ML18079A282

Corrosion of Matls in Spent Fuel Storage Pools.

ML18079A282
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Site:	Salem
Issue date:	07/31/1977
From:	Weeks J BROOKHAVEN NATIONAL LABORATORY
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ML17193A281	List: ML18079A282
References
BNL-NUREG-23021, NUDOCS 7905150196
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BNL-NUREG-23021 INFORMAL REPORT CORROSION OF MATERIALS IN SPENT FUEL STORAGE POOLS

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J.R. Weeks July 1977 Corrosion Group Department of Applied Science Brookhaven National Laborato"ry Upton, New York 11973

TABLE OF CONTENTS LIST OF TABLES------------------------------------------------- ii INTRODUCTION--------------------------------------------------- 1 I I MATERIALS------------------------------------------------- 2 t

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f. II WATER CHEMISTRY------------------------------------------- 3 t ~

1. BWR Fuel Pool Chemistry------------------------------ 3

2. PWR Fuel Pool Chemistry------------------------------ 3 I 3. Biocides--------------------------------------------- 4 I III CORROSION OF MATERIALS IN FUEL STORAGE POOLS--------------

1. Stainless Steels-------------------------------------

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2. Aluminum Alloys--------------------------------------- 5

3. Zircaloy Cladding------------------------------------- 6

4. Other Materials----------------*-*---------.------------ 6

5. Stress Corrosion------------------------------------ 6

6. Galvanic Corrosion--------------------------------- 7 IV SURVEILLANCE-------------------------------.---------------- 9 V

SUMMARY

AND CONCLUSIONS----------------------------------- 10 ACKNOWLEDGEMENTS----------------------------------------------- 11 REFERENCES~-=-===-------------.--------------------------------- 12 TABLE------------------------------~---~---~--~-------~~---~--- 13, 14, 15 i

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LIST OF TABLES TABLE 1 MATERIALS AND WATER CHEMISTRIES IN LWR FUEL STORAGE POOLS-----------------------------------13 & 14 ii

INTRODUCTION The current delays in establishing a national fuel reprocess-ing center have required many of the LWR licensees to expand their fuel storage capabilities either by modification of existing pools or addition of new fuel storage *pools. This report reviews the potential corrosion problems that might develop during the long-term {10 plus years) storage of nuclear fuels in these storage pools. A detailed review of the integrity of the fuel* in storage pools is being prepared by Johnson for ERDA, Cl) which has served as a basis for much of this report. Zircaloy-clad fuels with

• burnups up to 33,000 MWd/MTU have been successfully stored in fuel storage pools for periods up to 13 years in U.S. pools and 14 years (at lower burnups) in Canadian pools.

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I MATERIALS Three types of materials are_ generally in contact with the fuel storage pool water: the pool liner which is commonly stain-less steel, the storage racks which are commonly stainless steel or aluminum, and the materials present in the fuel element bundles which commonly include stainless steel, Inconel 718, 17-4 PH, and Zircaloy 2 or Zircaloy 4 cladding. Table 1 lists the materials and water*chemistry used in the fuel storage pools at a number of LWR nuclear stations, as available to the writer as of July 15, 1977.

Experience with storing these materials for long periods of time in reactor canals has been reviewed by A.B. Johnson, Jr. (l)

Maximum residence in U.S. Pools of spent zircaloy-clad fuel is 13 years. None of these materials should suffer significant corrosion in this environment in periods well in excess of 10 years, as has*

been borne out by experience.

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~ II WATER CHEMISTRY Because during the fuel unloading procedure the water in the fuel storage pool and the reactor primary coolant mix, an attempt is made to maintain water purity in the fuel storage pool to ap-proximately the same limits that are set for the primary reactor coolant.

1. BWR Fuel Pool Chemistry In a BWR this means that high purity demineralized water is typically maintained with a filter-demineralizer to a total heavy ion content of< 0.1 ppm, a pH range of 6.0 to 7.5, and a conductivity of < 1 µmho/cm. The water is sampled daily to meas-ure conductivity, and weekly for other impurities, including chlorides. The demineralizers primarily remove silicates from the water, and are typically checked for their capacity to remove this species once weekly. The primary source of the silicates may be dust from the air; the pools are normally uncovered. On the aver-age, fresh resin beds are installed monthly, primarily because of increased p~essure drops from silicate absorption. <2> The primary contribution to the conductivity is dissolved co 2 ; when the conduc-tivity exceeds i µmho/cm the demineralizers are changed. <2> During a visit in June, 1977, _the water in the Vermont Yankee fuel pool appeared extremely clear, with a distinct blue tinge to it, appar-ently as a result of scattering of the longer -light waves by the water and the use of mercury vapor lighting.

2. PWR Fuel Pool* Cherni*stry In a PWR, the fuel pool frequently contains several thou-sand ppm boric acid, .which is added to other otherwise highly pure water. No neutralization with LiOH is used* in the fuel storage pools; a typical pHC 3 > value is 4.5. A portion-of the fuel pool 3

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coolant is continuously passed through *a derriineralizer resin and impurities, .such as halides or sodium ions, maintained below 0.15 ppm. Periodically the demineralizer resins are checked for their ability to remove halides and sodium ions; resins have been devel-oped by Rohm and Haas that are specific for rernovi:rig halides.in the presence of boric acid. The manufacturer's claims in th.is mat-ter have been confirmed experimentally by one of the reactor ven-dors. <4 >

3.

• Biocides:

Biocides are not commonly used in fuel storage pools at nuclear power plants. Maintaining the water of the high purity needed for safe *storage of fuel appears to inhibit biol~gical

. growth, .and the use of stainless steel liners in the storage pool also tends to control biological growth. The radiation levels from the spent fuel stored iri. the pool also tend to sterilize the water, altho~gh radiation resistant bacteria are known. Finally, the continuous demineralization of a portion of the pool water serves to filter out any biological growth. No biol~gical fouling has been observed in 3 1/2 years operation of the* Prairie Island spent fuel poo1,< 3> in 3 1/2 years operation of the Vermont Ya.nkee,

>- 5 years operation of the Maine Yankee, and > 10 years operation of the Yankee-Rowe fuel storage pools,< 2> and no biocides have been added.

The use of biocides can lead to -the presence of chloride ions in the pool which are potentially harmful to the corrosion resistance of the materials stored in the pool, and would be unac-ceptable during the mixing with the reactor primary coolant that occurs during refueling. They have been used in the ICPT fuel pool at Idaho Falls, which is a painted concrete pool. (l) 4

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III CORROS'I'ON *oF MATERIALS" IN FUEL *s:TORAGE P-OOLS-The corrosion rates of zirconium, stainless steels and Inconel in water of the quality maintained in the fuel storage pools should be negligible during periods upwards of twenty years. Gener.al corrosion rate measurements for these materials in water of this quality and temperature are not generally available, and any esti-mates of corrosion rates must be extrapolated from measw::ements at much higher temperatures. The *primary difference between the water chemistry in the fuel pools and that in the reactor (other than the temperature) is that the pools are exposed to the air and are presumed to contain dissolved oxygeri up to the saturation point. Since all the materials used are passivated by oxide films, the presence of oxygen in the water should not affect their cor-rosion rates.

1. Stainless -Steels Since the stainless* steels *are used for the *primary pip-ing at substantially higher temperatures and in the presence of oxygen in BWR's where *stainless steels are deemed satisfactory for periods ~p to 40 years, corrosion in the fuel pool should be much less than in the reactor, because of the lower temperature.

2. Aluminum Alloys The anticipated corrosion of the aluminum alloys, 1100 or 6061, is negligible in water of this quality at temperatures up to the boiling point of water: at 12S0 c (257°F) a corrosion rate of 1.5 x 10- 4 mils/day(S) has been measured for alloy 6061 aluminum, in water of pH 7, which corresponds to a total corrosion of 1.1 mils in twenty years. Since the oxidation rate will con-tinue to decrease slightly over this period, this estimate should be conservative. At lower temperatures, the rate will be even 5

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lower. There is little difference in the corrosion rates of these two alloys at temperatures below 1so 0 c. The anodization of the aluminum components, which is occasionally used, .should protect them even further from corrosion.

3. *zircaloy Cladding The rate of corrosion of zircaloy in fuel storage *pool waters is very low. Berry(G) gives a corrosion rate in soo 0 water of 2 x 10- 2 mils/year, ~nd shows it to be continually de-creasing up to times* in excess of 10 or 15 yea*rs. At the *1ower temperatures that prevail in fuer storage pools, .the corrosion rates should be even lower. Morgan <7 > describes the corrosion rate of zircaloy in pool water as being sufficiently low to pro-vide an adequate containment barrier for at least 100 years.

The oxygen concentration in the pool water should not adversely affect corrosion of zircaloys. Zirconium and its alloys are protected from aqueous corrosion by a strongly passivating oxide film. The oxygen in the water* should serve to promote and maintain this passivation. Further, Uhlig (_S) has stated that this passivity is maintained both in strong acids and in strong alkalis.

4. Other Materials The fuel bundle and storage rack materials may also include type 17-4 PH stainless steel and Inconel 718. Neither of these alloys should undergo measurable. general corrosion in fuel storage pool waters.

5.

• Stress' Corrosion Stress corrosion of stainless steels and zircaloys in fuel storage pools is highly unlikely to occur provided the water 6

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chemistry is maintained within the specified l.imits. Stress cor-rosion of sensitized stainless steels that are highly stressed has been observed in oxygenated water acidified to pH 5 nitric acid at temperatures up to 140°F. C9 ) This is, however, a slow process which took 6 years to develop and occurred only in one highly*

stressed, highly sensitized area. While it is impossible to rule

• out completely that stress corrosion of the stainless steel or Inconel components will occur in the fuel storage pool, any such occurrence would be highly localized and rare, and not lead to serious problems with the storage racks or fuel bundle components.

No significant difficulties have been observed in fuel bundles examined from a number of reactors. Stress corrosion of 17-4 PH is unlikely to occur if the material has received an ll00°F heat treatment. This heat treatment is commonly specified for this material when it will be exposed to reactor coolants. Components of 17-4 PH given this heat treatment have been in service in the Brookhaven High Flux Beam Reactor (HFBR), which contains high purity o2 o acidified with nitric acid to a pD of 5 and containing greater than 8 parts per million of oxygen, for periods in excess of 12 years without any evidence of stress corrosion or pitting.Clo)

This water chemistry and temperature (145°F max.) are similar to that prevelant in PWR fuel storage pools. -

6. Galvanic Corrosion Galvanic couples between stainless steels, Inconel and zircaloy do not appear to give rise to any localized corrosion in fuel pool environments, since all of these materials are protected t

by highly passivating oxide films, and are, therefore, at similar potentials in pure water. Aluminum alloys, which are also protected by passivating films, nevertheless can be pitted in an acid environ-ment such as that present in PWR fuel storage pools, when coupled to stainless steel. The anodization of aluminum fuel storage racks 7

should minimize *this occurrence. In BWR storage pools, .the high electrical resi*stivity of the *water should also serve to prevent

. galvanic attack.

At the Oyster Creek Nuclear Power Station, aluminum racks were originally placed directly in contact with the stain-less steel pool liner. Some of these racks have been removed and examined after approximately 7 years of service in typical BWR pool water. (ll} No obsei:vable pitting of the aluminum was found at the point where it contacted the stainless steel. ( ll)

At least one nuclear utility (Vermont Yankee} has also elected to provide additional prote.ction against this potential probl'em by placing stainless steel feet on the racks, which, .in turn, .are electrically insulated from the aluminum with ABS *plastic inserts.

These have beeri determined to be *sufficiently far from the' radia-*

tion source to prevent their decomposition by high energy gamma fltix. <2 > These o~ganic inse*rts are, in my opinion, additional insurance that galvanic corrosion will not occur.

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IV SURVEILLANCE A spent UnReprocessed Fuel (SURF) program is under development by the ERDA Division of Waste Management, Production and Reprocess-ing, to be initiated in FY 1978. (l 2 ) Under this program, the char-acteristics and condition of spent fuel in storage will be evaluated on a continuing basis. Although the details of the examination to

.. be performed in this program have not yet been worked out, the national scope of this program, including periodic examination of a few selected fuel bundles from both PWR and BWR storage pools, will provide additional assurance to the NRC of the continued integ-rity of fuels in storage throughout the country.

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SUMMARY

Significant corrosion of nuclear fuel components is highly un-likely to occur during storage in fuel storage pools at the reactor sites in periods of upwards of 20 years, provided that the w~ter quality in the fuel storage pools is maintained within specifica-tions, and that chloride levels in the pool water are kept to minimum levels (< 1 ppm). Stress corrosion of stainless steel com-ponents or Zircaloy cladding cannot be entirely ruled out because of the lack of understanding of the stress states and the degree of sensitization of stainless steel. Should such a problem develop on the Zircaloy cladding it would be readily detected by routine monitoring of the fuel pool water for radioactivity. Should it develop on the stainless steel or Inconel components of the fuel bundles, it would be highly localized and unlikely to lead to sig~

nif icant overall deterioration. Periodic surveillance of the materials in storage at a number of nuclear utilities is being planned under the auspices of the U.S. Energy Research and Develop-ment Administration.

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t ACKNOWLEDGEMENTS The assistance of Dr. A.B. Johnson, Jr., of Battelle Pacific-Northwest Laboratory, in providing draft copies of his review (Reference 1) and in several useful discussions is gratefully ack-nowledged. Representatives of the Northern States Power Company, Yankee Atomic Electric Company, Duquesne Power and Light Company, Jersey Central Power and Light Company, and the Portland General Electric Company were very helpful in preparing this review. This work was performed under the auspices of the United States Nuclear Regulatory Commission 11

REFERENCES

1. A.B. Johnson, Jr., 1

• Behavior of; Spent Nuclear Fuel in Water Pool Storage", BNWL-2256, Draft, May, .1977, .also private communications, May, June *and July, 1977.

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2 .. John R. Hoffman, Yankee Atomic Electric Company, .Private communications, June 6 and 7, .and July 14, .1977.

3. Peter Jones, Northern States Power Company, .l?riva te communi-cations June *1 and July 15, 1977.

4. C. McCracken, Combustion Engineering, Private communication, June 3, 1977.

5. J.E. Draley and W.E. Ruther, Report No. ANL-5001, .February, 1953.

6.. W. E. _Berry, "Corrosion in Nuclear Applications", John Wiley

& Sons, N.Y., 1971, pages 107-116

7. W.w. Morgan, "The Management of Spent CANDU Fuel'", Nuclear Technology -24, 1974, .~ages 409-417,

8. H.H. Uhlig, "Corrosion and Corrosion Control", John Wiley &

Sons, N.Y., Second Edition, 1971, pages 367-371.

9. R.W. Powell, J.G.Y ... Chow, W.J. Brynda, M.. H. Brooks, J.R. Weeks "Experience With Stress* Corrosion Cracking and Materials Com-patibility at the High 'Flux Beam Reactor", CONF-730801, .166-180, 1973.

10. R.W .. Powell & J.G.Y. Chow, .Brookhaven National Laboratory, Private communications.

11. T.J. Madden, Jersey Central Power & Light Company, Private communication, May 19, .19 77.

12. C.R .. Cooley, .u.s.E .. R.D.A., Private communication, July 14, 1977 12

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TABLE I MATERIALS AND WATER CHEMISTRIES IN LWR FUEL STORAGE POOLS f

t PLANT MATERIAL USE ENVIRONMENT I.

L ARKANSAS 304 SS A-276-71 or Rack 1800 ppm bOron as t'*

1* (PWR) A-167-74

' boric acid

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308 or 308L Electrode 304L ASTM-A-167 Liner 120°F J

BEAVER VALLEY SS, 17-4 PH Racks, bolts 2000 ppm boron as

- (PWR) boric acid, Cl , F - < 0.15 ppm BRUNSWICK 304 SS Liner, racks 125°F (max 150°F)

. (BWR) E308 Electrodes cond < µmho/an 17-4 PH - Hll50, Bolts Hl025 pH 6.0 - 7.5 Cl < 0.2 ppm DRESDEN 1, Stainless steel Liner Demineralized water 2 and 3 Al-6061-T6 Racks cuno filters and (BWR) ASTM-B-209 deep bed deminer-i FT. CALHOUN 304 SS ASTM-A-276-71 Racks alizers 120°F or A-167-74 2000 ppm boron as 308 or 308L Weld

'** boric acid GINNA, R.E. 304 SS Racks Boric acid (PWR)

LACROSSE Borated SS and 304 SS Racks Demineralized water (BWR)

MILLSTONE 304 SS Liner, racks Demineralized water POINT I Filter and deminer-(BWR)

.... alizer I.i.

MILLSTONE POINT 2 (PWR}

304 SS Liner, racks Demineralized water

+ 2000 ppm boron as boric acid

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TABLE I (continued)

PLANT MATERIAL USE ENVIRONMENT NINE MILE 304 SS Rack Demineralized water POINT 1 of BWR primary cool-(BWR) ant quality 125°F

• . OYSTER CREEK Entire rack 304 SS Demineralized water (BWR) ASTM-A-240 Plate, bar Undissolved solids sheet AS'IM-A-193 Rivets, bolts

< 0.5 ppm ASTM-A-194 Nuts 308 SS, AS.ME SFA 5.9 Weld material PALISADES 304 SS Racks 122°F - 1570F (PWR) 2000 ppm boron as boric acid PILGRIM Same rack design as (BWR) Vermont Yankee POINT BEACH 304 SS Racks 2000 ppm boron as 1 and 2 boric acid 130°F (PWR}

PRAIRIE ISLAND 304 SS Racks, liner Demineralized water 1 and 2 Zircaloy, IN-718 Fuel bundles (Cl-, F- < 0.15 ppm

.... (PWR)

+ 2000 ppm boron as boric acid pH 4.5, 120°F QUAD CITIES Same rack design as 1 and 2 Dresden (BWR)

TROJAN 304 SS Racks, liner 2000 ppm boron as (PWR) Inconel Grid Mat'l.

boric acid 17-4 PH - HllOO Bolts and Module threaded 140°F feet Cl I F , 0.15 ppm maximum each 14

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TABLE I (continued)

PLANT MATERIAL USE ENVIRONMENT TURKEY POINT Entire rack 304 SS Demineralized water 3 and 4 Free standing rack with 1950 ppm boron (PWR) ASTM-A-240 Sheet, plate ASTM-A-276 Bar as boric acid AWS-E-308-15 Weld wire AWS-E-308-16 Weld wire VERMONT YANKEE 356-TSl ASTM-B-26 Alum. Grid castings , pH 6 - 7 .5 (BWR) 6061-0 or S052-H32 Alum. Cans (Cu, Ni, Fe, Hg, etc.)

6061-TGSl Alum. Plates 2024-T4 Alum. Bolts, Pins < 0.1 ppm All aluminum alloys, 12S°F anodized 4

304 SS Liner, feet Radionuclide < 10-ABS plastic insulators between feet & alum.

cans YANKEE ROWE 6061-T6 Alum. Rack 1300 F, some boron, Stainless Steel Liner chlorides < 0.5 ppm ZION 304 SS Rack Borated water

{PWR) lOS°F 15

DISTRIBUTION LIST L.C. Shao { 5)

R.J. Stuart w.s. Hazelton F.M. Almeter H. Levin (5)

W.Y. Kato D.H. Gurinsky Corrosion Group Files (10) 16