GB2577580A

GB2577580A - Graphite cores

Info

Publication number: GB2577580A
Application number: GB1906017.7A
Authority: GB
Inventors: Brennan Mark
Original assignee: Cavendish Nuclear Ltd
Current assignee: Cavendish Nuclear Ltd
Priority date: 2018-06-14
Filing date: 2019-04-30
Publication date: 2020-04-01
Anticipated expiration: 2039-04-30
Also published as: GB201809736D0; GB201906017D0; GB2577580B

Abstract

The present invention provides a method of modifying the core of an advanced gas-cooled reactor (AGR), comprising providing a graphite sleeve 12 inside one or more of a plurality of fuel channels so as to form one or more sleeved fuel channels 11, and positioning one or more control rods 20 so they are configured to be inserted into the sleeved fuel channels. The modified core therefore comprises: a plurality of fuel channels; a plurality of control rod channels 2 arranged to receive control rods 20; and one or more sleeved fuel channels 11 also arranged to receive control rods. The thick graphite sleeves 12 may be unitary structures or formed from a stack of several sleeve segments. The modified core structure allows the lifetime of the AGR to be extended by ensuring the free movement of control rods in the sleeved fuel channels, particularly in the event of an emergency, thereby negating the possible effects of graphite cracking in the existing control rod channels.

Description

GRAPHITE CORES

Introduction

The present application relates to graphite cores for advanced gas-cooled nuclear reactors (AGRs) In particular, it provides methods and apparatus for improving the safety of AGRs by modifying the graphite cores to reduce the risks posed by graphite cracking.

Background

Advanced gas-cooled reactors (AGRs) are a type of nuclear reactor that were first commissioned in the UK in 1976. The UK has seven AGR power stations comprising a total of fourteen reactors.

AGRs comprise a graphite core of around 3000 interlocking graphite bricks formed into a lattice. The graphite bricks are interconnected with graphite keys to give the core stability and maintain the bores at the correct orientation. A portion of the core structure is shown in Figure 1.

Over time, the graphite bricks in the core are exposed to high levels of radiation and to the reactor coolant. This can cause cracking in the graphite bricks as the reactors age. Core inspections of AGRs have shown that this cracking is present and that these cracks are a potential threat to the safe operation of the reactors.

There are two main types of graphite cracking that can occur: keyway root cracking; and bore cracking.

Keyway root cracking is likely to have the most significant impact on the safe operation of AGR cores. These cracks are caused later on in the reactor's life, when the stresses on the 30 outer surface of the graphite bricks become tensile rather than compressive, potentially resulting in cracks from the outside of the bricks towards the bore.

Bore cracking, or bore-initiated cracking, is a consequence of the early life conditions in the core. The graphite bricks shrink when they are initially irradiated and then later expand. Graphite that is closer to the fuel brick bore is exposed to a higher dose of radiation than the outside of the brick. This can cause cracking on the surface of the graphite brick bore, but this mechanism appears less extensive and less widespread than keyway root cracking.

To operate in a controlled and safe manner, an AGR core must allow free movement of fuel, control rods and coolant gas into the respective channels in the graphite bricks. The cracking of the graphite bricks (both keyway and bore cracking) can lead to distortion of the core 10 which could prevent or limit this free movement.

In particular, a significant safety concern is the structural stability of AGR cores in a seismic event or other emergency. If any of the control rod channels are blocked or distorted by cracked or displaced graphite bricks then this will potentially prevent a safe reactor shutdown 15 and hold-down.

The extent of graphite cracking is therefore a limiting factor for the lifetime of AGRs, as they cannot be operated if safety restrictions are breached. Currently, AGRs accounts for between 10-15% of the UK electricity supply. It is becoming increasingly important to extend their lifetime as far as possible to bridge the potential gap in UK electricity generation before the planned nuclear new build reactors come on stream.

As such, it is crucial to understand and predict the effects of graphite core ageing and implement effective solutions for extending the safe operation of AGR cores.

Summary

The present invention provides a method of modifying the core of an advanced gas-cooled reactor, the core comprising a plurality of fuel channels and a plurality of control rod 30 channels defined by graphite bricks, the method comprising: providing a graphite sleeve inside one or more of the fuel channels; and positioning one or more control rods such that they are configured to be inserted into the sleeved fuel channels.

The method of the present invention may advantageously increase the lifetime of AGR cores 5 despite the effects of graphite cracking. This is because the fuel channel has a greater inner bore diameter than the control rod channel, allowing the introduction of thick graphite sleeves with the same or a similar inner bore diameter to the control rod channels. Thus the sleeved fuel channels effectively provide new, alternative control rod channels with a think inner sleeve of graphite that is not subject to the effects of in reactor ageing. No fuel is 10 present in the sleeved fuel channels. In particular the sleeves are free from cracking, thus ensuring a clear pathway for the control rods to control, shutdown and hold-down the reactor, even in the event of core disruption such as a seismic event.

The graphite sleeves may be unitary sleeves or formed of several sleeve segments positioned 15 to substantially line the fuel channel.

Advantageously, the cost of manufacturing and installing the graphite sleeves would be minimal, making this a cost effective strategy.

The control rods are generally driven by a control rod drive system which controls the depth at which the control rods are inserted into the control rod channels, thereby controlling the nuclear reaction rate. The one or more control rods are positioned for insertion into the sleeved fuel channels.

Optionally, the control rods may be positioned such that they are at least partially inserted into the sleeved fuel channels. Optionally, the partially inserted control rods may be removable from the sleeved fuel channels.

In some embodiments, the method may comprise removing one or more of the existing 30 control rods from the control rod channels and repositioning the control rods to be inserted into the sleeved fuel channels. Thus, the method may not require any additional control rods.

In some embodiments, the method may require additional control rods to be provided. For example, the control rods inserted into the control rod channels may remain in place and additional control rods may be inserted into the sleeved fuel channels.

The graphite sleeves are preferably thick compared with a normal fuel element graphite sleeve which has a nominal thickness of about 23.55mm. Optionally, the graphite sleeves have a thickness of at least 30 mm. In some embodiments, the graphite sleeves may have a thickness of between 40 mm and 70 mm. Optionally, the graphite sleeves may be around 55.5 mm thick.

In some embodiments, the method may comprise operating the remaining fuel channels (i.e. the fuel channels without a graphite sleeve inserted) at a higher power. This may offset the power lost by converting the sleeved fuel channels into control rod channels.

Optionally, a plurality of graphite sleeves may be inserted into one or more fuel channel. For example, an axially aligned stack of graphite sleeves may be inserted into one or more fuel channels. The sleeves are stacked end on, one on top of the other.

Advantageously, manufacturing and installing a stack of shorter graphite sleeves is cheaper 20 and easier than manufacturing and installing a single continuous graphite sleeve per fuel channel.

Optionally, the graphite sleeves may be manufactured from graphite cylinders used to make standard fuel element sleeves.

Optionally, each graphite sleeve is axially interlocked with the sleeves immediately above and below it in the stack (i.e. with the adjacent sleeves).

In some embodiments, adjacent graphite sleeves may comprise complementary interlocking 30 members. This ensures that the stack of graphite sleeves in each fuel channel is a continuous stack of interlocked sleeves.

For example, adjacent graphite sleeves in the stack may comprise an end face geometry configured to interlock with the end face of the adjacent sleeves.

Optionally, adjacent graphite sleeves may comprise interlocking circumferentially 5 castellated end faces.

An AGR core is generally around 8 m in height, or more. An AGR fuel assembly usually comprises a stack of graphite bricks, each with a central bore defining the fuel channel. A plurality of fuel pins are arranged in clusters within a graphite fuel element sleeve to form a fuel element. Multiple fuel elements are linked to form a fuel stringer which is inserted into the fuel channel formed by the stack of graphite bricks. Each fuel element is around 1 m in height. Eight of the fuel elements are stacked one on top of the other in an axially aligned stack to form the fuel stringer.

The graphite sleeves of the present invention may be inserted into the fuel channels in addition to the fuel element sleeves. In other embodiments the fuel element sleeves (as well as the fuel pins) may be removed before inserting the graphite sleeves.

Optionally, the axial interfaces between the graphite sleeves in the stack may be misaligned 20 with the axial interfaces of the graphite bricks in the fuel stringer. For example, the interfaces between the graphite sleeves (i.e. the end faces of the sleeves) may not align with the end face of the graphite bricks defining the fuel channels.

This may be advantageous so that, if there was any disruption or movement in the core, for 25 example due to cracked graphite bricks in a seismic event, any impact loading on the graphite sleeves would be shared across adjacent sleeves, minimising the chances of them becoming misaligned. For example, this could be achieved by say using a graphite sleeve of 0.5 m or 1.5 m or + 0.5) m in height at the base of the stack of graphite sleeves inserted into the fuel channel.

Optionally, the one or more control rods positioned for nsertion into the sleeved fuel channels may be super articulated control rods (SACRs).

Optionally, the one or more sleeved fuel channels may be located towards the centre of the core. This may be beneficial as brick cracking in the AGR core will be more significant at the centre of the core.

In some embodiments, graphite sleeves may be inserted into at least eight fuel channels. For example, 16 central fuel channels may be sleeved.

In some embodiments, the graphite sleeves may be at least partially lined. For example, the 10 graphite sleeves may be lined with a zircaloy tube. The zircaloy tube may be continuous, such that the entire stack of sleeves in a fuel channel is fully lined.

Optionally, the method may comprise inserting one or more boron-ball shutdown devices into the one or more lined sleeved fuel channels.

The method may include providing one or more reduced diameter sleeves for fuel channel standpipes to accommodate control rod drive mechanisms. The reduced diameter sleeves may have a smaller diameter than a fuel stringer plug unit.

In a second aspect, the present invention provides a core for an advanced gas-cooled reactor (AGR), comprising: a plurality of fuel channels; a plurality of control rod channels; a plurality of control rods; and one or more sleeved fuel channels, wherein each sleeved fuel channel comprises one of the fuel channels with a graphite sleeve inserted therein; wherein one or more control rods are configured to be inserted into each of the sleeved fuel channels The advantages of the second aspect of the invention are as defined above in relation to the first aspect of the invention.

The core may be formed using the method of the first aspect of the invention to modify the core of an existing AGR.

The control rods are generally driven by a control rod drive system which controls the depth at which the control rods are inserted into the control rod channels, thereby controlling the nuclear reaction rate. Optionally, the control rods may be positioned such that they are at least partially inserted into the sleeved fuel channels. Optionally, the partially inserted control rods may be removable from the sleeved fuel channels.

Optionally, the core may comprise control rods inserted into each of the control rod channels. Additional control rods may be provided to be inserted into the sleeved fuel channels.

In some embodiments, one or more control rods are removed from the control rod channels and the control rods are repositioned to be inserted into the sleeved fuel channels.

Optionally, a plurality of graphite sleeves may be inserted into one or more fuel channels. For example, an axially aligned stack of graphite sleeves may be inserted into the one more fuel channels. The sleeves are stacked end on, one on top of the other.

Advantageously, manufacturing and installing a stack of shorter graphite sleeves is cheaper and easier than manufacturing and installing a single continuous graphite sleeve per fuel channel Optionally, the graphite sleeves may be manufactured from graphite cylinders used to make standard fuel element sleeves.

Optionally, each graphite sleeve is axially interlocked with the sleeves immediately above and below it in the stack (i.e. with the adjacent sleeves in the stack).

In some embodiments, adjacent graphite sleeves may comprise complementary interlocking 5 members. This ensures that the stack of graphite sleeves in each fuel channel is a continuous stack of interlocked sleeves.

Optionally, adjacent graphite sleeves may comprise interlocking circumferentially castellated end faces.

The graphite sleeves of the present invention may be inserted into the fuel channels in 15 addition to the fuel element sleeves. In other embodiments the fuel element sleeves may be removed before inserting the graphite sleeves.

Optionally, the axial interfaces between the graphite sleeves in the stack may be misaligned with the axial interfaces of the graphite bricks in the fuel stringer. For example, the interfaces 20 between the graphite sleeves (i.e. the end faces of the sleeves) may not align with the end face of the graphite bricks defining the fuel channels.

This may be advantageous so that, if there was any disruption or movement in the core, for example due to cracked graphite bricks in a seismic event, any impact loading on the graphite sleeves would be shared across adjacent sleeves, minimising the chances of them becoming misaligned. For example, this could be achieved by say using a graphite sleeve of 0.5 m or 1.5 m or + 0.5) m in height at the base of the stack of graphite sleeves inserted into the fuel channel.

Optionally, the one or more control rods positioned for insertion into the sleeved fuel channels may be super articulated control rods (SACRs).

In some embodiments, the modified core may comprise at least eight sleeved fuel channels. For example, around 16 centrally located fuel channels may comprise graphite sleeves.

In some embodiments, the graphite sleeves may be at least partially lined. For example, the graphite sleeves may be lined with a zircaloy tube. The zircaloy tube may be continuous, 10 such that the entire stack of sleeves in a fuel channel is fully lined.

Optionally, one or more boron-ball shutdown devices may be configured to be inserted into the one or more lined sleeved fuel channels.

Optionally, the core comprises one or more reduced diameter sleeves for fuel channel standpipes to accommodate the control rod drive mechanisms which have a smaller diameter than a fuel stringer plug unit.

Illustrative embodiments of the invention will now be described with reference to the 20 accompanying drawings, in which: Figure 1 is a prior art example of interlocked graphite bricks in an AGR core; Figure 2 shows a portion of an AGR core in accordance with an embodiment of the present 25 invention; Figure 3 shows an illustration of a control rod channel and a sleeved fuel channel according to an embodiment of the present invention; and Figure 4 shows a plan view of a cross-section of an AGR core in accordance with an embodiment of the present invention.

It should be appreciated that figures are illustrative drawings that are not shown to scale.

Figure 1 shows graphite bricks in an AGR core in accordance with the prior art. The graphite bricks comprise through bores which define fuel channels 1, gas coolant channels 3 and 5 control rod channels 2.

The graphite bricks are keyed to ensure that adjacent bricks lock together. The graphite bricks comprise filler keys 7, bearing keys 6, integral bricks 5 and filler bricks 4.

The fuel channels 1 are configured to receive fuel elements for powering the nuclear reactions in the core. The control rod channels 2 are configured to receive control rods which are inserted into the channels 2 to control the rate of reactions in the core.

Figure 2 shows a portion of an AGR core in accordance with an embodiment of the present 15 invention. Features in common with Figure 1 have been numbered accordingly.

As shown in Figure 2, a thick graphite sleeve 12 is inserted into one of the fuel channels, forming a sleeved fuel channel 11. The graphite sleeve 12 is around 55 mm thick as this ensures that the control rods have sufficient room to be inserted into the graphite sleeve 12.

Control rods 20 are inserted into the control rod channel 2 and the sleeved fuel channel 11. The control rods 20 may be super articulated control rods. The insertion depth of the control rods 20 is controlled by a drive system (not shown) to control the nuclear reaction rate in the core.

Figure 3 shows a simplified illustration of a control rod channel 2 and a sleeved fuel channel 11.

The control rod channel 2 is formed of a graphite brick with a central bore. The control rod 30 bore has an inner diameter, A, of around 127 mm.

The fuel channel 1 in Figure 1 is formed from a stack of a graphite bricks each having a height of around 1 m. The inner bore of the fuel channel 1 has a diameter of around 263.5 mm. Normally a fuel channel 1 would contain a stringer of eight approximately 1 m long fuel elements each with a fuel element sleeve 15.

The fuel element sleeve 15 has a thickness of around 23 55mm, with an inner diameter of 191 mm and an outer diameter of 238.1 mm.

For a sleeved fuel channel 11, an additional graphite sleeve 12 is inserted into the fuel 10 channel inside an empty (i.e. containing no fuel of fuel support structures) fuel element sleeve 15. The graphite sleeve 12 forms part of an axially aligned stack of graphite sleeves which is inserted into the fuel channel.).

In some embodiments, the graphite sleeves 12 are axially mis-aligned with the bricks forming the fuel channel, and the fuel element sleeve 15, as shown in Figure 3. This ensures that the axial interfaces of the graphite sleeve stack are misaligned with the interfaces between fuel element sleeve stack and/or the graphite brick stack. This may improve the stability of the two concentric stacks, graphite sleeves 12 and fuel element sleeves 15. For example, this could be achieved by say using a graphite sleeve of 0.5 m or 1.5 m or (n + 0.5)m in height at the base of the stack of graphite sleeves inserted into the fuel channel Each graphite sleeve 12 has complementary interlocking formations 13 disposed on the outer face of the sleeve 12, to allow adjacent sleeves in the stack to lock together. The graphite sleeves 12 are placed end-on in the stack. In the illustrated example the interlocking formations 13 are crenulations or keys which are inserted into complementary indentations in the adjacent sleeve 12.

The graphite sleeves 12 have a thickness of around 31.5 mm and an outer diameter of about 190 mm. This ensures that the graphite sleeves 12 fit within the fuel element sleeves 15 and 30 provide a central bore which is a similar size to the control rod channel, thereby allowing sufficient clearance for the control rods to be inserted into the sleeved fuel channels.

In other embodiments, the thicknesses of the two sets of sleeves, graphite sleeves 12 and fuel element sleeves 15, could differ while maintaining the overall thickness, outer diameter and inner bore diameter of the combined concentric stacks of sleeves.

In other embodiments, the fuel element sleeve 15 is removed from (or not inserted into) the fuel channel 11, as in Figure 2. A thicker graphite sleeve 12 can then be used, for example having an outer diameter of around 237 mm and a thickness of around 55.5 mm. This may provide a more robust structure (e.g. against impact damage from cracked fuel channel bricks in a seismic event) than concentric stacks of graphite sleeves 12 and fuel element sleeves 15.

Figure 4 shows a simplified plan view of a cross-section of an AGR reactor n accordance with the apparatus in Figure 2 and Figure 3.

Reflector bricks 9 are provided at the outer portion of the core. Seven of the fuel channels 15 have been converted into sleeved fuel channels 11 by inserting graphite sleeves 12. In other examples, the number and position of the sleeved fuel channels 11 may vary.

Control rods 20 are at least partially inserted into the sleeved fuel channels 11 and into a plurality of the control rod channels 2. Two of the control rod channels 2 are shown with the control rods removed, as these have been removed to be inserted into the sleeved fuel channels 1 1. In other examples, the number and position of the empty control rod channels may vary.

In practice, the sleeved channels 11 may not be adjacent to one another. In order to safely shutdown and holdown the nuclear reaction in all parts of the reactor core it may be necessary for the sleeved control rod 11 positions to be evenly distributed across the core, with some degree of concentration towards the centre of the core because this is where the neutron flux concentration driving the reaction is greatest, and where the levels of brick cracking are the highest.

In some embodiments, one or more of the graphite sleeves 12 may be lined with a zircaloy tube. The control rods 20 inserted into the lined sleeved fuel channels may be replaced by boron-ball shutdown devices.

The modified core structure allows the lifetime of the AGR to be safely extended by ensuring free movement of the control rods in the sleeved fuel channels, particularly in an emergency event, thereby negating the possible effects of graphite cracking on the control rod channels.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. In the present specification, "comprises" means "includes or consists of and "comprising" means "including or consisting of'. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS1. A method of modifying the core of an advanced gas-cooled reactor, the core comprising a plurality of fuel channels and a plurality of control rod channels defined by 5 graphite bricks, the method comprising: providing a graphite sleeve inside one or more of the fuel channels; and positioning one or more control rods such that they are configured to be inserted into the sleeved fuel channels.
2. The method of claim 1, wherein the one or more control rods are positioned such that they are at least partially inserted into the sleeved fuel channels.
3. The method of claim 1 or claim 2, comprising removing one or more of the control rods from the control rod channels before repositioning the control rods to be inserted into 15 the sleeved fuel channels.
4. The method of any of claims 1 to 3, wherein each graphite sleeve has a thickness of at least 30 mm, and/or between 40 mm and 70 mm.
5. The method of claim 4, wherein each graphite sleeve is around 55.5 mm thick.
6. The method of any preceding claim, wherein an axially aligned stack of graphite sleeves is inserted into the one or more fuel channels, wherein the graphite sleeves are stacked end on, one on top of the other.
7. The method of claim 6, comprising axially interlocking adjacent graphite sleeves within the stack.
8. The method of any preceding claim, wherein the one or more sleeved fuel channels 30 are located proximate the centre of the core.
9. The method of any preceding claim, comprising removing a fuel element sleeve from the one or more fuel channels before inserting the graphite sleeve.
10. A core for an advanced gas-cooled reactor (AGR), comprising: a plurality of fuel channels; a plurality of control rod channels; a plurality of control rods; and one or more sleeved fuel channels, wherein each sleeved fuel channel comprises one of the fuel channels with a graphite sleeve inserted therein; wherein one or more control rods are configured to be inserted into each of the sleeved fuel channels.
11. The core of claim 10, wherein the one or more control rods are positioned such that they are at least partially inserted into the sleeved fuel channels.
12. The core of claim 10 or claim 11, wherein control rods are at least partially inserted into each of the control rod channels in addition to the sleeved fuel channels.
13. The core of claim 10 or claim 11, wherein one or more control rods are removed 20 from the control rod channels and repositioned to be inserted into the sleeved fuel channels.
14. The core of any of claims 10 to 13, wherein each graphite sleeve has a thickness of at least 30 mm, and/or between 40 mm and 70 mm.
15. The core of claim 14, wherein each graphite sleeve is around 55.5 mm thick.
16. The core of any of claims 10 to 15, wherein the one or more sleeved fuel channels comprise an axially aligned stack of graphite sleeves inserted therein, wherein the graphite sleeves are stacked end on, one on top of the other.
17. The core of claim 16, wherein adjacent graphite sleeves in the stack comprise complementary interlocking members to axially align and interlock adjacent graphite sleeves.
18. The core of claim 17, wherein the complementary interlocking members comprise crenulat ons disposed on the end faces of the graphite sleeves.
19. The core of any of claims 10 to 18, wherein the one or more control rods positioned for insertion into the sleeved fuel channels are super articulated control rods (SACRs).
20. The core of any of claims 10 to 19, wherein the one or more sleeved fuel channels are located proximate the centre of the core.
21. The core of any of claims 10 to 20, wherein at least eight sleeved fuel channels are provided.
22. The core of any of claims 10 to 21, wherein the graphite sleeves are at least partially lined with a zircaloy tube.
23. The core of claim 22, wherein one or more boron-ball shutdown devices are positioned for insertion into one or more of the zircaloy lined sleeved fuel channels.
24. The core of any of claims 10 to 23, comprising one or more reduced diameter sleeves inserted into fuel channel standpipes to accommodate control rod drive mechanisms.
25. The core of any of claims 10 to 24, wherein the sleeved fuel channels comprise a fuel element sleeve in addition to the graphite sleeve inserted therein.