Nathaniel S. Bowden

Directional Neutron Detection Using a Time Projection Chamber

Measurement of the three dimensional trajectory and specific ionization of recoil protons using a hydrogen gas time projection chamber provides directional information about incident fast neutrons. Here we demonstrate directional fast neutron detection using such a device. The wide field of view and excellent gamma rejection that are obtained suggest that this device is well suited to searches for special nuclear materials, among other applications.

ANTINEUTRINOS FOR REACTOR SAFEGUARDS: EFFECT OF FUEL LOADING AND BURNUP ON THE SIGNAL

Various types of nuclear reactor related information, including relative power level and fuel evolution parameters, can be inferred remotely using antineutrino detectors. We show that it is possible to verify assembly-level burnup using information derived from an antineutrino detector if the nominal reactor fuel loading is known. Alternatively, if the core power is measured using an independent method, for example, a thermal hydraulic element, and the nominal core behavior is known, the antineutrino detector has a capability to determine previously unknown MOX loading in the core.

Pixellated NaI (Tl) Detector for Light Yield Nonproportionality Investigation

An 8 × 8 pixellated Nal(Tl) detector (1.7 cm × 1.7 cm × 1.0 cm) coupled to a multi anode PMT read out system was modeled to study the impact of light yield nonproportionality due to multiple photon scatterings and assess the contribution of delta rays in Nal(Tl) energy resolution. Geant4 simulation tool with modified nonproportional optical photons sampling routine was used in the study. A deconvolution technique was implemented to allow decoupling of the multiple photon scatterings contribution from the total energy resolution. Results from the simulation demonstrated that the effect of nonproportionality due to multiple photon scatterings is too small to be observed. However, the contribution of the delta rays was found to be a significant component of the evaluated Nal(Tl) energy resolution.

Reactors as a Source of Antineutrinos: Effects of Fuel Loading and Burnup for Mixed-Oxide Fuels

In a conventional light water reactor loaded with a range of uranium and plutonium-based fuel mixtures, the variation in antineutrino production over the cycle reflects both the initial core fissile inventory and its evolution. Under the assumption of constant thermal power, we calculate the rate at which antineutrinos are emitted from variously fueled cores, and the evolution of that rate as measured by a representative ton-scale antineutrino detector. We find that antineutrino flux decreases with burnup for Low Enriched Uranium cores, increases for full mixed-oxide (MOX) cores, and does not appreciably change for cores with a MOX fraction of approximately 75%. Accounting for uncertainties in the fission yields, in the emitted antineutrino spectra, and the detector response function, we show that the difference in core-wide MOX fractions at least as small as 8% can be distinguished using a hypothesis test. The test compares the evolution of the antineutrino rate relative to an initial value over part or all of the cycle. The use of relative rates reduces the sensitivity of the test to an independent thermal power measurement, making the result more robust against possible countermeasures. This rate-only approach also offers the potential advantage of reducing the cost and complexity of the antineutrino detectors used to verify the diversion, compared to methods that depend on the use of the antineutrino spectrum. A possible application is the verification of the disposition of surplus plutonium in nuclear reactors.