Two experts explain the facts and myths surrounding nuclear measurements
By H. M. Schwartz and Dr. Dirk Moermann
Nuclear measurement gauges for industrial process measurements have been around for many years. They have been a mainstay in making the most difficult level, density, bulk flow, and moisture measurements. Nuclear measurement gauges work where no other technology will. They give excellent performance under hostile and rugged conditions. High temperatures, pressures, and other difficult industrial processes usually pose no problem for a nuclear measurement gauge.
Despite thousands of satisfied users of this technology worldwide, nuclear measurement gauges are sometimes a pariah to companies and their employees. From many companies perspective, the move to be more environmentally friendly has sometimes superseded the facts about nuclear gauges, their benefits to process operations, and most important, the safety and environmental issues surrounding this measurement technology.
Adding to the confusion and misunderstanding about using nuclear measurement gauges has been the somewhat misleading information disseminated by the media. From "The China Syndrome" to Three Mile Island to dirty bombs, the word "nuclear" generally invokes a negative connotation in the minds of most people. Dirty, dangerous, environmentally non-friendly, and a whole host of other societal ills generally revolve around it. However, in the world of industrial process measurement, nuclear measurement gauges are routinely used in the world's most demanding process conditions.
Many companies have banned nuclear measurement gauges or dramatically restricted their use — often times to the detriment of making highly critical measurements. When this happens, companies can lose literally thousands of dollars a year in lost measurement productivity and increased maintenance required by having to use intrusive and contacting measurement methods.
But how dangerous is it to use nuclear measurement gauges? What are the safety considerations involved? Will employees receive undue exposure to radiation from working around these gauges? These questions should be answered, and the myths surrounding nuclear measurement gauges should be examined.
A nuclear density gauge is mounted at a 45-degree angle for small pipes or narrow measurement spans.
Nuclear measurement gauges operate on a simple yet sophisticated concept. The radioisotope (the size of a saccharine tablet) is placed in a rugged, steel-jacketed lead housing. The housing shields the radioactive beam except in the direction where it's supposed to travel. Using a small collimated aperture in the shielding, the beam can be projected at various angles into the pipe or vessel. On the other side of the pipe, vessel, conveyor, or other type of process container is a highly sensitive scintillation detector.
The detector contains a small crystal made from synthetic material or sodium iodide. The crystal is optically coupled to a photo multiplier tube, which converts light into electrical pulses. When the radioactive beam strikes the crystal, after having passed through the vessel/pipe walls and process, it generates thousands of light pulses that are recorded by the photomultiplier tube. This is converted into a signal, which can be used for a display or an analog output into a DCS or PLC.
Nuclear measurement gauges operate on the principle of attenuation — if there is nothing in the way of the beam, then the beam will be strong. If there is something in the way of the beam, its strength will be less. This principle applies to virtually any nuclear measurement.
A rod-type nuclear source is mounted on the outside of a vessel.
In addressing physical exposures and radiation, there are three important concepts that affect exposure. One is the amount of time a person is working in the area around a nuclear measurement gauge. The second is the distance between a person and the nuclear measurement gauge. And the third is the amount of shielding between a person and a nuclear measurement gauge. Time, distance, and shielding combined will have an effect on the body exposure.
In comparing body doses and exposures, millirems (mR) are used as a unit of measurement to compare the effect on the body by nuclear radiation. Typically, a person receives somewhere between 150-300 mR per year — including exposure working around nuclear measurement gauges. The part received working around nuclear gauges is low — only about 10-30 mR. (For more information, refer to the "myths" discussed below.)
The majority of exposure is from things having nothing to do with working near nuclear gauges — radiation from a variety of sources including medical X-rays, the earth itself, the sun, and even a person's own body.
Two rod-type nuclear sources are mounted on a horizontal vessel.
To help shed light on nuclear process measurement equipment, let's now review some common facts and myths.
MYTH: Working around nuclear gauges will cause cancer, leukemia, radiation sickness, etc.
FACT: Nothing could be further from the truth. The exposure received from working around a typical nuclear gauge is extremely minimal — and far under any ceiling for exposure to radiation. The average person gets more radiation exposure spending time on a beach in the summer than working around a nuclear measurement gauge.
MYTH: Working around nuclear gauges will make one sterile.
FACT: Not true! Once again, the exposures are no where close to providing harm to bodily organs.
MYTH: By using nuclear measurement gauges, you will contaminate any process in which they are used.
Here's a close-up of nuclear rod sources mounted on a horizontal tank.
MYTH: You cannot use nuclear measurement gauges in processes such as food or dairy.
FACT: There again, nuclear measurement gauges are used to measure processes in a variety of food processes. Measuring the solids density of tomato paste is one such application. As mentioned above, the radiation beam does not contaminate the process. Many people confuse irradiation of food with nuclear process measurement equipment. The systems used to irradiate meat and other food stuffs contain sources that are literally thousands of times stronger than the average nuclear measurement source.
MYTH: The source housings will leak out and disseminate dangerous radiation.
Rod-type nuclear sources are mounted on standpipes.
MYTH: You have to wear dosimeters to work around nuclear measurement gauges.
FACT: This is false. There is no federal or state requirement that dictates that workers must wear any dosimeters when working around nuclear measurement gauges. The dosages received when working around nuclear measurement gauges are so low that it's almost impossible to distinguish them from natural background radiation (radiation that occurs naturally from the earth) and exposure from nuclear measurement gauges. Many people confuse the requirement for wearing dosimeters in some areas of nuclear power plants with areas containing nuclear measurement gauges. This comparison cannot be as there is no way to equate the exposures received.
MYTH: When working on a nuclear measurement gauge (especially the source shielding) you must wash your hands directly after working on it.
FACT: Once again, as with most myths, nothing could be farther from the truth. There are no radioactive contaminants present that would require washing your hands or other part of your body after coming into contact with any part of a nuclear measurement gauge. The only reason you might need to wash your hands would be to remove the dirt and dust from the surrounding environment!
MYTH: Nuclear measurement gauges produce far more radiation exposure than medical X-rays and other natural occurrences.
FACT: Wrong again. Let's look at X-rays first. A typical chest X-ray produces about 40 mR, which incidentally is about the exposure the human body produces in a year by itself (from radioactive elements in the bones). Other types of medical X-rays, especially CAT scans, can produce up to 1,000 mR. More examples are cosmic rays from the sun (30 mR), the soil (30 mR), and living at higher altitudes — people living in Denver receive about 50 mR per year, 50 percent more than at sea level. By comparison, though it's difficult to predict because of all the variables involved, typical exposure of someone working around a nuclear measurement gauge would be somewhere in the 10-30 mR range. However, as mentioned earlier, the three factors (time, distance, and shielding) have more to do with exposures people receive than any other single factor or event.
ConclusionNuclear measurement gauges are an excellent solution for critical measurements where contacting or intrusive-type equipment will not work. They are attractive in today's process measurement landscape due to their simple yet elegant technical concept — and a few laws of physics thrown in. They can save thousands of dollars each year in reduced downtime and equipment and maintenance costs. Since nuclear measurement gauges have no moving parts or components inside pipes and vessels that need replacement, they are the ideal answer for accurate and repeatable measurements in the most rugged and hostile process environments.
H. M. Schwartz is the business unit manager of Berthold Technologies USA, LLC, and Dr. Dirk Moermann is the product manager of Berthold Technologies GmbH. More information about the technologies mentioned in this article is available by contacting Berthold Technologies at 865-483-1488 or visiting www.berthold-us.com/industrial.