Beryllium Oxide Anomaly

I computed the critical masses of the WG-HEU metal spheres with various reflectors found in:

Paxton, H. C.; Pruvost, N. L. 1986. Critical Dimensions of Systems Containing 235U, 239Pu, and 233U ”, LA-10860-MS, 1986 Revision. Table 28, p. 92.

using ENDF VIII and generally they were close, there is a systematic upward bias in the range of 2-15% which possibly is due to a difference between the effective experimental and the computational criterion of criticality, which is the sort of pattern I expected.

But there was one reflector material – BeO – that was an outlier. The computed values was 35-68% higher than experimental.

This was not observed with the other multi-element reflectors (WC, tungsten alloy*), so I believe I am configuring that correctly:
[material id=“2” name=“reflector”]
[density units=“g/cc” value=“2.69” /]
[nuclide ao=“0.5” name=“Be9” /]
[nuclide ao=“0.4998105” name=“O16” /]
[nuclide ao=“0.0001895” name=“O17” /]

Is there some known reason for this anomaly?

  • I had to get the actual tungsten alloy composition from the original report, LA-10860-MS is incomplete and inaccurate in its description:
    Paxton , H. C. 1964. Los Alamos Critical-Mass Data , LAMS-3067-MS. Los Alamos Scientific Laboratory, April 1964.

ENDF/B-VIII.0 has thermal scattering evaluations for Be in BeO and O in BeO — if you’re not using these already, make sure you specify those in the material:

reflector = openmc.Material()