I
F. J. BRUTSCHY, H.
S.
DREYER, and W. L. PEARL
Vallecitos Atomic Laboratory, Atomic Power Equipment Department, General Electric Co., Pleasanton, Calif.
Vallecitos Boiling Water Reactor
Coolant Technology First year results indicate that coolant chemistry problems in boiling water reactor systems are not as formidable as once believed
IN
APRIL 1955, the General Electric Co. contracted to build the 180,000 kw. Dresden nuclear power station (8) for the Commonwealth Edison Co. utilizing the principle of the dual-cycle boiling water reactor (Figure 1 ) . When the design was initiated, two basic sources of information on corrosion and coolant chemistry problems of such a reactor were available: studies and experience from fossil fuel-fired steam power plants and high temperature, high pressure nuclear power reactor plants, usually referred to as the PWR type. At that time, there were three main problem areas unique to boiling water reactors : Radiolytic water decomposition with resultant oxygen and hydrogen gases in steam to the turbine and in recirculating water. Material corrosion and resultant corrosion products Lchich might tend to deposit on heat transfer surfaces. to become irradiated and be transported and deposited throughout the system, and to foul high tolerance mechanisms. Deposition of radioactive corrosion products or fission products in the turbine, with its subsequent effect on contact maintenance.
In 1956, the Vallecitos Boiling Water Reactor (VBWR) was authorized primarily to provide a facility for testing fuel and other design facets for the Dresden station. Concurrently, VBiYR was available for an investigation of coolant chemistry problems associated with boiling water reactors in general and Dresden in particular. Reactor Water Quality
The Dresden clean-up s!stein (Figure 2) is designed to demineralize a small portion of the recirculatinq coolant. No filters will be used. The VBWR water circulates through a simple clean-up system consisting of a nonregenerative heat exchanger and a mixed bed demineralizer. The flow is returned to the suction of the fred water pumps. Dresden reactor water and feedwater quality specifications and normal ex-
perience with VBWR are compared in Table I . I n general, Dresden specifications were maintained in VBWR with the exception of the copper level and total solids. Some copper fittings exposed to high temperature \cater caused the former. Solids in VBIYR water were silica from inadequate treatment of cycled waste \cater and: at one time, boron carbide leached from a faulty control rod assembly. Resistivity measures only the ionic content of the water. T h e high total solids content found in combination with a high resistivity indicated that most of the solids Irere nonionic. This emphasizes the importance of specifying both a total solids limit as well as a resistivity minimum for reactor tvater quality. Performance of the clean-up demineralizer as a filter is also inferred. The primary coolant reference quality of 1 incgohm-cm. resistivity, neutral pH,
I The problem of radiolytic water decomposition, with resultant oxygen and hydrogen gases in steam to the turbine and recirculating water, as applicable to Dresden, was studied by careful selection of materials and careful control of water quality. No serious consequences are anticipated. Transport and deposition of activated corrosion products and fission products throughout the system, with potential maintenance problems, is still not completely resolved. Criticality of the problem, however, has been diminished b y favorable results obtained to date. No coolant chemistry problems have developed in operating the BORAX reactors, EBWR, or VBWR to detract from the boiling water reactor system as originally conceived.
I
I iJ ' SECONDARY S T E A M GENERATOR
FEED WATER H E A T E R S
Figure 1 . reactor
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I_
Dresden nuclear power station will use a dual-cycle boiling water
Steam-water mixture from boiling reactor core flows to seporoting drum; steam then flaws directly to turbine. Saturated water flows from drum through h e a t exchangers; heat is removed to produce lower pressure steam which i s then introduced into lower stage o f turbine. Combined condensate from turbine and condenser i s returned to reactor and secondary steam generators after passing through typical feedwater heaters
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 2. Dresden coolant clean-up system consists of regenerative heat exchanger, nonregenerative heat exchanger, and mixed bed demineralizer Demineralizer effluent solids
is
returned to primary steam drum; resin bed moves both suspended and dir-
solved
and no additives is not necessarily the optimum for a boiling water reactor. Tests sponsored by the Naval Reactors Branch for their pressurized water reactor systems indicated that operation of pressurized systems at p H above 9.5 produced less deposition on heat transfer surfaces than operation a t neutral p H (5). Deposition and fouling were increased by the presence of oxygen in the system (5). Wroughton and Cohen (9) concluded that less corrosion product was introduced into the circulating water at the higher pH. Some deposition on fuel elements has been reported (7) from Experimental Boiling Water Reactor (EBWR) operation. The deposit, consisting essentially of aluminum and nickel, was attributed primarily to the presence of aluminum1% nickel fuel dummies and nickelplated equipment. Similar materials are not used in most boiling reactor systems. No significant deposits on fuel elements have been noted in the first years’ operation of VBWR. A loosely adhering copper-colored film was present on Table I. Water Specifications for Dresden Were Generally Maintained in VBWR Dresden (6,8) VBWR Reactor water Additives 0 0 7.2 i 0.2 7.0 i 0.2 PH Resistivity, megohmcm. 1 >1 Total solids, p.p.m. 0.6 1-12 Feed water Resistivity, megohm10 lo* 1 em. Cu, p.p.rn. 0.002 0.01 0.01 0.002 0.002 0.001 Fe, p.p.m. c10.01 0.005f 0.005
* *
the Zircaloy cladding. There is no indication of a film build-up with time. The hope that neutral, nonadditive coolant quality may continue to provide satisfactory performance has been strengthened indirectly by some recent 1000-hour out-of-pile corrosion tests. Determination of corrosion product transferred to coolant, by calculation from weight of undescaled us. descaled corrosion coupons, indicated that with both carbon steel and stainless steel a boiling water system containing oxygen and hydrogen releases much less corrosion product than a pressurized system containing only hydrogen, both operated at p H 7 . Enough confidence has been developed as the result of experience with reference water quality that a program to study both inhibition of corrosion and inhibition of corrosion product deposition has been curtailed. Water Decomposition
rectly to the turbine is impractical. The amount of oxygen and hydrogen expected in steam going to the Dresden turbine was initially established in 1956 for design purposes at 60 p.p.m. of oxygen based on BORAX 111 data. Subsequent data from EBWR and VBWR have indicated that the Dresden system was substantially overdesigned for the expected quantity of noncondensable gases passing to the turbine. Some VBWR studies have aided better understanding of the water decomposition phenomenon. At a reactor pressur? of 1000 p.s.i. oxygen concentration in steam varied between 6.5 and 10.3 cc. per liter of condensed steam during a 1month period of operation. VBWR, EBWR, and BORAX I11 were quite different in their core design, and effects of such differences on water decomposition cannot be accurately predicted or determined. However, the significant difference in oxygen prcduction rate among the three reactors is attributable, at least in part, to their differences in pressure (Tablc 11). One run a t VBWR testing the effect of pressure (with corresponding change in temperature and steam density) confirmed this trend of less net water decomposition with increased reactor operating pressure. Specific water decomposition rate increases slowly with reactor power. This trend is in agreement with that found in EBWR (7, 3) but in contradiction with the decreasing effect reported by Whitham and Smith (7) for BORAX 111. A large effort aimed toward oxygen removal from the coolant and/or steam by recombination, scavenging, or chemical combination with hydrogen was considered in the early development program. Tests showed this was not necessary because of the corrosion resistance demonstrated by materials selected for Dresden in the presence of oxygen and hydrogen gases.
The problem of oxygen in the reactor coolant resulting from water decomposition was resolved early in the case of pressurized water reactors by utilizing an excess of hydrogen ( 5 ) . Radiolytic recombination of hydrogen and oxygen to water was accelerated with little to no residual free oxygen remaining in the reactor coolant. Maintaining excess hydrogen in a boiling reactor system sending steam di-
Table ll.
Separation Factor
Nonvolatile corrosion product and fission product activities carried with steam to the turbine have been effectively minimized in VBWR, EBWR, and BORAX 111, all cooled by natural circulation, as evidenced by the high ratio of reactor water activity to condensed steam activity. A decontamination factor of 0.6 X lo4 has been reported for the
Differences in Oxygen Production Rate Are Partly Due to Pressure Differences Power Operating 0, in Pressure, Steam, Production Rate, Level, P.S.I. P.P.M. Lb. On/Mw./Hr. Reactor Mw.
Borax III (2) EBWR (3) VBWR
8 20 20
300
600
35 24
1000
12
VOL. 51,
NO.
0.13 0.072 0.035
10
OCTOBER 1 9 5 9
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EBIVR ( 3 ) and 0.6 to 1.6 X l o 4 for BORAX I11 ( 2 ) . A portion of the decontaminating in EBIVR occurs across the dryer ( 3 ) . Separation of steam and water in the VBIVR reactor vessel \cas measured by adding a sodium salt to the reactor water and varying the reactor \cater level. T h e separation factor indicated by sodium-24 distribution was better than 10' but dropped suddenly belolc lo' ivhen the reactor water level \vas too high. Relative \cater and steam activity levels, as measured by distribution of radioactive isotopes of nonvolatile corrosion products during normal operation. ha\e confirmed the magnitude of thc \-B\VR separation factor (Table 111). Table 111. High Separation Factor Was Confirmed in Normal Operation !tl, _Radioactii __
Reactor
Constituent
CPI Mnb4 C O ~ ~
Feaq
nater 106
29 18 10
ppr
111
~
dtenin
coirdei~.~tc
0.01 0.004
0.006 0.008
Forced circulation cooling is used in the Dresden plant. An external steam separaring drum not only offers frre s i r face separation, as obtained in a natural circulation system. but also proLides properly designed drum internals to accomplish further steam drying. .A higher separation factor than i n \.B\VR is therefore expected in Dresden. Coinparison of gross activity i n reactor \vater and condensed steam: or their decay curves, as \vas done a t B O R h S I11 (?), does not give a true indication of separation factor, because of gaseous activities in the condensed steam. Contamination
Radioactive contamination problems of a dual-cycle boiling tvater reactor are twofold: those comparable to the primary recirculation s)-stcm of a pressurized water reactor and radioactivity carried to and deposited in the turbine system. Because common utility practice requires direct maintenance of the turbine and its auxiliaries, this second area requires further discussion. Actual low activity levels reported for EBIVR ( 7 ) and BORAX (7) turbines and found with the VBLVR turbine have been very encouraging. T h e \-BIVR turbine was opened after 900 and 3000 hours of operation. Radioactivity changes noted between the LIFO surveys \rere minor. hctivity levels throughout the turbine lvere lo\v avrraging about 1-2 mr. per hour of gamma and 20 Mrad. per hour of beta radiation. Major activities identified rvere activated corrosion products including cobalt-i7, cobalt-58, cobalt-60, iron-59, manganese-54, and chromium-51. Higher prrs-
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sure srages sho\ved some\chai higher readings than low pressure staqcs. A scan of the kvheel surfacrs has indicated that outside surfaces were about 3 times more radioactive than \\heel ccn tcrs .
T h e general lack of activity build-up throughout \'B\VR and BORAX turbinrs (71, despite some minor fuel element ruptures, has been extremely encouraging. C',orrosion coupons fastened in the. main stream linc and at seveiA points of rht~ recirculating \rater lint, have provided an excellent means of predicting activity build-up on components i n the primary s)-stem. Thesr coupons are easily removed and give reproducible, countable geometries that actual components do not. Corrosion product activity builds i i p rapidly on tl-iesc. sainplrs at first and then Icvels off to a lonlevel of build-up. I'irm conclusions have not been draLcn from these rtblaiivelv short term data. Condensate Demineralizers
Full floiv demincralization of condensate returning from condenser to wactor \\-as desirable for the Dresden plant : io rninimizr total solids goins to tht3 reactor that could b r irradiated and cause system actkit>- prohlems; to decrease the load on reactor clean-up d ~ mineralizer resins discarded a f t r r use because of high radioactivity contained therein; to permit ust: of a copper alloy condrnser and Stellite-tipprd blades o n thr turbine \vithout corrosion products qetting to the reactor; and t o prevent contamination from condenser cooling ivatrr from entering the r r a c t o ~ coolant in case of a condcnsrr leak. Because of the size of the equipment and the resin inventory rrquircd to handle the normal 3200 gallons per minute of feedwater floiv. utilizing normal design floiv rates of about 5 to 6 gallons per minute per square foot of resin bed surface would havr resulted in a large costly installation. .Idr:velopment program was insrituted latc in 1955 to exploit the economic incrntivc of using faster than normal demineralizer flow rates. The program definitely cstablished that higher flow rates up to 75 gallons per minute per square foot are technically and economically feasible Lcith substantially complete removal o f nletallic constituents (0.0 p.p.m. of cobalt and copper). .Although resin operating capacity decreases and system pressure drop and pumping cost increase, total rquipment cost and space requirements decrease at a more rapid rate to give a lower initial capital investinent and lo\i.er annual cost at higher flow rates. Economic gain levels off substantially with increase in flow rate above 50 gallons per minute per square foot of resin bed.
INDUSTRIAL AND ENGINEERING CHEMISTRY
