Methods for protection of nuclear reactors from thermal hydraulic/neutronic core instability

A method for protecting a nuclear reactor core from fuel damage due to thermal hydraulic instability in an extended operating domain for an extended power uprate, comprising:

• generating a power-flow operating map, comprising:
• establishing a natural circulation curve;
• establishing a Maximum Extended Load Line Limit Analysis (MELLLA) control rod line;
• and
• establishing a higher MELLLA+ control rod line positioned above the MELLLA control rod line;
• forming a MELLLA+ domain above the MELLLA control rod line, up to the higher MELLLA+ control rod line;
• identifying a predetermined core flow rate;
• calculating by a thermal hydraulic code simulator a thermal hydraulic stability limit for core flow rates less than or equal to the predetermined core flow rate;
• determining a power level that corresponds to the thermal hydraulic stability limit at different core flow rates that are less than or equal to the predetermined core flow rate;
• providing an Average Power Range Monitor (APRM) flow-biased scram line;
• lowering the APRM flow-biased scram line to below the thermal hydraulic stability limit based on the core flow rate being less than or equal to the predetermined core flow rate;
• and
• when operating in the MELLLA+ domain, actuating an APRM-based trip system based on a core flow decrease resulting from a flow decrease transient and power increases above the power level to provide an automatic anticipatory scram or a selected rod insertion trip to cause a power reduction.