The Consequences of a Sharp Temperature Change in the Fuel Pins of an Accelerator-Driven Subcritical System
Résumé
The effect of temperature changes and in particular
those that are accompanied by strong gradients was extensively
investigated for fast reactors. Subcritical systems
designed for their transmutation ability are to some
extent similar to critical power reactors in their subassembly
structure. However, they differ in two main
aspects. First, the coolant in a subcritical system is lead
or lead-bismuth eutectic (LBE) and not sodium, and second,
the main cause for steep temperature gradients in a
fast power reactor is sudden control rod insertion, or
scram, whereas in subcritical systems shutdown of the
accelerator and its proton beam is the main cause for
temperature gradients. Furthermore, the increased probability
of operational interruptions in an acceleratordriven
system is largely due to the instability of the
accelerator generating the proton beam.
This study uses the knowledge gained from fast reactors
as a preliminary reference and concentrates further
on the unique features of the proposed subcritical
systems.
In particular, the effect of beam trips on the fuel pin
integrity is evaluated as a function of the temperature
gradients and the duration of the beam trips. It seems,
however, that the largest hazard to the fuel pin integrity
is due to the lead (or LBE) coolant. In particular, the
stability of the protective oxide layer built on the clad
surface with the lead coolant appears quite sensitive to
sudden temperature changes. In the second part of this
study, several available experimental results show that
even very moderate temperature changes are sufficient to
cause crack formation in the oxide layer thereby exposing
the clad surface to enhanced LBE corrosion. In the
worst case, complete exfoliation of the magnetite outer
layer is observed. As a consequence, clad failure probability
due to corrosion is considerably increased.