Abstract: A pressurized water reactor (PWR) includes a pressure vessel containing a nuclear reactor core immersed in primary coolant water, control rod assemblies (CRA's), and control rod drive mechanisms (CRDM's) operating the CRA's. The reactor core has axially varying 235U enrichment and/or axially varying burnable poison concentration. A CRDM controller controls the CRA's over a burn-up cycle that does not include fuel assembly shuffling and is divided into a plurality of burn-up intervals. The CRDM controller is configured to, for each burn up interval: position the CRA's in accordance with a CRA pattern defining a set of fixed positions for the CRA's except for a sub-set of CRA's designated by the CRA pattern as floating CRA's, and control power level of the PWR by adjusting the floating CRA's without not adjusting the CRA's not designated as floating CRA's. The primary coolant water optionally does not contain soluble neutron poison.

Abstract: A control rod for nuclear reactors includes four wings including neutron absorbers containing hafnium, a front end structural member which has a cross shape in cross section and includes brackets bonded to the leading ends of the wings, and a terminal end structural member which has a cross shape in cross section and includes brackets bonded to the tailing ends of the wings. The four wings are bonded to a wing-bonding member including a cross-shaped center shaft so as to form a cross shape. The front end structural member and the wing-bonding member are made of a zirconium alloy. The wings include neutron-absorbing plates having neutron-absorbing portions and each have an outer surface which is opposed to a fuel assembly and at which a hafnium-zircaloy composite member covered with zircaloy is disposed. The neutron-absorbing plates are opposed to each other with trap spaces disposed therebetween.

Abstract: A control strategy for a pressurized water nuclear reactor that employs separate, independent control rod banks for respectively controlling Tavg and axial offset within corresponding deadbands. The strategy does not permit the control banks controlling reactor core power and the control banks controlling axial offset to move together, but normally gives preference to the control banks controlling the Tavg except when a demand signal is received simultaneously by both independent control rod banks to move in a same direction, in which case, the control bank compensating for the axial offset is given preference.

Abstract: The invention relates to a method for regulation of operational parameters of the core of a pressurised water nuclear reactor comprising: a step of acquisition of values (FH, FB, TBC, TBF, Q) which are representative of the conditions of operating of the core of the reactor; a step of evaluation of the actual values (Tmoye, AOe, {circumflex over (P)} maxe) of the operational parameters at least according to the values acquired (FH, FB, TBC, TBF, Q); a step of selection of a control law for the concentration of a neutron-absorbent component ([B]) and for the positions of insertion (Z1 to Z5) of the groups (P1 to P5) of rods selected from at least the first and second control laws which are different from one another; and a step of regulation of the operational parameters by means of the control law selected, according to set points (Tmoyc, AOc, {circumflex over (P)} maxc) relative to the said parameters and to the actual values (Tmoye, AOe, {circumflex over (P)} maxe) evaluated.

Abstract: An apparatus and method for safely controlling a control rod of a nuclear reactor for a nuclear power plant is provided. The apparatus may include a first controller to output a signal to insert or withdraw the control rod, a mechanical portion to perform insertion or withdrawal of the control rod in response to the signal output by the first controller, the mechanical portion including a movement process portion, a stop latch to restrain the control rod, a moving latch to move the control rod, and a lift coil to insert or withdraw the control rod, a detector to detect a position or a speed of the control rod when the control rod is inserted or withdrawn, and a brake to stop the control rod by force when the control rod is withdrawn irrespective of an intended control of the control rod.

Abstract: Cores include different types of control cells in different numbers and positions. A periphery of the core just inside the perimeter may have higher reactivity fuel in outer control cells, and lower reactivity cells may be placed in an inner core inside the inner ring. Cores can include about half fresh fuel positioned in higher proportions in the inner ring and away from inner control cells. Cores are compatible with multiple core control cell setups, including BWRs, ESBWRs, ABWRs, etc. Cores can be loaded during conventional outages. Cores can be operated with control elements in only the inner ring control cells for reactivity adjustment. Control elements in outer control cells need be moved only at sequence exchanges. Near end of cycle, reactivity in the core may be controlled with inner control cells alone, and control elements in outer control cells can be fully withdrawn.

Abstract: An integral pressurized water reactor (PWR) comprises: a cylindrical pressure vessel including an upper vessel section and a lower vessel section joined by a mid-flange; a cylindrical central riser disposed concentrically inside the cylindrical pressure vessel and including an upper riser section disposed in the upper vessel section and a lower riser section disposed in the lower vessel section; steam generators disposed inside the cylindrical pressure vessel in the upper vessel section; a reactor core comprising fissile material disposed inside the cylindrical pressure vessel in the lower vessel section; and control rod drive mechanism (CRDM) units disposed inside the cylindrical pressure vessel above the reactor core and in the lower vessel section. There is no vertical overlap between the steam generators and the CRDM units.

Abstract: A nuclear reactor includes a pressure vessel, and a control rod assembly (CRA) including at least one movable control rod, a control rod drive mechanism (CRDM) for controlling movement of the at least one control rod, and a coupling operatively connecting the at least one control rod and the CRDM. The coupling includes a connecting rod engaged with the CRDM and a terminal element connected with a lower end of the connecting rod and further connected with the at least one control rod. In some embodiments the terminal element includes a first portion comprising a first material having a first density and a second portion comprising a second material having a second density that is greater than the first density. In some embodiments the terminal element has a largest dimension parallel with the connecting rod that is greater than or equal to a largest dimension transverse to the connecting rod.

Abstract: The invention refers to a method for operating a nuclear light water reactor during an operation cycle including a cardinal cycle and a number of successive control rod cycles. The reactor includes a plurality of elongated fuel units containing a nuclear fuel in the form a fissible material, a burnable absorber, and a plurality of control rods introduceable in a respective control rod position. Substantially all control rods are introduced in the core before the reactor is started and an operation cycle is initiated. The reactor is operated during the cardinal cycle with a first control rod configuration with a first group of control rods at least partly introduced and the remaining control rods extracted. The reactor is operated during the subsequent control rod cycles with a respective control rod configuration which each includes a different group of control rods at least partly introduced, wherein the cardinal cycle is substantially longer than each of the subsequent control rod cycles.

Abstract: This document's object is to provide an axial power distribution control method in which only the control of an axial power distribution in a nuclear reactor with a simple operation with a clear operational target keeps the control of a xenon oscillation, thereby suppressing the xenon oscillation to an extremely small magnitude in advance at the same time.

Abstract: In the method, performance of a nuclear reactor may be improved by implementing an operational solution for the nuclear reactor using at least one control rod criteria in order to increase scram effectiveness during at least a portion of an operating cycle for the nuclear reactor.

Abstract: A control rod having a lower tip absorber material which exhibits substantially lower irradiation induced swelling than a second absorber material which extends above the lower tip absorber material. The lower tip absorber material having a substantially lower reactivity worth than the second absorber material, extends from a lower end plug of the control rod to an elevation just above a dashpot in a thimble guide tube in a nuclear fuel assembly when the control rod is fully inserted within the thimble guide tube.

Abstract: A control rod for nuclear reactors includes four wings including neutron absorbers containing hafnium, a front end structural member which has a cross shape in cross section and includes brackets bonded to the leading ends of the wings, and a terminal end structural member which has a cross shape in cross section and includes brackets bonded to the tailing ends of the wings. The four wings are bonded to a wing-bonding member including a cross-shaped center shaft so as to form a cross shape. The front end structural member and the wing-bonding member are made of a zirconium alloy. The wings include neutron-absorbing plates having neutron-absorbing portions and each have an outer surface which is opposed to a fuel assembly and at which a hafnium-zircaloy composite member covered with zircaloy is disposed. The neutron-absorbing plates are opposed to each other with trap spaces disposed therebetween.

Abstract: The invention relates to a control system for controlling an average temperature of a coolant at a reactor core outlet. The control system method detects an average temperature of the coolant at the reactor core outlet, compares the actual average temperature of the coolant at the reactor core outlet with a reference temperature thereby to generate an error signal, and adjusts the actual average temperature of the coolant at the reactor core outlet in response to the error signal. The invention extends to a cascade controller for a nuclear reactor, and to a nuclear power plant.

Abstract: In a boiling water reactor controlling each control rod hydraulically by driving a solenoid valve, the control system comprises an operation control means 41 having a duplicated data processing unit for generating sequence patterns based on the timing of the driving sequence based on the control information provided manually, a transmission control means 42 for creating a command word corresponding to each control rod being controlled based on said sequence pattern, and mutually communicating each command word between duplicated data processing units and computing the AND logic of said data within a predetermined time difference, and when the computed result coincide, transmitting the selected command word serially; a transmission unit 32 for receiving said command word and performing protocol conversion thereto, and transmitting the same to a plurality of transmission branch units positioned downstream as the control command; and a solenoid valve drive circuit 31 for driving the control rod drive unit corres

Abstract: According to the method, a reactor core is shut down during the operation cycle. One or more fuel bundles of the reactor core are then moved to new positions within the reactor core to increase a total energy, output of the reactor core as compared to continuing operation of the reactor core without the shutting down and moving steps.

Abstract: With this method it is possible to evaluate opposing friction forces occurring after a certain operating time on a mobile object in a guide. The method consists of measuring the variations in the speed of movement of the mobile object, of calculating the theoretical speed of movement without friction, of combining these two results and thereby deducing outside opposing forces through calculation of the acceleration of the mobile object. Particular application to the evaluation of additional friction forces acting on a mobile control cluster assembly in the core of a pressurized water nuclear reactor.

Abstract: A safe start-up of a boiling water reactor is achieved when two conditions are adhered to and monitored when the control rods are being moved out of the core: First, the drive of a control rod should be activated only when the directly and diagonally adjacent control rods remain at rest simultaneously, and, second, when all the directly adjacent control rods are still in the fully moved-in initial position.

Abstract: In order to provide a boiling water type nuclear reactor use control rod with a guide use roller which improves corrosive environment at a clearance in the guide use roller and suppresses a generation of stress corrosion cracking while maintaining sliding function of the control rod by the guide use roller, at least one of a handle 5, a lower portion supporting plate 2a and a dropping speed limiter 2 is provided with the guide use roller, and a space which causes water flow in a clearance between a pin 9 and a pin hole 12 in the guide use roller is provided adjacent the clearance.

Abstract: A method of controlling a nuclear power plant includes determining a first maximum linear heat generation rate and a first minimum critical power ratio by a core monitoring system, and determining a second maximum linear heat generation rate and a second minimum critical power ratio by utilizing the first maximum linear heat generation rate and the first minimum critical power ratio determined by the core monitoring system and plant data. The second maximum linear heat generation rate and the second minimum critical power ratio are compared with predetermined thermal limit values and a control signal outputted from an automatic power regulator system to a re-circulation flow control system and to a control rod control system is held when at least one of the second maximum linear heat generation rate and the second minimum critical power ratio exceeds the predetermined thermal limit values.

Abstract: A method of operating a nuclear reactor reduces the number of reload fuels to be loaded into the nuclear reactor in the second and the following operation cycles. The nuclear reactor has a reactor core in which a plurality of reload fuel assemblies respectively having different infinite multiplication factors are arranged. The method operates the nuclear reactor with control rods inserted in control cells each comprising four reload fuel assemblies having relatively large infinite multiplication factors among the plurality of reload fuel assemblies for a period longer than half of a period of an operation cycle.

Abstract: A control system, and a control method, of a nuclear power plant capable of easily executing automatic output regulation by an automatic power regulator system even when a lower-accuracy maximum linear heat generation rate and a lower-accuracy minimum critical power ratio are determined in a short cycle by using brief calculation. While a core monitoring system does not calculate a maximum linear heat generation rate and a minimum critical power ratio, they are determined by utilizing the maximum linear heat generation rate and the minimum critical power ratio calculated by the core monitoring system as well as plant data, and the lower-accuracy maximum linear heat generation rate and the lower-accuracy minimum critical power ratio so determined are compared with predetermined thermal limit values.

Abstract: Changes in the value of the control element assembly position signal (26,28) are monitored over a preselected time period, and a determination is made whether these changes conform to a regular, predetermined pattern (S.sub.1), characteristic of normal control element assembly motion. If any change deviates (S.sub.2,S.sub.3) from the pattern by more than a permissible tolerance, a position invalidity signal is generated.

Abstract: The present invention, in one aspect, is a method for identifying an optimum control blade positioning arrangement. The method generally has an initialization phase and a running, or search, phase. In the initialization phase, an initial control blade positioning arrangement is identified. Once the initial control blade positioning arrangement is identified, such arrangement is then optimized, within the defined constraints, in the running phase. More specifically, in the running phase, each control blade position is analyzed to determine whether the control blade position can be changed from the initial position to either satisfy a constraint or optimize cycle energy, or both.

Abstract: A control cluster for a water cooled and moderated nuclear reactor, comprises a spider and rods suspended from the spider. Each rod contains neutron-absorbing material. In a lower portion of each rod, the material is a metal neutron absorber. In an upper portion, it is a stack of pellets of boron carbide B.sub.4 C. In a middle fraction occupying a height lying in the range 35% to 45% of the total height, it is zirconium diboride ZrB.sub.2 isotopically enriched in boron 10 or hafnium diboride HfB2.

Abstract: A control rod driving apparatus adapted to drive a control rod assembly for a nuclear reactor is disposed in a housing mounted to a reactor pressure vessel of the nuclear reactor and includes a guide tube disposed in the housing, a connection pipe disposed inside the guide tube coaxially therewith and having one end to which a control rod assembly is connected, a ball spindle disposed inside the connection pipe and supported thereby so as to be rotatable by a ball nut assembly engaged with the ball spindle so as to be axially movable along the ball spindle, the ball nut assembly supporting another end of the connection pipe, a hydraulic drive operatively connected to the ball spindle to rotate the ball spindle, and a transmission mechanism operatively connected to the drive for transmitting a power of the drive to the ball spindle.

Abstract: A control system for avoiding coupled neutronic-thermal hydraulic instabilities during maneuvering of a boiling water nuclear reactor that determines the average elevation in the reactor core of the coolant boiling boundary, and avoids instabilities by maintaining the core average coolant boiling boundary elevation above a target value, which is dependent on reactor design specifics but may be about four feet, in core power-flow operating regions which are subject to instabilities. The system may be implemented without physical modification of existing reactors or reactor instrumentation and control systems through use of the reactor core monitoring computer, which typically has available all core parameters necessary for the computations required in the invention.

Abstract: A safety system grade dropped rod detection system for a pressurized water reactor (PWR) utilizes core exit thermocouples arranged in multiple trains and hot and cold leg RTDs to generate a safety system grade rod stop signal. The system generates from the temperature signals a relative power deviation (RD) and a curvature index (CI), which is the spatial second derivative of RD for each fuel assembly. The CI signatures not only provide rapid, reliable detection of dropped control rods, but also clearly identify failed and failing thermocouples.

Abstract: A safety system grade dropped rod detection system for a pressurized water reactor (PWR) utilizes core exit thermocouples arranged in multiple trains and hot and cold leg RTDs to generate a safety system grade rod stop signal. The system generates from the temperature signals a relative power deviation (RD) and a curvature index (CI), which is the spatial second derivative of RD for each fuel assembly. The CI signatures not only provide rapid, reliable detection of dropped control rods, but also clearly identify failed and failing thermocouples.

Abstract: A computed model of reactor power output distribution in space is read periodically to microprocessor based computer memory and retained in memory in a three dimensional matrix. This retention occurs between regular updates on the order of every two minutes. The reactor is conventionally monitored in groups of 16 fuel bundles each. Each 16 bundle group is monitored as to its thermal neutron flux by four vertical strings of local power range monitors, each string having one of four power monitors disposed at four different elevations extending the height of the active core. Each bundle group is controlled by four control rods and is assumed to be subject to uniform flow change with overall reactor flow change. The automated thermal limit monitor (ATLM) takes as inputs all power range monitor information from the BWR reactor core on a continuous basis to two channels one channel for determining operating limits the other channel for determining safety limits.

Abstract: A nuclear reactor, such as advanced pressurized water reactor, with controlled power output includes a reactor core having a cross section in a given plane with a center of area and axes of symmetry passing through the center of area. A multiplicity of control elements are disposed in groups, each having at least one of the control elements. The control elements of each of the groups with more than one control element are joined together. The groups are symmetrical to at least two of the axes of symmetry. Drive mechanisms each move a respective one of the groups of control elements. Each of the control elements has a plurality of control rods. A support structure joins the control rods to one another. Fuel assemblies are disposed in groups. Each of the groups of control elements has a given number of control elements being associated with one of the groups of fuel assemblies having the given number of fuel assemblies.

Abstract: A dashpot in a control rod guide thimble for a nuclear fuel assembly includes a lower tubular portion of an elongated main tube of the guide thimble, an auxiliary hollow tube inserted in the lower portion of the main tube, and an end plug attached to a lower end portion of the auxiliary tube. The auxiliary tube has an outside diameter slightly less than an inside diameter of the main tube to permit a close fitting relationship between an exterior surface of the auxiliary tube and an interior surface of the main tube lower portion. The auxiliary tube also has an upper end portion with an inside surface portion in axial cross-section flaring upwardly and outwardly to provide a tapered transition extending between and connecting an interior surface of the auxiliary tube with the exterior surface thereof.

Abstract: An apparatus for controlling the flow of coolant through a nuclear fuel assembly, in which a coolant flow restrictor 26 having longitudinal ducts 30 is provided with plug members 48, 49, 50 of different lengths. The plug members 48, 49, 50 are moved longitudinally by the effect of neutron induced growth on an actuating member 60. A connecting member 53 converts the growth of the actuating member 60 into a longitudinal movement of the plug members 48, 49, 50 so that they are progressively withdrawn from the flow restrictor 26 to increase the flow of coolant through the flow restrictor 26.

Abstract: A fuel assembly has a water rod which consists of a coolant ascending path and a coolant descending path, the coolant ascending path opening below a fuel support of a lower tie plate, the coolant descending path opening above the fuel support and adapted to guide downwardly the coolant that has flowed up the coolant ascending path to the upper portion of the water rod. The lateral cross section of the coolant ascending path is more than 25 times that of the coolant descending path.The nuclear reactor with these fuel assemblies loaded in the core is operated as follows. After startup of the reactor when the reactor power is low, all the control rods inserted in the core are withdrawn completely. Then, the reactor power is controlled by regulating a liquid level formed in the water rods. This reactor operation method prolongs the lifetime of the control rods, thereby reducing the number of times they have to be replaced.

Abstract: A system for determining the relative positions of control rods within a nuclear reactor is disclosed. When the position signal associated with the highest control rod in a group exceeds the position signal associated with the lowest control rod in the same group by more than a predetermined limit, an alarm and a fault indicator are activated.

Abstract: A core of a light water boiling reactor comprises a plurality of vertical fuel assemblies (10) and a plurality of control rods, each control rod comprising four vertical blades arranged in a cruciform. The control rods are arranged with each one of their blades between two fuel assemblies located in the same row, such that each control rod together with four fuel assemblies arranged around the blades of the control rod form a unit, the control rod unit (30, 30-o), having an at least substantially square cross-section. The control rod units are arranged in a symmetrical lattice with each control rod unit included in two rows of control rod units perpendicular to each other.

Abstract: A process for automatic regulation of the soluble boron content of the cooling water of a pressurized water nuclear reactor in which operating regions (2, 3) of the means of boration and of the means of dilution respectively are determined a priori, corresponding to pairs of values of two control parameters relating to the position of the reactor regulating group in the core and to the deviation of the axial power imbalance relative to the reference axial imbalance. The momentary value of the control parameters is determined continuously during the operation of the reactor, and the stopping or the triggering of the means for boration or for dilution is commanded when the operating point crosses a boundary of an operating region (2, 3).The invention applies in particular to a nuclear reactor controlled in G mode.

Abstract: A mechanical spectral shift reactor comprises apparatus for inserting and withdrawing water displacer elements having differing neutron absorbing capabilities for selectively changing the water-moderator volume in the core thereby changing the reactivity of the core. The displacer elements may comprise substantially hollow cylindrical low neutron absorbing rods and substantially hollow cylindrical thick walled stainless rods. Since the stainless steel displacer rod have greater neutron absorbing capability, they can effect greater reactivity change per rod. However, by arranging fewer stainless steel displacer rods in a cluster, the reactivity worth of the stainless steel displacer rod cluster can be less than a low neutron absorbing displacer rod cluster.

Abstract: A rod drive control system for nuclear such as boiling water reactors is comprised of a plurality of stepping motors for inserting and withdrawing control rods disposed within the nuclear reactor, which the stepping motors are each provided for each control rod, terminal units which are each provided for each stepping motor and are each provided with a motor controller to drive the related stepping motor and to check the operation of the motor, a central processing unit for transferring an operation signal to the motor control unit and for detecting faults in the terminal units, and transmission lines between the terminal units and the central processing unit.

Abstract: A mechanical spectral shift reactor comprises apparatus for inserting and withdrawing water displacer elements having differing neutron absorbing capabilities for selectively changing the water-moderator volume in the core thereby changing the reactivity of the core. The displacer elements may comprise substantially hollow cylindrical low neutron absorbing rods and substantially hollow cylindrical thick walled stainless rods. Since the stainless steel displacer rods have greater neutron absorbing capability, they can effect greater reactivity change per rod. However, by arranging fewer stainless steel displacer rods in a cluster, the reactivity worth of the stainless steel displacer rod cluster can be less than a low neutron absorbing displacer rod cluster.

Abstract: A method of operating a pressurized water nuclear reactor comprising determining the present core power and reactivity levels and predicting the change in such levels due to displacer rod movements. Groups or single clusters of displacer rods can be inserted or withdrawn based on the predicted core power and reactivity levels to change the core power level and power distribution thereby providing load follow capability, without changing control rod positions or coolant boron concentrations.

Abstract: The mechanical spectral shift reactor provides a method and apparatus for controlling a nuclear reactor comprising inserting a plurality of reactor coolant displacer members into the reactor at the beginning of the core life. The displacer members reduce the volume of reactor coolant-moderator in the core at the start-up. As the reactivity of the core declines with fuel depletion, a selected number of displacer members are withdrawn from the core at selected time intervals. The withdrawal of the displacer member allows reactor coolant water to enter the core which increases core moderation at a time when fuel reactivity is declining. Thus for a given amount of nuclear fuel the life of the core can be extended or for a given life of a core the uranium fuel requirements can be reduced.

Abstract: Closed-loop controller for altering the power level of nuclear reactors in a safe manner and without overshoot. Apparatus is provided for moving a control element such as a control rod or for adjusting the concentration of a soluble neutron absorber (chemical shim) for altering the nuclear reactor power level. A computer computes at short time intervals the two functions [.rho.(t)-.vertline..rho..sub.c .vertline./.lambda..sub.e '(t)]/.vertline..rho..sub.c .vertline. and .tau.(t)ln(P.sub.F /P(t)). The direction of motion of the control element is altered when these two functions become equal thereby resulting in the attainment of a new power level without overshoot. In a preferred embodiment, these two functions are computed at intervals of approximately one second.

Abstract: The setpoints for control systems in a pressurized water reactor are adjusted by an amount corresponding to the expected change in the controlled process variable resulting from rapid fluctuations in load to reduce the duty time of the control system components such as the rod drive mechanisms in the rod control system and the spray and heater units in the pressurizer pressure control system. In addition, the deadband in the response of the rod control system is continuously varied as a function of the variance of the magnitude of the rapid fluctuations in load to further reduce wear while maintaining good response during load following.

Abstract: A method of operating a pressurized water nuclear reactor comprising determining the present core power and reactivity levels and predicting the change in such levels due to displacer rod movements. Groups or single clusters of displacer rods can be inserted or withdrawn based on the predicted core power and reactivity levels to change the core power level and power distribution thereby providing load follow capability, without changing control rod positions or coolant boron concentrations.

Abstract: Control regulation and shutdown of gas-cooled, high-temperature nuclear reactors require precise adjustment of core reactivity through the use of absorber rods. Fine adjustment without permanent damage to the reactor core having a bed of spherical fuel elements is accomplished by a process and arrangement of the reactor such that two groups of absorber rods are independently controlled for insertion into the reactor cavity between the roof reflector and the bed of spherical fuel elements and into the fuel element bed itself. Adjustments can be made for rapid shutdown and/or complete shutdown as well as rapid shutdown required due to water or oil penetration into the reactor core.

Abstract: In the operation of a boiling water reactor, when the core flow rate is decreased due to a trip of the circulating pump, the control rod pattern is suitably changed to avoid unnecessary scram of the control rods. The change of the control rod pattern is made by driving control rods which have been inserted to small depth deeper into a depth near 12/24 of the core height. In addition to this control rod operation, the control rods which have been inserted to large depth are withdrawn to a depth near 12/24 or, alternatively, the control rods which have not been inserted at all are inserted to the depth of about 12/24 of the core height.

Abstract: A method of operating a pressurized water nuclear reactor comprising determining the present core power and reactivity levels and predicting the change in such levels due to displacer rod movements. Groups or single clusters of displacer rods can be inserted or withdrawn based on the predicted core power and reactivity levels to change the core power level and power distribution thereby providing load follow capability, without changing control rod positions or coolant boron concentrations.

Abstract: A method of operating a nuclear reactor adapted to use cross-shaped control rods. The control rods are grouped into a plurality of groups having a first group consisting of a control rod located at the center of the core, a second group consisting of 8 control rods surrounding the control rod of the first group, a third group consisting of control rods located adjacent to and outside of the control rods of the second group, and other groups determined successively in the same manner as in the third group. During the operation period other than the period in which the number of the control rods to be inserted for realizing the critical condition of said nuclear reactor is less than 6, the nuclear reactor is operated with control rod patterns in which the inserted control rods are selected from control rods of alternate or every other groups of said groups so as to include at least a pair of control rods which are located in a similar manner to that of the KNIGHT's movement on a chess board.

Abstract: A method and arrangement for utilizing mixed oxide fuel in the core of a nuclear reactor controlled by rectilinearly movable control rod elements which decreases the maximum single control rod element worth in the core. Fuel elements containing the mixed oxide fuel are positioned at discrete core locations, particularly at those locations receiving high worth control rod elements, thereby decreasing the worth of those control elements.