FLiBe

Li₂BeF₄

Moderators & Coolants

Density: 1.94 g/cm³

Atom density: 0.0825753 atoms/b·cm

Lithium fluoride–beryllium fluoride molten salt, the coolant and fuel carrier of fluoride-salt-cooled and molten-salt reactors.

Where these numbers come from

2LiF·BeF₂ stoichiometry with lithium enriched to 99.995 at% Li-7. Density carried over from the previous NRDP entry (≈1.94 g/cm³ near 950–1000 K); not in the compendium.

Material compositions from PNNL-15870 Rev. 2 (April 2021), R.S. Detwiler, R.J. McConn Jr., T.F. Grimes, S.A. Upton, and E.J. Engel, Compendium of Material Composition Data for Radiation Transport Modeling, Pacific Northwest National Laboratory. Full report (DOI 10.2172/1782721)

Composition

Nuclide (ZAID)	Weight fraction	Atom fraction	Atom density (atoms/b·cm)
Li-6 (3006)	6.0736e-6	1.4286e-5	1.1797e-6
Li-7 (3007)	1.4168e-1	2.8570e-1	2.3592e-2
Be-9 (4009)	9.0998e-2	1.4286e-1	1.1796e-2
F-19 (9019)	7.6732e-1	5.7143e-1	4.7186e-2

This is the material as formulated. The cards below carry weight fractions, written negative, which is what MCNP and Serpent read as a mass basis.

Generated material cards

MCNP

c FLiBe
c ReactorMC NRDP derived material - reactormc.net/nrdp/material/flibe; rho =
c 1.94 g/cm3 (use -1.94 on the cell card)
m21    3006.80c  -6.0736e-6  $ Li6
       3007.80c  -0.141676  $ Li7
       4009.80c  -0.090998  $ Be9
       9019.80c  -0.767320  $ F19

MCNP and Serpent cards use .80c (ENDF/B-VII.1 at 293.6 K) — renumber m1 and change the suffix to match your deck. SCONE compositions are atom densities in atoms/barn-cm with the .03 suffix matching temp 300. See the library reference before changing a suffix: the mapping is not portable between vendors.

Notes

Li-7 enrichment is not a detail: Li-6(n,α)T is both a tritium source and a large parasitic absorber, which is why MSR designs specify 99.99%+ Li-7. Density falls with temperature roughly as ρ = 2.413 − 4.88×10⁻⁴·T(K) g/cm³, so set it for your salt temperature.

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