In the failed assembly the pressure may attain 1000 psia locally which reduces to 150 psia when the whole subassembly is voided. Thus the pressure transient is roughly a very rapid rise to the peak followed by an exponential decay over the next 8-10 msec to the lower value.

If the failure is in the center of the subassembly, then explosion work (28a) has shown that the subassembly duct may only experience an atten­uated value of a third of the peak value of pressure. Static calculations for the response of the assembly duct show that even in the irradiated state the duct can withstand several hundreds of psia without failure. However, if the original failure is close to the subassembly duct wall, then the original localized pressure of 1000 psia could cause failure in the corner of a hex­agonal boundary (28b).

In any particular case, inertial calculations may be performed on the subassembly duct since the usual duct thickness is sufficient for the duct to have a natural damped vibrational mode with a period of about 1 msec. It will therefore not respond to acoustic waves generated in shorter times. Thus the duct essentially sees a steady pressure acting upon it even though this pressure may be localized. Any acoustic wave will have a very small invested energy (1 or 2 W-sec) because the relaxation time is so short (40 psec for a central pin). The questions to be answered include:

(a) Does the duct wall fail for localized pressures and over what axial extent does it do so?

(b) If the duct fails, what pressure deformation does the adjacent duct wall experience and does this deformation cause boiling of the coolant in the second subassembly?

If boiling is caused in the second subassembly, then of course this con­stitutes propagation of the failure, since there is nothing now to stop the process of damage from repeating itself though on a slower time scale.

However, calculations to date appear to show that even with considerable deformation, the adjacent assembly does not experience boiling, even when the outer coolant channels are completely constricted (28b).