Nuclear safety guide TID-7016 Revision 2 Page: 4 of 9
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-3-
CRITERIA FOR OPERATIONAL LIMITS
keff-= 1 + na
------------------------k Potential )
ok Minimum margin of critical ty5
eff subcriticality for
operation
---- -------------------------------_ k2 Maximumlit
Akff Normal and credible
eff abnormal contingencies
------------------------------ k3 floal
ktAllowable}
7 imi t
I regard the definition of criticality for the fissile material as
essential whether experimental or calculational. The associated stan-
dard deviation, a, embodies the uncertainty in the definition of
criticality with some multiple, n, of these establishing k1, a value at
which a system must be regarded as critical. It may or may not result
in actual criticality, but a judgement is made that its potential for
critical is sufficient to regard the state characterized by k1 as
critical. The maximum limit for the operation, k2, is to be a sub-
critical state. The associated first okeff is also expressible as some
multiple of the uncertainty in establishing the k2, either experimental
or calculational. The allowable limit, k3, is sufficiently below k2 to
protect against reactivity changes which may occur in the operation
because of contingencies symbolized by the second Akeff. It may be that
administrative controls or limits on other controlled parameters would
be adequate to permit k2 and k3 to be equal.
The result of an assessment is the k3 which is usually translated
into a limit on mass, dimension, concentration, or some other quantity
recognizable by the operators. It is considered poor practice for an
authority remote from the operation or outside the defined safety pro-
gram to specify the Akeff's to be used. These should be the product
of detailed analyses and overall evaluation of the operation.
As an illustration of the effort to produce data in the Guide, let
us examine single unit limits for simple geometries such as those which
appear in Chapter II. Figure 1 is typical and shows the variation of
mass as a function of the Uranium concentration. The masses corre-
sponding to a particular concentration are applicable to any 23SU
content of the uranium. These curves represent the formerly defined
'nominal' and 'full' reflector conditions. The 'minimal' reflector
condition is absent and we will return to this later. The computer
codes and cross sections were validated against representative
experiments performed over the density range and, with interpolative
calculations, the curves defined. They describe uranium systems that
are expected to attain a keff of about 0.95 for the respective reflector
conditions. The densities associated with the metal-water mixtures
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Thomas, J T. Nuclear safety guide TID-7016 Revision 2, article, January 1, 1980; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1055855/m1/4/: accessed May 30, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.