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Japan's nuclear situation explained -

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LEIGH SALES, PRESENTER: To learn more about the nuclear threat, I spoke earlier to one of the
world's foremost experts on nuclear power. Professor Richard Lester is the head of the Department
of Nuclear Science and Engineering at the Massachusetts Institute of Technology. I asked him what
Japanese authorities need to do to control and contain the release of any radioactive materials.

RICHARD LESTER, NUCLEAR SCIENCE, MIT: What the operators have to do at this point have is really
three things. They have to try to make sure that the fuel in the reactor remains cool. Cool enough
to prevent any further damage to the fuel and that requires keeping the fuel covered at all times
with water. The second thing they have to do is to make sure that pressure is not rising to risky
levels throughout the reactor system. And the third thing that they have to do is to ensure that
hydrogen, which may be being produced in the reactor as a result of reactions between the chemical
reactions between the steam in the reactor and the fuel cladding, they have to make sure that
hydrogen doesn't further accumulate and risk another explosion.

LEIGH SALES: We know the Japanese authorities are pumping sea water laced with the chemical element
Boron into two of the reactors to try to cool them down. When will we know if that's working, along
with those other measures that you just outlined?

RICHARD LESTER: Probably the most direct way to tell whether it's working or not is for
measurements to be made - and of course they are making these measurements - in the near vicinity
of the plant of radiation level, radioactivity levels. The latest figures that I saw suggest that
if you were at the site boundary, it would take a couple of hours for you to actually receive a
radiation dose equivalent to what you would get from a routine chest X-ray. If that's correct, if
those figures are correct, that means that further away from the plant, the radiation doses would
be extremely low. Nevertheless, given the possibility of further radiation releases, I do think
it's prudent for the authorities to have taken the actions that they've taken to evacuate the
population to what is actually a fairly substantial distance, 12.5 miles.

LEIGH SALES: Most viewers would've heard of Chernobyl and the nuclear disaster there in 1986. Can
you just give us some context by explaining how this situation differs to that one?

RICHARD LESTER: The Chernobyl reactor essentially had no containment. It had no secondary barrier
outside the reactor vessel. That is not the case at Fukushima. And the secondary and actually
tertiary barriers have actually played so far an important role in maintaining the radioactivity to
a very great extent inside the reactor system as opposed to what happened at Chernobyl where very
large amounts of radiation were released from the reactor vessel and had no place to go other than
out into the atmosphere.

LEIGH SALES: Japan's nuclear industry provides about 30 per cent of the country's electricity. What
sort of struggle does that mean Japan's going to have over the short term to meet its energy needs?

RICHARD LESTER: I think the Japanese nuclear fleet provides something like a third of Japan's
electricity. Some considerable portion of that, not only the units at Fukushima Daiichi, but also
Fukushima Daini and probably at several other sites that are within the zone affected by the
earthquake and the tsunami will probably have to remain closed for a very substantial period. Of
course, the two reactors that have been worst affected at Fukushima Daiichi units one and three
will almost certainly never re-open as a result of the action by the operators to flood these
reactors with saltwater. Those two reactors are essentially written off.

The number of reactors that will be able to restart at some point in the future is unclear. I've
seen one estimate that the current contribution of about 30, 35 per cent of Japan's electricity
from the nuclear plants may be reduced to something like 15 to 20 per cent for at least the
foreseeable future as a result of the earthquake. If that's the case, the Japanese will certainly
have to struggle to come up with replacement power, and I would guess that that would be a real
challenge for the electric power system in the coming months.

LEIGH SALES: Professor Richard Lester there from MIT.