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by Ε. L. Clark 6551 Dalzell Place, Pittsburgh 17, Pa.
EQUIPMENT AND DESIGN A
W O R K B O O K
F E A T U R E
Test Loops —Pilot Plants for Nuclear Reactors r \
COMMON WORD, the "loop," has
recently acquired special meanings in applied research laboratories whose efforts are dedicated to reducing the cost of nuclear energy. "Corrosion loops," "dynamic corrosion loops," "heat-transfer loops," "pressure-drop loops," and "in-pilc loops" are terms most commonly used. In all cases, the loop refers to some sort of re circulated system, with the modifying nouns and adjectives denoting its experimental use or location. These loops vary tremendously in size and complexity, depending on the re quirements of the problem to which they arc applied. In many cases they involve development of new types of equipment, new assembly methods, and new experimental techniques. The experimental portion of a loop may be placed within a nu clear reactor to examine corrosion, heat transfer, fluid stability, and pressure drop under the com plicating influence of a neutron flux of varying intensity. Pressure-Drop and Heat-Transfer Loops
These units consist of many com ponents familiar to all chemical en gineers. They are, in principle, identical to the experiments forced on all of us in the first few months spent in the Unit Operations Laboratory at most universities. Instead of eval uating pressure drop through com ponents of a simple piping system using water as a test fluid, these more esoteric problems might involve pres sure drops through assemblies of nu clear fuel elements, instantaneous pressure surges due to valve opera tion, or flow distribution patterns in a multichannel system. Heat trans fer problems might involve use of very high heat fluxes, complex shapes and assemblies, and new fluids for transferring energy. Basic components of the systems used in nuclear research are the same
as those familiar to the student. A pump and flow controller for cir culating the fluid and measuring its rate of circulation must be provided. An easily assembled test section for varying types of resistances to mo mentum or heat energy transfer is designed so that the basic system may be utilized for a multitude of experiments. Finally, the extent of heat transfer or flow resistance pro vided by the test piece or assembly must be measured. Problems in setting up a particular experiment are usually encountered in providing the mechanical gadgetry for the test section. Another source of difficulty is the use of high pres sures and/or high temperatures for the system in order to duplicate actual operating conditions for a particular reactor. Still another complicating problem is the use of special fluids such as liquid metals, for which pumps, valves, and exchangers had to be specially designed. These items have been described in consid erable detail in a "Liquid Metals Handbook" published by the Atomic Energy Commission and containing many important data collected by AEC laboratories and contractors. In designing a test section every effort is made to duplicate the ge ometry and heat-release character istics of the reactor system. In pres sure-drop experiments the actual fuel element plus a duplicate of a reactor core section may be utilized. In this manner velocities in the sec tion above the reactor core and within the fuel-clement flow channel are simulated in the test section, as are the entrance and exit conditions. The test section is then inserted into the loop and pressure drop-data are obtained at the flow rates that have been selected for the reactor cooling system. For heat-transfer experiments the heat-release characteristics of a tubular fuel element may be sim ulated by passing an electric current through a stainless steel tube. McI/EC
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Adams and coworkers are responsi ble for the design of such a unit. [McAdams, others, IND. ENG. CHEM.
41, 1946 (1949)]. A direct current generator rated at 15 volts and 1000 amperes provides the heating current for this test unit. The tube being tested is 0.250 inch in diameter and 3.75 inches long in a circulating water stream. The Bureau of Mines has reported experiments in which the heat re lease was caused by an electrical induction system with graphite ele ments as simulated fuel. A helium loop is being operated at tempera tures of 1000° to 2500° F. with this method of heat release to study hightemperature heat transfer. At elevated pressures and tem peratures more specialized equipment is used. Centrifugal pumps are pre ferred over reciprocating units to avoid pulsations in flow. High pres sure casings and seals are available from several manufacturers. A re ciprocating pump is included as a make-up or pressurizing unit. At elevated temperatures, a pressurizing vessel is often used in which the fluid is vaporized by heaters at a tempera ture somewhat above loop operating temperature. The vapor pressure in this vessel maintains the operating pressure of the entire system above saturation pressure at the operating temperature and thus also prevents a mixed phase in the loop circulation system. Typical conditions for a water system might be 2000 p.s.i.a and 550 ° F. for the circulating fluid and 2000 p.s.i.a. and the correspond ing saturated-steam temperature of 636 ° F. for the prcssurizer. Problems of leakage and main tenance of fluid quality are not too critical in these systems when non toxic or nonhazardous fluids are used. Most heat-transfer and pressuredrop experiments are of short dura tion and are not usually seriously affected by minor variations in fluid purity. One important ex ception is the presence of dissolved ORKBOOK
FEATURES
65 A
I/EC
EQUIPMENT AND DESIGN
gases in heat-transfer fluids. Large changes in heat-transfer coefficients have been observed due to gas bubble formation in water cooling at high fluxes. It is usually good practice to utilize pure fluids which have been degassed and periodically checked for cleanliness. To avoid corrosion and variation in surface properties, most units arc constructed of stainless steel or other corrosion-resistant materials. Corrosion Loops
Problems of corrosion and wear are constant economic factors for all industrial enterprises. These problems are particularly severe in nuclear plants, because of the necessity for long life of the large capital investment and the difficulties of maintaining and replacing equipment under radiation conditions. A great deal of this work has been done in static fluid systems to determine both corrosion and wear. Literally hundreds of high pressure autoclaves were utilized in development of materials and components for pressurized water reactors. In addition to testing strips and coupons of structural and nuclear materials, these autoclaves also contained components in motion being tested for use as control-rod drives and other actuating devices. Primary problem in operating such static systems was precise control of all process variables, including fluid quality. These static systems had several drawbacks. Absence of fluid motion, which can be a major contributor to varying corrosion rate by removing surface films, was an obvious fault of the static system. Maintenance of original fluid properties was very difficult, as reactions involved in the corrosion process tended to deplete the fluid of its dissolved oxygen content and increase the content of metal ions. These changes affected the accuracy of the data on corrosion rates. Adding reagents to the autoclave and removing samples of fluid were difficult because of the small volume of fluid available. Several important added features distinguish the corrosion loop. To avoid confusing the results of the test work through corrosion of the loop itself, these loops arc built of corrosion-resistant materials, usually Type 304, 316, or 347 stainless steels. 66 A
INDUSTRIAL AND ENGINEERING CHEM
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A Workbook
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Precise control of fluid quality is a major design feature. In most cases a side stream of circulating fluid is passed through a purification and sampling train in which dissolved and entrained impurities are removed, samples taken for analysis, and reagents added. In effect these testing units are pilot plants, duplicating the fluid circulation system of a nuclear reactor. Process variables of flow rate, temperature, and pressure are in the same ranges for the loop as anticipated for the nuclear reactor. The same provisions are made for purifying the circulating fluid. The same quality of fluid is used. Two major differences are the absence of radiation effects and large variations in the surface and volume relationships between test unit and reactor. Design problems in preparing a suitable corrosion test system center around the two main features which distinguish these units from the simple pressure-drop or heat-transfer loops. Control of fluid quality requires the absence of all lubricants which might contaminate the system. Use of corrosion-resistant materials involves more careful choice of components. Methods of handling and supporting test pieces to avoid contact of dissimilar materials and insulate against electrolytic effects have been developed. This requires use of insulating substances unaffected by the fluid at operating conditions. These requirements result in the use of many components specially developed for this purpose or other nuclear uses. "Canned-motor" pumps avoid stuffing boxes or packing glands which might contaminate the circulating fluid. These pumps are available for pressures up to 2500 p.s.i. and 650° F. from several suppliers in a variety of capacities. Differential pressure-sensing units which utilize diaphragms rather than liquid manometers are frequently specified to avoid contaminating the system with sealing or manometer fluids. The purification and sampling system is, by itself, a special unit. It usually includes a filter, a mixed-bed ion exchange unit, a sampling manifold, and a valve system which provides for insertion of a small vessel containing gas or solutions of reagents through which the circulating
stream is channeled to permit addition of these substances. For loops operating at elevated temperatures, a cooler must precede the ion exchange unit to avoid decomposing the resins. Often a heat exchanger is added to reduce the thermal load on the loop due to the cooler water returning from the purification train. Continuity of operation is important, because many corrosion tests require months to provide any reliable data. Thus spare units are usually included with provision for maintenance during operation. Major leakage must be avoided and considerable thought given to valve packings, vessel closures, and glands used for operating mechanical devices within the test sections. A tremendous amount of gadgetry is required in the various components and particularly in the test sections. Design of sample holders and test rigs to provide motion during the test period is a time-consuming job. Magnetic drives for providing reciprocating and rotary motions are often used to simulate component action in a test rig. Water-lubricated glands and all sorts of bellows-sealed actuating devices have been built as required. Most engineers with experience in the design of these units have a large library of techniques which have been successfully used for a variety of purposes. Although most of this discussion is concerned with water systems, the same problems, only more so, apply to liquid-metal or organic-liquid systems. A recent announcement [Chem. Eng. News 37, 138 (March 2, 1959)] lists the availability of a "packaged" NaK heat transfer loop also suitable for corrosion studies and mass transfer investigations. Such research is still an active problem. Similar versatility is available in most water corrosion loops which may be utilized for obtaining heat-transfer and pressure-drop data during corrosion studies.
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