Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 24, 1992
NRC INFORMATION NOTICE 92-21: SPENT FUEL POOL REACTIVITY CALCULATIONS
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 addressees to potential errors in reactivity calculations
for spent fuel pools. 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
On February 14, 1992, the NRC was notified by Northeast Utilities of a
discrepancy between reactivity calculations performed for the Millstone,
Unit 2, spent fuel pool by ABB Combustion Engineering (CE) and the
licensee's contractor (Holtec). The licensee has indicated that the keff
calculated by Holtec was approximately 5 percent higher than that previously
calculated by CE.
The NRC has recently learned that Houston Lighting and Power (HLP) has
identified a discrepancy between the reactivity calculations performed for
the South Texas, Unit 1, spent fuel pool by Pickard, Lowe and Garrick (PLG)
and the licensee's contractor (Westinghouse). The licensee has indicated
that the keff calculated by Westinghouse was approximately 2 to 2.5 percent
higher than that previously calculated by PLG.
Boraflex is utilized as a neutron absorber between spent fuel pool rack
cells in both the Millstone, Unit 2, and South Texas, Unit 1, spent fuel
pools.
Discussion
The computer code analyses performed by CE to predict neutron transport for
the Millstone, Unit 2, spent fuel storage racks used the two-dimensional,
discrete ordinates code DOT. CEPAK was used to generate the neutron cross
sections for DOT. The computer code analyses performed by Holtec used KENO
(Monte Carlo method). The source of the discrepancy between the CE and
Holtec calculations has been attributed by CE to two approximations made in
the generation of neutron cross sections. First, a transport cross section
was used by CE as
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an approximation for the total cross section. While this approximation is
valid for most materials, it is not valid for materials having large thermal
cross sections. Therefore, applying this approximation to regions
containing a strong neutron absorber (such as Boraflex) results in an
overestimation of the neutron absorption and a corresponding lower
calculated keff in that region. Second, a geometric buckling term
corresponding to a sparsely populated and weakly absorbing (unpoisoned)
array was utilized by CE as an approximation of buckling in the highly
absorbing configuration. This approximation, however, is not valid for the
specific configuration found in the Millstone racks where the assembly pitch
is small and the fuel assembly is completely surrounded by a strong neutron
absorber. After these approximations were corrected, the results of the CE
analyses were in good agreement with Holtec's.
The original computer code analyses performed by PLG to predict neutron
transport for the South Texas, Unit 1, spent fuel storage racks used the
two-dimensional diffusion theory code PDQ. LEOPARD was used to generate the
cross sections for PDQ. Computer code analyses performed by Westinghouse
utilized KENO (Monte Carlo method). The lower value of keff calculated by
PLG has been attributed by HLP to the inaccuracies inherent in using
diffusion theory to predict neutron attenuation through a thin region that
strongly absorbs neutrons (such as Boraflex).
Both the CE and PLG methodologies had been benchmarked against criticality
experiments that have been reported to closely represent the characteristics
of the spent fuel storage racks. However, it should be noted that the
number of criticality experiments that included a strong neutron absorber
(such as Boraflex) was limited.
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.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: Jack Ramsey, NRR
(301) 504-1167
Larry Kopp, NRR
(301) 504-2879
Attachment: List of Recently Issued NRC Information Notices
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