I
ARCHIE L. BILADEAU Atomic Energy Commission, Idaho Operations Office, Idaho Falls, Idaho
Re-use of Cooling Water in an Atomic Energy Commission Installation High-capacity ion exchange resins hold to a minimum the development of radioactivity in the primary coolant stream of a reactor
T H E AVAILABILITY of water, now as in the past, governs to a great extent the location and growth of a community. Besides availability, however, a water must have certain qualities to be attractive for industrial and community use. Purity, both chemical and bacteriological, is the quality most desired. Water of the purity essential to encourage industrial and community growth is not the plentiful item that it once was. As an area grows, the quality of the available water usually decreases because streams and rivers are utilized for disposal of community waste. With decrease in purity comes the added cost of increased treatment. These factors have been instrumental in building up a major industry in water treatment. O u r water problem is not so much one of scarcity as one of quality, and as costs of water treatment go up, conservation by re-use becomes economically attractive. Atomic Energy Commission installations have definite water requirements. The acceptance or rejection of a site by the commission is governed in part by the amount and condition of the available water. Water-cooled reactors are in wide use at present, and the volume of water required to absorb and dissipate the heat is considerable. I n some areas chemically treated raw water is used as the coolant, but its use is limited to the once-through type of system, where the waste coolant can be returned to a stream of adequate size to provide a high dilution factor. Oncethrough systems are few in number, but only because of their limited availability.
Economically they are very desirable. Because of limited sites, most watercooled reactors use the standard primary and secondary type system with heat exchangers and cooling towers. The primary cooling loop is usually a closed system, and the water is recirculated through the reactor and the heat exchangers by pumps (Figure 1). Generally the water used in the primary coolant loop is of high purity, and to ensure the purity desired, it must receive a high degree of treatment. At present ion exchange demineralization is used for this purpose. Water with a purity of 0.5 p.p.m. total solids or less is not uncommon. Waters of such high purity are needed because all impurities circulated through a n atomic reactor's cooling system are a potential source of induced radioactivity. I t does not take much radioactivity to require protective shielding. Personnel protection, where radioactivity is involved, cannot be neglected, regardless of cost. It is much less costly to eliminate the source of activity, where possible, then to construct shielding and other operation controls. Where pos-6 sible, it is generally economical to hold the activity in water to within the tolerance limits. The use of a high-purity water as a primary coolant in an atomic reactor limits but does not eliminate the amount of induced radioactivity. Water, even if it contains 0.5 p.p.m. of total solids or less, will break down or pick u p some matter by dissolution, disassociation, erosion, and/or corrosion. Some of these particles become radioactive and build
up the activity in the water. T o retard or to hold this activity a t a n acceptable minimum, a small and continuous purge is necessary. For every ounce of purge discharged to waste, a n equal amount of high-purity water must be added. As demineralized water of the required purity costs about '/3 cent per gallon, a purge of only 50 gallons per minute costs approximately $240 per day. T h e synthetic resins used to remove cations and anions from coolant waters are rather costly, but they are fairly stable and durable. Although there are some breakdown and loss of exchange capacity with time, these are minor considerations. Because water is a good solvent, the recirculation piping, storage facilities, and valving system are generally made of stainless steel, which adds to the general over-all cost of the system. Once-through use of high-purity water is costly, and the primary coolant is generally in a closed circuit, with secondary coolant facilities absorbing and dissipating the heat. This added equipment cost is more than offset by savings realized from use of lesser amounts of demineralized water. At the Idaho Operations miterial testing reactor cation and anion resins serve to salvage or recover demineralized water previously purged to waste. This waste is now run through a bed of mixed resins which absorbs the activity and frees the water to its original purity for re-use. T h e resin bed becomes highly radioactive in time, but should stabilize a t a certain activity level, in accordance with the rates of feed and decay of the isotopes removed. L
VOL. 48, NO. 12
0
DECEMBER 1956
2 159
Figure 1 .
A cleanup unit using resins is rcrKally referred to as a bypass demineralizer. I t was noted at the material testing reactor that the use of resins influences the p H of the water, especially when more or less cation resins are used than anion resins. The control of pH in a primary cooling water loop is essential where aluminum-clad fuel elements are used. These aluminum-type elements: which are predominant in thermal reactors, require a coolant p H of around 6.0 to hold corrosion to a minimum. A corroded fuel elem-ent often results in a ruptcre, which means reactor shutdown and a time of cooling water cleanup. A purge with demineralized water is generally required. For this reason, p H is closely regulated. Water flowing through a cation unit will normally have its p H lowered; a similar flow through an anion unit \vi11 generally raise the pH. By varying and regulating the flow through two 2 160
Flow diagram of primary cooling loop
such separate units. any desired p H can be maintained, within limits. Radioactive material may exist in the primary coolant water stream (1) absorbed in solid materials, (2) in solution as free ions, and (3) in colloidal form. The major elements contributing to the background activity in most rractor coolant systems, including many fission products, exist in the cation form. The anions are generally of minor consideration and decay or are present in such small quantities as to be of little significance. The use of cation resins only in a bypass cleanup unit therefore works very well, but lowers the pII, as mentioned above. One of the reactors at the National Reactor Testing Station in Idaho tried a single cation bypass unit. The cleanup was excellent. but the p H in the system dropped from 6 to less than 5. The corrosion rate was closely checked and an effort made to raise the p H chem-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ically. This met with failure, as would be expected. Anion resins were then added with the cation resins and a slight rise in p H was noted. It is planned to add more anion resins to the unit in an effort to reach the p H desired. The use of separate anion and cation exchange units in a combined unit appears to offer more flexibility in p H control; therefore a n anion resin bed is generally included in bypass systems and enough water is diverted from the effluent of the cation resin unit through the anion resin unit to regulate the pH. In some installations pre- and postfilters. to remove gross particles, such as sand and broken resin particles, are included as part of the bypass cleanup unit, but the value of this procedure is in doubt. A few installations include regeneration facilities as part of the complete unit, with added facilities to remove and store the radioactive liquids resulting therefrom. Regeneration of the cation
R E - U S E O F W A T E R BY I N D U S T R Y and anion resins is economically feasible, but the nuisance involved in handling radioactive by-products and regenerants, and the apparent inability of the resins to be regenerated to their original capacity, generally offset the small savings realized. The preferred practice is to remove the spent radioactive resins and replace them with new ones. The spent resins are easily removed hydraulically into simply constructed sheet metal containers that can be temporarily shielded for handling and hauling to a burial area. The container is usually buried with the resins. Bypass flows through resin cleanup treatment units are regulated to permit as many as 10 complete volume changeovers of the primary loop in 24 hours. These flows vary, depending on the water purity required. In experimental reactors, where experiments are often carried on in the cooling water passage, the use of bypass cleanup units is proving to be essential. Such experiments increase the probability that radioactive contaminants will get into the water, and, for safety reasons, bypass cleanup units are now considered standard equipment for such water-cooled reactors. Unscheduled reactor shutdown periods are costly and low-cost facilities that will reduce the shutdown time can generally be justified.
Operating Data Reliable operating data on bypass demineralizers are still not available. There is evidence of resin breakdown, but whether it is due to attrition, radioactivity, or high temperatures, is not known. Primary cooling water is usually on the hot side thermally and the durability of resins a t these elevated temperatures varies considerably. Anion resins appear to be more susceptible to thermal breakdown than cation resins. For this reason some installations are using heat exchangers ahead of the bypass demineralizers to cool the water prior to its being recirculated through the bypass unit. A constant increase in the exchange capacity of these resins is being developed, but at present the physical stability of the resins seemingly decreases as the exchange capacity increases. As the use of these special resins increases, it is probable that they will develop along lines of greater resistance to heat and radioactivity damage. The life of resins now used in bypass units is extremely long, probably because the water treated has less than 2 p.p.m. of total solids in it. One such unit has been in constant use about a year and still is as effective as when first installed. One may wonder where so much radioactivity can come from
in water of such purity. Unfortunately, it does not take much activity to make a water radioactive enough to require shielding and other safety measures. One gram or less of fission products in 300,000 gallons of water will make the water radioactively hot; but 1 kg. of these same products in 300,000 gallons of water is equivalent to 1 p.p.m. Therefore the activity can build u p in the water without any appreciable increase in total solids or conductivity. I t has been noticed that the conductance of this water remains almost constant, regardless of a marked increase in activity.
Mechanical losses are indeterminate and depend on the design, operation, and care taken during construction. Thus, about 3% loss is the minimum that can be expected in secondary cooling systems. However, this 3% l o p can be appreciable because of the large circulation flows normally required. Fortunately, raw water is generally used in secondary cooling systems, and only minor treatment is required. The amount of purge is based on that needed to keep the concentration of solid a t about 1500 p.p.m.
Cost of Treatment
The secondary cooling system as at present used by the Atomic Energy Commission was a development of necessity, because of the need to dissipate the enormous amount of heat generated by its reactors. The economic value of such heat was realized from the start, but to harness it in those early days of reactor development was not feasible. T h e present development is along lines to utilize this now wasted heat, and as the commission and industry become satisfied as to the safety and economy of reactor heat generation, this heat energy will be harnessed for beneficial use. An atomic reactor is probably the greatest potential source of electric power as yet unused commercially by industry. With improvement in reactor design and operation will come the biggest boom of heat energy utilization in our history. Its use is almost unlimited and water, as always, will play a n important part in its growth and utilization.
The use of cation and anion resins for treating water to remove radioactivity is relatively new. Only a few such installations are in use. The cost of resins for this purpose can be easily justified. The resins are generally used to the complete depletion of their exchange capacities, and are then removed and discarded to waste. T h e water thus treated, instead of being purged to waste, is available for re-use. Considering cost of resins, operation, depreciation, and disposal of the radioactive resins on depletion, the cost of treating 1000 gallons is about 25 cents. T h e over-all cost of producing demineralized water of 1-mho conductivity is about 1/3 cent per gallon or $3.35 per 1000 gallons. A considerable saving can thus be realized if the original demineralized water is treated to remove the activity.
Dissipation of Heat
Conservation of Water By treating the primary coolant water to remove the activity, the loss of water can be held to a minimum, and a considerable saving realized. As economy is the prime reason back of conservation, it is easy to realize why the primary coolant water is continuously re-used and its loss is held to a n absolute minimum. At each reactor shutdown a certain amount of primary cooling water purge is required. T o date none of this purge is salvaged, but future development of bypass-type treatment facilities may see the complete recovery of this purged flow. The conservation of water in the secondary cooling systems is not so easily justified as that for the primary system in water-cooled reactors. Where water is a critical item, it is often economical to use air as the secondary coolant. When water is used, the loss is rather high. Generally about 2% of the water flow can be assumed lost by evaporation and about 1% to windage and purge.
Summary I n most water-cooled atomic reactors, the primary cooling system is a closed circuit. Water losses are held to a n absolute minimum, but on occasion even the primary coolant system must be purged. Purged primary coolant waters, in some installations, are now being salvaged by the use of highcapacity exchange resins. Water, now being dissipated to the atmosphere as steam, will soon be used as a source of heat-producing power.
References (1) Heath, R. L., “Fission Product Monitoring in Reactor Coolant Streams,” Atomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho, I.D.O. Bull. 16213 (1955). (2) Sivetz, Michael, Scheibelhut, C. H., IND.ENC.CHEM. 47, 1020-2 (1955). (3) Swope, H. G., Anderson, Elaine, Zbzd., 47, 78 (1955). RECEIVED for review April 10, 1956 ACCEPTEDOctober 22, 1956 VOL. 48, NO. 12
DECEMBER 1956
2161
