Civil Engineering Reference
In-Depth Information
Fig. 2.9
Post-shut down
afterheat of a PWR core as a
function of time (initial
enrichment 3.2 % U-235,
burnup 32,000 MWd(th)/t)
[
11
]
2.10 Non-steady State Power Conditions and Negative
Temperature Feedback Effects
Power reactors are generally operated at constant criticality or steady power.
Exemptions are: startup conditions and power rise, transition from partial to full
load power, reactor shut down and accidental conditions. Accidental conditions
must be analyzed and presented to licensing authorities prior to begin of reactor
operation. Such accidental conditions are, e.g. inadvertent or faulty withdrawal of
absorber/control elements (increase of the criticality or effective multiplication
factor, k
eff
, above 1.0) or coolant loss as a consequence of pipe rupture or a faulty
opening of valves followed by primary coolant pressure loss. An increase of the
criticality or effective multiplication factor, k
eff
, provokes an increase of fission
reactions and a rise of power as a function of time. The increase of power results in
an increase of fuel temperature and by thermal conduction—with a certain time
delay—also to an increase of the coolant temperature. As already mentioned this
results in important negative feedback effects which counteract the power increase.
These negative feedback effects and the delayed neutrons allow the safe control of
nuclear reactors. These important negative feedback effects—already mentioned
above—will be explained now in more detail.
