Page 328 - Corrosion Engineering Principles and Practice

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the manufacture of marketable products, simply do not give adequate
technical attention to the details of boiler feedwater chemistry. Many
chemical converters, catalytic converters, or other waste heat boilers have
failed from steam-generating side corrosion due to such inattention.

8.5.5 Nuclear Boiling Water Reactors
In a nuclear boiling water reactor (BWR), the nuclear fuel boils the
water and the steam goes directly to the turbine (Fig. 8.18). Temperatures
are approximately 230 to 290°C, and with saturated steam from 2.8 to
7.2 MPa. As with conventional boilers, silica and copper content of the
water must be kept low to prevent their transport by steam and
deposition on the turbine. Additionally, high-purity neutral water
must be used because of the possible deposition of solid chemicals on
the fuel elements and the stripping of volatile components from any
chemicals that would otherwise be used in water treatment.
Although the water and steam contain appreciable amounts of
oxygen, carbon steels and stainless steels exhibit equivalent or
superior corrosion resistance to that observed in pressurized water at
the same temperatures. However, special problems do exist. Stainless
steel fuel cladding has failed by SCC in boiler water reactors.
SCC has also occurred in sensitized stainless steels (i.e., those
with chromium carbide precipitation at the grain boundaries) as a
result of heating the steel in the 425 to 825°C range by welding or
stress relieving. For this reason, Zircaloys are the preferred cladding
materials. However, care must be exercised that alkaline components
do not enter the water and become concentrated by boiling on the
cladding surface because Zircaloys have poor corrosion resistance in
high-temperature caustic solutions.

Containment structure
Steam (nonradioactive)
Nuclear
reactor
Generator
Core
Turbine

Control Condensor cooling water
rods (Nonradioactive) Cooling
tower

FIGURE 8.18 Boiling water reactor.