The previous sections have outlined small parts of the whole picture: fuel failure, molten fuel jet impingement on adjacent pins, fuel fragmenta­tion, fuel velocities in channel, and voiding mechanics. It is now important to try to draw these pieces into a whole description of the sequence of events. The consequences of fuel failure should be determined in sufficient detail to establish what protection can be provided and what probability there is of a propagation of the failure.

Section 4.4.1.1 has shown that the propagation of failure due to fission — gas blanketing alone is unlikely and only in some certain circumstances could a secondary failure be caused. In this case for primary ruptures in the region of 10-4 in.2 area, a secondary rupture could be formed imme­diately opposite the primary one across the subchannel. The only place a third rupture could be formed would be back on the original pin. This A to В and В to A sequence is unlikely to spread the damage across the sub­assembly, especially since each rupture size must be that critical size to just give rise to the necessary conditions for continuing the process of failure.

Section 4.4.1.3 showed that molten fuel could eject out of a pin which already contained fuel and cause a jet failure on the next pin. However, again the failure sequence would be of the A to В and В to A type, which is unlikely to provide a tertiary rupture.

Section 4.4.1.4 has however discussed the voiding of the subassembly and the mechanism for failure throughout the subassembly is provided. Table 4.6 suggests a failure sequence using the information from the pre­vious sections (24a). (Essentially, it provides more detail for the earlier portions of Table 5.8, which describes the same sequence.)

TABLE 4.6

Overenriched Fuel Pin Failure Sequence0

Time

(msec)

Overenriched hot pin ruptures as molten fuel contacts cladding during minor reactivity transient

Subchannel voided around failed pin, pressure about 1000 psia

Whole assembly voided, pressure about 150 psia

(Failure of adjacent cladding due to molten fuel jet impingement)

Cladding failure on the adjacent enriched peak pin

Cladding failure on all enriched peak pins in assembly

Pins adjacent to original failure melt (about four or five of them)

Film on voided channels at maximum thickness following condensation

Film dry-out following reduction in thickness.

Molten fuel ejection ends following an intermittent ejection Sodium reentry into voided channels (vapor explosion?)

Enriched pins molten and start to slump in contact with assembly can Normally enriched pins molten and start to slump Assembly duct experiences heat fluxes up to 2 x 109 Btu/ft2 hr

° See Graham and Versteeg (23b).

b Cannot occur if the reentering flow reestablishes itself and is not blocked.

The overenriched hot pin is presumed to fail as molten fuel contacts the cladding due to some minor transient and the molten fuel is ejected. The subchannel voids rapidly in 1 msec with immediate pressures of 1000 psia. Then the void spreads more slowly across the subassembly, so that the whole subassembly is voided in about 8 msec and the pressures have been reduced to 150 psia. At about this time the molten fuel might also, by jet impingement on the adjacent cladding, have caused a secondary failure.

Due to the voiding, the cladding will fail on the adjacent enriched peak pins in about 25 msec and on all pins in the assembly in about 35 msec. However this is not significant since little molten fuel is present. Then the
pins nearest the failure begin to melt and molten fuel may appear from at most 4 or 5 near pins in about 80 msec.

As the void is growing, it is condensing on pins above the failure, and the film on these pins is growing and heating up those components. Later, however, the process reverses and the film dries out in about 200 msec. By 300 msec, the entire fuel ejection process is over from the primary failed pin as well as those near it that melted. Then the sodium vapor-liquid interface reenters in about 600 msec.

At this point, several things could occur, and although detailed calcula­tions might help to clarify this point, experimentation on fuel element failure propagation will be the only way to clarify the actual course of events. The following could occur:

(a) The sodium reentering could impinge upon the molten fuel which is in the channel and cause a sudden vapor explosion much more violent than the original vaporization. It is considered that evidence shows this to be unlikely.

(b) The flow could reestablish itself and normal flow conditions could maintain cooling of the subassembly, even though cladding has largely ruptured.

(c) Molten cladding could have blocked four or five subchannels and the condition changes to a treatment of a local blockage. In this case more than 8-10 subchannels should be blocked before further failure can occur and calculations (25) have shown that the blockage should be coher­ent. Even 1% seepage through the blockage could provide adequate cooling to avoid anything but a slow subsequent continuation of the damaging process.

During this voiding process, the reactivity feedback is small, limited to less that 100 for an entire voided subassembly. However, if the fuel melting results in gross slumping, then the reactivity changes are likely to be larger. These fuel movement reactivity changes could be of either sign, as can the voiding effects. Previous failures in both DFR and Fermi (26, 27) have been marked by negative changes of power, due to failure-induced reactivity feedbacks.

Neglecting the uncertainty of effects at this point, if the pins are now com­pletely deprived of cooling, the enriched pins will be completely molten at the midpoint cross sections in about 3.5 sec, while the unenriched pins will reach the same state in 5.0 sec if we presume this assembly to be made up of a mixture of enriched and nonenriched fuel. Thus the assembly duct would begin to see slumped fuel in contact with it at about this time or shortly afterwards.