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I
A. P. GAVIN, C. R. BREDEN, R. E. BAILEY, M. H. THWS, and G. E. GORING Reactor Engineering Division, Argonne National Laboratory, Lemont, 111.
Water Decomposition in u
Direct-Cycle Boiling Water Reactor Increases Corrosion Rate Possible control procedures are suggested
WATER
decomposition is a major chemical problem encountered in directcycle boiling water reactors operated by Argonne ( 7 , Z ) . Radiolytic decomposition gases (hydrogen and oxygen) formed in the core are entrained by steam through the turbine and are removed from the condenser by air ejectors. Oxygenated steam is critical from a corrosion standpoint. The volume of hydrogen produced increases the load that air ejectors must handle, and also constitutes an explosion hazard in the air ejector exhaust lines. The net rate of radiolytic gas production is assumed to result from a decomposition reaction depending only on degree and type of irradiation and a recombination reaction that may be influenced by a number of other conditions. However, only net effects have been studied in the operating reactors. Accordingly, decomposition rate is defined as the volume of oxygen (STP) produced per liter of condensed steam.
Operating Conditions Effect of Pressure. For a specific reactor, decomposition rate is essentially constant at any given pressure over a range of power output but varies for each reactor (Figure 1). Owing to characteristics peculiar to each reactor concept (Table I), differences in decomposition rate cannot be attributed to a single factor. Figure 1 also shows that the rate for Experimental Boiling Water Reactor (EBWR) decreases as pressure increases. This relationship has not been determined for Argonne Low Power Reactor (ALPR) and Boiling Reactor Experiment-IV (BORAX-IV). Effect of Water Quality. Conductivity, pH, and the nature of dissolved solids in the water have pronounced effects on net decomposition. However, sufficient data are not available to evaluate these conditions independently. Effect of Chemical Additives. Chemical additives have been employed in the three reactors in an effort to reduce decomposition rate and also to gain information that will aid in understand-
Table 1.
Each Direct-Cycle Boiling Water Reactor Has Its Own Characteristics Power Pressure,
Reactor BORAX-IV EBWR ALPR
(Heat),
P.S.I.G. Mw. 300 600 300
1-20
5-62 3
Fuel Clad AI-Hi alloy Zircaloy-I1 Al-Ni alloy
Elements Meat UOz-ThO, ceramic
Operation Dates 12/563/58 1/5711/58-
U-Zr-Nb alloy AI-U matrix
ing the mechanism whereby the net rate can be controlled. Addition of hydrogen decreased decomposition rate, but the amount required to effect a given reduction varied considerably (Figure 2). Experiments reported in the open literature have indicated that under certain conditions of temperature and irradiation, hydrazine decomposes to hydrogen and nitrogen. Accordingly, hydrazine was injected into the EBWR feed water in hopes that this decomposition would form free hydrogen and decrease the rate of water decomposition. Addition of approximately 3 p.p.m. of ' hydrazine resulted in a sharp rise in conductivity of the water in the reactor and hot well and in activities measured at several points in the system. Decomposition rate decreased slightly and then increased to a value considerably above the initial rate. The p H of the hot well increased slightly, while that of the reactor decreased to a value of about 5.2. No hydrazine was detected in the reactor, and nitrate concentration increased from less than 0.1 to 0.8 p.p.m. These results indicated that ammonia was produced in the reactor during the injection period and that nitrate ions were formed and retained there. This was confirmed by the addition of 0.5 p.p.m. of ammonia which produced results of a similar nature. Addition of nitrogen to EBWR and BORAX-IV produced no noticeable changes in decomposition rate, reactor water conditions, or plant activities.
operation revealed the need for a catalytic recombiner that would recombine dissociated water in the exhaust stream from the air ejectors. During operation
Catalytic Recombiner
Figure 2. Much greater rates of hydrogen addition were required to reduce oxygen concentration in BORAXIV
Final design analysis of EBWR with respect to immediate and future mode of
='t?
0
200
300
0
400 500 P R E S S U R E PSIG.
600
700
Figure 1. Water decomposition rate varies for each reactor
MW
W G 74
4
300
,
'.
O I
o
1
'
I
'
l
'
l
!
l
20
70
28
70
300
IO
70
600
'
1
600
'
"
10 20 30 40 50 60 7c H Y D R O G E N ADDITION C C I - I T E Q O r F E E D h 4 T E R
VOL. 51, NO. 10
OCTOBER 1959
1265
3-CA i
200 DS'
S I T E STEAM
I O 0 PSI REACT3R STEAM
1
crnrl>.. a group of samplcs ivds i)laccd in a ditmrny fiicl elernc.nt to priividc. dai,i on corrosion of tort' inaterials ar o p e ~ . a ~ in: conditions.
Plant Activities
4%
Figure 3. EBWR catalytic recombiner eliminates explosion hazard by recombining dissociated water in the exhaust stream from air ejectors
a t 20 hl\v, the exhaust strearn compriscs a mixture of 1 cubic foot per minute of radiolytic gas a n d of air thar comes fimm inleakage to the condenser and in the turbine seals. '[his mixture is highl). explosive and! until diluted sufficiently \\.it11 more air, constiturrs a hazard to operation. Thus. reduction in explosive gas content that could be effecti,d by recombining radiolytic gases \vould alleviate. if not eliminate. the hazard. Finally. proposed operation with costly heavy xvater coolant mad? i t iinperative that virtuall>- all deuterium formed b y radiol\-sis of heavy \cater be iwovered. Initial attempts to install a catalytic bed of platinized alumina pellets \vith a manually controlled diluiinq steam supply resulted in exploiions in dir lines. Subwquent modifications dnd dcvelopnient effort resulted in a s!'stem (Figure 3 ) comprising a flou-meter to monitor the supply of diluent steam, temperature and pressure safety sivitches. and airoperated valves designrd to bypass the recombiner unit in the event of unsafe conditions. Entire operation of thtx unit is remotely controlled from thc reiictoi' control room. The system has been in ser\-ice for approximately six months. Hydrogen recombination is 99% complete.
Corrosion The most significant corrosion problem encountered in EB\VR i5 build-up of corrosion products on fuel plates. Il'hen the fuel has been handled in rhe reactor. oxide films approximately i mils thick
1 266
have flaked off' i n shcrts from thr: surfacc of fuel elements in areas of' hiqhrst heat Hux. .\nal>.ais of this oxidr. along ivith oxides collected fwin aliiiiiinum-nickel dummy furl elrinents gave a composition of 365; aluminum. l l r ; nickel. and 3 . , iron. Origin of respective. clemcnts the deposit includes aluminiim tubes in the condenser. aluminum-nickel dummy elements! Kaniqen nickel platinq on steam piping. and turbine casinq. and stainless strcl in the system. .An aluminum-nickel diimmy fuel element \vas removed from LhcA core approxitnately n c o ycai's aftrr initial startup. \-isual exarniiiaiion revealed a loose coating o f dark hro\cn t o $ray oxide over an adherent speckled bro\vn oxide. l l e a s u r c m r n t s indicated a loss of about 3 miis of aluminum in approximately 01ic year a t operatinq temperaturc. Build-up of corrusion products in control rod thimbles heneath t h r rcactor vessel has hcen rvidenced by a risr in activity levels in the umrking area. Chemical compo-irion of the deposit in tlie thirnbles closely rrseinblcs that of the deposit on the fuel element. Periodic cxainination of' the turbine has revealed no evidence of serious corrosion attack by \vet oxygenated steam. Both casing and rotor are in rscelknt condition. Current cormcion studi to provide quantitative data on respective sources of corrosion deposits in the steam sysrem. Specimens of various materials used in construction of the plant are being tested in a special holder installed in the strain lint.. 1Iore re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited
