TOFE Plenary III–Panel Discussion: The need and challenges for a Fusion Prototypic Neutron Source

The State of the Technology for a Fusion Prototypic Neutron Source

Eric Pitcher, Los Alamos National Laboratory

The history of proposed neutron facilities for testing fusion materials span at least four decades. In the 1980s, the International Energy Agency led an effort to evaluate and downselect between three leading technologies for meeting the mission: d-Li stripping, nuclear spallation, and a beam-plasma source. In 1992, the option selected for an International Fusion Materials Irradiation Facility (IFMIF) was d-Li stripping. Research and development efforts on this option has been ongoing in the ensuing three decades. Currently, no country has accepted to host the IFMIF, which has an estimated construction cost exceeding $1B.

The recent fusion community effort culminating in the APS DPP Community Planning Process and the FESAC 10-Year Plan identified a Fusion Prototypic Neutron Source (FPNS) as one of the highest priority facilities needed for near-term realization of a fusion power plant. The FPNS is recognized as a step toward IFMIF, not a replacement for it. As such, it is smaller in scale and capability with the expectation that FPNS will be substantially cheaper and faster to build than IFMIF. The APS DPP CPP report states, “The proposed FPNS facility would differ from international facilities, including IFMIF-DONES, by being operational on a shorter timescale at significantly lower capital cost and thus enabling a more aggressive timeline for an [Fusion Power Plant].”

A community workshop held in August 2018 identified the high-level goals that FPNS should meet. It concluded that its focus should be on scientific understanding of materials in prototype neutron spectrum and temperature. Near-term realization of an FPNS means it should be based on existing technology with low R&D requirement, and to keep its cost moderate and construction time short, FPNS will have limited sample volume as compared to IFMIF.

Consequences of Neutron Energy Spectrum on Radiation Damage, Gas Production, and Transmutations in Fusion Materials

Laila A. El-Guebaly, Univ. Wisconsin, Madison

Mohamed E. Sawan, Univ. Wisconsin, Madison

The inclusion of test modules in the U.S. Fusion Prototypic Neutron Source (FPNS) offers the opportunity to test a wide variety of materials in a representative radiation environment of fusion Pilot Plant, DEMO, and power plant. The testing may include various generations of structural materials for conventional and advanced blanket and divertor concepts. Since all structural materials derived from the fission industry are inadequate for fusion applications (due to the more damaging effects of the 14 MeV fusion neutrons), radiation-resistant reduced-activation structural materials (RAFM, vanadium alloy, W alloy, and SiC/SiC composites) were specifically developed for fusion and could be tested in FPNS to qualify for the highly irradiated fusion components surrounding the plasma.

Upon exposure to fusion neutrons, all materials become radioactive at different degrees, depending on their activation and decay characteristics, neutron flux level, and service lifetime. Energetic fusion neutrons displace atoms, generate high levels of hydrogen and helium gases and other transmutation products that tend to cause swelling, embrittle and degrade the integrity of structural materials, and ultimately limit their service lifetime in fusion facilities. Testing fusion materials with a fission spectrum (such as of HFIR) is misleading due to the significant difference in spectrums. Even though the atomic displacement by fission and fusion neutrons are equivalent in metals, the large helium generation by fusion neutrons is unique to fusion materials. The most important attribute for the FPNS would be the typical fusion-relevant He/dpa ratio of ~10 for steel in particular. By comparison, irradiation in the fission spectrum of HFIR would provide very low He/dpa ratio of ~0.3, which is irrelevant to fusion. This paper reviews the neutron irradiation impacts and presents a few examples of dpa and transmutation products for steel, W, and SiC based on research modeling in several fusion design studies. The operating conditions of advanced U.S. fusion power plants were considered along with the credible lifetime goal of 200 dpa and 20 MWy/m2 fluence that could be achieved with directed R&D programs coupled with the construction of the FPNS 14 MeV neutron facility.

Transmutation Science and Its Application to Fusion Materials Development

Lance L. Snead, Stony Brook Univ.

Neutrons of energy above approximately 10 MeV produce both cascade-damage and significant transmutation in solid materials, distinguishing the fusion-born neutron spectrum from that of the fission neutron spectrum. Helium production in structural steel and the resulting impact on swelling and fracture toughness is the most widely discussed transmutation issue, but represents only one example of a array of materials and phenomenon with make up the field of transmutation science. In this presentation a more comprehensive discussion of the range of fusion materials impacted by fusion-born neutron transmutation will be provided. By understanding this range of materials, and the scientific and data needs necessary to support the materials development of next-generation power reactors, we can suggest the specific tools, models, and community development required to engage in transmutation science studies. This will be presented in combination with a summary of the community workshops on a fusion-prototypic-neutron-source (FPNS) and recommendations of the recent FESAC Report Powering the Future of Fusion and Plasmas

Materials Modeling as a Bridge between Fusion Reactor Irradiation Conditions and FNPS Capabilities

Jaime Marian, Univ. California, Los Angeles

As consideration of several options for the Fusion Prototypical Neutron Source (FPNS) by DOE gets underway, the materials community must carefully weigh the strengths and weaknesses of each alternative and prioritize the parameter space susceptible of being explored in each case. In this context, theory, modeling and simulation can act as a bridge to understand the differences in material behavior between irradiations in a nominal 14-MeV neutron environment and in FPNS candidates. The last decades have experienced substantial progress in terms of the accuracy, scale, and relevance of materials modeling under fusion reactor operation. Increasingly more complex and realistic material microstructures can now be simulated under fusion-representative conditions, complementing experiments and solidifying our understanding of materials behavior under meaningful dose rates, temperatures, gas atom-to-dpa ratios, and spectral details. As well, while many challenges still remain, the experience acquired by modeling teams over the last few decades has now resulted in a set of 'best-practices' supported by a relatively wide community consensus with applications in a wide range of different operational scenarios. Current models can effectively capture irradiation damage buildup coupled to microstructural evolution, dose-rate and temperature dependent regimes, nuclear transmutation and gas atom evolution, solute mobilization by radiation enhanced diffusion, as well as the change in derivative quantities such as hardening, swelling, or creep. While important gaps in our understanding remain, particularly in terms of high dose and high temperature materials behavior, synergistic He/H effects in ferritic materials, pulsed irradiation, or chemical effects, we believe that modeling is presently in a good position to issue qualified materials behavior predictions in the anticipated operational range gap between a fusion facility and FPNS. In particular, we will discuss potential differences in He- and H-to-dpa ratios, irradiation flux and pulsed regimes, probe volumes, and irradiation temperatures, as well as the potential implications of modeling long term transmutation-induced chemical composition changes, swelling, and creep. Finally, we will discuss the potential of new and improved techniques, including data-driven approaches, aided by an increased availability of computational resources, to push the envelope of our current understanding limits and improve error and uncertainty estimation of model predictions.

Discussion

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