Averaging of Radiation Levels Over the Detector Probe Area
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