Under pressure - extending nuclear plant service-life
Brian Hall, Materials Project Engineer at the Westinghouse Materials Center of Excellence, USA, describes new initiatives to extend nuclear plant service-life.
Safe operation of the existing fleet of nuclear plants is of paramount importance to the nuclear industry. As these plants age, and as their operating licenses come up for extension, a better understanding of structural integrity becomes even more important. Materials scientists are at the forefront of helping to evaluate and care for this nuclear fleet through a variety of techniques.
One current area of focus is the nuclear reactor pressure vessel (RPV), which is a key safety-related pressure boundary component of nuclear power generating systems. In a pressurised water nuclear reactor, which is one of three types of light water reactors (LWRs), this boundary must retain pressurised water-based coolants operating at temperatures of around 300°C for many years. Currently, such plants that operate around the world were generally designed for 40 years of life, although in many regions there are strong initiatives to extend licences up to 60 years. Confirmation of the pressure-retaining capabilities of the RPV under such circumstances is crucial in supporting the case for extension of plant licences.
In general, the RPVs of LWRS are fabricated from thick section, low-alloy steel forgings or formed plates that are welded together to form the cylindrical pressure vessel. It has been known for many years that these manganese, molybdenum and nickel-containing steels can suffer microstructural ageing due to exposure to thermal and irradiation effects. Such ageing is a natural consequence of the bainitic structure of the steels and is manifested not only in hardening of the steel, but also in reduction of its resistance to fracture. This change in the fracture behaviour is, in turn, reflected in the behaviour of the energy absorbed during Charpy impact testing (a high strain-rate test determining the amount of energy absorbed by a material during fracture) of material samples at different temperatures.
Ageing alters the material’s Charpy curve (below) to reflect a reduction in the toughness on the upper shelf and a shift to higher temperatures of the ductile-to-brittle transition temperature (DBTT). The DBTT shift is measured at a somewhat arbitrary 41J.
The ageing response of the steels depends on the material chemistry and the accumulated irradiation fluence. The ageing response reflects the reaction of the microstructure to atomic displacements caused by interaction of the neutrons from the core with the atomic lattice of the steel in the RPV wall. Atoms that are displaced from their original sites may cause local lattice disturbances in the forms of vacancies or interstitials, or these defects may themselves enhance diffusion within the lattice with the potential of promoting further hardening by atomic clusters, pre-precipitates and precipitates. Both of these manifestations of lattice displacements can induce structural hardening in these types of steels, with the potential to increase embrittlement.
It is not just the intentional alloying elements that play key roles in promoting such hardening. For example, small amounts of copper incorporated in the alloy from scrap sources or welding processes have been found to be particularly deleterious in enhancing irradiation embrittlement. Advanced electron microscopy and atom probe methods are required to provide a better understanding of the underlying mechanisms.
For many years, the characteristic behaviour of the material’s Charpy curve in response to ageing has been employed to follow the effects of service ageing for LWR pressure vessels. From the early phases of LWR plant builds, ‘surveillance capsules’ have been incorporated into the RPV installations to allow practical following of ageing effects. Materials to be included in the surveillance capsules are representative of the key forgings or plates and welds that comprise the critical region of the vessel.
The RPV materials adjacent to the core are generally of greatest interest. Representative material samples are obtained by taking sections from regions of excess material in the forgings or plates. Materials from these sections are machined into mechanical and fracture test specimens prior to enclosure in specially designed surveillance capsules which are then installed inside the RPV before plant start-up. When strategically located, the capsules experience a neutron flux that is typically 1.5 to 4 times higher than the RPV wall itself. This higher fluence rate enables periodic withdrawal of the capsules and testing of their specimens, for predictive estimates of the changes in the properties of the RPV wall.
Historically, four to six capsules have been installed in each operating reactor. Periodic withdrawal and testing in formal RPV surveillance programmes have allowed for not only the following of the behaviour of individual RPVs, but also the better understanding of material and fluence effects by correlating the behaviours between different materials. Mechanical and fracture testing of the surveillance capsules is initially used to demonstrate that the specific pressure vessel has the capacity to safely withstand the plant’s operating conditions for years to come.
The shift in DBTT is employed to demonstrate continuing compliance with the pressurised thermal shock criteria to which the vessel has been licensed, and to identify the pressure–temperature conditions under which the plant may be operated. This has particular effect when cooling the plant for service outages, as well as heating up after outage.
For many years, Charpy specimens were the only means of following fracture resistance behaviour. Most of the capsules being withdrawn and tested today were installed in the 1970s and 1980s, but improved fracture mechanics have since allowed better methods of assessing fracture behaviour. Using extensive fracture databases for the class of RPV steels has enabled the development of ‘master curve’ approaches, via statistical techniques and established databases that support the testing of only a limited number of specimens. To accommodate these new methods, the compositions of surveillance capsules have been modified over the years. The newest surveillance capsules developed by US nuclear services provider Westinghouse Electric Company incorporate both fracture toughness specimens and Charpy specimens. For better monitoring of the long-time behaviour of such plants, the number of capsules has also been increased to eight per vessel for the company’s AP1000(R) nuclear reactor. Inclusion of such specimens and updating of testing practices has been in accordance with international consensus standards, such as those promoted by the ASTM (American Society for Testing and Materials) E10.02 subcommittee.
As existing plants have approached and exceeded their design life, situations have occurred in which individual vessels have had insufficient capsules to allow full characterisation of their materials’ behaviours. Consolidation of property information from many plants has allowed for more complete assessments of materials and irradiation effects. More effective management of the remaining surveillance capsules has led to coordination of the individual RPV surveillance programmes.
In the US, through the Electric Power Research Institute (EPRI), pressurised water reactor (PWR) owners have established a coordinated reactor vessel surveillance programme with the goal of producing higher fluence data for the fleet that would not otherwise have been available. This coordinated programme delays the withdrawal and testing of a number of capsules while still meeting the needs of each specific plant. It will provide higher fluence data, which will lead to a better understanding of embrittlement of the RPV at higher fluences, which will enable all plants to operate to 60 years and beyond.
The development of data to support this long-term service may also be addressed by fabricating new capsules and inserting those into the empty slots in operating reactors. However, this is dependent on the original equipment manufacturers having preserved pieces of the RPV materials from initial plant fabrication. The refurbished capsules must be inserted for many years to accumulate sufficient irradiation fluence to reflect 60 years of vessel exposure.
As an alternative to long-time exposures of previously unirradiated materials, new initiatives are assessing the potential for re-irradiating previously irradiated materials. One US Department of Energy-sponsored programme is directed at irradiating small samples of materials taken from pressure vessel samples and re-irradiating them in a test reactor, to assess the effect of higher levels of fluence on the microstructure. Even more ambitiously, EPRI has initiated a programme to fabricate a supplemental surveillance capsule from specimens reconstituted using previously tested materials, and to provide additional in-service exposure of these materials. The Westinghouse Materials Center of Excellence has testing facilities that have retained previously tested samples, enabling the samples to be handled and prepared for incorporation into a supplemental surveillance capsule.
Combining the test results from reconstituted specimens with the mechanistic understanding of microstructural studies, will provide a way to predict long-term service irradiations of LWR RPVs, and increase data to support plant licence renewal cases.
Image above right: Fracture toughness specimens are readied for assembly in a surveillance capsule. A newly completed surveillance capsule awaits final assembly and shipment from the USA to China, where it will be inserted in an AP1000 nuclear reactor being built there.