PLENTIFUL ENERGY

shutdown system will be activated. All else failing, the operator can manually shut
the reactor down. The unprotected loss-of-flow test in EBR-II simulated the
ultimate scenario where all these safety systems and operator actions have failed
and the reactor is ―on its own.‖
The sequence of events is outlined in the graphs in Figure 7-1. With the reactor
at full power the power to the primary pump was cut off. This immediately reduced
the coolant flow as shown in the bottom left plot. With the reactor producing at full
power, this caused the coolant outlet temperature to rapidly increase (about 200 o C
in thirty seconds) as shown in the top left plot. The rising coolant temperature
causes thermal expansion of the core components, in particular the fuel assembly
hardware, increasing the reactor size a miniscule amount. Slightly less dense than it
was before, neutrons now find it easier to escape, and neutron leakage from the core
increases. This reduces reactivity.
Figure 7-1. Unprotected loss-of-flow test results
During the initial tens of seconds, the mechanical pump inertia provided the flow
coast down necessary to keep coolant temperatures well below local sodium
boiling, enabling gradual transition to natural convection flow through the core. The
negative reactivity feedback is shown in the bottom right plot; the consequent
reduction in reactor power is shown in the top right plot.
As negative reactivity comes in the coolant temperature stops rising, and after
some minutes an asymptotic temperature is reached at equilibrium with the natural
heat loss from the system. The predicted coolant outlet temperature response during
the loss-of-flow without scram test is compared with the actual data from the test in
Figure 7-2. The excellent agreement shown demonstrates the ability to accurately
calculate these events.
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