Pressurized Water Reactor
Combustion Engineering Design
Notes from 2007

==Secondary Water Objectives==
1. Minimization or elimination of secondary side corrosion
2. Minimization of corrosion-product transport and accumulation 
within steam generators, and
3. Minimization of impurities in the steam
Why?
 - Protect the public
 - Maximize plant availability
 - Maintain long term plant and component integrity

===Maintain Steam Generator (SG) Integrity and Longevity===
 - Very agressive environment
 - Boiling concentrates impurities
 - High temperature accelerates corrosion rates

Secondary system has a large surface area of carbon steel
Carbon Steel -> Corrosion -> Transports Iron Oxide -> SG

Oxides of Iron do not induce or accelrate localized corrosion 
but they do:
 - provide flow starved regions (in which  corrosive materials 
can concentrate)
 -reduce heat transfer
Sludge piles concentrate aggressive species 
 -lead to piting
 -intergranular corrosion
 -stress corrosoin cracking

2.0 part per billion (ppb) Fe in SG feedwater (fw) -> 23# Fe / 
month
 -or 100# sludge to each SG each year (2 SGs per unit)

===Secondary System Components===
Feedwater Heaters - Material of Construction (Stainless Steel)
U1 Condenser Tubes - MoC (2 of 6 are Titanium other Stainless 
Steel)
*Plant modification since these notes were written has replaced 
2 more so that 4 of 6 are Titanium
U2 Condenser Tubes - 6 are Titanium
Condenser Tube Sheet is made of copper-nickel and is exposed to 
steam
Steam Generators - MoC Alloy 690 SS; design minimizes crevices

===Secondary System pH Control===
Corrosion of carbon steel is directly related to the solubility 
of iron which in turn is strongly dependent on the pH at temp. 
The solubility curve is a bath tub curve with the local minimum 
at pH25C 9.8 SU.
Iron Solubility
pH25C < 9.3 Increased solubility noticeably
pH25C = 9.8 minimum
pH25C > 10.5 Increased solubility noticeably

To minimize the accumulation of iron oxide corrosion products 
and achieve the industry prescribed limit for corrosion product 
transport in EPRI guidelines, the feedwater iron concentration 
must be maintained below 5 ppb. Operation at high pH in 
all-ferrous plants particularly with an amine treatment that 
provides better protection against corrosoin in regions of 
two-phase flow will permit achievement of this 5 ppb iron limit.