(497f) Integrated Dynamic Optimisation and Safety Modelling of Hydrogen Isotope Separation Systems in the Fusion Fuel Cycle

Global distribution of fusion power plants has the potential to provide abundant, low-carbon energy, playing a transformative role in addressing climate change. Tritium – one of the two primary fuels of the fusion reaction – introduces unique challenges to commercialisation of fusion energy due to its scarcity, cost, and radioactivity. To ensure the viability of fusion energy, fuel cycles are required to achieve “tritium self-sufficiency”. Tritium self-sufficiency is a condition where the fuel cycle must be designed to produce and recycle sufficient tritium to sustain the fusion reaction while generating a surplus inventory for the start-up of successive power plants [1]. A pivotal factor towards achieving self-sufficiency is minimising tritium losses within the fuel cycle, tritium losses mainly occur due to poor separation efficiency or excessive tritium inventory held in subsystems. The hydrogen isotope separation system, responsible for separating deuterium-tritium fuel from excess protium and deuterium, is a major contributor to tritium inventory hold-up, particularly when employing cryogenic distillation technologies [2, 3].

In this work, a mixed-integer non-linear programming (MINLP) optimisation model was developed to support the design and integration of cryogenic distillation columns with isotopic equilibrator reactors used for hydrogen isotope separation within the fuel cycle. Results indicate that reducing equilibrator temperatures to 77.4 K enhances the separation efficiency of the system, thereby enabling a reduction in the number of theoretical stages, and associated tritium inventory, required for a given hydrogen isotope separation criterion. These findings demonstrate the critical role of equilibrator placement and operating conditions in balancing separation performance and safety in isotope separation system design.

In addition, dynamic pressure transient models were developed to evaluate safety under loss-of-cryogen scenarios. According to API Standard 521, systems that exceed pressure limits within 10–30 minutes do not allow adequate time for operator response. A case study assuming total cryogen loss and heat input solely from tritium beta decay predicts that a column reaches overpressure at 6.8 bara within less than 15 minutes, even when the cryostat vacuum is maintained, and no external heating is present. These results highlight the need for robust pressure relief system design and further support minimising internal inventory to mitigate transient overpressure risks.

The approach of this work establishes a framework for the design of hydrogen isotope separation systems that have both high-efficiency tritium separation and are intrinsically safer for deployment in future fusion reactors.

References:

1. Abdou, M., et al., Physics and technology considerations for the deuterium–tritium fuel cycle and conditions for tritium fuel self sufficiency. Nuclear fusion, 2020. 61(1): p. 013001.
2. Day, C., et al., The pre-concept design of the DEMO tritium, matter injection and vacuum systems. Fusion Engineering and Design, 2022. 179: p. 113139.
3. Schwenzer, J., et al., Operational tritium inventories in the EU-DEMO fuel cycle. Fusion Science and Technology, 2022. 78(8): p. 664-675.