Nuclear reactor and method for opening the reactor

Abstract

A nuclear reactor is provided. The reactor includes a reactor pressure vessel containing a plurality of fuel rods containing fissionable material, the reactor pressure vessel having an upper, removable vessel head. The reactor further includes control rods, each made of a neutron-absorbing material. The control rods are inserted into the reactor and between the fuel rods through the vessel head to control the rate of nuclear fission reactions in the fuel rods. The control rods are movable relative to the vessel head through a normal range of insertion positions to control the power output of the reactor when the reactor is in a critical state and producing useful power, and to bring the reactor into a subcritical shutdown state. The reactor further includes a control rod drive mechanism carried by the vessel head and operable to drive the movement of the control rods. The control rod drive mechanism is controllable to release the control rods when a vessel opening operation is performed, during which the reactor is in a shutdown state and the vessel head is lifted upwardly from the reactor pressure vessel such that the control rods slide through the vessel head and remain stationary relative to the fuel rods to maintain the shutdown state. The reactor further includes a monitoring unit for identifying whether the control rods have been accidentally lifted together with the vessel head.

Description

[Technical field]

[0001] The present disclosure relates to nuclear reactors and methods of opening nuclear reactors. [Background technology]

[0002] Nuclear power plants convert thermal energy from the nuclear decay of fissile material contained in fuel assemblies into electrical energy. Pressurized water reactor (PWR) nuclear power plants typically have a primary coolant circuit that connects the following pressurized components: a reactor pressure vessel (RPV) that houses the fuel assemblies, one or more steam generators, and one or more pressurizers. Coolant pumps in the primary circuit circulate pressurized water through piping between these components. The RPV houses the nuclear reactor, which heats the water in the primary circuit. The steam generators act as heat exchangers between the primary circuit and a secondary system that produces steam to power the turbines. The pressurizers maintain a pressure of about 155 bar in the primary circuit.

[0003] The fuel assembly includes fuel rods formed of sintered pellets of fissile material. The fuel assembly also includes spaces for control rods. For example, a conventional fuel assembly provides housing for a 17×17 grid of rods, i.e., a total of 289 spaces. Of these total of 289 spaces, 24 may be reserved for control rods and one for instrumentation tubes. The control rods are movable in and out of the reactor by a control rod drive mechanism located in the head of the reactor pressure vessel, and control in real time the fission process that the fuel undergoes by absorbing neutrons released during fission. A typical nuclear reactor contains about 100 to 300 fuel assemblies. Fully inserting the control rods usually results in a subcritical state where the reactor shuts down.

[0004] In some reactors, the head of the reactor pressure vessel and its associated assemblies (including the control rod drive mechanism) are collectively called the reactor head package, which must be removed to refuel the reactor or to rearrange the fuel rods within the reactor.

[0005] Conventionally, to lift the head package, the control rods are lowered into the reactor to reduce the criticality of the reactor. After unscrewing and removing the vessel head studs that connect the head package to the reactor body and disconnecting the power and monitoring cables from the head package, the reactor head package is usually lifted by a crane. However, it is important to maintain a high safety shutdown margin in case of any other accidents that may compromise the shutdown margin, such as the inadvertent removal of the control rods. In particular, during the lifting of the reactor head package, the drive rods can get stuck in the control rod drive mechanism and pull out the control rods together.

[0006] Therefore, the conventional approach has been to "flood" the reactor by submerging it in deep water (the primary circuit coolant), introduce or increase the concentration of neutron poisons such as soluble boric acid solutions in the water, and allow the "poisoned" coolant to circulate through the reactor. This coolant is contaminated in the sense that it has a very large neutron capture cross section, depleting the fissile material for neutrons that would cause another fission event.

[0007] Unfortunately, boric acid is highly toxic and corrosive, and its use complicates reactor operation and design. It would be desirable to provide the necessary safety margin in a manner that does not require the use of boric acid. Summary of the Invention [Means for solving the problem]

[0008] Thus, in a first aspect, the present disclosure provides a nuclear reactor, comprising: a reactor pressure vessel containing a plurality of fuel rods containing fissile material, the reactor pressure vessel having a removable upper vessel top; control rods, each made of a neutron absorbing material, inserted through the vessel head into the reactor and among the fuel rods to control the rate of nuclear fission reactions in the fuel rods, whereby the control rods are movable through a normal range of insertion positions relative to the vessel head to control the power output of the reactor when the reactor is in a critical state and producing useful power, and to bring the reactor into a subcritical shutdown state; a control rod drive mechanism carried by the vessel head and operable to drive movement of the control rod; Equipped with the control rod drive mechanism is controllable to release the control rods when a vessel opening operation is performed, in which the reactor is at a shut-down state and the vessel head is lifted upwardly from the reactor pressure vessel such that the control rods slide through the vessel head and remain stationary relative to the fuel rods to maintain the shut-down state; The reactor further comprises a monitoring unit for identifying whether the control rods are erroneously lifted together with the vessel head; A nuclear reactor is provided.

[0009] In this way, the monitoring unit allows stuck control rods (i.e. those that are lifted together with the vessel head) to be identified during the lifting process, thereby preventing excessive rod withdrawal that may lead to unwanted additional fission reactivity (i.e. reduced shutdown safety margins). In this way, the reactor can be safely opened without contaminating the reactor coolant (e.g. by introducing a boric acid solution).

[0010] Additionally, the monitoring unit helps to avoid damage that a stuck rod could cause to the control rod drive mechanism and other components.

[0011] Optional features of the reactor of the first aspect are now described, which may be applied alone or in any combination.

[0012] The monitoring unit may include any of a number of position sensors, one or more neutron sensors, and one or more load sensors.

[0013] The monitoring unit may include a position sensor that detects the position of the control rod relative to the vessel head over a normal range of insertion positions and also detects the position of the control rod relative to the vessel head over an additional range of insertion positions beyond the normal range to identify whether the control rod is inadvertently lifted together with the vessel head as the vessel head is lifted and the control rod slides therethrough during a vessel opening operation. For example, each position sensor may be formed by an array of induction coils that detect the local presence or absence of a respective control rod. Advantageously, the position sensor may reveal the identity of a stuck rod.

[0014] The position sensor may be located adjacent to the control rod drive mechanism, below the control rod drive mechanism, or integrated into the control rod drive mechanism.

[0015] Additionally or alternatively, the monitoring unit may include a load sensor that measures the weight of the reactor head package to determine if the control rods have been erroneously lifted with the vessel head.

[0016] The load sensors may be integrated into either the crane head, the lifting points of the reactor head package, the connection between the reactor head package and the crane, or between the reactor pressure vessel head and the control rod drive mechanisms. In another embodiment, each of the control rod drive mechanisms may be attached to the reactor vessel head via a load sensor. Thus, by locating the load sensor that records the expected increase in mass, the location of the stuck control rod can be identified.

[0017] The load sensor may include one or more load cells.

[0018] Additionally or alternatively, the monitoring unit may include a neutron sensor that measures the number of neutrons to determine if the control rods have been erroneously lifted with the vessel head.

[0019] The neutron sensor may be located on or in the reactor head or reactor body, or may be mounted externally thereto. In particular, the neutron sensor may be located at, near, or above the level of the flange between the reactor vessel and the reactor head. The neutron sensor may be mounted on top of the inner housing that contains the individual fuel rods. Multiple neutron sensors may be used (e.g., around the reactor vessel or on top of each housing), and the relative intensity of the neutrons detected by each sensor may be used to approximate the position of the stacked rods during lifting.

[0020] The neutron sensors may include one or more of a scintillation neutron detector, a gas proportional detector, and / or a semiconductor neutron detector.

[0021] Thus, the monitoring unit may include any one, any two, or all three of the position, load, and neutron sensors. Having two, and preferably three, independent methods for identifying if the control rods have been inadvertently lifted with the vessel head increases the operational safety of the reactor.

[0022] The reactor may further include a control system (e.g., a computer-based control system) programmed to control the vessel opening operation and command an abort of the vessel opening operation (e.g., return of the vessel head to the reactor pressure vessel) if the control system receives an indication from the monitoring device that the control rods have erroneously been lifted with their heads. In this manner, the abort operation may be automated.

[0023] Conveniently, where the monitoring system includes a position sensor, the control system may be further programmed to compare the rate of change of the detected position of the control rod relative to the head with the rate of rise of the vessel head to identify control rods that are erroneously lifted with the head from their detected positions. Typically, the control system will include an output device, such as a monitor and / or printer, to return the positions of control rods that have been identified as being lifted with the head.

[0024] If the monitoring system includes a load sensor, the control system may be further programmed to compare the measured weight with a value for an expected weight of the reactor head package to identify from the measured weight whether the control rods have been erroneously lifted together with the head.

[0025] If the monitoring system includes a neutron sensor, the control system may be further programmed to compare the measured neutron count with a predetermined neutron count level and / or compare the rate of increase of the measured neutron count with a predetermined rate of increase to identify from the measured neutron count whether the control rod has been erroneously lifted together with its head.

[0026] The reactor may further include at least one cable umbilicus that remains attached to the vessel head during vessel opening operations to provide power and a communication path to the monitoring unit. Alternatively, power may be provided by a battery and the communication path may be provided wirelessly.

[0027] The vessel head, the control rod drive mechanisms, and any other components of the reactor which are lifted with the head form the reactor head package.

[0028] In a second aspect, the present disclosure provides a method of opening a pressure vessel of a nuclear reactor, comprising: the reactor pressure vessel houses fuel rods containing fissile material and has a removable upper vessel head, the reactor includes control rods each made of a neutron absorbing material, the control rods being inserted through the vessel head into the reactor and among the fuel rods to control the rate of nuclear fission reaction in the fuel rods, whereby the control rods are movable through a normal range of insertion positions relative to the vessel head to control the power output of the reactor when the reactor is in a critical state and producing useful power, and to bring the reactor into a subcritical shutdown state, the reactor further includes a control rod drive mechanism carried by the vessel head and operable to drive movement of the control rods, The method comprises: moving the control rod using a control rod drive mechanism to an inserted position where the reactor is shut down; Releasing the control rod from the control rod drive mechanism; lifting the vessel head upwardly from the reactor pressure vessel so that the control rods slide through the vessel head and remain stationary relative to the fuel rods and remain stationary; monitoring whether the control rods are inadvertently lifted together with the vessel head while the head is being lifted to open the pressure vessel; if it is monitored that the control rods are erroneously lifted together with the vessel head, stopping the lifting of the vessel head; Includes.

[0029] Thus, the reactor used in the method of the second aspect may be the reactor of the first aspect.

[0030] Optional features of the method of the second aspect are described herein, which may be applied alone or in any combination.

[0031] The aborting step may include returning the vessel head to the reactor pressure vessel if the control rod is monitored to be erroneously lifted with the head.

[0032] The monitoring step may include detecting a position of the control rod relative to the vessel head, and comparing a rate of rise of the vessel head to a rate of change of the detected position of the control rod relative to the head from the detected position to identify control rods that are erroneously lifted with the head. The method may then further include returning positions of the control rods identified as being lifted with the head.

[0033] Additionally or alternatively, the monitoring step may include measuring a weight of the reactor head package to identify whether the control rods have been erroneously lifted with the vessel head. For example, the method may further include comparing the measured weight to an expected value of the weight of the reactor head package to identify from the measured weight whether the control rods have been erroneously lifted with the head.

[0034] Additionally or alternatively, the monitoring step may include measuring a neutron count to identify whether the control rods have been erroneously lifted with the vessel head. For example, the method may further include comparing the measured neutron count to a predetermined neutron count level and / or comparing a rate of increase of the measured neutron count to a predetermined rate of increase to identify from the measured neutron count whether the control rods have been erroneously lifted with the vessel head.

[0035] The invention may include or be configured as part of a nuclear reactor power plant (herein referred to as a nuclear reactor). In particular, the invention may relate to a pressurized water nuclear reactor. The nuclear reactor power plant may have a power output of between 250 MW and 600 MW, or between 300 MW and 550 MW.

[0036] The nuclear reactor power plant may be a modular reactor. A modular reactor may be considered as a reactor made up of many modules fabricated off-site (e.g., in a factory), which are assembled on-site into a nuclear reactor power plant by connecting the modules together. Any of the primary, secondary, and tertiary circuits may be formed of modular construction.

[0037] The nuclear reactor of the present disclosure may include a primary circuit including a reactor pressure vessel, one or more steam generators, and one or more pressurizers. The primary circuit circulates a medium (e.g., water) through the reactor pressure vessel to extract heat generated by nuclear fission in the reactor core, which is then delivered to the steam generators and transferred to the secondary circuit. The primary circuit may include one to six steam generators, or two to four steam generators, or three steam generators, or any range of the aforementioned numbers. The primary circuit may include one, two, or more than two pressurizers. The primary circuit may include a circuit extending from the reactor pressure vessel to each of the steam generators, the circuit may carry hot medium from the reactor pressure vessel to the steam generators and cool medium from the steam generators back to the reactor pressure vessel. The medium may be circulated by one or more pumps. In some embodiments, the primary circuit may include one or two pumps for each steam generator in the primary circuit.

[0038] In some embodiments, the medium circulated in the primary circuit may include water. In some embodiments, the medium may include a neutron absorbing material (e.g., boron, gadolinium) added to the medium. In some embodiments, the pressure in the primary circuit may be at least 50 bar, 80 bar, 100 bar, or 150 bar during full power operation, and the pressure may reach 80 bar, 100 bar, 150 bar, or 180 bar during full power operation. In some embodiments where water is the medium in the primary circuit, the heating water temperature of the water leaving the reactor pressure vessel may be between 540 K and 670 K, or between 560 K and 650 K, or between 580 K and 630 K during full power operation. In some embodiments where water is the medium in the primary circuit, the cooling water temperature of the water returning to the reactor pressure vessel may be between 510 K and 600 K, or between 530 K and 580 K during full power operation.

[0039] The reactor of the present disclosure may include a secondary circuit that includes a water circulation loop that extracts heat from the primary circuit of a steam generator and converts water to steam to drive a turbine. In an embodiment, the secondary loop may include one or two high pressure turbines and one or two low pressure turbines.

[0040] The secondary circuit may include a heat exchanger to condense the steam into water as it is returned to the steam generator. The heat exchanger may be connected to a tertiary loop that may include a large body of water to act as a heat sink.

[0041] The reactor vessel may include a steel pressure vessel, which may be between 5m and 15m in height, or between 9.5m and 11.5m in height, and between 2m and 7m, or between 3m and 6m, or between 4m and 5m in diameter. The pressure vessel may include a reactor body and a reactor head disposed vertically above the reactor body. The reactor head may be connected to the reactor body by a series of studs passing through a flange on the reactor head and a corresponding flange on the reactor body.

[0042] The reactor head may include an integrated head assembly in which many elements of the reactor structure may be integrated into a single element, including the pressure vessel head, cooling shrouds, control rod drive mechanisms, missile shields, lifting rigs, hoist assemblies, and cable tray assemblies.

[0043] The movement of the control rods may be moved by a control rod drive mechanism, which may command and power actuators to lower and raise the control rods in and out of the fuel assemblies and hold the position of the control rods relative to the core. The control rod drive mechanism may rapidly insert the control rods to quickly shut down (scram) the reactor.

[0044] The primary circuit may be housed within a containment structure to retain steam from the primary circuit in the event of an accident. The containment may be between 15 m and 60 m in diameter, or between 30 m and 50 m in diameter. The containment structure may be formed from steel or concrete, or formed from steel-lined concrete. The containment may house one or more lifting devices (e.g., a pole crane). The lifting device may be housed on top of the containment above the reactor pressure vessel. The containment may contain inside or support outside of a water tank for emergency cooling of the reactor. The containment may include equipment and facilities to allow for refueling of the reactor, for storage of fuel assemblies and transport of fuel assemblies between inside and outside of the containment.

[0045] A power plant may include one or more civil structures to protect the reactor components from external hazards (such as missile attacks) and natural disasters (e.g., tsunamis). The civil structures may be made of steel, or concrete, or a combination of both. [Brief description of the drawings]

[0046] [Figure 1] FIG. 1 is a schematic diagram of a PWR nuclear power plant 10. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] The embodiment will now be described, by way of example only, with reference to FIG.

[0048] The RPV 12 containing the fuel assemblies is located centrally in the power plant 10. Clustered around the RPV are three steam generators 14 connected to the RPV by pressurized water piping 16 of a primary coolant circuit. Coolant pumps 18 circulate pressurized water around the primary coolant circuit, take heated water from the RPV to the steam generators, and circulate cooled water from the steam generators to the RPV.

[0049] The pressurizer 20 maintains the water pressure in the primary coolant circuit at approximately 155 bar.

[0050] In the steam generator 14, heat exchangers transfer heat from the pressurized water to supply water that circulates in the piping 22 of the secondary coolant circuit to produce steam that is used to drive a turbine, which in turn produces steam that drives an electrical generator. The steam is then condensed before returning to the steam generator.

[0051] In one arrangement of the power plant 10, the RPV 12, steam generator 14, and pressurizer 20 are each contained in their own pressure containment silo. This allows each silo to be significantly smaller and easier to manufacture than a conventional containment building for the entire power plant. Alternatively, the RPV 12, steam generator 14, and pressurizer 20 may be contained in a single pressure containment silo.

[0052] The RPV 12 has an upper, removable vessel head 24 fixed in place on the vessel on head studs 26, and control rods which are inserted into the RPV through the vessel head and can move through a range of normal insertion positions relative to the vessel head to control the power output of the reactor and place the reactor in subcritical shutdown when the reactor is critical and producing useful power. This movement is actuated by control rod drives (not shown) located in the vessel head which, together with the vessel head and other equipment, form the reactor head package.

[0053] Associated with the control rod drive mechanism are rod position indicators, usually in the form of a row of inductive loop position sensors distributed along the channel in which the control rods reside. These sensors allow the range of insertion positions of the control rods to be monitored in normal operation of the reactor. However, the sensors are also adapted to form part of a monitoring system for monitoring stuck control rods during RPV 12 opening operations (e.g. in case of refuelling).

[0054] More specifically, to open the RPV 12, the control rods are first fully inserted, bringing the reactor to subcritical shutdown. The head studs 26 of the vessel head 24 are unhooked and removed, and the power and monitoring cables of the reactor head package are disconnected except for at least one cable umbilicus that provides power and communication paths to the rod position indicators so that they remain connected and active. This umbilicus may be coiled or bellows-shaped to accommodate lifting and repositioning of the reactor head package.

[0055] The reactor head package can then be lifted by a crane. As the head package is lifted, the drive rods must remain stationary relative to the fuel assemblies and the rod position indicators must move up. Thus, the tops of the drive rods should appear to move down. This downward movement is monitored by position indicators that can extend the bank of inductive loop position sensors to detect additional ranges of rod movement. Conversely, rods that get stuck in the drive mechanism and lift together with the head package will appear to have no such movement. Thus, stuck rods can be identified by the difference in position between unstuck and stuck rods, even after a small lift distance. Another option is to compare the speed of apparent movement of the rods with the lift rate applied by the crane; unstuck rods will appear to move at the same speed (albeit in the opposite direction) as the lift rate, while stuck rods will show a different (usually zero) speed of movement.

[0056] This approach allows stuck rods to be identified early during lift, thereby preventing excessive rod withdrawal that would lead to undesirable reactivity additions. Because each rod may have its own rod position indicator, the location of a stuck rod may be easily identified by plant operators and corrective action may be taken. In turn, this improves the feasibility of operating the reactor without the use of soluble boron that would otherwise be required to provide sufficient neutron absorption margin in the event that a rod is inadvertently lifted away from the fuel assembly.

[0057] Fuel assemblies typically incorporate a few centimeters of low fissile material structure at their base, providing a "safety area" at the bottom of the RPV12. If the control rods are inadvertently lifted out of this safety area (but not beyond it), no significant neutron count increase should occur. Therefore, an initial "proving" lift limited to this safety area can be used to confirm the presence of inadvertent control rod lift such that safety limits are not violated in a soluble boron free environment, providing a high level of safety.

[0058] The power plant may have a computer-based control system programmed to control the opening operation. If such a system identifies a control rod lifting with its head package from a monitored position, it may quickly and automatically command an abort of the vessel opening operation by returning the vessel head to the reactor. It may also return the location of the stuck rod to an appropriate output device. Additionally, the system may be programmed to compare the rate of rise of the vessel head with the rate of change of the detected position of the control rod relative to the head, or to identify differences in the detected positions, so that, for example, a control rod that is erroneously lifted with its head can be identified.

[0059] Another option that can be implemented instead of, but is preferably in addition to, the rod position indicators is for a stacked control rod monitoring system to include a load sensor that measures the weight of the reactor head package. In particular, the mass of the head package is a known amount, and if a control rod is inadvertently lifted along with the package, its apparent mass will weigh more than the known amount.

[0060] Thus, a computer-based control system can be linked to the lift controller of the crane. If a crane payload exceeding a known head package mass is sensed, the control system will determine that the control rods are erroneously lifting with the vessel head and will override the crane controller to abort the lift.

[0061] Yet another option that can be implemented instead of, but preferably in addition to, either or both of the rod position indicators and load sensors is for the stuck control rod monitoring system to include a neutron sensor that measures the neutron count in the core. This can be used by the control system to determine if the neutron count is increasing, decreasing, or stable, and to identify if the control rods are erroneously lifting with the vessel head.

[0062] The computer-based control system can analyze the input from each of the sensors in the sensing unit to determine the location of the stuck fuel rod (i.e., which control rod drive mechanism in the head package is lifting the control rod). Determining the location of the stuck fuel rod can include identifying a signal from a sensor whose signal identifies the stuck control rod; consulting a data file that associates each sensor with a location; i.e., outputting to a user the location within the reactor head or fuel assembly associated with that particular sensor.

[0063] If the reactor is critical and producing useful power, such neutron sensors may be used in the reactor control system to insert control rods into the core, thereby causing a "scram" to rapidly abort the fission reaction if the sensed neutron count is too high or increasing too quickly. However, in the monitoring system of the present disclosure, detection of an increase in neutron count (e.g., above a predetermined level and / or rate of increase) by the neutron sensor may be indicative of a stuck control rod and may therefore be used by the control system to override the crane controller and abort the lift.

[0064] It will be understood that the present invention is not limited to the above-described embodiments, and various modifications and improvements can be made without departing from the concepts described herein. Any of the features can be used individually or in combination with any other feature, except where mutually exclusive, and the present disclosure extends to and includes all combinations and subcombinations of one or more features described herein. [Explanation of symbols]

[0065] 10 Pressurized Water Reactor (PWR) Nuclear Power Plants 12 Reactor Pressure Vessel (RPV) 14 Steam generator 16 Piping 18 Coolant Pump 20 Pressurizer 22 Piping 24 Container head 26 Head stud

Claims

1. A nuclear reactor (10), A reactor pressure vessel (12) containing a plurality of fuel rods containing fissile material, the reactor pressure vessel having an upper, removable vessel top; control rods, each made of a neutron absorbing material, inserted through the vessel head into the reactor and between the fuel rods to control the rate of fission reaction in the fuel rods, whereby the control rods are movable through a normal range of insertion positions relative to the vessel head to control the power output of the reactor and to bring the reactor into subcritical shutdown when the reactor is critical and producing useful power; a control rod drive mechanism carried by the vessel head and operable to drive the movement of the control rod; Equipped with the control rod drive mechanism is controllable to release the control rod when a vessel opening operation is performed, in which the reactor is in the subcritical shutdown state and the vessel head is lifted upwardly from the reactor pressure vessel such that the control rod slides through the vessel head and remains stationary relative to the fuel rods to maintain the subcritical shutdown state; The nuclear reactor (10), further comprising a monitoring unit for identifying whether a control rod is erroneously lifted together with the vessel head.

2. The nuclear reactor (10) of claim 1, wherein the monitoring unit includes any of a plurality of position sensors, one or more neutron sensors, and one or more load sensors.

3. 3. The nuclear reactor (10) of claim 2, wherein the monitoring unit includes a position sensor that detects a position of the control rod relative to the vessel head over the normal range of insertion positions and further detects the position of the control rod relative to the vessel head over an additional range of insertion positions beyond the normal range to identify whether a control rod is inadvertently lifted with the vessel head as the vessel head is lifted and the control rod slides through the vessel head during the vessel opening operation.

4. 4. A nuclear reactor (10) according to claim 2 or 3, wherein each said position sensor is formed by an array of induction coils which detect the local presence or absence of a respective said control rod.

5. 5. The nuclear reactor (10) of claim 1, wherein the vessel head and other components of the reactor that are lifted together with the vessel head form a reactor head package, and the monitoring unit includes a load sensor that measures the weight of the reactor head package to identify whether control rods are erroneously lifted together with the vessel head.

6. The nuclear reactor (10) of any one of claims 1 to 5, wherein the monitoring unit includes a neutron sensor that measures a number of neutrons to identify if a control rod has been erroneously lifted together with the vessel head.

7. 7. The nuclear reactor (10) of any one of claims 1 to 6, further comprising a control system programmed to control the vessel opening operation, the control system commanding an abort of the vessel opening operation if it receives an indication from the monitoring unit that a control rod has erroneously been lifted together with the vessel head.

8. 8. The nuclear reactor (10) of claim 7, wherein the control system returns the vessel head to the reactor pressure vessel if the control system receives an indication from the monitoring unit that a control rod has been erroneously lifted with the vessel head.

9. The nuclear reactor (10) of claim 7 or 8 when relying on claim 3 or 4, wherein the control system is further programmed to compare a rate of rise of the vessel head with a rate of change of the detected position of the control rods relative to the vessel head to identify control rods that are erroneously lifted with the vessel head from the detected position.

10. The nuclear reactor (10) of any one of claims 7 to 9 when relying on claim 5, wherein the control system is further programmed to compare the measured weight with an expected weight value of the reactor head package to identify from the measured weight whether a control rod has been erroneously lifted together with the vessel head.

11. The nuclear reactor (10) of any one of claims 7 to 10 when relying on claim 6, wherein the control system is further programmed to compare the measured neutron count with a predetermined neutron count level and / or compare a rate of increase of the measured neutron count with a predetermined rate of increase to identify from the measured neutron count whether a control rod has been erroneously lifted together with the vessel head.

12. 1. A method of opening a nuclear reactor pressure vessel, comprising: the reactor pressure vessel contains fuel rods (201) containing fissile material and has an upper, removable vessel head, the reactor includes control rods (202) each made of a neutron absorbing material, the control rods being inserted through the vessel head into the reactor and between the fuel rods to control the rate of fission reaction in the fuel rods, whereby the control rods are movable relative to the vessel head through a normal range of insertion positions to control the power output of the reactor when the reactor is in a critical state and producing useful power, and to bring the reactor into a subcritical shutdown state, the reactor further includes a control rod drive mechanism carried by the vessel head and operable to drive movement of the control rods, The method comprises: moving the control rod using the control rod drive mechanism to an inserted position where the reactor is in the subcritical shutdown state; releasing the control rod from the control rod drive mechanism; lifting the vessel head upwardly from the reactor pressure vessel such that the control rods slide through the vessel head and remain stationary relative to the fuel rods to maintain the subcritical shutdown condition; during the step of lifting the vessel head to open the reactor pressure vessel, monitoring whether the control rods are inadvertently lifted together with the vessel head; aborting the step of lifting the vessel head if a control rod is monitored to be erroneously lifted together with the vessel head; A method comprising:

13. 13. The method of claim 12, wherein the aborting step includes returning the vessel head to the reactor pressure vessel if a control rod is monitored to be erroneously lifted with the vessel head.

14. 14. The method of claim 12 or 13, wherein the monitoring step includes detecting a position of the control rod relative to the vessel head, and comparing a rate of rise of the vessel head to a rate of change of the detected position of the control rod relative to the vessel head from the detected position to identify a control rod that is erroneously lifting with the vessel head.

15. 15. The method of any one of claims 12 to 14, wherein the vessel head and any other components of the reactor that are lifted together with the vessel head form a reactor head package, and the monitoring step includes the step of measuring a weight of the reactor head package to identify whether a control rod has been erroneously lifted together with the vessel head.

16. 16. The method of any one of claims 12 to 15, wherein the monitoring step includes measuring a neutron count to identify if a control rod has erroneously lifted with the vessel head.

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