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82

Information Notice No. 93­82: Recent Fuel and Core Performance

Problems in Operating Reactors

UNITED STATES

NUCLEAR REGULATORY COMMUNICATION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

October 12, 1993

NRC INFORMATION NOTICE 93‐82: RECENT FUEL AND CORE PERFORMANCE PROBLEMS

IN OPERATING REACTORS

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors and all NRC‐approved fuel suppliers.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees of recent fuel and core performance problems in

operating reactors resulting from deficiencies and oversights in evaluating

the impact of fuel design changes and mixed core behavior in the fuel and

reload core design process. 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

Salem Unit 2 and Beaver Valley Unit 1

During shutdown inspection activities after Cycle 7 at Salem Unit 2 and

Cycle 9 at Beaver Valley Unit 1, the licensees at both plants discovered

numerous fuel rods that had developed fretting wear and perforation. The fuel

vendor attributed the degradation to grid‐to‐rod fretting resulting from flow‐

induced vibration of the fuel bundles. All but one of the affected fuel

assemblies were Westinghouse twice‐burned VANTAGE 5H fuel located next to the

core baffle or with a history of previous operation at a peripheral location.

The fretting wear occurred at the zircaloy mid‐grid spacers rather than at

lower grid locations where debris‐induced fretting wear typically occurs. In

some of the affected assemblies, secondary hydriding also was evident.

Wolf Creek

During the Cycle 5 shutdown at Wolf Creek, the licensee discovered 44 failed

fuel rods in three Westinghouse Batch F (standard) fuel assemblies that had

completed two cycles of operation with accrued assembly average burnups of

24,000 MWd/MTU. The most severely degraded fuel rod fragmented into three
segments during fuel handling operations while offloading the core.

9310070

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October 12, 1993

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Inspection of the failed fuel rods revealed the failures to be grid‐to‐rod

fretting wear at the lower three grid locations on in‐core interior fuel

assemblies. During Cycle 6 operation, additional fuel failures occurred in

three Batch F assemblies in their third cycle of operation with assembly

average burnups of 38,500 MWd/MTU. The occurrence was a virtual repeat of the

Cycle 5 experience, including the discovery of a broken fuel rod during core

offloading. These fuel failures have been attributed to high cross flows,

caused, in part, from mixed fuel designs which induced fuel rod vibration with

fretting wear at the lower grids. The mixture of standard and VANTAGE 5H fuel

(with debris filter bottom nozzles) resulted in axial mismatches between the

bottom nozzles and the grid spacers of the two fuel types. The failed

assemblies were all standard fuel and had been subjected to high cross flow.

The transition to VANTAGE 5H fuel had commenced with Cycle 5.

Palisades

Before Cycle 11 startup at Palisades, three separate segments of a damaged

fuel rod were discovered in the tilt pit area of the reactor cavity and a

0.15 m [6 in] segment containing the upper end cap was found in the fuel

assembly upper grid spacer. The upper 1.68 m [5.5 ft] segment in the tilt pit

was split with the plenum spring exposed and about 1/3 of the full fuel rod

inventory of fuel pellets was missing. The second segment (1.37 m [4.5 ft] in

length) and the third segment (0.46 m [1.5 ft] long with the lower end plug)

appeared to have fuel pellets intact. The fuel rod was from a corner rod

location in a peripheral fuel assembly which had been located in the corner of

the core adjacent to the core baffle during Cycle 10 operation.

The defective fuel assembly was one of a batch of 16 fuel assemblies,

fabricated by Siemens Nuclear Power Co. (SNP), located on the core periphery

since Cycle 9 that had 8‐rod hafnium absorber clusters inserted for vessel

fluence reduction. Inspection of this fuel assembly and others from the batch

of 16 fuel assemblies revealed that the spacer springs had relaxed and there

was up to 90‐percent through‐wall fretting wear at mid‐grid locations,

predominantly on the corner rods of affected fuel assemblies. Although the

typical exposure for these fuel assemblies was only 80 percent of the design

life, they had been subjected to the reactor environment and fast flux

exposure during five cycles of operation. Tests by SNP confirmed that the

lengthy service life was a contributory cause to the spring relaxation.

During Cycle 10, the licensee operated the plant with two batches of the SNP

fuel assemblies with high thermal performance flow mixing spacers. Flow

mixing spacers in neighboring fuel assembly locations and the low power of the

peripheral fuel assemblies adjacent to the core baffle produced a hydraulic

mismatch that caused cross flow into the peripheral fuel assemblies. In

addition, the flow forces had increased when primary coolant flow increased

7 percent after the replacement of steam generator tube bundles before Cycle 9

startup. Limited inspection by the licensee indicated that corner fuel rods

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October 12, 1993

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closest to the baffle were the most severely worn. The licensee also found
corresponding wear locations between the most severely damaged fuel assembly

and the core baffle, indicating that the fuel assembly had rubbed against the

baffle, resulting in severe spacer damage.

Hatch Unit 2, Peach Bottom Unit 3, and Perry

Recent fuel failures found at Hatch Unit 2 and Peach Bottom Unit 3 in General

Electric (GE) zirconium‐barrier fuel have been attributed to debris fretting

or to undetected manufacturing defects. At Perry, leaking fuel rods in two

bundles during Cycle 1, one bundle during Cycle 2, and two bundles during

Cycle 3 are believed to have been the result of bad end cap welds. However,

during the recently completed Cycle 4, one severely damaged fuel rod (with a

50.8 cm [20 inch] long large axial crack as a result of secondary hydriding)

and possible leaking fuel rods in several other bundles were noted. These

fuel bundles were fabricated to avoid repetition of the end cap problem.

GE believes that the root cause of the failures is undetected manufacturing

defects, possibly exacerbated by the Perry operating practice of using control

rod movement rather than flow control for minor power adjustments.

DISCUSSION

Recent operating experience of pressurized‐water reactors has identified

debris‐induced fretting as a leading cause of fuel failure. However, current

experience also indicates that a new type of vibrational fretting is emerging.

During ultrasonic examination of VANTAGE 5H failed fuel assemblies at Salem

and Beaver Valley, Westinghouse confirmed grid‐to‐rod fretting at the zircaloy

low‐pressure drop mid‐grids. Most of the failures occurred near the core

baffle or in fuel that had resided near the core baffle in a preceding cycle.

Through several tests, Westinghouse determined that the occurrence of flow‐

induced vibrational fretting was likely. This vibrational fretting involves

the natural frequency and flow condition for fuel assemblies adjacent to the

core baffle. To avoid this type of fretting wear, Westinghouse proposed

corrective actions for existing fuel assemblies adjacent to the core baffle to

dampen the vibrations. The long‐term corrective actions will include design

modifications and more extensive vibration testing to ensure that the natural

frequency of the fuel assembly does not correspond to flow conditions that may

exist in the reactor.

For Wolf Creek, the fretting wear involved a mixed core of standard fuel and

VANTAGE 5H fuel with inclusion of intermediate flow mixer grids and debris

filter bottom nozzles. The failures were located at the lower grids,

especially near debris filter bottom nozzles. Westinghouse performed a

sensitivity study to analyze the flow condition near the debris filter bottom

nozzle region for mixed cores. The analysis indicated that excessive

vibration of the fuel rods of the standard fuel assembly may have been caused

by cross flow from VANTAGE 5H fuel to the standard fuel and that the mixed

core of different bottom nozzle designs may have contributed to such a cross

flow phenomenon.

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IN 93‐82

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An issue involving the compatibility of mixed cores in a boiling‐water reactor

was identified during the NRC evaluation of the Washington Nuclear Power

Unit 2 (WNP‐2) instability event of August 15, 1992. At that time, it was

noted that an abnormally large mismatch in flow and pressure drop between

neighboring fuel assemblies in a mixed core was further exaggerated by the

core reload operating scheme and may have contributed to thermal‐hydraulic

instability of the core. Similarly, it now appears that core designs

comprised of mixed‐fuel types sometimes result in physical mismatches of

neighboring fuel assemblies that may be contributing to flow‐induced fretting
wear failure of fuel rods in pressurized‐water reactors. The mixing of fuel

of different types is further complicated by design variations introduced by

fuel reconstitution and by the purchase options offered by at least one fuel

vendor (Westinghouse) to include design features such as intermediate flow

mixer grids and debris filter bottom nozzles with the fuel type of customer

choice. This has resulted in one known instance (at Turkey Point Unit 3) of

an axial offset in power distribution as a result of a small axial mismatch

between burnable poison rods and the active core. The mismatch occurred when

debris filter bottom nozzles were included with the fuel type chosen by the

licensee, but a corresponding revision of burnable poison rod specifications

was overlooked.

The above problems raise questions about the effectiveness of design quality

assurance, as implemented by some licensees and vendors to ensure that the

reactor core continues to conform to its design bases when new and modified

fuel designs are introduced during core reload. The degree of core design

responsibility exercised by the fuel supplier varies significantly depending

to a large extent on the wishes of each individual licensee. Defining reload

core design safety evaluation and quality assurance responsibilities,

especially the interaction with fuel suppliers, can help assure that licensee

and vendor design responsibilities are properly implemented.

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No specific action or written response is required by this information notice.

If you have any questions regarding this matter, please contact one of the

technical contacts listed below or the appropriate Office of Nuclear Reactor

Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contacts: S. L. Wu, NRR

(301) 504‐3284

L. E. Phillips, NRR

(301) 504‐3232

Attachment: List of Recently Issued NRC Information Notices

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