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