Patent Number: 
Section: claims

1. A method for determining an operating margin for a nuclear fuel core of a particular 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) determining a generic transient change-in-critical-power-ratio (xcex94CPR/ICPR) distribution of values, wherein said generic transient xcex94CPR/ICPR distribution is based at least upon a pre-computed nominal xcex94CPR/ICPR bias value and an xcex94CPR/ICPR uncertainty;  b) determining initial critical power ratios (ICPRs) for each fuel rod in the core corresponding to a particular BWR plant;  c) determining a plurality of transient Minimum CPRs (MCPRs) for all fuel rods in the fuel core by applying the generic transient xcex94CPR/ICPR distribution to each ICPR determined in (b); and  d) effecting said Boiling Water Reactor operation by applying a selected OLMCPR as an operational control parameter, said OLMCPR selected for the particular BWR by computing a mean NRSBT (number of fuel rods subject to boiling transition) corresponding to a histogram of said plurality of transient Minimum CPRS and selecting an OLMCPR value as a transient Minimum CPR which results in a corresponding mean NRSBT value that is less than a predetermined cutoff value. 2. The method of  claim 1  wherein the pre-computed nominal xcex94CPR/ICPR bias value and uncertainty is determined by conventional statistical analysis of one or more computer simulations of at least one type of reactor transient operational occurrence for a least one class of BWR plant type and for at least one fuel type. claim 1 3. The method of  claim 1  wherein said ICPRs are determined for a particular transient event during a particular fuel cycle. claim 1 4. A method for determining an operating margin for a nuclear fuel core of a particular 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: determining initial critical power ratios (ICPRs) for each fuel rod in a fuel core of the particular BWR;  using a predetermined generic transient change-in-critical-power-ratio (xcex94CPR/ICPR) distribution of values to compute transient Minimum CPRs (MCPRs) for all fuel rods in the fuel core based upon said ICPRs;  computing a mean NRSBT (number of fuel rods subject to boiling transition) corresponding to a histogram of said transient Minimum CPRs;  selecting an operating limit minimum critical power ratio OLMCPR) for the BWR from a histogram of fuel rod transient Minimum CPUs for a particular set of initial conditions that result in a mean NRSBT value that is less than a predetermined cutoff value. 5. The method of  claim 4  wherein e predetermined generic transient xc3x97CPR/ICPR distribution of values is determined by statistical analysis of one or more computer simulations of at least one type of reactor transient operational occurrence for a least one class of BWR plant type and for at least one fuel type. claim 4