Patent Number: 
Section: claims

1. A method of estimating when dryout may occur in a nuclear light water reactor of the boiling water reactor kind, which reactor includes a nuclear fuel arrangement, which method is characterised in that it includes:receiving data that indicates operation parameters for the reactor; and processing the data by the use of a formula which expresses the local dryout property of the nuclear reactor,wherein said formula includes at least a first and a second factor, wherein the first factor is a first function that describes how the dryout property depends on the flow of the cooling medium through the nuclear fuel arrangement andthe second factor is a second function that describes how the dryout property depends on the axial power profile of the nuclear fuel arrangement,wherein said formula also includes a third function, wherein the third function describes how the dryout property depends on the R-factor of the nuclear fuel arrangement,wherein the R-factor is the weighted local power influence from the neighbour nuclear fuel rods,wherein said third function is a factor in the formula,wherein said formula also includes a fourth function, wherein the fourth function describes how the dryout property depends on the pressure of the cooling medium in the nuclear fuel arrangement, andwherein said fourth function is a factor in the formula,wherein the first, second, third and fourth functions independently of each other describe said flow dependence said axial power profile dependence, said R-factor dependence and said pressure dependence respectively. 2. A method according to claim 1, wherein the fourth function includes an entity hƒg, where hƒg is the latent heat for evaporation of the cooling medium. 3. A method according to claim 1, wherein the fourth function includes an entity proportional to the density of the vapour of the cooling medium at the actual pressure. 4. A method according to claim 1, wherein said formula includes an exponential function, which, in the exponent, includes another exponential function. 5. A method according to claim 1, wherein the formula includes a number of variables, a number of constants, and a number of fitting coefficients, wherein said fitting coefficients are considered as constants for the used nuclear fuel arrangement within certain limited variations of the operation conditions for the nuclear reactor. 6. A method according to claim 5, wherein the formula is such that only the fitting coefficients have to be adjusted in order for the formula to be applicable to a particular nuclear fuel arrangement. 7. A method according to claim 5, comprising using fewer than 10 fitting coefficients that play an important role in order for the formula to express the dryout property. 8. A method of estimating when dryout may occur in a nuclear light water reactor of the boiling water reactor kind, which reactor includes a nuclear fuel arrangement, which method is characterised in that it includes receiving data that indicates operation parameters for the reactor, and processing of the data by the use of a formula which expresses the local dryout property of the nuclear reactor,wherein said formula includes at least a first and a second factor, wherein the first factor is a first function that describes how the dryout property depends on the flow of the cooling medium through the nuclear fuel arrangement, andthe second factor is a second function that describes how the dryout property depends on the axial power profile of the nuclear fuel arrangement,wherein said formula also includes a third function, wherein the third function describes how the dryout property depends on the R-factor of the nuclear fuel arrangement,wherein the R-factor is the weighted local power influence from the neighbour nuclear fuel rods,wherein said third function is a factor in the formula,wherein said formula also includes a fourth function, wherein the fourth function describes how the dryout property depends on the pressure of the cooling medium in the nuclear fuel arrangement, andwherein said fourth function is a factor in the formula,wherein the first, second, third and fourth functions independently of each other describe said flow dependence said axial power profile dependence, said R-factor dependence and said pressure dependence respectively, andwherein said second function comprises a single mathematical transformation to the axial power profile which includes at least one variable related to a longitudinal direction of the nuclear fuel arrangement, parallel to the direction in which a fuel rod in the fuel arrangement extends. 9. A method according to claim 8, wherein the transformation is of the following form:      I    2    =            1              x        ⁡                  (                      Z            DO                    )                      ⁢                  ∫        ZXO                  Z          DO                    ⁢                        x          ⁡                      (            z            )                          ⁢                                  ⁢                  ⅆ          z                    where x(z) is the steam quality profile along the fuel bundle,z is a local spatial variable related to a longitudinal direction of the nuclear fuel arrangement, parallel to the direction in which a fuel rod in the fuel arrangement extends, andwherein the integration extends from the onset of net boiling, ZX0 and the integration ends at ZDO, which is defined as the axial position for dryout. 10. A method according to claim 8, wherein the transformation takes transient phenomena into account and has the following form:            I      2        ⁡          (              t        DO            )        =            1              x        ⁡                  (                                    Z              DO                        ⁡                          (                              t                DO                            )                                )                      ⁢                  ∫                  ZXO          ⁡                      (                          t              in                        )                                                Z            DO                    ⁡                      (                          t              DO                        )                              ⁢                        x          ⁡                      (                                          t                ⁡                                  (                                                            t                      in                                        ,                    z                                    )                                            ,              z                        )                          ⁢                                  ⁢                  ⅆ          z                    where z is a local spatial variable related to a longitudinal direction of the nuclear fuel arrangement, parallel to the direction in which a fuel rod in the fuel arrangement extends,t is the time during the transient,tin is the time during the transient when the particle experiencing dryout at tDO entered the bundle of fuel rods,tDO is the time when a fluid particle is experiencing dryout,ZDO(tDO) is the time-dependent axial position for dryout for this fluid particle, andZX0(tin) is the point where this fluid particle reaches the point of net boiling. 11. A method according to claim 8, wherein said formula includes an exponential function, which, in the exponent, includes another exponential function, and wherein the transformation forms part of the exponent in the first mentioned exponential function. 12. A method according to claim 1, wherein said first function includes an entity G, where G is the mass flow of cooling medium through the nuclear fuel arrangement per area and time, wherein G is constant in a stationary state of the operation of the nuclear reactor. 13. A method according to claim 12, wherein said formula includes an exponential function, which, in the exponent, includes another exponential function, and wherein the entity G, appears in the exponential function that appears in the exponent of the first mentioned exponential function. 14. A method according to claim 1, wherein the formula has exactly, or at least substantially, the following form:      X    DO    =            e              (                              1                          1              +                              e                                  (                                                            α                      1                                        +                                                                  α                        2                                            ⁢                      G                                                        )                                                              +                                    α              3                                                      I                2                            +                              f                ⁡                                  (                  Acc                  )                                                              +                                    α              4                        ⁢            R                          )              ⁢          ρ      rel              α        5              ⁢          h      fg              α        6            where XDO is the predicted local steam quality at dryout,G is the mass flow of cooling medium through the nuclear fuel arrangement per area and time,I2 is the transformation of the axial power distribution,ƒ(Acc) is a function of a flow acceleration term, which function describes transient effects captured through the flow acceleration term,R is the weighted local power influence from the neighbour nuclear fuel rods,ρrel is the relative steam density,hƒg is the latent heat for evaporation of the cooling medium,α1 to α6 are fitting coefficients specific for the specific fuel arrangement used. 15. A method according to claim 14, whereinƒ(Acc)=α7Acc2 where the acceleration term Acc is defined as  Acc  =            ∫      0              Z        DO              ⁢                  1                              u            f            2                    ⁡                      (                          t              ,              z                        )                              ⁢                        ∂                      u            f                                    ∂          t                    ⁢                          ⁢              ⅆ        z            where uƒ is the axial velocity of the liquid film on the nuclear fuel rods or its approximation,t is the time during the transient,z is a local spatial variable related to a longitudinal direction of the nuclear fuel arrangement, parallel to the direction in which a fuel rod in the fuel arrangement extends,ZDO is defined as the time dependent axial position for dryout, andα7 is a fitting coefficient specific for the specific fuel arrangement used. 16. A method of operating a nuclear energy plant comprising a nuclear light water reactor of the boiling water reactor kind, wherein said method comprising:receiving information concerning operation parameters of the nuclear reactor,using this information to estimate the dryout time of the reactor in a method according to claim 1, andcontrolling the operation of the nuclear reactor in dependence on the result of the estimate. 17. A method according to claim 8, wherein said first function includes an entity G, where G is the mass flow of cooling medium through the nuclear fuel arrangement per area and time, wherein G is constant in a stationary state of the operation of the nuclear reactor. 18. A method according to claim 8, wherein the formula has exactly, or at least substantially, the following form:      X    DO    =            e              (                              1                          1              +                              e                                  (                                                            α                      1                                        +                                                                  α                        2                                            ⁢                      G                                                        )                                                              +                                    α              3                                                      I                2                            +                              f                ⁡                                  (                  Acc                  )                                                              +                                    α              4                        ⁢            R                          )              ⁢          ρ      rel              α        5              ⁢          h      fg              α        6            where XDO is the predicted local steam quality at dryout,G is the mass flow of cooling medium through the nuclear fuel arrangement per area and time,I2 is the transformation of the axial power distribution,ƒ(Acc) is a function of a flow acceleration term, which function describes transient effects captured through the flow acceleration term,R is the weighted local power influence from the neighbour nuclear fuel rods,ρrel is the relative steam density,hƒg is the latent heat for evaporation of the cooling medium,α1 to α6 are fitting coefficients specific for the specific fuel arrangement used.