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

Abstract

The invention relates to methods for protecting a nuclear reactor core, such as a boiling water reactor core, from fuel and cladding damage due to thermal hydraulic instability in extended operating power flow conditions and, in particular, when an extended power uprate is implemented. The methods employ existing licensed stability methodologies and incorporated minor changes, e.g., to the Average Power Range Monitor (APRM)-based trip system to preclude operation inside the stability vulnerable region of the power/flow map. The APRM-based trip system is modified to set down the APRM flow-biased scram line when core flow is less than a predetermined core flow to prevent the core from entering an unstable region of operation.

What is claimed: 
     
         1 . A method for protecting a nuclear reactor core from fuel damage due to thermal hydraulic instability in an extended operating domain, comprising:
 calculating a thermal hydraulic limit;   identifying an unstable region of operation in a power/flow map;   identifying a predetermined core flow level;   modifying the APRM-based trip system to set down the APRM flow-biased scram line when core flow is less than the predetermined core flow level; and   preventing the nuclear core from entering the unstable region of operation by causing a APRM scram or a selected rod insert when the set down APRM flow-biased scram line is exceeded.   
     
     
         2 . The method of  claim 1 , wherein a set point for the set down of the APRM flow-biased scram line is determined by employing 3D core analyses of the thermal hydraulic stability limit. 
     
     
         3 . The method of  claim 1 , wherein operation of the nuclear reactor is conducted beyond a MELLLA rod control line to a MELLLA+ rod control line above the predetermined core flow level. 
     
     
         4 . The method of  claim 1 , wherein existing OPRM instrumentation is operable to detect and suppress global and local unstable operation in a MELLLA+ region. 
     
     
         5 . The method of  claim 1 , wherein the predetermined core flow level is defined by the thermal hydraulic stability limit. 
     
     
         6 . The method of  claim 1 , wherein the predetermined core flow level is 55% of rated core flow.