Abstract
In this work, Rossi-alpha measurements were simultaneously performed with a 3He-based detection system and an organic scintillator-based detection system. The assembly is 15 kg of plutonium (93 wt% 239Pu) reflected by copper and moderated by lead. The goal of Rossi-alpha measurements is to estimate the prompt neutron decay constant, alpha. Simulations estimate keff = 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 3He system estimated α = 37 μs. The known slowing down time of the 3He 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.
Highlights
AND MOTIVATIONReactivity estimates are an important facet of nuclear criticality safety
It takes the organic scintillator arrays (OSCARs) less than 30 minutes to be within 50% of its asymptotic value, while it takes the Neutron Multiplicity 3He Array Detector (NoMAD) approximates 120 minutes
WORK In this work, the organic scintillator array (OSCAR), comprising 21 total operational trans-stilbene detectors, and the Neutron Multiplicity 3He Array Detector (NoMAD), comprising 15 3He tubes embedded in a polyethylene matrix, simultaneously measured 15 kg of plutonium (93 wt% 239Pu) moderated by lead and reflected by copper with keff = 0.624 and α = 52.3±2.5 ns
Summary
Reactivity estimates are an important facet of nuclear criticality safety. Reactivity cannot be directly measured and is instead inferred from the prompt neutron decay constant, α. The value of α is estimated using Rossi-alpha measurements that are predicated on measuring the time difference between neutron detections [1]. Rossi-alpha measurements are traditionally conducted with 3He detectors, which use polyethylene to moderate neutrons to improve the detection efficiency. Because neutrons take time to moderate in the polyethylene, timing properties change (the decay of prompt neutrons and neutron slowing down time are convolved) and information can be lost. Organic scintillation detectors can detect neutrons directly and are fast compared to 3He systems. A fast plutonium assembly is simultaneously measured by 3He detectors and organic scintillators, and the detection systems are compared
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