Year
2019
Abstract
The reactivity of critical and near-critical fissile systems is a quantity of interest in nuclear nonproliferation, criticality safety, and emergency response applications. In nuclear nonproliferation applications, reduced reactivity could indicate the diversion of special nuclear material (SNM). Criticality safety relies on in-situ reactivity estimates to determine if a reactor will regain subcriticality during normal and upset conditions. Emergency responders are interested in determining whether a sample is multiplying, which could be rapidly determined by a statistically-significant estimated nonzero-reactivity. The Rossi-alpha method is a measurement and analysis technique used to estimate the reactivity of critical and near-critical systems. Traditionally, Rossi-alpha measurements rely on the detection of thermal neutrons using 3He-based detectors. Organic scintillators are an alternative type of detector sensitive to both neutrons and gamma-rays. Furthermore, organic scintillators can detect fast-neutrons from fission directly, whereas 3He-based systems require moderation to obtain appreciable efficiency. This work examines the feasibility of Rossi-alpha measurements that rely on fast-neutrons and gamma rays. The paper will compare the accuracy and precision of 3He and organic scintillator systems; organic scintillators have a much shorter “die-away” time and thus can record thousands of times more signals than 3He detectors can for a fixed measurement time. The use of gamma rays offers a quasi-independent estimate of reactivity; however, gamma rays are more susceptible to self-shielding. This paper examines the agreement between gamma-ray and neutron estimates of reactivity as well as the limits of gamma-ray Rossi-alpha measurements for high-Z SNM due to self-shielding. Additionally, the effect of lead shielding on the estimated reactivity is investigated.