ORNL-4191 - the Molten Salt Energy Technologies Web Site

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Fig. 4.12. Time Dependence of Samarium Poisoning and Approximate Control Rod Reactivity During the First
Period of Operation of MSRE with 233U Fuel Loading.
and 23R1J burnup effects.) The resultant curve in
Fig. 4.12 indicates that regulating rod insertion
will be required for about the first 79 days of
operation.
Because the samarium acts as a burnable poison,
the amount of excess uranium required for operation
will be largely governed by the requirements for
calibration of the control rods. There should he
considerable latitude in this choice, determined in
part by criteria of control rod sensitivity at the
operating point and by the maximum time OC opera-.
tion desired before refueling.
4.4 MSRE DYNAMICS WITH 233u FUEL
S. J. Ball
The dynamic behavior of the NKRE has been
analyzed for the case of 2,"3U-bearing fuel salt by
using the MSHE frequency response (:ode MSFR.
The only differences in input data for the 233U
and the reference 23sU calculations are in those
parameters compared in Table 4.5. The most im-
port.ant difference is the lower delayed-neutron
fraction for the 233U1 which makes the neutron
level mote responsive to changes in reactivity.
For a given fast change in rod reactivity, the
immediate flux response would be two to three
times greater with 233U than with 235U. ?'his
effect will probably require a minor modification in
'OS. J. Ball arid T. W. Krrlin, Stability Arinfysis of thr
Molten Salt React3r Experiment, ORNl,-TR.I-1070 (Decem-
ber 19665).
the present MSRE rod control system tu compensate
for the higher system gain.
The predicted effect of 233U on the MSKE inherent
stability, as indicated by the phase margin,
and the natural period of oscillation are shown as
a function of power level in Fig. 4.13. The phase
margin l1 is a typical measure of system stability,
the smallet phase margins indicating reduced stability.
A general rule of thumb in control practice
is that a phase margin of at least 30" is desirable;
phase margins of 20" or less indicate lightly
damped oscillations and thus poor control. Hence
the predicted inherent stability for the "U system
is greater than for 2 3 5 for ~ a11 power levels.
The faster response oE the 233W system, as indicated
by the smaller natural periods of oscillation,
is due to the higher gain of the neutron kinetics.
Experimentally determined values of period of
oscillation for the 23sU system ate also shown in
Fig. 4.13 for comparison. ' Although direct
measurements of phase margin were not made, the
stability chatacteristics as indicated by frequency
response measurements were also in good agreement
with the predictions.
It is concluded that no serious operational difficulties
are to be expected due to the differences in
dynamic hehavior resulting from the 233U fuel
loading.
"J. E. Gibson, Nonlinear Automatic ContrOf, chap. 1,
McGraw-Hill, New York, 1963.
"T. W. Karlin and S. J. Bell, Experimentaf Dynamic
Arialysis of the Molten Salt Reactor Experiment, ORNrd-
TM-1647 (October 196b)-