|Fig 1: Bus passing through a radiation portal monitor. (Source: Wikimedia Commons)
Radiation portal monitors (RPMs) are passive, non-intrusive devices used to screen objects and persons passing through them for nuclear and radiological materials.  These devices can detect various types of radiation emanating from nuclear devices, dirty bombs, special nuclear materials, natural sources, and isotopes commonly used in medicine and industry.  RPMs usually have gamma radiation detection ability and neutron detection ability. RPMs were originally used to screen individuals and vehicles at secure locations such as weapons facilities, but are now commonly used in monitoring traffic across borders, in airports, in urban centers, near nuclear facilities, in steel mills, in garbage incineration plants, and at import/export terminals.  Currently, over 1400 RPMs (like the one shown in Fig. 1) are deployed at US borders and a similar number are deployed in other locations around the world for the purpose of intercepting illicit nuclear and radiological material. 
As mentioned previously, the earliest RPMs were deployed at sensitive secure locations such as weapons facilities.  They were used to ensure that nuclear weapons being manufactured or stored at those facilities were not being illicitly taken out of the facilities. Over time, RPMs began to be deployed in health and safety applications, such as preventing spread of radioactive material contamination in scrap metal recycling facilities.  Some drastic changes in the world in the late 20th century brought grave concerns and dangers around nuclear and radiological material. When the Soviet Union fell in 1991, many nuclear facilities were left abandoned and unsecured, prompting concerns about being unable to contain or track nuclear material that could be used for harm. This, in addition to the increased availability of dangerous weapon information through the newly created internet and the growing presence of terrorism prompted a strong need for security systems. In 1999, the DOE NNSA made the first radiation detection equipment deployments of their new Second Line of Defense Program, installing devices at 39 sites in the former Soviet Union.  Once that situation was contained, deployments of portal detectors stalled until the terrorist attacks of September 11, 2001. These attacks shined light on the US and other global nuclear vulnerabilities. In 2002, the US's Customs and Border Protection (CBP) began installing radiation portal monitors at US borders, and the DOE NNSA soon began installing monitors at ports and other areas of concern (such as nuclear facilities and tunnels).  Since then, all US large points of entry and most global megaports have radiation portal monitors.
With the investment of extensive resources and structural changes to allow for radiation portal monitoring and the potential for grave consequences in the case of system failure, it is critical that radiation portal monitoring is accurate and comprehensive. The IAEA has established strict performance guidelines for approved radiation portal monitoring technologies. RPMs must be able to be operated 99 percent of the time and should have no greater than 10 missed positives for every 10,000 exposures. These guidelines are important for safety standards, but have hindered newer and cheaper technologies from being developed or implemented. An example of this is the failed "Advanced Spectroscopic Portal (ASP) program", which was heavily invested in by the US government but was concluded as not meeting accuracy standards in 2013 and was abandoned. Whereas the success of many post-9/11 global security measures in preventing attacks has been criticized (such as the Transportation Security Administration, also known as the TSA), the effectiveness of radiation portal monitoring is empirically significant. The IAEA Incident Trafficking Database (ITDB) 2015 Fact Sheet showed that since 1993, there have been a total of 2734 confirmed interceptions of nuclear or radioactive materials in the now 131 member states.  Of those,"442 incidents involved unauthorized possession and related criminal activities, 714 incidents involved reported theft or loss and 1526 incidents involved other unauthorized activities and events". 
© Greg Wolk. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "United States Customs and Border Protections Radiation Portal Monitors at Seaports ," U.S. Department of Homeland Security, OIG-13-26, January 2013.
 "Detection of Radioactive Materials at Borders," International Atomic Energy Agency, IAEA-TECDOC-1312, September 2002.
 "Incidents of Nuclear and Other Radioactive Material Out of Regulatory Control," International Atomic Energy Agency, 2015.