Overview of the LIBRA project

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

  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

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

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

Neutrons are detected with a combination of techniques

A8 Diamond proton recoil telescope

cividec.at/neutron_diagnostics-A8.html

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

LIBRA-toolbox

Contains tools to streamline our experimental analysis:

  • LSC counting
  • Tritium modelling
  • Parametric optimisation
  • Neutronics models (OpenMC)
  • Neutron detection
    • Activation foil analysis
    • Count manipulation
    • Proton Recoil Telescope (coincidence)

Everything (98%...) we do is open-source

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...

Measured

Modelled

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)

Impact of H2 in the purge gas

  • Adding H2 released tritium from previous runs
  • Isotopic exchange H2 + T -> HT + H
  • "Enabled" wall permeation (never seen before)

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

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-ONE

BABY-1L

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)

Irradiate FLibe, LiPb, and LiOx

Neutronics simulations LiOx

  • Collaboration with UKAEA
  • Led by Ross MacDonald (secondee at MIT)

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

Will it be soon enough to build a successful FOAK fusion power plant by 2030-2040?

Thank you!

Any question?

✉️   remidm@mit.edu