Nuclear warhead monitoring : a study of photon emissions from fission neutron interactions with high explosives as a tool in arms control verification

Since the signing of the Nuclear Nonproliferation Treaty, the technical community has been working to develop verification options that provide confidence in the reduction or elimination of nuclear warheads, while respecting countries' requirement of limited access to national secrets. This dissertation used a simplified open-source warhead model as a vehicle to investigate the use of secondary gammas, generated passively by neutron interactions inside high explosive (HE), as a signature for the presence of a warhead-like object. Analytical calculations were done to estimate the detectability of radiative capture and inelastic scatter emissions generated within the warhead model. Results showed the emission of gammas from nitrogen, between 5-7 MeV, to be detectable above background with dwell-times exceeding 90 minutes. These calculations motivated the systematic study of the signal experimentally using surrogate materials to represent the warhead's weapons-grade plutonium and HE. The experiment did not show the expected signals. This motivated a simulation of the mock-up experiment using the radiation transport code MCNP6 to help understand the observed results. The experimental and simulation data suggest that correlated backgrounds from neutron interactions with environmental materials dominate the signal. This finding helped provide a basis for understanding the feasibility and challenges to detecting this neutron-induced gamma signal. Three sets of pulse-height spectra have been analyzed: experimental spectra that looked at the effect of the HE surrogate on the overall detected counts; simulated spectra that helped to understand the underlying contributors to the observed experimental result; and a data-MCNP6 comparison that assessed the accuracy of the simulated results. Each set contributed to the quantification of detectability for the emissions of interest. The findings suggest the passive detection of the expected high-energy gamma signal is not feasible, unless backgrounds can be better controlled. The difficulty is attributed to low solid-angle coverage of the neutron source by the melamine explosive surrogate, and competing backgrounds produced by neutron-source interactions with surrounding materials. This thesis also examined the benefits and tradeoffs of this particular verification approach by investigating the non-technical context of the verification, such as the preferences of negotiators. The tradeoffs between confidence and intrusiveness highlight the need for technical verification solutions that span the diversity of options. Factors limiting the development of warhead verification systems, from the bias of researchers to issues of classification and sensitive geometries, were discussed.

Department dc:contributor.department

Massachusetts Institute of Technology. Department of Nuclear Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2017

Author dc:creator

• Snowden, Mareena Robinson

Advisor dc:contributor.advisor

• Richard C. Lanza and Adam Bernstein.

• Nuclear Science and Engineering.

Statement dc:rights

• MIT theses are protected by copyright. They may be viewed, downloaded, or printed from this source but further reproduction or distribution in any format is prohibited without written permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/113723

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/113723