US interest in water-cooling of reactors stemmed from the Hanford reactors and was
furthered by the submarine PWRs. It was known that water allowed to boil is more
effective in removing heat, but boiling was thought likely to trigger instabilities in a
reactor core. The water in such a core serves also as moderator; if a steam bubble forms,
the local effect on reactivity is swift and its consequences difficult to predict. But
experiments in the mid-1950s demonstrated that water could indeed be allowed to boil in
a reactor core. Accordingly, a new design of reactor was developed, which is by far the
simplest in concept of all power reactors: the boiling water reactor, or BWR.
BWRs and PWRs are often mentioned in the same breath, as 'light water reactors' or
LWRs. In a BWR the water serves as moderator, reflector and coolant - and in addition,
when boiled, produces steam which is ducted directly to drive a turbo-generator. Once
through the turbines, the coolant water is condensed and pumped again into the 'boiler' -
that is, the reactor pressure vessel.
The pressure which the vessel must contain need not be much more than the pressure of
the steam being produced - usually less than half that in a PWR. Accordingly, the
pressure vessel need not be so thick. A BWR pressure vessel also includes the whole
steam collection and processing array, above the core. The control rods therefore enter a
BWR core from below. The cooling circuits of a BWR bear little resemblance to those of
a PWR. In a BWR water boils inside the fuel assemblies, and there are no external steam
generators. The consequent saving in capital cost has long been billed as a major 43
advantage of the BWR over the PWR.
Since a BWR is coupled directly to the turbine of a generating set, special provision must
be made to dispose of steam if the turbo-generator cannot for any reason accept it, or if
any malfunction should occur. A BWR is therefore enclosed - pressure vessel, attached
piping and all - inside a primary containment, which consists of a huge flask-shaped
concrete housing called, confusingly, a 'drywell'. Cavernous pipes lead from the bottom
of the drywell down into a ring~ shaped tunnel, amply large enough to walk through,
half-filled with water. This tunnel is called a 'pressure-suppression pool'. If for any reason
steam or water escapes from the reactor vessel or the pipe -work, it is confined in the
drywell and channelled down through the pipes leading into the water in the pressuresuppression pool. Any steam which gets this far is thereupon condensed, and any excess 44
pressure it would otherwise exert on the containment is - as the name suggests -
'suppressed'.
The function of the BWR containment is closely associated with that of the emergency
core-cooling systems, provided, like those in a PWR, to prevent overheating of the
reactor core in the event of an accident. Once again, we shall defer further description of
these features until Chapter 6.
Like a PWR, a BWR is refuelled off load, with the reactor shut down and cooled.
Refuelling of a BWR is somewhat more of a chore; as well as flooding the reactor well,
and unbolting and removing the lid, it is also-necessary to lift out and set aside a motley
assortment of steam-processing fittings.
Like the coolant in a PWR, the coolant in a BWR may become slightly radioactive. Since
the primary coolant in a BWR supplies steam directly to a turbine, some of the
radioactivity in the coolant may reach the turbines. However, in practice most of the
radioactivity in BWR coolant stays in the liquid water, and does not travel with the steam
to the turbine.
The BWR shares with the PWR the drawback of comparatively low coolant temperature,
and resulting inefficiency of conversion of heat to electricity. A typical BWR output
temperature is less than 300 C. On the other hand the BWR also shares with the PWR the
problems associated with relatively high power density, as we shall discuss further in
Chapter 6 . The BWR is also more susceptible to 'burn-out' or 'steam blanketing', which
arises if a layer of steam forms next to the hot fuel cladding. The low heat conductivity of
the steam means that the heat is no longer so effectively removed from the fuel, and the
fuel temperature may rise suddenly and dangerously.
Design and operation of all types of reactor must take into account the possibility of
sudden surges, called transients: temperature transients, pressure transients and so on.
This is particularly true for reactors of high power-density, like the light water reactors.
Comments
Post a Comment