Reflector thick

ITEM MEAN CONCENTRATION (g/liter) HEIGHT OF LATTICE,h (cm) TEMPERATURE (°C) DjO REFLECTOR THICKNESS (cm) MEAN LATTICE RADIUS. R (cm) LATTICE VOLUME, V (1 i ters) CRITICAL MASS OF U (g) LATTICE AS IN FIGURE... 

Critical-Mass Measurements on Beryllium-Reflected, Enriched-Uranium, Spherical Systems, by O. Kolar and H, R. Ralston (VCBL-L). A scries of critical mass determinations has been made for spherical beryllium reflected systems. Cores were enriched (93.17S U-235) uranium spheres ranging in mass from 10.6 to 32.6 kg. The corresponding reflector thicknesses were 20.3 to 2.2 cm. All results were nonnalized to a core density of 18.6 g/cc. Some Los Alamos data are included for completeness and have been similarly normalized. These two sets of experiments were found to be in good agreement. The experiments and results arc discussed, together with the fit to the experimental data obtained with the spherical Si, 5-group code in use at UCRL. 

The second set of experiments was devoted to measurements of reflector and geometrical effects. A large number of assemblies with Na Nsi = 1200 x 10 and Vk /% O.Swere constructed with different height to diameter ratios and different end and side reflector thicknesses. Measurements on each assembly include the critical dimensions and radial and axial flux traverses with bare and cadmium covered gold and Dy-Al foils. 

Nuclear parameters of 4%-enriched UO lattices moderated by Q2O-H3O mixtures were calculated and these results were compared with experimental measuremeids. The critical assemblies used were clean, uniform lattices of af roxlmate cylindrie geometry. Physical properties commdb to all assemblies are given in Tablel." The assemblies differ inD20 concentration, boron concentration, core radius, and reflector thickness. 

Experiments with varying refiector thicknesses and materials were also performed in a 90° sector around haif the core length. Extrapolating the results linearly to a full reflector, It was found that doubling the above nickel reflector thickness to 4 In. would be only 1.3 times as effective. Further results are given in Table t. 

These data permit an evaluation of the critical mass for a hypothetical plutonium-water mixture in the 14-in. sphere reflected with 4-in. of concrete. The In (critical mass) is plotted vs total nitrate in Fig. 3. From these data, the critical mass for the plutonium-water mixture is then estimated to be 633 g Pu (Pu concentration of 28 g Pu/f). These results indicate the 4-in. concrete reflector to be essentially equivalent to an effectively Infinite water reflector. In subsequent experiments an additional layer of concrete will be used to increase the reflector thickness to 10-in. 

Fig. 1. Dependence of Number of Units in a Critical Array on Surface-to-Surface Separation and Paraffin Reflector Thickness.

TABLE I. Dependence of Constants N and s on Paraffin Reflector Thickness... 

Density exponents were also computed for plutonium atoms in water at various H Pu ratios as a function of water-reflector thickness. 

Uranium Reflector Thickness (in.) Material Wtthin the Annulus Height of Fuel for Radial hicrements of (in.) Average Density (i em ) ... 

Within the constraints imposed by the requirements of symmetry and simplicity, and by the ZPR-9 facility, FTR-3 was intended to apprcndmate the design of the FTR. The core he ight was 91.6. cm. The radii of the timer core zone, outer core zone, and radial reflector were 37.6, 60.3, and 85.4 cm, respectively, hi addition, a 12-cm-thick stainless steel and sodium shield surrounded the radial reflector. The axial reflector thickness was 31.4 cm. The total volume of the Inner and outer core zones was 1045 liters. The U peripheral boron control zones of the FTR-3 were approximately evenly spaced around the outer edge of the outer core. Each control zone was 11.2 liters in volume, extended the full height of the core, and were similar in size and composition to the peripheral boron control rods in the FTR design. 

The criticality of arrays of parallel cylinders sitting upon a fissile solution slab were investigated. These experiments were performed under minimum reflector conditions as. well as various reflector thicknesses of Plexiglas. The number of cylinders in arrays varied from 1, 4, 9, and 16. Cylinder diameters ranged from 4.S to 8 in. 

A cadmium sheet, 0.05 cm diick, placed between core and reflector, has an effect equivalent to re-duclng the reflector thickness from 20 to <2 cm. 

The assembly, as described, was subcrltical, but exhibited an apparent neutron source multiplication greater than 5. Criticality was achieved by increasing the outer-surface concrete reflector thickness from 20.3 to 30.5 cm of the array on the movable table i.e., the surface perpendicular to the direction of table motion. Criticality occurred at a table separation of 0.39 cm, and at table closure the keff of the assembly was measured as 1.0007. A second similar addition to the outer reflector surface on the stationary table resulted in criticality at a table separation of 7.36 cm. A summary of these data aj ears in Table U along with a schematic diagram of the experimental arrangement. 

Since, by assumption, the reflector thickness is also small compared to fto, the right-hand side of (8.17) is very large, and it follows from the approximation (8.14) that the above relation may be written... 

The general approach used above may also be applied to a system having a thick reflector. In this case we impose the restriction that xr R — Rq) 1 and furthermore that xrRq > > 1 i.e., both the reflector thickness and the core size are to be much larger than the diffusion length of the reflector material. Because of the former requirement, coth xr Ri — Ro) 1, and (8.11) is approximated by... 

An accurate calculation for the reflector savings based on the solution of the criticality equation for the one-velocity model will yield a curve of the shape shown in Fig. 8.3. The approximations (8.20) and (8.22) apply for the extreme values of reflector thickness. 

The core is surrounded by a graphite reflector (i.e., normal graphite, not graphite foam). The fuel is to be loaded into the foam s pores, thus minimizing the thermal path length between the fuel and the heat sink. The uranium-impregnated foam would be encased in a superalloy steel. The reflector/clad interface temperature is expected to be limited to 900 K. The reflector thickness is approximately 30 cm. 

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