Robert D. Busch

Two-region kinetic model for reflected reactors

Reflected reactors constitute one of the most important classes of nuclear reactors. Yet, experimental data clearly shows that the kinetic behavior of some types of reflected systems cannot be adequately characterized using the standard point kinetic model. In this work, a two-region, nodal kinetic model for reflected systems is developed that appears to explain the anomalous behavior of these systems in terms of integral parameters that are both measurable and calculable. The model is based on previous work by Avery and Cohn. We augment the Avery-Cohn model with the introduction of simple probability relationships essential to calculating the coupling parameters between the core and the reflector. Using these probability relationships, we then derive the definition of the effective multiplication factor keff, in terms of the multiplication factor of the core region, kc, and the reflector return fraction, f. We also derive the reflected-core inhour equation and introduce expressions to calculate the neutron lifetimes characterizing the systems static and dynamic behavior. The model is then tested using Rossi-α data from the University of New Mexicos AGN-201 reactor.

The equivalent fundamental-mode source

Abstract In this work, we describe the concept of an equivalent fundamental-mode source, and we derive an expression for a factor, g∗ , that converts any arbitrary source distribution to its equivalent fundamental-mode source strength. We also present a new experimental method that can be employed to measure the equivalent fundamental-mode source strength in a multiplying assembly. We demonstrate the method on the zero-power, XIX-1 assembly at the Fast-Critical Assembly (FCA) Facility, Japan Atomic Energy Research Institute (JAERI).

TSUNAMI Primer: A Primer for Sensitivity/Uncertainty Calculations with SCALE

This primer presents examples in the application of the SCALE/TSUNAMI tools to generate k{sub eff} sensitivity data for one- and three-dimensional models using TSUNAMI-1D and -3D and to examine uncertainties in the computed k{sub eff} values due to uncertainties in the cross-section data used in their calculation. The proper use of unit cell data and need for confirming the appropriate selection of input parameters through direct perturbations are described. The uses of sensitivity and uncertainty data to identify and rank potential sources of computational bias in an application system and TSUNAMI tools for assessment of system similarity using sensitivity and uncertainty criteria are demonstrated. Uses of these criteria in trending analyses to assess computational biases, bias uncertainties, and gap analyses are also described. Additionally, an application of the data adjustment tool TSURFER is provided, including identification of specific details of sources of computational bias.

Thermal neutron detectors based on gadolinium-containing lanthanide-halide nanoscintillators

A novel concept for detection of thermal neutrons based on lanthanide halide nanocrystals containing gadolinium, an element with by far the highest thermal neutron capture cross section among all stable isotopes, is presented. Colloidal synthesis of GdF3 nanocrystals, GdF3 nanocrystals doped with Ce, and LaF3 nanocrystals doped with Gd is reported. The nanocrystals were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), dynamic light scattering (DLS) analysis, and steady state UV-VIS optical absorption and photoluminescence spectroscopy. Neutron detection has been confirmed in experiments with Gd-containing nanocrystalline material irradiated with 252Cf neutron source.

Detection of thermal neutrons using gadolinium-oxide-based nanocrystals

The concept of detection of thermal neutrons using gadolinium oxide nanocrystals is explored. Gadolinium is an element with by far the highest thermal neutron capture cross section among all stable isotopes. Colloidal synthesis of Gd2O3 nanocrystals, Gd2O3 nanocrystals doped with Ce, Gd2O3 nanocrystals doped with Eu, and Gd2O3 nanocrystals co-doped with Ce and Eu is reported. The nanocrystals were characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, dynamic light scattering analysis, and steady-state UV-VIS optical absorption and photoluminescence spectroscopy. Neutron detection has been modeled with MCNPX and confirmed in experiments with Gd-containing nanocrystalline samples irradiated with 252Cf neutron source.

Characterisation of potassium bromide loaded with dysprosium fluoride nanocrystals for neutron detection

We explore a novel concept of passive optically-enabled detection of thermal neutrons that exploits transmutation of 164 Dy into 165 Ho. The concept relies on significant differences in optical properties of Dy and Ho and on our ability to find the most sensitive optical method of differentiating between Dy and Ho. While the concept applies equally well to bulk materials and to nanocrystals (NCs), the nanocrystalline approach is much more attractive due to its significantly lower cost, relative ease of colloidal synthesis of high quality NCs with controlled composition, and superior optical and mechanical properties of NCs compared to their bulk counterparts. One particular advantage of NCs for neutron detection is that in principle they can be integrated into a transparent host without causing optical scattering. Since Ho is known to have strong emission lines in mid-infrared, we considered potassium bromide (KBr), transparent in mid-IR spectral range, to be a suitable host for Dy-containing NCs. Here, we report on synthesis and characterisation of DyF 3 :10%Ce, HoF 3 :10%Ce, and DyF 3 :10%Ho,10%Ce NCs, their insertion into KBr matrix, and optical characterisation of the obtained nanocomposites, both non-irradiated and subjected to neutron irradiation.

Thermal neutron detection with PMMA nanocomposites containing dysprosium fluoride nanocrystals

Naturally occurring dysprosium is attractive as a neutron detector because of its high thermal neutron capture cross section and high natural abundance. Neutron-induced transmutation of 164Dy results in production of stable isotopes of holmium and erbium (the latter only at sufficiently high neutron fluxes), due to beta decays caused by nucleus instability. This mechanism, unaffected by gamma radiation, can be used to unambiguously detect neutrons, without having to discriminate against an accompanying gamma flux. Optically-enabled thermal neutron detection can be based on significant differences in optical properties of Dy and Ho or Er, which allows to determine the relative fractions of Dy, and Ho, and E in an irradiated sample. In our search for the most sensitive method of differentiating between Dy and Ho residing in the same host material, we produced various Dy- and Ho-containing nanocrystals and uniformly dispersed them in a PMMA polymer matrix. Optical properties of the nanocomposites were analyzed by means of absorption and PL spectroscopy. We also report on neutron irradiation experiments with Dy-containing nanocrystals and our attempts to optically detect neutron-induced conversion of Dy into Ho.

Effects of Gamma Irradiation on Optical Properties of Colloidal Nanocrystals

The effects of {sup 137}Cs gamma irradiation on photoluminescence properties, such as spectra, light output, and lifetime, of several types of colloidal nano-crystals have been investigated. Irradiation-induced damage testing was performed on CdSe/ZnS, LaF{sub 3}:Eu, LaF{sub 3}:Ce, ZnO, and PbI{sub 2} nano-crystals synthesized on a Schlenk line using appropriate solvents and precursors. Optical degradation of the nano-crystals was evaluated based on the measured dependence of their photoluminescence intensity on the irradiation dose. Radiation hardness varies significantly between various nano-crystalline material systems. (authors)

The equivalent, fundamental-mode source

In 1960, Hansen analyzed the problem of assembling fissionable material in the presence of a weak neutron source. Using point kinetics, he derived the weak source condition and analyzed the consequences of delayed initiation during ramp reactivity additions. Although not clearly stated in Hansen`s work, the neutron source strength that appears in the weak source condition actually corresponds to the equivalent, fundamental-mode source. In this work, they describe the concept of an equivalent, fundamental-mode source and they derive a deterministic expression for a factor, g*, that converts any arbitrary source distribution to an equivalent, fundamental-mode source. They also demonstrate a simplified method for calculating g* in subcritical systems. And finally, they present a new experimental method that can be employed to measure the equivalent, fundamental-mode source strength in a multiplying assembly. They demonstrate the method on the zero-power, XIX-1 assembly at the Fast Critical Assembly (FCA) Facility, Japan Atomic Energy Research Institute (JAERI).

Modeling of central reactivity worth measurements in Lady Godiva

The central reactivity worth measurements performed in Lady Godiva were duplicated using TWODANT, a deterministic neutron transport code, and the 16-group Hansen-Roach cross-section library. The purpose of this work was to determine how well the Hansen-Roach library predicts the reactivity worths for a fast neutron system. Lady Godiva is a spherical uranium metal (93.7 wt% [sup 235]U) critical assembly with a neutron flux distribution dominant in the first five groups of the Hansen-Roach energy structure (0.1 MeV and up). Provided that the cross sections of the replacement material do not undergo large variations (less than an order of magnitude) in any of the aforementioned groups, the calculated reactivities were within 10% of the experimental values. For cases where the reactivities were outside this range, a large variation in the cross section was found to exist in one of the groups, which was not fully accounted for in the Hansen-Roach group structure. However, even in the cases where the agreement between calculation and experiment was not good, the calculated reactivity appeared to be an extremum in that the effect was found to be either more negative or more positive than the experimental value.