KURENAI : Kyoto University Research Information Repository

I.1 INTRODUCTION
One of the most important phases of nuclear reactor transients is
a power transient where the reactor power increases exponentially with
time due to an excessive reactivity in the reactor core. When such a
transient occurs, the temperature of the fuel surface increases rapidly and
then transient boiling occurs. Further increase of the power will result
in a transition from nucleate boiling to film boiling (transient burnout).
Therefore, in evaluating the behavior of reactor coolant and fuel
temperature in a reactivity accident, it is quite important to know
the forced convective transient boiling heat transfer process for expone-
tial power increases.
Usually, the steam quality of a two-phase flow in normal operating
conditions of a nuclear reactor is very low. Furthermore, under the rapid
power increase, the transient boiling process is considered to be a local
phenomenon. This means the phenomena are determined only by the local
conditions and not dependent on the integrated effects of the upstream
conditions.
Therefore, the boiling and two-phase flow phenomena under the reactivity
accident in a nuclear reactor correspond to the phenomena with a small
length to diameter ratio, Q/dh e (Q; heater length, dhe;heated equivalent
diameter of a flow passage), where the boiling phenomena are localized-add
occur in a low quality two-phase flow.
In view of these, in this chapter, the transient boiling phenomena
under exponential power increase have been studied experimentally, using
small Q/dhe heaters and under low steam quality conditions.
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