Recent progress of LIBRA project and new TBR measurements
Remi Delaporte-Mathurin, Nikola Goles, Davide Pettinari, James Dark, Collin Dunn, Emily Edwards, Sara Ferry, Ross MacDonald, Stefano Segantin, Weiyue Zhou, Kevin Woller
Plasma Science and Fusion Center, MIT, USA
UK Atomic Energy Authority, UK
2025 Symposium on Fusion Engineering, June 23 - 26
LIBRA: derisking fusion power plants
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
Ferry, Sara E., et al. ‘The LIBRA Experiment: Investigating Robust Tritium Accountancy in Molten FLiBe Exposed to a D-T Fusion Neutron Spectrum’. Fusion Science and Technology 0, no. 0 (17 June 2022): 1–23. https://doi.org/10.1080/15361055.2022.2078136.
FLiBe
14 MeV neutron source
Li + n → T + He
Neutron multiplier
The LIBRA experiment
~1 m
Sweep/sparge gas
Tritium detection
The LIBRA experiment
Tritium transport
Transport mechanisms:
- Diffusion
- Advection
Release pathways:
- Release gas/liquid interface
- Permeation through walls
The LIBRA experiment
Half-life: 12 years
\mathrm{T} \rightarrow \mathrm{He} + \mathrm{e}^-
☢
Consumption of a 1 GWth fusion reactor (1 year)
50 kg
The breeding blanket
\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
\mathrm{D} + \mathrm{T} \rightarrow \mathrm{He} + \mathrm{n}
LIBRA experiment
You are here
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
The BABY 100 mL experiment
Neutron generator
Bubblers
Molten salt capsule
Gas handling system
Temperature control
14 MeV neutron irradiations
The BABY 100 mL experiment
Delaporte-Mathurin, Rémi, et al. ‘Advancing Tritium Self-Sufficiency in Fusion Power Plants: Insights from the BABY Experiment’. Nuclear Fusion, 2024. https://doi.org/10.1088/1741-4326/ada2ab.
T release dynamics
- TBR measurements
- Tritium release dynamics
- Tritium permeation
- Tritium speciation
- ...
Upgrade: BABY 1 L
Autumn 2024
1 L of salt
ClLiF
650 °C
More ports for diagnostics
Outer-vessel for permeated tritium
New crucible
Furnace
Outer-vessel
Power supply for furnace
Inner Vessel
Lid
Insulation
Only one neutron generator below the crucible
One bubbler per gas line
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!
Tritium was measured using
Liquid Scintillation Counting
HTO, TF and TCl
(soluble in water)
HT, T2
(insoluble)
Poster: Nikola Goles, Tritium Collection and Measurement in Small-Scale Molten Salt Breeding Experiments. Tue 24/06
LSC counter
Liquid Scintillation Counting
Counts between 0-18.6 keV for tritium detection
Neutrons are detected with a combination of techniques
Nb/Zr activation foils
provides real-time neutron rate
A8 Diamond proton recoil telescope
Poster: Emily Edwards, Neutron Detector and Generator Characterization for the BABY Experiment. Mon 23/06
A8 Diamond proton recoil telescope
supported by Cividec
DD peak
(2 MeV)
DT peak
(14 MeV)
×6
Previous TBR record broken!
Tritium Breeding Ratio
- Increased solid angle of salt 100 mL: 2%, 1L: 7%
- Now we just need x500...
All the BABY 1L runs
New DT source nGen®
(Starfire Industries)
neutron rate \(\approx 10^{10} \ \mathrm{n/s}\)
previously \(\approx 10^{8} \ \mathrm{n/s}\)
Using LIBRA to validate models
OpenMC model
Is our neutronics model an accurate representation?
support lead bricks
?
Romano, Paul K., et al. ‘OpenMC: A State-of-the-Art Monte Carlo Code for Research and Development’. Annals of Nuclear Energy, 82 (1 August 2015): 90–97. https://doi.org/10.1016/j.anucene.2014.07.048.
DT generator
Salt
Epoxy
Tritium Breeding Ratio
Measured
Modelled
Our measurements agree very well with neutronics models
- ~ 10% uncertainty on neutron fluence measurement
- discrepancy on 100 mL attributed to permeation losses
- Ongoing effort on uncertainty quantification
\Omega = 0.25 \ \mathrm{sr}
\Omega = 0.9 \ \mathrm{sr}
Neutron dose rates
Gamma dose rates
<𝟐 𝐦𝐫𝐞𝐦/𝐡
outside the vault ✅
\(3.2 \times 10^{6}\) 𝐦𝐫𝐞𝐦/𝐡
\(5.3 \times 10^{3}\) 𝐦𝐫𝐞𝐦/𝐡
\(10^{-2}\) 𝐦𝐫𝐞𝐦/𝐡
\(10^{-2}\) 𝐦𝐫𝐞𝐦/𝐡
The BABY experiment was modelled in OpenMC
ClLiF salt gives the highest TBR at the 100 mL scale
Using natural Li
V \frac{d c_\mathrm{salt}}{dt} = S - \textcolor{#438343}{Q_\mathrm{OV}} - \textcolor{#2a7eb8}{Q_\mathrm{IV}}
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
\(k_i\) mass transport coefficient (m/s)
neutron rate
Salt volume
Inner vessel
Outer vessel
- \( \mathrm{TBR} = 4.71 \times 10^{-4} \) (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
The 0D model reproduces the experimental data
Irradiation
Model
Measurements
Cumulative T release (Bq)
The results agree with the model
Very long experimental time
Cumulative T release (Bq)
No permeated T was detected
J_\mathrm{permeation} = K_r \ c_T ^ 2
Inert sweep gas (He) = low recombination rate
Cumulative T release (Bq)
Run #3: adding H2 to the sparge gas
J_\mathrm{permeation} = K_r \ c_T ^ 2 + K_r' \ c_T \ P_\mathrm{H_2}
\mathrm{H_2 + T (ad)} \rightleftarrows \mathrm{ HT +H (ad)}
Releasing T trapped in piping from previous runs
Pure He:
- No permeation
- Previous parameterisation is predictive ✅
Adding 1000 ppm H2:
Salt volume
IV metal surfaces
OV metal surfaces
IV bubbler
OV bubbler
S = \Gamma_n \cdot \mathrm{TBR}
A_\mathrm{OV} \ k_\mathrm{OV} \ c_\mathrm{salt}\ c_\mathrm{H2}
(1-f) \ A_\mathrm{IV} \ k_\mathrm{IV} \ c_\mathrm{salt}
f \ A_\mathrm{IV} \ k_\mathrm{IV} \ c_\mathrm{salt}
k_\mathrm{exchange,IV} \ I_\mathrm{piping, IV} \ c_\mathrm{H2}
k_\mathrm{exchange,OV} \ I_\mathrm{piping, OV} \ c_\mathrm{H2}
Run #4: 1000 ppm H2 from the start
×10
From 60 days to 4 days!
Mass transport coefficient
Generating data faster than we can publish it
Run #8 Influence of sweep gas humidity
Shielding experiments
So many other things we could do:
- Temperature scans
- Flow rate scans
- Gas composition
- Neutron source position
- ...
Multiphysics validation
LIBRA produces validation data for multiphysics models
Tritium production map from neutronics (OpenMC)
Tritium concentration field (FESTIM)
Max conc. 1.4E13 T/m3
[T/n/cm3]
Natural convection neglected
new coupling between OpenMC and FESTIM!
Outgassing flux
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
LIBRA produces validation data for multiphysics models
\varphi = K_r \ c_T ^ 2 + K_r' \ c_T \ P_\mathrm{H_2}
\mathrm{H_2 + T (ad)} \rightleftarrows \mathrm{ HT +H (ad)}
c= 0
\mathrm{2 T (ad)} \rightleftarrows \mathrm{ T_2}
Volumetric source from DT neutrons
\frac{c^-}{K_H} = \left(\frac{c^+}{K_S}\right)^2
Towards a multi-material model
Interface discontinuity
Surface recombination
LIBRA-toolbox
Experimental analysis
Tritium release models
Neutron detection
Neutronics models
Parametric optimisation
Tritium detection LSC counting
Modelling
Materials
Activation foil analysis
Diamond detector analysis
PRT analysis
spectrum analysis
Open-science and reproducibility are core values of LIBRA
- GitHub organisation:
github.com/LIBRA-project
- Zenodo community:
zenodo.org/communities/libra-project
- FAIR principles, open-science, 100% reproducible
Where will we go from here?
A staged approach to \(\mathrm{TBR} \approx 1\) ...
Tritium Breeding Ratio
LIBRA ONE (700 L)
LIBRA ONE
Measured
Expected
LIBRA-Pi
BABY
100 mL
1 L
ClLiF
FLiBe
More science experiments
June 2025
Our roadmap
Talk: Kevin Woller, Radiological and thermal management for large-scale tritium breeding experiments with molten salts. Thur 26/06
100 mL
1 L
ClLiF
FLiBe
LiPb
LiOx
Tritium production map in LiOx packed bed
Irradiate LiPb and LiOx
100 mL
1 L
LIBRA Pi
ClLiF
FLiBe
LiPb
LiOx
manufacturing
coating
operation
BABY informs the design and operation of LIBRA-Pi
~100 L of ClLiF
100 mL
1 L
LIBRA Pi
LIBRA ONE
ClLiF
FLiBe
LiPb
LiOx
manufacturing
coating
operation
design
LIBRA ONE
FESTIM model
~700 L of FLiBe
TBR > 1
100 mL
1 L
LIBRA Pi
LIBRA ONE
Modelling
ClLiF
FLiBe
LiPb
LiOx
manufacturing
coating
operation
design
validation data
informs design & operation
2025
Thank you!
Any question?
✉️ remidm@mit.edu
SOFE 2025
By Remi Delaporte-Mathurin
