Page 441 - Fundamentals of Magnetic Thermonuclear Reactor Design

418 Fundamentals of Magnetic Thermonuclear Reactor Design

The third confinement barrier includes the constructional elements of the
tokamak building, equipped with air ventilation, conditioning and detritia-
tion systems, which, where necessary, provide indoor air decontamination and
detritiation.
Fig. 14.3 illustrates also the system of barriers for tritium migration from
tritium building and hot cells (where activated and tritium-containing materials
and equipment are stored and processed). The first barrier for tritium contained
in the fuel preparation and feed systems consists of the walls of technologi-
cal equipment, transport casks and pipelines. The second barrier includes glove
boxes and coaxial jackets enclosing pipes. Spaces between the barriers are
equipped with detritiation filters that control tritium release outdoors. The third
barrier includes the constructional elements of the buildings, equipped with air
ventilation, conditioning and detritiation systems.
The radioactivity in the reactor, tritium and hot cell buildings and radioac-
tive waste storage rooms is under constant radiological monitoring. Premises,
in which radioactivity level exceeds the project guidelines, are isolated from
normal ventilation and connected to air decontamination and detritiation sys-
tems. Air flows are directed from lower contaminated to higher contaminated
premises, from where air is drawn into a blowdown stack and released to the at-
mosphere. Under accident conditions, exhaust air is detritiated and cleaned from
activated dust. Detritiation systems and dust removal filters are expected to have
an efficiency of no less than 99% and 99.9%, respectively. Radioactive leaks and
discharges from the cooling systems are controlled by water detritiation system.
Safety is also ensured by a continuous pressure control in spaces between
the confinement barriers and the control of radioactive decay heat.
Maximum design pressure in the vacuum vessel and the cryostat is 200 kPa.
Because this pressure may be exceeded under some accidents, such as a coolant
leak, the design provides for an emergency steam escape into a pressure-suppress-
ing tank and its condensation there. This tank, located inside the reactor building,
is leak-tight. The cooling systems are equipped with safety valves and drain tanks.
Maximum FW temperature 1 h after an accident should be within 600°C to
avoid (1) tritium release through the beryllium cladding micropores, leading to
an in-vessel tritium content increase, and (2) accumulation of hydrogen from
the beryllium–water steam reaction, giving rise to an explosion risk. Tempera-
ture increase under accident may be limited by suppressing the discharge and
removing radioactive decay heat. To this end, independent active and passive
vacuum vessel cooling systems are provided. The passive cooling system can
remove ∼3 MW by natural convection and radiation. This, together with the
active systems, prevents the decay heat from becoming an aggravating factor.
The decay heat density is low enough not to melt the vessel wall through, as its
design temperature is within 500°C.
Activated removable components of the reactor should be brought into the
hot cell for treatment. After the end of the reactor operation, all components are
de-activated before handing the facility over to a specialised organisation.