Rod Shadow

Rod shadow Is an effect in which the repositioning of one control rod changes the reactivity worth of adjacent rods or causes a change in power level indication on neutron detectors when power level has remained constant. Rod shadow is caused by flux redistributions within the core as a result of rod motion. 

Figure 7.5 Rod Shadow Between Adjacent Control Rods...

Figure 7.6 Rod Shadow Between Control Rods and a Neutron Detector...

Explain the rod shadow effect and what this means to reactor operation in terms of effects on rod worths and detector power level indication. 

Rod shadow is the shifting in the spherical isoflux lines in and around the reactor core by rod motion. It can cause a change in the reactivity worth of nearby rods by changing the flux patterns cutting those rods. It can also cause a change in indicated reactor power with no change in actual power by shifting flux patterns around detectors. 

Glenny JR. Rotary metal shadowing for visualizing rod-shaped proteins, in Electron Microscopy in Molecular Biology. A Practical Approach (Sommerville J, Scheer U, eds.), IRL Press, Oxford, UK, 1987, pp. 167-178. 

This efifect of unbalanced electrode surface area is often neglected under the shadow of the self-biasing effect. It is considered that the unbalanced electrode area is the dominant factor that influences the deposition in luminous gas phase, of which the majority of reactive species are neutral. If a small-diameter rod is inserted to the center of the basketlike tumbler as the hot electrode, an extremely high deposition of black carbonaceous film occurs to the rod, and virtually no deposition occurs to the small-size substrates placed in the basket due to the unbalanced electrode surface effect. The extent of the unbalanced electrode effect is dependent to some extent on the nature of monomer. The monomers that deposit easily by LCVD show a pronounced effect and hence cannot be deposited on the substrate. 

A more reliable and rapid technique for evaporating both shadowing metals and carbon is via electron beam bombardment. This method makes use of a focused flux of high-energy electrons to provide the necessary power for the evaporation of a small metal or carbon source. Generally, this is used to form thin films of Pt/C, W/Ta, and pure C. The platinum films contain some carbon (usually <596) due to the presence of the carbon support rod. The electron beam gun evaporation approach greatly improves throughput and consistency compared to the other approaches. 

Figure 2.2. Schematic for excluded volume (shadowed area) of the rod 1 (a) parallel to the neighbor rod 2 (b) perpendicular to the neighbor rod 2.

FIG U R E 2 Metal-shadowed smooth muscle myosin in the folded and extended conformation. Upper panel At physiological ionic strength in the presence of MgATP, myosin preferentially forms a folded monomer where the tail is bent into approximately equal thirds. Note the heads often bend down toward the rod. Lower panel For comparison, extended monomers formed at high ionic strength are shown. Bar-50 nm. Reprinted from Trybus, K. M., and Lowey, S., Journal of Biological Chemistry 259, 8564-8571, 1984. 

When such features exist, they are penetrated by the electron beam so the material is represented by a three-dimensional point lattice and diffraction only occurs when the Ewald sphere intersects a point. This produces a transmission-type spot pattern. For smooth surfaces, the diffraction pattern appears as a set of streaks normal to the shadow edge on the fluorescent screen, due to the interaction of the Ewald sphere with the rods projecting orthogonally to the plane of the two-dimensional reciprocal lattice of the surface. The reciprocal lattice points are drawn out into rods because of the very small beam penetration into the crystal (2—5 atomic layers). We would emphasize, however, that despite contrary statements in the literature, the appearance of a streaked pattern is a necessary but not sufficient condition by which to define an atomically flat surface. Several other factors, such as the size of the crystal surface region over which the primary wave field is coherent and thermal diffuse scattering effects (electron—phonon interactions) can influence the intensity modulation along the streaks. 

Figure 4. Electron micrographs of crystals of cellulose prepared at 90 C after shadowing with W/Ta. 4A from Avicel (DP 120) cellulose solution the structure consists of rod-like elements together with granular aggregates (arrows). Insert corresponding X-ray diagram. from microcrystalline cellulose (DP 34)solution. Insert corresponding electron diffraction diagram.

Results with the previous calculation scheme were giving C/E values of about +5 to 20%. The fact that the comparison is significantly improved indicates that both the method and the nuclear data have been improved and that there remain probably no compensating effects. The results obtained with the ERANOS calculation scheme are therefore satisfactory and this scheme can be considered to be reliable as an explicit treatment of all shadowing and transport effects is taken into account. This good behaviour is also observed for the prediction of control rod reactivity worth in the Phenix reactor. There is only a ring of 6 control rods in this reactor. The results are gathered in Table 5. 

Expose 1.5 mm of the Pt-carbon rod and increase the beam current to l(X) tA for 10 s, for the vapor deposition of platinum this process corresponds to shadowing. The vapor deposition must be carried out at a pressure of 10 Pa. 

The short, medium and long axis of the enclosing box gives S, M and L, respectively (Figure 5.2). The ratios of these axes are representative of the shape of the molecule. A low SjL ratio corresponds to a flat molecule, a low MjL ratio to a rod-shaped molecule. These axis ratios showed much stronger correlation than the absolute values of S, M and L. Analogous descriptors are referred to as shadow descriptors in the QSAR literature. 

---