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Unstable flow can occur in the form of flow
oscillations or flow reversals. Flow oscillations are
variations in flow due to void
formations or mechanical obstructions from design andmanufacturing.
A flow oscillation in one reactor coolant channel sometimes
causes flowoscillations
in the surrounding coolant channels due to flow
redistribution. Flow oscillations areundesirable
for several reasons. First, sustained flow oscillations can
cause undesirable forcedmechanical
vibration of components. This can lead to failure of those
components due to fatigue.Second,
flow oscillations can cause system control problems of
particular importance in liquidcoolednuclear
reactors because the coolant is also used as the moderator.
Third, flowoscillations
affect the local heat transfer characteristics and boiling.
It has been found throughtesting
that the critical heat flux (CHF) required for departure from
nucleate boiling (DNB) canbe
lowered by as much as 40% when flow is oscillating. This
severely reduces the thermal limitand
the power density along the length of the reactor core.
Again, it has been found throughtesting
that flow oscillations are not a significant problem for some
pressurized water reactorsunless
power is above 150% for the normal flow conditions. Flow
oscillations can be a problemduring
natural circulation operations because of the low flow rates
present.
During natural circulation, the steam bubbles
formed during a flow oscillation may have enough of
an effect to actually cause complete flow reversal in the
affected channel.Both
the flow oscillations and flow reversals lead to a very
unstable condition since the steamblankets
formed on heated surfaces directly affect the ability to
transfer heat away from those surfaces.
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