A Critical Review of Radiolysis Issues in Water-Cooled Fission and Fusion Reactors: Part II, Prediction of Corrosion Damage in Operating Reactors

Abstract

The radiolysis of water is a significant cause of corrosion damage in the primary heat transport systems (PHTSs) of water-cooled, fission nuclear power reactors (BWRs, PWRs, and CANDUs) and is projected to be a significant factor in the evolution of corrosion damage in future fusion reactors (e.g., the ITER that is currently under development). In Part I of this two-part series, we reviewed the proposed mechanisms for the radiolysis of water and demonstrate that radiolysis leads to the formation of a myriad of oxidizing and reducing species. In this Part II, we review the role that the radiolysis species play in establishing the electrochemical corrosion potential (ECP) and the development of corrosion damage due to intergranular stress corrosion cracking (IGSCC) in reactor PHTSs. We demonstrate, that the radiolytic oxidizing radiolysis products, such as O2, H2O2, HO2−, and OH, when in molar excess over reducing species (H2, H, and O22−), some of which (H2) are preferentially stripped from the coolant upon boiling in a BWR PHTS, for example, renders the coolant in many BWRs oxidizing, thereby shifting the ECP in the positive direction to a value that is more positive than the critical potential (Ecrit = −0.23 Vshe at 288 °C) for IGSCC in sensitized austenitic stainless steel (e.g., Type 304 SS). This has led to many IGSCC incidents in operating BWRs over the past five decades that has exacted a great cost on the plant operators and electricity consumers, alike. In the case of PWRs, the primary circuits are pressurized with hydrogen to give a hydrogen concentration of 10 to 50 cm3/kgH2O (0.89 to 4.46 ppm), such that no sustained boiling occurs, and the hydrogen suppresses the radiolysis of water, thereby inhibiting the formation of oxidizing radiolysis products from water. Thus, the ECP is dominated by the hydrogen electrode reaction (HER), although important deviations from the HER equilibrium potential may occur, particularly at low [H2]. In any event, the ECP is displaced to approximately −0.85 Vshe, which is below the critical potential for IGSCC in sensitized stainless steels but is also more negative than the critical potential for the hydrogen-induced cracking (HIC) of mill-annealed Alloy 600. This has led to extensive cracking of steam generator tubing and other components (e.g., control rod drive tubes, pressurizer components) in PWRs that has also exacted a high cost on operators and power consumers. Although the ITER has yet to operate, the proposed chemistry protocol for the coolant places it close to a BWR operating on Normal Water Chemistry (NWC) without boiling or, if hydrogen is added to the IBED-PHTS, close to a BWR on Hydrogen Water Chemistry (HWC). In the current ITER technology, the concentration of H2 in the IBED-PHTS is specified to be 80 ppb, which is the concentration that will be experienced in both the Plasma Flux Area (PFA) and in the Out of Plasma Flux Area (OPFA). That corresponds to 0.90 cc(STP) H2/KgH2O, compared with 20–50 cc(STP) H2/KgH2O employed in a PWR primary coolant circuit and 5.5 to 22 cc(STP) H2/KgH2O in a BWR on hydrogen water chemistry (HWC). We predict that a hydrogen concentration of 80 ppb is sufficient to reduce the ECP in the OPFA to a level (−0.324 Vshe) that is sufficient to suppress the crack growth rate (CGR) below the practical, maximum level of 10−9 cm/s (0.315 mm/a) at which SCC is considered not to be a problem in a coolant circuit but, in the PFA, the ECP is predicted to be 0.380 Vshe, which gives a calculated standard CGR of 2.7 × 10−6 cm/s. This is more than three orders in magnitude greater that the desired maximum value of 10−9 cm/s. We recommend that the HWC issue in ITER be revisited to develop a protocol that is effective in suppressing both the ECP and the CGR in the PFA to levels that permit the operation of the IBED-PHTS in accordance with the experience gained in fission reactor technology.

A Critical Review of Radiolysis Issues in Water-Cooled Fission and Fusion Reactors: Part I, Assessment of Radiolysis Models

“…The concentrations of the major radiolysis products (H 2 , O 2 , and H 2 O 2 ) were used to estimate the ECP and CGR for IGSCC in sensitized Type 304 SS and, to the authors' knowledge, Macdonald et al [216,218] were the first to have calculated the ECP and CGR using the MPM [217] and the CEFM [225], respectively. The CGR was then used to estimate the accumulated corrosion damage (crack length vs. time) as the reactor moves along the corrosion evolutionary path (CEP) [216,[225][226][227][228][229][230][231][232][233][234][242][243][244], as discussed in detail in Part II [291]. These same calculations should be made for ITER because they provide a rational basis for specifying the optimal water chemistry protocol for minimizing corrosion damage and activity transport.…”

“…The features described above have, in general, been predicted and observed for all fourteen BWRs that have been modeled in our group over the past three decades, with only relatively minor differences between different reactors. In Part II of this review [291], data of the type shown in Figures 13-15 are used to predict the ECP and CGR for IGSCC in sensitized Type 304 SS in operating plants. It is shown that HWC can have a significant impact on the mitigation and the accumulation of IGSCC damage in certain locations (e.g., RS, LP) but not in others (e.g., UD) and that the reasons for the differences are well understood in terms of the electrochemistry of the system.…”

“…Thus, the strategy for avoiding future corrosion damage requires the use of a water chemistry protocol together with an appropriate alloy that places the ECP as being more negative than the critical potentials for PC, CC, SCC, and CF, but more positive than the critical potential for HIC. A discussion of this important topic is beyond the scope of this review, but the principles supporting this approach will be discussed in Part II [291].…”

“…Accordingly, H 2 O 2 is much more aggressive towards general and localized corrosion phenomena than is O 2 . We argue in Part II [291] that the prudent approach to managing water chemistry in the mitigation of corrosion would be to place a limit on the electrochemical corrosion potential (ECP), as has been done in the operation of BWRs.…”

“…For the present purposes, the "standard crack" is defined as one having a crack length of 0.5 cm loaded to a stress intensity factor of 25.0 MPa•m 1/2 . The "standard crack" concept is addressed further in Part II of this series [291]. As seen in Figures 36 and 37, stabilization of the concentrations of the radiolysis species already occurs after the first cycle.…”