K. Kobayashi and S. Tsumura

AOI–AOX
0.002
0
–0.002
B C
A
D E
–0.004
–0.01 –0.005 0 0.005
AOP–AOX
Figure 4. Trajectory of axial offset for t3 = 25h
2. A point of offsets moves anticlockwise on the trajectory as a function of time.
3. A point of offsets moves quickly horizontally parallel to the horizontal axis by insertion
or withdrawal of control rods.
The reason responsible for item 3 is the fact that the neutron production rate changes very
quickly with time constant ω0 as seen in Eq.(100), while iodine and xenon densities change
very slowly.
In order to terminate the xenon oscillation, t3 = 25h or t3 = 28h was put in Eq.(107), and
t4 =27.6h and δΣ C2 = −1.85 × 10 −6 cm −1 or t4 =55.4h and δΣ C2 = −3.74 × 10 −7 cm −1 were
obtained for the cases of Figs.3 and 5, respectively. Using these values, the xenon oscillations
are completely terminated as seen in Figs.3 and 5.
In Fig.4, for example, the point of offsets moved in negative direction from the origin to A
almost parallel to the horizontal axis in a short time by withdrawal of a control rod, and
move in positive direction quickly from point B to C by returning the control rod to the
original position. At the time t3 chosen appropriately, the rod is inserted and the point
moved quickly in positive direction from point D to E, and at time t4, the point moved
quickly from point F to the origin and the oscillation is terminated. The distance from
point F to the origin is nearly the same as that from point D to E. In Figs.3 and 5, the
difference between the time t3, 25h and 28h for the insertion of control rods is 3h, however,
the difference of the time t4, 27.6h and 55.4h for the withdrawal between both cases is fairly
large.
19
O
F