Detecting radioactive materials in scrap.
Warning! Some scrap may be radioactive and dangerous to your health and pocketbook. Scrap, the life source of foundries, could also be the source of some major headaches for those foundry managers who are unaware of the source of their scrap sources. The culprit: scrap contaminated with radio-active elements.
Whether the radiation is natural or man-made, the risks exist for injury and financial loss. The chance of getting 'hot' materials in scrap deliveries is seen by some experts as a minor threat now, but it is one that is beginning to attract trade and government attention as contamination incidents recur.
Radiation is a naturally occurring event present in many of the materials around us. We are exposed every day to low levels of radiation without ill effect. The problem (for foundries) is not in detecting radiation in scrap metal, rather it is in detecting excessive radiation in the scrap," explains Richard Smola of Ludlum Measurements, Inc.
Should foundrymen unknowingly use 'hot" metal Scrap in their operations, the results could not only pose health dangers, but would most certainly prove to be very costly. A foundry using radioactive scrap could face casting recalls, workmen's and whatever other compensation expenses that might be involved, plus the certainty of stiff cleanup costs. It has been estimated that the disposal of low-level radioactive waste, less transportation charges, is currently [$100/ft.sup.3] or $750/55-gallon drum.
According to Wayne Kerr of the office of radiation safety for the Illinois Department of Nuclear Safety, radioactive waste as a scrap contaminant has probably been around for quite some time. But, he added, the sheer volume of scrap used (iron foundries in the first two months of this year processed over two million tons of scrap) combined with an environmentally aware citizenly and several instances of radioactive metals accidentally used in production operations have focused attention on the problem.
Kerr recalled a 1983 incident in which an Auburn, New York, steel plant received. a piece of radioactive hospital equipment as part of a scrap shipment that was subsequently included in a melt. The radioactive contaminant was 60cobalt, After the company discovered the contamination, it was forced to shut down its production for 28 days while it spent more than $2 million in cleanup costs.
It was the first publicly documented incident of a steel mill closing due to radiation contamination. In a survey by Kerr for the years 1985-1989, 30 responding states reported 92 incidents' of radioactive materials found in steel scrap, melted in a steel-making facility, or contained in slag or other byproducts of steel or aluminum smelting and foundry operations.
Though decontamination costs are substantial, the loss of production and customer confidence raises the monetary stakes substantially. This has led many companies to investigate the practicality of screening scrap deliveries and opting to accept radiation monitoring costs both for incoming scrap and outgoing waste materials as a form of operation insurance.
The Auburn contamination incident caught the federal Nuclear Regulatory Commission and New York state hazardous materials monitoring and safety agencies by surprise. Most experts now agree that scrap metal handling and shipments require closer inspection and supervision to guard against the recurrence of such contaminations. No specific federal regulations existed then and are not forthcoming as quickly as the scrap industry would have them.
Kerr said a later contamination occurred the following year (1984) in Texas from contaminated scrap originating in Mexico. Scrap containing cobalt became part of a melt used by an American foundry to cast iron table pedestal bases. The contamination went undetected until it was discovered by an Illinois state trooper whose patrol car was equipped with a radiation detection device. The contaminated pedestals triggered the patrolman's monitor as the truck carrying the pedestals passed his car. Kerr said that incident serves as an example of how quickly radioactive contamination can spread. All of the radioactive pedestals eventually-were recovered and returned to the Mexican scrap source. How Likely the Threat?
The problem is very real, and the incidence of radioactive materials in scrap metal is becoming more commonplace. But because of the lack of disposal and transportation regulations dealing with the problem, metal scrap users and handlers are, for the most part, on their own in handling contaminated scrap," Kerr said.
Anthony LaMastra, a certified health physicist and an early researcher of the problem, said that a substantial number of radioactive material sources in scrap metal are from radium-contaminated oil well pipe. The radium is contained in the scale deposited on the wall of the pipe.
Industrial radioactive material applications, however, actually carry the greatest hazard potential for causing broader contamination within a foundry and pose the risk for both internal and external human exposure. LaMastra warned that scrap handlers need to be aware of shielded and unshielded (loose) radiation sources, the latter being easier to detect.
Of the industrial scrap classifications, the one most likely to contain sealed radioactive sources, he said, is cut plate or structural scrap, often referred to as railroad or demolition grade scrap. It tends to include heavier structural and plate sections that might have been part of an industrial structure to which a radioactive gauging device remains attached.
The problem with industrial gauges in scrap is that the protective shielding (mostly lead encased in a steel jacket) may be damaged, shredded or removed from the radioactive source material, exposing personnel to radiation hazards. Alternatively, the shielding may be intact, making detection by monitoring equipment difficult or impossible, thereby boosting the chances of the device being included in a furnace charge.
Of the seven most recent contamination events in the U.S., five involved industrial gauges mixed with or affixed to scrap. Industrial gauges usually contain [137cesium, 60 cobalt, 26 radium or 241 americium, each of which could be harmful in the event of protracted exposure, with the latter two being of a more serious nature. LaMastra estimates that there are 500,000 radioactive gauging devices in the U.S., and they present the greatest contamination threat because of their sheer numbers and the amount of radioactive materials they contain.
137Cesium is probably the most common radioactive material used in process control gauges such as those used to measure the level of material in a steel tank or the density of material flowing through a pipeline or on a conveyor belt. Other radioactive materials that could easily find their way into scrap metal include isotopes of americium, radium and strontium which, unlike 137 cesium or Ocobalt, tend to settle into the bone ("bone seekers") and remain in the body for several decades. They emit penetrating gamma radiation that is much more of a hazard than the alpha or beta radiation from cesium, cobalt, strontium or polonium,' LaMastra said.
Some scrap may become dangerous by incorporating materials made radioactive through their use in particle accelerators or nuclear reactors. With the increased use of high-energy accelerators for product sterilization, medical diagnostics and cancer therapy applications, it is possible for such user equipment to be scrapped without the proper disposal methods being observed.
Though there are many different radioactive materials that are potential scrap metal contaminants, much of the radioactivity encountered in scrap handling is considered naturally occurring radioactive material' (NORM). Scrap containing NORM can be found in the scale that attaches to crude oil transmission pipe walls or to oil drilling pipe as the pipe passes through oil basin depths. Radiation levels in drilling pipe can be quite high, but this class of scrap is not covered by any federal and only a few state disposal regulations.
After the Auburn incident, interest grew in screening scrap for radioactivity, and subsequent studies proved that effective screening was feasible but difficult. The problem became one of determining how to obtain a reading on scrap radiation separate from natural, or background, radiation. Background radiation affects the sensitivity of a detection system. The higher the background radiation level, the more difficult it is for systems to detect a radiation source. Shielding around the detectors reduces the count rate from background radiation and improves system sensitivity.1
According to Ludium's Smola, it would seem simple enough to set a radiation detection alarm point just above the background radiation level. However, background radiation changes, varying from hour to hour and point to point. Too low an alarm point will mean that fluctuations in the background will produce false alarms; too high an alarm point-will render-the instrument insensitive. The false alarm rate can be very high (one a day) or very low (one a year). Where one desires maximum available sensitivity, the one a day rate is good; in other situations desiring assurance that an alarm is real, a false alarm rate of one per year may be acceptable.
Smola lists five factors that aid the detection of small amounts of radiation above background radiation. The detector should:
* minimize the distance from the scrap to the detector to increase system sensitivity;
* extend the length of time the scrap is in "view" of the detector to increase detection probability;
* reduce the amount of scrap that can block the radiation (reduce the cover density of the scrap where possible);
* shield the detectors to reduce background radiation coming from other directions to increase the sensitivity to radiation fluctuations in scrap loads;
* increase the size or volume of the detectors (large detectors are more sensitive to small changes in radiation).
In summary, Smola says the ideal detection system would have a very large, well-shielded detector located close to a slow moving, small quantity of scrap.
There are two basic radiation detector designs that are the most efficient for detecting radiation in scrap metal: those using a sodium iodide (Nal) crystal or a plastic scintillator. Both are sensitive, differing principally in size, and both are capable of detecting gamma radiation. Plastic detectors are more rugged and available in much larger geometries than Nal detectors. A 6-7 in. Nal crystal has the equivalent detection capability of a 10 ft x 2 ft x 2 in. plastic detector, but it covers less detection area than the larger geometry of the plastic panel allows.
Both stationary gate detector-systems use photomultiplier tubes, magnetic shields, voltage divider assemblies and microprocessors to link the individual detector arrays together and monitor detection results. Both systems are available in hand-held, battery operated models for examining small scrap lots or to verify and locate radiation sources detected in large loads.
The plastic scintillator is an engineered substance that uses either a molded polystyrene or polyvinyltoluene to which fluor is added. The fluor gives the plastic the ability to emit light, or scintillate, when hit by radioactive photons. The rugged, highly sensitive plastic can be molded into virtually any-size panel and can withstand wide swings in temperature and inclement weather conditions.
The scintillations are read optically and amplified by a photomultipliertube that turns the light amplifications into electrical signals that can be fed to any type of recording device. The larger detection arrays-monitor-any radioactivity via a microprocessor that displays alarms on a CRT screen or sends them to a digital or analog meter that records radiation counts per minute. A hand-held detector, equipped with a much smaller Nal crystal, works in a similar manner. It detects light emissions from its built-in scintillator and provides a direct analog readout of the radiation contact.
Whereas the detectors are generally stock units, the detection system and number of detectors in any given scrap gating system are usually custom designed. Larger scrap handlers may use stationary detection areas in which trucks or gondola cars pass through the detection station. The detector, the most important part of the radiation detection system, comprises 80% of the cost of a large, sensitive system.
Lead shielding for a large plastic detector panel is far larger than that required to shield a Nal crystal, but the plastic panels are less likely to be disrupted by thermal or physical shock than are the crystal diodes. The Nal detectors are generally less costly than are the plastic detectors.
1. J.G. Yusko, Will Monitoring for Radioactivity in Metal Scrap Help Me?' ISRI Workshop (Sep 1990).
2. G.W. Kerr, CRCPD & State Activities on Radioactivity Scrap Metal, Conference of Radiation Control Program Directors Report, ISRI Workshop (Sep 1990).
3. A. LaMastra, Radioactive Material in Steel Scrap; its Occurrence, Consequences and Detection.'
4. R. Wrobel, Defining Scrap Detection,' Bicron Corp. (1991).
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||scrap metal|
|Date:||Sep 1, 1991|
|Previous Article:||Aurora Industries' strategic approach to business building.|
|Next Article:||Selecting a thermal reclaimer for green sand. (Guidelines for selecting a reclaimer, part 2).|