Tritium Fuel Cycle: Challenges and Advances for Fusion Power Plants
Remi Delaporte-Mathurin
Half-life: 12 years
\mathrm{T} \rightarrow \mathrm{He} + \mathrm{e}^-
☢
\mathrm{n} + ^6\mathrm{Li} \rightarrow \mathrm{T} + \mathrm{He} + 4.8 \ \mathrm{MeV}
\mathrm{n} + ^7\mathrm{Li} \rightarrow \mathrm{T} + \mathrm{He} + \mathrm{n} - 2.5 \ \mathrm{MeV}
\mathrm{TBR} = \mathrm{\frac{tritium \ produced}{tritium \ consumed}} > 1
❓How ❓
Lithium is used to breed tritium
\mathrm{D} + \mathrm{T} \rightarrow \mathrm{He} + \mathrm{n}
\mathrm{n} + ^6\mathrm{Li} \rightarrow \mathrm{T} + \mathrm{He} + 4.8 \ \mathrm{MeV}
\mathrm{n} + ^7\mathrm{Li} \rightarrow \mathrm{T} + \mathrm{He} + \mathrm{n} - 2.5 \ \mathrm{MeV}
\mathrm{D} + \mathrm{T} \rightarrow \mathrm{He} + \mathrm{n}
Magnet
Breeding blanket
\mathrm{n} + \mathrm{Li} \rightarrow \mathrm{T} + \mathrm{He}
The breeding blanket
Plasma
The breeding blanket is only one component of the fuel cycle
The fuel cycle can be simulated with the residence time model
\frac{{dI_i}}{{dt}} = \sum_{{j \neq i}} \left( f_{j \to i} \frac{{I_j}}{{\tau_j}} \right) - \left(1 + \epsilon_i\right) \left( \frac{{I_i}}{{\tau_i}} \right) - \lambda I_i + S_i
Component \(1\)
\(\tau_1\), \(I_1\), \(S_1\)
\( f_{1 \rightarrow 2} \frac{{I_1}}{{\tau_1}}\)
\( f_{1 \rightarrow 3} \frac{{I_1}}{{\tau_1}}\)
\( f_{0 \rightarrow 1} \frac{{I_0}}{{\tau_1}}\)
All inputs
Outputs + losses
Radioactive decay
Source term
Let's play with a simple fuel cycle model
Burns tritium
Breeds tritium (TBR)
Tritium Extraction System
Breeding Blanket
Plasma
Storage
neutrons
TBR
(constrained by technology)
Doubling time
(driven by economics)
Startup inventory
(constrained by safety)
Startup inventory
Tritium Extraction System
Breeding Blanket
Plasma
Storage
ARC will require ~1 kg of start-up tritium inventory
Doubling time ~1-2 year(s)
The most important parameter for TBR is the Tritium Burn Efficiency (TBE)
TBR Sensitivity index
Most important parameters
Required TBR
❓How hard is it to achieve TBR ~ 1.1 ❓
The most important parameter for TBR is the Tritium Burn Efficiency (TBE)
We are here
TBR ~ 1.1
Plasma improvements
What are the different blanket concepts?
Water Cooled Lithium Lead
Tritium Breeding Ratio
Water Cooled Lithium Lead
Tritium Breeding Ratio
They just made it worse!
⚠️Pipes generate a build up of tritium
Tritium transport FESTIM simulation (2021)
Dual Cooled Lithium Lead
Tritium Breeding Ratio
Helium Cooled Pebble Bed
Tritium Breeding Ratio
ARC Liquid Immersion Blanket
Tritium Breeding Ratio
Sorbom et al, Fus Eng Design, Volume 100, November 2015, Pages 378-405
⚛️Breed tritium
🛡️Shield from neutrons
🔥Extract heat
Liquid Immersion Blanket
FLiBe
ARC power plant
Sorbom et al, Fus Eng Design, Volume 100, November 2015, Pages 378-405
Tritium Breeding Ratio
How much do we trust these numerical results?
design target +9.5 %
DEMO requirement 1.05
Problem: current reactors don't breed T
Breeding Blanket design
Fusion Power Plant
will test...
is needed for...
The EU-DEMO plan
Step 1. Build ITER [pause for dramatic effect]
Step 2. Plug some mockups in the Tritium Breeding System (TBS)
Step 3. Pick the best one and build a FULL SCALE prototype for DEMO
Step 4. Hope scales well 🤞
Plan B
There's no plan B.
ITER
TBS
DEMO
The LIBRA plan:
let's test it before ARC
What is the smallest breeding blanket that can demonstrate a TBR of 1?
🎯Demonstrate T self-sufficiency with DT neutrons
🎯Validating neutronics and tritium transport models
🎯Investigate tritium extraction from liquid breeders
LIBRA experiment
500L FLiBe
14 MeV neutron source
Inconel
double wall
Li + n → T + He
Neutron multiplier
The LIBRA experiment
Tritium transport
Transport mechanisms:
- Diffusion
- Advection
Release pathways:
- Release gas/liquid interface
- Permeation through walls
The LIBRA experiment
He
Tritium detection
The LIBRA experiment
A staged approach to \(\mathrm{TBR} \approx 1\) ...
Tritium Breeding Ratio
LIBRA full-scale
The BABY programme studies tritium breeding at a small scale
- \(14 \ \mathrm{MeV}\) neutrons generated
- tritium created from nuclear reactions
- tritium transport in the salt
- tritium released into the gas phase
- tritium collection and accountancy
Molten salt @ 700C
neutron generator
Tritium collection
reentrant heater
How to measure TBR?
\mathrm{TBR} = \mathrm{\frac{T \ produced}{T \ consumed}}
= \mathrm{\frac{T \ produced}{n \ produced}}
We need to measure these two numbers!
The BABY 100 mL experiment
The BABY experiment was modelled in OpenMC
ClLiF salt gives the highest TBR at the 100 mL scale
Neutrons are detected with a combination of techniques
Niobium activation foils
Diamond detector
\( (n,\alpha)\) peak provides real-time data
Neutrons are detected with a combination of techniques
A8 Diamond proton recoil telescope
supported by Cividec
DD peak
(2 MeV)
DT peak
(14 MeV)
Neutron detection
Tritium was measured using
Liquid Scintillation Counting
HTO, TF and TCl
(soluble in water)
HT, T2
(insoluble)
Liquid Scintillation Counting
Counts between 0-18.6 keV for tritium detection
The TBR measurement somewhat agreed with neutronics simulations
Hypothesis: tritium was lost to permeation
\mathrm{TBR} = \frac{1.17\times10^{10} \ \mathrm{T}}{3.35\times10^{13}\ \mathrm{n} }
= {3.57\times10^{-4}}
Modelled TBR (OpenMC)
Measured TBR
V \frac{d c_\mathrm{salt}}{dt} = S - \textcolor{#438343}{Q_\mathrm{wall}} - \textcolor{#2a7eb8}{Q_\mathrm{top}}
Q_i = A_i \ k_i \ (c_\mathrm{salt} - c_\mathrm{external}) \\ \approx A_i \ k_i \ c_\mathrm{salt}
S = \mathrm{TBR} \cdot \Gamma_n
A transient 0D model is used to simulate the tritium release
\(k\) mass transport coefficient
neutron rate
100 mL
salt
Top release
Wall release
- \( \mathrm{TBR} = 4.71 \times 10^{-4} \) (from OpenMC)
- \( \Gamma_n = 3.88 \times 10^8 \) n/s (from measurement)
- Mass transport coeffs. \( k_i \) (fitted)
100 mL
salt
Top release
Wall release
A transient 0D model is used to simulate the tritium release
Cumulative release (Bq)
Neutron generators on
Reproducibility
Varying the mass transfer coefficients only affects the dynamics
Redox potential affects the release dynamics
Cumulative release (Bq)
Redox potential affects the tritium speciation
Total tritium release
Upgrade: BABY 1 L
1 L of salt
Top release gas sweep
Outer-vessel for permeated tritium
New crucible
Only one neutron generator below the crucible
One bubbler per gas line
×6
New world record! 🥇
The previous TBR record has been broken again!
Tritium Breeding Ratio
Now we need ×500...
The BABY 1L results also agree with the model
But:
- Is the 0D assumption valid?
- Tritium production not homogeneous
- Temperature not homogeneous
- Why is there no permeation?
- Is this reproducible?
BABY 1L run 2 (as of 24 Feb 2025)
Where will we go from here?
LIBRA will help validate FESTIM models
Delaporte-Mathurin et al, International Journal of Hydrogen Energy 63, 2024, 786-802
Temperature
(steady state)
Velocity
(steady state)
Tritium concentration
The FESTIM model highlights a qualitative discrepancy
- Very sensitive to diffusivity
- Wall release > Top release
Different than measured
Hypotheses
- Advection not correctly taken into account?
- Permeation barrier: oxide layer?
- Complex chemistry?
Top release
Wall release
Diffusivities of FLiBe and FLiNaK
OpenMC model
Is our neutronics model an accurate representation?
4 lead bricks → +11 % TBR
?
BABY will inform the design and operation of LIBRA-Pi (100 L)
Take aways
- Tritium self sufficiency is the next big risk item on the fusion roadmap
- \( Q_\mathrm{plasma} = 40+ \) is useless without fuel for the reaction
- Safety factor is 9.5 %, but the real uncertainty may be larger...
- Up to now, R&D was mostly (only?) focused on computational studies
- Some initiatives give us hope
- LIBRA: world record measured TBR
- LIBRTI: UK-led programme for tritium breeding
- VNS: 2.5 m major radius tokamak (see Giannini et al 2024 10.1109/TASC.2024.3365097)
Will it be soon enough to build a successful FOAK fusion power plant by 2030-2040?
Thank you!
Any question?
✉️ remidm@mit.edu
Tritium Fuel Cycle Seminar (UCSD)
By Remi Delaporte-Mathurin
Tritium Fuel Cycle Seminar (UCSD)
The tritium fuel cycle presents one of the most critical challenges for achieving sustainable fusion power. Tritium, a key fuel for fusion reactions, is both scarce and radioactive, requiring efficient production, handling, and recycling to support the operation of future fusion power plants. This seminar will explore the scientific and engineering hurdles associated with the tritium fuel cycle, including production, transport, storage, and recovery, as well as the stringent safety and environmental considerations. We will highlight ongoing research at the Plasma Science and Fusion Center (PSFC) aimed at addressing these challenges, with a focus on the development of tritium breeding blankets. These blankets are designed to produce tritium in situ by leveraging neutron interactions with lithium-containing materials. Our work includes experimental investigations, such as the LIBRA project, and advanced modeling efforts using the open-source hydrogen transport code FESTIM. Key topics will include the optimisation of tritium production and recovery, the role of material selection, and the integration of tritium systems with fusion reactor designs. By advancing these technologies, we aim to pave the way toward achieving tritium self-sufficiency, a cornerstone of viable fusion energy systems.
