United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-38: Degradation of Boraflex Neutron Absorber in Spent Fuel Storage Racks

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

                               September 8, 1995

                               SPENT FUEL STORAGE RACKS


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a potentially significant problem pertaining to
degradation of the Boraflex neutron absorber material in spent fuel storage
racks.  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.


Degradation of Boraflex has been previously addressed by the NRC in
Information Notice (IN) 87-43, "Gaps in Neutron-Absorbing Material in High-
Density Spent Fuel Storage Racks," September 8, 1987, and IN 93-70,
"Degradation of Boraflex Neutron Absorber Coupons," September 10, 1993.  The
Electric Power Research Institute (EPRI) has been studying the phenomenon of
Boraflex degradation for several years and recently issued EPRI report  
TR-103300, "Guidelines for Boraflex Use in Spent-Fuel Storage Racks,"  
December 1993, identifying two issues with respect to using Boraflex in spent
fuel storage racks.  The first related to gamma radiation-induced shrinkage of
Boraflex and the potential to develop tears or gaps in the material.  The
second concerned gradual long-term Boraflex degradation over the intended
service life of the racks as a result of gamma irradiation and exposure to the
spent fuel pool environment.  This second issue has previously been observed
in degradation of Boraflex surveillance coupons at the Palisades plant      
(IN 93-70), but further testing of the actual Palisades storage racks
indicated no similar degradation.  Because of the relatively watertight
Boraflex panel enclosures in most spent fuel storage rack designs, this type
of degradation was typically not previously considered.

The potential exists for a gradual release of silica and boron carbide from
Boraflex following gamma irradiation and long-term exposure to the spent fuel
pool environment.  When Boraflex is subjected to gamma radiation in the 
aqueous environment of the pool, the silicon polymer matrix becomes degraded
and silica filler and boron carbide are released.  Because Boraflex is   

9509050009.                                                            IN 95-38
                                                            September 8, 1995
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composed of approximately 25 percent silica, 25 percent polydimethyl siloxane
polymer, and 50 percent boron carbide, the presence of silica in the pool
provides an indication of depletion of boron carbide from Boraflex.  The loss
of boron carbide (washout) from Boraflex is characterized by slow dissolution
of the silica from the surface of the Boraflex and a gradual thinning of the
material.  In a typical spent fuel pool, the irradiated Boraflex represents a
significant source of silica (several thousand kilograms) and is the most
likely source of pool silica contamination.  The boron carbide loss will
result in an increase in the reactivity of the matrix of fuel and Boraflex in
the spent fuel pool. 

EPRI report TR-103300 has identified several factors that influence the rate
of silica release from Boraflex.  The presence of water around the Boraflex
panels is perhaps the most significant factor influencing the rate of silica
dissolution from Boraflex.  Because of the different rack designs, this rate
will vary from plant to plant.  The rate of dissolution also increases with
higher pool temperature and gamma exposure, suggesting that Boraflex
degradation can be reduced by keeping pool temperatures low and by not placing
freshly discharged fuel assemblies in the same storage cells at each refueling

Description of Circumstances

The South Texas Project, Unit 1, has fuel storage racks installed in the spent
fuel pool that use Boraflex as a neutron absorber.  The pool contains two rack
types.  The Region 1 racks are designed to receive high reactivity fuel
assemblies, including fresh fuel, and use Boraflex panels in a removable
stainless steel box.  The Region 2 racks are designed for low reactivity spent
fuel assembly storage and contain fixed Boraflex panels between the cell
walls.  The Boraflex panels were designed to ensure that adequate negative
reactivity would be maintained if the pool were accidentally flooded with
unborated water.    

Blackness (neutron absorption) testing was performed during August 1994 on
selected South Texas Project Unit 1 spent fuel pool storage racks to determine
the condition of the Boraflex and to determine the size and location of any
gaps that may have developed.  However, in addition to gap development, which
is a known phenomenon, the results also indicated that the Boraflex had
significantly degraded due to a decrease of the boron content in several of
the storage cells tested.  Of the eight cells that had been designated to
receive an accelerated gamma dose in Region 1, five cells exhibited large
areas of degradation (0.9 to 1.4 meters [3 to 4.5 feet] in length) postulated
to result from accelerated dissolution of the Boraflex caused by pool water
flow through the panel enclosures as well as the high accumulated gamma dose.

Similar Boraflex degradation was discovered at the Fort Calhoun Station.  As
part of their rerack project, the old spent fuel storage racks containing
Boraflex were removed and disassembled in December 1994 to determine the
condition of the Boraflex.  Two cells from the removed Boraflex racks which
had experienced the highest gamma flux since 1983 were inspected.  Only      
40 percent of the Boraflex remained in one of the panels from these cells
while another panel in the same cell exhibited no loss of Boraflex.  An  .                                                            IN 95-38
                                                            September 8, 1995
                                                            Page 3 of 3

adjacent cell had a panel which had some Boraflex loss but subsequent
attenuation and density tests confirmed that the average boron-10 areal
density still exceeded the material minimum certifications.  The new Fort
Calhoun Station storage racks do not contain Boraflex.


Because Boraflex is used in the South Texas Project spent fuel storage racks
for absorption of neutrons, a reduction in the amount of Boraflex could result
in an increase in the reactivity of the spent fuel pool configuration, which
may approach, or even exceed, the current NRC acceptance criterion of keff no
greater than 0.95.

In response to the identified Boraflex problem, Houston Lighting & Power
Company, the licensee for the South Texas Project, developed restrictions to
not use the substantially degraded storage cells in Region 1 for discharged
spent fuel.  In addition, the licensee is developing a long-term neutron
absorption panel management plan, as well as a dose-to-degradation
correlation, which will aid in establishing restrictions for use of the spent
fuel racks in both Units 1 and 2.  The licensee also cited criticality
analyses that showed that the fuel will remain subcritical by at least 
5 percent, even with no Boraflex, as long as the soluble boron concentration
is at least 2,500 ppm.

Although pressurized-water reactor spent fuel pool water is normally borated
to approximately 2,000 ppm of boron, current regulatory requirements do not
allow credit for the soluble boron except under accident conditions.  Many
boiling-water reactor (BWR) storage racks also contain Boraflex.  Because BWR
spent fuel pool water does not contain boron, any significant Boraflex
degradation in a BWR pool may challenge the 5 percent subcritical margin.

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 DMCrutchfield

                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Laurence I. Kopp, NRR
                     (301) 415-2879

                     K. I. Parczewski, NRR
                     (301) 415-2705
Page Last Reviewed/Updated Monday, November 18, 2013