Environmental Assessment of Ionization Chamber Smoke Detectors Containing Am-241 (NUREG/CR-1156)
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Manuscript Completed: October 1979
Date Published: November 1979
R. Belanger, D. W. Buckley and J. B. Swenson
Science Applications, Inc.
1200 Prospect Street
La Jolla, CA 92038
Division of Fuel Cycle and Material Safety
Office of Nuclear Material Safety and Safeguards
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
Under Interagency Agreement DOE 40-550-75
Residential fires continue to pose a major problem in the United States, ranking as the second most frequent cause of accidental death in the home. Between 7,500 and 12,000 lives are lost in fires every year with 70 percent of these occurring in residential fires. Residential property loss due to fires is over one billion dollars annually. A key element in reducing loss of life and property is early detection of fires. Various devices are available that detect flames, heat, or products of combustion. In recent years devices that detect products of combustion have been introduced to the consumer market and their sales have increased dramatically. These so-called "smoke" detectors are of two types. The majority have been of the ionization chamber smoke detector (ICSD) type which contains small amounts of radioactive material. The remainder have been of the photoelectric type which contains no radioactive material. A combination detector has been recently marketed that contains both an ICSD and a photoelectric detector. Based on theoretical studies and case histories, the estimated percent of residential fire-related deaths that smoke detectors could save is between 41 and 89 percent.
Most ionization chamber smoke detectors contain americium-241 (Am-241), a radioactive substance that emits ionizing radiation. Other radionuclides could be used in place of Am-241. In the past, radium-226 (Ra-226) has been used in detectors, but at present it is not employed in the consumer product. Nickel-63 (Ni-63) is presently being studied for use in consumer smoke detectors, having already been used in commercial applications such as in large warehouses. The user of an Am-241 ICSO is exempt from regulations, but the manufacturer must apply for and obtain a specific license from the Nuclear Regulatory Commission to distribute the product. The license is granted only after the applicant has demonstrated that the product is designed and will be manufactured in a manner such that specific requirements and safety criteria are met.
Since 1972-1973, the residential market for Am-241 ICSD's has increased rapidly. Fourteen million units containing a total of 41 curies of Am-241 were distributed in 1978 and 26 million units have been distributed since 1972. Based upon present projections, about 90 million units will be distributed by 1986. The peak year of distribution should be 1978. The average activity per unit in 1978 was 3 uCi and projections indicate this will become lower in the future.
The distribution of photoelectric detectors has been minimal compared to ICSD's. Early photoelectric units were not as reliable as comparable ICSO's because of various reasons, in particular, the use of incandescent bulbs. The introduction of low-power light emitting diodes has improved their reliability considerably. The cost of photoelectric units is greater than the cost of comparable Am-241 ICSD's, basically because of slightly more complex circuitry. The photoelectric units have been found to react quicker to smoldering or slow-burning fires while the ICSD's react quicker to fast-burning fires. Both appear to have responses comparable in terms of saving lives. However, it is. felt the ICSD units may have a slight edge over the photoelectric units since response is more critical to fast-burning fires.
The nonradiological impacts due to use of both ICSD's and photoelectric detectors have been determined to be slight and indistinguishable from one another. For a typical residential installation, the risk of an occupant being exposed to nonradtological toxic products, due to burning for example, is judged to be extremely small.
The use of Am-241 ICSD's does result in exposure of people to low levels of radiation. Analysis shows that the manufacture, distribution, normal use, and disposal of 14 million Am-241 ICSD's each containing 3 uCi of Am-241 will result in a collective total body dose of 110C.oerson-rem. The useful life is assumed to be ten years. Disposal is by either sanitary landfill or incineration. Fourteen million ICSD's will service about 21 million people. Analysis also shows the risk to the exposed population is about 0.1 fatal cancer. The normally-occurring cancer mortality rate for the total population of the United States is about 370,000 per year or about 35,000 per year for a group of 21 million people.
A comparison of these numbers illustrates the relatively small risk involved in using an Am-241 ICSD. It should be noted the 370,000 cancer deaths per year are actual deaths while the 0.1 fatal cancer over ten years of ICSD use was calculated using conservative assumptions. The ratio of the potential lives saved to the possible fatal cancers due to use of ICSO's ranges from 15,000 to 51,000.
Analysis of potential accidents with Am-241 ICSD's showed the dose commitments received by maximally exposed individuals to be significantly within the safety criteria standards required by the Nuclear Regulatory Commission.
Comparative analyses based upon cost, risk, and benefits, found present day Am-241 ICSD's to be preferable to ICSD's. with other radionuclides and preferable to present day photoelectric detectors. The best available unit for fire protection was found to be the recently marketed combination detector. The cost of a combination unit is considerably more than the Am-241 ICSD and data is lacking on how much more sensitive it is. Therefore, the cost-effectiveness of combination units compared to Am-241 ICSD's is unknown.