Stainless steel, chemical changes

Control of the core is affected by movable control rods which contain neutron absorbers soluble neutron absorbers ia the coolant, called chemical shim fixed burnable neutron absorbers and the intrinsic feature of negative reactivity coefficients. Gross changes ia fission reaction rates, as well as start-up and shutdown of the fission reactions, are effected by the control rods. In a typical PWR, ca 90 control rods are used. These, iaserted from the top of the core, contain strong neutron absorbers such as boron, cadmium, or hafnium, and are made up of a cadmium—iadium—silver alloy, clad ia stainless steel. The movement of the control rods is governed remotely by an operator ia the control room. Safety circuitry automatically iaserts the rods ia the event of an abnormal power or reactivity transient. 

In addition, the magnetic characteristics of a material can change as a function of stress (e.g., unannealed series 316 stainless steel can be magnetic after machining), temperature, pressure, and physical and chemical treatment. Therefore, when two paramagnetic materials with similar magnetic susceptibilities are to be separated, the possibility that pretreatment will facilitate subsequent separation should be studied. 

Greater deviations which are occasionally observed between two reference electrodes in a medium are mostly due to stray electric fields or colloid chemical dielectric polarization effects of solid constituents of the medium (e.g., sand [3]) (see Section 3.3.1). Major changes in composition (e.g., in soils) do not lead to noticeable differences of diffusion potentials with reference electrodes in concentrated salt solutions. On the other hand, with simple metal electrodes which are sometimes used as probes for potential controlled rectifiers, certain changes are to be expected through the medium. In these cases the concern is not with reference electrodes, in principle, but metals that have a rest potential which is as constant as possible in the medium concerned. This is usually more constant the more active the metal is, which is the case, for example, for zinc but not stainless steel. 

Thus sodium chloride has the property of changing into a soft metal, sodium, and a greenish-yellow gas, chlorine, when it is decom-posed by electrolysis. It also has the property, when it is dissolved in water, of producing a white precipitate when a solution of silver nitrate is added to it and it has many other chemical properties. Iron has the property of combining readily with the oxygen in moist air, to form iron rust whereas an alloy of iron with chromium and nickel (stainless steel) is found to resist this process of rusting. It is evident from this example that the chemical properties of materials are important in engineering. 

A major change in the design of the low-pressure zone was introduced in 1955. Dry ice used as coolant was replaced with mechanically cooled low-temperature condensers, which proved much easier to use and maintain. The earliest condensers were made of stainless steel. As the scope of freeze-drying expanded, a need for a chemically resistant type of trap arose. In response, titanium traps that proved much more resistant to corrosion were developed. 

After refining, many metals are alloyed, or mixed with other elements, to change their physical and chemical characteristics. In some cases, certain impurities are allowed to remain during refining because their presence improves the properties of the metal. For example, a small amount of carbon in iron greatly enhances its hardness. Examples of alloys include brass (Cu and Zn), bronze (Cu and Sn), duralumin (Al, Cu, Mg, and Mn), and stainless steel (Fe, Cu, Mn, plus others). 

Fuel composition may change from uranium to plutonium, and cladding from aluminum to zirconium to stainless steel. In some cases blankets, moderators, and coolants must be processed, and these will introduce thorium, beryllium, NaK, and bismuth to the chemical process. These changes in materials will present new chemical and corrosion problems in waste treatment processes and waste storage procedures. 

Ip Yes. It s important to be familiar with the 1 properties of chemicals—to know what can be safely mixed and what can t. The container used for mixing is important chemically, too. For example, stainless steel containers can erode and change the makeup of chemicals so they won t have the same effects. Heavy plastic is used instead. 

Chemistry. Chemical analysis methods range all the way from simple to sophisticated. See Chapter 6 under Field Identification. The exact chemical composition is generally not required. The identification method should distinguish the correct material from materials that may have been substituted. For example, a moly (molybdenum) spot test is a chemical color change technique that is sometimes used to distinguish 304 (no moly) from 316 (small percentage of moly) stainless steel. 

The existence of a chemical isomer shift produced by the change in nuclear radius and differing chemical environments was first demonstrated by Kistner and Sunyar [15] in 1960 for the case of a stainless steel source and an absorber of a-FeaOs. Early work quickly showed that the chemical isomer shift measured was related to the formal oxidation state of the iron. This is illus-... 

---