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.
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.
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
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
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.
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.
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.