The nuclear debate - surface and particulate engineering in nuclear power generation

Materials World magazine
3 Jul 2011
Plasma facing tiles in a fusion containment

The opportunities for surface and particulate engineering in nuclear power generation were debated at a recent town meeting.

Enhancing safety, rather than power generation efficiency, will be the major driving force in the next round of new nuclear build, concluded a town meeting to debate a study on the potential of surface or particulate engineering solutions in fulfilling the needs of the nuclear power sector.

The review, conducted by the UK Materials Knowledge Transfer Network (KTN), recognises that safety in all aspects of the nuclear power cycle is always going to be a critical issue in gaining public acceptance but, with the meeting on the report occurring at the time of the Japanese earthquake and tsunami and its after-effects on the nuclear power station, the issue was in even greater focus.

Powder power

The report by David Whittaker and Keith Harrison of the KTN’s Powders Sector was stimulated by a resurgence in interest in investing in nuclear capability, both in the UK and globally. Nuclear power generation sources can deliver power on a reliable and consistent basis while avoiding CO2 emissions. Indeed, a recent study by the UK Royal Academy of Engineering has concluded that some combination of nuclear and carbon capture and storage-equipped fossil-fuel generation will play a leading role in the future UK ‘energy mix’.

The interim KTN report has considered future needs from two perspectives –

  • The potential for surface and particulate engineering solutions to help secure a viable nuclear power sector in the UK in the period up to 2030.
  • Opportunities for UK companies in the nuclear power generation supply chain to gain competitive advantage, given the predicted significant growth in the global market opportunities over this same period. A recent review has valued the global nuclear market over the next 20 years at around £600bln for new nuclear build and £250bln for decommissioning, waste treatment and disposal.

The meeting gave a brief introduction to the anticipated developments in nuclear reactor design and the predicted drivers in the nuclear power sector. Opportunities for surface engineering and particulate engineering in the sector were identified in the following areas –

  • The full fuel cycle for nuclear fission – Mining/ extraction of the primary ores, fuel enrichment, fuel processing and fuel re-processing.
  • Nuclear fission reactor hardware.
  • Nuclear fission plant de-commissioning.
  • Nuclear waste management.
  • Coatings on nuclear fusion plant components.

In the subsequent discussions, delegates from industry and academia provided important pointers for the content of the final full White Paper and emphasised that enhancement of safety, rather than of power generation efficiency, will be the major driving force in the next round of nuclear build.

Indeed, it was recognised that the Generation III systems that will dominate the forthcoming new build globally are more robust derivatives of systems already proven but with enhanced safety arrangements. Improved efficiency will not become an objective until the introduction of the subsequent Generation III+ and IV systems, which is unlikely to happen in the UK in the first half of this century.

Reactor designs

The leading examples of the Generation III ‘near-term thermal systems’ are the Westinghouse AP1000 system and the Areva EPR system. Both have been developed over the past 20 years as evolutionary improvements on earlier light water reactor (LWR) systems. These include innovative passive safety features, molten core catchers, performance characteristics, modular construction techniques, and more robust safety control.

The later Generation III+ ‘medium-term thermal systems’ will be either high-temperature gas-cooled reactors (HTRs) or novel LWRs. The high-temperature gas-cooled reactors (HTRs) will offer improved levels of safety through inherent design features. They will operate at around 800°C, and even higher temperatures are possible (cf. 300°C for Generation III LWRs), meaning efficiency will then be emerging as a further objective.

Generation IV systems will be based on concepts that include ‘very high temperature’ evolutions of Generation III+ designs, and a series of designs based on fast breeder reactor technology. These systems will be characterised by significant improvements compared with existing systems in terms of economics, safety, environmental performance and proliferation resistance. They will offer a complete nuclear system (including fuel, fuel cycle and waste management systems).

The current fleet of nuclear power stations in the UK comprises a total of 19 reactors. Between 2014 and 2023, all but the Sizewell B PWR plant will have been closed. So, safe decommissioning of the retired reactors will be a significant concern and a major business opportunity in this period. The lost capacity will be replaced by new build Generation III reactors, the first of which may come on stream around the end of this decade.

Coaxing the last years of operation out of the existing plants will mean extending their lifetimes well beyond those previously advertised, and the new build Generation III plants will be targeted for at least 60 years of operational lifetimes. All of these projected lifetimes are well beyond current experience and ensuring safe operation for these periods will generate a range of material performance challenges.

Waste management

As is well recognised, waste management also carries major safety concerns. Management of irradiated fuel will initially involve storage in water-filled ponds. Much of the AGR fuel and all of the Sizewell B fuel will be stored in this way for many decades. The remainder of the AGR fuel and all of the Magnox fuel is scheduled to be reprocessed to recover uranium and plutonium as fissile material for new fuel (at Sellafield).

In the case of waste from reprocessing for overseas customers, this will be returned to the customer along with the separated uranium and plutonium. For the waste arising from the reprocessing of UK fuels, after separation, the ‘high level’ fission product waste will be immobilised in cement or glass matrices and placed in sealed containers for long-term storage.

Oxide fuel reprocessing (in THORP) will continue as a commercial operation for overseas customers to treat both AGR and LWR fuels. There is an opportunity to manufacture mixed oxide (MOX) fuel at the Sellafield MOX plant; mixed oxide fuels would be suitable for water reactors.

Surface engineering

The opportunities for surface engineering in nuclear power plants will be greatly influenced by health, safety and environmental issues. In particular, cobalt replacement is a major issue because of its half-life and the high gamma radiation produced by Co60 and its impact upon refit and maintenance schedules.

Surface engineering solutions must also take into account extended maintenance intervals and also the ability to repair some components at overhauls. Cobalt-based alloys have good high temperature wear properties and have been used for components in pumps, valves and so on. There are opportunities for surface engineering to provide wear resistant and/or corrosion solutions on substrates that do not contain cobalt, for example, high velocity oxy fuel (HVOF) coatings of WC/Ni-Cr or Cr3C2/Ni-Cr show excellent wear resistance with good corrosion resistance. Nickel based alloys with little or no cobalt content are being evaluated to replace the high cobalt overlay coatings on valve internal components such as seats, wedges and discs.

Compounds of noble metals such as platinum and rhodium have been injected into the primary circuit reactor water. These noble metals deposit on the surfaces in contact with the reactor water and catalyse the recombination of oxygen and hydrogen produced by radiolysis in the core. This reduces the local oxygen concentration at the surfaces and helps suppress intergranular stress corrosion cracking. The noble metals can also be deposited underwater remotely by plasma spraying or by underwater weld cladding to produce high quality, uniform deposits.

Stress corrosion cracking can also be mitigated by introducing a compressive residual stress into the surface using various peening techniques, such as laser, water jet peening or shot peening. Stress corrosion cracking can be significantly reduced or eliminated by peening the surface.

Surface engineering solutions for the nuclear industry must be robust, reliable and consistent. It is important that coatings and surface treatments are used to enable predictive design of components. To achieve this goal, the engineering functionality of coatings needs to be accurately defined and robust property data must be generated to give the designer confidence to specify surface engineering. Variation in the resultant coating needs to be kept to a minimum so the manufacturing process must be stable and the sensitivity of the process accurately determined.

Safety in layers

There are many examples of surface engineering solutions in existing nuclear power plants. Coatings have also been used on plasma facing components in nuclear fusion reactors such as JET.

The outbreak of fires in reactors and associated plants is of major concern and it is very important to prevent these happening. Fire retardant layers, such as intumescent paints and mineral fibres or cementitious/gypsum-based coatings, may help reduce the risk of uncontrolled fires. Intumescent or endothermic coatings are used to protect electrical cables.

Concrete surfaces in primary containment areas, such as ceilings, walls, floors and the reactor cavity, are coated with organic layers formulated from epoxy, phenolic epoxy and zinc to combat nuclide degradation. The experience of these coatings in Generation II plants will lead to their use in Generation III plants, but at a lower level. This is because new design safety features are there to prevent adverse effects, such as detached coating blocking screens and nozzles on the emergency cooling water systems used in case of a loss-of-coolant accident.

An important safety feature of the V/HTR is the type of nuclear fuel it uses – small fuel kernels 500μm in diameter are coated with four layers of ceramic to create a miniature fission product containment vessel, which is much safer than placing fuel pellets in metallic rods. This fuel, known as a TRISO (tristructural isotropic) coated fuel particle, is made of a 90μm buffer layer of low density pyrolytic carbon, inner pyrolytic carbon at 40μm, 35μm of silicon carbide and an outer pyrolytic carbon coating of 40μm. All of these coatings are deposited by fluidised bed chemical vapour deposition.

Surface engineering has an important role to play in the safe transportation and storage of nuclear materials and nuclear waste, especially for corrosion protection. In Sweden, storage vessels have been clad with copper to give long term corrosion resistance. In the UK, Poeton, a group of surface coating contracting companies, has developed the Apticote system to protect the surfaces of low level waste storage drums against long term corrosion – over 50 years). All surfaces of the drum, the inner container and the lid are coated with a sacrificial ‘super–zinc’ base coating followed by a proprietary epoxy polymer topcoat. These techniques could be developed for use on storage vessels used for the transportation and disposal of nuclear waste.

The authors are now preparing a full White Paper report, which will be published electronically this summer.

Further information