Simulating the Passive Neutron and Gamma Signatures of Containerized Nuclear Warheads for Disarmament Verification

For nuclear disarmament verification, measuring passive neutron and gamma signatures is discussed for confirming the presence of weapons-grade plutonium. Using the Geant4 code, the effects of neutron and photon interactions with the various materials of containerized items are explored for (i) notional fission and thermonuclear warheads waiting for dismantlement, (ii) intentionally shielded plutonium in a scrap container. Due to strong neutronic linking of the various warhead materials neutron multiplicity measurements can not be expected to give correct results. Gamma emissions of the plutonium may even be completely shielded by a tamper. Gamma spectrometry could verify the presence of explosives from (n,?) activation of hydrogen and nitrogen as well as of fission processes from their prompt fission gamma emissions. Limiting diameters of scrap containers together with long-time gamma measurements of the absence of photons produced by (n,?) activation of shielding materials will provide an effective approach for detecting an intentional diversion of plutonium.