Linking Microstructure to Neutron Irradiation Defects in Advanced Manufacture of Steels

The UK plans to build a new fleet of nuclear power plants starting with two units at Hinkley Point in Somerset. The UK government has also recently announced in the autumn 2015 statement that £250M will be set aside for in innovative nuclear technologies. More specifically it has stated that the UK will invest in small modular reactor designs. The large reactors and many small modular reactor designs are based around a reactor type called a pressurised water reactor. These reactor designs have a steel reactor pressure vessel to enclose the nuclear fuel and act as a key barrier to the release of radiotoxic materials to the environment. The integrity of the vessels is paramount to the safety and continued operation of the reactor. Unfortunately, neutron irradiation from the nuclear fuel damages the steels over their 40-60 year design life. Understanding the role of neutron damage to these steels is therefore key to continued operation beyond the design life.

This programme of work will study commonly used reactor pressure vessel forging grade steels (A508 class 3), under neutron irradiation damage, at the OPAL test reactor, at Lucas Heights in Australia. The steels will be manufactured by processes not commonly used in nuclear reactors i.e. hot isostatic pressing (HIP) of powdered material and then welded using electron beams (EB). These new manufacturing processes could potentially be used to manufacture parts for the reactor pressure vessels of future small reactor designs. As yet there is no information on how changing the manufacturing routes from arc welding of forged material to EB welding of HIPed material will change the neutron irradiation response of the material. In this case the chemistry of the material remains unchanged so the key variable is the so-called "microstructure" of the material.

It is planned to irradiate samples, at the OPAL reactor, for up to 1 year, to achieve doses of neutron embrittlement equivalent to 40-60 years reactor operation. The irradiated material will then be mechanically tested, in hot cells, at the Australian Nuclear Science and Technology Organisation before material is shipped to the new Materials Research Faclility at UKAEA Culham site in the UK. Here, it will be prepared for state-of-the-art characterisation, by atom probe tomography on the new LEAP 5000 atom probe recently installed at Oxford University, Chemi-STEM transmission electron microscopy at Manchester University, together with atomic scale models developed at Imperial College London and Manchester University. The project will also have management and input from the National Nuclear Laboratory and Rolls-Royce and international links to the University of New South Wales, University of California Santa Barbara and Oak Ridge National Laboratory.

The overall output from this work will be much improved mechanistic understanding and models of how neutron irradiation effects steels manufactured by HIP and EB welding, lead to a new generation of engineers in the UK who can perform work on irradiated materials and help direct the use of such technologies for the building of future small reactor designs. It will also be a crucial driver in the effort to rebuild the physical and knowledge based infrastructure, for dealing with neutron irradiated steels, that has been missing for a generation in the UK.

The UK has just invited the Chinese to invest £6Bn on nuclear new build with EDF investing the remaining £16Bn. The UK plan to build six new PWR units as a result. The electricity generated by a 1.6 GWe reactor is worth about £1-1.5M per day and the design life of new reactors is estimated to be at least 60 years, with potential to extend to 80, or even 100 years. 20 years of life extension even at a modest load factor, for a modern reactor, of 90%, is worth about £7-9 Bn per reactor. The science to underpin such life extensions will largely focus on the irradiation embrittlement behaviour of the reactor pressure vessel (RPV), the most safety critical part of the system. Once the RPV is deemed not safe the reactor is shutdown.

In tandem the UK government in the Autumn 2015 statement said it will invest up to £250M in innovative nuclear technologies with a strong focus on small modular reactor designs to make the UK a competitive world leader in the nuclear power sector. The work proposed here is targeted specifically at small reactors but using material that is common for existing large reactors and hence will have an impact on the future safety cases of both. The other great societal impact comes from the huge savings in CO2 emissions that a 20 year life extension of a low carbon (only 16 gCO2e kWh-1) nuclear power station can make. With both improved air quality and climate change high on the agenda of the UK government new nuclear build and life extension research will have a huge impact.

The work will also impact on UK infrastructure for handling and understanding nuclear materials as there has been a generation of researchers in the UK without this capability. Therefore, this programme will have impact in rebuilding the UKs capability, in this area, and train a new generation of academics, Post-doctoral researchers and PhD students. We will be a big user of the recent government capital investment under the National Nuclear Users Facility at Culham maximising the impact this facility and UK Government investment in it will have. The Materials Research Facility, for active materials, at Culham will receive the material and allow the investigators the ability to store it and further prepare samples for advanced analytical analysis at Oxford and Manchester Universities. Here we will again leverage to maximum effect the use of government investment in new instruments such as the new LEAP 5000 atom probe at Oxford and the Titan-chemi-STEM machines at Manchester to perform state-of-the art investigations into the materials irradiated at the atomic scale.

Impact for the nuclear industry is also expected through utiliisation of the results at National Nuclear Laboratory and Rolls-Royce for the qualification of the neutron dose-damage relationships used to predict RPV life. The proposal will provide research to underpin current models and provide new models for new product forms.

The final impacts will be on manufacturing of nuclear components, by methods such as hot isostatic pressing and electron beam welding. These materials have different microstructures to the forged and arc welded materials and may therefore have different response to neutron irradiation. The response is critical to licensing them for new reactors, such as small modular designs, and therefore this research will provide the first insights into their behaviours in a reactor environment.