Typical levels of primordial radioactivity measured in bulk samples taken from the existing mine horizon are less than typical hard-rock mines:
Free neutron flux levels were measured by OSU (Boyd, et. al., Nuclear Instruments and Methods in Physics Research, A 399, 1997, pp 269-274) at 332 +/- 148 neutrons/m2/day. Thermal and epi-thermal neutron flux levels were measured at 115 +/- 22 neutrons/m2/day.
With such low levels of natural Uranium, and with high constant ventilation, Rn-222 levels in the WIPP underground are dominated by surface air concentrations (Rn-222 < 7 Bq/m3).
For overburdens greater than about 100 mwe, the neutrons from fission and (alpha,n) reactions dominate the thermal and epi-thermal energies. However, the much harder to measure flux of energetic neutrons created by nuclear capture of negative muons in deep inelastic muon-nucleus scattering is chiefly a function of muon flux. At WIPP's depth, we expect a muon flux (this has not been measured yet) of about 2x10-3 /m2/s