I
JOSEPH THOMPSON and A. C. REENTS Illinois Water Treatment Co., Rockford, 111.
Ion Exchange Processes for Nuclear Power Plants Most of the processes described are at present used in full-scale plants. Of particular interest are treatment methods recommended for plants controlling start-up and shut-down of reactors by neutron absorption with boric acid
BORON
has a high-capture cross section for thermal neutrons. By the addition of approximately 1% boric acid to primary loop water, the atomic reaction in a reactor may be stopped. This method of reactor control has been termed boron poisoning. During a shutdown period of a pressurized water reactor, boric acid may be used in two systems of the plant. The reactor core is flodded with boric acid and it is also used in fuel storage pits during changing of fuel rods in the reactor. Boric acid must be removed from the primary loop water during the start-up period. The boric acid in canal water (usually 10,000 to 12,000 p.p.m.) may become contaminated with radioactive corrosion products such as chlorides, chromates, and traces of molybdates. As the concentration of boric acid in this solution is relatively high compared to that of the primary loop water, its removal by anion exchange resins would be expensive, and the equipment for this purpose would be space consuming. Experimental
The first phase of this study involved removal of boric acid from water by means of a strong base anion exchanger. Illco A-284, a quaternary ammoniumtype strong base anion exchanger, was converted to the hydroxyl form by passing 5% sodium hydroxide through a column of the resin. Deionized water containing less than 1 p.p.m. of dissolved salts was used to rinse the resin free of sodium hydroxide. Boric acid (c.P.) was added to deionized water in the amount of 200 p.p.m., a concentration similar to that of the primary loop coolant. The solution was passed downflow through the hydroxyl form of the anion exchange resin, and the effluent was tested for the presence of boric acid. Effluent data for a typical operational cycle (Figure 1) indicate that the strong base anion exchanger, hydroxyl form, will remove
boric acid to the low levels required for successful operation of the reactor. To determine if ion exchange resins could be used to remove impurities from a boric acid solution of higher concentration, a column of mixed resins was used. One part (volume) cation exchange resin, Illco C-211, on the hydrogen cycle was mixed with two parts (volume) the borate form of a strong base anion exchanger, Illco A-284. The anion exchanger was converted to the borate form by passing a 1% solution of C.P. boric acid through a bed of its hydroxyl form. After rinsing, and mixing with the hydrogen form cation exchanger, an aqueous solution of boric acid (12,000 mg. per liter) containing sodium chloride (175 mg. per liter) was passed downflow through the mixed resin bed. The boric acid effluent was tested for sodium, chlorides, and boric acid. Sodium chloride was removed, and the concentration of the boric acid was essentially the same as the influent. Tests with the same resins using sodium chloride-contaminated boric acid (2 ma. per liter) showed chloride leakage below 0.02 p.p.m. (Table I). Experimental work on this solution was repeated, using a different resin combination. The hydrogen form of cation exchanger was mixed with a styrene-type weak base polyamine anion exchanger. Although this combination of resins removed most of the contaminants, chlo-
ride leakage was greater than if the borate form of the strong base anion exchanger was used. Leakage values found were very low. However, during purification of boric acid solutions, hydrolysis of the borate form of a strong base anion exchanger occurred when the resin was rinsed with deionized water (Figure 2). A column of borate form exchanger anion 35 X 900 mm., was rinsed with deionized water at a rate of 3 gallons per minute per square foot. Effluent from the column was tested for boric acid. Prior to introducing deionized water, the resin had been treated with a 1.25% boric acid solution During rinsing with deionized water, it
Table 1. Mixed-Bed Removal of Sodium Chloride from Boric Acid Solutions Chloride leakage with the borate form was below 0.02 p.p.m.
Concentration, P.P.M. Constituent Influent Effluent H+and Borate Form (Strong Base)
&BO, Na
12,000 68 104
c1
11,800 0.035 0.017
H + ttnd Weak Base Anion Exchanger HaBOa
12,000
11,700
68 104
0.070
Na
c1
0.25
c
Figure 1. Hydroxyl form of strong base anion exchanger removes boric acid from water to low levels required for reactor operation
! I
+: . 0 3
l
LL
1
6 02z
I
+ Y
Influent H$Os = 200 me./ liter; water temp. = 7'5' F.; resin column contained OHform
VOL. 51, NO. 10
OCTOBER 1959
1259
r
--
-
1
Handling Spent Radioactive Mixed-Bed Resins Expendable Cartridge Unit. After exhaustion of the resins, a new cartridge is substituted for the spent unit. Disposal of the highly radioactive resin cartridge i s b y burial on land or a t sea. Expendable Basket-Type Cartridge. Resins are enclosed in a perforated container or basket, fitted into a permanent shell. When exhausted, only the inner basket i s replaced. Expendable Mixed Resins in Permanent Unit. Used resins are flushed from the column through a nozzle located at the bottom of the unit. Resin disposal i s accomplished b y burning in an incinerator, or by transporting in a lead coffin for burial on land or at sea. In some instances, the radioactivity of the resins i s permitted to decay in storage. Thereafter, spent ion exchange materials are mixed with cement in steel drums and moved to a storage area or buried at sea.
Figure 2. Deionized water hydrolyzed borate form of strong base anion exchanger was evident that hydrolysis of borate form of the resin occurred.
Other Ion Exchange Applications Ion exchange provides the most satisfactory technique in other facets of reactor operation involving water treatment. M u c h work has been accomplished in recent years in the field of nuclear water treatment technology. Water. A regenerable, mixed-bed deionizing system (5, 6, 10, 73, 75) should be installed to provide water for cleaning and flushing the entire reactor system prior to placing it in operation. I t should supply water for filling the shielding water containment vessels and canals a t the start of operation and for make-up thereafter. I t should provide make-up water for primary and secondary loops free of turbidity, lo\v in organics, free of corrosive gases, and containing less than 1.0 p,p.m. of toral ionized solids.
Literature Background Application of ion exchange processes to radioactive waste disposal Cation exchange removal of radioactivity from waste solution Mixed-bed ion exchange for water purification of nuclear reactors Experience with primary water coolant of pressurized water nuclear power plant Removal of deuterated boric acid from heavy water by ion exchange Preparation, use, and disposal of water at Shippingport Water treatment from Army Package Power Reactor
(I)
(12)
I n addition, in the operation of sonic reactors, boric acid is used as a control adjunct a t the time of start-up and shutd0n.n. This has introduced additional water and waste handling problems as discussed above. T h e most effective method for coping with the corrosion by-product problem uses filtration and ion exchange. A small portion of the loop recirculating water is diverted through a heat eschanger and then through a deionizing unit. .\lthongh a filter unit may be located ahead of the deionizer, more oftcn the ion exchange equipment itself serves a dual role; namely, removal of both suspended and dissolved contaminants. An ion eschange rrsin provides a coilvenient carrier for a substantial amount of radioactive corrosion products in ii small volume. T h e resins are not rrgenelated but are discarded \\.hen bed effluent no loriger meets qualit)- specifications. < b o d engineering practice does not encourage use of a miued-bed deionizer as a filter. T h e trend, therefore, has been totvard incorporation of a hydrogen c>clr cation exchange unit ahead of the misedbed. I n addition to acting as an escc.1lent filter for the mised-bed, the cation unit \vi11 reduce the load on the miscdbed as a n appreciable portion of thc
(9, 1I ) (16)
(7) (171
(41
~~~
Generally, \ \ a t e r from deep \veils does not contain troublesome organic contaminants or suspended solids and seldom requires extensive pretreatment prior to ion exchange purification. IVater from most surface supplies, hoivever, usually does contain these impurities which cause fouling of ion exchange materials ( 3 ) .
1 260
I\'ater pretreatment may involve chlorination, coagulation and sedimentation, filtration, or activated carbon purification (floivsheet). Various arrangements of deionization equipment should be considered for most economical operation. For example, if the concentration of dissolved salts in raw water is high, two-bed ion exchange equipment preceding the mixed-bed deionizer may be advisable. Because radioactivity should not be encountered in this ion eschange system, standard materials of construction can be used. Conventional How rates are recommended for make-up water ion exchange columns. Automation facilitates regeneration of the units. Pressurized W a t e r Reactor. PKIWARY LOOP. I n the primary loop of a pressurized \cater reactor, treatment of recirculating cooling Ivater is also rrquired. Even though reactors are constructed of stainless steel, the temperatures and pressures involved, coupled with the high recirculation flow rate? result in pick-up of heavy metal corrosion products. Because thesc suspended and dissolved corrosion products become radioactive \vhcn passing through the r e actor corr, an efficient contaminant rcmoval method is of vital importance.
Raw + Water
INDUSTRIAL AND ENGINEERING CHEMISTRY
Chlorination Step for Removal of Organic Matter
Granular Activated Carbon Filtration
:
Coagulation or Cold time Softening for Removal o f Turbidity a n d Alkalinity
Make-up Water Deionization System
-
Anthracite or Quartz Filtration Step
t o Deionized W a t e r Storage
lt
WATER TECHNOLOGY metal contamination will be in the form of hydroxides. Furthermore the exchange capacity of a nuclear grade cation resin is approximately 2.5 times that of mixed resins, and the cost of the former is about one half the cost of the latter. As a result, lower cost of operation is assured. Nonregenerable ion exchange equipment usually is designed to handle normal bypass water at conservative flow rates-i.e., approximately 25 gallons per minute per square foot. Units are sized to permit treatment of sufficient water to hold contapination in the loop at a tolerable level. A current practice designed to decrease rate of corrosion is to control the pH of the primary coolant in the range of 9.5-10.5 by means of ion exchange. This is accomplished by nonregenerable mixed-bed ion exchange units involving cation exchange resin in the lithium, ammonium, or potassium form coupled with the hydroxyl form of the anion exchange resin. Control of pH may be obtained by operating one of these resin combinations in parallel with a hydrogen-hydroxyl mixed-bed. SECONDARY LOOP. The secondary system, comprising steam generators, turbines, condensers, and feed water equipment, is basically the same as conventional power plants. Many of the pretreatment and ion exchange techniques already mentioned may be involved. To date, however, the majority of nuclear boilers have been fabricated of stainless steel, requiring maintenance of extremely low chloride and oxygen residuals in boiler water to prevent stress corrosion. At one major plant, a maximum concentration of 1.0 p.p.m. of chlorides for occasional operation and 0.1 p.p.m. for continuous operation is prescribed for the boiler water. To prepare make-up water of the required quality, the utility has taken a unique approach. Raw water supply is first treated in a split stream ion exchange softening and dealkalizing plant then passed into an evaporator. Condensed evaporator overheads are then deionized in a mixed-bed unit. In addition, the possible need for condensate purification is apparent. Because of the large volumes of water involved, the most economical approach would indicate the use of high flow rate deionization. Sufficient information has been accumulated in a field pilot plant unit over the past 20 months to confirm the practicability of high flow, mixedbed deionization at rates up to 100 gallons per minute per square foot (2, 74). Boiling Water Reactor. This type of reactor eliminates the coolant loop used with PWR. Steam is formed in the reactor core and flows through the fuel
Table II. Nuclear Grade Resins Are Recommended for Reaction Water Purification Chemlcolly equlvolent mixture of hydrogen and hydroxyl form
Capacity, MeW Exchanger Cation
Gram"
(Dry) 4.7
Conversion, % 95% t o H +
Type Fe
Impurities Amount
Moisture, %
Size,b Mesh
55
16-50
OHC15 equiv. % 60 60 form COS-15 equiv. % Column capacity for NR-6 nuclear grade resins = 12 kilograms/cu. ft. 0.3% fines passing through 60 mesh: U.9. standard screen sizes.
16-50
cu
form
Pb Anion
3.5
200 p.p.m. 100 p.p.m. 100 p.p.rn.
Particle
80% to
elements. This steam is moderately radioactive so that all equipment must be shielded. Here, the coolant is also the moderator. Both heavy and light water may be used for this purpose. Make-up water for this type of reactor may be prepared by the methods outlined previously. A small amount of make-up water is required, but it must be of very high quality. In this type of reactor, steam is generated in the reactor itself, and solids concentrate in the reactor water. These become radioactive causing operating hazards and may deposit on heat transfer surfaces. High flow rate deionization of all the condensate returned to the reactor has been suggested as the best method of solving this problem (8). In such a system substantial quantities of ion exchange resins are required. These resins are used in regenerable mixed-bed ion exchanger units to achieve economical operation. Nuclear Grade Resins. During the development of ion exchange processes for nuclear reactor water purification, the quality and purity of ion exchange resins came under surveillance. Technical grade ion exchange resins contain organic and inorganic contaminants harmful in nuclear reactor waters. Organic contaminants are unreacted soluble sulfonic acids, amines, polymers, and solvents. If they are allowed to circulate through the radiation field of the core, corrosive materials are produced. Inorganic contaminants in both cation and anion exchange resins are heavy metals such as iron, lead, and copper. Sodium and chloride content of the cation and anion resins must be minimized. Nuclear grade resins are recommended for use in reactor water purification systems, both for regenerable and disposable resin handling systems (Table 11). Styrene-base sulfonic type cation exchangers and strong base anion exchangers are treated to eliminate contaminants. These resins are generally shipped in the mixed condition on a 1 to 1 equivalent basis. Cation exchange I
* Maximum
of
resins may be in hydrogen, lithium, ammonium, or potassium forms. The anion exchanger is converted to the hydroxyl form. Acknowledgment
The authors are grateful for the cooperation and contributions of H. w. Keller and Frances Nelson for their aid in laboratsry work on boric acid studies and nuclear grade ion exchange resin preparation. literature Cited
(1) Ayres, J. A., IND. END. CHEM.43, 1526 (1951). (2) Dick, I. B., Silliman, P. L., 19th Annual Water Conference Enp. SOC. West. Pa., December 1958. (3) McGarvey, F. X., Reents, A. C., Power 98, 76 (1954). (4) Medin, A. L., IND.ENC. CHEM.50, 989 (1958). (5) Reents, A. C. (to Illinois Water Treatment Co.), U. S. Patent 2,605,084 (July 29, 1952). ( 6 ) Reents, A. C., Kahler, F. H., IND. END.CHEM.43, 730 (1951). (7) Silverman, L., Bradshaw, W., Zbid., 48, 1242 (1956). (8) Sission, A. B., Reid, R. C., Fraser, H. W., Proc. Am. Power Conf., March 1958. (9) Sivetz, M., Scheibelhut, C. H., IND. ENG.CHEM.47, 1020 (1955). (10) Stomquist, D. M. (to Illinois Water Treatment Co.), U. S. Patent 2,771,424 (Nov. 20, 1956). (11) Swope, H. G., J. Am. Water Works Assoc. 49, No. 8, 1085 (1955). (12) Swope, H. G., Anderson, E., IND. ENG.CHEM.47, 78 (1955). (13) Thompson, J., McGarvey, F. X., Chcm. Eng. Progr. 49, 437 (1953). (14) ThomDson. J.. Reents. A. C.. Am.
-
RECEIVED for review April 6, 1959 ACCEPTED August 3, 1959 Division of Water, Sewage, and Sanitation Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959. VOL. 51, NO. 10
OCTOBER 1959
1261
