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Steven Biegalski, The University of Texas at Austin
Transcribed from video by Drew Masada and Sarah Williams, The University of Texas at Austin
Nuclear explosion monitoring is a timely topic due to clandestine nuclear activities recently observed in Iran and the Democratic Peoples Republic of Korea (DPRK; North Korea). Recent discoveries reveal a covert uranium enrichment facility in Iran that may indicate activities leading towards the development of nuclear weapons. DPRK has two recent nuclear weapons tests: one on October 9, 2006 and the other on May 25, 2009.
The Comprehensive Nuclear Test Ban Treaty (CTBT) was adopted by the United Nations General Assembly on September 10, 1996. The CTBT bans all nuclear explosions in all environments. The treaty has not yet entered into force, but the Provisional Technical Secretariat (PTS) has been established in Vienna, Austria. [Editor’s note: The US Senate rejected ratification of the treaty in October, 1999. The US and eight other countries that possess nuclear power or research reactors must ratify the treaty before it enters into force.] Within the PTS an International Monitoring System (IMS) has been developed, as defined in the treaty, to monitor the world for nuclear explosions. The IMS contains sensors for four monitoring technologies: radionuclide (80 stations and 16 laboratories), seismic (50 primary and 120 auxiliary stations), hydroacoustic (11 hydrophone stations), and infrasound (60 surface stations). The network of IMS stations was configured to obtain a near-uniform monitoring capability around the world. Once the treaty goes into force, CTBT Member States may request an on-site-inspection (OSI) to be carried out if data from the IMS stations indicates that a nuclear explosion has occurred. On-site-inspection is limited to an area of 1,000 square kilometers in regions controlled by CTBT participating nations. This article focuses on the radionuclide monitoring component of the IMS.
On April 10, 2009, a panel discussion on CTBT monitoring was held during the 8th International Conference on Methods and Applications of Radiochemistry (MARC VIII) to discuss the data from the DPRK event and the general radionuclide monitoring capability of the IMS. This article summarizes the panel discussion. The second North Korean nuclear explosion took place after the panel discussion occurred.
The panel was moderated by the author. The six panel members were, in alphabetical order,: 1) Dr. Guy Brachet of the Commissariat à l'Energie Atomique (CEA) in France, 2) Mr. Fitz Carty, Senior Program Manager at General Dynamics Advanced Information Systems, USA, 3) Dr. Harry Miley from Pacific Northwest National Laboratory, USA, 4) Dr. Anders Ringbom from the Swedish Defense Research Agency, Sweden, 5) Dr. Robert Werzi of the PTS of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), Austria, and 6) Dr. Matthias Zähringer of the PTS of the CTBTO, Austria.
The 2006 DPRK Test
Figure 1 shows part of the planned worldwide IMS network of radioisotope detectors. The complete IMS includes 80 air-monitoring radionuclide detection stations, 40 of which will have noble-gas detection capability. Sixteen radionuclide laboratories support the detection network. Data from the IMS is sent to the International Data Center (IDC) in Vienna, where it is processed, analyzed, and disseminated to member states. (Data is available for scientific use upon request.) Radioxenon from the North Korean test was detected at the Yellowknife, Canada, station (CA16 in Figure 1) with a peak 133Xe concentration of 0.74 ± 0.06 mBq m-3 [1 Bq = 1 decay/sec; 133Xe has a beta-decay half-life of about 5.2 days]. While this is a low environmental quantity of radioxenon, it is significantly above the detection capability (~ 0.3 mBq m-3 for 133Xe) of radioxenon monitoring systems developed for CTBT monitoring. All panel members concurred with the opinion that the planned full IMS network would have measured the DPRK event more accurately than the partial network that was operational at the time (only eleven noble gas stations were operating at the time of the event), and placed a high priority upon completion of the network.
Rather than one station detecting the event, the planned network would have had two or three stations capable of detecting radioxenon in the range of 10-100 mBq m-3 of 133Xe. More stations able to record significant amounts of activity in the atmosphere following a nuclear event allows for better atmospheric backtracking and definition of a possible source region and hence increases the likelihood that the event will be correctly identified and located. Data gathered by the Yellowknife station after the DPRK event was used to predict the capabilities of the planned network to detect significant amounts of xenon in the atmosphere. The images shown in Figure 2 are the modeled average activity concentration of radioxenon around the North Korean test site 12 days after the event.
The four stations listed below, if operational, would have detected the indicated corresponding amounts of 133Xe, all of which are significantly higher than the 1 mBq m-3 detection limit defined by the CTBT for IMS noble gas stations for this isotope. Actual noble gas systems have significantly better 133Xe detection limits that can range down to values as low as 0.3 mBq m-3 or even lower.
• RU58 (maximum calculated activity ~ 80 mBq m-3)
• JP38 (maximum calculated activity ~ 12 mBq m-3)
• JP37 (maximum calculated activity ~ 7 mBq m-3)
• PH52 (maximum calculated activity ~ 5 mBq m-3)
Figure 2. Atmospheric transport of radioxenon following DPRK event utilizing the Lagrangian model FLEXPART with NCEP FNL 1°x1° metrological data (units of mBq m-3).
Other stations in Fig. 1 were not sensitive to this test because of dominant wind patterns in the area at the time following the explosion. Stations in the US and Mexico are too far from the test site to detect the event. While a fully-operational IMS network would have had two or three stations detect atmospheric radioxenon following the DPRK event, increased sensitivity at existing stations might also have allowed the detection of radioiodine.
The nature of radionuclide backgrounds is controversial. Some panel members emphasized that backgrounds are complex and site-specific with average concentrations varying over several orders of magnitude. This may require site- specific criteria to separate normal background from treaty-relevant detections. Others suggested that backgrounds are mostly composed of 133Xe, normally distributed over stations, and maintain some consistency across station sites. Occasionally 131mXe (half-life 11.9 days) is present. These two xenon signatures would compose the majority of the backgrounds. The panelists believed that the state of technology and equipment has improved, allowing them to measure radioxenon concentrations not previously detectable.
The second issue regarding radionuclide backgrounds is identifying and cataloging known sources of radionuclide production that are part of the world-wide background. Excluding nuclear tests, there are two main sources of radioxenon isotopes in the environment: radionuclide production facilities and nuclear power plants. Monitoring stations in close proximity to emitting sources will pick up 133Xe and 135Xe. The panelists believed that future research should gather data on what these civil sources produce, in what quantities, and their emissions on a daily basis.
Finally, experts place a high priority upon completing the network as soon as reasonably achievable. A completed network will have superior detection capabilities. The CTBTO has fourteen stations planned to be installed in the in the next few years that will expand the network and draw near to the 40 noble gas system network.
Further investment aiming at having a noble gas detector at each of the 80 radionuclide stations would further enhance the detection capability of the network. A few stations may be difficult to install due to political reasons such as non-signature of potential host countries. Efforts should be directed at avoiding gaps that degrade the homogeneous performance of the network.
Future Development of the International Monitoring System
While the current IMS radionuclide monitoring system meets CTBT requirements, it is expected that technology will improve. These advancements should be integrated into the IMS monitoring system to increase performance and reliability. A process for approving new system components for the network and the factors that might facilitate or hinder the process must be considered. Assuming that the CTBTO would update the network either by upgrading the pre-existing system or installing new detectors, it could be ten or more years from the time the details are worked out to the point where a new detection system is deployed.
Issues in examining radionuclide backgrounds include factoring in new discoveries, cataloging known sources of radionuclides, and completing the network. It is expected that radionuclide backgrounds will change due to the addition of new radiopharmaceutical production facilities and new commercial nuclear reactors. Since this background is not constant, it must be continually monitored, analyzed, and documented.
Detection techniques for OSI have yet to be fully developed. There are many scientists in the field who are waiting for the opportunity to contribute to OSI, and have developed related projects. Improved monitoring and detection capabilities for OSI teams are gaining support and funding from the policymaking community. The decision to rely on one type of monitoring originated with a cost-benefit analysis going back to the conception of the CTBT detection network. This situation fostered an environment in which radiochemistry is not the priority detection method of the network. The panel is not aware of better analytical techniques given the operational considerations and necessary detection limits. However, should such technology become available, steps should be taken to incorporate the technology as appropriate.
In addition, improvements to the Atmospheric Transport Model (ATM) software have created better tracking and modeling of potential sources. This is another issue relevant to obtaining a broad acceptance of calibration techniques for noble gas systems and would help to improve the reliability of the data. Most of all, the IMS needs to continue to collect data. The more data that is collected, the more it helps to prove the capability of the system. In Vienna, the CTBTO is leading efforts to organize the International Scientific Symposium (ISS 2009). The goal of the symposium is to assess the current capability and future potential of the verification regime.
To address concerns about the perceived lack of alternatives or new technology being incorporated into the IMS, it should be noted that available new cooling technologies for HPGe detectors are not capable of withstanding the technical and environmental pressures of the current IMS network. (Detector cooling to ~ 70 K is necessary to reduce electronic noise in the detector.) Network engineers rely on what is currently on the market, however, lab-tested systems are not necessarily robust enough for field deployment. New developments have also increased the availability of ultra-low background HPGe detectors. This technology could potentially lead to significant detection limit improvements for the measurements of aerosol samples at IMS stations and laboratories. Consequently, the feasibility of measuring radioiodine releases from underground nuclear tests could be improved. This panel highly recommends that funding be made available for testing new technology, research, and development for feasibility of field deployment as well as improvements to detection limits.
Over the past decade, the IMS has progressed significantly, steadily increasing the number of certified stations worldwide. The CTBTO and its related scientific community continue to look for opportunities to improve detection techniques and means to improve the technical capacity of the network. The efforts of the CTBTO and others in the field have helped to bring the CTBT verification regime closer to becoming fully operational. Specifically, past efforts by the scientific community have led to better understanding of the radionuclide background and to overcome deficiencies in the IMS. On the issue of radionuclide background, scientists now have evidence that radionuclide production facilities are the major sources that contribute to these background levels. They know the location of most facilities and what they produce. What is now needed are the exact quantities of daily emissions from these facilities.
Regarding deficiencies in the IMS network, there has been progress towards becoming fully operational. High quality data is available and system reliability is significantly improved over the last seven years. System detection limits may now be calculated with post priori data to better asses IMS capabilities.
Steven Biegalski is Director of the Nuclear Engineering Teaching Laboratory at The University of Texas at Austin.
This contribution has not been peer refereed. It represents solely the view(s) of the author(s) and not necessarily the views of APS.