A method for calculating a PCI margin associated with a loading pattern of a nuclear reactor including a core into which fuel assemblies are loaded according to the loading pattern is implemented by an electronic system. The fuel assemblies include fuel rods each including fuel pellets of nuclear fuel and a cladding surrounding the pellets. This method includes calculating (100) a reference principal PCI margin for a reference loading pattern of the fuel assemblies in the core; calculating (110) a reference secondary PCI margin for the reference pattern; calculating (120) a modified secondary PCI margin for a modified loading pattern of the fuel assemblies in the core, and calculating (130) a modified principal PCI margin for the modified pattern, depending on a comparison of the modified secondary PCI margin with the reference secondary PCI margin.

A method for calculating a PCI margin associated with a loading pattern of a nuclear reactor including a core into which fuel assemblies are loaded according to the loading pattern is implemented by an electronic system. The fuel assemblies include fuel rods each including fuel pellets of nuclear fuel and a cladding surrounding the pellets. This method includes calculating (100) a reference principal PCI margin for a reference loading pattern of the fuel assemblies in the core; calculating (110) a reference secondary PCI margin for the reference pattern; calculating (120) a modified secondary PCI margin for a modified loading pattern of the fuel assemblies in the core, and calculating (130) a modified principal PCI margin for the modified pattern, depending on a comparison of the modified secondary PCI margin with the reference secondary PCI margin.

A fuel element has a ratio of area of fissionable nuclear fuel in a cross-section of the tubular fuel element perpendicular to the longitudinal axis to total area of the interior volume in the cross-section of the tubular fuel element that varies with position along the longitudinal axis. The ratio can vary with position along the longitudinal axis between a minimum of 0.30 and a maximum of 1.0. Increasing the ratio above and below the peak burn-up location associated with conventional systems reduces the peak burn-up and flattens and shifts the burn-up distribution, which is preferably Gaussian. The longitudinal variation can be implemented in fuel assemblies using fuel bodies, such as pellets, rods or annuli, or fuel in the form of metal sponge and meaningfully increases efficiency of fuel utilization.

In an apparatus for evaluating reliability of a nuclear power plant operator, an expected task finishing time for each task of the operator is calculated based on input data for reliability evaluation of the operator to determine a task completion time for each task of the operator based on the calculated expected task finishing time and the input data. An initial time for the tasks is determined based on the input data and the task completion time for a current task. A total execution time of the operator is calculated based on the determined task completion time and initial time. An available time allowed for the operator to complete the tasks is calculated based on a predetermined allowed time and the determined initial time. A task failure probability is obtained from the difference between the calculated total execution time and the calculated available time based on probability distribution information.

A method is for monitoring a nuclear reactor comprising a core in which fuel assemblies are loaded, each assembly comprising nuclear fuel rods each including nuclear fuel pellets and a cladding surrounding the pellets. The method includes determining (100) at least one operating time limit (T.sup.FPPI) for the extended reduced power operation of the reduced power nuclear reactor, so as to avoid a rupture of at least one of the claddings, operating (102) the nuclear reactor at reduced power for an actual time strictly less than the time limit (T.sup.FPPI), and relaxing (104) at least one threshold for protecting the nuclear power plant as a function of a difference between the time limit (T.sup.FPPI) and the actual time.

In an apparatus for evaluating reliability of a nuclear power plant operator, an expected task finishing time for each task of the operator is calculated based on input data for reliability evaluation of the operator to determine a task completion time for each task of the operator based on the calculated expected task finishing time and the input data. An initial time for the tasks is determined based on the input data and the task completion time for a current task. A total execution time of the operator is calculated based on the determined task completion time and initial time. An available time allowed for the operator to complete the tasks is calculated based on a predetermined allowed time and the determined initial time. A task failure probability is obtained from the difference between the calculated total execution time and the calculated available time based on probability distribution information.

A nuclear-reactor control-absorber drive mechanism includes a device for monitoring a potential situation of increase to overspeed of the absorber, configured to measure the number of control steps delivered to at least one of the first, second and third phases of the stator during a time window of preset duration or the number of rotation steps of the rotor during a time window of preset duration. The drive is also configured to compare the number of measured control steps with a preset maximum or the number of measured rotation steps with a preset maximum.

The present invention concerns a fuel assembly for a nuclear power boiling water reactor. The fuel assembly comprises fuel rods. At least 95% of the fuel rods comprise nuclear fuel material in the form of U enriched in 235U. At least 20% of the fuel rods belong to a first set of fuel rods. The fuel rods in this first set comprise both U enriched in 235U and Th. The first set comprises a first and a second subset of fuel rods. The ratio, with regard to weight, between Th and U, in each fuel rod of said first subset, is higher than the ratio, with regard to weight, between Th and U, in each fuel rod of said second subset. The invention also concerns a nuclear power boiling water reactor and a manner of operating such a reactor.

A nuclear reactor power regulator adjusts reactor output based on a reactor output target value and a reactor output change rate. The regulator includes a reactor output calculating device that performs computation based on a thermal equilibrium from power signals of plant parameters to calculate a reactor output signal. A correcting device corrects a continuously obtained reactor output equivalent signal that is considered to be equivalent to a reactor output at a calculation interval of the output signal, so that the output equivalent signal coincides with the output signal. The correcting device calculates a continuous corrected output equivalent signal. A reactor output controlling device calculates a reactor output control signal for controlling the output of the reactor, using the corrected reactor output equivalent signal, the reactor output target value, and the reactor output change rate. A reactor output controller is operated based on the reactor output control signal.

A nuclear reactor power regulator adjusts reactor output based on a reactor output target value and a reactor output change rate. The regulator includes a reactor output calculating device that performs computation based on a thermal equilibrium from power signals of plant parameters to calculate a reactor output signal. A correcting device corrects a continuously obtained reactor output equivalent signal that is considered to be equivalent to a reactor output at a calculation interval of the output signal, so that the output equivalent signal coincides with the output signal. The correcting device calculates a continuous corrected output equivalent signal. A reactor output controlling device calculates a reactor output control signal for controlling the output of the reactor, using the corrected reactor output equivalent signal, the reactor output target value, and the reactor output change rate. A reactor output controller is operated based on the reactor output control signal.