Principle of Rationalizing the Criteria for Abnormal Transients

1 Principle of Ensuring the Fuel Integrity at Abnormal Transients 

Abnormal transients are defined as events that will lead to the situation in which the nuclear plant cannot maintain the normal operation due to an external disturbing factor that may occur during the life span of the nuclear plant under the operational conditions including single failure or malfunction of the devices or single operational errors by operators, and to the abnormal situation in which the nuclear plant is not planned to operate and that may occur with the same probability as the former. A set of abnormal transients and accidents as standard safety analysis of the current LWRs is studied for the Super LWR, including the loss of coolant accident (LOCA) and the anticipated transients without scram (ATWS) (see Chap. 6 for details). The requirements for the Super LWR are same as those of LWRs  

The above three requirements are not directly related to parameters that can be easily obtained in the plant safety analyses. Instead of directly examining satisfaction of these requirements, the following criteria had been typically adopted in the safety analyses of the Super LWR. To meet requirement 1, the criterion of the peak 

Among the criteria, the criterion for the peak cladding temperamre had been estimated by simple but conservative calculations. However, it is expected to depend on the material of the cladding and the fuel rod design. Based on the above requirements, the following four principles are adopted to derive rationahzed new criteria for abnormal transients. 

The above principles can be rewritten quantitatively as in the case for LWRs. For example, the same or similar values that have been used for LWRs may be directly adopted as follows the pressure difference on the cladding should be less than 1/3 of the collapse pressure of the cladding, the average plastic strain of the cladding in the radial direction should be less than 1%, the fuel enthalpy is less than 170 cal/ gU02, and the system pressure does not exceed 28.9 MPa (105% of the maximum pressure for normal operation). 

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