GB2609628A - A nuclear reactor - Google Patents

Abstract

There is provided a nuclear reactor 21, 23 comprising a reactor vessel (37, Fig. 6) having a central longitudinal axis (30) defining an upper (40) and lower (42) end. Within the reactor vessel is an inner conduit (31) for transporting fluid, e.g. light water. The reactor 21 is configured to stack end-to-end along the central longitudinal axis with one or more further nuclear reactors 23 such that when stacked, fluid can flow from the inner conduit of the reactor to the one or more further reactors. Also disclosed is a method of installing a nuclear reactor in a borehole, comprising: positioning a reactor above a borehole, wherein the borehole is lined with a tubular casing; connecting the reactor to one or more further reactors to provide a reactor stack 5; and lowering the stack into the borehole such that an annulus is provided between the exterior of the stack and the tubular casing.

Description

A Nuclear Reactor

Field of the Invention

The present invention concerns nuclear reactors. More particularly, but not exclusively, the present invention concerns stackable nuclear reactors. The invention also concerns methods of installing stackable nuclear reactors in boreholes.

Background of the Invention

It is known to site nuclear reactors underground. Benefits provided by locating nuclear reactors underground include inherent safety in the event of reactor failure, protection against external threats, and ease of containing nuclear waste.

US4851183 discloses a small nuclear reactor located underground. The reactor is self-regulating, and shaped to fit within a narrow straight-sided borehole of depth -200- 500m. The system has gas-driven doors for fast closure of the borehole. The reactor type is a compact, fast neutron breeder reactor using highly enriched uranium and/or plutonium fuel with pressurised gas heat pipes for energy recovery from and neutron control of the reactor. The gas can be lithium fluoride, lithium or beryllium difluoride. Drawbacks inherent in fast neutron reactors such as this, tend to be the expense of build and operation, complexity of operation and repair, and potentially problematic by-products.

Other types of nuclear reactor have been proposed for underground deployment.

For example, us201eio1s9212A1 discloses a hybrid geothermal-nuclear power system utilising a modified pebble bed reactor and a water heat transfer system. Wells are drilled into bedrock at -4400m-5000m (HDR zone), and a reactor is positioned within the well to heat the rock. Water is pumped at low pressure (30-40psi) from a pumping station above ground along a single U-shaped conduit extending underground beneath the bottom of the boreholes, and back up to be extracted as vapour at high pressure (3000-6000psi at -500degC) at a power station above ground. The nuclear reactor is situated in one or more vertical shafts adjacent the water conduit, and used to combat temperature losses in the water during extraction by heating the surrounding bedrock. Drawbacks associated with pebble bed reactors such as this tend to centre on issues of waste handling (high volumes), and the use of combustible graphite fuel casing (which also serves as a moderator). In addition, very high temperatures can pose challenges for materials. Further problems that may be encountered when using a pebble bed reactor underground include difficulties in inspecting fuel for damage, and providing adequate temperature control of the reactor. The benefits of siting nuclear reactors underground are not fully recognised and there is a general desire to explore the concept further, for example, using alternative reactor types. In particular, there is a need to expand on the locational possibilities for underground reactors, and provide economically viable methods of construction and installation. The present invention seeks to provide an alternative and improved, nuclear reactor for underground use. The present invention also seeks to provide a suitable method of installing such a nuclear reactor underground.

Summary of the Invention

In a first aspect of the invention there is provided a nuclear reactor according to claim 1.

The present invention recognises that a nuclear reactor can be configured to be stackable (for example, vertically) in a modular fashion wherein a plurality of individual reactors can be connected end to end to provide a reactor system. The reactor can function as a single standalone unit, and moreover, can function when stacked as part of the larger reactor system. The inventors have recognised that the alignment and interconnection of the individual reactor units when stacked enables the overall reactor system to operate as a single, larger, reactor wherein fluid can flow in a pathway through the reactor system. Since each reactor unit has an inner conduit which is open-ended at both ends, when the reactor units are aligned and connected, fluid can flow through from one reactor unit to the next. Any number of individual reactor units may be joined end to end to provide a larger functioning reactor system, thus enabling custom solutions for different locations and operating requirements. Compared to small and micro reactors installed underground as described in the prior art, the power output capability of the present reactor system is significantly increased, whilst retaining the passive safety, security, environmental protection and decommissioning benefits of underground deployment. The reactor of the present invention may be sacrificial, wherein after use, it can be safely abandoned downhole. In a further advantage, the present invention also recognises that a vertically stackable reactor may be suitable for siting in an underground borehole (newly drilled or a repurposed wellbore). Thus, existing decommissioned oil and gas and geothermal wellbores can be re-used for nuclear generation using the reactor of the present invention.

The nuclear reactor may preferably be a small modular reactor (SMR), meaning a relatively small, compact, nuclear reactor which is manufactured and assembled at factory and transported as a self-contained pre-fabricated unit, rather than being constructed on-site in the traditional manner. SMRs typically generate in the region of 300M We or less.

Use of SMRs enables central (rather than onsite) construction and shipping in self-contained units. Thus, when the present invention is realised using SMRs, further benefits in the form of construction quality, economic savings and efficiencies are provided. The reactor may be compact and substantially cylindrical for ease of installation in a borehole.

The inner conduit may be open ended at each of the upper and lower ends of the reactor. The inner conduit may be a straight-sided, longitudinal conduit. The inner conduit may be positioned centrally within the reactor vessel. The inner conduit may have a boundary wall formed of stainless steel. The inner conduit may additionally have a thermally insulating lining. The inner conduit may have an inner diameter in the region of 12 mm to 686 mm. The inner conduit may be configured to transport pressurised water.

The upper end may be configured to attach to a complementary shaped end of a second reactor. The upper end may comprise box threads for receiving complementary shaped pin threads of the second reactor. Alternatively the upper end may be configured to receive an end of the second reactor via a push fit, or slide fit.

The lower end may be configured to attach to a complementary shaped end of a third reactor. The lower end may comprise pin threads for being received in complementary shaped box threads of the third reactor. Alternatively the lower end may be configured to be received in an end of the third reactor via a push fit, or slide fit. Alternatively the lower end may be rounded (not configured to attach to a further reactor).

The reactor vessel may be substantially tubular. The reactor vessel may be substantially cylindrical tubular and hollow. The reactor vessel may comprise walls formed of carbon steel. The walls may be clad with a zirconium alloy. The reactor vessel may contain a fuel assembly.

The reactor may further comprise one or more sensors. The reactor may further comprise a data cable.

The reactor may further comprise a gripping mechanism. The reactor may further comprise a centralising mechanism, for example one or more fins.

The reactor may be a small modular reactor (producing 300MWe or less). The reactor may be a micro-reactor (producing 20MWe or less). -4 -

In a second aspect of the invention there is provided a reactor system according to claim 17.

The inner conduits of the reactors may align along a common longitudinal axis.

The system may further comprise a joining element configured to attach to one end of the reactor stack, the joining element having a hollow core.

The system may further comprise a pipe section configured to attach to the joining element.

The reactor stack, joining element and pipe section may align to provide a reactor string, wherein fluid can flow through the reactor string in a pathway between the inner conduits of the reactors, the hollow core of the joining element and the pipe section.

In a third aspect of the invention there is provided a method of installing a nuclear reactor in a borehole according to claim 22.

The tubular casing may comprise carbon steel. The tubular casing may comprise a stainless steel cladding.

The method may further comprise the step of calculating a depth wherein a hydrostatic pressure exerted within the borehole matches a predicted reactor internal pressure during operation, wherein the reactor stack is lowered to the calculated depth. The borehole may be a purpose drilled borehole or a re-purposed wellbore, for example an oil & gas wellbore or a geothermal wellbore.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa.

Description of the Drawings

Three embodiments of the present invention will now be described by way of example only, with reference to the accompanying schematic drawings of which: Figure 1 is a schematic side view of a reactor system situated in a borehole according to the first example embodiment; Figure 2 is a schematic side view of the reactor system of Figure 1 in use, with circulating fluid flowing through the reactor system; -5 -Figure 3 is a schematic side section of a reactor stack, and circulating pipe of the reactor system of Figure 1; Figure 4 is a schematic side section of the lower three reactor units of the reactor stack of Figure 3; Figure 5 is a schematic side section of the upper reactor unit of the reactor stack of Figure 3, with joining element, and circulating pipe sections; Figure 6 is a schematic side section of a reactor shoe of the stack of Figure 3; Figure 7 is a schematic side section of a reactor joint of the stack of Figure 3; Figure 8 is a schematic side section of a reactor joint of the second example embodiment; Figure 9 is a schematic side section of a reactor joint of the third example embodiment; Figure 10 is a schematic perspective view of a reactor site above ground, according to the first, second or third example embodiments; and Figure 11 is a flow chart showing stages of a borehole drilling and installation process according to the first, second or third example embodiments.

Detailed Description

A reactor system 1 (Figure 1) according to the first example embodiment of the invention is situated in a borehole 6 beneath ground level 3. As is known in the art, the borehole 6 consists of progressively smaller concentrically drilled holes (6a, 6b, 6c) extending vertically underground. The narrowest end 6c of the borehole 6 is situated deep underground (-6km beneath ground) in cap rock 2. In the first example embodiment, the borehole 6 is associated with an oil reservoir 4 (however in another example embodiment, the borehole 6 may be associated with a water or gas reservoir, or there may be no reservoir). In the first example embodiment, the reactor system is installed vertically in a vertical borehole. In an alternative embodiment, the reactor system could be installed in a different orientation, for example horizontally within a lateral borehole.

The reactor system 1 comprises a reactor stacks (shown situated in the narrowest end 6c of the borehole 6), and a circulating pipe 7 connected to, and extending away from the reactor stack 5 towards ground level 3. The circulating pipe 7 has an outer diameter of between 101.6 mm (4") and 177.8 mm (7") (in an alternative embodiment, it may have an outer diameter in the range of 25.4 mm (1") to 762 mm (30")). Above ground 3, the reactor system 1 comprises a wellhead structure 13 into which the circulating pipe 7 extends. The wellhead 13 allows circulating fluid to be pumped into the reactor and returned to surface -6 -level, and diverted to other surface equipment (such as steam generators, pumps/circulators, pressurisers, turbines, generators, condensers and backup generators). The wellhead 13 comprises a series of valves 15a and valve outlets 15b. The valves connected to the annulus 17, are used to transport steam returning up the annulus 17 to a steam turbine or to a steam generator (not shown). The remaining valves and valve outlets are used to monitor pressures in the borehole structure (ensuring well integrity is maintained). The circulating pipe 7 is installed in the centre of the wellhead 13. The circulating pipe 7 is arranged with specialised packing elements (not shown) which are expandable to allow controlled movement of the pipe up and down with no loss of structural integrity. Above the wellhead 13, additional safety elements (not shown) protect the circulating pipe including one or more of valves, expandable packers, inflatable packers, and closing rams.

The reactor stack 5 and circulating pipe 7 together form a reactor string 16. The string is enclosed within a first cylindrical casing 8, there being an annulus 17 between the exterior of the string, and the inner walls of the casing 8. In the lowermost portion 6c of the borehole 6, cement 10 fills the gap between the exterior casing wall and the sides of the borehole 6. In the mid portion 6b, the string is additionally enclosed within a second cylindrical casing 12 arranged concentrically outside of the first cylindrical casing 8, with cement 10 filler therebetween. In the uppermost portion 6a, the string is additionally enclosed within a third cylindrical casing 14 arranged concentrically outside of the first, second and third cylindrical casings, with cement 10 filler therebetween. In alternative embodiments, filler material other than cement may be used.

The first cylindrical casing 8 has an outer diameter of 339.725 mm (13-3/8") placed within a 444.5 mm (17-1/2") open hole. The gap is left open to form the annulus 17. The second cylindrical casing 12 has an outer diameter of 508 mm (20") placed within a 660 mm (26") open hole. The gap is filled with cement. The third cylindrical casing 14 has an outer diameter of 762 mm (30") placed within a 914.4 mm (36") open hole. The gap is filled with cement. In alternative embodiments, the dimensions, for example the thicknesses of the cement filled gaps, or open hole sizes may differ.

The casings are made of carbon steel of a suitable grade. Carbon steel provides an economical yet practical solution for containing the circulating fluid in the annulus. In alternative embodiments, one or more of the casings can be made of stainless steel or a composite material. The casing material should in any event withstand the high temperatures expected from the reactor in operation. Materials for the casing threads and -7 -couplings (not shown) should also withstand the high temperatures in the borehole 6 and should be suitable to handle the thermal cycling throughout the reactor lifecycle. The materials should also have the flexibility to compensate for thermal expansion.

In certain other embodiments, it may be beneficial to line each casing with a thermal coating or cladding to retain heat within the fluid column throughout the borehole 6. This may also enhance the structural properties of the casing and the barrier material (e.g. cement) used behind the casing. It may be particularly beneficial to use stainless steel as cladding to prevent corrosion. Any materials chosen for the coating/cladding should be selected with thermal conductivity and low thermal expansion in mind. Materials should also be selected to limit radiation damage.

Typical cements used in constructing wells may not have the suitable structural properties to withstand the expected temperatures of the vertically stacked reactor system of the present invention. New cement formulations to withstand high thermal cycles may be used. Such formulations may comprise classes of cement such as API Class1 or high alumina cement, and additives such as calcium aluminate phosphate. Such formulations may have seal-healing properties. Other materials may be used instead of cement which have low permeability and also have a good elastic deformation limit to prevent any impact on well integrity from expansion and contraction. It may be beneficial to use in-situ formations as a barrier such as creeping shales, salt and claystone. It may also be beneficial to use thermosetting polymers and composites, metals (such as steel, bismuth or bismuth-based alloys), gels, glass, or any other suitable substance.

The reactor system 1 also comprises, directly above the reactor stacks and arranged between the circulating pipe 7 and surrounding annulus 17, a thermoelectric generator 9.

The thermoelectric generator 9 functions to provide independent power for downhole equipment. The thermoelectric generator 9 generates electricity utilising a heat differential between circulating fluid flowing in the circulating pipe 7 and in the annulus 17.

The reactor system 1 also comprises safety equipment 11. In the example embodiment, the safety equipment 11 comprises valves associated with the circulating pipe 7. The safety equipment 11 is installed in two positions, the first position between the reactor stacks and the thermoelectric generator 9, and the second position above the thermoelectric generator 9 (in alternative embodiments the safety equipment may be located elsewhere). The safety equipment 11 functions autonomously and also via human intervention (on receipt of signals from surface) to instantaneously shut the wellbore in an emergency. In alternative embodiments the safety equipment may alternatively or -8 -additionally comprise any of: bleed of valves, safety relief valves, swellable packers, inflatable packers, shear pins/assemblies, shut off valves, blind shear rams, casing shear rams, variable bore rams, and/or explosives. The safety equipment may be used to shut-in the annulus, circulating pipe, or both. Some of the equipment may be used temporarily to seal the well and then return to production.

In the example embodiment of the invention, the reactor system 1 is a boiling water reactor. In use, circulating fluid (i.e. light water) is pumped down (Figure 2) from the wellhead 13, through the circulating pipe 7, through the reactor stack 5, and out through the bottom of the reactor stack 5. The fluid circulates back up to ground level 3 through the annulus 17. The circulating fluid warms as it passes the reactor stack 5, and increases in pressure as the temperature rises. The circulating pipe 7 is thermally coated to ensure minimal heat losses arising from the temperature differential between the colder fluid inside he pipe (being pumped from surface) and the warmer fluid outside the pipe (in the annulus, returning to surface).

As the circulating fluid returns towards ground level 3, it begins transforming into steam. In the example embodiment, the circulating fluid boils once it has reached a depth in the borehole 6 wherein the local hydrostatic pressure permits boiling. The steam is transported to ground level 3 in the annulus 17 and released directly into a turbine (above ground -not shown) for electricity generation. The steam then passes into a condenser (not shown) to be converted back into a liquid state. Seawater or other available nearby bodies of water can be used to aid with cooling the steam to condense it back into a liquid state. The circulating fluid is then pumped downhole again to be re-used (any losses in the system are topped up). Thus in use, there is a continuous flow of circulating fluid down through the circulating pipe 7 and reactor stack Sand up through the annulus 17.

In the first example embodiment, the circulating fluid is contained within a single closed loop circuit. Advantageously, as the reactor is sited down a borehole, the length of the fluid path through the annulus 17 ensures that any radioactivity in the circulating fluid is decayed before reaching surface level (the half-life of the radioactive fluid is sufficiently short).

There is no steam generator required, because the circulating fluid boils directly in the annulus. The circulating fluid has multiple functions. It removes heat from the reactor stack to prevent overheating (acting as a reactor coolant) and also provides energy for thermoelectric generation within the turbine (as steam). As a control measure, the circulating fluid flow rate may be increased or decreased as required to control cooling. In -9 -alternative embodiments, the circulating fluid also acts as a moderator (described in more detail below). In some embodiments, the steam is choked on reaching ground level to increase the pressure for thermoelectric generation within the turbine. In some embodiments, the steam is passed through ancillary equipment prior to being released into the turbine. In some embodiments, the steam is fed into a heat exchanger or heat exchangers. In an alternative embodiment, a higher pressure primary loop is used to heat a lower pressure secondary loop to create steam. In such an embodiment, the primary loop transfers heat (via conduction through materials) to the lower pressure loop to boil water therein. Any energy conversion system suitable for electricity generation may be used.

In a traditional boiling water reactor, direct release of steam tends to be avoided.

This is because the coolant fluid is subjected to a high neutron flux producing Nitrogen-16 which has a seven second half-life and emits a gamma ray when it decays back to oxygen-16. In a traditional boiling water reactor in a surface plant, the gamma rays would be emitted within the steam turbine, causing safety issues and restrictions. However, in the example embodiment of the invention, the Nitrogen-16 decays during the circulation of the circulating fluid within the annulus back to the surface systems. Thus, the gamma rays are emitted in a safe downhole environment.

In the example embodiment of the invention the circulating fluid (i.e. light water) has multiple functions. In addition to cooling, it also provides steam for electricity generation. Furthermore, the hydrostatic pressure provided by the circulating fluid in the borehole counteracts at least in part, the reactor internal pressure. In alternative embodiments, the circulating fluid also provides a moderator.

The fluid flow in the first example embodiment is down through the circulating pipe 7 and back up through the annulus 17. In an alternative embodiment, reverse circulation is possible, with fluid being pumped down through the annulus, and returning up via the circulating pipe. The majority of the heating of the circulating fluid occurs in the inner conduits of the reactor units. Some additional heating may occur in the annulus 17.

The reactor string 16 (shown in part in Figures 3,4 and 5) made up of the reactor stack 5 and the circulating pipe 7, comprises a plurality of units (21) 23, 25, 27) connected end to end. The reactor stacks comprises a plurality of reactor units (21, 23). The circulating pipe 7 comprises a plurality of circulating pipe units 27 and a joining element 25. In the first example embodiment, there are four reactor units (a reactor shoe 21 and three reactor joints 23), a joining element 25, and four circulating pipe units 27 (further circulating -10 -pipe units are not shown). In alternative embodiments, there may be more or fewer reactor units, and/or more or fewer circulating pipe units.

The reactor units (21, 23) of the reactor stacks comprise centralisers 44 in the form of solid fins for centralising the reactor stacks within the borehole. In alternative embodiments blades may be used instead of fins. The fins or blades can be flexible or expanding instead of solid. The fins 44 are fixed to the exterior of the reactor units, but in alternative embodiments can be accessories attachable to the reactor units.

The reactor stack 5 additionally comprises a gripping mechanism (not shown) to lock the reactor units in place downhole. The gripping mechanism in the first example embodiment comprises a mechanical arm comprised of a metal. The gripping mechanism is activated when required, by an operator (for example on receipt of a signal that the stack has landed). In an alternative embodiment, the gripping mechanism may be provided by an inflatable packer which is activated automatically when pressure data exceeds a preprogrammed pressure threshold. In an alternative embodiments, the gripping mechanism may be provided by a swellable packer.

The reactor shoe 21 is the lower most unit in the string 16. As described in more detail below, it is shaped to interconnect with an adjoining unit. In the first example embodiment, the adjoining unit is a reactor joint 23. In an alternative embodiment having only one reactor unit (i.e. only a reactor shoe and no reactor joint), the adjoining unit could instead be the joining element 25. As also described in more detail below, each of the reactor joints 23 is shaped to interconnect with two adjoining units -in the first example embodiment the relevant adjoining units are either the reactor shoe 21, a further reactor joint 23 or the joining element 25.

The joining element 25 provides an intersection between the reactor units (21, 23) and the circulating pipe units 27 which have a different size of connector thread. In the example embodiment of the invention, the joining element 25 comprises a disconnect mechanism (not shown) to seal the circulating pipe 7 in the event of an emergency. In an alternative embodiment, the disconnect mechanism may be located on the circulating pipe. The disconnect mechanism is provided by a shear release joint with a non-return valve.

Once a predetermined amount of overpull is applied from surface, the circulating pipe 7 shears away, allowing the non-return valve to close and therefore stop any circulating fluid being able to come to surface level. The mechanism is very fast, providing an almost immediate closure of the circulating pipe 7. The mechanism can additionally be used to remove the circulating pipe 7 from the borehole 6 at the end of the reactor lifecycle. The disconnect mechanism is located at or above the top of the joining element 25 (above the safety equipment 11). In use, whilst the shearing safely secures the circulating pipe 7, the safety equipment can at the same time seal the annulus 17, to quickly close the entire borehole 6.

The circulating pipe units 27 are each shaped to interconnect with two adjoining units (in the example embodiment) a further circulating pipe unit 27 or the joining element 25).

The reactor shoe 21 (Figure 6) comprises an inner conduit 31 extending centrally and longitudinally through the unit. The reactor shoe 21 is substantially cylindrical tubular, with a central longitudinal axis 30. The inner conduit 31 is open at the upper end 40 of the reactor shoe 21 and the lower end 42 of the reactor shoe 21. At its lower end 42, the reactor shoe 21 comprises a base 32 which is affixed as an attachment prior to lowering, and has a rounded shape (i.e. known as a "bullnose shoe"). Such a rounded shape is beneficial for installation of the reactor stack downhole because the reactor shoe 21 can more easily slide past restrictions which may be encountered (for example a ledge) when the reactor stacks is lowered into the borehole. At its upper end 40, the reactor shoe 21 comprises box thread connectors 34, shaped to interlock with complementary shaped pin thread connectors of an adjoining unit (not shown). Instead of box and pin threads, alternative means of connection may be used, for example, welded or push-fit connectors.

Advantageously, the connectors are gas-tight.

The inner conduit is surrounded by a shielding wall 38. Thus, in the first example embodiment the inner conduit 31 does not expose the reactor core directly to the circulating fluid. In alternative embodiments, it may be beneficial for the shielding wall 38 to have perforations, slots, or another means of communication allowing the circulating fluid to directly contact the core.

Surrounding the inner conduit 31 in the mid portion of the reactor shoe, are a plurality of vertically arranged fuel rods 33. Interspaced between the fuel rods 33 are a plurality of moderator elements 35. The moderator elements 35 comprise graphite. In alternative embodiments, other solid materials may be used which are capable to reduce the speed of fast neutrons. In alternative embodiments of the invention, the moderator elements may for example comprise water, heavy water, sodium, carbon dioxide, beryllium or lithium-7. As is known in the art and not discussed in detail herein, the fuel rods 33 release energy by nuclear fission which heats the circulating fluid in the inner conduit 31 and annulus 17. The moderator elements 35 moderate the neutrons, to an energy level suitable -12 -to sustain the nuclear chain reaction. The fuel rods are arranged as a cylindrical fuel assembly. Other fuel assembly geometries are known in the art. In the first example embodiment, the fuel rods comprise ceramic uranium oxide pellets in zirconium alloy cladding. The fuel is enriched to 20% (but could be enriched up to 85% due to the decreased risk of proliferation on site). In alternative embodiments of the invention, fuel rods may instead comprise coated particles or other suitable fuel types. The fuel material may be suitable for operation in a fast reactor. For example, the fuel material may comprise thorium, uranium, or uranium/ transuranic (TRU) oxide or uranium/TRU/Zr metal alloy. The fuel material may be particulate UO2 fuel kernels, or TRISO type UO2 fuel. Fuel may be in the form of: UO2 granules in an inert matrix, UO2 in silumin matrix; U-Zr metal ceramic; UO2 TRISO fixed bed; U/TRU/Zr Metal (TRU = Transuranic waste); U/TRU oxide; U/TRU/Nitride. In embodiments of the invention the fuel may be surrounded by a neutron reflector. In the first example embodiment, no re-fuelling is envisaged. The reactor is designed for single use. No nuclear waste will be generated or handled on site at any time.

The fuel rods 33 and moderator elements 35 are contained within a hollow cylindrical reactor vessel 37, providing a pressurised (-5,000psi), fully enclosed reactor. In alternative embodiments, the reactor pressure may be between 14.5-10,000psi. The reactor vessel in the first example embodiment comprises carbon steel. In alternative embodiments the reactor vessel alternatively or additionally comprises alloyed metals (for example low-alloy ferritic steel), stainless steel (to prevent corrosion), and/or cementitious materials. In the example embodiment, the reactor vessel 37 is clad with a zirconium alloy to limit neutron and gamma irradiation embrittlement, thermal ageing, low-cycle fatigue, thermal fatigue and corrosion. Alternative cladding materials include carbon steel, alloyed metals, stainless steel, zirconium-magnesium alloy, and carbide compounds. Materials used within the reactor vessel should have low thermal expansion coefficients and high thermal conductivity to resist against thermal shock. Materials should also be used which limit damage due to radiation, temperature, and corrosion. In addition, cladding materials may be chosen to meet corrosion resistance, mechanical properties, fluid compatibility and fabrication requirements.

Positioned above the reactor vessel 37 are a plurality of sensors 39, and a data cable 41 for transporting data signals on reactor conditions above ground. In use, the data signals can be communicated to ground level in real time. The sensors include but are not limited to: temperature sensors, pressure sensors, flowrate sensors, visual sensors, acoustic sensors, vibration sensors, time sensors, fluid sampling and analysis sensors. There may also -.3 -be a means of recording borehole depth and geometry in the form of rollers and/or callipers. The data cable is a permanent fixture attached to each reactor unit, respective lengths of which are connected following the connection of reactor units. In an alternative embodiment, the data cable is provided by a separate reel of continuous cable which can be affixed to the pipe as it is lowered into the borehole.

In the first example embodiment, there are no control rods. As the vertically stacked reactor is located in a downhole environment, there is advantageously no requirement for control rods or burnable/soluble neutron absorber materials. In an alternative embodiment, there may be a single control rod which can be lowered into the borehole using wireline, slickline or coil tubing into the circulating pipe and into the reactor units. This may be used in combination with an inner conduit wall which is thinner than standard. The single control rod may be hollow to allow continuous flow through of circulating fluid. A perforated or slotted conduit wall may be beneficial in this example, to enable circulating fluid to reach the reactor core. Materials which may be used for the control rod include, but are not limited to: cadmium, hafnium, boron, boron carbide, and dysprosium titanate. The control rod may be incorporated in a control rod assembly which can be lowered via pressure, mechanical means or magnetically. The control rod may cover in the region of 50% of the reactor length within each reactor unit. Alternatively, a permanent control rod or rods may be incorporated within the reactor unit. The permanent rod or rods may be located in a sealed compartment which is rotated when required to expose the rods to the core. This may be done autonomously or by human intervention. Alternatively, or additionally, an injection fluid such as boron solution may be circulated for reactor control. A dedicated valve may exist to allow solutions or fluid to be pumped directly into the reactor core. The above methods may be used for immediate abandonment or decommissioning of the wellbore in a safety critical case. If required, the control rods can be left downhole.

In use, the inner conduit 31 functions to transport cool circulating fluid from an adjoining unit above (not shown), through the reactor shoe 21 running parallel to the fuel rods 33, through the base 32 of the reactor shoe and out into the annulus 17 as warm circulating fluid (not shown in Figure 6) for return up to ground level 3.

The reactor joint 23 (Figure 7) is substantially the same as the reactor shoe but instead of a bullnose shoe, the lower end 42 comprises pin thread connectors 36 shaped to interlock with complementary shaped box thread connectors of an adjoining unit. The reactor joint 23 comprises an inner conduit 31 extending through the full height of the unit, -14 -to transport circulating fluid from an adjoining unit above, along the length of the reactor joint, to an adjoining unit below. The reactor joint 23 is substantially cylindrical tubular, with a central longitudinal axis 30.

Thus, the reactor units (reactor shoe and reactor joints) can be stacked one on top of another, in a manner which ensures alignment of the inner conduits for the flow through of circulating fluid and prevents lateral displacement of the reactor units. Moreover, since the pin threads are guided into the box threads by the tapered inner surfaces of the box threads, the units self-align. Once in position, the units are held in position by the box/pin thread connections which are torqued, and which may additionally be sealed with an adhesive.

For ease of manufacturing, and further efficiencies, in the first example embodiment, the reactor shoe 21 is in essence the same as the reactor joint 23 but with a (separate) bullnose shoe attached to it. Each reactor unit (reactor shoe and reactor joint) in the first example embodiment is 9144 mm to 9448.8 mm (30 to 31 ft) in length. In alternative embodiments the length of each unit may be as high as 13,716 mm (45 ft), or as low as 304.8 mm (1 ft). In another embodiment, the reactor and circulating pipe may be of a design contained within a flexible pipe such as coil tubing, in which case the reactor length may be significantly increased. The diameter of each reactor unit is 1,219 mm to 43,891 mm (4" to 144").

In the second example embodiment of the invention, the reactor joint 23' (Figure 8) is similar to in the first example embodiment; however, it comprises an inner conduit 31' with slotted fluid communication channels 46' to allow the circulating fluid to enter the reactor vessel 37' from the inner conduit 31'. The circulating fluid then circulates within the vessel 37', and acts as a neutron moderator around the fuel rods 33', before passing back into the inner conduit 31' to continue circulation towards the annulus (not shown). In the second example embodiment of the invention, there are no solid moderator elements, only a water moderator 35' (in an alternative embodiment, there may in addition be solid moderator elements present).

In the third example embodiment of the invention, the reactor joint 23" (Figure 9) is similar to in the first and second example embodiments; however, it comprises an inner conduit 31" with perforated fluid communication channels 46" to allow the circulating fluid to enter the reactor vessel 37" from the inner conduit 31". The circulating fluid then circulates within the vessel 37", and acts as a neutron moderator around the fuel rods 33", before passing back into the inner conduit 31" to continue circulation towards the annulus -15 - (not shown). In the third example embodiment of the invention, there are no solid moderator elements, only a water moderator 35" (in an alternative embodiment, there may in addition be solid moderator elements present). In the second and third example embodiments, the circulating fluid may be changed to a fluid which is more efficient than water at absorbing neutrons, yet nevertheless can boil to drive the turbine, and serve to cool the reactor.

Above ground, the reactor site 43 (Figure 10) of the first, second or third example embodiment comprises a central hub 45, and nine reactor stations 47 connected via pipelines. Each reactor station 47 is associated with a different downhole reactor. Either the central hub 45 or the multiple smaller reactor stations 47 may be connected to the electrical grid.

The reactor system of the first, second or third example embodiment of the invention is installed in a newly drilled borehole. The drilling and installation process (Figure 11) comprises several stages.

In a first stage 51, a site suitability assessment is undertaken. The primary function of the borehole is to act as a containment structure. The site is selected based on, but not limited to, the following criteria: surface topography and logistics; environmental impact assessment; proximity to nearby water sources (such as rivers, lakes, sea, etc.) for emergency well control purposes (such as loss of wellbore fluid to a downhole environment) or steam escape), for the primary circuit circulating fluid and, if required, for coolant for the primary circuit circulating fluid; subsurface geology at reactor depth; structurally stable formations; low to zero permeability (such as a reservoir cap rock); low geological activity; full analysis of geology, geophysics, geochemistry, hydrology, geomechanics information which is existing and newly gained from seismic surveys etc.; information of aquifers and groundwater sources; proximity to end users/electrical grid; proximity to external emergency response facilities; public and political acceptance (visual, noise) aesthetics, wildlife, archaeology, traffic, contamination of soil, etc); potential of co-benefits such as district heating, desalination, etc.; existing infrastructure (especially in the case of an offshore sited reactor); external factors (such as defence from proliferation, naturally occurring natural disasters) etc.); conformity to all relevant regulatory requirements (may be Oil & Gas requirements); site restoration and aftercare.

In a second stage 53, a desired borehole depth is calculated such that hydrostatic pressures in the circulating fluid match the expected reactor internal pressures during operation. Pressure-balancing as such eliminates the need for a robust and costly reactor -16 -pressure vessel, meaning a cheaper more lightweight material can be used without compromising safety. The risk of a pressure-related failure may be significantly reduced. Hydrostatic pressure increases linearly with depth and is based on the density of the fluid. It can therefore be determined at which depth the reactor stack should be positioned. The reactor stack length may be such that the hydrostatic pressure balances differently on the reactor stack at different depths (i.e. on different units). In an embodiment wherein the reactor stack is positioned horizontally, the hydrostatic pressure may advantageously be constant along the length of the reactor stack. Such an arrangement may be especially beneficial in cases where there are many reactor units.

The depth of the reactor may in addition be based on geological features (ensuring suitability for future decommissioning). In some cases, the reactor may be positioned temporarily at a certain depth for production and then moved to a final location for decommissioning. This may be required if the geological features which are beneficial for decommissioning are at a different depth to the hydrostatic location for pressure balance.

In a third stage 55, a first section of the borehole is drilled, using the largest of a series of drill bits. The largest section will typically have a diameter of 914 mm (36") or larger. It may be beneficial for the vertically stacked reactor to be positioned at a depth which has a larger casing diameter than a typical wellbore will have at that depth. As such, the first section may be larger than usual to accommodate larger hole sections deeper in the borehole.

In a fourth stage 57, a casing is inserted into the borehole. It may be beneficial for the gaps between casings to be larger than in a typical wellbore to achieve a larger thickness of set barrier material (e.g. cement).

In a fifth stage 59, cement is inserted into the gap between the casing and borehole walls The third, fourth and fifth stages are then repeated using successively narrower drill bits until the sixth stage 61 wherein the desired depth is reached.

In a seventh stage 63, the reactor system is installed in the borehole. For installation, the reactor units are housed within a suitable temporary containment housing.

The housing is used to position the reactor units above the borehole in a drilling rig. The housing also protects against radioactive emissions at surface level.

The reactor units are connected to adjacent modular units (above ground), and lowered into the borehole. The drilling rig is held in place to allow connection of numerous modular sections at the rotary table using slips or other methods known to those in the art.

-17 -The drilling rig then lifts the reactor stack vertically and lowers it down the borehole. Spacing joints are used to ensure the reactor stack has been lowered to a sufficient depth into the borehole to ensure safe operations at surface. Once the full reactor stack has been lowered into the borehole, it is crossed over (using a crossover sub or section which changes thread size or type) to a suitable tubing or pipe to act as the circulating pipe for carrying fluids pumped from surface, through the reactor stack and back to surface via the annulus. This circulating pipe can then be lowered into the borehole to the final depth of the reactor stack. The reactor stack then permanently locks itself in position in the borehole via bridge plugs or other mechanisms described herein (which can be set automatically, or via an operator).

In the first, second and third example embodiments, a custom wellhead is then installed to allow circulating fluid to be pumped into the reactor stack and returned to surface and diverted to other surface equipment. Finally, the drilling rig is demobilised.

It some cases, the joining element 25 may be pre-attached to a shorter length of reactor joint 23 and a shorter length of circulating pipe unit 27. This may help to space out and position the reactor in a deeper, safer location. Pre-attaching may save time on the wellsite. If the joining element is fitted in the factory, this can avoid a radioactive reactor unit sitting at surface for a long time while it is connected to an adjacent unit and torqued to the required value.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein By way of example only, certain possible variations will now be described.

In the first, second and third example embodiments of the invention, the borehole is purpose drilled. In an alternative embodiment of the invention, the reactor is sited in an existing wellbore (i.e. a borehole previously used for the extraction of oil or gas). Any vertical shaft of a suitable size may be used to site the nuclear reactor of the present invention, provided that the geology is suitable and the shaft and well materials are fit for purpose. When an existing wellbore is repurposed, a suitable site inspection is undertaken prior to installation of the nuclear reactor to determine that the wellbore is fit for purpose.

Factors examined include: age of the well; location of the well; well integrity history; current well integrity status; current well status and completions status; drilling design; geological studies; a study of the daily drilling reports, end of well report, geology data, cementing -18 -reports, wellbore fluids, etc.; casing and other wellbore tubular material; other wellbore accessories. Other criteria may also be applicable.

In the first, second and third example embodiments of the invention, there is only a single circulating fluid loop, which is advantageous for servicing. The steam is converted back to a liquid state and re-used. In an alternative example embodiment, the system may have an open loop design in which the steam is allowed to vent to atmosphere. The system may alternatively include a dry steam cycle, flash steam cycle or binary cycle. In alternative embodiments, there may be two or more overlapping closed loops for extracting heat from the reactor.

In the first, second and third example embodiments, the reactor type is a boiling water reactor. In alternative embodiments, the reactor could be any of: a pressurised water reactor; a light water reactor; a pressurised heavy water reactor; an advanced gas-cooled reactor; a fast neutron reactor; a fixed bed nuclear reactor; a compact high temperature reactor; a molten salt reactor; a lead and/bismuth fuelled fast reactor; a gas cooled fast reactor; or a breeder reactor (or any other reactor known to those in the art).

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims (25)

1. -19 -Claims 1. A nuclear reactor comprising a reactor vessel having a central longitudinal axis defining an upper and a lower end, and within the reactor vessel an inner conduit for transporting fluid, the reactor being configured to connect end-to-end along the central longitudinal axis with one or more further nuclear reactors such that when connected, fluid can flow from the inner conduit of the reactor to the one or more further reactors.
2. 2. A reactor according to claim 1, wherein the inner conduit is open ended at each of the upper and lower ends of the reactor.
3. 3. A reactor according to any preceding claim, wherein the inner conduit is configured to transport pressurised water.
4. 4. A reactor according to any of claims 1-3, wherein the upper end is configured to attach to a complementary shaped end of a second reactor.
5. 5. A reactor according to claim 4, wherein the upper end comprises box threads for receiving complementary shaped pin threads of the second reactor.
6. 6. A reactor according to any preceding claim, wherein the lower end is configured to attach to a complementary shaped end of a third reactor.
7. 7. A reactor according to claim 6, wherein the lower end comprises pin threads for being received in complementary shaped box threads of the third reactor.
8. 8. A reactor according to any of claims 1-5, wherein the lower end is rounded.
9. 9. A reactor according to any preceding claim, wherein the reactor vessel is substantially tubular.
10. 10. A reactor according to any preceding claim, wherein the reactor vessel comprises walls formed of carbon steel.
11. -20 - 11. A reactor according to any preceding claim, wherein the vessel contains a fuel assembly.
12. 12. A reactor according to any preceding claim, further comprising one or more sensors.
13. 13. A reactor according to any preceding claim, further comprising a data cable.
14. 14. A reactor according to any preceding claim, further comprising a gripping mechanism.
15. 15. A reactor according to any preceding claim, further comprising a centralising mechanism.
16. 16. A reactor according to any preceding claim, being a small modular reactor.
17. 17. A nuclear reactor system comprising a plurality of reactors according to any preceding claim connected end to end to form a reactor stack, wherein fluid can flow through the reactor stack in a pathway between the inner conduits of the reactors.
18. 18. A reactor system according to claim 17, wherein the inner conduits of the reactors align along a common longitudinal axis.
19. 19. A reactor system according to claim 17 or 18, further comprising a joining element configured to attach to one end of the reactor stack, the joining element having a hollow core.
20. 20. A reactor system according to claim 19, further comprising a pipe section configured to attach to the joining element.
21. 21. A reactor system according to claim 20, wherein the reactor stack, joining element and pipe section align to provide a reactor string, wherein fluid can flow through the reactor string in a pathway between the inner conduits of the reactors, the hollow core of the joining element and the pipe section.
22. 22. A method of installing a nuclear reactor system in a borehole, the method comprising the steps of: i. Positioning a nuclear reactor above a borehole wherein the borehole is lined with a tubular casing; ii. Connecting a further nuclear reactor to the nuclear reactor to provide a reactor stack; iii. Lowering the reactor stack into the borehole, such that when lowered, an annulus is provided between the exterior of the reactor stack and the tubular casing.
23. 23. A method according to claim 22, wherein the tubular casing comprises carbon steel.
24. 24. A method according to claims 22 or 23, wherein the tubular casing comprises a stainless steel cladding.
25. 25. A method according to any of claims 22-24, further comprising the step of calculating a depth wherein a hydrostatic pressure exerted within the borehole matches a predicted reactor internal pressure during operation, wherein the reactor stack is lowered to the calculated depth.