Radiation and Smoke Detectors

In the late 1930s, a smoker inadvertently made a discovery for detecting smoke. The Swiss physicist Walter Jaeger tried to invent a sensor for poison gas. His device failed: small concentrations of gas had no effect on the sensor’s conductivity. Frustrated, Jaeger lit a cigarette—and noticed that a meter on the instrument registered a drop in the current. Smoke particles had apparently done what poison gas could not. Jaeger’s experiment was one of the advances that paved the way for the modern smoke detector.

Here’s something else surprising: Smoke detectors work because of radiation. They are an example of the beneficial uses of radiation and radioactive materials.

The first significant installations of commercial smoke detectors started in the US around 1969. Since then, the installation of smoke detectors has saved thousands of lives, numerous injuries, and millions of dollars. It has been reported that smoke detectors are installed in 93 percent of US residences. However, it is estimated that more than 30 percent of these alarms don’t work, as users remove the batteries or forget to replace them in a timely manner.

In the US, while smoke detector manufacturers and distributors are subject to NRC regulation, end users of smoke detectors (consumers) are typically not because of the small amount of radioactive material used in each detector.

The most common type of smoke detector consists of an ionization chamber, electronic circuitry, a power source that is usually a battery, an alarm mechanism, and an outer case. The ionization chamber is the main component. It consists of a source of ionizing radiation, usually Americium (Am-241) positioned between two oppositely-charged electrodes. The radiation source is a very small metallic foil disc about 3 to 5 millimeters in diameter.

To give you an idea of the small amount of radiation that is emitted by this disc, a person flying coast-to-coast gets more radiation from cosmic sources in one trip than a person sitting in the close proximity of an ionization smoke detector gets in a whole year.

Here is how the device works: Particles emitted during radioactive decay of the Am-241 interact with neutral air molecules flowing through the chamber and convert them to positive ions by removal of electrons. The removed electrons then form negative ions by attachment to other neutral molecules. The resulting positive and negative ions are attracted toward the electrodes, causing a small, reasonably steady current between the electrodes. The electronic circuitry monitors this current and, if the current drops below a preset level, which it will if the air entering the chamber contains enough smoke, it triggers an audible alarm.

If you are interested in the technical evaluations the NRC has done on smoke detectors and other consumer products containing radioactive material please see NUREG-1717 “Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials” .

Ujagar Bhachu
Mechanical Engineer

Japan Task Force Report Now Public

While finding that events like the Fukushima accident are unlikely at U.S. reactors and U.S. reactors can be operated safely, the NRC’s Japan Task Force report made public today proposed improvements in a variety of areas, including “loss of power” response, spent fuel pools and preparedness for natural events.

The report has been given to the Commissioners, who will be formally briefed on it next Tuesday. On July 28, the task force will hold a public meeting on the report, and members will appear before the Advisory Committee on Reactor Safeguards on Aug. 17. Additional meetings may be scheduled to seek public input on the recommendations. Any action on the report’s recommendations is up to the Commission.

The report, which noted that over the years “patchwork of regulatory requirements” developed and suggested it be replaced with a more logical, systematic and coherent regulatory framework, was produced by team of in-house experts who collectively had over 130 years of reactor regulatory experience. This report will be followed about six months later by a more in-depth report as additional information about the Fukushima reactors becomes available.

Other highlights from the report:

The current NRC approach to regulation includes requirements for protection and mitigation of design-basis events, requirements for some “beyond-design-basis” events through regulations, and voluntary industry initiatives to address severe accident issues. “Consistent with the NRC’s organizational value of excellence, the Task Force believes that improving the NRC’s regulatory framework is an appropriate, realistic and achievable goal.”

Continued operation and continued licensing activities do not pose an imminent risk to public health and safety, the report added.

The report, among other things, recommends:

• Requiring plants to reevaluate and upgrade as necessary their design-basis seismic and flooding protection of structures, systems and components for each operating reactor and reconfirm that design basis every 10 years;

• Strengthening Station Black Out (SBO) mitigation capability for existing and new reactors for design-basis and beyond-design-basis natural events – such as floods, hurricanes, earthquakes, tornadoes or tsunamis – with a rule to set minimum coping time without offsite or onsite AC power at 8 hours; establishing equipment, procedures and training to keep the core and spent fuel pool cool at least 72 hours; and preplanning and pre-staging offsite resources to be delivered to the site to support uninterrupted core and pool cooling and coolant system and containment integrity as needed;

• Requiring that facility emergency plans address prolonged station blackouts and events involving multiple reactors;

• Requiring additional instrumentation and seismically protected systems to provide additional cooling water to spent fuel pools if necessary; and requiring at least one system of electrical power to operate spent fuel pool instrumentation and pumps at all times. The Task Force noted it will take some time for a full understanding of the sequence of events and condition of the spent fuel pools. The report said based on information available to date the two most cogent insights related to the availability of pool instrumentation and the plant’s capability for cooling and water inventory management;

• Requiring reliable hardened vent designs in boiling water reactors (BWRs) with Mark I and Mark II containments;

• Strengthening and integrating onsite emergency response capabilities such as emergency operating procedures, severe accident management guidelines and extensive damage mitigation guidelines.

The full report can be found here: http://pbadupws.nrc.gov/docs/ML1118/ML111861807.pdf. Broad recommendations are contained in the Executive Summary, and details on recommendations can be found in Appendix A.

Eliot Brenner
Public Affairs Director
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