Environmental Engineering Reference
In-Depth Information
boundary leads to the classic LOCA (or small break LOCA) and the possi-
bility of eventual destruction of the core due to the inability of the reactor
to provide continued cooling to remove the still considerable heat from
the decay of residual fi ssion products (about 7 MWt after 2 min for a 1000
MWe reactor). Once breached and after the ability to provide cooling to
the core is lost, the loss of integrity of the primary system leads to the
release of fi ssion products from the core to the reactor containment build-
ing. Maintaining the integrity of the primary system is therefore considered
a key safety (and regulatory) issue.
Of equal importance is the maintenance of clean heat transfer surfaces.
Heat transfer is the reason why nuclear plants exist - ultimately to produce
electric power for sale. Heat transfer issues, usually due to deposit forma-
tion, lead to a temperature rise on the surface of the fuel and on the sec-
ondary side of the PWR steam generator. Issues other than heat transfer
are the build-up of boron in the fuel deposits in a PWR and of radioactive
materials on both BWR and PWR fuels. The general response by PWRs
has been cleaner chemistry (less Al, Si, and Ca in the primary water) and
tighter pH specifi cations. Lately both BWRs and PWRs have explored the
addition of materials such as Pt and Zn to maintain the integrity of system
components.
The factors involved in maintaining this boundary intact have evolved
over the last 50 years of reactor operation. Initially, the discipline of main-
taining nuclear plants was viewed by both the utilities and the vendors as
being similar to that of coal fi red boilers. Due to the build-up of solids on fuel
rods, primary side water specifi cations for PWRs and BWRs saw a drop in
allowed levels of dissolved solids in the order of a factor of 1000 to low ppb
levels. In the 1960s and early 1970s secondary side cooling water chemistry
for PWRs was similar to that of any coal fi red boiler which used phosphate
chemistry. As deposit build-ups on the secondary side of steam generators
occurred in PWRs (multiple tons in the early 1970s), problems began to
emerge with the cracking of tubes at the support plates and U-bends of
the steam generator tubes which necessitated the mass plugging of tubes in
highly radioactive environments and the derating of some PWRs. It was then
realized that different standards of water cleanliness would be required and
all volatile chemistry was introduced, air leaks in condensers were repaired
(and brass condensers were replaced with stainless steel ones), and feed-
water specifi cations increased dramatically. Meanwhile cracking in coolant
piping and in the welds of steam dryers (Fig. 19.4), bowing of water chan-
nels in BWRs (which would hinder the insertion of the control blades) also
began to reinforce the notion that the care and feeding of nuclear plants
required a very different approach than the care of fossil fi red plants where
boiler tubes might be routinely replaced every ten years with no worries
about radioactivity.
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