12.91. The temperature of the fuel rod cladding would rise to a maxi­mum in the reflood stage after an LOCA. The integrity of the cladding must be maintained so that it would not fragment under the thermal stress subsequently imposed when the very hot fuel rods are quenched. Embrit­tlement of zircaloy, which could lead to fragmentation, is a function of the temperature and degree of oxidation. Criteria (1) and (2) are intended to preclude the embrittlement (and melting) of the cladding.

12.92. The objective of criterion (3), which limits the amount of hy­drogen gas produced by the zirconium-steam (or — water) reaction, is to keep the concentration of the gas in the containment vessel well below that at which a hydrogen-air mixture would ignite.

12.93. Local swelling (or ballooning) of the cladding, which might result from the expansion of contained fission-product gases, could affect the flow of coolant water through the core. According to criterion (4), cal­culations must show that despite changes in its internal geometry, the core should remain coolable during the reflood stage. The purpose of criterion (5) is obvious and does not require discussion.

12.94. The conservative nature of the foregoing criteria assures an ad­equate margin of performance of the ECCS should a design basis LOCA

occur. This margin is provided by the criteria themselves as well as by conservative features of the evaluation models (§12.132 et seq.). The con­servative features include the following:

1. The calculation of stored heat in the fuel is based on a preaccident reactor power level of 102 percent of the maximum operating power, with the highest allowed peaking factor (§9.167) and the lowest estimated thermal conductance between the fuel pellets and the cladding (§9.50).

2. Heat transfer during blowdown must be calculated using NRC-approved realistic models having an extensive experimental basis.

3. The fission-product decay heat is taken to be 20 percent greater than in the ANS standard (§2.217). The use of the Baker-Just relationship (§7.130) for calculating the heat generation rate from the zirconium-water reaction is also conservative.

4. The peak cladding temperature of 2200°F (1204°C) refers to the hottest region of the hottest fuel rod. This criterion ensures that there would be very little damage to the reactore core.