Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
November 23, 1994
NRC INFORMATION NOTICE 94-79: MICROBIOLOGICALLY INFLUENCED CORROSION OF
EMERGENCY DIESEL GENERATOR SERVICE WATER PIPING
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to degradation resulting from microbiologically
influenced corrosion in carbon steel piping systems that supply service water
to emergency diesel generators. It is expected that recipients will review
the information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems. However, suggestions contained in
this information notice are not NRC requirements; therefore, no specific
action or written response is required.
Description of Circumstances
Haddam Neck Plant
On February 12, 1994, a through-wall leak developed in the service water
system supply piping to the "A" emergency diesel generator. The leak occurred
in a weld in an area of low flow, where the licensee was doing light surface
grinding to prepare for ultrasonic test (UT) inspection. Previously, in March
1993, the licensee had found a similar leak associated with the "B" emergency
diesel generator. After removing the leaking section and examining the pipe,
the licensee determined that the leak was caused by poor initial weld quality
and microbiologically influenced corrosion. Lack of penetration of some welds
created a crevice condition. Radiographic tests (RT) of additional emergency
diesel generator service water system piping revealed three additional welds
that could form similar leaks. It was during preparation of one of these
welds for UT examination that the through-wall leak associated with the "A"
emergency diesel generator was found.
The licensee had previously performed a structural integrity determination in
1993 by radiography. Though based on a uniform wall loss instead of a sharp
disruption, the licensee had at that time concluded that the worst-case
degraded cross-section of the as-found pipe welds, located in the unisolable
portions of the emergency diesel generator supply piping, met the Generic
Letter 90-05 acceptance criteria for structural integrity and that the
degraded piping would have sufficient mechanical and structural integrity to
remain operable.
9411170039. IN 94-79
November 23, 1994
Page 2 of 4
Beaver Valley Power Station, Unit 1
On May 6, 1994, the licensee found a through-wall leak (an 8 mm [5/16 inch]
diameter hole) on the "A" train reactor plant river water system header to the
emergency diesel generators. The leak developed on the below-grade portion of
the 15-cm [6-inch] diameter A106 Grade B carbon steel piping. The licensee
excavated the "A" and "B" train headers, ultrasonically examined them using
the "autoscan" area mapping technique, and found localized pitting to less
than minimum required wall thickness in three additional locations along the
"A" train piping. The pits were not clustered or closely spaced, but were
located along the pipe length between the five and seven o'clock positions.
For reference, the nominal pipe wall thickness is 6.4 mm [0.25 inch] and
minimum code wall thickness is 1.5 mm [0.06 inch].
The licensee evaluated the pitting in accordance with the American Society of
Mechanical Engineers (ASME) Code of record. The basis for the localized
thinning evaluation was conducted in accordance with industry guidance, EPRI
NP-5911SP, "Acceptance Criteria for Structural Evaluation of Erosion-Corrosion
Thinning in Carbon Steel Piping." This evaluation was reviewed by personnel
in NRR and deemed as an acceptable conservative means of demonstrating ASME
Code conformance. The licensee determined microbiologically influenced
corrosion to be the cause of the pitting and through-wall leak. The licensee
removed a 0.61 m [24 inch] long section of piping to culture and characterize
the microbial activity and perform additional ultrasonic testing.
The general inside surface of the piping was moderately corroded, with
localized pits and tubercles. Cultures from the pits contained sulfur-
reducing bacteria and the anaerobic bacteria Clostridium. The tubercle
formation was the result of the growth of the iron-oxidizing bacteria
Gallionella. Ultrasonic examinations of a heat-affected zone indicated that
the weld examined was not subject to preferential microbiologically influenced
corrosion attack.
Discussion
Stagnant or intermittent-flow conditions, as in the case of emergency diesel
service water supply headers, are conducive to the growth of microorganisms
that can accelerate corrosion rates. Service water supply lines to emergency
diesel generators are stagnant because motor-operated isolation valves are
normally maintained shut (except during monthly surveillance testing).
Crevices such as those in piping welds that lack penetration can enhance
microbiologically influenced corrosion attack by giving a place for deposits
and, therefore, for the bacteria to collect. Microbial films form when
aerobic species, such as iron-oxidizing bacteria, create anaerobic conditions
underneath them for microorganisms, such as sulfate-reducing bacteria, to
accumulate at the metal surface. Sulfate-reducing bacteria attack the metal
surface, produce corrosive chemicals, and cause deep pitting.
. IN 94-79
November 23, 1994
Page 3 of 4
Microbiologically influenced corrosion on carbon steel will increase general
corrosion, through-wall pitting, and the formation of tubercles. Tubercles
consist of corrosion products, microbes, and debris. Tubercle growth could
restrict cooling water flow to equipment.
Stainless steel piping is not immune to microbiologically influenced corrosion
because microbes can attack at the weld heat affected zone (HAZ) in stainless
steels when this zone becomes sensitized. Microbiologically influenced
corrosion can also damage metals lined with polymeric materials, typically at
coating imperfections.
Once microbial films are established on metal surfaces, they are extremely
difficult to eliminate because of the resiliency of the individual
microorganisms. Biocides are applied by some licensees in areas where
continuous flow conditions cannot be maintained. However, biocide treatments
are not always effective against established microorganism colonies because
the biocide cannot penetrate through the tubercles or aerobic biofilms.
Treatment against established colonies involves a combination of mechanical or
chemical pipe cleaning, continued water treatment and regular maintenance.
Continuous flow conditions have been found to prevent the attachment and
growth of microbial films.
It may be necessary to replace materials if microbiologically influenced
corrosion severely damages them or where mitigation measures cannot bring the
system condition under control. Possible alternatives include replacing
carbon steel with stainless steel or replacing stainless steel with more
resistant materials, such as 6-percent molybdenum stainless steels, nickel
base alloys, titanium, or nonmetallic materials.
The licensee did not regularly treat the Beaver Valley River water supply
lines to the diesel with biocide or corrosion inhibitors. The chlorination
injection point for the main river water headers is downstream of the branch
lines to the emergency diesel generators. It also appears that the existing
program at Haddam Neck of hypochlorite injection was not successful in
mitigating the microbiologically influenced corrosion problems in stagnant
dead-end lines at such locations as the emergency diesel generator supply.
. IN 94-79
November 23, 1994
Page 4 of 4
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
Original signed by B.D. Liaw for
Brian K. Grimes, Director
Division of Project Support
Office of Nuclear Reactor Regulation
Technical contacts: Michael Modes, RI James A. Davis, NRR
(215) 337-5198 (301) 504-2713
Peter P. Sena, RI Vern Hodge, NRR
(412) 643-2000 (301) 504-1861
Attachment:
List of Recently Issued NRC Information Notices
