Design and Development of Stroke Limiting Device for Control and Safety Rod Drive Mechanisms (CSRDMs) of future FBRs

For Anticipated Transients Without Scram (ATWS), the time margin leading to core damage is relatively small in comparison with Loss of Heat Removal System (LOHRS). Significant mechanical energy release, derived from prompt criticality in a core disruptive accident, has to be practically eliminated, as it has the potential to breach the reactor vessel and to significantly affect the integrity of the containment. Thus, means for passive or inherent reactor shutdown are necessary for core damage prevention. In this context the following are the important decisions related to shutdown systems for next generation SFRs to facilitate practical elimination significant mechanical energy release due to Anticipated Transients Without Scram. i) Strengthen the first two shutdown systems by addition of passive/active features. ii) Introduce an ultimate shutdown system, which is completely diverse, independent, passive & confined within core sub-assembly. This shall come into action on failure of first two systems. In this paper, design augmentation of first shutdown system by addition of an active safety device is discussed. The first shutdown mechanism of future FBR is Control and Safety Rod Drive Mechanism (CSRDM). A Stroke Limiting Device (SLD) is provided in CSRDM to prevent inadvertent withdrawal of Control & Safety Rods beyond a pre-set level and thereby limit the consequences of inadvertent control rod withdrawal event well within limits even with the failure of other safety actions. SLD limits the consequences of inadvertent withdrawal of CSR by physically limiting the withdrawal stroke length of CSR to 20 mm. Two different concepts of SLD have been conceived, manufactured and standalone endurance tested. One of them has also been integrated with CSRDM and tested. Based on the above, Mark-II design of SLD with some additional design improvements is being developed. The final design of SLD (Mark - II) will be adopted in CSRDM for future Fast Breeder Reactors. The details of two concepts developed, testing carried out as part of design validation and design improvements being incorporated in Mark-II design are presented in this paper. (author)