Award Date

May 2017

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Mechanical Engineering

First Committee Member

William Culbreth

Second Committee Member

Alexander Barzilov

Third Committee Member

Yi-Tung Chen

Fourth Committee Member

Brendan O'Toole

Fifth Committee Member

Steen Madsen

Number of Pages



The behavior of symmetrical coupled-core systems has been extensively studied, yet there is a dearth of research on asymmetrical systems due to the increased complexity of the analysis of such systems. In this research, the multipoint kinetics method is applied to asymmetrical zero-power, subcritical, bare metal reactor systems. Existing research on asymmetrical reactor systems assumes symmetry in the neutronic coupling; however, it will be shown that this cannot always be assumed. Deep subcriticality adds another layer of complexity and requires modification of the multipoint kinetics equations to account for the effect of the external neutron source. A modified set of multipoint kinetics equations is derived with this in mind. Subsequently, the Rossi-alpha equations are derived for a two-region asymmetrical reactor system.

The predictive capabilities of the radiation transport code MCNP6 for neutron noise experiments are shown in a comparison to the results of a series of Rossi-alpha measurements performed by J. Mihalczo utilizing a coupled set of symmetrical bare highly-enriched uranium (HEU) cylinders. The ptrac option within MCNP6 can generate time-tagged counts in a cell (list-mode data). The list-mode data can then be processed similarly to measured data to obtain values for system parameters such as the dual prompt neutron decay constants observable in a coupled system. The results from the ptrac simulations agree well with the historical measured values.

A series of case studies are conducted to study the effects of geometrical asymmetry in the coupling between two bare metal HEU cylinders. While the coupling behavior of symmetrical systems has been reported on extensively, that of asymmetrical systems remains sparse. In particular, it appears that there has been no previous research in obtaining the coupling time constants for asymmetrically-coupled systems. The difficulty in observing such systems is due in part to the inability to determine the individual coupling coefficients from measurement: unlike the symmetrical cases, only the product of the values can be obtained. A method is proposed utilizing MCNP6 tally ratios to separate the coupling coefficients for such systems.

This work provides insight into the behavior of asymmetrically-coupled systems as the separation distance between the two cores is changed and also as the asymmetry is increased. As the separation distance increases, the slower observable prompt neutron decay constant increases in magnitude while the faster decay constant decreases. The coupling time constants are determined from the measured decay constants. As the separation distance increases, both coupling coefficients decrease as expected. As the asymmetry increases, the difference between the faster and slower decay constants and between the coupling time constants increases as expected.

Based on these findings, an effective computational method utilizing MCNP6 and the Rossi-alpha technique can be applied to the prediction of asymmetrical coupled system measurements.


Monte Carlo simulations; neutron noise measurements; neutron transport; reactor kinetics; Rossi-alpha; subcritical reactor


Nuclear Engineering