Polyethylene reflections

The previously reported work with annular cylinders of uranium metal (p = 18.76 g/cm ) enriched to 93.2% in U-235 has been extended to include the effects of reflecting materials. Annuli with outside diameters as large as 15 in. and inside diameters as small as 7 in. were assembled to delayed critical with the central cavity filled with HLM-type gnq>hite (p = 1.73 g/cm ), beryllium (p = 1.83 g/cm ), or Plexiglas (CjHgO , p = 1.18 g/cm ). In some cases the outer surfaces of the annuli were reflected with up to 18 in. of gr hite or at least 5.75 in. of polyethylene (CH2, p - 0.92 g/cm ), both with and without a reflecting material in the center of the annuli. Some of the results for the graphite- and polyethylene-reflected experiments are shown in Fig. 1. 

A series of experiments and complementary calculations using the KENO program on boOi bare and thick-polyethylene-reflected, graphite-moderated cubic arrays of 8 nominally 15.7 kg 17(93.2) metal cylinders, has been started. Data from these experiments supplement that on unmoderated and hydrogenously moderated arrays of the same metal cylinders, and so will be valuable in the analysis of the effects of array moderation and reflection. Also, the data provide valuable check points lor validation of calculational techniques, such as the KENO program, that are used in nuclear criticality safety analysis. This summary presents data calculational results on the first several ejqperlmentsiin the series. 

Calculations performed assuming the same contents in all tanks indicate that the reactivity of the tank next to the concrete reflector exceeds that of the other tanks. This is due to enhanced reflection of epithermal neutrons by the concrete. Solutions of U, which are more reactive in slab geometry than solutions of Pu or U, will only be placed into the new tanks located away from the concrete wall. (This prevents contamination of other tanks by radioactive decay daughters.) Calculations assuming that the two tanks closest to the wall are filled with Pu(N03)4 and that the two outer tanks are filled with U02(N03)2, yield keff = 0.8S8 0.006. This is about the same as when all tanks are Filled with Pu solution, and indicates that the reactivity of a polyethylene-reflected tank of solution docs not exceed the reactivity of a conciete-refleeted tank of Pu solution. Additional calculations show that the subcriticality of the four-tank grouping is not jeopardized by off-normal construction well beyond that to which control is readily achievable. 

Fig. 2 Chemical composition distribution (CCD) of a typical Ziegler-Natta linear low-density polyethylene, reflecting the composition heterogeneity of these copolymers...

There are two common ways of representing the temperature dependence of the shift factor. One is as an exponential in reciprocal absolute temperature [exp( /Rr)], as done with the polystyrene data this functionality is usually vahd at temperatures well above the glass transition temperature, which is 104 °C for the polystyrene. The polystyrene data are fit with E/R = 9,300 K a value of order 4,000 K is typical of polyethylene, reflecting the wide variation in this parameter. The other commonly used functionahty is the Williams-Landel-Ferry (WLF) equation, which is written... 

Criticality Precautions. The presence of a critical mass of Pu ia a container can result ia a fission chain reaction. Lethal amounts of gamma and neutron radiation are emitted, and a large amount of heat is produced. The assembly can simmer near critical or can make repeated critical excursions. The generation of heat results eventually ia an explosion which destroys the assembly. The quantity of Pu required for a critical mass depends on several factors the form and concentration of the Pu, the geometry of the system, the presence of moderators (water, hydrogen-rich compounds such as polyethylene, cadmium, etc), the proximity of neutron reflectors, the presence of nuclear poisons, and the potential iateraction with neighboring fissile systems (188). As Httle as 509 g of Pu(N02)4 solution at a concentration Pu of 33 g/L ia a spherical container, reflected by an infinite amount of water, is a critical mass (189,190). Evaluation of criticaUty controls is available (32,190). 

By the mid-1990s capacity for polyethylene production was about 50 000 000 t.p.a, much greater than for any other type of plastics material. Of this capacity about 40% was for HDPE, 36% for LDPE and about 24% for LLDPE. Since then considerable extra capacity has been or is in the course of being built but at the time of writing financial and economic problems around the world make an accurate assessment of effective capacity both difficult and academic. It is, however, appeirent that the capacity data above is not reflected in consumption of the three main types of material where usage of LLDPE is now of the same order as the other two materials. Some 75% of the HDPE and LLDPE produced is used for film applications and about 60% of HDPE for injection and blow moulding. 

Compatibility and various other properties such as morphology, crystalline behavior, structure, mechanical properties of natural rubber-polyethylene blends were investigated by Qin et al. [39]. Polyethylene-b-polyiso-prene acts as a successful compatibilizer here. Mechanical properties of the blends were improved upon the addition of the block copolymer (Table 12). The copolymer locates at the interface, and, thus, reduces the interfacial tension that is reflected in the mechanical properties. As the amount of graft copolymer increases, tensile strength and elongation at break increase and reach a leveling off. 

However it has to be conceded that after battery life cycle tests at such temperatures polyethylene separators also reach their limits, although this fact does not yet reflect in failure-mode studies [49], even in locations with extreme ambient temperatures. The tendency towards using ever-thinner backwebs cannot be continued, however, without seeking protective measures. Suitable provisions have to be made espc-... 

Muller et al. focused on polybead molecules in the united atom approximation as a test system these are chains formed by spherical methylene beads connected by rigid bonds of length 1.53 A. The angle between successive bonds of a chain is also fixed at 112°. The torsion angles around the chain backbone are restricted to three rotational isomeric states, the trans (t) and gauche states (g+ and g ). The three-fold torsional potential energy function introduced [142] in a study of butane was used to calculate the RIS correlation matrix. Second order interactions , reflected in the so-called pentane effect, which almost excludes the consecutive combination of g+g- states (and vice-versa) are taken into account. In analogy to the polyethylene molecule, a standard RIS-model [143] was used to account for the pentane effect. 

Figure 4. A set of pole figures for three reflections of a highly oriented linear polyethylene specimen. The concentric circles are parallels of latitude, which represent constant values of x from 0 deg (outermost circle) to 90 deg (north pole) in 15 deg steps. The straight lines are meridians of longitude, which represent constant values of < > from 180 degrees to +180 degrees.

For semicrystalline isotropic materials a qualitative measure of crystallinity is directly obtained from the respective WAXS curve. Figure 8.2 demonstrates the phenomenon for polyethylene terephthalate) (PET). The curve in bold, solid line shows a WAXS curve with many reflections. The material is a PET with high crystallinity. The thin solid line at the bottom shows a compressed image of the corresponding scattering curve from a completely amorphous sample. Compared to the semicrystalline material it only shows two very broad peaks - the so-called first and second order of the amorphous halo. 

Much more information can be obtained from the DSC experiment than simply an observation of the transition from a solid to a liquid phase. A plot of heat flow against temperature is a true depiction of the continuity of the heat capacity at constant pressure (Cp). If the entire temperature range of a given process is known, the physical state of a material will reflect the usefulness of that material at any temperature point on the plot. For polyethylene terephthalate (see Fig. 4.9), a stepshaped transition is interpreted as a change in Cp resulting from a... 

Recently, polyethylene and Teflon mesh sample holders have been used. A drop of sample is placed on the mesh and spread to a relatively uniform thickness for analysis. These holders can often be rinsed and reused. A very convenient alternative to liquid sample holders is the technique called attenuated total reflection or ATR. The ATR cell is a crystal of gallium arsenide, GaAs and the infrared radiation enters one end of the trapezoidal crystal. With the angles adjusted to obtain total internal reflection, all of the IR radiation passes through the crystal and exits the other end as shown in Fig. 5.14. 

Macromolecules are found in nature. Cellulose, wool, starch, and DNA are but a few of the macromolecules that occur naturally. Carbons ability to form these large, complex molecules is necessary to provide the diversity of compounds needed to make up a tree or a human being. But many of the useful macromolecules that we use every day are created in the lab and industrial complex by chemists. Nylon, rayon, polyethylene, and polyvinyl chloride are all synthetic macromolecules. They differ by which repeating units (monomers) are joined together in the polymerization process. Our society has grown to depend on these plastics, these synthetic fabrics. The complexity of carbon compounds is reflected in the complexity of our modern society. 

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