Criteria for Fuel Rod Integrity

The fuel rod cladding material of the Super LWR is being developed and tested. For fuel rod design of the Super LWR in the cmicept development phase, typical austenitic stainless steels or Ni-alloys are applied as described in Chap. 2. The principle of the safety criteria for fuel rod integrity is shown in Table 6.6. Since heat transfer deterioration is a much milder phenomenon than boiling transition, the minimum deterioration heat flux ratio was eliminated from the transient criterion related to fuel rod heat-up [7]. The types of abnormalities are separated into loss of cooling and overpower . 

For the loss of cooling type accidents, the requirement is to maintain a cool-able geometry, as in LWRs. The limiting failure mode is expected to be oxidation of the cladding. The criterion of the cladding temperature is set at 1,260 C for stainless steels, taken from the criterion for LOCA of early US PWRs with stainless steel cladding [8]. The criterion for Ni-alloys is also set at 1,260 C because the oxidatimi behavior of Ni-alloys is expected to be similar to that of stainless steels. 

For the overpower type transients, the Umiting failure mode is expected to be burst or pellet cladding mechanical interaction (PCMI). The maximum allowable powers listed in Table 2.8.1 are determined through the fuel rod thermal and mechanical analyses in Chap. 2, taking several conservatisms so as to prevent melting of the pellet centerline and keep the circumferential strain on the cladding 

Accident No excessive damage Enthalpy Limit (RIA) Oxidation Umit MSCT Limit 

Transient No systematic damage aP on ciad. Limit Plastic strain Limit Pellet temp. Limit Plastic strain Limjl Enthalpy Liit f  

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Table 1.9 Principle of safety criteria for fuel rod integrity...

Table 1.11 New criteria for fuel rod integrity for abnormal transients...

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