Page 219 - Fundamentals of Magnetic Thermonuclear Reactor Design

Vacuum and Tritium System Chapter | 6 201

To sum it up, the vacuum pumping duct conditioning process should include
a pre-installation annealing of parts in an ‘oil-free’ vacuum, chemical treatment,
physical and chemical cleaning, and a high-temperature GDC using argon fol-
lowed by a thermal vacuum training allowing argon removal.

6.6.5 Vacuum Pumping Equipment
Vacuuming technologies using liquids and lubricants are unsuitable for fusion
devices, because they evaporate and become sources of plasma impurities.
Therefore, the designers of MFR vacuum systems give absolute priority to oil-
free pumps [15].
A pumping system of an experimental facility is based on the use of com-
mercial oil-free pumps. Notable exceptions are the open-end magnetic mirrors
and the fast neutral beam injectors, which typically employ built-in surface-
action pumps (mostly cryogenic ones).
The move to D–T fusion reactors places additional restrictions on can-
didate pumping means and vacuum system configurations. They are associ-
ated with the tritium’s high cost and radioactivity, and environmental safety
requirements that include the need for a tritium closed-cycle technology,
minimisation of tritium irreversibly depositing in pumps, and inexpensive
and fast pump regeneration methods; the ban on substances and materials
changing their physical and mechanical properties after contact with tri-
tium; and compliance with procedures for precluding tritium release into the
atmosphere.
Pumps used for electrical physics applications are unsuitable for the MFR
due to difficulties with removing absorbed tritium. Condensate and cryosorp-
tion pumping, and sometimes pumping based on non-pulverisable metal get-
ters (Table 6.5), proved effective for removing surface impurities in large-scale
experimental facilities and fusion reactors. These technologies offer an almost
unlimited pumping speed, which is critical for fast atom injectors and linear
magnetic mirrors. In addition, high-vacuum surface-action pumps have superior
technical and economic characteristics.
The key problem with the D–T fuel cryogenic pumping is the volumetric
heat release inside the condensate layer, caused by the tritium β-decay. The
β-decay electron-stimulated desorption of tritium is insignificant, and the
condensate volumetric heating is low, but if the condensate layer is more than
3 mm thick, tritium self-heating may give a considerable rise in temperature.
3
A prolonged pumping may be complicated by the release of the He isotope,
which cannot condense at 4 K. To remove it, an additional pump is necessary.
The condensate pumps’ key strengths are structural and operating simplicity,
performance stability with respect to a wide range of removed gases, and
an unlimited number of regeneration cycles. Their key weaknesses include
the low operating temperature level and inability to withstand intensive heat
loads.