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70

Information Notice No. 93­70: Degradation of Boraflex Neutron

Absorber Coupons

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON D.C. 20555

September 10, 1993

NRC INFORMATION NOTICE 93‐70: DEGRADATION OF BORAFLEX NEUTRON ABSORBER

COUPONS

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 recipients to a potentially significant problem pertaining to

degradation of Boraflex neutron absorber coupons. 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.

Background

Palisades has high‐density fuel storage racks installed in the spent fuel pool

that use Boraflex, a proprietary neutron absorbing material that was

manufactured by Brand Industrial Services, Incorporated (BISCO). The storage

racks are supplied by Westinghouse. The Boraflex is attached to the walls of

each canister and is held in place by a stainless steel wrapper, which is spot

welded to the walls.

The licensee has a surveillance program using Boraflex coupons to indicate the

status of the Boraflex contained in the high density storage racks in the

spent fuel pool. Recently observed degradation of several of these coupons,

which may be due to exposure to high level gamma radiation in conjunction with

interaction with pool water, raised questions about the integrity of the

Boraflex contained in the storage racks. Significant loss of Boraflex in the

high‐density fuel racks could result in loss of the subcriticality margin in

the spent fuel pool.

Through a commitment to the NRC, Palisades was required to test and inspect

the Boraflex coupons after 5 years of use. The tests include opening the

coupons for visual observation, neutron attenuation determination, and a

Boraflex hardness test.
Description of Circumstances

During a period from August 17 through 19, 1993, 5 of the existing 10 Boraflex

coupons were removed from the spent fuel pool, 4 being full‐length coupons

9309070206.

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September 10, 1993

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and 1 a short‐set coupon (see Figure 1). During removal of the full‐length

coupons a powdery substance and a grey debris cloud were observed emanating

from the coupons. Further investigation of the full‐length coupons revealed

that one coupon had lost an estimated 90 percent of its Boraflex, two others

50 percent, and one 38 percent. Investigation of the short‐set coupon showed

that it had retained 100 percent of its Boraflex. The licensee has determined

that the Boraflex in the 4 full‐length coupons and the short‐set coupon came

from different lots of material.

A principal difference between the short‐set and the full‐length coupons is

their geometrical design. The short‐set coupon consists of eight compartments

encapsulating the Boraflex with pool water accessing only the edges of the

coupons. However, in the full‐length coupon Boraflex is sandwiched axially

over the entire length of coupon and bolted between two 0.51‐mm [0.020‐inch]

stainless steel metal strips. The licensee postulated that a much larger area

of the full‐length coupon was exposed to the pool water environment and flow.

In addition, the full‐length coupon has a 12.7‐mm [0.5‐inch] hole through the

metal strip on the top portion of the coupon, which contributes to the pool

water flow around the Boraflex. The licensee has no immediate plans for

removal or testing of the remaining five coupons in the spent fuel pool, but

does intend to conduct neutron "blackness" testing in the spring of 1994.

The licensee also had initiated measurements of the silica content of the

spent fuel pool water. No increase in silica above normal 1‐4 ppm levels have

been observed except in instances when the pool boron concentration was

increased. Because silica filler material is a constituent of Boraflex, the

presence of silica in the water may be an indication of degradation of the

Boraflex in the fuel pool.

Discussion

Potential degradation mechanisms for Boraflex include (1) gamma flux, which

changes the material characteristics of the base polymer, and (2) chemical

environment, namely the accessibility of water to the Boraflex. The licensee

has not drawn firm conclusions as to (1) the root cause of the observed

degradation of Boraflex in the coupons, or (2) the correlation of the behavior

of Boraflex in the coupons to that in the storage racks.

Degradation of the Boraflex in fuel storage racks could reduce the

subcriticality margin in the spent fuel pool. The design basis assumes a

5 percent subcriticality margin on the basis of lowest pool temperature, no

boron concentration in the pool water, and minimum spacing between fuel

assemblies. In its preliminary analysis, the licensee assumed no boron in the

pool water and a complete loss of Boraflex from the fuel storage racks. This

reduced the subcriticality margin from greater than 5 percent to about

2 percent. However, the licensee used the burnup for the currently stored

fuel in the criticality calculation. This is higher than the design burnup

value, implying less reactive fuel in the fuel pool than assumed in the design

calculation.

The licensee took several compensatory measures, including (1) making the

operating staff aware not to dilute the spent fuel pool, (2) increasing the.

IN 93‐70

September 10, 1993
Page 3 of 3

spent fuel pool chemistry sampling frequency to daily, and (3) keeping the

boron concentration in the spent fuel pool above 1800 ppm, which exceeds the

technical specification requirement for boron concentration. With these

precautions a subcriticality margin greater than 5 percent would be

maintained.

Degradation of Boraflex has been previously addressed by NRC in Information

Notice 87‐43, "Gaps in Neutron‐Absorbing Material in High‐Density Spent Fuel

Storage Racks," September 8, 1987. The Electric Power Research Institute,

which has been studying this phenomenon for several years, has recently

published an interim report, "Boraflex Test Results and Evaluation,"

TR‐101986, February 1993.

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.

/s/'d by BKGrimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Regulatory Regulation

Technical contacts: Kombiz Salehi, RIII

(708) 790‐5782

K. I Parczewski, NRR

(301) 504‐2705

Larry Kopp, NRR

(301) 504‐2879

Vern Hodge, NRR

(301) 504‐1861

Attachments:

1. Figure 1, Palisades Boraflex Coupon Types

2. List of Recently Issued NRC Information Notices

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