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WIPP's Salt Formation is Very Low in Naturally Occurring Radioactivity


Uranium/Thorium/Potassium: The salt layers of the WIPP underground contain significantly less natural radioactivity than the rocks exposed in other mines.  In the accompanying photgraph, a miner's lamp held against the salt shows the diffuse glow of backscattered light from within the rock formation.  However, the WIPP salt contains some impurities.  Note the lighter and darker bands.  These indicate varying levels of other materials present in the salt, such as clay or wind-blown dust deposited during salt-bed formation more than 250 million years ago.  Interval-weighted composite samples from the WIPP horizon exhibit about 3-5% of the naturally occurring radioactivity that is found in average crustal rocks: 
Uranium Thorium Potassium (K-40)

30 - 50 ppb

70 - 80 ppb

0.1 - 0.8 ppm

Neutron Flux: Free neutron flux levels were measured by Ohio State University (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.

Radon: 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).  Learn how the ventilation controls provide fresh air from the surface and keep it flowing through the experiment gallery, and separate it from air that flows near waste handling and disposal operations.   

Muon Flux: For overburdens greater than about 100 mwe, 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, a muon flux  of about 2x10-3 /m2/s (6x10+4/m2/yr) is expected.

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