UNITED STATES

NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555-0001

March 5, 2019
Mr. Bryan C. Hanson
President and Chief Nuclear Officer
Exelon Generation Company, LLC
4300 Winfield Road
Warrenville, IL 60555
SUBJECT:

THREE MILE ISLAND NUCLEAR STATION, UNIT 1 - REVIEW OF
THE FALL 2017 STEAM GENERATOR TUBE INSPECTIONS
(EPID L-2018-LR0-0014)

Dear Mr. Hanson:
By letter dated March 19, 2018 (Agencywide Documents Access and Management System
(ADAMS) Accession No. ML 18085A168), as supplemented by letter dated September 21, 2018
(ADAMS Accession No. ML 18264A008), Exelon Generation Company, LLC (Exelon) submitted
information summarizing the results of the Fall 2017 steam generator tube inspections at the
Three Mile Island Nuclear Station, Unit 1. These inspections were performed during the
22nd refueling outage.
The U.S. Nuclear Regulatory Commission (NRC) staff has completed its review of the
information and determined that Exelon provided the information required by its technical
specifications. In addition, the NRC staff did not identify any technical issues that warrant
followup action at this time. Enclosed is the NRC staff's review of the Three Mile Island Nuclear
Station, Unit 1, I steam generator tube inspection report.
If you have questions, please contact me at 301-415-2048 or Justin.Poole@nrc.gov.
Sincerely,

........

2_--···~
. --~···~
"

-

Justin C. Poole, Project Manager
Plant Licensing Branch I
Division of Operating Reactor Licensing
Office of Nuclear Reactor Regulation
Docket No. 50-289
Enclosure:
Review of Fall 2017 Steam Generator
Tube Inspections
cc: Listserv

 REVIEW OF THE FALL 2017 STEAM GENERATOR TUBE INSPECTIONS
EXELON GENERATION COMPANY, LLC
THREE MILE ISLAND NUCLEAR STATION, UNIT 1
DOCKET NO. 50-289

By letter dated March 19, 2018 (Agencywide Documents Access and Management System
(ADAMS) Accession No. ML 18085A168), as supplemented by letter dated September 21, 2018
(ADAMS Accession No. ML 18264A008), Exelon Generation Company, LLC (Exelon, the
licensee) submitted information summarizing the results of the Fall 2017 steam generator (SG)
tube inspections performed at the Three Mile Island Nuclear Station, Unit 1 (TMI) during
Refueling Outage 22.
TMI has two AREVA Enhanced Once-Through Steam Generators with 15,597 thermally treated
Alloy 690 tubes. Each tube has a nominal outside diameter of 0.625 inches and a nominal
thickness of 0.0368 inches. The tubes are supported by 15 stainless steel tube support plates
(TSPs ). The TSPs have trefoil broached holes, except for the 1,740 drilled holes in the
uppermost TSP on the outer periphery of the tube bundle.
The licensee provided the scope, extent, methods, and results of its SG tube inspections in the
documents referenced above. In addition, the licensee described corrective actions (e.g., tube
plugging) taken in response to the inspection findings.
Based on U.S. Nuclear Regulatory Commission (NRC) staff's review of the information
submitted by Exelon, the staff has the following observations and comments:
•

In its response to Request for Additional Information 1, the licensee clarified that the
inspections were limited to SG B.

•

In its response to Request for Additional Information 2 requesting that the licensee
discuss the results of condition monitoring and operational assessment (OA) that provide
assurance that tube integrity will be maintained for SG A, the licensee stated the
following:
o

SG A is not expected to be subject to the same TSP-to-shroud locking
mechanism as observed in SG B over its operational life. If present in SG A, the
susceptibility to TSP-to-shroud locking would have manifested itself as high wear
near the "W-axis" and/or peripheral tubes in the first three inspections performed
after SG replacement, as was the case with SG B. The condition has not
manifested itself during the first three cycles of operation, and the assumptions
have been confirmed by the observation of the deepest TSP wear indication
reported in SG A as being 33 percent through-wall.

o

In addition, SG A has exhibited a more uniform distribution of wear depths as a
function of radius from the center of the SG, and the majority of wear flaws near
the W-axis in SG A are of the same magnitude as the rest of the population.
Therefore, the licensee believes that the TSP-to-shroud locking mechanism will
not become active in SG A in the future.

Enclosure

 -2-

o

Operational Assessments have been performed with fully-probabilistic methods
on both SGs A and B since the first inservice inspection. Predictions from the
OA models have bounded the actual inspection results with respect to wear
depths and growth rates. With respect to the methodology implemented for the
QA for SG A, the same methods used to justify a single cycle of operation for
SG B were applied to SG A, and two operating cycles were supported, which is
why the licensee was able to defer inspections for SG A to Refueling Outage 23.

Based on a review of the information provided, the NRC staff concludes that the licensee
provided the information required by its technical specifications. In addition, the NRC staff
concludes that there are no technical issues that warrant followup action at this time since the
inspections appear to be consistent with the objective of detecting potential tube degradation,
and the inspection results appear to be consistent with industry operating experience at similarly
designed and operated units.

 -2SUBJECT:

THREE MILE ISLAND NUCLEAR STATION, UNIT 1 - REVIEW OF
THE FALL 2017 STEAM GENERATOR TUBE INSPECTIONS
(EPID L-2018-LR0-0014) DATED MARCH 5, 2019

DISTRIBUTION:
PUBLIC
PM File Copy
AHuynh, NRR
RidsNrrPMSeabrook Resourc
AJohnson, NRR
RidsACRS_MailCTR Resource
RidsNrrDorlLpl1 Resource
RidsRgn1 MailCenter Resource
RidsNrrLALRonewicz Resource
PKlein, NRR
RidsNrrDmlrMccb Resource

ADAMS A ccess1on No.: ML 19059A028
DORL/LPL 1/PM
DORL/LPL 1/LA
DMLR/MCCB/BC*
OFFICE
SBloom
NAME
JPoole
LRonewicz
DATE
03/01/2019
02/28/2019
1/22/2019
OFFICIAL RECORD COPY

*b,y memoran d um
DORL/LPL 1/PM
DORL/LPL 1/BC
Joanna
JPoole
03/05/2019
03/05/2019