快猫短视频

ONCE upon a time, the grass was green, the sky was blue and there were no
stockpiles of weapons-grade plutonium or nasty nuclear waste lying around
waiting for a final resting place. If physicists are to be believed, those
halcyon days could soon be coming back. They think they have come up with the
ideal solution to the nuclear industry鈥檚 biggest image problem. Their fix will
destroy the existing inventory of plutonium and minimise the threat of nuclear
terrorism. It will also reduce the amount of hazardous radioactive waste, making
underground storage a cinch. It could even generate electricity into the
bargain.

This panacea is called transmutation. Add a neutron or two to some of the
most dangerous radioactive elements and you destroy them. Plutonium, for
example, is split asunder, while the most intractable fission products are
rendered harmless.

Consider what transmutation can do to one of the most noxious constituents of
radioactive waste. Technetium-99 is a fission product of uranium and reactors
around the world throw out about 6 tonnes of it a year. It has a
half-life鈥攖he time it takes for its radioactivity to decay to half its
initial level鈥攐f 200 000 years, and because it dissolves easily in water,
technetium-99 accumulates in the food chain. Thanks to the nuclear industry,
concentrations of technetium-99 in some areas of ocean have increased a
hundredfold since the 1960s.

But add a neutron to this isotope and you transmute it into technetium-100,
which has a half-life of just 15.8 seconds. In the time it takes to whip out
your Geiger counter, it decays into ruthenium-100, which is stable and harmless
(See Diagram).

It sounds too good to be true, and there is certainly a degree of scepticism
about the whole notion. Nuclear power, after all, has turned out to be a
double-edged sword. But just now there is a great surge of interest in
transmutation. The governments of Spain, France and Italy are about to receive a
report outlining what is needed to build a prototype transmutation reactor, and
the US Department of Energy has just released $4 million for research and
development.

There is, in fact, a race going on between European and American teams. Both
hope that transmutation鈥檚 time has come. For, although it is not a new idea
(see 鈥淣uclear alchemy: France鈥檚 impossible dream?鈥�, 快猫短视频, 20 June
1992, p 12), it has never been seen as technologically and economically
feasible. What has changed? Over the past few years researchers have rejected
specially designed nuclear reactors as the natural home of transmutation, and
turned their attention towards particle accelerators.

Nuclear reactions driven by a stream of subatomic particles can be made
鈥渟ubcritical鈥濃�攚hich means they are not self-sustaining, so they stop when
the accelerator is turned off. More importantly, the reactions can be tuned to
destroy more radioactive materials than they create. Research on this idea began
in earnest in the late 1980s at Los Alamos National Laboratory in New Mexico.
Then, in 1993, scientists at CERN, the European centre for particle physics,
near Geneva, announced that they were working on a similar project. Both
projects have evolved quickly, and both teams believe the other has copied its
ideas. The Europeans have turned down an opportunity to collaborate with the
Americans because they are confident they can go it alone.

Persuasive energies

The main challenge facing the teams is to persuade isotopes to change their
nature, which is no easy task. Whether or not a long-lived fission product, such
as technetium-99, captures a neutron and transmutes is a matter of chance. But
each isotope has a series of high, resonant energies at which that chance
increases. If it encounters a neutron with one of these energies, it is more
likely to transmute.

The other class of long-lived nuclear detritus is the transuranic elements
(TRUs), such as plutonium, uranium and americium. Rather than transmute by
capturing a neutron, these elements are destroyed by fission鈥攁nd the
probability that they will disintegrate increases as the neutron energy
increases. 鈥淎t high energy all the TRUs fission,鈥� says Jean-Pierre Revol, a
member of the CERN team.

Disappearing waste

The problem facing physicists has been to produce neutrons with high enough
energies. The latest generation of particle accelerators has made this feasible.
These accelerators are smaller and more reliable than their predecessors.
Whereas older accelerators turned only about 5 per cent of power from the grid
into a particle beam, today that figure is 50 per cent.

The result is a real possibility of making at least some radioactive waste
disappear. The researchers calculate that, at the very least, the volume of
waste can be reduced by a factor of 100. In theory, the volume could be cut
further still, but that would be costly. 鈥淚t will be a political decision as to
how far you go with cleaning up,鈥� Revol says.

As the volume decreases, so too does the hazard. The governments of most
nuclear nations plan to bury their long-lived waste in underground repositories
designed to stay isolated from the world above for hundreds of thousands of
years. After this time, the radioactivity would have decayed to a 鈥渟afe鈥� level.
The trouble is that on such long timescales no one can predict what might
happen. 鈥淚f you have something with a half-life of 10 000 years, it essentially
means relying on the storage for a million years,鈥� says Robert Klapisch, one of
Revol鈥檚 colleagues at CERN. 鈥淵ou really have to worry about earthquakes if you
want to be sure these things don鈥檛 return to the biosphere.鈥�

But with the TRUs split up into short-lived fission products and obstinate
isotopes transmuted into stable elements, a repository would need to stay intact
for a much shorter period. 鈥淲e鈥檙e talking about 300 years, compared to 1 or 2
million years,鈥� says Revol.

In the US, transmutation鈥檚 ability to clean up waste is a strong selling
point. Los Alamos researchers want to put plutonium into their system, fission
it, and transmute the fission products. 鈥淥ne hundred years from now we could
have less than 10 per cent of the existing inventory of plutonium,鈥� says
Francesco Venneri, head of the transmutation programme.

Transmutation could also, he argues, go a long way to solving the nation鈥檚
repository problem鈥攁 hot potato for the Department of Energy. Back in 1982
the DoE promised to find a safe place to store waste by February last year. But
when the deadline came, no site existed. Today, all that does exist is an
investigation into the suitability of Yucca Mountain, Arizona, as a dumping
ground. And if American reactors keep spewing out radioactive waste at their
present rate, such a site would be full within 50 years anyway. Venneri wants to
add waste from nuclear reactors to the plutonium and give it the neutron
treatment too. By transmuting the waste first, Venneri reckons that Yucca
Mountain would not fill up for hundreds of years.

So how would a transmutation machine work? The radioactive isotopes are
packed into long tubes and lowered into channels inside a huge lead block. A
beam of protons from an accelerator is then fired at the lead. The colliding
protons generate a hail of neutrons with energies high enough to fission the
TRUs. And as the neutrons collide with lead nuclei, they gradually lose energy,
sweeping down the energy scale. As they pass through the resonant energies of
isotopes such as technetium-99, the neutrons are more likely to trigger
transmutation.

As well as generating neutrons, the lead also acts as the system鈥檚 coolant.
Heat from fission melts the lead, which rises in the reactor vessel
(see Diagram).
It then passes through a heat exchanger, where it cools and
sinks again. The waste heat is converted into electricity.

Although lead is efficient at cooling by convection, it also has a downside.
American scientists abandoned previous attempts to use liquid lead as a coolant
because it is so corrosive. But, they now have what the CERN scientists
reluctantly admit is a great asset: the Russians. Five years ago, the Russian
Navy announced, to everyone鈥檚 surprise, that it already had a fleet of
submarines powered by reactors which used molten lead.

The Russians have solved the corrosion problem by bubbling oxygen into the
lead, which encourages the reactor鈥檚 structural alloys to grow a protective,
self-healing film of oxide. They still regard their technique as a military
secret. But, in exchange for research funding, they are sharing it with the Los
Alamos scientists.

Assuming the lead can be made to behave itself, the machine would cook up its
charge of waste and plutonium for perhaps three years. Afterwards, what鈥檚 left
would go to be reprocessed by a technique pioneered at Argonne National
Laboratory in Idaho, called pyrochemical separation. This turns the waste into a
molten electrolyte and draws off any unfissioned TRUs at a superheated
electrode. The TRUs can then be mixed with any remaining long-lived fission
products and new waste, and returned to the reactor. With each pass, the
Americans estimate that at least 20 per cent of the radioactive isotopes would
be destroyed. Short-lived fission products are sent to the repository.

Icing on the cake

That鈥檚 the American pitch. The Europeans have added the icing on the cake. On
top of destroying plutonium and reducing hazardous waste, they also want to
produce competitively priced electricity. Their proposed machine has been dubbed
the Energy Amplifier by its designer, Nobel prize winning physicist Carlo
Rubbia. 鈥淭he cost of building an Energy Amplifier will only be known after the
main technical options have been chosen and a demonstration prototype has been
built,鈥� admits Klapisch. 鈥淏ut the cost of electricity should be comparable with
a pressurised water reactor because there is less maintenance and the fuel is
肠丑别补辫.鈥�

This cheap fuel is a mixture of thorium and oxides of the TRUs. Although
thorium is not used as a nuclear fuel, it is considered to be attractive because
it produces smaller amounts of TRUs鈥� especially plutonium鈥攖han the
uranium used in today鈥檚 reactors.

The political climate in the US doesn鈥檛 encourage generating electricity with
nuclear energy. 鈥淭here will be electricity production, and it will offset a
large portion of the cost, but that鈥檚 not the prime purpose,鈥� says Venneri.

He argues that aiming for efficient electricity production would make the
reactor too complicated, and that the idea is a ploy to help the European scheme
gain acceptance. 鈥淓veryone would like to make it look as good as possible, but
to insist on getting the best efficiency doesn鈥檛 look to me like the best thing
to do,鈥� he says.

Klapisch disagrees that efficiency means complexity. 鈥淭here鈥檚 absolutely no
reason why it should be more complex,鈥� he says. And there is nothing wrong with
presenting the most cost-efficient machine. The American plan is just as
political in its approach, he says, it鈥檚 just that the political priorities are
different.

So far, the researchers only have a simulation and a series of experiments on
isolated aspects of the system. The CERN researchers have put technetium-99 into
a lead block and transmuted it, verifying the principle. But they have not built
any of the reactor鈥檚 structures. The Americans are building a target block, and
aim to construct a prototype plant within five years. 鈥淚 think we鈥檝e made enough
inroads in the last year to get started now,鈥� Venneri says.

Both teams are bullish about their prospects, but those watching from the
sidelines are more cautious. 鈥淚 don鈥檛 think it鈥檚 an idea that should be
dismissed, but it requires advances in a whole range of technologies,鈥� says
Richard Bush, the fuel processing manager at AEA Technology in Oxfordshire. He
suggests that too many untested claims are being made. Lawrence Lidsky, a
professor of nuclear engineering at MIT, is equally cautious. 鈥淭echnically, it鈥檚
on the edge of possible,鈥� he says. 鈥淲hether it makes any economic sense is
another question.鈥�

Lidsky sat on a panel reviewing the Los Alamos work. Although impressed by
the technology, the panel also had concerns. Stealing plutonium from the
proposed machine would, in principle, be just as possible as from one of today鈥檚
reactor. Once pyrochemical separation has recovered the TRUs from the machine鈥檚
waste, the concentration of plutonium is actually higher than in spent reactor
fuel. These plants would need high security, Venneri admits.

Lidsky is also worried about reactor safety. Although the researchers claim
that their machines would be safe because they would work subcritically, he is
concerned by their complexity. Today鈥檚 reactors are already too complicated.
鈥淭his is every bit as complicated, if not more so,鈥� he says. 鈥淭here鈥檚 more
things to go wrong.鈥�

Short-lived

He predicts that the new optimism about transmutation will be short-lived.
鈥淲hat finally happens . . . is that somebody does a real analysis of how long it
will take to accomplish any of these highly desirable political and social
goals,鈥� he says. 鈥淭hen we鈥檒l look at the real costs, where these things go, the
shipping of plutonium to the sites, the degree of processing you need to do on
site. You start doing an environmental impact study and it begins to look a lot
less interesting.鈥�

But the CERN team is convinced that transmutation programmes will get the
green light. 鈥淚 think the EA prototype will certainly be built, and my guess is
that it will be deployed,鈥� says Revol. 鈥淚t鈥檚 almost out of our hands now. The
fundamental research and development is done.鈥�

Several European governments have expressed interest in transmutation, and
nuclear companies such as Siemens and British Nuclear Fuels are carrying out
their own research. The American company Westinghouse is backing the Los Alamos
project. This, after all, is the break the nuclear industry has been waiting
for鈥攕ocially responsible action with nuclear power at its core.

Forgetting political caution for a moment, Venneri envisages a new era for
power generation. 鈥淓ventually we鈥檒l be able to expand the role of nuclear power
from the present-day 5 per cent to 25 per cent,鈥� he says. If you were looking
forward to an era of calm, green technologies as an answer to global power and
environmental crises, this may not have been exactly what you had in mind.