Real-time Verification of Very Low-yield Tests With Prompt Gamma-rays Spectrometry

Very low-yield tests conducted by states to maintain and modernize their nuclear arsenals have recently become the source of tensions. The United States has accused Russia and China of possibly breaching the zero-yield standard by conducting supercritical tests at very low yields. The zero-yield standard stipulates that only explosive tests involving a subcritical fission chain reaction are allowed by the Comprehensive Nuclear-Test-Ban Treaty (CTBT) and the current moratorium. However, the current network of seismic stations and laboratories operated by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) cannot detect tests at very low yield let alone discriminate between a subcritical test and a supercritical test. This represents a serious flaw in the verification regime of the CTBT and a destabilizing factor for the current moratorium. This study explores the feasibility of using prompt fission gamma rays emitted by fission events during the test to distinguish between a subcritical and supercritical test. While an increasing rate of measured prompt gamma rays over time would indicate that the experiment involves a supercritical fission chain reaction, a decreasing rate would indicate a subcritical test. Preliminary results have been obtained by modeling very low-yield tests of various criticality levels with computer simulations. The open-source Monte Carlo code OpenMC is used to model the neutronics and the production and transport of photons throughout the system. This verification method can help strengthen the commitment to the current moratorium and could be integrated into the CTBT verification regime once enforced.