SUBJECT: Control Rod-Power Characteristic
For purposes of instrument and control-rod design
it is necessary to know the effect of changes in
normally obtained empirically; but lacking a precise graph
The power is shown to be approximately proportional
to the excess reactivity; the latter in turn is given by an
S-shaped curve which is similar to a displaced sine curve.
The variation with position of rod of the power is shown in
the attached figure, adjusted to fit a total value of
The arguments loading to this graph are given
in the appendix of this note.
The effect of a cadmium or boron control rod is to depress the neutron flux,
usually to zero at the boundary. If such a rod were inserted axially in a
circular cylinder of length much greater than the diameter, the neutron flux
would be represented, in a bare reactor, by the function
|where Jo is the Bessel function and the dimensions
are given in the sketch, Fig. 1.
(This is to be contrasted with the undisturbed flux
The critical condition for such a system would be written
where M² is an effective migration length for neutrons.
(Undisturbed, the k is given by the same relation without the a)
In a finite cylindrical bare pile, there is an axial sinusoidal variation
in flux density,
|The approximation is now made that the
effective k of a finite cylinder with a
rod partially inserted is the average of
the undisturbed value kv and the disturbed
value kd, with weighting factors
namely the respective areas under the flux
|It is easy to show that the effective k by
this criterion is
If r1 is the excess reactivity value of the
rod and the "rod-in" position corresponds to
r = 0, then the excess reactivity r at any
distance the rod is pulled up, Z, is given by
In order to translate this into a power graph it is necessary only to note
that if the pile is just critical at zero power with the rod in and the
solution at the temperature of the cooling water, for any other steady state
at power P the excess reactivity due to the rod must just balance the drop due
to the solution temperature rise, to again reach k = 1.
Calculations on heat removal and experimental data from
an essentially linear rise in allowed power with temperature. Thus
P/P1 = T/T1
where (P1,T1) is the maximum operating point, and (P,T) is any other.
If the temperature effect on reactivity is written
where [alpha] is the temperature coefficient, then the rod-power correlation is
This is plotted for the following assumed values of the constants