Helium

Moderators & Coolants

Density: 0.000164 g/cm³

Helium gas at STP. Coolant in high-temperature gas-cooled reactors (HTGR, VHTR) and fuel-cladding gap fill gas.

Composition

Nuclide / Element	Fraction
He-4	1.000000

MCNP

c Helium Gas (STP)
c Density: 0.000164 g/cm3 (use -0.000164 on cell card)
m27    2004.80c  1.000000  $ He-4

Density at STP. HTGR helium coolant operates at ~7 MPa, where density is ~3.8 kg/m³. Also used as fill gas in fuel-cladding gaps.