Development and Verification of Nuclear Calculation Methods for ...

- St -
Verification of the over-all burn-up method will be given in section It,
where over-all depletion calculations for the first core of the Yankee reactor
(ret. 20 ) will be compared with experiments.
Macroscopic cross sections as a function of burn-up, power density
and flux spectrum can be provided by means of the LASER-CROSS MACRO
complex.
A typical 30 x 30 mesh,2 group quarter core over-ail depletion calculation
for the Yankee reactor with a direct representation of control rods
and followers requires about 40 seconds per time step.
5.4. Study of Spatial Xenon Oscillations
A summary of xenon-induced spatial power oscillation studies for the
BHWR-800 NORDIC STUDY (ref. 21) is given.
RZ and XY geometry calculations were performed with an early
version of the DBU programme in the modified one-energy group approximation.
It was observed that time step lengths of the order of one hour or less
were required for adequate accuracy. Furthermore, the use of a flux convergence
criterion of 10" was required instead of 10~ , which is normally
used in static problems, to ensure sufficient accuracy of the calculated
xenon stability characteristics.
Criticality was maintained by use of the eigenvalue poison search that
distributes the poison uniformly over the core.
It was demonstrated that a flat power distribution was mostly exposed
to xenon oscillations due to the small interior leakage.
The axial free-running mode oscillation induced by insertion of four
control rod rings is given in fig. 14. The power densities are damped and
convergent. Fig. 16 shows a power density map three hours after insertion
of four control rod rings, where the power maximum, owing to xenon
effect, is in its upper position.
The azimuthal free-running mode oscillation induced by control rod
insertion is shown in fig. IS. The power densilies oscillate harmonically
and are divergent with a period of 36 hours. However, these oscillation«
can be controlled with control rods. Power densities at different times after
the perturbation appear from fig«. 17 through 19.