Criticality safety issues in the disposition of BN-350 spent fuel Page: 4 of 11
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CRITICALITY SAFETY ISSUES IN THE DISPOSITION OF BN-350 SPENT FUEL
R. W. Schaefer
Argonne National Laboratory
P. O. Box 2528
Idaho Falls, ID 83403-2528
(208) 533-7054R. T. Klann
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois 60439-4803
(630) 252-4305S. M. Koltyshev
National Nuclear Center
Republic of KazakhstanS. Krechetov
Atomic Energy Committee
Republic of KazakhstanABSTRACT
A criticality safety analysis has been performed as part
of the BN-350 spent fuel disposition project being con-
ducted jointly by the DOE and Kazakhstan. The
Kazakhstan regulations are reasonably consistent with
those of the DOE. The high enrichment and severe
undermoderation of this fast reactor fuel has significant
criticality safety consequences. A detailed modeling
approach was used that showed some configurations to be
safe that otherwise would be rejected. Reasonable require-
ments for design and operations were needed, and with
them, all operations were found to be safe.
I. INTRODUCTION
The U.S. Department of Energy (DOE) is engaged in
a cooperative effort with the Republic of Kazakhstan on the
disposition of spent fuel from the BN-350 reactor.1
BN-350 is a 900 MWT, liquid-metal-cooled, fast breeder
reactor. The disposition involves several thousand fuel
assemblies. The spent fuel is being packaged currently.
This stage will be followed by transportation and dry
storage for =50 years.
The purpose of this paper is to describe criticality
safety analyses issues and their influence on designs and
operations in the disposition project. Background neces-
sary for understanding the issues is given in Section II.
General issues, such as guidelines for the safety assess-
ment, are described in Section III. Specific technical
analysis issues are presented in Section IV. The impact of
criticality safety on design and operations is the subject of
Section V. Some of these topics are relevant to other
interactions between DOE and former-Soviet-Union (FSU)
countries regarding spent fuel. Others are germane to the
disposition of DOE-owned spent fuel.
II BACKGROUND
In excess of 600 fuel assemblies comprise a BN-350
reactor loading,3 Each assembly is =3.5 m long and has
an hexagonal cross section with -100 mm flat-to-flatdimension. This cross section consists of a close-packed
bundle of fuel pins surrounded by an hexagonal duct.
Each driver assembly has an enriched uranium oxide fuel
section with depleted uranium oxide axial blanket sections
above and below. There are more than 200 driver assem-
blies in a loading, divided into three radial enrichment
zones: 17%, 21% and 26%. The inner zone includes a
dozen control rod assemblies, which also have 17% en-
riched fuel. Depleted uranium oxide radial blanket assem-
blies surround the 26% enrichment zone. In addition to
this standard fuel, experimental fuel was tested in the
reactor. Some experimental fuel differed from standard
fuel only in the composition of the stainless steel compo-
nents. Other experiments used different fuel types, includ-
ing MOX and uranium enrichments as high as 33%.
All spent fuel from the reactor was placed in a spent
fuel pool at the BN-350 facility. Until the late 1980s,
driver assemblies were removed for reprocessing off site,
once they cooled sufficiently. Nearly 2000 spent fuel
assemblies - drivers, control rods, radial blankets and
experiments - were in the pool when the spent fuel disposi-
tion project was conceived. It was decided recently to shut
down the reactor permanently and all the fuel in the reactor
was added to the disposition inventory.
The packaging phase of the project consists of condi-
tioning assemblies, sealing them in storage canisters and
temporarily storing the canisters in the pool until transpor-
tation begins. For normal assemblies, conditioning con-
sists of drying, evacuating and filling with inert gas. Six
normal assemblies are placed in a storage canister "clever-
ly" named a 6-pac. A horizontal cut through a loaded 6-
pac is shown in Fig. 1. It has a central lifting post sur-
rounded by a ring of six steel tubes containing the assem-
blies, all enveloped by an =400 mm-diameter steel shell.
Abnormal assemblies are those that are experiments,
damaged, fragile or disassembled (loose pins). All abnor-
mal assemblies are packaged four to a storage canister
named a 4-pac. Among the abnormal assemblies, each one
that is damaged, fragile or disassembled must be sealed in
a failed fuel stabilization canister (FFSC) before it is
placed in a 4-pac. Conditioning includes an initial step of
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Schaefer, R. W.; Klann, R. T.; Koltyshev, S. M. & Krechetov, S. Criticality safety issues in the disposition of BN-350 spent fuel, article, February 28, 2000; Argonne, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc702087/m1/4/: accessed July 15, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.