the end of last week and over the
weekend there has been this terrible
unfolding tragedy in Japan first of all
the earthquake and tsunami and then the
difficulties were there nuclear reactors
I have quite a personal interest in this
because I went to Sendai six years ago I
was joined organizer of a UK Japan
workshop which was held in Sendai which
is just about here and so we have
friends there and very happily they're
all okay but we've had several messages
from viewers several from Japan asking
us about nuclear reactors why do you
need boron why do you need iodine so I
thought be worth telling you a bit about
it
let's begin by explaining what happens
inside the nuclear reactor inside
nuclear reactor you have uranium you can
imagine this as an atom of uranium and
uranium is radioactive and if a uranium
atom is hit by a neutron it can split
into two so when uranium atom splits
into two other atoms which are smaller
and in fact the masses of these two when
you add them together is slightly less
than the mass of the uranium and the
difference in mass between the start and
the product is the energy and mass can
be converted into energy by this very
famous equation of Einstein e equals
mc-squared and even if you lose only a
little bit of mass it's a huge amount of
energy so not only do you form the two
products you also produce another
Neutron or sometimes more than one
Neutron and these neutrons can go on and
cause further uranium atoms to react if
you have uranium by itself this reaction
just builds up and builds up and you
have an atomic bomb but in a nuclear
reactor you control the flow of neutrons
so the uranium splits or so-called
undergoes fission fission is just
another word for splitting at a rate
that gives you energy you want and you
pump water through the reactor to
generate steam and the steam generates
your electricity which is the purpose of
having a nuclear reactor now the way
they control the reactors is by having
control rods which are lowered into the
reactor the uranium is usually in the
form of a powder inside the tube made
out of Ciccone ium
between the tubes or holes which you can
raise and lower the control rods and the
control rods contain boron which absorb
the neutrons if all the control rods are
in then the nuclear reaction stops
because all the neutrons or nearly all
the neutrons are absorbed so as soon as
the earthwork weight happened I presume
in went the boron rods and the reactor
stopped reacting so why can you ask
didn't it cool down why did you need to
keep it cooling if the reaction has
stopped and the answer is that the bits
that are formed the nuclear fragments
are themselves radioactive some of them
very highly radioactive and they go on
decaying and giving out energy
spontaneously they don't need neutrons
to set them off and it is this decay
that produces more heat and this is the
problem that you face having to cool the
reactor for several days afterwards in
fact you have to go on cooling spent
nuclear fuel for years until it's
absolutely cold but the danger period
for reactor I believe is just a few days
now this brings us on to where you might
have explosions many modern nuclear
reactors have mortar round the roads and
the first thing is that as you heat up
the water as more and more heat comes
out of the rods the water generates
steam and the steam generates pressure
and if it gets too hot the whole thing
can just pop through a pressure of steam
however zirconium the outer cladding of
the nuclear fuel rods can react with
water when it gets to high temperature
perhaps fifteen hundred degrees
centigrade
zirconium plus h2o goes to zirconium
oxide
Plus hydrogen and hydrogen gives a
higher pressure than steam and also of
course hydrogen plus oxygen can explode
you've all seen explosions probably on
our video and you get a much bigger bang
if you pressurize a whole nuclear power
station with hydrogen than you will from
the small balloon some of you may have
heard that because of the emergency
they're using seawater to cool the
reactor rather than the ordinary
deionized water with pure water that
they use and you will have read the
people saying well then that's the end
of the reactor they can't use it anymore
and the reason for this I believe is
nothing to do with the fact that it's a
nuclear power station but merely that
the chloride from the sodium chloride
the salt in seawater can attack
stainless steel and other metal
components at high temperature and
weaken them in such a way that you can
never be sure that it's going to be safe
afterwards it causes cracks through the
this steel which are not obvious but
which when they're pressurized could
give way at any moment now the other
question that was asked by one of our
Japanese viewers and I think some of the
others is what is the ideal tablets that
people are get talking about well some
of the fission products that you get
from the reactions are a radioactive
form of iodine iodine 131 the isotope
that has mass 131 and if people absorb
ideal 131 it can get into their thyroid
gland and then when it decays by
emission of an electron
these can attack damage the cells and
cause radiation sickness but this is
very specifically occurs in the thyroid
gland which concentrate
it's iodine in the body so if you give
somebody potassium iodide tablets
beforehand their body takes in the
iodine and so the thyroid gland is
saturated and can't absorb any more
iodine so the radioactive iodine just
passes through the body with doing
relatively little damage so the final
point is with so much destroyed why are
the people worrying about the nuclear
reactors and the answer is that if the
radioactivity gets distributed in the
environment it's very difficult to clean
up and if people absorb it it can then
cause health problems for years and
years to come so every effort must be
made to contain these things because
well they're contained
they're much easier to deal with so
finally I should say that all our
thoughts are with our colleagues and
friends and everyone else in Japan and
we hope that things will get back to
normal
as soon as possible and you have our
best wishes from all of us