Patent Number: 
Section: claims

1. A method of determining the R-factor for a bundle of nuclear fuel rods in a nuclear boiling light water reactor, which reactor comprises a plurality of bundles of nuclear fuel rods, wherein the fuel rods in the bundle are arranged side by side, at least substantially parallel to each other, and extend essentially in an axial direction, wherein said R-factor is a factor that accounts for a weighted local power influence on a fuel rod, including contributions from neighbouring fuel rods, the method comprising determining a local R-factor (Ri(z)) for each fuel rod (i) in said bundle and for each of a plurality of levels (z) in said axial direction, wherein an individual axial heat generation profile for a certain fuel rod (i) is taken into account when determining the local R-factors (Ri(z)) for said fuel rod (i), and determining a total R-factor (Rz) for each of said levels (z) for the whole bundle of fuel rods, wherein said total R-factor (Rz) at a level (z) is determined as a maximum of said local R-factors (Ri(z)) at said level (z) in said bundle of nuclear fuel rods. 2. A method according to claim 1, wherein said number of levels is at least equal to 10. 3. A method according to claim 1, wherein said bundle of nuclear fuel rods includes at least 15 fuel rods. 4. A method according to claim 1, wherein the determination of said local R-factor (Ri(z)) at a level (z) for a certain fuel rod (i) includes the determination of a double integrated heat generation rate of said fuel rod (i) up to the level (z) relative to a double integrated average heat generation rate of all fuel rods in the bundle up to the level (z). 5. A method according to claim 4, wherein said determination also includes a normalisation by means of the integrated heat generation rate of said fuel rod up to the level (z) relative to the integrated average heat generation rate of all fuel rods in the bundle up to the level (z), wherein the normalised determination of the double integrated heat generation rate of said fuel rod (i) up to the level (z) relative to the double integrated average heat generation rate of all fuel rods in the bundle up to the level (z) is carried out by determining:      [                            ∫                      z            0                    z                ⁢                                            q              i                        ⁡                          (                              z                ′                            )                                ⁢                                          ⁢                      ⅆ                          z              ′                                ⁢                                    ∫                              z                0                            z                        ⁢                                          ∫                                  z                  0                                                  z                  ′                                            ⁢                                                                    q                    _                                    ⁢                                                                          (                                      z                    ″                                    )                                ⁢                                  ⅆ                                      z                    ″                                                  ⁢                                                                  ⁢                                  ⅆ                                      z                    ′                                                                                                            ∫                      z            0                    z                ⁢                              q            _                    ⁢                      (                          z              ′                        )                    ⁢                                          ⁢                      ⅆ                          z              ′                                ⁢                                    ∫                              z                0                            z                        ⁢                                          ∫                                  z                  0                                                  z                  ′                                            ⁢                                                                    q                    i                                    ⁢                                                                          (                                      z                    ″                                    )                                ⁢                                  ⅆ                                      z                    ″                                                  ⁢                                                                  ⁢                                  ⅆ                                      z                    ′                                                                                            ]    awherez=axial positionz′=integration variable representing the axial positionz″=integration variable representing the axial positionqi(z)=linear heat generation rate of rod i at the level z q(z)=average linear heat generation rate of all rods in the bundle at the level zz0=the axial position of bulk boiling boundary (zero steam quality limit) in said bundlea=a constant, 0<a<1. 6. A method according to claim 1, wherein the determination of said local R-factor (Ri(z)) at a level (z) for a certain fuel rod (i) includes the determination of the sum of an integrated heat generation rate of said fuel rod up to the level (z) and a fraction of integrated heat generation rates of fuel rods neighbouring said fuel rod up to the level (z), relative to an integrated average heat generation rate of all fuel rods in the bundle up to the level (z). 7. A method according to claim 6, wherein said determination of the sum of the integrated heat generation rate of said fuel rod up to the level (z) and the fraction of the integrated heat generation rates of fuel rods neighbouring said fuel rod up to the level (z), relative to the integrated average heat generation rate of all fuel rods in the bundle up to the level (z) is carried out by determining:                              (                                    ∫                              z                0                            z                        ⁢                                                            q                  i                                ⁡                                  (                                      z                    ′                                    )                                            ⁢                                                          ⁢                              ⅆ                                  z                  ′                                                              )                b            +              c        ⁢                              ∑                          j              ∈                              S                i                                                                                    ⁢                                          ⁢                                    (                                                ∫                                      z                    0                                    z                                ⁢                                                                            q                      j                                        ⁡                                          (                                              z                        ′                                            )                                                        ⁢                                                                          ⁢                                      ⅆ                                          z                      ′                                                                                  )                        b                              +              d        ⁢                              ∑                          k              ∈                              D                i                                                                                    ⁢                                          ⁢                                    (                                                ∫                                      z                    0                                    z                                ⁢                                                                            q                      k                                        ⁡                                          (                                              z                        ′                                            )                                                        ⁢                                                                          ⁢                                      ⅆ                                          z                      ′                                                                                  )                        b                                              (                  1          +                      cN                          S              i                                +                      dN                          D              i                                      )            ⁢                          ⁢                        (                                    ∫                              z                0                            z                        ⁢                                          q                _                            ⁢                              (                                  z                  ′                                )                            ⁢                                                          ⁢                              ⅆ                                  z                  ′                                                              )                b              ⁢        where the symbols are as explained in claim 6 and whereqj(z)=linear heat generation rate of side neighbouring fuel rod j at the level zqk(z)=linear heat generation rate of diagonal neighbouring fuel rod k at the level zSi=the set of side neighbouring fuel rods for the fuel rod iDi=the set of diagonal neighbouring fuel rods for the fuel rod iNSi=the number of side neighbouring fuel rods for the fuel rod iNDi=the number of diagonal neighbouring fuel rods for the fuel rod ib=a constant, 0<b<1c=a constant, 0<c<1d=a constant, 0≦d≦c. 8. A method according to claim 1, wherein the determination of said local R-factor at a level (z) for a certain fuel rod (i) is carried out by determining:            R      i        ⁡          (      z      )        =                              (                      1            +                          e              i                                )                ⁡                  [                                                    ∫                                  z                  0                                z                            ⁢                                                                    q                    i                                    ⁡                                      (                                          z                      ′                                        )                                                  ⁢                                                                  ⁢                                  ⅆ                                      z                    ′                                                  ⁢                                                      ∫                                          z                      0                                        z                                    ⁢                                                            ∫                                              z                        0                                                                    z                        ′                                                              ⁢                                                                                            q                          _                                                ⁢                                                                                                  (                                                  z                          ″                                                )                                            ⁢                                              ⅆ                                                  z                          ″                                                                    ⁢                                                                                          ⁢                                              ⅆ                                                  z                          ′                                                                                                                                                                        ∫                                  z                  0                                z                            ⁢                                                q                  _                                ⁢                                  (                                      z                    ′                                    )                                ⁢                                                                  ⁢                                  ⅆ                                      z                    ′                                                  ⁢                                                      ∫                                          z                      0                                        z                                    ⁢                                                            ∫                                              z                        0                                                                    z                        ′                                                              ⁢                                                                                            q                          i                                                ⁢                                                                                                  (                                                  z                          ″                                                )                                            ⁢                                              ⅆ                                                  z                          ″                                                                    ⁢                                                                                          ⁢                                              ⅆ                                                  z                          ′                                                                                                                                                  ]                    a        ⁢                                        (                                          ∫                                  z                  0                                z                            ⁢                                                                    q                    i                                    ⁡                                      (                                          z                      ′                                        )                                                  ⁢                                                                  ⁢                                  ⅆ                                      z                    ′                                                                        )                    b                +                  c          ⁢                                    ∑                              j                ∈                                  S                  i                                                                                                  ⁢                                                  ⁢                                          (                                                      ∫                                          z                      0                                        z                                    ⁢                                                                                    q                        j                                            ⁡                                              (                                                  z                          ′                                                )                                                              ⁢                                                                                  ⁢                                          ⅆ                                              z                        ′                                                                                            )                            b                                      +                  d          ⁢                                    ∑                              k                ∈                                  D                  i                                                                                                  ⁢                                                  ⁢                                          (                                                      ∫                                          z                      0                                        z                                    ⁢                                                                                    q                        k                                            ⁡                                              (                                                  z                          ′                                                )                                                              ⁢                                                                                  ⁢                                          ⅆ                                              z                        ′                                                                                            )                            b                                                            (                      1            +                          cN                              S                i                                      +                          dN                              D                i                                              )                ⁢                                  ⁢                              (                                          ∫                                  z                  0                                z                            ⁢                                                q                  _                                ⁢                                  (                                      z                    ′                                    )                                ⁢                                                                  ⁢                                  ⅆ                                      z                    ′                                                                        )                    b                    where the symbols are as explained in claims 6 and 8 and whereei=the dryout sensitivity constant for the fuel rod i (also denoted “rod constant”). 9. A processor configured for automatically determining the R-factor for a bundle of nuclear fuel rods in a nuclear boiling light water reactor, which reactor comprises a plurality of bundles of nuclear fuel rods, wherein the fuel rods in the bundle are arranged side by side, at least substantially parallel to each other, and extend essentially in an axial direction, wherein said R-factor is a factor that accounts for a weighted local power influence on a fuel rod, including contributions from neighbouring fuel rods, wherein the processor is configured with an input receiving data concerning a linear heat generation rate (qi(z)) of the different fuel rods (i) at the different levels (z) in said bundle, and the processor is configured to determine the R-factor in accordance with the method of claim 1. 10. A computer program product comprising a computer-readable medium on which is recorded a computer program that is directly loadable into an internal memory of a computer, which computer program is configured to cause the computer to carry out the method of claim 1. 11. A method of determining a critical power for a bundle of nuclear fuel rods in a nuclear boiling light water reactor, wherein said method includes determining an R-factor according to the method of claim 1. 12. A method of determining a critical power by means of a local steam quality at dryout (XDO) for a bundle of nuclear fuel rods in a nuclear boiling light water reactor, which method comprises:(1) determining how the local steam quality at dryout (XDO) depends on flow of cooling medium through the nuclear fuel bundle (f1(G)),(2) determining how the local steam quality at dryout (XDO) depends on axial power profile of the nuclear fuel bundle (f2(I2)),(3) determining how the local steam quality at dryout (XDO) depends on R-factor of the nuclear fuel bundle (f3(R)),(4) determining how the local steam quality at dryout (XDO) depends on pressure of the cooling medium in the nuclear fuel bundle (f4(P)), and(5) determining the local steam quality at dryout (XDO) based on determinations made in steps (1) through (4), characterised in that the R-factor dependence is determined by using the method of claim 1 for determining the R-factor. 13. A nuclear energy plant comprising a nuclear boiling light water reactor, which plant comprises a control unit arranged to carry out a method according to claim 11. 14. A nuclear energy plant according to claim 13, including operation parameter detectors, arranged to detect operation parameters of the nuclear reactor during operation, wherein the control unit is arranged to receive information concerning said operation parameters from the detectors and to use these operation parameters when carrying out the method. 15. A nuclear energy plant according to claim 13, wherein the control unit comprises control outputs arranged to control the operation of the nuclear reactor in dependence on said method carried out by the control unit. 16. A computer program product comprising a computer-readable medium on which is recorded a computer program that is directly loadable into an internal memory of a computer which forms part of a control unit, which computer program is configured to cause the computer to carry out a method according to claim 11. 17. A method of operating a nuclear energy plant comprising a nuclear boiling light water reactor, wherein said method includes the following steps:(a) providing information concerning operation parameters of the nuclear reactor,(b) using this information in a method according to claim 11, and(c) controlling the operation of the nuclear reactor in dependence on step (b).