Designing Study on large-sized Steam Generators of Sodium cooled FBR; Measures to diminish the influence of water leak

Kisohara, Naoyuki ; Hori, Toru; Soman, Yoshindo; not registered

In case of heat transfer tubes failure in sodium heated Steam Generators (SG), the secondary sodium pressure rapidly increases due to the hydrogen generated by Na/water reaction. The Na/water reaction mitigation system terminates this phenomena immediately without any influence on the integrity of the sodium circuit. Therefore, the water leak has no effect on the reactor and plant safety. However, not only safety but also economy and public acceptance are required for practical FBRs. As for economy, it is necessary to protect the investment of SGs and to avoid the decline of plant availability due to water leak. In addition, decreasing the possibility of Na/water reaction accidents is also taken into consideration in order to promote the public acceptance of FBRs. For these purposes, a double wall straight tubes (DWT)-SG and a single wall helical coil tubes SG are selected as the candidates for the FBR's SG. The DWT-SG has the potential to exclude Na/water reaction by its dual boundaries between Na and water. Although the helical coi1 SG provides single wall tubes, this SG have proven to be developed in Japan and the high reliability can be attained by a lot of knowledge and experience of the SG test facility and the prototype reactor. In terms of the avoidance of Na/water reaction, the DWT-SG is the first candidate. However, there are many issues to be solved for the DWT-SG, the single wall helical coil tubes SG is regarded as the second candidate for an alternative. This report describes the method to prevent or minimize Na/water reaction for both of the SGs. The ultra sonic test (UT) method during the periodical plant inspection is applied to DWT-SG to prevent inner and outer tube simultaneous failure. The preliminary ISI test and failure analysis of the DWT indicate the potential of avoiding the penetrated failure of DWT. However, crack detection tests by UT and crack development analysis due to DNB are indispensable to confirm this methodology to exclude ...