ATWS IMPROVEMENTS AT BFN

摘要

In the summer of 2017, Browns Ferry Nuclear Plant (BFN) was preparing to implement Extended Power Uprate (EPU), raising reactor power to 120% of original rated reactor thermal power. One of the goals for implementation of EPU was to identify any areas that could cause a challenge for the operators. One area in particular was Anticipated Transient Without Scram (ATWS) in which timely operator response at EPU is required. Studies have shown that, for the worst case scenarios, flow induced power oscillations may start in 60 seconds and large irregular power oscillations may cause fuel damage in 120 seconds. This requires prioritization of terminating and preventing injection to the Reactor Pressure Vessel (RPV) in order to lower core inlet subcooling. This prioritization supersedes even injection of Standby Liquid Control (SLC). An additional issue with operation at EPU is that an ATWS with Main Steam Isolation Valve (MSIV) isolation could result in reactor power being greater than all available injection sources to the RPV. This would result in RPV water level being below the level defined as providing adequate core cooling. Both of these issues would be even more critical for any future approval and operation with Maximum Load Line Limit Analysis - Plus (MELLLA+). A two-fold approach was used to address these issues. The first was to reorder and prioritize the actions taken by operators at the onset of the ATWS. The second was to evaluate the adequacy of the current Emergency Operating Instructions (EOI) covering ATWS, and to make recommendations the BWR Owners Group (BWROG) for future revisions to the EOIs.