A new reactor burnup strategy CANDLE (Constant Axial shape of Neutron flux,
nuclide densities and power shape During Life of Energy producing reactor) was proposed, where
shapes of neutron flux, nuclide densities and power density distributions remain constant but
move upward (or downward) along its core axis.
This burnup strategy can derive many merits. The change of excess reactivity along burnup is
theoretically zero for ideal equilibrium condition, and shim rods will not be required for this
reactor. The reactor becomes free from accidents induced by unexpected control rods withdrawal.
The core characteristics, such as power feedback coefficients and power peaking factor, are not
changed during life of operation. Therefore the operation of the reactor becomes much easier
than the conventional reactors. The infinite- medium neutron multiplication factor of replacing
fuel is less than unity. Therefore the transportation and storage of replacing fuels becomes easy
and safe, since they are free from criticality accidents.
Application of this burnup strategy to neutron rich fast reactors makes excellent
performances. For this reactor some fissile material such as plutonium is required only for the
nuclear ignition region of the initial core, but only natural or depleted uranium is required for the
other region of the initial core and for succeeding cores. The average burnup of the spent fuel is
about 40 % that is equivalent to 40 % utilization of the natural uranium without the reprocessing
and enrichment. However, the application to fast reactors requires considerable design changes.
The application of CANDLE burnup strategy to block-type-fuel high temperature reactor
requires little design change. By introducing diluted burnable poison to replacing fuel makes
possible to perform this burnup strategy.
The fast reactor and thermal reactor give very different characteristics. Mechanism for
realizing CANDLE burnup is also very different. Principles of CANDLE burnup and comparison
of these applications are presented.
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