Optimising water chemistry30 July 1998
A recent EPRI report* describes how water chemistry is moving from the general to the specific.
The competitive pressures on operators as the power market is de-regulated are leading them to take measures to reduce O&M costs and extend the life of major components. Corrosion is particularly relevant in this regard – historically this has caused substantial losses of power generation, resulted in large repair and maintenance costs, and contributed to significant personnel radiation exposures. The control of plant water chemistry is the main tool available for limiting or preventing corrosion problems.
As concern over water chemistry grew, a number of industry-wide actions were initiated. Among EPRI-led activities is the development of Water Chemistry Guidelines which were originally introduced to standardise water chemistry regimes in PWR primary, PWR secondary and BWR systems. As new technology develops and operational experience accumulates, the guidelines are continually revised. Their focus has shifted from a prescriptive set of specifications to true guidelines for developing site-specific optimised water chemistry programmes.
Given this revision process, EPRI felt that it was time to prepare a single overview document which provides an historical basis for both the technical advances which have taken place in the industry and the change in philosophy that has driven the revisions to the guidelines.
According to EPRI, this document, Plant specific optimization of LWR water chemistry, meets a number of needs and addresses a number of audiences. This includes, “utility managers who need to understand the importance of water chemistry to the operation of LWRs, regulators and other industry custodians who need to understand the basis for the evolutionary nature of the guidelines, and the technical staff in the industry who need an historical basis for the current water chemistry guidelines.”
The report, prepared by EPRI staff, provides an historical summary of corrosion problems driving changes in water chemistry programmes, followed by the latest strategies for controlling corrosion and radiation fields. It reviews the major developments in the field, with emphasis on water chemistry optimisation principles.
As indicated above, the first versions of the EPRI guidelines were prescriptive; impurity limits were specified and few treatment options were available. As treatment programmes were developed to address emerging corrosion and radiation field control issues, the guidelines were revised to allow utilities to select the most applicable options for their plants.
More recently, the guidelines have reflected a trend towards plant specific optimisation. Utilities must not only select among the appropriate control options and strategies, but also optimise or tailor the water chemistry program based on site specific considerations as well as one that is most suitable to their philosophy and long term strategy. This includes cost considerations as corrosion is a major contributor to the O&M costs.
This has also led to a refocusing of R&D which has shifted from the development of more expensive, prescriptive solutions to corrosion problems to the development of cost effective mitigation options.
Evolution of chemistry control
Ideally, a water chemistry specification should be established that minimises all adverse effects. Of course, since each of the major systems in LWRs contains a number of different materials and are exposed to varied environmental conditions, no one chemistry can be best for the entire system. This means that water chemistry parameters must be selected to mitigate the most important problem areas, without worsening less significant problems. In practice such a conflicting requirement demands an optimised strategy or approach.
The concept of optimisation applied to mitigation strategies means that the overall economic impact on the entire plant is considered when pursuing a given strategy. Much emphasis is being focused on developing tools which the operators can use to perform these evaluations.
The philosophy behind the guidelines is also changing as chemistry becomes more complicated. Instead of “one rule for all plants”, the guidelines have to accommodate several options. The previous panel shows the various approaches available.
Overall, the US utility industry has been quite successful in reducing the impact of corrosion in nuclear power plants in recent years. However, with the growing competitive pressures, determining the optimum chemistry programme has become more crucial than ever for the successful operation of a plant, and it will become even more complicated in the future.
Some technical developments on the horizon will help, particularly the move towards automated chemistry control and expert systems. On-line monitoring techniques are improving and computerised data trending is widely used successfully. The CHECWORKS family of computer codes are used for predicting corrosion, especially flow-accelerated corrosion, and the EPRI chemWORKS family of computer codes is used for determining and optimising water chemistry strategies.
|Range of water chemistry treatments|
|The water chemistry problems that nuclear plants face include: • In PWRs, problems affecting steam generators, the largest single source of output loss in PWRs, were primarily due to intergranular attack/stress corrosion cracking (IGA/IGSCC). • In BWRs, stress corrosion cracking of recirculation piping systems was the major source of output loss 10 years ago; the main area of concern is now IGSCC and irradiation assisted stress corrosion cracking (IASCC) of reactor internals. • On occupational radiation exposures, poor water chemistry can significantly increase radiation fields, such as through the transport of active corrosion products. The range of treatments now available include the following: PWR secondary side In contrast to the near-universal “all-volatile treatment” (AVT) of a few years ago, the plant chemist now has choices of boric acid, high hydrazine, morpholine, several advanced amines, and additional corrosion inhibitors such as titanium oxide. Attention is focused on controlling chemistry in the crevices of the steam generator, which has resulted in recommendations on molar ratio control. The optimum chemistry in local regions where concentrated solutions form is a slightly alkaline pH and a reducing condition. However, controlling the bulk water chemistry to produce such a local environment is challenging. Additionally, interactions with other parts of the system must be considered. PWR primary system The main interactions in PWR primary system are between the requirement to maintain radiation exposures as low as reasonably achievable, which in turn requires low radiation fields, the need to minimise stress corrosion cracking of alloy 600 tubing and penetrations, and the need to avoid excessive oxidation of Zircaloy fuel cladding and axial offset anomaly. In this case, pH and lithium and boron are the important parameters. Elevated lithium is required to maintain pH in the presence of high boric acid concentrations at start of cycle, which could impact tube integrity. If pH is reduced, crud will build up on the fuel, possibly increasing cladding temperatures at a time when fuel corrosion margins are being eroded away by higher burnups. Enriched B-10 boric acid is an option, but this has a heavy up-front investment. Clearly chemistry issues need to be fully considered in determining fuel cycle strategy. BWR systems For BWRs the prime need has been to control IGSCC in the recirculation piping system, but here again the requirements of fuel integrity and radiation control must be considered. Electrochemical potential is the most significant parameter, but metallic impurities/additives are also important. High purity oxygenated water chemistry may still be the choice of some plants. However, there is a growing rate of implementation of hydrogen water chemistry, originally used to control stress corrosion cracking of recirculation piping, and now being applied (with higher hydrogen concentrations) to reduce cracking of internal components in the reactor vessel. Options now available to deal with these issues include, different rates of hydrogen injection, oxygen addition to control flow- assisted corrosion in the feedwater system, zinc injection (including depleted zinc-64 to minimise radioactive zinc-65 formation) and iron/nickel control to minimise radioactivity transients at shutdown, depending on specific plant design features. A new water chemistry technology, Noble Metal Chemistry (NMC), has been demonstrated at one US BWR with others to follow. The advantage of NMC is that protection can be provided to in-core components at much lower hydrogen concentrations.|
|EPRI tools up for controlling water chemistry|
|EPRI carries out water chemistry R&D programmes in several areas, including the development of software tools for plant-specific chemistry management programmes. It is now setting up a Chemistry Data Center as a central repository for historical chemistry data. EPRI chemWORKS™ The EPRI chemWORKS family of codes was introduced four years ago in response to the need for tools which the plant chemist and engineer can use to optimise water chemistry. These codes were well received by the industry and have been incorporated into the day to day duties of many plant chemists. The individual programs that comprise EPRI chemWORKS is shown in the table. Codes have been developed to address BWR chemistry as well as PWR Primary and Secondary chemistry. Year 2000 compatibility testing of all the chemWORKS codes is currently underway. A users Group was formed in 1995 in which there are now 28 members who discuss plant use of the currently released codes, make suggestions for future code enhancements and preview upcoming releases. EPRI will continue to identify and write new modules as needs emerge. New modules are being considered in the area of BWR iron control and amine optimisation. EPRI SMART chemWORKSTM The next stage of EPRI chemWORKS is rolling out this year. This involves the integration of EPRI chemWORKS with plant chemistry data management systems (CDMS) and real time expert data analysis techniques. This product, dubbed EPRI SMART chemWORKS, is currently being alpha tested at the Waterford 3 station where its capability of providing on-line assistance to the plant chemists is being assessed. These systems will provide real time plant and chemistry data to an intelligent chemistry diagnostic and management system integrated with EPRI chemWORKS. EPRI SMART chemWORKS promises improved diagnostics and reduced sampling requirements translating to lower O&M costs and fewer forced outages. Over the last year, chemistry data management systems developed by Duke Power and Entergy have been installed at the PWRs and BWRs of ten member utilities under a tailored collaboration agreement. Modifications are being made to the CDMS codes to allow seamless integration with EPRI SMART chemWORKS. Two additional installations will take place this year, one at a BWR and one at a PWR. An EPRI report will be published this year detailing the technology and describing the specifications for communication between SMART chemWORKS and chemistry data management systems. The SMART chemWORKS system will be installed at approximately 24 plants next year. Commercialisation of this technology will make it available to utilities worldwide by mid 1999. There is now a chemWORKS web site on the internet (www.smartchemworks.com) which offers up to date information on the codes, as well as technical support. Chemistry Data Center EPRI is planning on forming a Chemistry Data Center (CDC) in 1999. This center would act as a central respository for historical chemistry data from all nuclear plants. Reviews of historical chemistry data versus corrosion experience, or dose rates is a critical step in determining the cause and effect relationships for many of our water chemistry control strategies. In the past, these efforts have been costly due to the large effort required to assemble the necessary data. Additionally, the efforts of utility staff have been duplicated in many instances to provide the same data for the many studies we perform. The goal of the data center effort is to collect the data once from each utility, store it in an organised and retrievable manner, and make it available to the industry projects that have a need for historical data. Phase 1 of this project is already underway. Sponsored by the SGMP, the infrastructure for the chemistry database was put in place and secondary chemistry data collection began in 1997. Phase 2 (primary chemistry data) and Phase 3 (BWR chemistry data) are envisioned to begin next year.|