Study on safety characteristics of a sodium-cooled fast reactor with negative void reactivity during initiating phase in severe accident

負のボイド反応度を有するナトリウム冷却高速炉の炉心損傷初期過程の安全特性に関する研究

石田 真也  ; 深野 義隆  ; 飛田 吉春; 岡野 靖

Ishida, Shinya; Fukano, Yoshitaka; Tobita, Yoshiharu; Okano, Yasushi

One of the effective design measures against core disruptive accident (CDA) is to decrease void reactivity, and a sodium-cooled fast reactor (SFR) with low void reactivity has been developed to improve reactor safety for future SFR. The evaluation of small SFRs, which can have a negative void reactivity coefficient, is useful to examine the reactor characteristics for future research and development. The event progression of unprotected loss of flow (ULOF), which is a typical initiating event of CDA, was analyzed by the SAS4A code. In comparison with a general behaviour of large SFR with relatively higher positive void reactivity, it was confirmed that the low void reactivity reactor has the following characteristics: (1) Event progression becomes slow and mild. (2) Positive reactivity insertion by a cladding melting and relocation has larger importance. (3) Generating mechanical energy during the initiating phase becomes less likely to occur.