Re: Global Warming


Alan Lichtenstein a écrit :


I would like to know on what basis do you make the assertion that
nuclear energy will not solve our energy problems. Would you kindly
elaborate? Nuclear energy does not produce any Greenhouse Gases. There
are other problems associated with nuclear energy, true, but not those
which contribute to Global Warming, which can cause a major change, for
the worse, in our climate.

I think a big problem is that the current fission reactors use up
U235 at a rate which gives this type of nuclear energy a relatively
short lifetime, the depletion times are estimated to be maybe
30 years but have remained that figure with new discoveries. But
like oil, the depletion time extensions eventually become real. The
logical
step to breeder reactors which produce more new fuel (Plutonium)
than U235 consumed has not been taken because of the
technical problems connected with breeders (they are faster and
more capable of going out of control). If the nuclear route is
selected breeders will have to be developed. The French failed
with this with their Super-Phoenix power reactor. The problems
however, are solvable.

Next, the fusion reactor. Still under "study" and even in the most
optimistic minds would not come on line until the latter part of
this century. It holds promise but like all promises, can be broken.
There is an international research effort ITER, in southern France.

The most recent news article is yesterday, below.

It should be noted that our sun's core runs its reactor
at 15 million degrees. It is a very slow fusion, which is
why the sun will last 10 billion years before running out
of hydrogen and passing on to helium fusion, which
is hotter. However an earth reactor must run fast and
requires about 100,000,000 degrees plasma. The big
problem is with the neutron densities generated, can
materials handle it. The plasma can be confined
using a magnetic field, but the neutrons can not be.


*****

Fusion reactor shows its metal
Agençe France-Presse

Monday, 22 May 2006

Glowing plasma inside a fusion test reactor (Image: Princeton Particle
Physics Laboratory)

Physicists say they have cracked a problem facing nuclear fusion,
touted as the cheap, safe, clean and almost limitless energy source of
the future.

The US researchers say they have found a way to cut down erosion of the
metal reactor wall, which would be a crucial step to improving
efficiency.

They publish their work online today in the journal Nature Physics.

In fusion, atomic nuclei are fused together to release energy, as
opposed to fission, the technique used for nuclear power and atomic
bombs, where nuclei are split.

In a fusion reactor, particles are rammed together to form the charged
gas plasma, contained inside a doughnut-shaped chamber called a
tokamak, by powerful magnetic coils.

A consortium of countries signed a deal last year to build the
International Thermonuclear Experimental Reactor (ITER) in southern
France as a testbed for an eventual commercial design.

But many experts have been shaking their heads at the many challenges
facing the ITER designers.

One challenge has been the phenomenon of edge localised modes, or ELMs,
sudden fluxes or eddies in the outer edge of the plasma that erode the
reaction chamber's inner wall.

The tokamak's inner wall is an expensive metal skin that absorbs
neutrons emitted from the plasma. And erosion would mean that the wall
would have to be replaced more often.

Eroded particles also have a big impact on the plasma performance,
diminishing the amount of energy it can deliver.

A team led by Todd Evans of General Atomics, California, believes that
the problematic ELMs can be cleverly controlled.

The scientists found that a small resonant magnetic field, derived from
special coils located inside a reactor vessel, creates 'chaotic'
magnetic interference on the plasma edge, which stops the fluxes from
forming.

The experiments were conducted at the General Atomics' DIII-D National
Fusion Facility, a tokamak in San Diego.

Turbulence inside a tokamak (Image: US National Energy Research
Scientific Computing Center)
Nuclear fusion is the same process used by the Sun to radiate energy.
In the case of our star, hydrogen atoms are forced together to produce
helium.

On Earth, the fusion would take place in a reactor fuelled by two
istopes of hydrogen, deuterium and tritium, with helium as the waste
product.

Deuterium is present in seawater, which would make it a virtually
limitless resource. Tritium would be derived from irradiating the
plentiful element lithium in the fusion vessel.

The US$12.8 billion (A$21.6 billion) ITER scheme entails building the
largest tokamak in the world at Cadarache, near the southern French
city of Marseille.

The partners are the European Union, the US, Japan, Russia, China,
India and South Korea.

It is designed to be a testbed of fusion technologies, with a
construction period of about 10 years and an operational lifespan of 20
years.

If ITER works, a prototype commercial reactor would be built, and if
that works, fusion technology would be rolled out across the world.

Other problems facing fusion technology include the challenge of
creating a self-sustaining plasma and efficiently containing the plasma
so that charged particles do not leak out.

In existing tokamaks, no one has achieved a self-sustaining fusion
event for longer than about five seconds, and at the cost of using up
far more energy than is yielded.

A huge jolt of heat of nearly 100 million°C is needed to kick-start
the process, which then has to be sustained by tiny amounts of fuel
pellets.

.

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