GB2544114A

GB2544114A - Detection Unit

Info

Publication number: GB2544114A
Application number: GB1519737.9A
Authority: GB
Inventors: Mitchell Alex
Original assignee: Rolls Royce Power Engineering PLC
Current assignee: Rolls Royce Power Engineering PLC
Priority date: 2015-11-09
Filing date: 2015-11-09
Publication date: 2017-05-10
Also published as: GB201519737D0

Abstract

A nuclear spent fuel detection and handling unit has thermocouples 50 for monitoring the temperature and hence energy being emitted by a nuclear fuel rod. If the temperature is below a certain value then the nuclear fuel rod is considered to be spent. The handling unit comprises jaws 46 that are adapted to grip a hexagonal rod which allows a spent fuel rod to be removed and replaced. A detector is attached to or integrated in the shroud member for measuring an indicator of the energy output of the fuel assembly.

Description

DETECTION UNIT

TECHNICAL FIELD

The present disclosure concerns a spent fuel detection unit, a spent fuel removal device and/or system, a nuclear power plant, and/or a method of detecting and/or removing spent fuel from a reactor.

BACKGROUND A nuclear power plant generally includes a primary circuit and a secondary circuit. The primary circuit includes a reactor vessel where nuclear fuel is used to heat fluid flowing in the primary circuit. Fluid of the primary circuit is used to heat fluid of the secondary circuit via a heat exchanger. The heated secondary fluid is then typically used to drive a turbine so as to generate electricity.

Once the nuclear fuel has reached its useful life it is often referred to as “spent fuel”. Spent fuel is generally removed from the reactor, sent for storage and then reprocessing and/or disposal. The spent fuel is removed by gripping an end of the fuel assembly and moving it vertically out of the reactor. Once the spent fuel is removed from the reactor fresh fuel will be provided.

The decay of radioactive elements in the spent fuel creates heat long after the fuel assembly ceases to be used to generate power. The heat generated by the fuel assembly must be removed to prevent the element overheating.

For the fuel to be safely removed from the reactor it must not emit more heat than the fuel handling system is capable of removing. Often the decay heat power output of each individual fuel assembly is estimated based on measurable variables in the core; knowledge of the power extracted from the core over its life and details of the composition of the fuel before its entry to the reactor.

The estimate generated must by definition be very conservative and therefore the amount of time the fuel is left to cool down is much longer than would be required if a more accurate measure of the decay power of the fuel was known.

SUMMARY

According to a first aspect there is provided a nuclear spent fuel detection unit. The detection unit comprises a shroud member for at least partially surrounding a fuel assembly. A detector is attached to or integrated in the shroud member for measuring an indicator of energy output of the fuel assembly.

The energy output of the fuel assembly may be detected either directly or indirectly. Alternatively an indicator of energy, e.g. of heat energy, may be measured instead of detecting the energy output. For example, the temperature of the fuel assembly may be detected.

The detector may comprise a temperature sensor, e.g. thermocouples.

The detector may contact the fuel assembly from a longitudinal side of the fuel assembly.

The shroud member may be arranged to have a detection (or closed) configuration where the indicator of energy output of a fuel assembly is detected and a release (or open) configuration where the shroud member is positioned to ease removal of the fuel assembly from the unit.

The shroud member may be configured to fully circumscribe a fuel assembly so as to encapsulate the fuel assembly in the detection configuration. For example, the shroud member may be operable to fully circumscribe a fuel assembly so as to encapsulate the fuel assembly in the detection configuration.

The shroud member may be configured to grip the fuel assembly when in the detection configuration.

The shroud member may include a fixed portion and a moveable portion, the moveable portion being arranged to be moveable so as to define the detection configuration and the release configuration.

The unit may be arranged for use with a rotary fuel assembly transfer device, and the unit may be configured such that rotation of the fuel assembly transfer device actuates the unit between the detection configuration and the removal configuration.

The unit may comprise gears that engage the rotary fuel assembly transfer device.

The gears may rotate a cam that engages a follower for moving at least a portion of the shroud member between a position defining the detection configuration and a position defining the removal configuration.

The follower may be biased for contact with the cam, e.g. using a spring.

The follower may be pivotally connected to two arms that are each connected to a moveable portion of the shroud member. In this way, linear movement of the follower may cause rotational movement of the arms.

An elongate member may be connected between each of the arms and the moveable portion of the shroud member.

The unit may be for use with a fuel assembly having a hexagonal cross section and the member defines six planar faces for contacting the fuel assembly.

Two adjacent planar faces may form part of a moveable portion of the member and the remaining four adjacent planar faces may form part of a fixed portion of the member.

According to a second aspect there is provided a fuel assembly replacement system comprising a rotary fuel assembly removal device and a unit according to the first aspect.

According to a third aspect there is provided a method of detecting the energy output of a spent fuel assembly, the method comprising using a mechanical sensor to detect an indicator of the energy output from the spent fuel assembly prior to removal from a fuel assembly removal device.

According to a fourth aspect there is provided a fuel assembly replacement system comprising a rotary device arranged to support one end of a fuel assembly; and a gripper arranged to grip a fuel assembly from an elongate (or longitudinal) side of the fuel assembly.

The gripper may be configured to fully circumscribe the fuel assembly when gripping the fuel assembly.

The gripper may include a fixed portion and a moveable portion. The moveable portion may be arranged to move the gripper between a gripping position and a release position.

The gripper may be configured such that rotation of the rotary device actuates movement of the moveable portion between the gripping position and the release position.

The gripper may comprise gears that engage the rotary device.

The gears may rotate a cam that engages a follower for moving the moveable portion of the gripper between the gripping position and the release position. The follower may be biased for contact with the cam, e.g. using a spring.

The follower may be pivotally connected to two arms that are each connected to a moveable portion of the gripper, such that linear movement of the follower causes rotational movement of the arms. An elongate member may be connected between each of the arms and the moveable portion of the gripper.

The system may be for use with a fuel assembly having a hexagonal cross section and gripper may define six planar faces for contacting the fuel assembly. Two adjacent planar faces may form part of a moveable portion of the member and the remaining four adjacent planar faces form part of a fixed portion of the member.

The gripper may be the nuclear spent fuel detection unit of the first aspect.

According to a fifth aspect there is provided a method of removing a fuel assembly from a reactor, the method comprising gripping the fuel assembly from a longitudinal side, and removing the fuel assembly from the reactor and positioning the fuel assembly in a fuel removal device.

The fuel assembly may be gripped using the gripping member of the fuel assembly arrangement of the fourth aspect. The fuel assembly may be removed using the fuel assembly replacement system of the fourth aspect.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.] - wording used in final draft as part of summary.

DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with reference to the Figures, in which:

Figure 1 is a schematic of a nuclear power plant;

Figure 2 is a sectional view of a fuel assembly replacement system; Figure 3A is a plan view of a spent fuel detection unit of the fuel assembly replacement system of Figure 2 positioned for receiving a fuel assembly;

Figure 3B is a plan view of a spent fuel detection unit of the fuel assembly replacement system of Figure 2 positioned for releasing a fuel assembly for removal from the unit;

Figure 4A is a plan view of an alternative spent fuel detection unit for the removal device of Figure 2 positioned for receiving or removal of a fuel assembly;

Figure 4B is a plan view of the spent fuel detection unit of Figure 4A positioned for detecting the power output of the fuel assembly; and

Figures 5A to 5D show plan views of the unit of Figures 4A and 4B at various positions during a rotation of the fuel assembly replacement system.

DETAILED DESCRIPTION

Referring to Figure 1, a sodium fast reactor plant is indicated generally at 10. The reactor plant includes a first sodium circuit 13, a second sodium circuit 15 and a nitrogen gas cycle 17. The first sodium circuit 13 includes a reactor core 12 where heat energy is generated. A first flow of liquid sodium is circulated to the core for cooling. In the cooling process the liquid sodium is heated. The first flow of liquid sodium is then circulated from the core to a first heat exchanger 14 before being circulated back to the core. The liquid sodium is circulated around the first sodium circuit by a pump 16.

The second sodium circuit 15 includes a second flow of liquid sodium. The second flow of liquid sodium is circulated to the first heat exchanger 14 where heat is transferred from the first flow of liquid sodium to the second flow of liquid sodium. The second flow of liquid sodium then flows to a second or intermediate heat exchanger 20 before being circulated back to the first heat exchanger. The second flow of liquid sodium is circulated around the second sodium circuit by a pump 18.

The nitrogen gas cycle 17 includes a flow of nitrogen gas. The nitrogen gas is circulated to the intermediate heat exchanger 20 where heat is transferred from the second flow of liquid sodium to the nitrogen. Nitrogen then flows from the heat exchanger to a turbine 22. The turbine 22 is connected to a compressor 24 and an electrical generator 26. Nitrogen gas flows from the turbine 22 to a recuperator 28, then to a heat sink 30, before being directed to the compressor 24. The nitrogen gas flows from the compressor to the recuperator where the nitrogen gas is pre-heated before being directed to the intermediate heat exchanger 20. In this way electricity can be generated from a nuclear core.

During the life of a nuclear power plant such as the described plant 10, the fuel assemblies used in the reactor 12 become depleted (referred to as “spent fuel”) and need to be replaced with new fuel assemblies. Removal of the fuel assemblies can be done in a number of ways. One such way is to use a fuel assembly replacement system, such as the one indicated generally at 31 in Figure 2. The system includes a rotary removal device 32 and a spent fuel detection unit 44.

Referring to Figure 2, it can be seen that the nuclear fuel assemblies 34, 36 in this example are hexagonal in cross section and extend along a longitudinal axis. Flowever, in alternative embodiments the fuel assemblies may have an alternative cross section, for example the fuel assemblies may have a circular cross section, e.g. the fuel assemblies may be cylindrical.

When it is expected that the power output from a fuel assembly 34 has reduced to a predetermined level, the fuel assembly is removed from the reactor and replaced with a new fuel assembly 36. To remove the fuel assembly 34 from the reactor, the fuel assembly is gripped from one end and positioned in rotary fuel removal device 32 (which may also be referred to as a rotor unit). A new fuel assembly 36 is positioned in the removal device at an opposing side of the device. The removal device is rotated so as to present the new fuel assembly for positioning in the reactor and to present the spent fuel assembly for removal from the reactor.

The removal device includes a central drive shaft 38 which connects to a base 40. The base 40 includes recesses 42 that are shaped and dimensioned for receiving an end portion of the fuel assemblies.

Referring to Figures 3A and 3B, one example of the spent fuel detection unit 44 includes a shroud member 46 for surrounding at least a portion of the fuel assembly 34. In this example, the fuel assembly covers a portion of four of the six faces of the fuel assembly 34. The shroud member 46 in this example includes only members that are in a fixed position with respect to the rest of the unit and the fuel assembly removal device 32. The shroud member is connected to the central drive shaft 38 of the removal device. The shroud member is connected to the drive shaft via a connection member 48. As the drive shaft rotates the connection member 48 rotates, which rotates the shroud with the fuel assembly removal device and the fuel assembly.

The unit 44 includes a sensor for detecting the energy output of the fuel assembly so that the power output can be calculated. The sensor in this example is a plurality of thermocouples 50 that are positioned on the shroud member on each face that faces the spent fuel assembly.

The thermocouples 50 are connected to a control system 52 that calculates the power output and outputs this information to an operator or to a further control system.

Providing a physical detector for the power output of a spent fuel assembly mitigates the risk of a fuel assembly being removed before it has depleted to the predetermined minimum energy output.

The control system 52 may comprise: at least one application specific integrated circuit (ASIC); and/or at least one field programmable gate array (FPGA); and/or single or multi-processor architectures; and/or sequential (Von Neumann)/parallel architectures; and/or at least one programmable logic controllers (PLCs); and/or at least one microprocessor; and/or at least one microcontroller. By way of an example, the control system 52 may comprise at least one processor and at least one memory. The memory may store a computer program comprising computer readable instructions. The computer program may be software or firmware, or may be a combination of software and firmware.

Referring to Figures 4A and 4B, an alternative unit 144 will now be described. Similar to the previously described unit 44, the unit 144 includes a shroud 146 and a sensor is integrated into the unit, for example thermocouples are provided on each of the faces that contact the fuel assembly. The shroud 146 is connected to the drive shaft 38 of the removal device via a connection member 148, such that the shroud (and the rest of the unit) rotate with the removal device.

The unit 144 is configured to fully circumscribe the fuel assembly during detection, e.g. to encapsulate the fuel assembly 34 at the position of the unit 144. In the example of Figures 4A and 4B, the unit 144 is configured to grip the fuel assembly, which optionally means that the fuel assembly can be positioned in the removal device using the unit 144, avoiding the need for the fuel assembly to be gripped from one end and vertically inserted into the removal device.

The shroud 146 of the unit 144 includes a fixed portion 154 and two moveable portions 156. The fixed portion is similar to the shroud member 46 of the previous example and is arranged to contact four adjacent faces of a spent fuel assembly 34. One of the two moveable portions is moveable to contact one of the remaining faces of the fuel assembly and the other of the moveable portions is moveable to contact the other of the remaining faces of the fuel assembly, in this way the shroud can fully circumscribe the fuel assembly. The moveable portions 156 may be considered to be jaws of the shroud. In this example, the shroud 146 also grips the fuel assembly and so the moveable members are moved to an extent that they exert a pressure onto the fuel assembly so as to grip the fuel assembly.

The moveable portions 156 of the shroud 146 are connected to the drive shaft 38 of the fuel removal device via a gear arrangement and a cam and follower arrangement. A ring gear 158 is provided concentric to and driven by and at the same rotational velocity as the drive shaft 38. A planetary gear 160 engages with the ring gear. In this example the gear ratio is 1:4, however in alternative embodiments an alternative gear ratio may be used. The gear ratio is selected such that the moveable portions 156 of the shroud 146 move between the release (or open) position and the detection/gripping (or closed) position at the desired locations. A cam 162 is connected to the planetary gear 160 such that the cam rotates with the planetary gear. In this example, the cam is a pear-shaped cam. A follower 164 contacts the cam and linearly reciprocates as the cam rotates. A guide 170 is provided to restrict movement of the follower to substantially as single axis. The follower 164 includes a roller 168 for contacting the cam 162. The follower further includes a spring 166 that contacts the guide and is biased to keep the follower in contact with the cam, i.e. to ensure positive engagement of the follower with the cam.

The follower 164 is pivotally connected to two arms 172, such that linear movement of the follower causes rotational movement of the arms. That is, it may be considered that a hinge is provided between the follower and the two arms. A joining member 174 is provided transverse to the arms and connects to the respective moveable portion 156 of the shroud.

Operation of the unit 144 will now be described with reference to Figures 5A to 5D. When the unit 144 is in a position for receiving the fuel assembly 34, the unit 144 has a release configuration, so as to ease insertion of the fuel assembly 34. In the release configuration, the follower 164 is positioned furthest from the drive shaft 38 of the removal device. This means that the arms 172 are pivoted and the moveable portions 156 of the shroud 146 are positioned away from the fuel assembly region.

Once the unit 144 is in position, a fuel assembly 34 is inserted into the removal device. As described previously this may be done vertically (longitudinally), or because of the unit’s ability to grip the fuel assembly the unit 144 may be moved to the fuel assembly, grip the fuel assembly, and then move it to the removal device in a horizontal direction (or radial direction with respect to the unit).

The removal device 32 and unit 144 are then rotated. In Figures 5A to 5D, the position where the fuel is received is indicated at 270°. Rotation through 45° moves the follower 164 to a position nearest to the drive shaft 38. This movement of the follower pivots the arms 172 such that the moveable portion 156 of the shroud 146 contacts the fuel assembly. That is, the shroud can be considered to be in a detection and/or gripping configuration. The gear ratio and the shape of the cam are such that at 45° and 135° the moveable portion is in the detection configuration. When the shroud is in the detection configuration, the temperature of the fuel assemblies is detected using the thermocouples to indicate the power output of the fuel assembly.

Further rotation of the removal device commences movement of the moveable portions 156 to arrange the shroud 146 in the release configuration, such that at a position indicated at 90° in Figures 5A to 5D the shroud is in the release configuration. At the position indicated at 90° (which is 180° from the position of insertion of the fuel assembly) the fuel assembly is removed, if the power output has reduced to the predetermined level.

The described units 44 and 144 enable in-situ physical measurement of the power output of a spent fuel assembly.

The unit 144 fully surrounds the fuel assembly encapsulating it in a uniform manner, whilst still enabling radial transfer to the rotor unit. Full encapsulation of the fuel assembly improves the accuracy of the power output measurement and provides an additional point to secure the fuel assembly. The described arrangement also provides the option of the unit gripping the fuel assembly.

The use of the drive shaft to drive the positioning of the unit and the moveable portions of the shroud means that the mechanism can be simplified and there is no need for pneumatics or additional motors to move the shroud around the fuel assembly.

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 subcombinations of one or more features described herein.

For example, the unit 144 may be used only for gripping a fuel assembly. In such embodiments no sensor may be provided in or connected to the unit 144.

The unit 44, 144 may be modified to receive multiple fuel assemblies. In such embodiments, the unit will include a plurality of planetary gears; e.g. one planetary gear associated with each shroud.

The units 44, 144 have been described for use in a nuclear fuel replacement system, however in alternative embodiments the unit may be provided in other locations of the nuclear power plant, for example in a transfer cask that receives spent fuel from the reactor. In such embodiments, the rotary fuel assembly device may be replaced with an alternative rotary fuel assembly transfer device.

Claims

1. A nuclear spent fuel detection unit having: a shroud member for at least partially surrounding a fuel assembly; and a detector attached to or integrated in the shroud member for measuring an indicator of energy output of the fuel assembly.

2. The unit according to claim 1, wherein the detector comprises a temperature sensor.

3. The unit according to claim 1 or 2, wherein the detector contacts the fuel assembly from a longitudinal side of the fuel assembly.

4. The unit according to any one of the previous claims, wherein the member is arranged to have a detection configuration where the indicator of energy output of a fuel assembly is detected and a release configuration where the shroud member is positioned to ease removal of the fuel assembly from the unit.

5. The unit according to claim 4, wherein the shroud member is configured to fully circumscribe a fuel assembly so as to encapsulate the fuel assembly in the detection configuration.

6. The unit according to claim 4 or 5, wherein the shroud member is configured to grip the fuel assembly when in the detection configuration.

7. The unit according to any one of claims 4 to 6, wherein the shroud member includes a fixed portion and a moveable portion, the moveable portion being arranged to be moveable so as to define the detection configuration and the release configuration.

8. The unit according to any one of claims 4 to 7, wherein the unit is arranged for use with a rotary fuel assembly transfer device, and wherein the unit is configured such that rotation of the fuel assembly transfer device actuates the unit between the detection configuration and the removal configuration.

9. The unit according to claim 8, wherein the unit comprises gears that engage the rotary fuel assembly transfer device.

10. The unit according to claim 9, wherein the gears rotate a cam that engages a follower for moving at least a portion of the shroud member between a position defining the detection configuration and a position defining the removal configuration.

11. The unit according to claim 10, wherein the follower is biased for contact with the cam.

12. The unit according to claim 10 or 11, wherein the follower is pivotally connected to two arms that are each connected to a moveable portion of the shroud member, such that linear movement of the follower causes rotational movement of the arms.

13. The unit according to any one of the previous claims, wherein the unit is for use with a fuel assembly having a hexagonal cross section and the member defines six planar faces for contacting the fuel assembly.

14. A fuel assembly replacement system comprising a rotary fuel assembly removal device and a unit according to any one of the previous claims.

15. A method of detecting the energy output of a spent fuel assembly, the method comprising: using a mechanical sensor to detect an indicator of the energy output from the spent fuel assembly prior to removal from a fuel assembly removal device.

16. A fuel assembly replacement system comprising: a rotary device arranged to support one end of a fuel assembly; and a gripper arranged to grip a fuel assembly from an elongate side of the fuel assembly.

17. The fuel assembly according to claim 16, wherein the gripper is configured to fully circumscribe the fuel assembly when gripping the fuel assembly.

18. A method of removing a fuel assembly from a reactor, the method comprising: gripping the fuel assembly from a longitudinal side, removing the fuel assembly from the reactor and positioning the fuel assembly in a fuel removal device.

19. A unit, system and/or method substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.