WEOD-S: Westinghouse expanded operating domain stability solution

As Extended Power up-rates (EPUs) are implemented in BWR plants, the flow window at full power decreases due to the extension of the rod line. It is thus desirable to raise load line limits to realize increased power generation at a wider flow range offering operational flexibility and fuel cycle efficiency. However, when load lines are raised, the power/flow operating map is changed in a direction that can cause core power instability at its lower left corner (high power/low flow) if a flow reduction transient (i.e. pump trip) occurs. Unstable operation of the reactor core can result in diverging neutron flux (and power) oscillations, and through the thermal hydraulic/neutronic feedback challenge the Safety Limit Minimum Critical Power Ratio (SLMCPR). In many BWRs the SLMCPR in a power oscillation event is already protected by a detect and suppress system. The methodology to determine the set point of this system, the DIVOM methodology (Delta CPR over Initial MCPR versus Oscillation Magnitude), is defined and applicable up to, but not beyond, the thermal hydraulic stability limit. The DIVOM methodology is used to determine the channel power oscillation magnitude that will challenge the SLMCPR. It is defined as the relationship between ΔCPR/ICPR and the Hot Channel Oscillation Magnitude (HCOM). The DIVOM calculations are typically performed at the end state following a design basis two pump trip from rated power and minimum flow. When approaching the thermal hydraulic (T/H) instability limit, the DIVOM curve can become chaotic and the DIVOM approach breaks down. At T/H-instability, small power fluctuations give rise to large flow oscillations and the non-linear dynamic properties emerge. The newly developed Westinghouse Expanded Operating Domain Stability (WEOD-S) solution proactively prevents entry into the regions of the power/flow map that are vulnerable to thermal hydraulic instability. This is achieved automatically, without any dependence on operator action, by adjusting the existing average power range monitor (APRM) flow biased trip system to block entry and provide power suppression scrams or selected rod insert (SRI) trips if the core flow falls below and the core power is above pre-determined set points. WEOD-S is applicable to all fuel types and core designs, and has a limited impact on the current hardware. The thermal hydraulic stability limit and set down of the APRM flow biased scram is verified on a cycle specific basis