In this work, Rossi-alpha measurements were simultaneously performed with a 3 He-based detection system and an organic scintillator-based detection system. The assembly is 15 kg of plutonium (93 wt% 239 Pu) reflected by copper and moderated by lead. The goal of Rossi-alpha measurements is to estimate the prompt neutron decay constant, alpha. Simulations estimate k eff = 0.624 and α = 52.3 ± 2.5 ns for the measured assembly. The organic scintillator system estimated α = 47.4 ± 2.0 ns, having a 9.37% error (though the 1.09 standard deviation confidence intervals overlapped). The 3 He system estimated α = 37 µs. The known slowing down time of the 3 He system is 35-40 µs, which means the slowing down time dominates and obscures the prompt neutron decay constant. Subsequently, the organic scintillator system should be used for assemblies with alpha much less than 35 µs.

The work presented in this paper focuses on the first 10 years (2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020) of Godiva IV operations at the National Criticality Experiments Research Center (NCERC). Godiva IV is a fast burst critical assembly constructed of approximately 65 kg of highly enriched uranium fuel alloyed with 1.5% molybdenum for strength. Godiva is one of the last such critical assemblies in the United States and can be used for studies of super-prompt-critical behavior as well as irradiations and demonstrations. An overview of the startup of Godiva IV at NCERC is given followed by a summary of the experiments performed utilizing Godiva IV over the first 10 years of operation at NCERC.

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