PLENTIFUL ENERGY

It is suspected that the lead corrosion problem was the main culprit in shortening
the life of the Alfa-class submarines. A prototype Alfa-class was launched in 1972,
but a coolant freezing accident destroyed the reactor and it was dismantled in 1974.
Six additional of the Alfa-class were launched between 1977 and 1983. Eventually,
four of seven Alfa-class submarines experienced reactor failures. [4-5] In 1982, for
example, a steam generator failure leaked about two tons of lead-bismuth into the
reactor compartment, damaging the reactor irreparably.
Lead alloys will be considered as an option if a very high outlet temperature is
required for specific advanced applications, always providing the corrosion problem
is satisfactorily solved. However, for conventional power reactor applications, its
disadvantages make it unlikely to replace sodium as the coolant of choice.
14.2.2 Helium Gas
Gas-cooled fast reactors were considered in early days but abandoned in the late
‗70s. However, the interest in helium gas cooling has resurfaced in recent years. [2]
Thermophysical properties of helium are also listed in Table 14-1. The specific heat
of helium is about four times that of sodium, but the density is very low—five
thousand times less. The volumetric heat capacity, therefore, is less by about three
orders of magnitude. Even if helium is pressurized to 100 atmospheres, the coolant
velocity would have to be increased by a factor of ten for the typical lattice design
parameters used in sodium cooling. The helium coolant velocity required is in the
100 m/s range, introducing the risk of flow-induced vibration, and attention to this
is necessary in the reactor‘s structural design. The thermal conductivity is very
low—four hundred times less than that of sodium—so the heat transfer is poor even
at high coolant velocity and the cladding surface needs to be roughened to increase
the effective surface for heat transfer.
Certainly, from a thermal-hydraulics point of view, helium is not a good coolant
for the high specific power of fast reactors, and justification for its use would have
to come from elsewhere. One rationale for considering a helium-cooled fast reactor
stems from the fact that it could be a natural extension of thermal-spectrum hightemperature
gas-cooled reactors. For thermal-spectrum reactors, graphite is
employed as moderator and the large volume of graphite in all such reactor designs
provides a passive heat sink lasting for a period of days in the event of loss of
coolant, for example. For fast reactors, such a graphite heat sink is not possible;
coolant must be available, and active safety systems have to be relied upon to shut
down the reactor and remove the decay heat. For a high-pressure, high-velocity
helium coolant system, loss of coolant flow and even loss of pressurization are
credible possibilities. Even for protected (with scram) transients, in the fast reactor
an active emergency cooling system might be required to prevent core melting.
Anticipated transients without scram will result in core disruptive accidents.
305