Onsite Verification Of The Comprehensive Nuclear Test Ban At Very Low Yields

The United States has accused Russia and suspects China of violating the Comprehensive Nuclear Test Ban Treaty (CTBT) at very low yields - in the kilogram-range rather than kiloton range of chemical explosive equivalent energy yields. The accusation is that the tests involved supercritical chain reactions that are forbidden under the U.S. interpretation of the treaty. Offsite, there would be no detectable physical evidence to differentiate such tests from permitted subcritical tests except satellite images of suspiciously large containment vessels being emplaced in tunnels. However, if onsite inspections are allowed, it might be possible to measure gamma emissions from the fissions and neutron-activation product concentrations in a containment vessel to infer the energy released through fission during the test. Because subcritical tests have fission yields of less than a milligram of chemical explosive equivalent, the gamma activity beyond levels induced by natural background and diagnostic neutron and gamma-ray bombardment during the test might not be detectable. This could make the measurements more acceptable to the inspected party. Results, calculated with the open-access OpenMC transport code and the ONIX depletion code, are presented, identifying the most promising fission and/or activation products for onsite forensic inspections.