Primary side conditions for PWR steam generators

The primary coolant in the RCS serves as a moderator and is a medium for transporting heat from the core to the steam generators. Hence, it must not endanger plant operation by the corrosion of materials and consequences thereof. Beside of the function as a moderator, the task of water chemistry can be divided into the following main points:

 Metal release rates of the structural materials should be minimal.

 The occurrence of localized forms of corrosion should be counteracted.

 The transport and deposition of corrosion products must be influenced in such a manner, that contamination of the primary coolant system is kept low.

 The deposition of corrosion products on heat transfer surfaces, particularly on fuel assemblies, should be prevented as far as possible.

 Radiolytic formation of oxygen should be suppressed.

Historically, the starting point for all discussions about the correct pH in PWR primary coolant can be found in the work of Sweeton, et al. [134], who have reported measurements of the solubility of Fe from magnetite (Fe₃O₄). These data suggested that under the conditions of a PWR primary system the optimum pH should be pH 6.9 at 300°C. At these conditions magnetite iron solubility is at a minimum and thus the transport of iron based crud should also be a minimum. Furthermore, 2 mg/kg of lithium were sufficient to achieve a pH of 6.9 at begin of cycle, BOC (≈ 900 mg/kg B for an annual cycle).

This 2 mg/kg lithium was also considered to be low enough to avoid any corrosion attack on the fuel elements.

However, later on it was recognized, that the contribution of nickel is much more important to the primary side corrosion product inventory than the iron. Further on it was found, that nickel ferrite is a major constituent. Consequently the solubility behaviour of nickel ferrite was investigated and it was found, that a pH of 7.4 at 300°C should be the solubility minimum. However, a pH of 7.4 could not be adjusted at BOC since 2 or 2.2 mg/kg lithium was at the upper specified limit in order to prevent lithium induced corrosion of the fuel element cladding.

Boron is added in the form of boric acid (H₃BO₃) as a neutron absorber for reactivity control. The boric acid concentration is changed throughout a reactor cycle to compensate for other changes in reactivity and is not varied independently. The boron levels are relatively high (1000 to 2000 mg/kg using natural boric acid) at the beginning of the fuel cycle. Then, they are gradually reduced by ~100 mg/kg per month. The concentration of lithium hydroxide (LiOH) is co-ordinated with the boric acid concentration to achieve the desired pH.

At field different B/Li chemistry treatments were and are still applied:

 Coordinated lithium/boron chemistry

 Modified lithium/boron chemistry

 Elevated lithium/boron chemistry

 Constant elevated lithium/boron chemistry.

Initially most of the PWRs were operated with coordinated B/Li chemistry treatment, i.e. the pH300

was kept constant at 6.9 during the cycle. The second mode is the modified lithium/boron chemistry, where at the begin of cycle in annual cycles a concentration of 2–2.2 mg/kg Li is used and kept constant till reaching a desired pH₃₀₀ of e.g. 7.4 and then the pH₃₀₀ 7.4 line is followed by the appropriate Li/B-coordination. The third alternative is to operate with an elevated lithium/boron chemistry where a level of 3.5 mg/kg Li is used at begin of cycle till reaching a pH300 of e.g. 7.4. At this pH the Li/B coordination is adjusted to stay at pH300 = 7.4 till the end of the cycle. The fourth alternative is the constant elevated lithium/boron chemistry where at the beginning of cycle a high Li-level of e.g. 5 mg/kg is adjusted and during the cycle a Li/B-coordination keeps the pH constant throughout the cycle. Nevertheless in all cases the fuel vendor stipulates the maximum lithium concentration.

The application of enriched boric acid (B-10 fraction of ~30%) was introduced due to changes in the core design (so called ‘high duty cores’) to achieve the criteria for reactivity control. The use of B-10 has the advantages that the target pH₃₀₀ = 7.2 to pH₃₀₀ = 7.4 is achieved earlier than using natural boric acid.

The key component concerning primary coolant water chemistry is therefore given by restrictions due to the fuel elements and dose rate reduction issues, and not the steam generators, except for Alloy 600MA SG tubes. The restrictions imposed due to the above mentioned issues are then automatically

given for the SGs and sufficiently restrictive. The only steam generator problem arising from the primary side has been PWSCC, which can mainly be completely solved by material replacement (i.e.

replace of Alloy 600MA).

Cracking on the primary side has occurred in areas where tensile stresses existed at the inner side of the tube. Such locations have been eliminated in the design of replacement steam generators. In addition, Alloy 690TT and Alloy 800NG seem to be immune against PWSCC. Regarding the corrosion resistance of the materials used up to now in steam generators it seems based on results of EPRI [135], that Alloy 690TT might have a better corrosion resistance than Alloy 800NG for some (but not all) secondary side extreme environmental cases. The slightly better corrosion resistance of Alloy 690TT is compensated by the longer operation experience of Alloy 800NG. Primary water stress corrosion cracking of Alloy 600MA and Alloy 182/82 weld metal seem to be the biggest challenge currently faced by the PWR Industry. These materials are widely used in the RCS of PWRs (see Figure 5.27, Figure 5.28, Figure 5.29) and the incidences of cracking have increased sharply in recent years. The well-known incidences for such cracks are V.C. Summer, Oconee 1 and Davis-Besse [136].

In order to get a better control about such incidents, EPRI is managing a Materials Reliability Program (MRP) [137]. This programme addresses the following aspects:

 Accessibility. Many of the susceptible locations are very difficult to access.

 Consequences. Safety and operability assessments are not available for many of the susceptible components.

 Inspection. Procedures and capabilities need to be defined for many of the susceptible components.

 Assessment. Crack growth data for these materials show a lot of scatter, the origins of which are not well understood.

 Mitigation. Stress reduction methods are available but need to be adapted for the susceptible components. No fully qualified chemistry mitigation method is available.

 Repair/replacement. Resistant materials are known and some components can be replaced, but repair options need to be defined and qualified for many of the susceptible components.

Figure 5.27. Typical Alloy 600 locations in Westinghouse plants [137].

Figure 5.28. Typical Alloy 600 locations in Combustion Engineering plants [137].

Figure 5.29. Typical Alloy 600 locations in Babcock&Wilcox plants [137].

In view of such issues, an effective water chemistry mitigation option would be highly appreciated.

Zinc addition appears to be the most promising possibility for the time being. At higher concentrations (typically 15 to 40 µg/kg) Zn has palliative effect against PWSSC, by providing more stable protective layers thus reducing the influence of tensile stresses in the corrosion process. Nevertheless, up to now it is the only chemistry related measure available to counteract PWSCC. Based on laboratory and field experience with SG Alloy 600MA tubes, it is confirmed that, zinc definitely mitigates the RWSCC initiation. However, its influence on PWSCC crack growth rate is somewhat not conclusive: Even though, mitigating effect of zinc on PWSCC crack growth rate of Alloy 600MA SG tubing material was observed at field (e.g. Diablo Canyon unit 1&2), its clear beneficial effect for reactor pressure vessel (RPV) Head penetrations could not be confirmed. Even though, mitigating effect of zinc on

PWSCC crack growth rate of Alloy 600MA SG tubing material was observed at field (e.g. Diablo Canyon unit 1&2), it is clear beneficial effect for reactor pressure vessel (RPV) Head penetrations could not be confirmed. Recently, based on the results of materials reliability programme (MRP), EPRI concluded that zinc addition, due to inconsistent and limited effect of zinc on thick walled Alloy 600MA and Alloy 182 weld metal, is not a reliable way to mitigate stress corrosion crack growth in thick walled nickel alloys [138].

Recently, based on international laboratory tests, it was found that coolant Dissolved Hydrogen (DH) concentration influences significantly the PWSCC of Alloy 600MA. Based on investigations sponsored by EPRI, PWSCC crack growth rate is at maximum in the range of 20–30 cc/kg DH concentration. At DH concentrations either less than 15 cc/kg or above 40 cc/kg, the PWSCC crack growth rate decreases. In addition to these experiences, Studvik test results confirmed that the PWSCC initiation is insignificant at DH concentrations below 15 mg/kg, however, it increases with increasing DH concentrations. Therefore, the intended strategy regarding the DH concentration to mitigate PWSCC of Alloy 600MA, Alloy 182/82 welds is contradictory: Whereas, EPRI recommends to increase the DH concentration to about 60 cc/kg [138] based on the assumption that PWSCC cracks already exist, Japanese PWR industry intends to decrease the DH concentration below 25 cc/kg (lower specification value) to avoid the PWSCC initiation. All these strategies are still under discussion and a clear common strategy is not established yet.

Although zinc was injected in the beginning to suppress the PWSCC risk, it was found, that low concentrations of approximately 5 µg/kg in the primary coolant reduces the dose rate build up. Zn injection results in incorporation of zinc into oxide layers (see Figure 5.30). Thus zinc can delay the incorporation of activated products (e.g. radio-cobalts) in the system surfaces, and even displace already embedded nuclides out of the oxide layer, which is corroborated by an increase of radionuclide concentration in the coolant associated with start of Zn injection. Many units nowadays use zinc as a dose rate reduction measure.

Figure 5.30. Incorporation of zinc in oxides layer.