Reactor high-alloy steel

Selection of Corrosion-Resistant Materials The concentrated sofutions of acids, alkalies, or salts, salt melts, and the like used as electrolytes in reactors as a rule are highly corrosive, particularly so at elevated temperatures. Hence, the design materials, both metallic and nonmetallic, should have a sufficiently high corrosion and chemical resistance. Low-alloy steels are a universal structural material for reactors with alkaline solutions, whereas for reactors with acidic solutions, high-alloy steels and other expensive materials must be used. Polymers, including highly stable fluoropolymers such as PTFE, become more and more common as structural materials for reactors. Corrosion problems are of particular importance, of course, when materials for nonconsumable electrodes (and especially anodes) are selected, which must be sufficiently stable and at the same time catalytically active. 

The central part of the plant is the reactor pressure vessel housing the reactor core with its components as well as the water-steam separators and the steam dryers (see Fig. 2.2.). The dimensions of this component (inner diameter 6.62 m, height 22.35 m, total weight 785 Mg) exceed the dimensions of a PWR reactor pressure vessel considerably. The reactor pressure vessel is made from the high-alloy steel 22NiMoCr37 with an austenitic weld overlay on the inner surface. In its cylindrical section, the wall thickness of the vessel is 16.3 cm plus a weld overlay of 0.8 cm. 

Replacement of the high-temperature, high-alloy steels required in the reforming reactor with less expensive materials... 

To minimize the require development program associated with the SSR, the use of a high-alloy steel for a core jacket was selected. There were two consequences of selecting this clad material on the reactor design. First, the selection of clad determines the temperature at the core/clad boundary and thus sets certain balance-of-plant design criteria. Second, given that the SSR concept is based on a monolithic right circular cylinder, a system to shut down the reactor should be located external to the core to maximize the temperature of the coolant at the exit from the reactor. 

The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. 

The Tj-carbides are not specifically synthesized, but are of technical importance, occurring in alloy steels, stelUtes, or as embrittling phases in cemented carbides. Other complex carbides in the form of precipitates may form in multicomponent alloys or in high temperature reactor fuels by reaction between the fission products and the moderator graphite, ie, pyrographite-coated fuel kernels. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Since corrosion is a severe problem of hydrothermolytical oxidation, corrosion control deserves close attention in the course of further development. The presented process offers the possibility to use common reactor materials e.g. high-grade steel 1.4541 or in selected applications even carbon steel St 1203. If calciumborate as corrosion inhibitor is used special alloys like hastelloy or inconel are not absolutly necessary for wet oxidation in presence of chloride-ions. 

Heat transfer to the tubes on the furnace walls is predominantly by radiation. In modern designs this radiant section is surmounted by a smaller section in which the combustion gases flow over banks of tubes and transfer heat by convection. Extended surface tubes, with fins or pins, are used in the convection section to improve the heat transfer from the combustion gases. Plain tubes known as shock tubes are used in the bottom rows of the convection section to act as a heat shield from the hot gases in the radiant section. Heat transfer in the shield section will be by both radiation and convection. The tube sizes used will normally be between 75 and 150 mm diameter. The tube size and number of passes used depend on the application and the process-fluid flow rate. Typical tube velocities will be from 1 to 2 m/s for heaters, with lower rates used for reactors. Carbon steel is used for low temperature duties stainless steel and special alloy steels, for elevated temperatures. For high temperatures, a material that resists creep must be used. 

The approach to the problem of low-alloy, high-tensile steel for heavy-wall reactor vessels, on the other hand, has been different between the United States and Europe. The objective was the same — to produce a steel with high room temperature strength which is maintained at elevated temperature and with good notch ductility and weldability. This requires a fine-grain steel with low carbon and sufficient alloying elements to maintain strength at elevated temperature. 

Table 4-16. High Tensile as Rolled or Normalized and Tempered Alloy Steels for Heavy-Wall Reactors and Pressure Vessels...

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