The global fleet of nuclear power plants includes more than 80, which have been in operation for at least 40 years. Reactors are built with a design life of around 50 years, but advances in technology and changes in the supply chain mean that replacements for some of the components used within the nuclear and turbine islands may no longer be commercially available – they are obsolescent. Age is the primary cause of equipment obsolescence as the original equipment manufacturer (OEM) may have discontinued production of the product or no longer be operating in the industry.
Vast numbers of components go into every nuclear power plant. Some are ‘off the shelf’ while others are bespoke. Good record keeping and standardisation make for ease of maintenance management. The design records and documentation of some older stations may be patchy, which can create problems. A procurement challenge facing operators is to find a reliable source of replacements for the ageing and obsolescent parts in their plant.
Valves: variety, volume and continuity
Gate, globe, check, butterfly, control, speciality – valves come in many shapes and sizes and a nuclear power plant is likely to deploy over 1000 different ones. They may be used to stop and start flow, reduce or increase flow, control the direction of flow and regulate or relieve a flow or process pressure. The fundamental design of valves has not changed much in decades, but advances in material technology and manufacturing techniques and metallurgy have increased the pressure, temperature and frequency at which it is possible for modern valves to operate.
Valves may not be big-ticket items but the number and variety mean their maintenance is generally outsourced to specialist contractors. A $15 million contract awarded last year to refurbish main steam isolation valves and reactor recirculation cooling system valves at Mexico’s Laguna Verde 1 and 2 nuclear plants gives an indication of the scale. The two 800MW boiling water reactors (BWRs) came into operation in 1989 and 1994 respectively. In 2013 the operating lives of the stations were extended to 40 years following the completion of a $605 million upgrade programme, which retrofitted new turbines and generators.
Valves are typically made by forging or casting metals and then applying hard facings to provide resistance to wear and galling. Cobalt-based hard facing alloys are used because of their ‘weldability’ and wear resistance. However they are not suitable for use in primary circuits of nuclear power stations (breakdown releases elemental cobalt, which is transported through coolant flow streams into the fuel core where it is irradiated and converted to radioactive cobalt-60). Stainless steel-based hard facing alloys are an alternative, but they are harder to weld and less reliable at temperatures above 200°C. Recent advances in powder metallurgy have enabled the application of hard facing alloys to components without welding, enabling the application of cobalt- free hard facings.
Each valve is controlled by its own actuator, a motor mounted to a gearbox, which moves the valve at a specific time or speed or opening rate. The basic design of the mechanical part of the actuator has remained unchanged over the last three decades but rapid advances in electronics have transformed the range of potential control and communication functions of the actuator.
Refurbishment, replacement or repair?
Electro/mechanical actuators consist of a gearbox with an electric motor and electrical switchgear. Actuators were often supplied to power stations with a design life of 25-30 years as ‘fit and forget’ items. Soft parts such as oil seals and O-rings had a shelf life of between 10 and 15 years but often the actuator did not have recommended service intervals. The service life of an actuator is affected by the ambient conditions. Those operating in high temperature or pressure environments will have a shorter life than those working in temperatures of around 40 degrees.
Each actuator has a mechanical interface with the valve which defines torque, flange size and output speed. The interface with the environment must meet specifications and standards for vibration and temperatures encountered. The connection with the control system is provided by an electronic interface for communications.
Rapid developments in electronics have driven advances in control systems and state- of-the-art actuator/valve combinations are now installed in new plants. Refurbishment does offer an opportunity to add functions to existing systems, but operators may decide that remaining with the original design intent is less problematic than retrofitting
an equivalent. When comparing the cost of overhauling an existing unit with the supply and retrofitting of an equivalent unit of different design, the apparent cost benefit of the new unit may be outweighed by the cost of the technical justification, installation and plant control system alterations.
Refurbishment provides an opportunity to consider modern corrosion protection enhancements which can be effective in
the harsh environment of high temperature and steam areas such as cooling towers. For example, powder-coating processes provide protection, enabling actuators to better withstand such highly corrosive environments. Other adaptations include special lubrication or sealing which can better withstand the higher temperatures.
A workshop refurbishment of an actuator involves a full strip down, evaluation of any damaged or obsolete components, upgrade to the current design specification, rebuild and test. Where the original electrical equipment is out of production, technicians can either perform a complete re-wire and fit all new electric components or leave the wiring looms intact and just fit new electrics. Over the years actuators have incorporated increasing amounts of electronics for control and communications. Because of the speed of development in electronics small components installed years or decades ago may not be available. Good design practice means enabling new elements to replace the functionality of old electronics which are no longer available. Where appropriate, data sets such as operating parameters, speed etc can be transferred from older systems to new replacements and the data sets downloaded from the old actuator onto the new actuator, avoiding the time consuming need for new programming.
One of the challenges of refurbishing old plant valves and actuators is that at the time when they were installed there was no standardisation in mechanical adaptations for mounting valves onto their actuators. In some cases documentation is inadequate, inaccurate or non-existent and the adaptations can only be seen once they are dismounted. If the old valve is to remain in service a bespoke like- for-like replacement mounting is required to enable the new actuator to control the valve. Replacing the actuator means dismounting the electrical equipment from the valve, studying the coupling and devising a way to fit the new actuator to the old valve. This painstaking work requires skilled and experienced workshop technicians.
Maintenance models
Refurbishing old valves and actuators is a specialist area, combining a problem-solving approach with traditional engineering skills. Weir Engineering Services (WES), part of the Weir Group, repairs and refurbishes valves and actuators for the power industry at its workshop in Scotland. The company has a contract with operator EDF to deliver a major outage valve scheme across the UK’s advanced gas cooled fleet of 14 nuclear reactors. These reactors were a unique British design built in the 1970s and 80s which are expected to remain in service into the next decade. In the course of a major outage around 500 valves will be refurbished by a team of up to 100 people working on the power plant. The cost of a valve refurbishment scheme on a typical major nuclear outage is over £2 million.
Where possible, component parts are prepared in the workshop, ready for installation during a planned outage. The spares may be part of a service exchange scheme, they may be re-engineered, or possibly even recycled from
a redundant power station and refurbished before being installed elsewhere.
WES says that lead time for new valves is generally around 26 weeks while valve refurbishment can be accomplished in half that time, including data capture if the valve has no documentation. Continuing advances in technology including additive manufacture, composites and 3D printing may speed up the process in the future.
While a power station typically has hundreds of actuators (one for each valve) this is usually made up of four or five main different types, each with any number of sub types, amounting to perhaps three dozen different actuators with slight differences. Specialist supply chain companies offer service exchange programmes which reduce customer outage times. A pool of new, spare or reclaimed actuators is available and the appropriate model can be configured for a specific application to fit a plant where a requirement has been identified. If the actuators are prepared ahead of an outage
or maintenance window, the unit can then simply be exchanged which may take as little as two to three hours. The unit that has been replaced then goes back to the workshop for refurbishment and then into the pool of spare actuators in preparation for future use.
Reactive maintenance is being supplemented by more proactive approaches. Replacement of high-pressure parallel valves has traditionally involved removing pipework in order to access the damaged valve and cut it out ready for return to the workshop for repair and welding. Such valves can weigh tonnes and extracting and moving from their location can be tricky and result in considerable downtime. The development of new welding techniques along with advances in remote control and onboard camera feeds have enabled development of proactive in situ valve seat replacement where technicians work within the confines of the power plant to replace the valve seats without having to move the valves. Careful preparation is key and technicians will use a practice rig in the workshop to prepare for the in situ replacement. Developed by Doosan Babcock and WES and first offered in 1999 this halves the cost of valve replacement and reduces downtime to three or four days.
Finding replacement parts
If the original component is no longer in production, the quickest and least expensive solution is to find a commercially available equivalent component, which meets required safety and verification standards. The commercial grade dedication (CGD) process was developed by the US commercial nuclear industry and is regulated by the Nuclear Regulatory Commission. Its role is to approve existing commercial grade component for use in the nuclear industry. It was set up in response to a decline in the number of ASME Nuclear Quality Assurance (NQA-1) accredited suppliers, leading to a supply shortage of replacement items for nuclear applications.
If this is not possible, then a range of options exist. Modifying current OEM manufactured equivalent components to meet the form, fit and function of the original component may be an option. The selected modified component then needs to have its safety function verified and accepted. Depending on the nature of the modification this can be a relatively quick and inexpensive solution.
If no replacement can be found then it may be necessary to reverse engineer and fabricate a component. This means identifying technical information of the original component in order to produce a replacement. If adequate documentation does not exist then reverse engineering involves analysing the component’s geometry, design, construction and operation to identify the functional requirements of the component and design. Modern technology such as remote inspection devices, portable, coordinate measuring machines (CMM), computer-aided design (CAD) and 3D software are invaluable.
If none of these options is appropriate then a design change may be required. The new component is selected based on the plant application, but can provide new features, accuracy and benefits as a result of the newer technology. This may be expensive and there may be a long lead-time. Field changes may be required and new documentation and approvals must be put in place.
Ensuring continuity of supply
The US fleet of 99 nuclear power stations has an average age of 36 years. Over 22 reactors are over 40 years old. Ensuring the appropriate parts are in place for planned maintenance outages requires strategic procurement planning. Tools and services to manage obsolescence issues include the Proactive Obsolescence Management System (POMS); Readily Accessible Parts Inventory Database (RAPID); the Obsolete Items Replacement Database (OIRD); and the Configuration Management Interface System (CMIS).
The Proactive Obsolescence Management System (POMS) is a database with more than 12 million equipment records. Its current membership includes 130 nuclear units mainly in North America but also in South America and further afield. Managed by Rolls-Royce, the system is designed to retrieve equipment and vendor obsolescence information and match manufacturer/model numbers to identify whether possible replacement solutions are available from the OEM recommended replacements.
Areva’s Solutions Complex in Virginia is a ‘one stop shop’ to help nuclear operators tackle ageing and obsolescence challenges. The facility includes commercial grade dedication and component testing and qualification services, to verify new replacement components. The company offers integrated procurement solutions, which provide customers with safety-related equipment that is no longer available on the market.
Summary
Extending the life of ageing nuclear power plants poses particular maintenance problems. Records may be inadequate and obsolescence means that replacement parts may be hard to acquire. The problem will not go away, and operators must be assiduous in procurement planning to ensure that ageing and obsolete parts will be available at the right time. Advances in technology bring new materials and techniques into play. However in the high hazard safety-critical environment of the nuclear industry regulatory approval is required before they can be incorporated. With more
life extensions in sight, the supply chain will be called upon to respond to the challenge, developing innovative approaches and new maintenance models to keep the veteran plant
