Tidal Current Energy

250 D. Matzner
in the German AVR reactor (in 1970) and, more recently, in the Chinese 10 MW e
prototype.
Even with the reactor shut-down, however, temperatures continue to rise due
to heat still being generated by the highly radioactive fission products within
the fuel spheres. By now, the reactor pressure vessel, normally maintained at
about 300°C, is also heating up and is radiating more and more heat to what is
known as the reactor cavity cooling system (RCCS) that surrounds it. This is a
water-filled system which, even if power supplies fail, will continue to evacuate
radiant heat from the pressure vessel to the outside atmosphere by natural
circulation and eventually boiling.
After a day or so, the increasing rate at which heat is being radiated from the
pressure vessel (now at about 500°C) becomes equal to the diminishing rate at
which heat is being generated by the fission products within the core. The temperature
of the fuel stabilizes at rather less than 1600°C and thereafter gradually
declines. The ‘ crisis ’ is over. So far, the operators have had nothing to do. Only
now, perhaps 72 hours after the initiating event, may they have to replenish
water contained in the RCCS.
The type of accident in which fission product heating melted much of the
Three Mile Island reactor core is thus impossible, as is the melt-through ‘ China
syndrome ’ .
This has been a ‘ loss of coolant ’ or ‘ loss of cooling ’ accident. The other conceivable
type of major reactor accident is the so-called ‘ reactivity ’ accident,
which caused the initial burst of energy at Chernobyl. Reactivity is a concept of
basic importance for the reactor designer. If the reactivity coefficient is exactly
unity, for every fission in the reactor core just one fission neutron goes on to
cause a further fission and the reactor power level remains constant. If, for
every 100 fissions, 101 fission neutrons cause further fission the chain reaction
diverges and the power level rises.
Many factors affect reactivity. If, as we have seen, the fuel temperature rises,
there are fewer neutrons to cause further fission, reactivity drops below unity
and the power level falls away. If, on the other hand, the control rods are withdrawn
somewhat, they absorb fewer neutrons. There are then more neutrons in
the core to cause fission, reactivity rises above unity and the power level rises in
consequence.
In the PBMR, two aspects of reactivity combine to ensure nuclear safety.
Firstly, the on-load fuelling regime makes it possible to design the core to perform
all necessary evolutions with very little ‘ excess reactivity ’ . If, due to
some disturbance, the power level starts to rise it will do so relatively slowly.
Secondly, the temperature coefficient of reactivity is always strongly negative.
In other words, there is no other factor to override the Doppler feedback mechanism
as there was at Chernobyl. Again, in other words, if the temperature of
the fuel rises, the power level will inevitably fall. The reactivity type of accident
seen at Chernobyl is therefore also impossible.
Such considerations justify the PBMR claim to ‘ inherent safety ’ . Fuel integrity
under accident conditions in no way depends on operators making correct