Monitoring communities have unfortunately used the terms “containment,” “venting,” and “seeping” in different ways. The formal use of these terms may originate from wording adopted in the United States and a “Containment Evaluation Panel” (CEP) that was established to review the containment of U.S. nuclear-explosion tests, though communities other than the past U.S. testing community may have different definitions of the terms. For example, the CEP (Carothers, 1995) defined successful containment as:
“Successful Containment: Containment such that a test results in no radioactivity detectable off site as measured by normal monitoring equipment and no unanticipated release of radioactivity on site within a 24 hour period following execution. Detection of noble gases which appear on site at long times after an event due to changing atmospheric conditions is not unanticipated. Anticipated releases will be designed to conform to specific guidance from DOE/DASMA (NV-176, Revision 5, Planning Directive for Underground Nuclear Tests at the Nevada Test Site (U))” (p. 7).
During the time of active testing, it was in fact not unanticipated to have noble gases measurable on-site,1 and “normal” offsite monitoring was far less sensitive than is today’s equipment. In addition, at the time, “late-time seepage” of noble gases was expected after operations ceased at the test site. Because the noble gasses are produced by radioactive decay of fission products produced in the explosion, the maximum amount of radioactive xenon actually occurs a few days after the shot time, and therefore seeps could be appreciable.
The definition used by the CEP is not particularly relevant for CTBT monitoring. First, for both IMS monitoring and NTM, the measurement technology is significantly advanced from even a decade ago. Second, if radioactivity is released at 24 hours, it would not be considered containment failure, though the IMS and NTM assets would still be usable for determination of a CTBT violation. Third, even though noble gases escaping from a nuclear test were not considered containment failure by the CEP, measurement of the radioactive noble gases is a key way to verify the CTBT.
Approximately 50 percent of all Soviet nuclear tests were measured off-site using noble-gas measurement technology (Dubasov et al., 1994). With the improvements of detection sensitivity, and in-field measurements, it is possible that the number of tests that would have been detectable off-site would be higher. Because of this, one might consider 50 percent as the limiting case for a mature nuclear weapon state with a lot of practice and somewhat lower for new proliferators. However, as the number of active scientists with experience with nuclear testing and nuclear test containment decreases, it is likely that the probability for successful containment of nuclear tests may end up lower because of the lost experience base, much of which is not documented.
Another data point is the U.S. experience with the trapping of radionuclides from underground nuclear testing. In reporting from the U.S. Department of Energy, from nuclear tests conducted between 1961 and 1992 on the release of radioactive debris into the atmosphere, of the 723 underground nuclear tests conducted during this period, 105 (14.5
1 Ward Hawkins, personal communication, 2009.
percent) had “containment failures,” 287 (39.7 percent) had operational releases, and 322 (44.5 percent) were “contained.”2 This means that of the 427 nuclear tests since 1961 where no release was expected, approximately 25 percent did vent according to the conservative definition used by the CEP.
Because there is little detailed data available, it appears that the U.S. experience with containment of nuclear tests does not seem radically different than the Soviet containment experience. Therefore, as a rule of thumb, we judge that in at least 50 percent of nuclear tests near 1 kiloton or larger, even those carried out by experienced testers, xenon noble gases may be detectable offsite above the detection limits of the IMS (0.1 mBq/m3) from prompt venting of nuclear tests; also, long-term seepage of appreciable noble gases would be expected that could be detectable, both offsite and onsite.
2 Of the 723 tests, 9 (1.2 percent) were either Plowshare or other late time releases (U.S. Department of Energy, 1996).