Information Notice 2013-18, Refueling Water Storage Tank Degradation

Refueling Water Storage Tank Degradation

ML13128A118
Person / Time
Issue date:	09/13/2013
From:	Cheok M C, Kokajko L E Office of New Reactors, Division of Policy and Rulemaking
To:
Klos L J
References
IN-13-018
Download: ML13128A118 (6)

ML13128A118 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION OFFICE OF NEW REACTORS WASHINGTON, DC 20555-0001 September 13, 2013 NRC INFORMATION NOTICE 2013-18: REFUELING WATER STORAGE TANK DEGRADATION

ADDRESSEES

All holders of an operating license or construction permit for a nuclear power plant issued under Title 10 of the

Code of Federal Regulations (10 CFR) Part 50, "Domestic Licensing of Production and Utilization Facilities," except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel.

All holders of and applicants for a power reactor early site permit, combined license, standard design certification, standard design approval, or manufacturing license under 10 CFR Part 52,

"Licenses, Certifications, and Approvals for Nuclear Power Reactors."

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is is

suing this information notice (IN) to inform addressees of potential issues associated with leakage due to flaws in refueling water storage

tanks. The NRC expects recipients to review the information for applicability to their facilities

and consider taking action, as appropriate. Suggestions contained in this IN are not NRC requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

Kewaunee Power Station

The refueling water storage tank (RWST) at Kewaunee Power Station is a stainless steel tank of welded construction with a capacity of 272,500 gallons that is located in the auxiliary building. On July 10, 2012, the licensee identified a boric acid deposit adjacent to the weld connecting

the tank wall to the floor plate. No actual leakage of water from the tank was observed. The

licensee postulated that the boric acid deposit was due to a pinhole leak that resulted from an

original weld defect or some form of stress corrosion cracking, e.g., chloride stress corrosion cracking. The licensee justified continued operation of the tank through the use of American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code) Case N-705,

"Evaluation Criteria for Temporary Acceptance of Degradation in Moderate Energy Class 2 or 3 Vessels and Tanks," until permanent repairs could be made to the tank. Palisades Nuclear Plant

The Palisades Nuclear Plant safety injection refueling water tank (SIRWT

1) is an aluminum tank of welded construction with a capacity of 300,000 gallons that is located on the roof of the

auxiliary building. The tank was constructed in accordance with ASME Standard B96.1, Welded Aluminum-Alloy Storage Tanks, which is inactive and has no replacement. Part of the tank is located directly above the control room. Numerous pipes exit the floor of the tank through nozzles.

Leakage into the auxiliary building concrete roof structure (catacombs) was observed beginning in the spring of 2011. Despite repair efforts in 2012, leakage from the tank, sometimes accompanied by rainwater leaking through the roof around the tank, was present until the majority of the tank floor was replaced during the summer of 2013.

Inspection and repair efforts revealed the following issues:

1. Inspections were conducted using visual, surface and electromagnetic techniques. These techniques revealed numerous flaws in the tank floor welds and floor to shell weld

seam. Some flaws in the floor plates were also identified. These flaws were generally

associated with arc strikes and/or spatter, were not through wall, and may have been

original construction defects.

2. Through wall defects (leaks) were located in the tank floor welds. Some of these defects were associated with tank floor to penetration nozzle welds. As part of the repair

process the penetrations were redesigned to improve nozzle strength and to permit the

nozzles to move in response to flexing of the tank floor.

3. ASME Standard B96.1 does not contain specific guidance for the construction of tank floor penetration as used in this tank.

4. Leakage from the tank recurred in 2013, requiring plant shutdown. This leakage was attributed to the failure of a nozzle repair weld made in 2012. A destructive evaluation of

the weld indicated significant lack of fusion that could be the result of inadequate cleaning of the weld surfaces, and/or low weld heat input.

5. Design drawings for the tank indicated that the tank shell was supported by a concrete ring foundation and that the tank floor rested on a sand bed. During tank floor

replacement in 2013 it was determined that neither the ring foundation nor the sand bed was present. Given that the entire tank bottom sits on the concrete roof structure of the auxiliary building, these differences in the "as designed" and "as built" tank are not

structurally significant.

6. Some of the nozzles which were encased in concrete exhibited denting. Denting appears to be the result of a two-step process. In the first step aluminum corrodes as a result of contact with moist concrete. In the second step, the resulting corrosion products, which are more voluminous than the metal from which they are formed, force

the nozzle inward.

1 SIRWT is a site-specific term that is essentially equivalent to the generic term RWST. South Texas Project, Unit 1

The RWST at South Texas Project, Unit 1, is an ASME Code,Section III, Class 2 seismically- qualified stainless steel tank of welded construction with a capacity of 550,000 gallons. It is

located inside the Unit 1 auxiliary building and has drains connected to the radioactive drain

waste collection system. In September 1997, the licensee observed a spot of rust, traces of moisture, and some boric acid crystals at the RWST floor to tank wall weld. Since no actual leakage of water was observed, it was initially suspected the boric acid residue may have come

from a tank connection above this location. Based on an engineering evaluation, the licensee

initiated a periodic monitoring program of the RWST but took no additional action.

In February 1999, the licensee observed boric acid crystals in the same location as identified in 1997. The licensee performed a failure analysis using in-situ replication metallography and determined that the observed boric acid crystals were the result of a through wall crack at the

base plate weld (tank floor to tank shell). Additionally, the licensee determined that the crack was transgranular and branched, consistent with chloride stress corrosion cracking.

In 2001, the licensee performed a visual examination from the inside of the RWST using a video

camera on a remotely controlled submersible device. No evidence of base plate or side wall

cracking was observed, and no repairs were perform

ed. Periodic monitoring of the exterior of the tank continued.

In September 2011, boric acid residues were discovered under the base lip of the RWST in two

new locations near the RWST discharge line. Following discussions with the NRC, STP conducted the necessary calculations to demonstrate the structural adequacy of the tank in

accordance with ASME Code Case N-705. The licensee made repairs to the tank in

accordance with the ASME Code,Section XI, in the fall of 2012.

BACKGROUND

RWSTs serve the dual purpose of supplying water to flood the refueling cavity during refueling

operations and supplying water to the emergency core cooling system and the containment

spray system in emergency situations. RWSTs are fabricated from stainless steel or aluminum to minimize corrosion resulting from the borated water stored in the tank. These tanks are generally classified as ASME Class 2 for purposes of inspection. Leaks from the tanks may

result in failure to meet the tank's intended safety function due to loss of inventory, contamination of soil and or groundwater due to release of tritium to the environment, and/or

damage to safety related equipment due to water spray or flooding.

DISCUSSION

The above events demonstrate that leaks from RWSTs can result in disruption of plant operations. Based on the above events, the NRC believes that the following observations may

be of value to licensees:

1. Tank inspections have determined that all of the leaks have occurred in welds which are in or near the bottom of the tank.

2. Although additional degradation modes appear possible, the subject leaks have been attributed to weld fabrication flaws, stress corrosion cracking, and/or high stress low cycle fatigue. 3. The degradation mechanisms which have been identified are not those described for above ground tanks in NUREG 1801, the Generic Aging Lessons Learned (GALL)

Report (ADAMS Accession No. ML103490041). Based on the events cited, the NRC is

considering whether modifications to the GALL Report are warranted.

4. Although the events described in this IN resulted in leakage, none of the events has challenged the structural integrity of the tank.

5. As demonstrated by two of the events described above and documented in IN 2012-20, "Potential Chloride-Induced Stress Corrosion Cracking of Austenitic Stainless Steel and Maintenance of Dry Cask Storage System Canisters" (ADAMS Accession No. ML12319A440), cracking of 300 series stainless steel may occur in environments consisting of moist air and chloride containing surface deposits at temperatures less

than 140 degrees F.

6. Due to the age of some of the tanks under consideration, differences in the "as designed" and "as built" tank may exist. An understanding of the existence and extent of

these differences may be significant in determining the susceptibility of the tank to

degradation.

7. The interaction of aluminum and moist concrete may play a role in the long term performance of aluminum components.

In summary, recent events have indicated that RWSTs may undergo degradation which may

lead to disruption of plant operations and could result in release of tritium to the environment.

These events to date have been attributed to a variety of causes and have resulted in leakage issues, but have not challenged the structural integrity of the tanks. Licensees are encouraged to consider a wide range of environmental and mechanical degradation modes for these tanks

when developing or revising inspection or aging management programs.

CONTACT

S This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contact listed below or the appropriate project managers in the Office of Nuclear Reactor Regulation (NRR) and Office of New Reactors (NRO).

/RA/ (Sher Bahadur Acting for) /RA/

Lawrence E. Kokajko, Director Michael Cheok, Acting Director

Division of Policy and Rulemaking Division of Construction Inspection Office of Nuclear Reactor Regulation and Operational Programs Office of New Reactors

Technical Contact:

David Alley, NRR/DE/EPNB 301-415-2178 E-mail: David.Alley@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under the NRC Library.

ML13128A118 TAC No. MF1357 OFFICE NRR:DE:EPNB NRR:DE:EPNB BC:RIII/DRP/RPB4 Tech Editor NAME JTsao DAlley JGiessner CHsu DATE 8/29/2013 8/23/2013 8/29/2013 5/8/2013 OFFICE RES:/DE/CIB/BC BC:DE:EPNB BC:DE:EVIB DE:SLS NAME ACsontos TLupold SRosenberg RHardies DATE 8/30/2013 8/29/2013 8/30/2013 8/29/2013 OFFICE DLR:RAPB BC:NRR:DLR: RAPBD:NRR:DE D:NRO/DCIP (Acting) NAME WHolston KGreen PHiland MCheok DATE 5/7/2013 5/7/2013 8/29/2013 9/3/2013 OFFICE PM:NRR:PGCB LA:NRR:PGCB BC:NRR:PGCB DD:NRR:DPR D:NRR:DPR NAME JKlos CHawes (TAlexion for) SStuchell SBahadur (S.Bahadur for) LKokajko DATE 9/3/2013 9/4/2013 9/11/2013 9/12/2013 9/ 13 /2013