The International Monitoring System managed by the Comprehensive Nuclear-Test-Ban Treaty Organization relies on detecting one or more of four distinct signatures from a nuclear explosion. Seismic detectors continuously listen for the shock waves passing through the earth from underground nuclear tests. Hydro-acoustic monitors listen for sound waves in the oceans from underwater tests. Infrasound detectors scan for pressure waves in the atmosphere. The fourth kind of signal involves radioactive gases generated by a nuclear explosion and released into the atmosphere.
Detecting an unusually high concentration of such airborne radionuclides is the only reliable way to prove, from a distance, that an explosion was nuclear. The so-called noble gases—a group of rare gases that exhibit very low rates of chemical reaction—play a special role in this type of detection, since they do not bind to other elements, can escape from the site of an underground nuclear test, and, if they are radioactive, can be easily detected. Radioactive xenon is the most important, because it is produced in high enough yields during the explosion that it can be detected even days after it has escaped into the atmosphere and travelled great distances.
There have been three detections of nuclear weapon tests in the past decade, all by North Korea, and all of them have been captured by detectors that are part of the monitoring system managed by the Comprehensive Nuclear-Test-Ban Treaty Organization. But new analysis suggests that the part of this monitoring system—the part that looks for the radioactive gases that are tell-tale signals of nuclear testing—may have blind-spots in some regions of the world, and in other areas the traces amounts of the gases released from a nuclear test may be lost in the background created by emissions from commercial nuclear activities. Assuring global coverage and a higher probability of detecting nuclear tests is possible, but only if the international community is willing to expand the monitoring system and put limits on radioactive releases from nuclear facilities.
Uncertainties in the system. When completed, the test ban organization's monitoring system will have 321 stations; 80 of these will monitor the air for suspicious concentrations of radioactive particulates. Forty of these are to be additionally equipped to detect noble gases. Already, 30 such stations are taking data; the other 10 are under construction or in planning. The number and locations of these stations was agreed during the Comprehensive Nuclear Test Ban Treaty negotiations in the early 1990s and were subject not only to meteorological considerations, but also geography and politics.
The North Korean nuclear tests provide good examples of the operational capacity of the International Monitoring System and the difficulty of detecting and identifying nuclear tests. For the North Korean test in 2006, the seismic signature suggested an explosion with a yield of .65 to 1.1 kilotons (kt) at the Punggye-ri nuclear test site. The closest downwind station equipped to detect airborne radioactive tracers was in Takasaki, Japan, but it was not yet operational. Two weeks later, an elevated concentration of radioxenon was detected at a noble gas station in Yellowknife, Canada. The reading was consistent with a possible release from the seismically established test site and time in North Korea.
The North Koreans' 2009 test was also detected by its seismic signal, which suggested a yield of 1.5 to 4.5 kt. Even though the Takasaki station was open by then, no noble gases were detected. The inferred size of the explosion and its location— the same as the previous test—led to the assumption that it might have been a well-contained nuclear explosion, with little or no leakage of radioactive particles and gases into the atmosphere.
In May 2010, multiple radionuclide and noble gas stations in East Asia picked up traces of various radionuclides that have been interpreted as a release from a nuclear test. But some of those radionuclides have never been detected before in the history of the International Monitoring System, and the possible source became the subject of controversy. Seismic analysis published in January 2015 seems to support the idea that it was a nuclear test. But it's important to note that the radionuclide-noble gas component of the monitoring system detected something contemporaneously that the waveform detection methods missed at that time.
For the North Koreans' nuclear test of 2013, seismic data suggested a yield of 6 to 9 kt, but radioxenon was detected only about six weeks later, at the Takasaki station. One possible explanation for the delay: The noble gases formed by the explosion may have remained trapped underground until seismic aftershock activity or on-site construction work opened a crack to the outside world.
The difficulties of detection. The North Korean cases underscore the importance of the radioxenon detection capability, but they also point to the difficulties of detecting the noble gases. Four main factors affect the successful detection of atmospheric radionuclide emissions from a nuclear explosion.
Leakage is the first, and whether byproducts from an underground nuclear test leak into the atmosphere is often out of control of the testing party. Without leakage no remote detection of radioactive particles or gases is possible; they must vent into the atmosphere to be detectable.
The second factor is the distribution of the International Monitoring System stations themselves. Ideally, there should be at least one monitoring station downwind from any given test site and within the distance the radioxenon can travel before it decays. The most important radioxenon isotope decays with a half-life of about five days. Generally speaking, the further a monitoring station is away from the nuclear test explosion, the less likely is a successful detection.
The third factor is wind. Wind patterns govern the dispersion of particles and gases in the atmosphere and determine whether releases from a nuclear test site reach a monitoring station. The plume of radioxenon released into the atmosphere from a test spreads over time, becoming ever more dilute. This dilution happens alongside the natural radioactive decay of the gas.
A fourth consideration that can affect detection of a nuclear test is the radioxenon background in the atmosphere. Radioxenon is regularly emitted from commercial medical isotope production facilities and to a much smaller degree from nuclear power plants. The same noble gas characteristic that makes radioxenon difficult to contain in an underground cavity after a nuclear explosion makes it hard to contain in these civilian nuclear facilities. A high background from commercial facilities can potentially mask the signal from a nuclear test.
Blind spots. Our research has found that the noble gas detection part of the International Monitoring System is unlikely to work as it should because of the limited distribution of noble gas stations, neglect of important meteorological patterns in some areas, and the radionuclide background from emissions from the commercial production of medical isotopes.
This conclusion is based on computer simulations of the trajectories of radioxenon emissions from hypothetical nuclear explosions all around the globe over the course of one year. The simulation assumed that all 40 International Monitoring System noble gas stations were operating as planned.
The first blind spot in the detection system stems from interplay of globally diverse wind patterns. Large parts of the Earth's surface, especially the global mid-latitudes—including North Korea—are well covered by the noble gas monitoring system. However, along and around the equator there are areas with much lower probabilities for detecting radioxenon from a nuclear test.
These gaps are caused by special meteorological conditions surrounding the equator that create zones of lowered wind speeds and slower transport mechanisms for radioactive tracers. In the intertropical convergence zone—a low-pressure belt of a few hundred kilometers width, close to the equator—trade winds carrying air masses from lower and higher latitudes toward the equator collide. Air masses get trapped in loops—known as Hadley cells, after George Hadley, an 18th century English scientific writer—between the equator and 30 degrees north and south latitude. The simulations show that the 40 International Monitoring System stations will not fully cover parts of these equatorial areas. Radioxenon emissions from a nuclear test in equatorial Africa, South America, or South-East Asia will be trapped in equatorial wind patterns and decay before reaching a monitoring station.
The second blind spot emerges from medical isotope production facilities that emit radioxenon. At the moment, the largest emitting facilities are located in Australia, Belgium, Canada and South Africa. Simulations show that radioxenon emissions from these facilities cloud regions downwind, and stations there would have a significantly lower probability of detecting nuclear test signals. The regions affected by radioactive emissions production of medical isotopes are not fixed; old facilities close down and new ones open up in response to market forces and government policies. For example, the Chalk River Laboratories in Canada are expected to be closed in 2016, which will possibly lead to a new facility in the United States to supply the North American demand for medical isotopes. Similarly, with increasing demand in Asia, one or more medical isotope facilities will probably emerge in that region; a new facility in Indonesia is just starting up.
Filling in blind spots. Much has been learned about radioxenon background sources and their atmospheric transport since the Comprehensive Test-Ban Treaty negotiations of decades ago. The two primary blind spots in the treaty organization's International Monitoring System can be filled in.
Closing the blind spot caused by wind patterns is by far the easier task. It would require adding more noble gas stations in equatorial regions. Preliminary estimates suggest that 10 to 20 additional stations would be needed. A new station costs about $1 million to $2 million, plus annual maintenance of $100,000 to $200,000, putting the total cost in the range of a few tens of millions of dollars, a small sum compared to government budgets even in developing countries.
It would be possible in some cases to save money and upgrade the existing system stations in equatorial regions by adding noble gas samplers to their suite of instruments. Further research using atmospheric transport modeling would enable a more efficient selection of sites where new monitoring stations are needed and identify which station upgrades would have the most significant impact on detection capabilities.
Addressing the problem of commercial background emissions is more challenging. The CTBTO recommends a voluntary upper limit on emissions per day and per commercial facility, to avoid blinding the International Monitoring System network. But existing facilities follow commercial interests and are reluctant to spend money changing already state-approved process lines to meet emission levels recommended by an organization that does not even have an in-force treaty to monitor. Since the emissions from medical isotope facilities are generally well below levels that might pose a health risk, most governments see no immediate need to act.
Medical isotope production companies could take the lead to resolve this problem. One option would be for new facilities to install a relatively cheap system of activated charcoal to capture their emissions. The Petten Nuclear Reactor in the Netherlands already offers an example of this approach, with an effective system of selective gas separation in place. Alternately, these companies could provide detailed data on their emissions to the CTBTO, allowing the organization to gain a better understanding of the radioxenon background and so to make more accurate predictions of expected concentrations at monitoring stations. So far, most companies avoid providing such data out of competitive business concerns. Just recently, the five permanent members of the UN Security Council announced they intend to release a joint statement on minimizing the impact of medical isotope production on the International Monitoring System.
The problems affecting noble gas monitoring to detect nuclear explosions are not static, and they don't have a static solution. Meteorological patterns and, to a greater extent, background sources are bound to change in the future. This means that the number and locations of the monitoring stations should undergo a regular review process.
The ultimate responsibility for improving the coverage and reliability of the International Monitoring System however lies with the 183 states that have signed the Comprehensive Nuclear-Test-Ban Treaty and with the CTBTO, which is charged with the treaty’s verification. States must find the right balance between their national economic interests and their international security obligations and provide the resources to build the extra monitoring stations where they are required and to curb activities that might limit the global capability to monitor possible nuclear tests. Of course, marshaling these resources would be easier if the United States and other "nuclear capable" countries would ratify the treaty, so it could enter into force.