Information Notice No. 85-99: Cracking in Boiling-Water-Reactor Mark I and Mark II Containments caused by Failure of the Inerting System
SSINS No.: 6835
IN 85-99
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
December 31, 1985
Information Notice No. 85-99: CRACKING IN BOILING-WATER-REACTOR MARK I
AND MARK II CONTAINMENTS CAUSED BY
FAILURE OF THE INERTING SYSTEM
Addressees:
All boiling-water-reactor (BWR) facilities having a Mark 1 or Mark II con-
tainment.
Purpose:
Information Notice No. 84-17, "Problems with Liquid Nitrogen Cooling Compon-
ents Below the Nil-Ductility Temperature," was issued to describe a large
crack in the vent header of Hatch Unit 2 that was attributed to brittle
fracture caused by the injection of cold nitrogen during inerting system
operation. The purpose of this information notice is to advise licensees and
applicants of a crack discovered in the nitrogen inerting and purge line of
Hatch Unit 1, and to re-emphasize the concern regarding inerting system
failures that may result in cooling of components below the nil-ductility
temperature (NDT) of associated materials susceptible to brittle fracture.
It is expected that the recipients of this notice will review the
information for applicability to their facilities and consider actions, if
appropriate, to preclude similar problems at their facilities. However,
suggestions contained in this notice do not constitute NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances:
Hatch Unit 2 Vent Header Crack and Subsequent Actions - On February 3, 1984,
Georgia Power Company reported a through-wall crack almost completely around
the vent header within the containment torus of Hatch Unit 2. Later that day
IE Bulletin 84-01, "Cracks in Boiling Water Reactor Mark 1 Containment Vent
Headers," was issued for action to licensees of BWR facilities with Mark 1
containments that were in cold shutdown. The bulletin required inspection
for cracks in the containment vent header and in the main vents in the
region near the intersection with the vent header. The inspections by the
licensees revealed no cracks. The bulletin also suggested that the operating
BWR plants with Mark I containments should review their plant data on
differential pressures between the wetwell and drywell for anomalies that
could be indicative of cracks. The licensees who performed the review
reported no anomalies.
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Following the event, the Regulatory Response Group (RRG) of the BWR Owners
Group (BWROG) met twice with the NRC staff, on February 6 and 23, 1984, to
present the results of their investigation into,the cause of the event and
the industry's plans to voluntarily take measures to prevent this type of
event. The large crack in the Hatch 2 vent header was confirmed to be the
result of brittle fracture caused by the injection of cold nitrogen into the
torus during inerting.
As a part of the industry plan, the RRG sent General Electric (GE) Service
Information Letter (SIL) No. 402, "Wetwell/Drywell Inerting," to the BWROG
representatives on February 17, 1984, and advised the owners to contact
their respective NRC project manager on implementation status. The SIL
contained several recommended actions to be taken by owners to confirm that
equipment damage had not occurred and that inerting system design and
operation was proper so that damage would not occur in the future. On March
5, 1984, Information Notice No. 84-17 was issued to describe the cause
and safety significance of the vent header crack.
During the meetings with the RRG, the NRC staff emphasized the need for each
utility to contact its NRC project manager and provide a schedule and plans
for actions relative to the SIL in order to ensure the regulatory closeout
of the issue. By October 1, 1984, all utilities had provided written
responses concerning their plans with respect to the SIL. The written
responses varied significantly both in content and in level of detail.
However, they all indicated that the SIL recommended actions or appropriate
alternatives had already been taken or were planned. This led to a general
conclusion by the NRC staff that the industry was taking all the necessary
actions regarding this important safety issue.
Hatch Unit 1 Nitrogen Inerting and Purge Line Crack - On December 15, 1984,
inservice inspection (ISI) tests using a magnetic particle inspection method
were being performed on selected pipe welds of Hatch Unit 1. During this
testing, a linear through-wall crack approximately 2-3/4 inches long was
discovered in a weld located in the 18-inch nitrogen inerting and purge line
between the drywell penetration and inboard containment isolation valve
(Ref: LER 84-25). The crack was ground out, and the weld was repaired and
satisfactorily tested.
The crack represented a breach of containment integrity. The licensee's
investigation as described in the LER indicated that the crack was likely
caused by thermally induced stress. The location of the crack is shown on
the attached Figure 1. A 2-inch nitrogen makeup line enters the 18-inch
purge line approximately 2 feet upstream from the crack. Cold nitrogen
entering the purge line from the makeup line makes contact with the purge
line in the area of the weld, the contour of which makes it a likely spot
for a thermally induced failure to occur.
Discussion:
The NRC staff met with Georgia Power Company and its consultants on November
8, 1985, to discuss the safety and regulatory issues stemming from the Hatch
Unit 1 crack. The primary conclusions reached at the meeting were as
follows:
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1. The likely cause of the Unit 1 crack Was the growth of a pre-existing
weld defect. It is possible that cold nitrogen injection from the
makeup system contributed to the growth of the crack. The nitrogen
temperature is not monitored during a makeup evolution and there is no
automatic shutoff on low temperature. A Nitrogen Inert System Makeup
Low Temperature alarm exists, but this alarm was not covered in any
annunciator response procedures.
The licensee's corrective actions included raising the nitrogen makeup
low temperature alarm set point to 10F from 0F and revising
the annunciator response procedures to terminate nitrogen makeup when
the annunciator is actuated.
2. The licensee performed only a visual inspection of the Unit 1 nitrogen
inerting and purge line welds rather than the ultrasonic testing
recommended by the SIL. The visual inspections would not have
discovered the crack found during the ISI. Therefore, the crack could
have existed at the time the SIL was implemented.
3. The nitrogen makeup line was not considered by the licensee to be
covered within the scope of the SIL as a source of cold nitrogen (less
than 40F) into containment systems, because of: the use of rela-
tively low flow rates; and the long run if bare piping inside the warm
reactor building. The licensee did confirm, however, that the makeup
portion of the inerting system was in use for an 18 1/2-hour period in
February 1984 when the ambient temperature did not get above 22F.
Makeup line nitrogen temperatures during this period were not known.
No specific action or written response is required by this information,
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: R. Singh, IE
(301) 492-4149
Attachments: 1. Figure 1: Hatch Unit 1 Nitrogen Inerting and Purge System
2. List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021