I
THOMAS F. DEMMITT Coolant Systems Development Operation, General Electric Co., Richland, Wash.
Coolant Purification and pH Control. for Low pH, Recirculating W a t e r Cooled Reacfors Recircula t ing cool ant systems permit
/ higher fuel temperatures
d higher coolant temperatures d lower water make-up T H E PRESEiiT TREND in nuclear reactor design is toward systems cooled by high-temperature, recirculating fluids. This is generally true whether the reactors are designed for research tools, power production, or fissionable materials production. This trend represents a major departure from the early reactor designs which incorporated low temperature, single-pass cooling s).steins. Many of the reactors employing recirculating coolant systems use high purity water as the primary coolant-with or without the addition of corrosion inhibitors. This report is limited to a discussion of these systems only. The incentive for using a recirculating coolant system is to permit higher fuel element and coolant temperatures and to reduce the over-all amount of water required for make-up. The decision to
+
concentrate on reactors of this type was, however, preceded by a rather extensive amount of research work to determine the magnitude of the problems associated with these systems. The primary problem areas, from the coolant standpoint, stem from the fact that the corrosivity of water increases substantially a t elevated temperatures and the impurities initially present in the water are continually transported through a nuclear flux region, becoming radioactive. Thus, the water treatment systems for these reactors must maintain the coolant at a high state of purity to minimize the amount of coolant-borne activity. I n addition, the coolant must be compatible with the components of the primary piping system, which may necessitate a corrosion inhibitor. Experimental studies have been con-
I REACTOR
FILTER
PRESSURIZER
n
REGENERATIVE HEX
k+
HEATER
MOCK-UP TUBE
MAKE-UP WATER Schematic flow sheet shows coolant routing and major equipment in an in-reactor loop
642
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ducted in neutral, acidic, and basic systems at Hanford to resolve these problems. These studies have usually involved the use of ion exchange resins to effect simultaneous coolant purification and p H control. The operating characteristics of ion exchange resins in the neutral and high p H systems have been widely reported in the literature by other investigators. The quantity of information available for similar operations in loiv p H systems is, however, quite limited. This field should not be completely overlooked since systems involving aluminum components perform best in acidic systems, particularly at high tempcratures. The work described here, tlierefore, dealt only with recirculating watercooled systems in the acidic region. The general procedure used at Hanford to evaluate proposed systems changes or modifications is threefold. First the procedure is given a preliminary laboratory investigation to obtain basic data under controlled conditions. T h e second phase involves a series of tests in an outof-reactor system that simulates the proposed reactor operating conditions as closely as possible, with the exception that no nuclear radiation is present. The final step in the evaluation is to perform in-reactor tests under prototypical conditions. As a result of the evaluation described here, an ion exchange clean-up system \vas developed for use in low p H recirculating reactor systems. The regeneration requirements have been defined, and the resin has been tested in in-reactor and out-of-reactor facilities. I n general, the resin performance is satisfactory, although the coolant total solids concentration and radiation level are somewhat higher than in neutral or high p H systems under otherwise comparable operating conditions. The system is also subject to temporary increases in background activity level Lvhen some of Ihe film sloughs off the system piping. This problem could become much more serious if the system is subiected to frequent thermal shocks. There
are still a large number of unknown factors, although sufficient data are now available to permit long-term operation of low pH systems. Reactor Design
Basically the in-reactor test loop of the 100-KE reactor a t Hanford (p. 642) consists of an in-reactor Zircaloy-2 process tube that contains the fuel elements, a Zircaloy-2 mock-up tube that simulates the in-reactor tube except for the absence of the nuclear flux, a primary heat exchanger to cool the Lvater prior to the next pass, a canned rotor recirculation pump, and an electrically heated pressurizer. The supplementary equipment in the clean-up system includes a regenerarive heat exchanger, a particulate solids filter, and two ion exchange columns in parallel. The clean-up stream is drawn from the pump discharge line and is returned to the pump suction with any required make-up water. There are four such loops currently in operation a t the 100-KE reactor. Three of these loops are predominantly stainless steel, while the other is predominantly carbon steel. The low p H studies were conducted in the stainless steel systems since acid environments are not compatible with carbon steel. One of the primary reasons for using low pH systems is to minimize the corrosion rate of aluminum. Previous work has shown that both HaPo4 (2, 3 ) and H4Si04 ( 4 ) are effective aluminum corrosion inhibitors. Since the presence of H4Si04 in the coolant introduces the possibility of forming undesirable silicate deposits on the heat transfer surfaces, it was eliminated from further consideration. The problem of finding the optimum H 3 P 0 4concentration was resolved in laboratory studies using autoclave systems. I n these tests, an acid concentration capable of reducing the room temperature pH of high purity water to 4.5 C 0.2 units was the most desirable from a corrosion standpoint. The problem was therefore reduced to finding a satisfactory ion exchange system to provide simultaneous coolant cleanup and p H control in this pH range. Since removal of both cation and anion impurities is required in a closed recirculating system, a mixed bed ion exchange resin is needed. In addition, the operation will be most effective if the resin does not appreciably change the H3P04 concentration. Some nuclear grade mixed bed resin was therefore converted to the hydrogen-phosphate form for preliminary testing. The resin chosen for investigation was Amberlite XE-150. This resin is an intimate mixture of a strongly acidic, nuclear sulfonic cation resin and a strongly basic, quaternary amine anion resin (5). This resin was chosen because it is a nuclear grade resin and has been
7.01
I
,,t =I 5.0
H3824
I
I
I
I
4% H3P04
4.0
I .o 0
2% H3P04
1
% 0 25 30
5 IO 15 20 GALLONS OF REGENERANTiCUBIC FOOT OF RESIN.
Figure 1. Regeneration curves illustrate effective resin exchange capacity and regeneration characteristics
used for several years in the hydrogenhydroxide form to provide high purity make-up water for the experimental facilities at the 100-KE reactor. Preparation of the hydrogen-phosphate form resin can be accomplished either by regenerating some mixed bed resin with HJ’04 or by regenerating the anion component with the acid and then mixing the cation and anion resin. To simplify the procedure, the mixed bed resin was regenerated directly. Evaluation and Discussion
The first step in the preliminary evaluation was to determine the regeneration requirements for the resin. Samples were regenerated in a 1-inch diameter column using regenerant solutions of 2 , 3 , 4 , and 8% (volume) of concentrated H8P04. The effluent solution was collected during the regeneration step, and the pH was determined as a function of the volume of acid passed through the column (Figure 1). All the regeneration curves are very sharp and well defined. Premature acid leakage did not occur, as evidenced by the nearly vertical portion of the curves prior to total leakage. These results show that the regeneration band width was very narrow under these conditions. Also the total amount of acid required to reach the breakthrough point was constant during each test indicating that the maximum possible conversion to the hydrogen-phosphate form occurred in each case. The acid flow rate used during each of these tests was approximately 0.5 gallon per minute per cubic foot of wet resin. The regenerated resin was then rinsed with deionized water to remove the excess acid and to determine the shape
of the effluent p H us. rinse water volume curve. The results are shown on Figure 2 with a pH us. phosphate concentration curve for H3P04 in deionized water. The resin effluent pH curve is composed of at least two distinct regions which have arbitrarily been designated as Regions A and B. Region A represents a resin condition in which the effluent pH changes quite rapidly during the rinse step. Region B represents a resin condition in which the p H change is much smaller for an equal amount of rinse water. The narrow region representing the transition between Regions A and B has been included in Region A for this discussion. The pH us. phosphate curve for the acid shows a further characterization of the two regions. If the phosphate concentration curve is extrapolated to correspond to the complete p H range on the rinse curve, it is apparent that Region A represents a relatively large phosphate concentration range while Region B represents a relatively small range. The resin effluent p H curve shows, however, that the amount of rinse water required to affect the concentration change in Region A is much smaller than the amount required in Region B. Clearly then there is a definite change in the acid removal mechanism between Regions A and B. The full significance of this phenomenon has not yet been determined. The first thought is that rinsing of excess acid probably has a great deal to do with the behavior noted in Region A . That other important considerations are also involved was demonstrated by varying the rinse water flow rate. The rinse curve shown on Figure 2 was obtained at a rinse water flow rate of 1 gallon per VOL. 53, NO. 8
a
AUGUST 1961
643
reactor loop recirculation facilities are located in a separatc building some dis30 tance from the reactor and are therefore I I I I I usually maintained at a relatively low contamination level. If, however, particulate matter from the in-reactor sec6 tion of the loop should slough off and be transported to the recirculation facilities and ion exchange columns, the general 5 background activity level might be increased considerably, violating the water p H VERSUS PHOSHATE treatment objectives. 1 4 Since there was no way to evaluate n this possibi1it)- without actually per-------, forming an in-reactor test, a test was 3scheduled. The results were quite similar to those previously described for the RESIN RINSE CURVE out-of-reactor tests. The problem of 2 increased background radiation arising from transported radioactive film particles was not as severe as had been anticI ipated. Apparently most of this film was removed when the loop was cooled rapidly following a fuel element rupture 01 I I and dumped into effluent catch tanks. 5 IO 15 There was only one major difference RINSE T I M E (HOURS) between the in-reactor and out-of-reactor Lests and that involved variations in the Figure 2. Acid removal mechanism changes as the effluent pH approaches 3 total phosphate concentration in the coolant. The phosphate concentration varied with time in both systems, but in the in-reactor tests the average concentration was about 1.5 times that observed Coolant total solids concentration minute per cubic foot of wet resin. The variations in the out-of-reactor tests after a comtotal time required to reach the p H e Film formation on the system comparable operating period. The reason range shown on Region B changes only ponents for this difference is not known. It may slightly at rinse flow rates between 0.5 have been caused by an increase in the and 2 gallons per minute per cubic foot I n general, the resin ion exchange peramount of insoluble salts or by polyof resin. All of these results indicate formance was satisfactory: although it merization of the phosphoric acid under that the behavior in Region A is probably was not as good as normally- encountered the influence of the high temperature and influenced considerably by acid diffusion. in neutral or high pH systems. Aluminuclear radiation. The flow rates of 0.5 and 2 gallons per num corrosion inhibition was quite good minute per cubic foot of resin correspond although general corrosion of the ferrouq to rinse volumes of approximately 90 and Acknowledgment metals was somewhat higher than had 300 gallons per cubic foot of resin, rebeen anticipated. The pH control and The author is deeply grateful to spectively. solids removal operations irere noticeCoolant Testing Unit, IrradiationProcessThe observed behavior is probably ably affected by the formation of ing Department, who operates the recircontrolled initially by removal of excess sparingly soluble, metal phosphate corculation facilities ; the Chemical and acid; then by a combination of acid rerosion products. These materials were Spectral Analysis Operation, Hanford moval and diffusion; and finally-in not completely removed by the resin at Laboratories Operation, for the analyses Region B-by diffusion alone. There the reduced temperatures (approxiperformed; and E. R. Wood for his help may also be a change in the diffusion mately 100’ F.) prevailing in the in performing much of the preliminary mechanism at the transition point beclean-up system and apparently exerted experimental work. tween Regions A and B. Under normal some buffering action in the coolant. conditions most ion exchange reactions This resulted in a slow accumulation of are diffusion controlled. At least one Literature Cited phosphate salts with time. investigation has actually demonstrated I n addition, some iron> nickel and (1) Boyd, G. E., Schubert, J., Adamson a change in the rate controlling mechaother undetermined phosphate comA. W’.,J . Am. Chem. Soc., 69, 2818 (1947) nism at different ionic concentrations (2) Dillon, R. L., Lobsinger, R. J., U. S. pounds were adsorbed on the high tem( 7 ) . Although liquid film diffusion Atomic Energy C3mmission NW-59778, perature magnetite film on the stainless May 1, 1959. would most probably be the controlling steel system components. This resulted ( 3 ) Draley, J. E.. others, Proc. U. N. Infactor in Region B, diffusion within the in a thicker film but did not interfere tern. Conf. Peaceful Uses At. Energy, resin beads may be of importance in the 2nd, Geneva, 1958 3 , 113 (1958). appreciably with heat transfer operavery low pH region. (4) Haag, R. M., Zyzes, F. C., U. S. tions. The adsorbed material was not Atomic Energy Commission KAPL-1741, The rinse step is discontinued when the very adherent, and occasionally high Feb. 28,1957. effluent p H reaches 4.3 i 0.1 units and particulate solids concentrations werc (5) Rohm & Haas Co., Resinous Prodthe resin is then ready to be charged into ucts Division, Philadelphia, Pa., “Amencountered for a short timz as some of a loop. The resin was evaluated iniberlite Ion Exchange Resins,’ 1949. the film sloughed off the piping walls. tially in an out-of-reactor loop. The This material was gradually filtered out RECEIVED for review September 26, 1960 following factors were considered in the ACCEPTED March 3. 1961 in the clean-up system. resin performance evaluation : This experience did cause some conDivision of Water and Waste Chrmistry. cern in considering the effects of such an 138th Meeting. ACS, New York, Septeme Corrosion inhibition effectiveness ber 1960. occurrence in the reactor. The ine pH control characteristics
PHOSPHATE CONCENTRATION (PPM.) 5 10 15 20 25
-
‘$
- \
644
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I
