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.

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2 Of the 723 tests, 9 (1.2 percent) were either Plowshare or other late time releases (U.S. Department of Energy, 1996).



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