Patent Number: 055241280
Section: claims

1. A method of operational control of the stability of a boiling water nuclear reactor, said reactor having a core comprising vertically oriented fuel bundles in a generally right circular cylindrical array, each fuel bundle having an open lattice of nuclear fuel pins enclosed by a flow channel through which coolant water flows upwardly and is heated by thermal power generated by said fuel pins, said core having on a core average basis a boiling boundary between an upper coolant phase in which boiling occurs and a lower coolant phase in which boiling does not occur, said method comprising the steps of: a. establishing a target value of elevation in the reactor core of the average coolant boiling boundary;  b. determining the actual elevation in the reactor core of the average coolant boiling boundary;  c. comparing said actual coolant boiling boundary elevation to said target value; and  d. controlling said actual coolant boiling boundary elevation to be greater than said target value, said controlling step being performed in a controlled region in core power-core flow space comprising a predetermined operating region of core power and core flow.  means for determining the average elevation in the core of the boiling boundary;  means for comparing said average core boiling boundary elevation with a target value of core elevation; and  means for controlling the operation of said reactor so as to maintain said average core boiling boundary elevation greater than said target elevation value. 2. The method of claim 1, wherein said controlling step is performed in a controlled region adjacent an instability region which comprises a predetermined operating region susceptible to power oscillations. 3. The method of claim 2, wherein said reactor has a rated core flow and core power, and said controlling step is performed in a controlled region which lies between about 20% and about 60% of rated core flow, and between about 20% and 90% of rated core power. 4. The method of claim 2, further comprising the step of controlling said core power and core flow to be outside said instability region. 5. The method of claim 1 wherein said establishing step includes establishing said target value at about one third of the height of said core. 6. The method of claim 1, wherein said establishing step includes establishing said target value at about four feet. 7. The method of claim 1, wherein said controlling step includes positioning of shaping control rods in said core. 8. The method of claim 7, wherein said controlling step includes positioning of shaping control rods in said core so as to provide an average axial power distribution in said core which peaks at a relatively high core elevation. 9. The method of claim 1, wherein said comparing step includes comparing the power generated by said core in the axial region below said target value with the power required for coolant water saturation. 10. The method of claim 9, wherein said comparing step includes determining the ratio of the power generated by said core in the axial region below said target value to the power required for coolant water saturation, and said controlling step includes controlling the operation of the reactor so that said ratio is equal to or less than one. 11. Apparatus for use in controlling the operation of a boiling water nuclear reactor to avoid reactor core power oscillations, said reactor having a power generating core through which coolant water flows upwardly, said coolant having on a core average basis an average boiling boundary between an upper phase in which boiling occurs and a lower phase in which boiling does not occur, comprising: 12. The apparatus of claim 11, wherein said target elevation value is about four feet. 13. The apparatus of claim 11, wherein said target elevation value is about one-third of the height of said core. 14. The apparatus of claim 11, wherein said means for determining said average core boiling boundary elevation includes means for determining the power generated by said core below said target elevation. 15. The apparatus of claim 14, wherein said comparing means includes means for comparing the power generated by said core below said target elevation value with the power required for saturation of said coolant water. 16. The apparatus of claim 15, wherein said comparing means includes means for determining the ratio of the power generated by said core below said target elevation value to the power required for saturation of said coolant water. 17. The apparatus of claim 11, further including means coupled to said comparing means for displaying the comparison of said average boiling boundary elevation with said target elevation value. 18. The apparatus of claim 11, wherein said apparatus comprises a core monitoring computer.