U.S. Nuclear Regulatory Commission
Averaging of Radiation Levels Over the Detector Probe Area
Title: Averaging of Radiation Levels Over the Detector
See the letter from L. V. Gossick to J. J. Munro (Tech/Ops,
Radiation Products Division) concerning PRM-20-9 and dated
March 23, 1979. The letter states that averaging of
radiation levels over the cross-sectional area of a probe
of reasonable size is acceptable for demonstrating
compliance with the requirements of 10 CFR 20.205 (c) (2).
This health physics position also applies to "new" 10 CFR
20.1906 (d) (2).
PRM-20-9 was a petition submitted to NRR requesting
amendment of 10 CFR Part 20.205 (c) (2) regarding surface
radiation level limits of packages for transport. It was
requested that 10 CFR Part 20.205 (c) (2) be amended so
that radiation levels found five centimeters from the
external surface of the package in excess of 100
millirem/hour or three feet from the package in excess of
10 millirem/hour would require the immediate notification
of the Director of the appropriate NRC Regional Office and
the final delivering carrier. In determining the radiation
levels, the measurements were to be averaged over a
cross-sectional area of ten square centimeters with no
linear dimension being greater than five centimeters.
As written, 10 CFR 20.205 (c) (2) required a licensee who
received a package of radioactive material in excess of
Type A quantity to monitor the external radiation levels
both at the surface and at three feet from the surface of
the package. If the radiation levels exceeded 200
millirems per hour at the surface or 10 millirems per hour
three feet from the surface, the licensee was to
immediately report to the Director of the appropriate NRC
Regional Office and to the final delivering carrier.
In denying the petition, the NRC stated that the proposed
changes to 10 CFR Part 20.205 (c) (2) would result in
increased costs to the licensee without a corresponding
benefit in improved public health or safety. In fact, the
proposed changes would result in higher collective hand
doses being delivered to package handlers.
In its ruling, the NRC stated that radiation levels
averaged over a cross-sectional area of a probe of
reasonable size is acceptable for demonstrating compliance
with the requirements specified in 10 CFR 20.205 (c) (2)
[or 10 CFR 20.1906 (d) (2)]. "A probe of reasonable size"
was defined as: (1) the sensitive volume of the probe being
small compared to the volume of the package being measured,
and (2) the largest linear dimension of the sensitive
volume of the probe being no greater than the smallest
dimension of the package. Geiger-Mueller tubes may be used
for both small and large packages but ionization chambers
should be used only for large packages. Averaging is not
acceptable for demonstrating cracks, pinholes, uncontrolled
voids, or other defects prior to the first use of any
packaging for the shipment of licensed materials as
required by 10 CFR 71.53.
Regulatory references: 10 CFR 20.201, 10 CFR 20.205, 10
CFR 20.1501, 10 CFR 20.1906, 10 CFR 71.53
Subject codes: 7.1, 12.17