Page 329 - Corrosion Engineering Principles and Practice

300 C h a p t e r 8 C o r r o s i o n b y W a t e r 301

Boiling water reactor designs can be modified to accommodate a
superheater to raise the steam temperature for improved efficiency.
Experimental nuclear superheaters have been built to superheat the
steam from the boiling water reactor. Corrosion rates for stainless steels
and nickel alloys under heat transfer in the 600°C temperature range are
about 25 µm/y. However, experience has shown that even minute
amounts of entrained moisture in the entering steam can carry sufficient
chlorides that cracking of the stainless steel cladding of the nuclear fuel
elements may ultimately ensue. Alloys with higher nickel contents (e.g.,
N08800, N06600, or N06690) have resisted SCC under these conditions.

8.5.6 Nuclear Pressurized Water Reactors
In a pressurized water reactor (PWR) (Fig. 8.19), high-purity water is
pumped past nuclear fuel elements where it becomes heated. The heated
water then goes to the tube side of a heat exchanger, giving up its heat to
boil water on the shell side. Steam is generated from the boiler water.
The water adjacent to the core is called primary water. It is usually
treated with hydrazine to remove oxygen, with lithium hydroxide or
ammonium hydroxide to pH 10 to 11 to minimize corrosion product
transport, and with 25 to 50 cc hydrogen per kg of water to suppress
radiolytic decomposition of water (thereby minimizing corrosion
product transport).
The nuclear fuel must be clad with a corrosion-resistant material to
prevent the release of radioactive gases and fission products to the
primary water. The fuel itself is usually uranium oxide, which is quite
resistant to the primary water. The fuel is clad with austenitic stainless
steel or Zircaloy-2 (R60802) or the extra-low nickel Zircaloy-4 (R60804).

Containment structure
Steam (nonradioactive)
Steam
Control generator
rods
Nuclear
reactor Generator

Turbine

Core
Condensor cooling water
Radioactive water (nonradioactive)
Pump
Cooling tower
FIGURE 8.19 Pressurized water reactor.