22 But isn't fusion nuclear? What about radioactive waste?
Description
This article is from the Fusion FAQ, by Robert F. Heeter heeter1@llnl.gov with numerous contributions by others.
22 But isn't fusion nuclear? What about radioactive waste?
Fusion *is* a nuclear technology, but there are significant
qualitative differences between fusion and fission. These
differences add up to both safety and environmental advantages
for fusion. (Safety issues are discussed in Section 2 Part 3)
On the environmental side, fusion differs from fission in that
one can control the waste products by controlling the fuels
used and the materials exposed to neutrons produced in the
fusion reaction. In fission, uranium or plutonium decays
in a random way and the "daughters" of the fission process are
scattered all over the periodic table, and there are lots of
nasty radioactive isotopes produced. Thus fission results in
large amounts of concentrated radioactive waste.
In fusion, one has the opportunity to minimize or even
perhaps to eliminate the radioactive waste problem.
"Aneutronic" fusion fuels (discussed in Section 1) would
produce little or no radioactive waste at all. Even in
"neutronic" (but much easier) deuterium-tritium fusion
(discussed in Section 1) most of the neutrons (which
are the primary source of radioactive waste) are absorbed in
a lithium blanket in order to replace the tritium fuel
burned in the reactor. The only sources of radioactive waste
in a D-T reactor are stray tritium atoms and the reactor
structure which is exposed to neutron radiation.
Tritium is a relatively benign radioactive element, because:
(a) It doesn't emit strong radiation when it decays, so it's
only hazardous if one breathes it in or ingests it.
(b) It generally shows up in one's body as water, and your body
flushes out its water fairly frequently, so tritium won't
build up in a living creature. (Unlike many fission
reaction products.)
(c) It has a moderate half-life, only 12 years or so. This
means that it won't be around forever, so it doesn't
create a long-term waste problem. On the other hand,
it probably won't decay in the few days that pass
while it's in one's body, which is also good.
Based on current tritium-handling knowledge and experience
scientists believe that incidental releases of radiation
from fusion reactors will be comparable to releases from
either fission or coal plants. In a fusion economy, the
contribution of tritium to one's radiation exposure will be
orders of magnitude less than natural exposure to things like
radon, cosmic rays, etc; and also much less than man-made
exposure to things like medical x-rays.
Radioactive waste in a fusion reactor can be minimized by
choosing special structural materials which can withstand
neutron bombardment without becoming highly radioactive.
Two strong candidate "low-activation materials" are vanadium
and silicon-carbide. Vanadium will be tested as a structural
material on the TPX tokamak to be built at Princeton.
If either vanadium or silicon carbide is used as a structural
material, the radioactive inventory of a fusion reactor will
be much less than that of a fission reactor with comparable
power output. In fact, with a low-activation fusion reactor,
one can wait ten or so years after shutdown, and the fusion
reactor will be 1,000 to 1,000,000 times *less* radioactive
than the fission reactor. The material in the fusion reactor
will actually be less radioactive than some natural minerals,
particularly uranium ores, and it would conceivably be safe
to *recycle* the fusion reactor structure into a new
fusion reactor, with little permanent waste at all. In these
circumstances one must compare the problems and hazards
posed by permanent *chemical* wastes from manufacturing and
operating other energy sources with the problems and hazards
posed by fusion energy.
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