Page 446 - Fundamentals of Magnetic Thermonuclear Reactor Design

Safety of Fusion Reactors Chapter | 14 423

conditions and dynamics of processes determining operational safety, including
accident conditions, may undergo significant changes. For example, a transition to
low-activated structural materials may result in a drastic decrease in radiotoxicity
and alleviate the decay heat problem and decrease the radwaste quantity.
It is interesting to review the conceptual safety-relevant design solutions for
advanced contemporary fusion machines, such as the Russian DEMO-S and the
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US ARIES-III. In this respect the D- He advanced fuel cycle designed for the
US machine is particularly worthy of note.
The DEMO-S reactor is a stationary DT cycle, single-zero divertor tokamak
with a tritium self-sufficiency and a large bootstrap current contribution, with a
rated fusion power of 2400 MW and 1145 MW of gross electric power. DEMO-S
has two blanket designs: a helium-cooled one using the Li SiO ceramics in
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the breeder region and ferritic steel as a structural material, and the other using
liquid lithium for breeding tritium and the vanadium–chromium–titanium alloy
as a structural material [20,21].
Desired safety level is attained using a system of independent LoDs, includ-
ing physical and functional barriers to RMs spreading, as well as barrier protec-
tion and control equipment. The barrier arrangement is similar to that adopted
for ITER. The vacuum vessel acts as the first confinement barrier; it must with-
stand any postulated accident without losing its leak-tightness.
Harmful impact on the staff must be as low as reasonably achievable using
socioeconomic and technical criteria. Radiological impact levels (dose criteria)
must be in line with currently accepted radiation protection standards, as must
be the premises housing the equipment.
Potential accident analysis has not been part of the next-step reactor design.
However, coolants have been selected such that the risk of hydrogen release due
to water (coolant) abnormal ingress in the vacuum chamber is ruled out. At the
same time, one of the proposed blanket designs poses problems associated with
a large amount of flammable lithium.
The project pays a great deal of attention to the handling of radioactive in-
ventories after the end of the reactor operation. It has been shown that all materi-
als have to be aged for long times prior to recycling and burial. Manual recycling
of the magnet coil materials, the support structure and, perhaps, part of the IVC
vanadium alloys (together accounting for up to 60% of the reactor materials) is
possible after a 30-year ageing. The vacuum vessel materials, the Li SiO ceram-
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ics and the blanket beryllium components (accounting for ∼25% of the reactor
materials) may be recycled after a 100-year ageing. The rest of the blanket and
the divertor cassette–activated materials are subject to geological disposal [21].
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The transition to a, conditionally speaking, neutron-free D- He fuel cycle,
proposed for the ARIES-III machine, is a big step forward on the way to en-
hancing the fusion reactor’s radiation safety. Because, tritium and high-energy
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neutrons would be produced in the primary and secondary D(d, n) He, D(d, p)
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T and T(d, n) He reactions, proceeding along with the dominant He(d, p) He
reaction, there would be accumulation of radioactive inventories and activation