Light Water

H₂O

Moderators & Coolants

Density: 0.997 g/cm³

Atom density: 0.099983 atoms/b·cm

Room-temperature light water — moderator and coolant in PWRs and BWRs.

Where these numbers come from

PNNL-15870 Rev. 2, "Water, Liquid" (id water-liquid): 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)
H-1 (1001)	1.1187e-1	6.6659e-1	6.6647e-2
H-2 (1002)	2.5714e-5	7.6667e-5	7.6653e-6
O-16 (8016)	8.8569e-1	3.3252e-1	3.3247e-2
O-17 (8017)	3.5856e-4	1.2667e-4	1.2665e-5
O-18 (8018)	2.0482e-3	6.8333e-4	6.8322e-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.

Bound scattering

ENDF/B-VII.1 has a thermal scattering law for this material as formulated, so the cards attach lwtr.20t. Free-gas scattering would misplace the thermal spectrum here.

What ENDF/B-VII.1 forced

O18 folded into O16: ENDF/B-VII.1 has no O18 evaluation (0.002048 by weight, so the spectrum does not care).

The cards keep the material's mass and atom count; a nuclide the library cannot name hands its share to one it can. That is why a card row can differ slightly from the table above.

Generated material cards

MCNP

c Light Water
c PNNL-15870 Rev. 2, "Water, Liquid" - reactormc.net/nrdp/material/light-water;
c rho = 0.997 g/cm3 (use -0.997 on the cell card)
c O18 folded into O16: ENDF/B-VII.1 has no O18 evaluation (0.002048 by weight,
c so the spectrum does not care).
m16    1001.80c  -0.111872  $ H1
       1002.80c  -2.5714e-5  $ H2
       8016.80c  -0.887740  $ O16
       8017.80c  -3.5856e-4  $ O17
mt16   lwtr.20t

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

Density is the compendium's room-temperature value. Reactor conditions are far from it: ~0.72 g/cm³ at PWR hot-leg temperature (300 °C), ~0.74 g/cm³ in a BWR core (286 °C). Scaling the density is the single most important correction on this page. Hydrogen needs lwtr.20t: free-gas hydrogen mis-moderates badly.

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