The maximum reactivity insertion rate of the control rod is limited so as to
prevent loss of coolant boundary integrity or core internal integrity. For that
purpose, the following measures are taken.
• Limit the number of control rods that can be simultaneously withdrawn.
• Limit the maximum withdrawal velocity.
(2) Self controllability
The core has an instantaneous negative feedback characteristic. That means
the following.
• Mixed oxide (MOX) fuel itself has negative temperature coefficients based on the Doppler effect and thermal expansion.
• By combining the temperature coefficients of fuel, structure, coolant and core support plate, the power reactivity coefficient is negative.
|
Table 4.1 Major design principles of LMFBR Core [6]
|
Item
|
Design principle
|
Example of criteria
|
|
Reactor shutdown
|
(a) Two independent shutdown systems are provided
(b) At least one system can shut down the reactor at low temperature with the required shutdown margin even if the control rod with the highest reactivity worth is fully withdrawn and stuck
(c) Reactor is kept subcritical at low temperature even if one shutdown system is assumed to fail
|
Reactor shutdown margin Over 0.001 Ak/k at low temperature
|
|
Limitation of
|
(a) Maximum reactivity insertion rate
|
Maximum reactivity insertion rate
|
|
reactivity insertion rate
|
of control rod is limited so that rod withdrawal at maximum possible velocity leads to neither loss of coolant boundary integrity nor failure of core, reactor internals etc which deteriorate core cooling
|
Below 8.5 x 10“ 5(Ak/k)/s
|
|
Self controllability
|
(a) Doppler coefficient and power reactivity coefficient, which is obtained by combining all kinds of reactivity coefficients such as temperature coefficients of fuel, structure, coolant and core support plate, is kept negative under all operating conditions in order to provide negative feedback characteristic
|
Doppler coefficient
dk
-5.7to -7.6 x 10-3T
dT
|
|
Fuel integrity
|
(a) Maximum temperature of fuel cladding is limited
(b) Maximum fuel centerline temperature is kept below melting point of fuel pellets
(c) Maximum burnup of fuel assembly is limited.
|
(a) Below 675 °C at normal operation and 830 °C at abnormal operational occurrences (center of wall thickness)
(b) Below 2 650 °C for unirradiated fuel
(c) Below 93,000 MWd/t
|
|
Limitation of
|
(a) At normal operation and anticipated
|
Maximum linear heat rate
|
|
power
distribution
|
operational occurrences, adequate power distribution is kept so that allowable design limit of fuel is not exceeded
(b) Power distribution is flattened in order to efficiently take out thermal power. Core is divided into two regions with different plutonium enrichments. Outer core has higher plutonium enrichment
(c) Maximum linear heat rate is limited.
|
Below 360 W/cm
|
|
Stability
|
(a) Throughout the operation period, power oscillation which leads to an
|
Abnormal oscillation of power distribution does not occur because
|
(continued)
|
|
Table 4.1 (continued)
|
Item
|
Design principle
|
Example of criteria
|
|
Limitation of coolant temperature
|
excess of allowable design limit of fuel is prevented at normal operation and anticipated operational occurrences
(a) Coolant temperature is kept below boiling point at normal operation, anticipated operational occurrences and design basis accidents
|
no fission products have large absorption cross section in the fast reactor energy range
|
|
Breeding
performance
|
(a) Adequate breeding ratio is achieved
|
About 1.2 as target
|
|
• Increase in the power is suppressed by the negative reactivity feedback even at anticipated operational occurrences.
