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

  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

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

cividec.at/neutron_diagnostics-A8.html

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

github.com/festim-dev/FESTIM

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

Thank you!

Any question?

✉️   remidm@mit.edu