Answer to Question #3199 Submitted to "Ask the Experts"

Plutonium-238 (²³⁸Pu) decays primarily by alpha-particle emission. An intimate mixture of ²³⁸Pu with beryllium becomes a good source of neutrons, however, through a nuclear reaction in which ⁹Be absorbs an alpha particle and forms ¹²C with the emission of a neutron.¹ Such sources are typically encapsulated, and if the integrity of the encapsulation is not compromised then shielding should be designed to address primarily the neutron radiation. Materials like paraffin and plastic (for example, polyethylene) are commonly used shielding materials for these neutron generators, moderating and absorbing the neutrons by interactions with protons in the shield material² and also absorbing some of the incidental photon radiation. Additional shielding of photons with lead or steel may be necessary, however. A simple storage container might consist of the source placed inside a capped steel pipe around which molten paraffin is allowed to solidify. Some additional shielding around the paraffin against photons released when moderated neutrons are absorbed may also be necessary. Measurements should be made to determine the effectiveness of your container in shielding both neutrons and photons.

Your email address suggests that you are in Australia. You should contact the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA)³ for ²³⁸Pu storage and physical protection requirements there.

In the United States, plutonium is Special Nuclear Material (SNM), as defined in the Atomic Energy Act of 1954, as Amended (NUREG-0980). Its use in the private or commercial sector is by license with the Nuclear Regulatory Commission or through an Agreement State. U.S. requirements for physical protection of SNM are found in Title 10, Code of Federal Regulations, Part 73 (10 CFR 73); requirements for material control and accounting are in Title 10, Code of Federal Regulations, Part 74 (10 CFR 74). Specific requirements are also included as part of the license.

Your best option for disposal, if the plutonium in your ²³⁸PuBe source was produced in the United States, is to arrange with the manufacturer to take back the source. Such sources can be accepted for disposal from within the United States by a government program called the Off-Site Source Recovery Project (OSRP). Contact the corresponding agency responsible for radioactive material control in your own country (ARPANSA in Australia⁴) if this is not an option. Note that the International Atomic Energy Agency (IAEA) defines your source as a Category 3 source (IAEA-TECDOC-1344) and therefore considered potentially dangerous if uncontrolled. You should not just throw away or dispose of this source except in an authorized manner.

James S. Bogard, CHP, PhD

1. James E. Turner, Atoms, Radiation, and Radiation Protection, Second Edition, John Wiley & Sons, Inc.; page 210; 1995.
     
2. For more at this Web site about neutron interactions with matter, see Bruce Busby's response to Question #609 in Ask the Experts category "Radiation Basics-Neutrons" and George Chabot's answer to Question #1094 in "Radiation Shielding."
     
3. We suggest Peter Colgan as a contact at ARPANSA at the time of this response (November 2003).
    
   
4. Australia has proposed the construction of a National Radioactive Waste Repository. Follow the link on the ARPANSA Web site for more information.