When the fuel center temperature attains 6500°F the vapor pressure starts to rise rapidly from 10 to 100 atm at 7250°F. Under these conditions the high pressures could force molten fuel through cracks into contact with cladding and cause cladding failure.

On the other hand, if there is a large amount of fuel melting the excessive fission gas release will cause excessive pressures on the cladding. The amount of molten fuel judged to be excessive depends, of course, on the condition of the cladding and the burn-up of the fuel.

Thus at start-, middle — and end-of-life, a fuel temperature of 6500°F may be judged to portend failure. However, 60 mil of fuel melting (25% areal extent) represents failure at the start-of-life while 20 mil may represent a similar failure at the end-of-life due to the weakening of the cladding and the rise of fission gas pressures.

The position of the failure depends on the pattern of fuel melting (5).

The modeling and the analysis is borne out by experimental results of failures and by radiographs of fuel structure and cladding cracking at various burn-up levels. This latter information is unfortunately only obtained when the fuel has cooled off, of course.