A Radiation Protection Primer

Joel O. Lubenau is senior project manager, health physics, with the U.S. Nuclear Regulatory Commission, state programs (Washington, D.C.). Anthony LaMastra is a certified health physicist with Health Physics Associates Inc. (Lenhartsville, Pa.). Michael S. Peters is manager, environment and energy, for Structural Metals Inc. (Seguin, Texas). James G. Yusko is western area health physicist, bureau of radiation protection, Pennsylvania Department of Environmental Resources (Pittsburgh).

Since 1983, there have been at least 77 incidents in the United States in which radioactive material was found in metal scrap. There have also been 10 reported instances in which radioactive materials were accidentally melted during recycling. Seven of these events involved steel mills, but aluminum, copper, and lead recyclers have also suffered incidents. Cleanup costs for the steel mills averaged in excess of $1 million and in one case the cost was about $4 million. Fortunately, there was no serious radiation exposure to employees or the public in these cases.

In a 1984 international case, cast iron table legs and rebar contaminated with radioactive cobalt-60 were imported from Mexico into the United States. Federal and state radiation protection agencies mounted a huge search for the material. Ultimately, 2,500 table legs and 1,500 tons of rebar were recovered and returned to Mexico for disposal as radioactive waste. Serious exposure to workers and the public did occur in this case.

At present, there is no central clearinghouse for collecting incident reports involving radioactive scrap, so any estimates about occurrences are bound to be conservative. One point is certain: The problem of radiation in metal scrap exists and it is evolving. To protect themselves, scrap recyclers should consider implementing protection programs at their operations, using radiation detection equipment and quality assurance procedures to prevent problems.

For a protection program to be effective, plant management must fully support it. Management must be willing to commit funds for radiation detection equipment, train personnel to carry out the program, and audit the program to ensure that it is working. In this sense, a protection program is no different than any other plant investment.

A quality assurance (QA) program is a vital foundation in all protection efforts. It should include the following elements:

Designation of a management representative responsible for the QA program. This individual ensures that the program is implemented and reports to the plant manager on the status and results of the program.

Training and retraining programs for supervisory and operating staff. All employees must be informed and committed if a program is to be effective. Training should cover operation of detection equipment, employee safety, alarm responses, contamination responses, and cleanup requirements and consequences. Such information will not only mitigate concerns but provide incentives for employees to diligently carry out the program. Professional safety specialists such as health physicists can assist in these efforts. Training should emphasize that the protection program is only an insurance policy, not a guarantee. No protective measures can be 100-percent effective.

Regularly scheduled feedback sessions with employees. This can be accomplished in retraining sessions and during informal visits by the QA manager around the plant.

Maintaining and reviewing operational logs. This is particularly important when radiation monitoring equipment is used. Records of daily and periodic performance tests, alarms (including false alarms), and repairs and maintenance can track the equipment's performance. The QA manager should review these records regularly to ensure good recordkeeping practices and stimulate feedback.

Periodic independent checks and tests of technical equipment by the QA manager. These checks are especially recommended whenever unscheduled maintenance or repairs are performed on the equipment.

One simple yet effective operational check can be performed using small radioactive sources such as a 10 microcurie cesium-137 sealed source. These "check sources" are safe enough to buy without a license, yet radioactive enough to test monitoring equipment. This test should be performed daily by operators and periodically by the QA manager. When records are kept, the data can provide information on the monitor's long-term performance.

Comparison of your plant's experience with experiences of comparable plants. The QA manager needs to keep up with developments. Awareness and familiarity with programs at other plants and sharing information on experiences helps everyone.

Increasingly, scrap processors are using radiation monitors as a primary protective measure. Keep in mind that not all radioactive materials emit radiation that can be detected externally, and there are physical limits on the sensitivity of detection instruments.

Radiation monitors are "event" detectors-that is, they detect an ionization event and produce an electronic signal. The most familiar radiation detector is the Geiger-Mueller detector. Radiation interacts with a gas in the detector and creates an electronic pulse. Geiger-Mueller detectors are inexpensive but relatively insensitive for scrap monitoring purposes. Monitors designed specifically for scrap applications usually feature a denser detection medium such as a liquid or solid, increasing the chance that radiation will interact with the media.

There is always some background radiation and its level varies. To account for this, the monitor alarm level is usually set two to three times higher than the background level. A lower setting will usually result in an unacceptable number of false alarms. More sophisticated systems can track the trends in the background level and, thus, can be set as low as 10-percent above background.

In general, monitor sensitivity can be increased by shielding the detector on all sides except the front, thereby decreasing external background radiation, and by electromagnetic shielding of the photomultiplier tube and cables, which reduces electronic noise.

Sodium iodide (NaI) crystals and plastic are the most common solid media used in radiation detectors. NaI detectors used for monitoring scrap typically feature a 2-by-2-inch crystal and typically are used singly, connected to a simple electronic alarm circuit.

Larger crystals are available, but they are more expensive and more susceptible to breakage from physical or thermal shock. NaI detectors must be waterproof, as moisture can damage the crystal, and should not be subjected to rapid temperature changes. NaI systems, which detect only gamma radiation, are readily available, relatively reliable, and inexpensive.

Plastic detectors can be large (up to thousands of cubic inches), which increases sensitivity. They can detect neutron sources as well as gamma radiation. In 1990, a U.S. steel plant that uses both a NaI and a newer plastic detector system found an americium-beryllium neutron source. The NaI detector did not pick up the source; the plastic detector did.

Plastic detectors are also generally more resistant to environmental stresses than NaI detectors. When large plastic detectors are aired and the outputs are coupled to a microprocessor, the system can be significantly more sensitive than Nal systems. However, such systems are also substantially more expensive.

One compromise arrangement is to reduce the plastic detector to roughly the same size as a 2-by-2-inch NaI detector but retain the microprocessor circuit. Another alternative is to use a plastic detector with an intermediate-sized volume (hundreds of cubic inches) connected to an advanced rate meter circuit. One such system has detected naturally occurring radioactive material in loose dirt and rocks in railcars.

Large liquid scintillation detectors are also used, though infrequently, in scrap settings; they are expensive and cannot function if any liquid is accidentally lost.

External factors can influence a system's ability to detect radiation in scrap. For example, radiation sources can be encased in lead or shielded by scrap. The greater the density and mass of the scrap, the greater the shielding factor. Thus, aluminum scrap processors face a smaller shielding factor than steel scrap processors. In addition, because trucks carry a smaller volume of scrap than gondola-type railcars, shielding is more likely in scrap carried in the latter.

A basic tenet of radiation detection is that the longer a potential source is monitored, the greater the chance of detecting any radiation present. Therefore, whether vehicles carrying scrap are stationary (as with most truck shipments)--and for how long--or whether they are moving (as with most rail shipments)--and how fast--will influence the effectiveness of the system.

The distance of detectors from the scrap also affects sensitivity. In general, the farther the monitor from the material, the less effective it is. Thus a single monitor mounted to detect radiation from below--because it must be placed well above vehicle height to avoid damage--is usually less sensitive than monitors installed to the side--which allows closer proximity to the vehicle.

Systems with detectors on both sides of incoming vehicles can be more sensitive than those with a single detector, provided that the signals from each detector are not summed. Summing the signals from two detectors doubles the background count and the alarm set point. When used properly, dual detectors can detect a source that is too far from one detector but close enough to the other to trigger an alarm.

Detection systems using NaI or plastic detectors should be equipped with "low-noise" photomultiplier tubes. These should be required in the purchase order or bid specifications.

Incoming and outgoing shipments should be monitored. Processing scrap may reconfigure a radioactive source that was undetected in an incoming shipment. Monitoring outgoing shipments gives recyclers a second opportunity to check for radiation in their products. For some facilities, monitoring outgoing shipments may mean either redirecting traffic patterns or purchasing additional monitors. .

The need for rugged monitors cannot be emphasized enough. In addition to environmental factors such as temperature changes and moisture, detectors are exposed to vibration and damage from vehicles and protruding scrap. Protective barriers for side-mounted detectors are a must. When charging-bucket monitors are used, sturdy detector shields, crane stops, and other physical protection must be considered.

Warranties and servicing of the units by the vendor is another important consideration. Downtime of a detector may be especially costly to a scrap supplier whose contract calls for monitoring of all scrap shipped to a customer. In such cases, spare parts or a back-up system should be available on-site.

After installing a detection system, periodic reexamination of its effectiveness and suitability is highly recommended.

Some mills now require their scrap suppliers to certify under contract that incoming scrap has been monitored for radioactivity. The contracts usually do not specify that there will be no radioactivity--this would be unrealistic. What is realistic--and advisable--is for the contract to outline who will be responsible for the costs of recovering and disposing of any radioactive material that might be found in a scrap shipment.

Companies demolishing factories and plants, or processors accepting scrap from known sites, can call the U.S. Nuclear Regulatory Commission (NRC) or their state radiation control agency to determine if radioactive sources were reportedly used at the sites. No record of radioactivity, however, does not mean that no radioactive material was used: Agency records may be incomplete or the installation of the source may have preceded the regulatory program. Scrap processors and consumers may wish to have demolition-scrap suppliers certify that such checks with regulatory agencies were made.

While industry and government efforts are under way to reduce the likelihood of radioactive materials appearing in metal scrap, implementation will take time. For the foreseeable future, radiation in metal scrap will remain a problem. Protective measures are possible and can be effective when fully supported by management.

 Note:  This article does not represent agreed upon staff positions of the NRC or the Pennsylvania Department of Environmental Resources, nor have the employers of the authors approved the technical content.

Radioactive Materials in Metal Scrap

The following are examples of radioactive materials that have been found in metal scrap:

Americium-241: Used for thin metal gauging and to measure the thickness of coatings on steel sheeting. Home smoke detectors contain small amounts of this material and are exempt from regulation.

Cesium-137: Used in irradiators, calibrators, medical teletherapy units, and nuclear measuring gauges.

Cobalt-60: Used in irradiators, industrial radiography, and medical teletherapy equipment.

Iridium-192: Used in industrial radiography.

Radium-226: Used in medical equipment, gauges, and calibrators. Radium also appears in metal scrap as a naturally occurring radioactive material (NORM), such as the scale found inside pipes used in gas and oil well production.

Uranium and thorium: These radioactive metals have uses other than as source material for nuclear energy. Uranium, a very dense material, can be an effective radiation shield when properly encased to contain its radioactive emissions. It is used as a shield in radiography devices, teletherapy units, and transportation containers. Thorium is often alloyed with other metals to improve their characteristics. Magnesium-thorium alloys are used in aircraft engines. Nickel-thorium and tungsten-thorium are other common alloys of thorium.

Radiation most frequently appears in scrap in the form of radium sources, NORM contamination, and Cesium- 137.

Radiation Information Resources

The NRC has published a poster that illustrates radioactive warning labels to look for on scrap materials. The poster also provides the addresses and telephone numbers of NRC regional offices and its 24-hour emergency operations center. For information on ordering the poster, contact the NRC, Office of State Programs, Washington, DC 20555; 202/492-7000.

The Institute of Scrap Recycling Industries has published a brochure called "Caution! It Could Be Radioactive Scrap," which provides contact information for the NRC's regional offices, as well as the state offices that handle radiation safety. Single copy cost: $5. Send orders to Publications Orders, ISRI, 1627 K St. N.W., Suite 700, Washington, DC 20006. Quantity discounts are available; for more information call Iris Harnage, 202/466-4050.

For further guidance, contact your state radiation control program. State personnel, who will often be the first to respond to an incident, can explain regulations and put you in touch with local health physicists. For additional information about state radiation programs, contact the Conference of Radiation Control Program Directors Inc., 205 Capitol Ave., Frankfort, KY 40601; 502/227-4543.•

More advanced radiation detection equipment and quality assurance procedures can help scrap recyclers prevent problems related to radioactive scrap.