Patent Number: 
Section: claims

1. A method for determining an operating margin for a nuclear fuel core of a Boiling Water Reactor (BWR), wherein fuel design or core configuration are contingent upon an operating margin for the reactor, said operating margin being determined by a process for evaluating an operating limit minimum critical power ratio (OLMCPR) comprising: a) developing in a memory of a computer system distributions of values for a change in critical power ratio to initial critical power ratio (xcex94CPR/ICPR) corresponding to a plurality of generic computer simulations of a plurality of transient operational reactor occurrences, said simulations each providing a xcex94CPR/ICPR distribution with respect to a corresponding generic transient operational reactor occurrence;  b) selecting, from the xcex94CPR/ICPR distributions generated by the simulations, the xcex94CPR/ICPR distribution corresponding to the generic transient operational reactor occurrence related to the BWR operating limit determination;  c) for the operating limit determination, developing in the memory of the computer system a histogram of NRSBT (number of fuel rods subject to boiling transition) corresponding to a plurality of transient minimum critical power ratios (CPRs) by determining initial critical power ratios (ICPRs) for one or more fuel rods and applying the selected xcex94CPR/ICPR to determine the transient minimum CPRs for said BWR;  d) selecting a nominal NRSBT value, based on a central tendency of NRSBT distribution, statistically determined by said computer system from the histogram of the developed NRSBT values;  e) selecting a confidence interval for the nominal NRSBT value;  f) selecting an OLMCPR value from a minimal CPR such that during a simulation of a limiting transient operational reactor occurrence the nominal NRSBT value remains less than a predetermined cutoff value; and  g) effecting said Boiling Water Reactor operation by applying the selected OLMCPR as an operational control parameter. 2. The method  claim 1  wherein said histogram of values for xcex94CPR/ICPR is developed on a data processing system using multi-dimensional modeling of transient operational occurrences, said data processing system including memory to store CPR data obtained from said simulations. claim 1 3. A method for determining an operating margin for a nuclear fuel core of a Boiling Water Reactor (BWR), wherein fuel rod design or core configuration are contingent upon an operating margin for the reactor, said operating margin being determined by a process for evaluating an operating limit minimum critical power ratio (OLMCPR), said method comprising: a) developing in a memory of said computer distributions of values for changes in critical power ratio to initial critical power ratio (xcex94CPR/ICPR) corresponding to a plurality of computer simulations of generic transient operational reactor occurrences, said simulations each providing a xcex94CPR/ICPR distribution corresponding to the generic transient operational reactor occurrences, said xcex94CPR/ICPR distributions each based on a predetermined probability distribution of change in critical power ratio (xcex94CPR/ICPR) for a corresponding reactor transient operational occurrence;  b) selecting a one of the xcex94CPR/ICPR distributions that corresponds to the generic transient operational reactor occurrence related to the BWR operating limit determination;  c) developing in the memory of the computer system a histogram of NRSBT (number of fuel rods subject to boiling transition) values corresponding to the operating limit determination by determining initial critical power ratios (ICPRs) for one or more fuel rods and applying the selected xcex94CPR/ICPR distribution to determine the transient minimum CPRs;  d) calculating a nominal NRSBT value, based on a central tendency of an NRSBT distribution, from the histogram of NRSBT values;  e) selecting a confidence interval for the nominal NRSBT value;  f) selecting an OLMCPR for the reactor as the initial minimal MCPR such that during a simulation of a transient the nominal NRSBT value remains less than a predetermined cutoff value; and  g) effecting Boiling Water Reactor operation by applying the selected OLMCPR as an operational control parameter. 4. For use in controlling a Boiling Water Reactor (BWR) having a nuclear fueled core characterized by an operating limit minimum critical power ratio (OLMCPR) value, a method for determining said OLMCPR value comprising: a) using a computer to simulate generic transient operational reactor occurrences based on predetermined generic occurrences which might occur during the operation of the Boiling Water Reactor;  b) developing distributions of values for a change in critical power ratio to initial critical power ratio (xcex94CPR/ICPR) corresponding to a plurality of the computer simulations of the generic transient operational reactor occurrences, said simulations each providing a xcex94CPR/ICPR distribution for a corresponding generic transient operational reactor occurrence;  c) selecting one of the xcex94CPR/ICPR distributions corresponding to the generic reactor transient operational reactor occurrence under consideration for the BWR operating limit determination;  d) for the operating limit determination, developing a histogram of number of fuel rods subject to boiling transition (NRSBT) corresponding to a plurality of transient minimum CPRs by determining an initial critical power ratios (ICPRs) for one or more fuel rods for multiple parametric quantities that are relevant to ICPR of said reactor and applying the identified xcex94CPR/ICPR distributions to determine the transient minimum CPRs;  e) determining a nominal NRSBT value, based on a central tendency of an NRSBT distribution, from the histogram of developed NRSBT values;  f) selecting a confidence interval for the nominal NRSBT value; and  g) selecting an OLMCPR for the reactor as the initial minimal MCPR such that during a simulation of a transient the nominal NRSBT value remains less than a predetermined cutoff value.