Tritium Breeding and transport in Molten Salts under 14 MeV neutron Irradiation:
Insights from the LIBRA Experiment
Remi Delaporte-Mathurin, Nikola Goles, Collin Dunn, Emily Edwards, Samuele Meschini, Stefano Segantin, Sara Ferry, Ethan Peterson, Dennis Whyte, Edward Lamere, Colin Weaver, Kevin Woller and LIBRA collaborators
The tritium issue
\mathrm{D} + \mathrm{T} \rightarrow \mathrm{He} + \mathrm{n}
1 \ \text{GWth} \approx 10^{20} \ \text{reactions/s} \approx 50\ \text{kg/year}
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}
⚛️Breeding 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
LIBRA: demonstrating the Liquid Immersion Blanket
What is the smallest blanket that can demonstrate a \(\mathrm{TBR} \geq 1\) ?
LIBRA
Liquid Immersion Blanket tritium Robust Accountancy
Objectives
🎯Lab demonstration of T self-sufficiency with DT neutrons
🎯Experience with molten salt handling
🎯Tritium extraction from molten salts
Ferry, S. E. et al. (2023), Fusion Science and Technology, 79(1), pp. 13–35. doi: 10.1080/15361055.2022.2078136.
LIBRA was designed to achieve TBR ≈ 1
\mathrm{TBR} = \mathrm{\frac{T \ produced}{T \ consumed}}
= \mathrm{\frac{T \ produced}{neutron \ produced}}
Neutronics simulations
\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}
The BABY experiment 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
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
The LIBRA experiment
He
Tritium detection
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!
Neutrons are detected with a combination of techniques
Niobium activation foils
Diamond detector
\( (n,\alpha)\) peak provides real-time data
Neutron detection
Neutrons are detected with a combination of techniques
A8 Diamond proton recoil telescope
supported by Cividec
DD peak
(2 MeV)
DT peak
(14 MeV)
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 agrees with neutronics simulations
Some of the tritium was lost
\mathrm{TBR} = \frac{1.17\times10^{10} \ \mathrm{T}}{3.35\times10^{13}\ \mathrm{n} }
= {3.57\times10^{-4}}
Modelled TBR (OpenMC)
Measured TBR
OpenMC model
OpenMC model
Comparison of TBR calculations with different breeders
At the 100 mL scale
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
\(A\) surface area
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
Salt T inventory
T fluxes
Cumulative release
1 \ \mathrm{Bq} \approx 10^{-15} \ \mathrm{mole}
Varying the mass transfer coefficients only affects the dynamics
Reproducibility
BABY can 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
Hypotheses
- Advection not correctly taken into account?
- Permeation barrier: oxide layer?
- Complex chemistry?
Top release
Wall release
\varphi = \int_{\partial \Omega} -D \nabla c \cdot \mathbf{n} \ dS
The flux is computed from the concentration field
\mathrm{cumulative \ release} = A \int_0^t \varphi \ dt
Diffusivities of FLiBe and FLiNaK
What is the effect of salt chemistry?
Redox potential affects the tritium speciation
Total tritium release
Redox potential affects the release dynamics
Cumulative release (Bq)
Where to go from here?
Road to \(\mathrm{TBR} \approx 1\) ...
Tritium Breeding Ratio
LIBRA
1 L of salt
Top release gas sweep
Outer-vessel for capturing permeated tritium
BABY 1 L about to be commissioned
Furnace
LIBRA Pi is being manufactured
100 L of salt
Re-entrant heaters
Inner vessel
Conclusions and next steps
So much more we can do
- Optimise neutron detection and tritium accountancy
- Reproduce speciation experiments
- Irradiate FLiBe
- Road to LIBRA
100 mL
1 L
100 L
500 L
BABY
LIBRA
- BABY is the first TBR measurement in molten salts
- Theoretical and technical knowledge on tritium breeding technologies
- Paper under review in Nuclear Fusion
- Data: github.com/LIBRA-project/insights-from-BABY-experiment-paper
Thank you!
Any question?
✉️ remidm@mit.edu
Tritium Breeding and transport in Molten Salts under 14 MeV neutron Irradiation: Insights from the LIBRA Experiment
By Remi Delaporte-Mathurin
