Reactor Status Update And Fukushima Risk Q&A

Tyler Durden's picture

The following summarises what is happening at each unit, and the major risks:

WHAT STOPS A RADIATION LEAK?

Each reactor is surrounded by the primary containment vessel. This is
made of strengthened steel four to eight inches thick. It provides the
most critical line of defence against leaking radiation from the
reactor.

Should there be a breach, there is a final line of
defence to prevent radiation leaks, a bigger containment building made
of steel and concrete. A breach of the containment building would
release radiation into the atmosphere.

WHICH REACTORS ARE MOST AT RISK?

REACTOR No 3: 784-MW

-- What is happening:

TEPCO said on Wednesday that resolving problems at this reactor was the
top priority because it had the highest radiation levels. This reactor
is the only one that includes plutonium in its fuel mix.

The
operator has been pumping sea water into the reactor to prevent
overheating. The building housing the reactor was hit by an explosion on
Monday.

An attempt by a military helicopter to drop water on
the reactor failed on Wednesday probably because radiation levels were
too high, Kyodo news agency reported. The Japan nuclear agency had said
earlier in the day that the pumping of sea water was proceeding
smoothly.

TEPCO said the spent fuel pool may have heated up,
producing steam. The temperature has risen to around 60 degrees Celsius
from the usual 30-40 degrees, but the change was not critical, it said.

-- What are the risks:

The International Atomic Energy Agency (IAEA) said on Tuesday that the
primary containment vessel, the first line of defence against a
radiation leak, appeared intact.

However, Chief Cabinet Minister
Yukio Edano said on Wednesday there is a "possibility" the vessel had
been damaged, Kyodo reported.

If that is the case, authorities
will be worried that radiation may leak through the first containment
wall into the secondary containment building.

The spent fuel
pools present a radiation risk if the spent fuel is exposed to the
atmosphere. When a rod is exposed to the air, zirconium metal on the
rods will catch fire, which could release radiation contained in the
fuel, said Arnie Gundersen, a 39-year veteran of the nuclear industry
who is now chief engineer at Fairwinds Associates Inc.

REACTOR No 4: 784-MW

-- What is happening:

TV on Wednesday showed smoke or steam rising from the facility after
flames were seen earlier. The reactor had been shut down for maintenance
when the earthquake and tsunami struck.

On Tuesday, a pool
where spent fuel is stored caught fire and caused an explosion. Japan's
nuclear safety agency says the blast punctured two holes around 8-metres
square in the wall of the outer building of the reactor.

TEPCO
has said it may pour water through the two holes within two or three
days to cool spent nuclear fuel that is inside. Workers cannot prepare
to pour water into the pool sooner because of high radiation levels,
Kyodo said.

Instead, TEPCO plans to bulldoze a road to the
reactor building so water-pump trucks can approach and hose water
inside, said Kazuya Aoki, a director of Japan's Nuclear and Industrial
Safety Agency.

-- What are the risks:

Exposure of spent
fuel to the atmosphere is serious because there is more radiation in the
spent fuel than in the reactor, said Gundersen. The spent fuel pool is
not inside a containment facility either.

"They need to keep
water in those pools because the roof over the building housing the
pools is already damaged and radiation will escape," he said.

The pools contain racks that hold spent fuel taken from the reactor.
Operators need to constantly add water to the pool to keep the fuel
submerged so that radiation cannot escape.

Exposing the spent fuel to the atmosphere will release radiation.

REACTOR No 2: 784-MW

-- What is happening:

An explosion rocked the plant on Tuesday, damaging a suppression pool,
into which steam is vented from the reactor to relieve pressure. The
roof of the reactor building is damaged, Jiji news agency reported.

TEPCO said on Tuesday the fuel rods were fully exposed. Kyodo reported
an estimated 33 percent of the nuclear fuel rods have been damaged at
the No 2 reactor.

However, on Wednesday, Japan's nuclear agency said the pumping of sea water into the reactor was proceeding smoothly.

-- What are the risks:

When fuel rods are no longer covered in coolant they can heat up and start to melt, raising the risk of a radiation leak.

The suppression pool is part of the primary containment vessel, which
is designed to prevent a leak, but the IAEA said the blast "may have
affected the integrity of its primary containment vessel."

Still, beyond the primary containment vessel is the containment
building, which is also designed to prevent radiation from escaping.

REACTOR No 1: 460-MW.

-- What is happening:

An explosion occurred at the reactor on Saturday. Kyodo reported on
Wednesday an estimated 70 percent of the nuclear fuel rods have been
damaged.

Authorities are pumping sea water into the reactor to
prevent overheating, and pressure levels were stable, Edano said on
Tuesday.

The Japan nuclear agency said on Wednesday the pumping was proceeding smoothly.

-- What are the risks:

The IAEA said on Tuesday the primary containment vessel appeared
intact. If the fuel rods in the reactor are not covered by coolant, they
can heat up and start to melt.

REACTOR No 5: 784-MW

-- What is happening:

The reactor had been shut down for maintenance at the time of the quake and tsunami.

TEPCO said on Wednesday water was being poured into the reactor and
that temperatures in the spent fuel pool were rising slightly.

-- What is the risk.

Reactor 5 and reactor 6 are seen less at risk than reactors 1 to 4.

REACTOR No 6: 1,100-MW

-- What is happening:

TEPCO said on Wednesday water was being poured into the reactor and
that temperatures in the spent fuel pool were rising slightly.

-- What is the risk.

Reactor 5 and reactor 6 are seen less at risk than reactors 1 to 4.

And a visual summary:

Source: Reuters