This article discusses about a global monitoring system that detects nuclear detonations either in remote areas or underground. Scientists at the US Department of Energy’s Pacific Northwest National Laboratory have developed the Automated Radioxenon Sampler/Analyzer (ARSA), and the Radionuclide Aerosol Sampler/Analyzer (RASA), to provide more detailed and accurate data on surreptitious nuclear detonations faster than other systems. According to the test ban treaty, in the event of a valid question regarding whether a nuclear detonation has taken place, any member state may request onsite inspections of any other state party, or in any area beyond government jurisdiction , such as the open ocean. ARSA analyzes air samples for radioactive xenon, or xenon isotopes, primarily xenon 133 and xenon 135, which are major indicators of fission that seep into the atmosphere from underground nuclear explosions. Detonating atomic weapons beneath the earth is the primary method of avoiding detection. RASA is designed to detect fission products from atmospheric nuclear explosions that attach themselves to dust particles.
Whent the United Nations General Assembly voted to adopt the Comprehensive Nuclear Test Ban Treaty on September 10, 1996, the world community was offered the opportunity to halt the spread of nuclear weapons and to stop their detonation in tests. A key provision of the treaty is a global monitoring system to detect nuclear detonations, whether in remote areas or underground. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory have developed ARSA, the Automated Radioxenon Sampler/Analyzer, and RASA, the Radionuclide Aerosol Sampler/Analyzer, to provide more detailed and accurate data on surreptitious nuclear detonations faster than other systems.
Of the 152 signatories of the test ban treaty, 36 have ratified it, not including the United States. By accepting the treaty, a country agrees to watch and listen for nuclear test explosions through an international monitoring system of 321 sensors at 260 stations worldwide. For example, under terms of the treaty, the United States is responsible for constructing, operating, and upgrading 38 stations and assisting with the construction of the others.
The monitoring stations will use four different technologies, three of which detect the vibration and sound created by nuclear detonations. Seismological instruments detect tremors in the earth, hydroacoustic devices pick up the waterborne sound waves, and infrasound equipment monitors atmospheric sound waves. The fourth technology, radionuclide systems, involves detecting radioactive isotopes in the air that would be generated by either underground or atmospheric nuclear explosions. Both ARSA and RASA are designed to fill a major role in the 80-station radionuclide network. Data from all networks will be sent to an international data center located in Vienna, Austria, and operated by the International Atomic Energy Commission.
According to the test ban treaty, in the event of a valid question regarding whether a nuclear detonation has taken place, any member state may request on-site inspections of any other state party, or in any area beyond government jurisdiction, such as the open ocean. However, as the often futile attempts to monitor Iraq’s chemical and nuclear weapons program have demonstrated, a government can delay or halt international inspections by simply denying the facts. Verification is essential to the test ban treaty, and that is where ARSA and RASA excel.
“These two systems allow us to capture a tiny part of the weapon,” said Ted Bowyer, a nuclear physicist and Pacific Northwest’s principal investigator for ARSA. “Radionuclides are a smoking gun. They are positive confirmation of recent nuclear fission.”
The development of ARSA and RASA is sponsored by the DOE’s Office of Nonproliferation and National Security, with the goal of transferring the technologies to commercial vendors so that any country can acquire the systems to meet its obligations under the treaty.
ARSA analyzes air samples for radioactive xenon, or xenon isotopes, primarily xenon 133 and xenon 135, which are major indicators of fission that seep into the atmosphere from underground nuclear explosions. Detonating atomic weapons beneath the earth is the primary method of avoiding detection. Prior to ARSA, radioxenon detection required technicians to collect air samples and transport them to laboratories. There, highly specialized instruments designed to detect xenon isotopes were used to examine the samples.
“In addition to being expensive, this strategy could take hours, which hampered the accuracy of the analysis because the half-life of xenon isotopes that indicate nuclear blast, such as xenon 135, are only nine hours,” said Bowyer.
ARSA automatically uses a piston compressor to pressurize air samples to 90 or 100 lbs. per square inch and send them through an aluminum oxide bed to remove moisture and carbon dioxide. Processed air is then passed through a charcoal sorption bed cooled to minus 100°C by an air chiller made by Polycold Systems International of San Rafael, Calif. This traps the xenon isotopes for analysis and removes radon, which can throw off measurement accuracy.
Electric band heaters raise the temperature of the charcoal to desorb the xenon. Nitrogen gas is directed through the charcoal bed to remove the xenon and carry it to ARSA’s radiation detection system.
“Radioactive xenon has a characteristic signature,” Bowyer explained. “It emits a photon in coincidence with a beta particle. If you detect both particles, you improve the accuracy of detecting fission products, which is why we equipped ARSA with two measurement systems.”
A sodium iodide gamma-ray spectrometer contains photo-multiplier tubes which measure the light that reflects the presence and energy of photons. A beta detector also measures light to determine the presence and concentration of beta particles. “Both techniques are decades old, but this is the first time they have been used together in this way,” noted Bowyer.
Bicron Inc., a designer and manufacturer of radiation detection and measuring equipment based in Newbury, Ohio, supplied both instruments for ARSA.
The ARSA system is controlled and monitored by a customized control system that was designed and assembled from SNAP input/output components made by Opto 22 of Temecula, Calif. The control system contains more than 100 digital and analog input/ output boards and other modules that interface with a personal computer made by Ziatech Corp. of San Luis Obispo, Calif.
Any data gathered by ARSA will be automatically transmitted to the appropriate organizations over telephone lines. The system continuously separates xenon from the atmosphere at a flow rate of 40 cubic meters per hour for an eight-hour collection period, and can measure four samples at the same time.
The laboratory began working on the concept of ARSA in 1993 and the U.S. Air Force, which will be responsible for the test ban monitoring stations, field tested a prototype in New York in 1996 for nine months. “Based on this experience, we simplified the unit, for example, changing the testing system to consume less power (4.2 kW), and lowering the overall number of parts to reduce the cost and complexity of the system,” recalled Bowyer.
In March and April 1999, the latest version of ARSA was tested by DME Corp. of Orlando, Fla. DME is a manufacturer of safety and diagnostic equipment that won the Air Force bid to build the systems for commercial use. “The system detected radioxenon to 0.1 milli- bequerels [a bequerel is a unit of measurement for radioactivity] per cubic meters of air, concentrations 10 times smaller than specified in the CTBT,” said Bowyer. “The U.S. Air Force selected ARSA in part because it is the only system that can detect xenon-135 to meet the CTBT obligations.”
The lab designed ARSA to operate automatically with remotely programmable functions. An electrical line, with backup generator in remote locations, powers the system.
A major challenge was fitting ARSA’s chemical processing function into a compact package. The entire system measures 0.9x2.1x2.1 meters, and requires minimal restocking or maintenance. Besides removing the need for operators on-site, ARSA also eliminates delays between sample collection and analysis. By measuring xenon 135, whose half-life is nine hours, before significant decay takes place, ARSA can differentiate between nuclear detonation debris and reactor releases. ARSA eliminates the need to transport samples to laboratories for analysis, saving time and money.
Looking for Clues in the Dust
RASA is designed to detect fission products from atmospheric nuclear explosions that attach themselves to dust particles. The basic technology was developed 30 years ago, but researchers have automated this technology to make it more than 100 times as sensitive as preceding detection systems.
Radionuclides are a smoking gun. They are positive confirmation of recent nuclear fission.
Those earlier detection systems used a pump about the size of a riding lawnmower, which fed air samples into a filter that was removed by an operator and brought to a laboratory for analysis by a gamma-ray spectrometer, about the size of an outdoor grille, to determine whether there were radionuclides that indicated a nuclear detonation had taken place.
“The strength of this procedure is that a human being could customize it as needed,” said Harry Miley, a physicist and principal investigator for RASA at Pacific Northwest National Laboratory. “For example, an operator seeing a filter blackened by heavy particulate collection could chemically process it before analysis.”
Labor costs for technicians, the time needed to transport samples from remote locations, and human error were drawbacks to this sampling technique.
In addition, the pre-RASA radionuclide samplers were equipped with high-pressure industrial blowers to draw sample air into the detection devices. These 15-hp motors consumed large amounts of energy and were noisy enough to require mufflers in populated areas. “We were determined to build a radionuclide detection system the size of a refrigerator, requiring the power of a hair dryer, and producing the noise of a vacuum cleaner,” remarked Miley.
At 1x2x1 meter, RASA is about the size of a commercial refrigerator. The Pacific Northwest engineers made RASA less energy-hungry than previous radionuclide detection systems by devising a large area filtering system that required a low-pressure drop to capture aerosols. A 1-kW blower powered by an electric line, with a backup generator in remote areas, automatically draws 15,000 to 20,000 cubic meters of air per day into a filtration system consisting of six strips of Substrate Blown Microfiber, made by 3M Corp. of Minneapolis.
The microfiber is made of porous polypropylene formulated so that it becomes electrically charged as air passes through it. “The electrostatic charge captures most dust particles in the filter,” said Bob Thompson, an electrical engineer and principal engineer on the RASA project at Pacific Northwest. “The remaining particles are captured on impact with the fibers.”
The six dust-laden filter strips are automatically drawn together radially to a single point by two rollers. Adhesive tape on the edge of one filter strip contacts Mylar on the edge of another strip, forming a single bundle. These filter bundles, each representing one air sample, are bar coded and passed to RASA’s radiation detection system. The unit typically runs 200 samples, one per day, with a restocking cost for the filter of $6 per sample.
RASA uses a large, germanium gamma-ray spectrometer to detect radionuclides. Companies that make these instruments suitable for RASA duty include Canberra of Meriden, Conn., Ortec of Oak Ridge, Tenn., and Princeton Gamma Tech in Princeton, N.J. “The spectrometer acts as a giant diode through which no current can pass unless photons of radioactive decay are absorbed,” Miley said. “The energy of photons indicates the presence, and concentration, of specific radionuclides, such as barium 140, that indicate an atomic detonation has taken place.” RASA’s findings are transmitted over a telephone line to the appropriate organizations, which will use it to report a violation in the test ban agreement.
The current Mark 4 RASA system being built commercially by DME Corp. was based on improvements made to the Mark 3 prototype that Pacific Northwest developed in 1995. The Mark 3 was field tested by the U.S. Air Force at McClellan Air Force Base in Sacramento, Calif., from May 1995 to February 1996. Among the bugs worked out at McClellan were the smooth rubber rollers that became dirty over time and had difficulty drawing the fiber strips. A mechanical engineering student researcher at Pacific Northwest lab solved the problem by replacing the smooth rollers with crepe rubber rollers.
“As most prototypes, the Mark 3 RASA incorporated a lot of expensive parts we made ourselves,” Thompson said. “For the Mark 4, which was intended for commercial production, we used as many off-the-shelf components and systems as we could.”
During tests performed by the U.S. Air Force and DME Corp. in 1997, the Mark 4 BASA was able to more than meet the treaty’s requirement for the detection of barium 140 in amounts of fewer than 30 microbequerels per cubic meter of air.