Modeling and analysis of the tritium fuel cycle for ARC- and STEP-class D-T fusion power plants

Abstract

The limited tritium resources available for the first fusion power plants (FPPs) make fuel self-sufficiency and tritium inventory minimization leading issues in FPP design. This work builds on the model proposed by M. Abdou et al. \cite{abdou2020physics}, which analyzed the fuel cycle of a DEMO-class FPP with a time-dependent system-level model. Here, we use a modified version of their model to analyze the fuel cycle of an ARC-class tokamak and two versions of a STEP-class tokamak. The ARC-class tokamak breeds tritium in a FLiBe liquid immersion blanket (LIB), while the STEP-class tokamak breeds tritium utilizing either a liquid-lithium blanket design or an Encapsulated Breeding Blanket (EBB). A time-dependent system-level model is developed in Matlab Simulink~\textregistered\hspace{.2em} to simulate the evolution of tritium flows and tritium inventories in the fuel cycle. The main goals of this work are to assess tritium self-sufficiency of the ARC- and STEP-class designs and to determine quantitative design requirements that can be used to analyze the adequacy of a proposed fuel cycle system. These design requirements are aimed at achieving a low tritium inventory doubling time ($t_d$) and a low start-up inventory ($I_{\mathrm{startup}}$) while keeping the required tritium breeding ratio (TBR$_r$) as low as possible. We also consider how improvements in fuel cycle technology and plasma operations affect TBR$_r$ and $I_{\mathrm{startup}}$. The model results show that TBR$_r$ for ARC- and STEP-class FPPs should be achievable if the tritium burn efficiency (TBE) reaches 0.5-1\% (TBR$_r <$ 1.2). This assumes significant, but attainable, improvements over current abilities. However, the model results indicate that a FPP must achieve ambitious performance targets, including FPP availability $>$70\%, tritium processing time $<$4~h, and the implementation of direct internal recycling. If future research yields major improvements to achievable TBE, it may be possible to achieve tritium self-sufficiency while operating at lower availability and without implementing DIR.

Integral analysis of the effect of material dimension and composition on tokamak neutronics *

“…The exact threshold TBR to maintain a self-sufficient tritium cycle is unclear because of uncertainties in nuclear data [15,16], neutronics modeling [17,18], and tritium loss and extraction mechanisms [19,20], which vary on the system temperature, surrounding materials, and tritium-breeding material. Additionally, the 'required' TBR strongly depends on the tritium fuel cycle design, technology, and reliability, availability, maintainability, and in-spectability (RAMI) [21] considerations [22][23][24]. Efforts are ongoing to understand tritium breeding and behavior for a molten salt blanket, namely the Liquid Immersion Blanket: Robust Accountancy (LIBRA) experiment at the Massachussets Institute of Technology (MIT) [25].…”

Modelling neutron damage effects on tritium transport in tungsten

“…Regarding safety, a higher quantity of the radioactive tritium may be trapped in components using a tungsten first wall. Economically, a higher trapped inventory means less of the tritium produced by the breeding blanket can be recovered, meaning a higher startup inventory and larger tritium breeding ratio will be required for the same tritium doubling time [4]. Furthermore, larger tritium inventories will produce more helium through decay, resulting in more bubble formation, contributing to embrittlement effects, and reducing the lifespan of the plasma-facing components.…”

“…Accurate simulation of the hydrogen isotope transport is imperative to ensure safe operation and optimal performance of the FPP's using tritium. Furthermore, accurate simulations of subcomponents can better inform system-level fuel cycle models, potentially leading to better economic performances of an FPP [4,5]. Tritium production in nuclear fusion devices presents a concern for designers due to its permeability through materials, and its radioactive nature requires accurate monitoring.…”

Plasma control for the step prototype power plant

“…, I (t − m∆t) , t, ∆t, m, Popt) . (7) In this way the model acts as a state observer which is constrained by the available measurements. The observed state variables can then be used in a traditional control scheme by computing the control error:…”

“…The plasma control system must be able to guide and stabilise this system using the limited external actuators. Four fifth of the power produced in the fusion reaction will leave the plasma in the form of 14 MeV neutrons and these will be absorbed in the breeding blankets surrounding the plasma, producing the tritium required for continuous operation [7]. The remaining fifth of the fusion power plus the external power injected into the plasma, will leave the plasma impacting the plasma facing components.…”