Helium

Moderators & Coolants

Density: 0.000166322 g/cm³

Atom density: 0.000025 atoms/b·cm

Helium at room conditions — HTGR and VHTR coolant, and the fill gas in a fuel-cladding gap.

Where these numbers come from

PNNL-15870 Rev. 2, "Helium, Natural" (id helium-natural): density and full isotopic composition are taken from the compendium unchanged.

Open the compendium entry for the isotope-by-isotope table and the compendium's own notes.

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)
He-3 (2003)	1.0097e-6	1.3400e-6	3.3532e-11
He-4 (2004)	1.0000e+0	1.0000e+0	2.5024e-5

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 Helium
c PNNL-15870 Rev. 2, "Helium, Natural" - reactormc.net/nrdp/material/helium; rho
c = 0.000166322 g/cm3 (use -0.000166322 on the cell card)
m27    2003.80c  -1.0097e-6  $ He3
       2004.80c  -0.999999  $ He4

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

At 1 atm, as here. HTGR coolant at ~7 MPa is about 3.8 kg/m³, twenty times this density. Natural helium is He-4 with a trace of He-3; the He-3 is negligible by mass but has a 5330-barn thermal absorption cross section, so the compendium keeps it.

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