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20 November 2009
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Introduction
Fuel sources
Uranium
Thorium
Plutonium
Recycling
nuclear fuel
Waste fuel
Transport
Further reading
Many, especially in the USA, claim that there is not enough uranium to
supply a proliferation of nuclear power stations. This is based on the fact
that the US Government, under President Carter's guidance, promulgated
legislation forbidding the recycling of nuclear fuel. This makes US reactors
"once-only", using virgin uranium. In contrast, most European, Japanese and
Russian reactors use recycled fuel. Obviously, the latter method is more
kind to uranium resources.
There are three radionuclides (or radioactive metals) that can be useful
for use as fuel, uranium, thorium and plutonium. It is not intended that
this page be a primer in radiochemistry, but a few basic facts may clarify
the situation.
This is quite a common metal found throughout the world but
usually in low concentrations. There are however "pockets" of rich uranium
ore in many countries, such as the USA, Canada, Australia, Russia etc. These
are easily exploited and produce uranium at a low cost. The ore has to be
processed and "enriched" (this means that the physical concentration of
highly radioactive uranium in the low-radioactivity uranium has to be
increased). It is estimated that the reserves of rich ore are limited and,
at the present rate of consumption, would last only a few decades. However,
there are many places with abundant supplies of low-grade uranium ore
throughout the world and could last hundreds of years with the most
pessimistic demand. The down-side is that the cost of extraction would
double or triple the price of enriched uranium. This sounds serious, but it
isn't; the repercussion on the price of electricity produced would be very
small, of less than $0.01/kWh. Even so, there is another source of uranium:
the oceans. There are about 4 gigatonnes (4 trillion kg) of uranium in the
sea. Research is ongoing on an economical and passive way of extracting it
by a process called adsorption. This is expensive but requires almost no
energy, with estimated costs similar to the extraction of low grade ore. To
date, less than 1 tonne has been extracted, but full scale pilot tests are
likely in the near future.
Thorium is about three times more abundant in the earth's
crust than uranium and is about the same as lead in quantity. It is easily
and inexpensively mined at about one-third the cost of uranium. One
reference states that there is more energy available from thorium than from
all the uranium and fossil fuels in the earth combined. Thorium itself is
not fissionable (it cannot sustain a chain reaction in a critical mass).
However, it can easily and cheaply be converted to a fissionable form
of uranium by bombarding it with neutrons.
Plutonium is almost non-existent in nature but is a
by-product of uranium fission. It is, itself readily fissionable. Plutonium
is therefore available in commercial quantities only from the spent fuel of
uranium. One kilogram of plutonium can produce the equivalent heat of 22
million kWh of electricity, enough to keep a town the size of Larnaca in
electricity for about 4 months! Plutonium is usable to produce small nuclear
weapons, hence the almost paranoid insistence of the Americans not to allow
some countries to have nuclear power. Plutonium, by itself, is unsuitable as
a nuclear fuel but may be mixed with other radionuclides or moderators to do
so.
Spent fuel rods contain a variety of transmuted
radionuclides. The most popular way of recycling them is to separate and
purify them, then to radiate and enrich them in the form of a mixture of
metal oxides, notably uranium and plutonium, to simulate similar effects as
uranium would have. This mixed oxide (MOX) recycling is 96 per cent
effective, so that only 4 per cent of the weight of radionuclides is medium
level waste (as opposed to 100 per cent of high level waste from the US fuel
rods). Most W. European and Japanese reactors use MOX technology, with
recycling in the UK and France. Interestingly, the USA has exported
weapons-grade plutonium to France, to enter into the MOX cycle as a civil
nuclear fuel.
Another possibility is the breeder reactor. As the name
implies, it is another form of nuclear reactor. This can take in some poorly
fissionable material, such as un-enriched uranium or thorium (or used fuel)
and produce a more highly fissionable material. In some cases, the fuel
extracted will contain hundreds of times as much energy as the input
material. Experimental breeder reactors have been built in a number of
countries but they have always given problems. The most common type is the
sodium-cooled liquid metal fast breeder reactor, but the simpler
water-cooled thermal breeder reactor (used for converting thorium to
uranium) is much more manageable. The infamous Super-Phénix reactor at
Creys-Malville in France was supposed to have been the first full-scale fast
breeder reactor, but it was taken out of service by government decree before
being fully commissioned, because of problems containing the liquid sodium
heat transfer fluid outside the reactor, as well as popular protests.
One of the most difficult points, often quoted by
anti-nuclear activists, is how to dispose of waste fuel safely. This is
especially problematic in the USA, which has hundreds of thousands of tonnes
of high-level waste (from weaponry, as well as spent fuel) because of their
political decision not to recycle it.
The usual way of disposing of high-level waste is to vitrify
it after cooling it by immersion in water for a number of years. The waste
is mixed with molten glass which is then cast into blocks in a thick
stainless steel drum. The quantity of radionuclide in a drum is a fraction
of that which could cause a chain reaction. The glass is cooled very slowly,
annealing it to prevent cracking. This inner drum is then encased in another
thick metal container with an interlayer of lead and/or concrete to allow
the whole to be safely manipulated. The ensemble is bulky but extremely
robust.
After vitrification, it is usual to store the waste for
several years where each container can be periodically checked. They can
then be placed in a definitive depot. This is where the real crunch lies.
Activists have created such fear of high level waste that nobody wants it
within hundreds of kilometres, even though it would be absolutely safe for
thousand of years in a properly engineered depot. Many countries have
researched the ideal conditions for underground disposal. This includes
reinforced tunnels pierced in a stable dry rock stratum in a seismically
inactive region. Geologists have found many such strata in various countries
where they estimate the risk of an accident occurring in the tens or
hundreds of thousands of years, by which time the radioactivity would be
negligible (remember that high-level radionuclides have a shorter half life
than some low-level ones). An ideal location would be in a thick anhydrite
stratum, which has the advantage that any fracture would be self-healing.
Some of these have been stable for millions of years and are at sufficient
depth that it would be extremely unlikely that they could ever reach the
surface. As water cannot penetrate anhydrite, by definition, there can be no
risk of water contamination.
The USA has chosen Yucca Mountain, Nevada, as such a
repository This choice is controversial because the rock is a soft tuff
which is not necessarily anhydrous. However, the thick stratum is sandwiched
between calcite which is impermeable. Furthermore, being (currently) in an
extreme desert climate, the risk of water penetration is reduced. What is
possibly more controversial is that there are ancient faults on either side
of the proposed area. Switzerland is another country that has chosen some
sites. The most important one is in the Zürcher Weinland near Schaffhausen.
Another one that was considered was at Ollon, Vaud, which has an ideal
anhydrite stratum, but opposition to this site was badly handled, forcing
its abandonment.
Another issue that has raised some opposition is the
transport of nuclear fuel. Some of the more stupid opponents have taken
physical action that, if it had succeeded, would have been dangerous for
themselves and others.
It is clear that recycling involves the transport of spent
fuel from the power station to the recycling plant and back again. In some
cases, this involves a sea voyage, often through crowded sea lanes; in
others, it is done by rail. However, the fuel is always put in special
containers designed for this job and offering a maximum of security. Tests
on these containers have involved dropping them off a bridge and driving a
locomotive into them, neither of which caused damage to the shell sufficient
to permit the contents to become accessible. It can be said that the chances
of a serious nuclear accident from this cause are very small, indeed. The
quantity of fissile material in a single container is small, so as to
prevent any risk of a chain reaction under any circumstances, so that, if
the worst scenario occurred, the escape of radionuclides would be small,
even if the three levels of "envelopes" were all breached. Even so, all
transport is accompanied by armed security guards, either with it or along
the route. Each container is externally checked for radioactivity at each
stop.
In the case of shipping, there is a danger of collision in
some places, such as the Malacca Straits, used between Japan and Europe. The
worst case scenario is that the ship would sink with its cargo. The
container would take centuries to corrode through, by which time much of the
high-level radioactivity will have decayed. It is unlikely that the small
quantity of remaining radionuclides will cause significant damage to the
marine environment. A much more significant danger would be from a terrorist
hijacking of the ship and its contents. The danger of their using it to make
a nuclear bomb is zero, because the quantity and type of materials are
totally unsuitable. The worst it could be used for is to make a "dirty"
conventional bomb of relatively small impact but of immense psychological
value because of the public fear.
Interestingly, the USA has sent shipments of weapons-grade
plutonium to France for use in the MOX cycle. In the hands of terrorists,
this would be very much more dangerous and extreme measures were taken to
prevent a hijack from happening. This included sending it in one of three
identical armed vessels, so that no one would know which was the "real" one.
The salient points
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