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 re­circulated system, with the modifying nouns and adjectives denoting its experimental use or location. These loops vary tremendously in size and complexity, depending on the re­quirements 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 nu­clear reactor to examine corrosion, heat transfer, fluid stability, and pressure drop under the com­plicating influence of a neutron flux of varying intensity.

Pressure-Drop and Heat-Transfer Loops

These units consist of many com­ponents familiar to all chemical en­gineers. 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 eval­uating pressure drop through com­ponents of a simple piping system using water as a test fluid, these more esoteric problems might involve pres­sure drops through assemblies of nu­clear fuel elements, instantaneous pressure surges due to valve opera­tion, or flow distribution patterns in a multichannel system. Heat trans­fer 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 cir­culating the fluid and measuring its rate of circulation must be provided. An easily assembled test section for varying types of resistances to mo­mentum 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 pro­vided 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 pres­sures 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 consid­erable 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 ge­ometry and heat-release character­istics of the reactor system. In pres­sure-drop experiments the actual fuel element plus a duplicate of a reactor core section may be utilized. In this manner velocities in the sec­tion 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 sim­ulated by passing an electric current through a stainless steel tube. Mc-

Adams and coworkers are responsi­ble 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 re­lease was caused by an electrical induction system with graphite ele­ments as simulated fuel. A helium loop is being operated at tempera­tures of 1000° to 2500° F. with this method of heat release to study high-temperature heat transfer.

At elevated pressures and tem­peratures more specialized equipment is used. Centrifugal pumps are pre­ferred over reciprocating units to avoid pulsations in flow. High pres­sure casings and seals are available from several manufacturers. A re­ciprocating 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 tempera­ture 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 correspond­ing saturated-steam temperature of 636 ° F. for the prcssurizer.

Problems of leakage and main­tenance of fluid quality are not too critical in these systems when non­toxic or nonhazardous fluids are used. Most heat-transfer and pressure-drop experiments are of short dura­tion and are not usually seriously affected by minor variations in fluid purity. One important ex­ception is the presence of dissolved

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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.

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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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