Shielding experiments

Interesting fundamental studies and analytical applications of RRDEs have been published by Bruckenstein and co-workers the attraction of collecting experiments with the RRDE lies more especially in the fact that metals with different oxidation states are becoming more accessible to analysis, e.g., Cu(II) and Cu(I)127, U(VI) and U(V), Fe(III) and Fe(H). Shielding experiments were carried out for Bi(III) and Bi(0)128. Special use of stripping voltammetry with collection at a glassy carbon RRDE for the determination of tin in the presence of lead was proposed by Kiekens et al.129 after cathodic electrode-deposition... 

Various circuits have been described to measure collection efficiencies based on galvanostatic control of the upstream electrode with the downstream electrode being held at the limiting current for the reaction taking place there. It is also possible to measure N0 by a potentiostatic shielding experiment. For, an irreversible electrode reaction, measurement of N0 in these two different ways will, in principle, give different results if the upstream electrode is not uniformly accessible. 

Comum K, Comum R, Storm W. Use of psychostimulants in extended flight operations a Desert Shield experience. In Advisory Group for Aerospace Research and Development conference Proceedings No. 579, Neurological Limitations of... 

Several species have been shown to not modify S-320 or MAA concentration in response to UVR, such as the zoanthid Zoanthus sociatus, in response to increased levels of UVR [101], the octocoral Clavularia sp. over a depth gradient [51], the coral Montastraea annularis on transplantation from 24 m to 12 m over 21 days [102] and the temperate anemone Anthopleura elegantissima in UVR-exposed versus UVR-shielded experiments [53]. Data such as these have been used to suggest that MAAs are not directly photoprotective but are rather a byproduct of other chemical reactions and that photoprotection is a secondary function. An alternative explanation is that, at least for all of the above studies that involve coelenterates, the MAAs are derived from diet and therefore rather than MAA concentration being dependent on the intensity of UVR is dependent on the concentration of MAAs in the food source. 

Below, we describe crystal-detector electrode shielding experiments for both the dissolution process alone and in the presence of polymaleic acid. Homo- and co-polymers of maleic acid are well-documented precipitation inhibitors [249-251]. Firstly, however, we consider the theoretical relationship between the shielding current and the heterogeneous rate constant for the reaction of H+ with the crystal surface. 

Several different types of experiments are possible at the RRDE, The most common are collection experiments, where the disk generated species is observed at the ring, and shielding experiments, where the flow of bulk electroactive species to the ring is perturbed because of the disk reaction. 

The background count rate at 5,000 feet above sea level measured in this laboratory is 22 percent higher than the rate at sea level, using a similar instrument and operational conditions. The increased background is caused by cosmic ray produced muons and nucleons. Shielding experiments and an analysis of the background components are presented. 

Lead bricks used for shielding experiments were fabricated from lead wool used by the U.S. Navy during World War II. The radioactive impurities of lead wool were prechecked before fabrication with Ge(Li) gamma detector to assure their low level. The laboratory is housed in a single story, concrete-block building in Arvada, near Denver, at an elevation of approximately 5,000 feet above sea level. 

These experiments were made to determine the contribution of cosmic-ray components to the background. Because of the large size of the NCMS and the limited number of lead bricks (approximately 200 pieces), no further shielding experiments were conducted. 

After a short period of operation of B, D and F Reactors, it became apparent that the graphite was growing as a result of irradiation. Evidences of this were distortion of the process tube and rod channels and movement of the shields. Experiments indicated that graphite expands when irradiated at low temperatures and contracts when irradiated at high temperatures. 

The development of a standard database for FBR shielding design was continued. In the database, the experimental and analytical information from JASPER and other shielding experiments is evaluated and compiled in a systematic and consistent manner. 

As a part of the efforts, typical axial shielding experiments of JASPER are now being reanalyzed with the latest version of nuclear data library, JENDL-3.2, and compared with the results of the former JENDL-2. Further, a cross-section sensitivity code, SWANLAKE, was applied to survey the cause of the difference between the two JENDL libraries. 

E. T. BOULETTE, Analysis of ZPPR/FTR Shield Experiments Neutron Distribution," WHAN-FR-12, WADCO Corporation (October 1970). 

Performing an inverse square law measurement using a low-level radioactive source and simple detector. Performing a shielding experiment. 

In a basic shielding experiment the potential of the ring and disc electrodes are held at the same value, where a reaction is mass transport controlled. The ring current under these conditions will be less than expected because some of the electroactive species which would normally be oxidised (or reduced) at the ring will be removed by reaction at the disc. The shielding ring current is given by [29]... 

Shielding experiments allow precise determination of the quantity of electroactive species consumed at the disc, including material lost in non-faradaic processes such as adsorption or a chemical reaction. Where such non-faradaic processes occur, the experimental shielded ring current will be less than that calculated by Equation (4.63). 

Two examples of the many possible applications of the shielding experiment will be given. The first is taken from the paper by Sherwood Bruckenstein... 

Important works including PFBR shielding experiments, testing of transfer arm in air, boron enrichment, post-irradiation examination of FBTR fuel after 125 GWd/t bumup, structural integrity testing, and reprocessing of carbide fuel are being carried out. 

Figure . Ring-shielding experiment with a RRDpj m E in 0.1 M HCIO4...

The concrete tank has 7 ft thick walls and is 28 ft long, x 8 ft wide and 24 ft deep internally and contains light water. Three large aluminium windows are fitted in the tank and these allow heavy shielding experiments to be set up outside. Three beam holes are also fitted. The reactor core is made up of uranium/aluminium plates clad in aluminium it can be moved through the water into position on the inner side of any window. The water is circulated for cooling and cleaning. 

Calamand, D. and Curl, LJ, A Review of Progress with the JANUS Programme of Fast Reactor Shielding Experiments, Proc. Int. Conf. on the Physics of Reactors, Vol. 1, p. V-1, Marseille (1990). 

Rotated electrodes are used for studies of the character of electrode reactions and their kinetics and for the estimation of electroanalytical parameters. Convection provides a constant mass transport of the analyte to the electrode surface with changed diffusion characteristics compared to quiescent liquids. The effect is similar to a stirring of the solution, but the rotation of an electrode can be controlled much more precisely than of a solution. Rotating ring-disc electrodes are practically used only for elucidating electrochemical reaction mechanisms where oxidation or reduction on the disc creates (collection experiment) or consumes (shielding experiment) electroactive substances which are monitored with the ring electrode. 

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