Hitachi’s Efforts in the Nuclear Sector (4)Injection of Noble Metal to Prevent SCC in Reactor Internal Structures

1. Introduction

Japan’s Seventh Strategic Energy Plan¹⁾ indicates that nuclear energy is to provide around 20% of electricity generation in 2040 to help achieve carbon neutrality. This will require that the operating life of existing nuclear power plants be extended and their capacity factor improved. One of the concerns associated with this scenario is the stress corrosion cracking (SCC) in the structural materials of reactors²⁾. Common practices adopted in the past to address SCC have included the use of corrosion-resistant materials and techniques such as water jet peening (WJP) for improving residual stresses. At boiling water reactors (BWRs) in the USA, however, the injection of hydrogen and noble metals into reactor water to alleviate the corrosive environment, known as On-Line NobleChem^* (OLNC), has successfully suppressed the initiation and propagation of SCC in reactor internals³⁾. It is hoped that the safety of BWRs in Japan can likewise be improved by adopting the technique.

This article summarizes OLNC and its central role as an environmental mitigation technique for inhibiting SCC, also describing how the technique has performed in the USA, the other new technologies developed for its use, and the outlook for the future.

*

On-Line NobleChem is a trademark of General Electric Company.

2. OLNC for Inhibiting SCC

It is known that SCC can be prevented by keeping the electrochemical corrosion potential (ECP) of stainless steel below -230 mV relative to the standard hydrogen electrode (SHE)⁴⁾. While hydrogen injection has been used in BWRs since 1979, keeping the ECP of reactor internals below the target requires that the feedwater have a hydrogen concentration of more than 1 ppm (see Figure 1). While injecting hydrogen at high concentrations keeps the ECP low, it also changes the chemical form of the ¹⁶N formed in the reactor by irradiation and increases the fraction present in the main steam feed. This has the side effect of making the turbine more radioactive, thereby increasing exposure for operators performing inspections⁵⁾. If instead a noble metal is injected into the reactor so that it adheres to the surface of structural materials, its catalytic properties will efficiently reduce the ECP even with a low concentration of hydrogen in the feedwater of about 0.2 to 0.3 ppm. As the increase in turbine radioactivity is negligible for hydrogen concentrations below 0.5 ppm in the feedwater, this overcomes the above problem of increased radiation exposure. That is, use of OLNC makes it possible to inhibit SCC without any major side effects. The following sections summarize how the injection of noble metal works and describe the associated technology developments.

Figure 1—Differences in Corrosion Potential Reduction Behavior for Hydrogen Injection and Noble Metal InjectionPLR: primary loop recirculation system, SHE: standard hydrogen electrode, OLNC: On-Line NobleChemAchieving the corrosion potential of -230 mV vs. SHE assumed necessary for inhibiting SCC by means of hydrogen injection requires increasing the amount of hydrogen injected via the feedwater. In contrast, use of noble metal injection can significantly reduce the amount of hydrogen injection needed.

2.1 Overview of OLNC

Noble metal injection was first used in US plants in 1996. While the initial practice was to inject aqueous solutions containing platinum (Pt) and rhodium (Rh) during plant shutdowns, this was changed to OLNC in 2006 to avoid the loss of plant utilization that resulted from the two days or so needed for the injection work. OLNC works by injecting an aqueous solution that contains Pt only and can be performed when the plant is operating at its rated output. Most US plants currently use OLNC⁵⁾.

Figure 2 shows the system configuration for using OLNC. With OLNC, the injected Pt solution is diluted with purified make-up water (MUWP) from the feedwater. A mitigation monitoring system (MMS) is also installed in the clean-up water (CUW) system to determine how much of the injected Pt has been deposited and the extent to which the ECP has been reduced. Similarly, an inductively coupled plasma mass spectrometer (ICP-MS) is used to make highly sensitive measurements of the concentration of Pt in the reactor water during application.

Limits are also placed on when OLNC injection can be performed. This is done to maintain fuel integrity when newly loaded and to reduce exposure during routine inspections. That is, to get the Pt to adhere to the surface of structural materials, the Pt aqueous solution is injected over an approximate two-week period at roughly one-yearly intervals, and this is done from two months after plant operation starts until three months prior to the next shutdown. Reactions catalyzed by the Pt deposited on surfaces promote the recombination of oxygen and hydrogen in the cooling water. Provided that the hydrogen/oxygen (H/O) mol concentration ratio remains above 2, the conditions in the water should keep the corrosion potential at material surfaces close to the theoretical potential of Pt (approximately −500 mV vs. SHE), which is below the -230 mV vs. SHE threshold at which SCC should be inhibited.

Figure 2—System Configuration when Using OLNCMUWP: make-up water (purified), F/D: filter/demineralizer, CUW: clean-up water system, ICP-MS: inductively coupled plasma mass spectrometer, MMS: mitigation monitoring system, ECP: electrochemical corrosion potential, SCC: stress corrosion crackingAdoption of OLNC also involves the use of equipment such as the Pt injection system, the MMS for quantifying Pt deposition amount and the reduction in ECP, and an ICP-MS for measuring the very low levels of Pt present during application.

2.2 Technology Development for Installation in Japan

Although OLNC is widely used in the USA, issues remain with the technology. These include the problem of differential pressure increase in the injection line during application, flow-accelerated corrosion (FAC) in the CUW pipes, and how to extend the life of ECP sensors. The following sections describe the work undertaken by Hitachi GE Vernova Nuclear Energy, Ltd. to address these issues.

1. Dealing with differential pressure rise in the injection line

1. Preventing FAC in CUW pipes

1. ECP sensor development

ECP is measured to provide an indication of how well environmental mitigation is working, including at plants in Japan that use hydrogen injection. While a variety of different types of sensors can be used to measure ECP, each has its own advantages and disadvantages. In practice, Pt sensors, iron/iron-oxide sensors (Fe sensors), and stainless steel (SS) sensors are used in tandem (see Table 1). While Pt sensors have a comparatively long life and provide reliable readings when the H/O mol ratio of hydrogen and oxygen is 2 or higher, they do not provide correct values when the H/O mol ratio is below 2. With Fe sensors, meanwhile, although they provide accurate measurements regardless of the H/O mol ratio, they suffer from a comparatively short sensor life. When noble metal injection is used, SS sensors show the same ECP as Pt sensors and there is no way of telling whether a SS sensor has failed when used in tandem with a Pt sensor. Hitachi GE Vernova Nuclear Energy has been working on improving the life of Fe sensors, successfully developing a sensor with an expected life of four years or more in an operating plant based on benchmark testing conducted under accelerated conditions at the US Electric Power Research Institute¹¹⁾. While ECP measurement in the past has been performed at the reactor water temperature of 288°C found in plants operating at rated output, there is also work on the development of zirconium (Zr) sensors and the potential for expanding the use of Fe sensors to meet the emerging demand for measurement at temperatures of between 150°C and 200°C that occur in the CUW system or during reactor startup.

Table 1—Pros and Cons of Different ECP SensorsSS: stainless steelWhile a variety of different ECP sensors exist, each has its own advantages and disadvantages. Recommended practice is to use a number of different sensors to provide redundancy.

3. Conclusions

OLNC inhibits SCC of reactor internals by mitigating the corrosive environment. At BWR plants in the USA, it is also helping to improve plant utilization in addition to enhancing the reliability of reactor structures. For BWR plants in Japan, meanwhile, the goals are to get plants restarted and then to keep them running for long periods with high utilization. Here, the use of OLNC is seen as desirable both to further improve nuclear power plant safety and to contribute to achieving carbon neutrality. Having analyzed how OLNC has performed in the USA and the issues it has raised, Hitachi GE Vernova Nuclear Energy has been supporting the smooth introduction of the technology into nuclear power plants in Japan. This has included the development of Pt nanoparticles as a measure for minimizing the differential pressure rise in injection lines, a technique for assessing FAC in CUW pipes when environmental mitigation measures are in use, and long-life ECP sensors that will be essential for assessing how well environmental mitigation is working.