A reduced flow rate results in reduced heat removal and eventually damage to the fuel pin. If the pin is sodium-bonded then this damage will first appear as bond vaporization with consequent insulation of the fuel. Thus failure of the fuel pin might be considered to start when the fuel surface heat flux exceeds a critical value for sodium vaporization. This is rather similar to pool boiling and the critical heat flux (CHF) is given by Eq. (3.1).

CHF = 534,000 р0Л1 Btu/hr-ft2 (3.1)

where p is pressure (psia) (lb).

Such a failure criterion corresponds to PWR burn-out conditions. The criterion can be modified by local conditions in any particular design and may possibly be correlated to fuel surface temperature for a particular bond pressure. In one design in which it was calculated that when the fuel surface temperature exceeded 1900°F, the critical heat flux was attained, this sur­face temperature could itself be used as a failure criterion.

Bond boiling would effectively insulate the hot fuel until fuel melting had occurred. The molten fuel then might come into contact with the cladding, so that although vaporization of the bond might not burst the cladding, molten fuel in contact with the cladding or fuel vaporization would.

This failure criterion does not however apply to gas-bonded oxide fuel pins and other failure criteria are required.