Patent Number: 047708433
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method and apparatus for accurately determining on-line, and controlling, the stability of the individual fuel assemblies in a boiling water reactor. More particularly, it relates to selecting the fuel assemblies most likely to exhibit instability and performing a stability analysis based on physical solution of the non-linear conservation equations which takes into account nuclear feedback as well as hydraulic effects on the individual fuel assemblies with the effects of cross-coupling included when appropriate. The invention further relates to determining control action required to return a core with unstable fuel assemblies to stable operation, thus providing an effective on-line expert system in real time. 2. Background Information Boiling flow instabilities must be considered in the design and analysis of many devices used in energy production. In particular, such instabilities should be avoided in most apparatus of interest. Sufficiently large excursions and/or oscillations from the steady state can affect the efficiency of the process, erode thermal margins, and may cause physical damage to mechanical components. Flow instabilities are of particular concern in boiling water reactor (BWR) cores. BWR plant operators are under strict Nuclear Regulatory Commission guidelines to be alert for, and to suppress, any flow/nuclear instability. Monitoring is typically done by observing neutron flux signals from local power range monitors (LPRMs), and simultaneously adjusting power/flow conditions to remain below a prespecified core stability limit. In the event that an oscillation is noticed in the LPRM signals, the operator inserts control rods or increases total core flow to attempt to suppress instabilities. Additional control action is then required to bring the plant into a desired configuration. The LPRMs in a BWR are arranged in strings distributed across the reactor core, and a typical BWR may have 160 such detectors. While this means that the operator must monitor a large numer of readings, there are still many fuel assemblies which are not adjacent to an LRPM. This can lead to a safety problem in the form of fuel damage, and release of fission products could also occur, should a fuel assembly some distance away from an LPRM string undergo sustained regional instabilities. Such instabilities may not be detected by the LPRM signals, and hence go unnoticed. Thus, the practice of controlling instabilities through monitoring LPRM signals is cumbersome at best, and could lead to increased downtime and hence economic loss. U.S. Pat. No. 4,319,959 suggests a system for supervising stability in a BWR in which flux readings from the LPRMs, and signals regarding such operating conditions as rod position, the flow quantity of recirculation water and the thermal power of the core determined by the heat balance of the plant, are used to determine values for the coolant flow quantity and thermal power in each assembly. Channel stability of each assembly is then determined using an equation which takes into account hydrodynamic factors such as inlet flow velocity, inlet subcooling, heat flux, and mean pressure. However, axial distribution of power, cross flow between channels of subchannels, and nuclear feedback effects are not considered. In one embodiment, a correlation of signals from LPRMs spaced along a channel is used to determine flow rate. Indications of channel stability can be used as a guide for adjusting control rod position to improve stability. Such an approach uses lump parameters for assemblies adjacent LPRMs and can not provide meaningful indications of stability in assemblies remote from an LPRM. It also requires calculation of stability for all of the assemblies to locate any that might be unstable. It is a primary object of the present invention to provide a method and apparatus for controlling the stability of the fuel assemblies in a boiling water reactor which provides an accurate quantitative evaluation of stability based upon physical laws in real-time for on-line implementation. It is a another object of the invention to achieve the first object by selecting for the accurate quantitative evaluation of stability a limited number of fuel assemblies most susceptible to instability based upon current observed operating conditions of the core. It is yet another object of the invention to provide such a method and apparatus which generates and uses, in said selection and evaluation, distributed values of pertinent core parameters. It is still another object of the invention to provide a method and apparatus which evaluates stability taking into account the nuclear feedback as well as hydrodynamic effects on the stability of the individual fuel assemblies. Finally it is an object of the invention to provide an expert system environment to the plant operator which suggests ways to obviate unstable operation and also to safely recover from unstable operating modes. SUMMARY OF THE INVENTION These and other objects are realized by the invention in which the stability of the fuel assemblies is controlled by measuring on an on-line basis reactor parameters including flow, temperature, control rod position and pressure. A digital computer utilizes these measurements to generate nodal distributions of selected reactor parameters with at least one node per fuel assembly in the radial plane and a plurality of nodes in the axial direction. The computer then calculates from these nodal distributions a stability index for selected fuel assemblies taking into account core physical parameters which are a measure of power level, axial power distribution, flow, enthalpy, void drift, detailed fuel rod dynamics, nuclear reactivity feedback, and where appropriate, flow cross-coupling in the radial and axial directions. The stability indexes, which in the preferred form of the invention are decay ratios, are reported to the operator. The stability of the least stable fuel assembly is, in addition, compared to a prdetermined stability index. If this stability index limit is exceeded by the least stable fuel assembly, the computer iteratively assumes incremental changes in either control rod position or coolant flow, as selected by the operator, and recalculates the stability index until the index is within the prescribed limit. While the calculations are detailed in taking into account the nuclear feedback as well as hydrodynamic effects on stability, they are carried out rapidly to provide recommendations for real-time adjustments in flow or control rod position. The cumulative adjustment needed to return the least stable assembly to stable operation is reported to the operator, who may make the recommended change in flow or rod position manually at his discretion. Alternatively, the recommended change in flow or rod position can be provided as a control signal to a controller which automatically makes the required parameter change. As another aspect of the invention, the number of fuel assemblies for which stability calculations need to be generated by the digital computer is reduced to those most susceptible to instability based upon current operating conditions. To this end, the power generated by each assembly, and the axial distribution of power, including the axial location of the peak power in each assembly, are determined. Only those assemblies generating power above a certain level, preferably above the average level of power generated in all the assemblies, and having their peak axial power occur at a location below the average location for all fuel assemblies are selected for stability calculations. This reduces the number of assemblies for which the stability calculations must be made from several hundred to about a dozen or less. With such a manageable number of fuel assemblies, the detailed calculations can be carried out in time for real-time control of fuel assembly stability. The invention relates to both the above method and apparatus for controlling stability in a boiling water reactor.