Civil Engineering Reference
In-Depth Information
Fig. 3.19
Cruciform
absorber element of a
boiling water reactor core
with a 7
7 fuel element
[
14
]
Steam is generated by water boiling in the upper part of the reactor core. To
provide sufficient core flow for ample heat transfer, BWRs employ internal
recirculation pumps which increase the core flow rate. The core with the absorber
plates is contained in a large steel pressure vessel (Fig.
3.20
). Above the core, there
are the steam separators and steam dryers. The reactor vessel head can be removed
for loading and unloading of fuel elements. A BWR pressure vessel has a diameter
of 6.6 or 7.1 m, a wall thickness of roughly 160 mm, and a height of 23 m. The
Kraftwerk Union BWR-1,300 and the AREVA SWR-1,000 (KERENA) pressure
vessels are made of, e.g. 22NiMoCr37 steel, with the inside being plated with
austenitic stainless steel.
The saturated steam flows from the reactor pressure vessel directly to the turbo-
generator system and is pumped back from the condenser and feedwater tank to the
pressure vessel. The condenser is cooled by water from a cooling tower or from a
river (see Fig.
3.17
).
A question of particular interest in BWRs with direct cooling systems is the
radioactivity of the cooling water. The amount of radioactivity is determined by the
impurities contained in the water and by an (n,p)-reaction of oxygen producing
nitrogen, N-16, with a half life of 7.2 s. Many years of experience in operating
boiling water reactors have shown that, because of the short half-life of the N-16,
maintenance work on the turbine, the condenser and the feed water pumps is not
impaired significantly by radioactivity.
The water circulation in the reactor pressure vessel and through the core can be
used for changing the reactor power. Reduction of water flow through the core will
result in a higher evaporation rate and in a larger volume of bubble formation.
Increasing the volume of steam in the core reduces the moderation of the neutrons,
and as a consequence, the criticality or effective multiplication factor k
eff
(see
Chap.
2
) and the reactor power will be reduced. In this way, changes in the water
