Advances in Microscopic, Time-Correlated Neutron Noise Techniques
Hua, Michael
2021
Abstract
Estimating keff is of broad interest and paramount importance in the nuclear community including. A keff equal to one represents a self-sustaining and stable chain reaction and is often desirable in energy-producing reactors. An uncontrolled/accidental keff greater than one can result in criticality accidents and operator death. Therefore, it is desirable to be able to control and monitor the keff of an assembly; the value of keff can be modulated and tailored by using reflector material. A challenge is that keff cannot be directly measured; however, it can be inferred from the prompt neutron decay constant, alpha, and its negative reciprocal, the prompt neutron period. Microscopic, time-correlated, neutron noise techniques are used to measure alpha and the two most popular methods are the Rossi-alpha and Feynman-alpha approaches. This dissertation advances the two methods by addressing shortcomings in detectors, models, and uncertainty quantification. Current measurements are performed with 3He detectors that are insensitive to fast assemblies (with prompt periods shorter than a microsecond). Fast assemblies are pertinent to criticality safety applications and modern fast reactor designs, for example. The measurements of this dissertation use and validate the new organic scintillator array (OSCAR) system, which measured up to 15 kg of weapons-grade plutonium, 22 kg of highly enriched uranium, and 6 kg of neptunium (0.45 < keff < 0.95). The data analysis demonstrates that OSCAR exhibits capabilities beyond 3He systems. For instance, OSCAR is sensitive to prompt neutron periods as fast as 8 ns, whereas 3He detectors are limited to ~1 us. Furthermore, the OSCAR exhibits much less noise and can achieve desired precisions faster than competing 3He systems by factors as great as 10^2. The Rossi- and Feynman-alpha techniques were originally developed for bare cores of fissionable material and the traditional one-region point kinetics model is inadequate for reflected assemblies that requires a two-region model. Since reflectors are of interest in the application space, new theory is required. New theory is also required for uncertainty propagation and quantification from a single measurement; currently, methods are either incorrect or rely on long, repeated measurements. This dissertation extends traditional point kinetics from one-region to two-region and rigorously derives uncertainty methods for both the Rossi- and Feynman-alpha methods. The new theory is validated with the OSCAR and 3He measurements of reflected assemblies. The results demonstrate that the new theory increases the accuracy of alpha estimates by over 10% and generalizes the existing models. The two-region model introduces new variables and it is shown that the parameters can potentially be exploited as signatures of reflection. The two neutron noise techniques are compared, and it is found that the Feynman-alpha method is more precise than the Rossi-alpha method by one-to-two orders of magnitude in relative uncertainty. In terms of accuracy, the Feynman-alpha approach outperforms the Rossi-alpha approach for keff < 0.92, whereas the opposite is true for keff > 0.92. The uncertainty methods based on a single measurement are validated and can reduce measurement times by a factor of 20 or more, therein reducing measurement costs. This dissertation advances microscopic, time-correlated neutron noise techniques by demonstrating the use of a fast organic scintillator detection system that has capabilities beyond the current state-of-the-art 3He systems, developing and validating new measurement theory to account for experiments on reflected assemblies, and developing and validating quantification of measurement uncertainty.Deep Blue DOI
Subjects
neutron criticality measurement
Types
Thesis
Metadata
Show full item recordCollections
Remediation of Harmful Language
The University of Michigan Library aims to describe library materials in a way that respects the people and communities who create, use, and are represented in our collections. Report harmful or offensive language in catalog records, finding aids, or elsewhere in our collections anonymously through our metadata feedback form. More information at Remediation of Harmful Language.
Accessibility
If you are unable to use this file in its current format, please select the Contact Us link and we can modify it to make it more accessible to you.