Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
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September 10, 1993
Page 2 of 3
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
.
