Answer to Question #7973 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

My question is about the calculation of a transport index (TI) used in package labeling of radioactive materials in transport. I would like to know what goes into the calculation of the TI and how it would properly be measured by first responders at the scene of an event/accident. I am aware that it represents the maximum allowable dose rate measured at one meter from the labeled package. This is a good piece of information, however, how would first responders to the scene of an accident (who need to determine if there is a breach in the packaging) measure this dose rate? If the material is emitting gamma radiation, I guess a gamma scintillator probe with a rate meter will do. But what if the material is emitting neutron radiation; how will this dose rate be determined? I assume alpha and beta radiation will probably be shielded within the packaging.

A

I’ll try to clarify what the transport index is and then attempt to address the remainder of your question. The transport index (TI) is the maximum dose equivalent rate at one meter from the surface of a package containing radioactive material. It is a number that is established by measurement after the radioactive material has been packaged for transport. The TI is then the actual dose rate at one meter; it is not the maximum allowable dose rate at one meter. It is true that there are restrictions on how great a value of the TI is allowed for legal transportation of a single package and also restrictions on the sum of TIs from multiple packages in a single shipment.

For packages with a white Radioactive I label, no transport index is necessary since such packages expectedly produce a negligible reading at one meter (and no more than 0.005 mSv h-1 at contact with the package surface). For packages with a yellow Radioactive II label, the TI must not exceed 0.01 mSv h-1, and for packages with a yellow Radioactive III label, the TI exceeds 0.01 mSv h-1. For common carriers (carriers not devoted exclusively to transporting radioactive material from a single shipper) the TI must not exceed 0.1 mSv h-1, and the dose rate at the surface of the package must not exceed 2 mSv h-1. Additionally, if multiple radioactive packages are being transported together, the sums of the TIs for all the packages must not exceed 0.5 mSv h-1. For exclusive-use transport vehicles, higher limits apply—e.g., for a closed exclusive-use vehicle, the dose rate may be as high as 10 mSv h-1 at the surface of the package, 2 mSv h-1 at the surface of the vehicle, and 0.1 mSv h-1 at two meters from the vehicle surface.

In the event of a transportation accident, if a radioactive package remains intact with no appreciable damage or shifting of the source and/or shielding within the package, the TI on the package label should provide a good indication of the dose rate at one meter from the package surface. If more than one radiation type is contributing to the dose rate, the TI represents the sum of contributions from all of the radiations, but the label with the TI will not tell you how much of the dose rate is from a specific radiation type.

For the very large majority of radioactive packages that are transported around the country, the major radiation contributing to dose at one meter form the surface of the package is gamma radiation, which can be reasonably easily measured with any of several detectors. You are correct in your assumption that alpha and beta radiation being emitted by the radioactive material will most likely not contribute to the dose rate at one meter. For many gamma-emitting radionuclides, an energy-compensated Geiger-Mueller detector may be adequate for assessment by first responders. Plastic scintillator detectors, calibrated to read tissue dose rate or exposure rate are frequently good choices if available; they have the advantage that the detector material is similar enough to soft tissue that it provides a tissue-like response over a wide range of energies. Inorganic scintillators, such as sodium iodide, must be used with caution because they usually exhibit a marked energy-dependent response quite different from tissue, thus making accurate dose measurements difficult. Many ionization chamber detectors are also desirable for gamma measurements and generally provide good energy response; just be sure that the selected instrument covers a sufficient range of dose rates, especially on the low end. There are numerous companies that provide radiation detectors of these and other types. A few representative ones are Ludlum Measurements, Inc., Thermo Scientific, Victoreen Instruments (Elimpex), Technical Associates, and Fluke Biomedical. There are numerous others and the listing here should not be interpreted as an endorsement of any particular company.

Neutrons are a concern for a tiny fraction of all the packages under transport by common carriers. Neutrons are emitted from relatively few radionuclide sources. One such type is referred to as an alpha, neutron source. It uses a mixture of an alpha-emitting radionuclide, such as 241Am, with an appropriate stable target species, most commonly beryllium, doubly encapsulated in stainless steel. Another source that has fairly widespread use is 252Cf, which produces neutrons through a process called spontaneous fission. There are a number of transuranic radionuclides (radionuclides beyond uranium in the periodic table), such as some isotopes of plutonium, that also undergo spontaneous fission and yield some neutrons. Such radionuclide species might be present in spent nuclear reactor fuel or in other sources, possibly including nuclear material that might be intended for or suitable for use in a nuclear weapon.

I do not believe that most first responders have neutron-measuring instrumentation immediately available and would probably not be able to obtain any firsthand measurements of neutron dose rates from packages that might emit neutrons. In such an event, where a package containing neutron-emitting materials appears to have been damaged, the best course of action for the first responders may be to remove any people from the vicinity of the package and to seek assistance from the state radiation control program, which should have appropriate instrumentation available. The shipping papers that are required with a shipment of radioactive material can be very helpful in confirming the nature of the material being shipped, and these should be reviewed.

If you, or another first responder, are looking to obtain neutron dose measuring instrumentation, there are limited practical possibilities. These detectors are considerably more pricey than many of the common gamma-measuring instruments. Among the most popular instruments are those that use rather large volumes of polyethylene to slow down fast neutrons so that they can be detected by the detector, which is sensitive to low-energy neutrons. These detectors commonly weigh in the range from about 9 to 14 kg and are designed to measure neutron dose over a wide range of neutron energies. Here are a links to a few examples—Thermo Scientific (1) and Thermo Scientific (2) and Elimpex/Victoreen (scan down the page to Model 190N). Here is another link to a somewhat lower mass (about 4 kg) instrument from Canberra.

Transportation regulations are quite restrictive, and as the amounts and hazards of radioactive materials increase the physical requirements for packaging also increase to provide enhanced safety in the event that a package is involved in a transportation accident. Over the years there have been numerous transportation accidents involving radioactive materials, and the packaging has fared very well. According to information from a Nuclear Regulatory Commission workshop, for the least restrictive type of packaging, which is simply called a strong tight container that requires no specific integrity testing, about 10 percent of those involved in accidents have failed to some extent. For what is referred to as Type A packaging that does require stringent testing, about 1 percent have failed. For Type B packaging, which must abide by the most demanding requirements for integrity, there has been only one case of package failure and that was associated with an industrial radiography source. Sources of any significant neutron production would likely be in a Type A, or possibly Type B package for particularly large sources, so loss of package integrity is likely a low-probability event.

Naturally, none of the packaging requirements would necessarily apply in the cases of illegal shipments, as might be associated with terrorist activities in which case there may be no knowledge on the part of innocent parties as to the contents of specific packages. We all hope that these are situations that we will not have to confront.

I hope this information is helpful to you.

George Chabot, PhD, CHP 

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