Physics and technology considerations for the deuterium-tritium fuel cycle and conditions for tritium fuel self-sufficiency AND physics and technology R & D challenges

Mohammed Abdou

UCLA

Wednesday, April 28, 2021

4:00pm

Virtual

PSFC Seminars

Abstract: The tritium aspects of the DT fuel cycle embody some of the most challenging feasibility and attractiveness issues in the development of fusion systems. This seminar presents brief review of a comprehensive study to understand and quantify these challenges and to define the phase space of plasma physics and fusion technology parameters and features that must guide a serious R & D in the world fusion program. We focus in particular on components, issues and R & D necessary to satisfy three ‘principal requirements’: (1) achieving tritium self-sufficiency within the fusion system, (2) providing a tritium inventory for the initial start-up of a fusion facility, and (3) managing the safety and biological hazards of tritium. A primary conclusion is that the physics and technology state-of-the-art will not enable DEMO and future power plants to satisfy these principal requirements. We quantify goals and define specific areas and ideas for physics and technology R & D to meet these requirements. A powerful fuel cycle dynamics model was developed to calculate time-dependent tritium inventories and flow rates in all parts and components of the fuel cycle for different ranges of parameters and physics and technology conditions. Dynamics modeling analyses show that the key parameters affecting tritium inventories, tritium start-up inventory, and tritium self-sufficiency are the tritium burn fraction in the plasma ( f_b ), fueling efficiency (η_f ), processing time of plasma exhaust in the inner fuel cycle (t_p), reactor availability factor (AF), reserve time (t_r) which determines the reserve tritium inventory needed in the storage system in order to keep the plant operational for time tr in case of any malfunction of any part of the tritium processing system, and the doubling time (t_d). Results show that η_ff_b > 2% and processing time of 1–4 h are required to achieve tritium self-sufficiency with reasonable confidence. For η_ff_b = 2% and processing time of 4 h, the tritium start-up inventory required for a 3 GW fusion reactor is ∼11 kg, while it is < 5 kg if η_ff_b = 5% and the processing time is 1 h. To achieve these stringent requirements, a serious R & D program in physics and technology is necessary. The EU-DEMO direct internal recycling concept that carries fuel directly from the plasma exhaust gas to the fueling systems without going through the isotope separation system reduces the overall processing time and tritium inventories and has positive effects on the required tritium breeding ratio (TBR_R). A significant finding is the strong dependence of tritium self-sufficiency on the reactor availability factor. Simulations show that tritium self-sufficiency is: impossible if AF < 10% for any η_ff_b, possible if AF > 30% and 1% ≤ η_ff_b ≤ 2%, and achievable with reasonable confidence if AF > 50% and η_ff_b > 2%. These results are of particular concern in light of the low availability factor predicted for the near-term plasma-based experimental facilities (e.g. FNSF, VNS, CTF), and can have repercussions on tritium economy in DEMO reactors as well, unless significant advancements in RAMI are made. There is a linear dependency between the tritium start-up inventory and the fusion power. The required tritium start-up inventory for a fusion facility of 100 MW fusion power is as small as 1 kg. Since fusion power plants will have large powers for better economics, it is important to maintain a ‘reserve’ tritium inventory in the tritium storage system to continue to fuel the plasma and avoid plant shutdown in case of malfunctions of some parts of the tritium processing lines. But our results show that a reserve time as short as 24 h leads to unacceptable reserve and start-up inventory requirements. Therefore, high reliability and fast maintainability of all components in the fuel cycle are necessary in order to avoid the need for storing reserve tritium inventory sufficient for continued fusion facility operation for more than a few hours. The physics aspects of plasma fueling, tritium burn fraction, and particle and power exhaust are highly interrelated and complex, and predictions for DEMO and power reactors are highly uncertain because of lack of experiments with burning plasma. Fueling by pellet injection on the high field side of tokamak has evolved to be the preferred method to fuel a burning plasma. Extrapolation from the DIII-D penetration scaling shows fueling efficiency expected in DEMO to be <25%, but such extrapolations are highly uncertain. The fueling efficiency of gas in a reactor relevant regime is expected to be extremely poor and not very useful for getting tritium into the core plasma efficiently. Gas fueling will nonetheless be useful for feedback control of the divertor operating parameters. Extensive modeling has been carried out to predict burn fraction, fueling requirements, and fueling efficiency for ITER, DEMO, and beyond. The fueling rate required to operate Q = 10 ITER plasmas in order to provide the required core fueling, helium exhaust and radiative divertor plasma conditions for acceptable divertor power loads was calculated. If this fueling is performed with a 50–50 DT mix, the tritium burn fraction in ITER would be ∼0.36%, which is too low to satisfy the self-sufficiency conditions derived from the dynamics modeling for fusion reactors. Extrapolation to DEMO using this approach would also yield similarly low burn fraction. Extensive analysis presented shows that specific features of edge neutral dynamics in ITER and fusion reactors, which are different from present experiments, open possibilities for optimization of tritium fueling and thus to improve the burn fraction. Using only tritium in pellet fueling of the plasma core, and only deuterium for edge density, divertor power load and ELM control results in significant increase of the burn fraction to 1.8–3.6%. These estimates are performed with physics models whose results cannot be fully validated for ITER and DEMO plasma conditions since these cannot be achieved in present tokamak experiments. Thus, several uncertainties remain regarding particle transport and scenario requirements in ITER and DEMO. The safety standard requirements for protection of the public and release guidelines for tritium have been reviewed. General safety approaches including minimizing tritium inventories, reducing tritium permeation through materials, and decontaminating material for waste disposal have been suggested.

Bio: Dr Mohamed Abdou is a Distinguished Professor of Engineering and Applied Science at University of California, Los Angeles (UCLA). He is the Director of both the Energy and the Fusion Science and Technology Centers at UCLA. He is the Founding President of the US Council of Energy Research and Education Leaders (CEREL). He is the Editor-in-Chief of Fusion Engineering and Design. Prof. Abdou is the Founding Chair of the International Standing Committee for Fusion Nuclear Science and Technology (FNST). He worked as Department Head at Argonne National Laboratory and Professor of Nuclear Engineering at Georgia Institute of Technology prior to joining UCLA.

Prof. Abdou is one of the world’s most recognized leaders of fusion science and technology. He has published >450 scholarly journal papers on experiment, modeling, and analysis  for neutronics, tritium transport, MHD thermofluids, thermomechanics and materials; as well as on creative designs of fusion nuclear components and power plants; and technical planning of fusion experimental facilities and development pathways.

Prof. Abdou has promoted and lead many collaborative activities among the US, Japan, Europe, Russia, China, Korea, and India. He led the US program on FNST and the Blanket Testing Program for ITER. Prof. Abdou chaired many high-level committees on important topics of science, technology, and energy for the US Government, the World Bank, many universities, industry, and other institutions in the US and worldwide.

Prof. Abdou won many honors and recognition for his pioneering technical contributions and leadership in nuclear and fusion science and engineering and other fields of energy and environment in the US and internationally. These include the American Nuclear Society Outstanding Achievement Award, Secretary of Energy Distinguished Associate Award, Fusion Power Associates Leadership Award, the International Conference for Energy & the Environment Award, the CAS 2010 Einstein Professorship Award, and the FPA Distinguished Career 2016 Award. He is a fellow of the American Nuclear Society, Fellow of the World Academy of Science (TWAS), and “Einstein Professor” of the Chinese Academy of Science. Prof Abdou received the 2018 Chinese Government Friendship Award.

For more detailed CV, please see web site:     http://www.fusion.ucla.edu/abdou/