CEGB analysis of a steam line break fault in the Sizewell B PWR

This paper describes the analysis of a postulated fault sequence in the Sizewell B PWR, consisting of a rupture of a main steam line, followed by failure of one rod cluster control assembly (RCCA) to insert on reactor trip and failure of two main steamline isolation valves (MSIVs) to close. A 4-loop RETRAN-02 model is used to predict the transient response. Reactivity feedback is modeled within RETRAN-02 by evaluating a reactivity function which has been derived from detailed reactor physics calculations using the multi-dimensional neutronics code AKTS. The sub-channel analysis code VIPRE-01 is then used to calculate the departure from nucleate boiling ratio (DNBR) at the time of peak power during the return to power phase of the transient. The minimum calculated DNBR is 3.08 which is considerably greater than the allowable limit of 1.3, indicating that dry-out will not occur in this fault sequence. The new calculation is compared with the analysis of the same fault presented in the Sizewell B pre construction safety report (PCSR), which was performed using proprietary Westinghouse codes