Answer to Question #8600 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

I need to perform leak testing on sealed sources of 57Co, 137Cs, and 133Ba. Can a leak-test sample be checked with a pancake GM probe prior to delivery to my lab for well-counter testing? Would the leak-test sample be exempt from Nuclear Regulatory Commission (NRC) regulations for transport/disposal because of the low amount of radioactive contamination, if any, of the sample taken? I reside in the state of Washington. One question the state representative asked was if there is a need for air-emission testing because of the samples.

My take on this is: The leak-test sample, if not detectable with a pancake probe, should be exempt of any air-emission testing and also should be an exempt quantity of radioactive material for transport and also disposal to regular trash if the sample is not greater than 185 Bq.

A

I believe most of your conclusions regarding the exemptions that apply to the leak-test samples of concern are valid. The specific radionuclides that you mention, 57Co, 137Cs, and 133Ba, have respective NRC license exemption limits of 3.7 x 106 Bq, 3.7 x 105 Bq, and 3.7 x 105 Bq , respectively, as tabulated in 10 CFR 30.71 Schedule B. Also, according to 10 CFR 71.14, certain quantities of radionuclides are exempt from the NRC transportation requirements of Part 71. The specific limits of the radionuclides you mention that are tabulated in Table A-2 of Appendix A of 10 CFR 71 are 1 × 106 Bq for 57Co, 1 × 104 Bq for 137Cs, and 1 × 106 Bq for 133Ba. Thus, at least in so far as abiding by NRC regulations, it appears that for most situations involving minimal source leakage, the wipe samples could be shipped to you. In the case of an actual leaking source that yielded test samples in excess of any of the cited limits, the samples could not be shipped without specific considerations of and abidance by the applicable regulations (probably a moot point since the initial results would have already confirmed a leaking source and further evaluation may not be required).

As to the use of a pancake GM probe for making initial measurements of the test samples, one should have no trouble seeing 185 Bq of 137Cs on a wipe sample placed directly below the window of the pancake probe. A typical thin-window pancake probe with an active area of about 15 cm2 will have an approximate 4 p efficiency of 20 percent for the beta radiation from 137Cs. Thus, 185 Bq of 137Cs would yield an approximate net count rate of 233 cps, a number easily distinguishable from a usual background rate of around 0.8 cps. The same may not be true for nuclides such as 57Co and 133Ba. Each of these decays by electron capture, and the only particulate radiations emitted are some low- and moderate-energy Auger and conversion electrons, the possibly detectable ones having low yields. Significant gamma rays are emitted by both radionuclides, but the detection efficiency for the gamma radiation is characteristically less than 1 percent. If you still wish to use the pancake detector, the licensee should arrange to have the detector tested against representative 57Co and 133Ba sources of known activity to determine whether the detector has adequate sensitivity for use for the intended purpose. If the detector is not suitable, the licensee may be able to use a different type of detector, such as a reasonably thick NaI(Tl) scintillator—e.g., a 2.54 cm × 2.54 cm NaI(Tl) detector would yield an expected net count rate of several tens of cps when a 185 Bq point source of 57Co was positioned directly below the flat face of the crystal. A comparable activity 133Ba source would likely yield an even higher net count rate with such a detector.

Of course, the exempt quantities noted above (both license-exempt and Part 71-exempt) are all values that one would have no difficulty seeing with the pancake probe. If the licensee can convince his regulator that, based on measurements with the pancake probe, in no case would any shipment exceed the pertinent exempt quantity, he might obtain permission to ship the test samples. If the pancake probe is to be used, I would recommend having it calibrated for detection of the radionuclides of interest, especially the 57Co and 133Ba.

Regarding emission of radioactive material into the air from the wipe-test samples, I do not believe this should be a concern. Most of the usual chemical forms of the three radionuclides of interest are not readily volatilized at ambient temperatures. Since wipe tests are being performed presumably to detect possible leakage from sealed sources, and since sealed sources of these radionuclides contain solid source material, any leakage would presumably be of particulate material and, except in the case of a major fracture of such a source, any particulates obtained on a wipe should be retained quite well on the wipe material. Enclosure of the test wipe in a closed plastic bag should provide acceptable assurance that any particulates that might be released from the wipe during transit would be appropriately contained.

George Chabot, PhD, CHP

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