In a gas-bonded fuel pin, failure might be defined as being coincident with cladding rupture, which, if the pin were originally unvented, would allow the release of fission gases.

The rupture will depend on conditions within the fuel pin that depend in
turn on the power history of the pin and its burn-up. The structural changes may be summarized as follows:

a. At start-of-life. Figure 3.1 shows a cross section of a gas-bonded, stainless-steel-clad oxide pellet with a fuel density of from 85 to 95% theo­retical density. The design includes a 5 mil cold gap between the fuel and the cladding.

Fig. 3.1. Structure of fuel, cold and at the start-of-life.

b. At low bum-up. Fuel above the sintering temperature (3272°F) sinters and lenticular voids migrate up the temperature gradient and a central void is formed. The fuel above the sintering temperature attains the theoretical density and columnar grains mark the migration routes of the lenticular voids. Fuel at less than the sintering temperature remains at its original density. The void migration rates fall off rapidly over a small radius in­crement.

c. At higher burn-up. Continued irradiation (to about 25,000 MWD/tonne) causes fuel swelling and fission gases are generated. The rate of fuel swelling is higher than that of the cladding and so the fuel meets the cladding and then there will be a fuel-cladding contact in the higher rated regions. Fission gases are released from the fuel and they diffuse to the fuel pin gas plenum (Fig. 3.2).

d. Later in life. At about 70,000 MWD/tonne the central void starts to decrease and could close eventually. The cladding at the end-of-life is weaker due to thermal cycling, erosion, and corrosion. Reductions in cladding tensile strength have been reported of between 15 and 30% over 90,000 hr irradiation {2a), and the strain on the cladding might be as much

as % (2b). At this stage the fuel pin is most sensitive to adverse conditions as in addition, the pressure of fission gases within the fuel pin plenum may be between 800 and 3500 psia depending on the size of plenum (2a).

It is assumed that the amount of fission gas released from the fuel is a function of the temperature of the fuel (3a, b): 100% is released at tempera­tures over 3272°F; 50% is released at temperatures between 2912°F and 3272°F; and 4% is released at temperatures less than 2912°F. Thus the total gas released can be calculated by integrating over the fuel temperature distribution. Values of about 65-70% can be expected. During a transient when the temperatures increase little additional gas will emerge.

In addition to this normal operational data, the TREAT facility has provided additional information on abnormal conditions in a transient (4):

(a) Cladding deformations of greater than 1% result in gross cracking on the inner cladding surface due to heavy grain precipitation.

(b) No foaming of irradiated oxide fuel occurs even when 70-80 vol % of the fuel is melted.

(c) Agreement with the calculated temperatures is fairly good. This indi­cates that the fuel condition can be successfully modeled.

(d) In irradiated fuel, the failure mechanism appears to be cladding melting due to contact with molten fuel.

With this information the following failure criteria may be derived.