Neutron multiplication experiments

J. T. MIHALCZO and J. J. LYNN. Neutron Multiplication Experiments with Enriched Uranium Metal in Slab Geometry, CF-61-4-33, Oak Ridge National Lab. (1961). 

In -the moderation-control method for S% enriched UFii, criticality safety is assured by control of two parameters- the U enrichment and the hydrogen moderation. Topical subjects to be reviewed in method development include UF , purity control in liquefaction operations the Influence of container size on internal and external moderntion effects decoupling properties of the cylinder wall In water-moderated UFa arrays special neutron multiplication experiments with large UFo assemblies derivation of moderation control limits large cylinder cleaning procedures and protective packaging. The administrative requirements for method application are al.so discussed briefly. 

Neutron reflectivity is ideally suited to this problem, since concentration profiles can be resolved on the nanometer level and since, for an infinitely sharp interface, Rkjf will approach asymptotically a constant value. In addition, neutron reflectivity is nondestructive and multiple experiments can be performed on the same specimen. Figure 4 shows a plot of Rk Q as a function of bilayer of protonated... 

The protein-solvent interface was studied in an explicit solvent environment of 3182 water molecules by MD simulations performed on metmyoglobin [31].Both the structure and dynamics of the hydrated surface of myoglobin are similar to that obtained by experimental methods calculating three-dimensional density distributions, temperature factors and occupancy weights of the solvent molecules. On the basis of trajectories they identified multiple solvation layers around the protein surface including more than 500 hydration sites. Properties of theoretically calculated hydration clusters were compared to that obtained from neutron and X-ray data. This study indicates that the simulation unified the hydration picture provided by X-ray and neutron diffraction experiments. 

The Virus House was finished in October. Besides a laboratory the structure contained a special brick-lined pit, six feet deep, a variant of Fermi s water tank for neutron-multiplication studies. By December Heisenberg and von WeizsScker had prepared the first of several such experiments. With water in the pit to serve as both reflector and radiation shield they lowered down a large aluminum canister packed with alternating layers of uranium oxide and paraffin. A radium-beryllium source in the center of the canister supplied neutrons, but the German physicists were able to measure no neutron multiplication at all. The experiment confirmed what Fermi and Szilard had already demonstrated that ordinary hydrogen, whether in the form of water or paraffin, would not work with natural uranium to sustain a chain reaction. 

A pencil-size neutron source, 0.22 gm RaBe, was placed between the fuel rods at the center of the lattice during approach measurements. Three BF, counters were used to monitor the neutron multiplication. Two safety channels with control and trip circuits from the TTR were used in conjunction with lour safety and two control rods for each experiment. A fast water-moderator dump system was also provided. 

Neutron Multiplication Measurements with Plutonium-Aluminum Alloy Rods in Light Water, V, I. Neeley, R. C. Uoyd, and E. D. Clayton (GE-HAPO). A series of near critical and exponential experiments have been performed with natural water moderated assemblies of Pu-Al alloy rods in order to establish bases for nuclear safety specifications lor handling and storage of this kind of fissile material. 

The experiments were carried out in a 4-ft diam.by 5-lt deep tank of light water. The lattices were hexagonal in shape and placed so as to provide effectively infinite water reflection on all sides. The lattice plates were constructed from Incite. Safety circuits, neutron sources, and BF, counters for monitoring neutron multiplication were adapted from previous experiments of this type on 3.1% enriched uranium-water systems. ... 

Many the lattices which were investigated at BNL as subcritical assemblies were also studied in critical assemblies at Westinghouse (Bettis). b some cases the techniques at the two laboratories were similar, and in other cases the techniques were different. There were also a few critical e]q >eriments at BNL. The subcritical experiments at BNL fall. into two classes miniature lattice measurements and exponential measurements. Values of e, p, and f were obtained in small assemblies (miniature lattices) which have a neutron multq>lication of about 3, and in which the central neutron spectrum is characteristic of a critical assembly. Values of M and B were measured in larger assemblies (exponential assemblies) in which the neutron multiplication is between 3 and 50. Thus, there are considerable data to compare reactor parameters as measured in similar lattices by subcritical and critical techniques. 

The bottles were placed in arrays on a remotely operated split-table device, using neutron multiplication techniques to estimate the number of bottles for criticality. Extrapolations were also made on spacing to determine the critical spacing for fixed-number square arrays of the bottles. Arrays were single tier, except in one experiment where a double-tier array was used. 

Both et riments and KENO code calculations have shown that the neutron multiplication factor for the heterogeneous assemblies were no more than 1% greater than keff for homogenized materials. These experiments wUl be described more fully in an ORNL report. ... 

A subsequent experiment was performed to confirm the calculation. The experiment established that the cask was subcritical, to the extent that no meaningful neutron multiplication data were observed during the loading. Other similar calculations and experiments were per> formed with this cask with the same result. While the experimental results were limited, they did show that calculational techniques may prove adequate tio determine the criticality condition of even nonuniform poisoned lattices. i... 

C. L. BROWN, R. C. LLOYD, S. R. BIE AN, and E. O. CLAYTON, Exponential Experiments and Neutron Multiplication Measurements with 1.25 w/o Enriched N Reactor Fuel Elements in Light Water," BNWL-52, Battelle-Northwest Labs. (March, 1969). 

The infinite-medium neutron multiplication factor k , and the neutron age to thermal energy for UFt-paraffln mixtures have been inferred from critical experiments with 2 and 3% U-enriched uranium. The volume fraction of paraffin was varied so that the H U atomic ratio was between 195 and 972 for the 2% enrichment and was 133 and 277 for the 3% enrichment. 

Experiments were performed in a 48-in.-diam x 72-in.-high water tank located in the PNL Critical Approach Facility. Using lattice plates with 0.45- and 0.60-in. triangular pitches, it was possible to measiure the critical number of pins and the material buddings for seven lattice spacings. For all but the 0.45- and 1.35-in. lattices, neutron multiplication measurements were carried out to within 96% of the critical number of pins. At this point, the neutron source was removed and a back-off experiment was performed, utilizing neutrons from the spontaneous fission of Pu and Pu as the neutron source. This technique produced a linear extrapolation to the critical number of pins to within 1 pin. Material buck-Ungs were determined by the flux shape method in which the flux distribution is measured perpendicular to the source plane. 

Any allowance (based on para. 674(b)) for a change in neutron multiplication assumed in the criticality assessment as a result of actual irradiation experience and... 

For single-crystal diffraction experiments the most common type of detector is the CCD. The area of the detector is typically of the order of 25-100 cm and it is held in a fixed position, so the sample must be rotated in order to collect more of the diffraction pattern. For neutron diffraction experiments it is common to fix the orientation of the sample and therefore use a bank of multiple CCD detectors, the data from which are then merged into one file. CCDs can also be used in electron diffraction experiments, but some modifications are required. First, the diffraction pattern contained in the beam of electrons must be converted to an optical signal, which is typically done by positioning a phosphor screen in front of the detector. Secondly, the diffracted beam of electrons must be removed before it strikes the surface of the detector, or charge build-up on the surface will result. This can be achieved by the addition of a second, conducting layer, such as a thin film of aluminum metal. 

It should be remembered that the structure is derived from neutron diffraction experiment and, as for all diffraction experiments, it represents a time-averaged summation through space of the contents of multiple unit cells. In the case of a fully ordered structure, such as that of room temperature phase of lithium phosphate, the averaging process is not apparent in the resulting structural model. However, in the case of... 

The first group in a rather arbitrary ordering of the experiments is devoted to studies of the behavior of neutrons from fission or source energy to thermalization. Moderation and diffusion properties, diffusion length, and Fermi age are measured in a water tank facility at a reactor. Distribution of thermalized neutrons can be measured in several uraniumbearing exponential facilities in which neutron multiplication occurs and from which material buckling and critical reactor size can be inferred. In another exercise, the effective neutron temperature in two of the critical training reactors is measured by several methods. 

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