12.90. To provide assurance that the ECCS is designed so that its op­eration can prevent a significant release of radioactivity to the environment, the NRC requires in 10 CFR 50.46 that, in a postulated design basis LOCA
in a light-water reactor (PWR or BWR), the following criteria should be met:

1. The calculated maximum fuel cladding temperature following the accident should not exceed 2200°F (1204°C).

2. The calculated total oxidation of the cladding, as a result of the interaction of the hot zircaloy with steam, shall nowhere exceed 0.17 times the total cladding thickness before oxidation. (The total oxidation is defined as the thickness of cladding that has been converted into oxide, assumed to be stoichiometric Zr02. A modified definition is used where the calculations indicate that swelling or rupture of the cladding is to be expected.)

3. The calculated total amount of hydrogen gas generated, by the chemical reaction of zirconium in the cladding with liquid water and steam, shall not exceed 1 percent of the hypothetical amount that would be generated if all the cladding material surrounding the fuel pellets, i. e., within the active core, were to react.

4. Calculated changes in the geometry, e. g., in fuel rod diameters and spacing, shall be such that the core remains amenable to cooling.

5. After successful initial operation of the ECCS, the calculated core temper­ature shall be maintained at an acceptably low value for the extended period of time required by the decay of the long-lived radioactivity remaining in the core.