As the fuel slumps against the subassembly duct wall, heat fluxes reach several million Btu/hr-ft2. The exact configuration of fuel against the wall does not matter because, even with small amounts, the fuel farthest away from the wall is soon at its vaporization temperature of 6500°F if no credit is taken for heat removal in the failed assembly.

A heat transfer calculation for this slumped fuel and the two adjacent subassembly walls and the neighboring subassembly may be calculated for a simple slab model to determine whether boiling (and thus an effective further failure) can be induced in the second subassembly. Figure 4.18 shows some typical results in which the adjacent subassembly is analyzed for its temperature distribution, a distribution due to an incoming heat flux from its neighbor. The COBRA code (29, 30) was used to compute the cross flows and the temperatures in each subchannel. In the particular case shown, the highest coolant temperatures occur in the shut-down case in which it is assumed that the reactivity feedback from the failed assembly has given a high flux signal and tripped the reactor. In this case a maximum tempera­ture of 1660°F is reached, which shows that boiling may occur only if the system is at atmospheric pressure. However, primary systems are usually slightly pressurized and boiling would not occur. Not only does the slumped

fuel result in high heat fluxes to the adjacent can wall, but it also melts a certain amount of the can wall itself, attaining some sort of steady state in about 10-20 sec.

The final consequence of the subassembly failure depends on whether the reactor has been tripped or not due to reactivity feedbacks, whether or not the subassembly duct has been ruptured due to high pressure pulses, whether or not high heat fluxes have caused adjacent channel boiling, and what the final configuration of fuel in the failed subassembly is.

These answers depend on some experimental information that is still required: How does the film remaining after voiding grow and dry out, how does fragmented fuel behave in a voiding environment, how does fuel fragment, how do molten fuel pins slump, what is the response of the duct to pressure pulses within the assembly and locally against the duct wall, etc. It is expected that when these answers are forthcoming, surety will be obtained that failure cannot propagate beyond a single subassembly.