades ago, U.S. Mark I plants installed vents, valves and piping, but the circumstances in the Fukushima accident suggest the vent designs should be improved. The NRC is also considering whether the vents should have filters to capture any radioactive material in the vented gas

On March 12, the NRC issued an Order to all U.S. Mark I plants, as well as similar Mark II reactors. The Order requires Mark I plants to ensure their vents are hardened and reliable, and it requires Mark II plants to install hardened, reliable vents.

“Hardened” means these vents must withstand the pressure and temperature of the steam generated early in an accident. The vents must also withstand possible fires and small explosions if they are used to release hydrogen later in an accident. The vents must be reliable enough to be operated even if the reactor loses all electrical power or if other hazardous conditions exist. The NRC staff will issue, later this summer, specific guidance on the requirements for containment vents.

In order to ensure these vent improvements are properly designed and installed, the NRC has set a deadline of Dec. 31, 2016, for the Mark I and II plants to comply with the Order.

Scott Burnell
Public Affairs Officer

Problems in the Aggregate – Literally

Economists and others like to talk about “problems in the aggregate,” or a big-picture view of the issues. But that phrase is taking on new meaning in the case of the Seabrook nuclear power plant, where there are concerns involving the aggregate used in the concrete.

More specifically, the problem at the New Hampshire facility is the intrusion of moisture into the foundation walls of certain structures, resulting in the degradation of some of the concrete.

The exact term for what is occurring at Seabrook is alkali silica reaction, or ASR, which involves the hydroxide ions in the pore solution in cement paste and the reactive silica in aggregate. (Aggregates are inert granular materials, such as sand, gravel or crushed stone that, along with water and cement paste, are an essential ingredient in concrete.) The main byproduct of ASR is a gel, which can expand and may cause micro-cracks in the concrete.

While the extent of the problem is still being evaluated, structures identified as being affected by ASR are considered “operable but degraded.”

What exactly does that mean? In layman’s terms, it means the NRC – while far from done with reviews of the issue – has determined the structures can continue to safely perform their function based on the following information:

1. Conservative safety load factors, or the extra safety margin that was included when the structures were designed and built;

2. Visual observations by qualified NRC inspectors;

3. The fact that the ASR is limited to localized areas; and

4. Because progression of the concrete degradation is occurring slowly.

That determination, contained in an NRC inspection report issued on March 26, was the result of reviews carried out by six of our inspectors over many months, dating back to last September. Among other things, we made use of concrete/structural integrity expertise at our headquarters office. We also had an inspector in our Region III office, in suburban Chicago, observe lab tests performed in Northfield, Ill., on concrete core samples taken from Seabrook.

An important next step for the NRC’s review of the Seabrook concrete degradation will be a public meeting scheduled for Monday, April 23, at our headquarters office in Rockville, Md. During that meeting, the NRC staff will discuss with NextEra, the plant’s owner and operator, its analysis of the issue, planned corrective actions and dates to fully correct the problem, as well as other details.

Based on the outcome of that session, the NRC will determine its next steps regarding the issue. One thing we have already made clear is that no decision will be made by the agency on a license renewal application for the plant until the extent of the concrete degradation is fully understood.

Members of the public who would like to listen in on the meeting but cannot travel to NRC headquarters will be able to do so by phone bridge. What’s more, the slides to be used during the session will be available via an online webinar.

More details regarding the April 23rd meeting are available at: http://www.nrc.gov/public-involve/public-meetings/index.cfm .

Neil Sheehan
Region I Public Affairs Officer
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