NUCLEAR REACTOR AND METHOD OF OPENING A NUCLEAR REACTOR

Abstract

A nuclear reactor is provided. The reactor includes a reactor pressure vessel housing plural fuel rods containing fissile 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 through the vessel head and between the fuel rods to control the rate of the fuel rods' fission reaction. The control rods are movable over a normal range of insertion positions relative to the vessel head to control the power output of the reactor when it is critical and generating useful power, and to put the reactor in a sub-critical shutdown state. The reactor further includes control rod drive mechanisms carried by the vessel head and operable to drive the movements of the control rods. The control rod drive mechanisms are controllable to release the control rods when a vessel opening operation is performed in which the reactor is in the shutdown state and the vessel head is lifted upwards from the reactor pressure vessel such that the control rods slide therethrough to remain stationary relative to the fuel rods to maintain the shutdown state. The reactor further has monitoring unit to identify whether a control rod is accidently lifting with the vessel head.

Claims

1. A nuclear reactor including: a reactor pressure vessel housing plural fuel rods containing fissile material, the reactor pressure vessel having an upper, removable, vessel head; control rods, each made of a neutron-absorbing material, the control rods being inserted into the reactor through the vessel head and between the fuel rods to control the rate of the fuel rods' fission reaction, whereby the control rods are movable over a normal range of insertion positions relative to the vessel head to control the power output of the reactor when it is critical and generating useful power, and to put the reactor in a sub-critical shutdown state; and control rod drive mechanisms carried by the vessel head and operable to drive the movements of the control rods; wherein the control rod drive mechanisms are controllable to release the control rods when a vessel opening operation is performed in which the reactor is in the shutdown state and the vessel head is lifted upwards from the reactor pressure vessel such that the control rods slide therethrough to remain stationary relative to the fuel rods to maintain the shutdown state; and wherein the reactor further comprises a monitoring unit to identify whether a control rod is accidently lifting with the vessel head.

2. The nuclear reactor according to claim 1, wherein the monitoring unit comprises any of: a plurality of position sensors; one or more neutron sensors; and one or more load sensors.

3. The nuclear reactor according to claim 2, wherein the monitoring unit includes: position sensors which detect the position of the control rods relative to the vessel head over the normal range of insertion positions, and which further detect the positions of the control rods relative to the vessel head over an additional range of insertion positions beyond the normal range when the vessel head is lifted during the vessel opening operation and the control rods slide therethrough to identify whether a control rod is accidently lifting with the vessel head.

4. The nuclear reactor according to claim 2, wherein each position sensor is formed by a row of induction coils which detect the local presence or absence of a respective control rod.

5. The nuclear reactor according to claim 1, wherein the vessel head and any other components of the reactor lifted with the head form a reactor head package, and wherein the monitoring unit includes a load sensor which measures the weight of the reactor head package to identify whether a control rod is accidently lifting with the vessel head.

6. The nuclear reactor according to claim 1, wherein the monitoring unit includes a neutron sensor which measures neutron population to identify whether a control rod is accidently lifting with the vessel head.

7. The nuclear reactor according to claim 1, further including a control system which is programmed to control the vessel opening operation, the control system commanding termination of the vessel opening operation if it receives from the monitoring unit an indication that a control rod is accidently lifting with the head.

8. The nuclear reactor according to claim 7, wherein the control system returns the vessel head to the reactor pressure vessel if it receives from the monitoring unit an indication that a control rod is accidently lifting with the head.

9. The nuclear reactor according to claim 17, wherein the control system is further programmed to compare a rate of lift of the vessel head with the rates of change of the detected positions of the control rods relative to the head to identify from the detected positions a control rod which is accidently lifting with the head.

10. The nuclear reactor according to claim 18, wherein the control system is further programmed to compare the measured weight with a value of an expected weight of the reactor head package to identify from the measured weight whether a control rod is accidently lifting with the head.

11. The nuclear reactor according to claim 19, wherein the control system is further programmed to compare the measured neutron population with a predetermined population level and/or to compare a rate of increase of the measured neutron population with a predetermined rate of increase to identify from the measured neutron population whether a control rod is accidently lifting with the head.

12. A method of opening a pressure vessel of a nuclear reactor; wherein the reactor pressure vessel houses fuel rods containing fissile material and has an upper, removable, vessel head; wherein the reactor includes control rods (202), each made of a neutron-absorbing material, the control rods being inserted into the reactor through the vessel head and between the fuel rods to control the rate of the fuel rods' fission reaction, whereby the control rods are movable over a normal range of insertion positions relative to the vessel head to control the power output of the reactor when it is critical and generating useful power, and to put the reactor in a sub-critical shutdown state; and wherein the reactor further includes control rod drive mechanisms carried by the vessel head and operable to drive the movements of the control rods; the method including steps of: using the control rod drive mechanisms to move the control rods to an insertion position in which the reactor is in the shutdown state; releasing the control rods from the control rod drive mechanisms; lifting the vessel head upwards from the reactor pressure vessel such that the control rods slide therethrough to remain stationary relative to the fuel rods to maintain the shutdown state; monitoring for whether a control rods is accidentally lifting with the vessel head during the lifting of the head to open the pressure vessel; and terminating the lifting of the vessel head if a control rod is monitored to be accidently lifting with the head.

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

14. The method of claim 12, wherein the monitoring step includes detecting the positions of the control rods relative to the vessel head, and comparing a rate of lift of the vessel head with the rates of change of the detected positions of the control rods relative to the head to identify from the detected positions a control rod which is accidently lifting with the head.

15. The method of claim 12, wherein the vessel head and any other components of the reactor lifted with the head form a reactor head package, and the monitoring step includes measuring the weight of the reactor head package to identify whether a control rod is accidently lifting with the vessel head.

16. The method of claim 12, wherein the monitoring step includes measuring neutron population to identify whether a control rod is accidently lifting with the vessel head.

17. The nuclear reactor according to claim 3, further including a control system which is programmed to control the vessel opening operation, the control system commanding termination of the vessel opening operation if it receives from the monitoring unit an indication that a control rod is accidently lifting with the head.

18. The nuclear reactor according to claim 5, further including a control system which is programmed to control the vessel opening operation, the control system commanding termination of the vessel opening operation if it receives from the monitoring unit an indication that a control rod is accidently lifting with the head.

19. The nuclear reactor according to claim 6, further including a control system which is programmed to control the vessel opening operation, the control system commanding termination of the vessel opening operation if it receives from the monitoring unit an indication that a control rod is accidently lifting with the head.

Description

[0058] Embodiments will now be described by way of example only, with reference to FIG. 1, which is a schematic diagram of a PWR nuclear power plant 10.

[0059] An RPV 12 containing fuel assemblies is centrally located in the plant 10. Clustered around the RPV are three steam generators 14 connected to the RPV by pipework 16 of the pressurised water, primary coolant circuit. Coolant pumps 18 circulate pressurised water around the primary coolant circuit, taking heated water from the RPV to the steam generators, and cooled water from the steam generators to the RPV.

[0060] A pressurizer 20 maintains the water pressure in the primary coolant circuit at about 155 bar.

[0061] In the steam generators 14, heat exchangers transfer heat from the pressurised water to feed water circulating in pipework 22 of a secondary coolant circuit, thereby producing steam which is used to drive turbines which in turn drive an electricity-generator. The steam is then condensed before returning to the steam generators.

[0062] In one arrangement of the plant 10, each of the RPV 12, steam generators 14 and pressurizer 20 is contained in a respective pressure-containing silo. This makes each silo significantly smaller and easier to fabricate than a conventional containment building for the whole plant. Alternatively, the RPV 12, steam generators 14 and pressurizer 20 may be contained in a single pressure containing silo.

[0063] The RPV 12 has an upper, removable, vessel head 24 fixed in place on the vessel with head studs 26, and control rods inserted into the RPV through the vessel head and movable over a normal range of insertion positions relative to the vessel head to control the power output of the reactor when it is critical and generating useful power, and to put the reactor in a sub-critical shutdown state. This movement is actuated by control rod drive mechanisms (not shown) located on the vessel head, the control rod drive mechanisms forming, with the vessel head and other equipment items, a reactor head package.

[0064] Associated with the control rod drive mechanisms are rod position indicators which are typically in the form of rows of induction loop position sensors distributed along the channels in which the control rods reside. These sensors allow the range of insertion positions of the control rods in normal operation of the reactor to be monitored. However, the sensors are configured so that they also form part of a monitoring system to monitor for stuck control rods during an opening operation of the RPV 12 (e.g. for refueling).

[0065] More particularly, to open the RPV 12, the control rods are first fully inserted to put the reactor in a sub-critical shutdown state. The head studs 26 of the vessel head 24 are unscrewed and removed, and power and monitoring cables for the reactor head package are disconnected, except for at least one cable umbilical which provides power and communication pathway for the rod position indicators so that they remain connected and active. This umbilical may be coiled or concertinaed to accommodate lifting and repositioning of the reactor head package.

[0066] The reactor head package can then be lifted by crane. As the head package is lifted, the drive rods should remain stationary relative to the fuel assemblies and the rod position indicators should move upwards. Hence, the tops of the drive rods should appear to move down. This downwards movement is monitored by the position indicators whose rows of induction loop position sensors can be extended to detect the additional range of movement of the rods. Conversely, a rod which is stuck in its drive mechanism and lifting with the head package will appear to have no such movement. Accordingly, even after a small distance of lift, the difference in positions between non-stuck rods and a stuck rod allow a stuck rod to be identified. Another option is to compare the rates of apparent movement of the rods with rate of lift applied by the crane; non-stuck rods will appear to move at the same rate (although in the opposite direction) as the rate of lift, while stuck rods will show a different (typically zero) rate of movement.

[0067] This approach allows a stuck rod to be identified early during the lift and thereby prevent excessive rod withdrawal leading to an undesirable reactivity addition. As each rod can have its own rod position indicator, the position of a stuck rod can be readily identified by the plant operator and remedial action taken. This in turn enhances the viability of operating the reactor without using soluble boron, which would otherwise have been required to provide sufficient margin of neutron absorption were a rod to have been accidentally lifted clear of the fuel assembly.

[0068] Fuel assembles typically incorporate several centimetres of low-fissile material construction at their bases, which provides a “safe zone” at the bottom of the RPV 12. If the control rods are inadvertently lifted out of this safe zone region (but not beyond), no significant neutron population increase should occur. Thus an initial “proving” lift restricted to this safe zone can be used to confirm the presence of inadvertent control rod lifting such that safety limits are not violated and high levels of safety are provided in an environment free of soluble boron.

[0069] The plant can have a computer-based control system programed to control the opening operation. Such a system can swiftly and automatically command termination of the vessel opening operation, e.g. by returning the vessel head back to the reactor, if it identifies from the monitored positions a control rod which is lifting with the head package. It can also return the location of the stuck rod to a suitable output device. In addition, the system can be programed to e.g. compare the rate of lift of the vessel head with the rates of change of the detected positions of the control rods relative to the head, or identify any differences in detected positions, so that a control rod which is accidently lifting with the head can be identified.

[0070] Another option, which can be implemented in place of but preferably is in addition to the rod position indicators, is for the monitoring system for stuck control rods 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 quantity, and if a control rod is inadvertently lifted with the package its apparent mass will read higher than the known quantity.

[0071] Thus the computer-based control system can be linked to a lift controller of the crane. If a crane payload is sensed which is beyond the known head package mass, the control system determines that a control rod is accidently lifting with the vessel head and overrides the crane controller to terminate the lift.

[0072] Yet another option, which can be implemented in place of but preferably is in addition to either or both of the rod position indicators and the load sensor, is for the monitoring system for stuck control rods to include a neutron sensor which measures neutron population of the reactor core. This can then be used by the control system to determine if the population is increasing, decreasing or stable to identify whether a control rod is accidently lifting with the vessel head.

[0073] The computer-based control system may analyse the input from each of the sensors in the sensing unit to determine a location of a stuck fuel rod (i.e. which control rod drive mechanism in the head package is lifting the control rod). Determining a location of a stuck fuel rod may comprise identifying a signal from a sensor for which the signal identifies a stuck control rod. Consulting a data file which associates each sensor with a location; outputting to a user the location within the reactor head or fuel assembly associated with that particular sensor.

[0074] When the reactor is critical and generating useful power such a neutron sensor can be used by the reactor control systems to cause a “scram”, in which the control rods are inserted into the core thereby rapidly terminating the fission reactions if the sensed neutron population is too high or increasing at too fast a rate. In the present monitoring system, however, detection by the neutron sensor of a rising population (e.g. above a predetermined level and/or rate of increase) can be indicative of a stuck control rod and thus may be used by the control system to override the crane controller and terminate the lift.

[0075] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.