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

  1. \(14 \ \mathrm{MeV}\) neutrons generated
     
  2. tritium created from nuclear reactions
     
  3. tritium transport in the salt
     
  4. tritium released into the gas phase
     
  5. 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

poster

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

cividec.at/neutron_diagnostics-A8.html

         Poster: Emily Edwards, Neutron Detector and Generator Characterization for the BABY Experiment. Mon 23/06

poster

Bjornstadt et al The Current Status and Future Trends on Radioisotope Application in Industry (2014) 10.13140/RG.2.2.10990.00322

A8 Diamond proton recoil telescope

cividec.at/neutron_diagnostics-A8.html

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

github.com/festim-dev/FESTIM

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

github.com/festim-dev/FESTIM

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

github.com/festim-dev/FESTIM

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

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

poster

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

github.com/LIBRA-project