Moderating ratio

Porous membranes with selective permeabiUty to organic solvents have been prepared by the extraction of latex films prepared with moderate ratios of PVA—PVAc graft copolymer fractions. The extracted films are made up of a composite of spherical cells of PVA, PVAc microgel, and PVA—PVAc graft copolymers (113). 

For moderate ratios (<3), the type of emulsion is decided by several factors (5), such as order of addition or type of emulsifier. One Hquid slowly added to the other with agitation usually results in the last-mentioned phase being the continuous one. Another factor is preferred solubiHty of the emulsifier the phase in which the emulsifier is soluble most probably is continuous. 

Moderator Slowing-down power, cm" Moderating ratio... 

Cycloaddition of vinylallene 201 having methyl and w-butyl substituents at the terminal allenic carbon gave a moderate ratio of geometric isomers owing to the small steric difference between the two substituents [167]. 

With the measurements subject to fluctuations of 20 or 30%, no accurate description of the profile is possible. All that can be said is that with moderate ratios of tube to particle diameter, the maximum velocity is about twice the minimum, and that when the particles are relatively small, the profile is relatively flat near the axis. It is fairly well established that the ratio of the velocity at a given radial position to the average velocity is independent of the average velocity over a wide range. Another observation that is not so easy to understand is that the velocity reaches a maximum one or two particle diameters from the wall. Since the wall does not contribute any more than the packing to the surface per unit volume in the region within one-half particle diameter from the wall, there is no obvious reason for the velocity to drop off farther than some small fraction of a particle diameter from the wall. In any case, all the variations that affect heat transfer close to the wall can be lumped together and accounted for by an effective heat-transfer coefficient. Material transport close to the wall is not very important, because the diffusion barrier at the wall makes the radial variation of concentration small. 

The unusual nucleophilic epoxidation of /i-hydroxyenones under Sharpless conditions (see Section 4.5.1.3.2.1.) is also applicable to compounds 1 with endocyclic double bonds. The. sj H-epoxides are produced with complete selectivity. The stereochemical outcome of the reaction under Weitz-Scheffer conditions significantly differs from that observed for acyclic compounds. While the acyclic enones afforded preferentially the 7i-epoxides32. 

Figure 1. Moderating ratio vs. hydrogen content for selected hydrides. (After Ref. 1.)...

D2O is the best moderator of thermal neutrons. D2O absorbs far less neutrons than H2O and heavy watermoderated reactors can operate with less reactive fissile material. Different moderators are compared in Table 1. A very large moderating ratio for D2O has... 

The value of i) depends on how much fissile material the fuel contains. The other three factors have a more complicated dependence of the reactor design including the fuel/moderator ratio, their amounts and shape some values are given for a practical case in Table 19.4. 

Atomic Nnmber Z Name Otemical Atonic Weight A Density (/>) gm/cc. 20°C 760 mm Hg Nuclei per ec (u 10-24) N (1-h) Avg. Lxtg Energy Loss f Tl Neut lermal (0.025 ev). ron Cross-Sections, Macroscopic Cross-Sections, Z oN Thermal (0.02S ev) Mean Free Path cm Slowing (town Moderating Ratio... 

Atomic- Number Z Name Chemical Atomic Weight A Denaity (P) gjyfcc, 20 C 760 an Hg Nuclei per cc (X 10-34) N (l-b) Arg. Lag Energy Ipsa ( n Neut ermal ron Cr cr) in (0.025 ev) MS Sections )sms Ibermal (0.025 ey) Macroscopic Cross Sections, oN Ibermal (0.025 er) Mean Free Path cm Slowing Down Power No-gf Moderating Ratio (crs/cr ) Xf... 

FIG. 6 is a set of curves representing reproduction factor for various natural uranium to moderator ratios ... 

A quantitative description of the moderating properties including neutron absorption is given by the moderating ratio (MR) ... 

Moderating properties of different compounds scattering cross section for epithermal neutrons (er in barn) absorption cross section for thermal neutrons (average logarithmic energy decrement slowing down power (SDP) (cm ) and moderating ratio (MR)... 

Fissionable uranium is present in the HTR-Module in spherical fuel elements. Each fuel element has a diameter of 6 cm and contains approximately 11 600 coated particles within the inner graphite matrix. Each of these coated particles consists of a fuel kernel (uranium dioxide, UO2) with a diameter of about 0.5 mm, which is coated with layers of pyrocarbon (PyC) and of silicon carbide (SiC). These layers enclose the fuel kernel, thus preventing a fission product release from the fuel element to the highest degree. One fuel element contains a total of 7 g of uranium (equivalent to a moderation ratio of approx. 69(X)) with a 235 U enrichment of 7.8%. 

Survey calculations were subsequently performed for systems containing 8, 15, 30, 50, and 100 wt% PuOa. The systems examined were water-reflected and unreflectcd spheres containing Pu02, and water-reflected spheres containing PuOz with 15, and 25 isotopic percent °Pu. The moderation ratios examined extend from dry, unmoderated oxide powders, to very dilute, overmoderatod, aqueous solutions. 

One of the more difficult tasks in the criticality safety evaluation of irradiated fael casks is that of determining tte most reactive fuel assembly type for both normal and accident conditions. In the criticality analysis of the NFS-4 and NFS-5 casks, the conservative approach used was that of analyzing Hie foel under the worst possible conditions. These conditions Included tael irin bowing (increase in fuel-to-moderator ratio and concomitant in-cr e in fuel assemMy envelope dimensions) and fuel pin drop out (increase in fuel-to-moderator ratio). 

These were performed by Phillips Petroleum and G.E. in 1964 and 1967. The lattices have nonmoderator-to-moderator ratios of 1.9 and 2.3, and H/U ratios of 2.7 and 2.3, respectively. The fuel ro are UO pellets clad in stainless steel, surrounded by thin aluminum sleeves to simulate the Zr insulators in the PBF core. These rods are described in Table I. The rods are placed in a square mesh of 15.656-mm <0.616-in.) and is,l89-mm (0.598-in.) pitch for the 1.9 and 2.3 ratio cells, respectively. The experimental arrays are approximately right circular cylinders, with radial water reflectors, containing 900 to 1568 pins. These vary firom simple clean arrays to configurations containing mocked-up test areas, control rods, voids, etc. Tite water height was adjusted to achieve criticality. 

Technical difficulties are rather easier to list than unusual events even if it is difficult to quantify some of them. For example, with sophisticated methods like Monte Carlo Codes, errors can be introduced by the use of too few number of stages, to use the techniques in areas where there are too few experimental check points, to oversimplify physical conditions (i.e., modeling errors), to minimize the total number of cases calculated when examining changes of reactivity as parameters such as-moderation ratio are changed. Care should be taken to avoid understudying a situation—never rely on one calculation. 

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