  It has been suggested that the anomalous Na, Mg and Al observed in Globular
Cluster Red Giant stars could be the result of a thermally unstable hydrogen
shell. Currently accepted reaction rates indicate that temperatures of
approximately 70-75 million K are required to produce the observed enhancements
in Na and Al along with depletions in Mg.
  The work presented here attempts to model the H shell instability by a simple
mechanism of altering the energy production in the region of the H shell. We
show that even extreme cases only give rise to small intermittent temperature
increases that have minimal affect on the surface abundances. Full evolutionary
modelling incorporating this technique simply accelerates the evolution of the
RGB phase producing the same surface abundances as other models but at an
earlier time. We conclude that, unless hydrogen shell instabilities manifest
themselves quite differently, they are unlikely to lead to the required
temperatures and alternative explanations of the abundance anomalies are more
promising.
