WO2020016566A1

WO2020016566A1 - High-temperature nuclear reactor

Info

Publication number: WO2020016566A1
Application number: PCT/GB2019/051979
Authority: WO
Inventors: Bruno Merk; Dzianis LITSKEVICH; Seddon ATKINSON
Original assignee: The University Of Liverpool
Priority date: 2018-07-16
Filing date: 2019-07-16
Publication date: 2020-01-23
Also published as: GB201811644D0

Abstract

A high temperature reactor (HTR) (100) is described. The HTR (100) comprises a first set of fuel columns (110), including a first fuel column 110A, comprising fissile fuel (120). The HTR (100) comprises a second set of burnable poison rods (130), including a first burnable poison rod (130A). The HTR (100) comprises a moveable moderator (140). The HTR (100) comprises a passageway (150) arranged to receive the moderator (140) therein. The HTR (100) comprises a first reflector (160) arranged to surround the first set of fuel columns (110), the second set of burnable poison rods (130) and the passageway (150). The HTR 100 is arrangeable in a first configuration C1, wherein the moderator (140) is received in the passageway (150) in a first position P1 thereby providing a first moderation M1 of neutrons produced by the fissile fuel (120). The HTR (100) is arrangeable in a second configuration C2, wherein the moderator (140) is received in the passageway (150) in a second position P2 thereby providing a second moderation M2 of the neutrons produced by the fissile fuel (120), whereby a change in criticality, in use, due to depletion of the fissile fuel and/or of the burnable poison rods (130) is attenuated.

Description

HIGH-TEMPERATURE NUCLEAR REACTOR

Field

The present invention relates to a nuclear reactor and a method of controlling a nuclear reactor.

Background to the invention

Current legislation requires a reduction of carbon-dependent energy sources in favour of low carbon or green alternatives. In the UK, for example, a minimum reduction of 80% in carbon footprint is required by 2050. Nuclear energy, as produced by a small modular reactor (SMR) for example, may be complementary to renewable energy sources.

A micro modular reactor (MMR), for example a U-Battery (RTM), is a type of SMR. Particularly, the MMR is a high temperature reactor (HTR) based on a high temperature engineering test reactor (HTTR), operational in Japan since 1990. The MMR has a relatively low power capacity of about 10MWth and is intended for remote location deployment, such as off grid facilities, including mining, oil and gas. A key requisite for this is a long life core. In order to maximise fuel life cycle and thereby reduce costs of refuelling, a high initial fuel loading is used, for example a 30% packing factor with 20% ²³⁵U. However, this high fuel loading presents excess reactivity that may adversely affect safety, for example accident risk, and/or operation, for example power distribution distortion.

Hence, there is a need to improve HTRs having long life cores, for example MMRs such as the U-Battery.

Summary of the Invention

It is one aim of the present invention, amongst others, to provide a HTR which at least partially obviates or mitigates at least some of the disadvantages of the prior art, whether identified herein or elsewhere. For instance, it is an aim of embodiments of the invention to provide a HTR whereby excess reactivity of the HTR may be controlled, for example reduced, without compromising, for example reducing, a lifetime thereof.

A first aspect provides a high temperature reactor, HTR, comprising:

a first set of fuel columns, including a first fuel column, comprising fissile fuel;

a second set of burnable poison rods, including a first burnable poison rod;

a moveable moderator; and a passageway arranged to receive the moderator therein; and

a first reflector arranged to surround the first set of fuel columns, the second set of burnable poison rods and the passageway;

wherein the HTR is arrangeable in:

a first configuration, wherein the moderator is received in the passageway in a first position thereby providing a first moderation of neutrons produced by the fissile fuel; and

a second configuration, wherein the moderator is received in the passageway in a second position thereby providing a second moderation of the neutrons produced by the fissile fuel, whereby a change in criticality, in use, due to depletion of the fissile fuel and/or of the burnable poison rods is attenuated.

A second aspect provides a method of controlling moderation in a HTR according to the first aspect, the method comprising:

moving the HTR from the first configuration to the second configuration by changing a position of the moderator received in the passageway from the first position to the second position, whereby a change in criticality due to depletion of the fissile fuel and/or of the burnable poison rods is attenuated.

Detailed Description of the Invention

According to the present invention there is provided a HTR, as set forth in the appended claims. Also provided is a method of controlling moderation in a HTR. Other features of the invention will be apparent from the dependent claims, and the description that follows.

High temperature reactor

The first aspect provides a high temperature reactor, HTR, comprising:

a second set of burnable poison rods, including a first burnable poison rod;

a moveable moderator; and

a passageway arranged to receive the moderator therein; and

wherein the HTR is arrangeable in:

a first configuration, wherein the moderator is received in the passageway in a first position thereby providing a first moderation of neutrons, for example a state or setup thereof, produced by the fissile fuel; and

a second configuration, wherein the moderator is received in the passageway in a second position thereby providing a second moderation of the neutrons, for example a state or setup thereof, produced by the fissile fuel, whereby a change in criticality, in use, due to depletion of the fissile fuel and/or of the burnable poison rods is attenuated.

In this way, excess reactivity of the HTR may be controlled, for example reduced, without compromising, for example reducing, a lifetime thereof. Particularly, the HTR is arrangeable to reduce neutron moderation, for example, by moving from the first configuration to the second configuration. By reducing the neutron moderation, the reactivity of the core is reduced, for example significantly, due to a lower number of thermal neutrons. Additionally, with a less thermal system as provided, ²³⁸U has a higher probability of producing ²³⁹Pu by breeding processes. The additional fissile material (i.e. ²³⁹Pu) may later be re-moderated and utilised for fission again. This trade off acts as a potential ‘pseudo’ neutron storage system, in which unrequired neutrons may be used for breeding and subsequently regained through the use of the bred fissile fuel.

Generally, a key aspect of HTR design for MMRs is being“meltdown proof such that core structural integrity is not compromised due to excessive temperatures, thereby reducing risk of radioactive substances being released from fuel. This is achieved, at least in part, by use of TRISO fuel particles to hold fission products in combination with a sufficient large surface to volume ratio of the core. Tristructural-isotropic (TRISO) fuel is a type of micro fuel particle, having a diameter of about 0.92 mm, including a fuel kernel composed of UO_x (also known as UC or UCO), coated with four layers of three isotropic materials. The four layers are a porous buffer layer made of carbon, by a dense inner layer of pyrolytic carbon (PyC), a ceramic layer of SiC to retain fission products at elevated temperatures and to give the TRISO particle more structural integrity and a dense outer layer of PyC. TRISO fuel particles are designed not to crack due to stresses from processes (such as differential thermal expansion or fission gas pressure) at temperatures up to and beyond 1600 °C, and therefore can contain the fuel in accident scenarios. TRISO particles eventually degrade at around 1900 °C, releasing fission products.

Excess reactivity is due to an excess (i.e. too many) neutrons being produced in the HTR, so as to be operable for a long time without adding new fuel. This excess reactivity is more prevalent when the fuel is completely fresh. However, this excess reactivity may be required to achieve long term operation, such that further fuel additions are not required. However, this may result in a runaway effect if the control system is not able to deal with. However, the required strong control system will imply on operational parameters like power distribution and/or can act as an accident initiator.

Thus, excess reactivity has implications for safety (possible accident initiation) and/or operation (distortion of the power distribution) of the HTR, for example. Figure 1 schematically depicts a transverse cross-section of a conventional high temperature reactor, in the special case the high temperature engineering test reactor (HTTR). Features shown in figures have their usual meanings, as understood by the skilled person.

Control of excess reactivity in HTTR mainly relies on use of, for example, sixteen control rods. In the initial core of HTTR, thirty pairs of 50 cm FBP rods are inserted into the top of the core, where the fuel is at the maximum enrichment, as shown in Figure 1. In more detail, control rods may be used to change an amount of power inside a HTTR by absorbing neutrons, using high neutron absorbing elements. Due to their positioning, the control rods tend to remove neutrons from the axial top and/or bottom of the core and thus distort the natural power profile of the core. Hence, the use of control rods should be minimal, yet sufficient such that the reactor is maintained in a state of controlled reactivity i.e. critical. This is particularly useful as the initial core of the HTTR uses a manipulated enrichment policy to control the power profile with the top of the reactor having as high as 10% enrichment and the bottom as low as 3.4%.

This means that the insertion of the control rods leads to flattening the flux across the axial height of the core.

Control of excess reactivity may be achieved, at least in part, for prismatic HTRs through a combination of fixed burnable poisons (FBPs) and control rods. FBPs are inserted at the beginning of a lifetime of an HTR fuel block and then burn out slowly, for example, during the lifetime. In the case of the MMR, one of the main criteria is along operating fuel cycle lifetime of at least five years. A reduced enrichment and/or packing factor of the fissile fuel would in turn reduce the overall total lifetime of the core. Hence, a harder burnable poisoning regime is required to balance an increased enrichment and/or packing factor, so as to achieve the required overall total lifetime.

However, a problem of both of these methods of control of excess reactivity is loss of neutrons due to absorption in the poison rods, resulting in reduced core lifetime and their influence on the power distribution.

The HTR of the first aspect solves this problem, controlling, for example reducing, the excess reactivity without compromising, for example reducing, the core lifetime. Particularly, the HTR of the first aspect achieves this solution by reducing neutron moderation, for example, by moving from the first configuration to the second configuration. By reducing the neutron moderation, the reactivity of the core is reduced significantly, due to a lower number of thermal neutrons. Additionally, with a less thermal system as provided, ²³⁸U has a higher probability of producing Pu by breeding. The additional fissile material (i.e. Pu) may later be utilised for fission again in a ideally moderated system. This trade off acts as a potential‘pseudo’ neutron storage system, in which unrequired neutrons may be used for breeding and subsequently regained through the use of the bred fissile fuel.

MMR for example a U-Battery

In one example, the HTR comprises a MMR, for example a U-Battery, adapted as described herein, particularly to include the moveable moderator and the passageway therefor.

Fuel columns

The HTR comprises the first set of fuel columns, including the first fuel column, comprising the fissile fuel.

It should be understood that the fissile fuel comprises fissile fuel capable of sustaining a nuclear fission chain reaction. By definition, fissile fuel can sustain a chain reaction with neutrons of any energy. Fissile fuel is a subset of fissionable material. For example, ²³⁸U is fissionable but not fissile. In contrast, for example, ²³⁵U is fissile. The predominant neutron energy may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. In one example, the fissile fuel comprises and/or is ²³⁵U, ²³³U, ²³⁹Pu, ²⁴¹Pu and/or mixtures thereof. ²³⁵U is preferred, for example having an initial enrichment in a range from 15% to 100%, preferably from 15% to 25%, for example 17% or 20%. It should be understood that a composition of the fissile fuel changes during fission and hence compositions and/or masses, for example, of the fissile fuel are initial compositions and/or masses, respectively. In one example, a mass of the fissile fuel is in a range from 50 kg to 5,000 kg, preferably in a range from 100 kg to 200 kg, more preferably in a range from 150 kg to 1 ,500 kg, for example 208 kg or 1 ,040 kg.

In one example, the first fuel column of the first set of fuel columns comprises TRISO fuel comprising the fissile fuel. In one example, the TRISO fuel comprises a set of TRISO fuel compacts, comprising the fuel compacts. In one example, a TRISO fuel compact comprises a toroid having a diameter of 26 mm, a length of 39 mm and an axial passageway therethrough having a diameter of 8 mm.

In one example, the fuel columns of the first set of fuel columns are arranged in fuel blocks, for example in 6 x 4 or 30 x 4 fuel blocks.

In one example, the first set of fuel columns includes M fuel columns, wherein M is a natural number in a range from 1 to 10,000, for example 216. In one example, each of the M fuel columns is as described with respect to the first fuel column. In one example, fuel columns of the first set of fuel columns are mutually spaced apart. In one example, the first set of fuel columns are arranged in an array, for example a regular array. In one example, the first set of fuel columns are arranged symmetrically.

In one example, the first set of fuel columns is arranged to surround the passageway.

Burnable poison rods The HTR comprises the second set of burnable poison rods, including a first burnable poison rod. It should be understood that the burnable poison rods are fixed burnable poisons (FBPs). Burnable poisons are also known as neutron absorber or neutron poisons. In nuclear reactors, absorbing neutrons is normally an undesirable effect. However, burnable poisons may be intentionally inserted into some types of reactors in order to lower a relatively high reactivity of an initial fresh fuel load. Burnable poisons are materials that have a high neutron absorption cross section that are converted into materials of relatively low absorption cross section as the result of neutron absorption. Burnable poisons deplete as they absorb neutrons during reactor operation. In more detail, to control a large excess of fuel reactivity, for example without using control rods, burnable poisons may be loaded into the core. Due to a burn-up of the poison material, a negative reactivity of the burnable poison decreases over a lifetime of the core. Ideally, the negative reactivity decreases at the same rate as the fuel's excess positive reactivity depletes, so as to maintain a constant or substantially constant activity. Fixed burnable poisons are generally used in the form of compounds of boron or gadolinium that are shaped into separate lattice pins or plates, or introduced as additives to the fuel. Since they can usually be distributed more uniformly than control rods, these poisons are less disruptive to the core's power distribution. Fixed burnable poisons may also be loaded in specific locations in the core in order to shape or control flux profiles to prevent excessive flux and power peaking near certain regions of the reactor.

In one example, the first burnable poison rod comprises B₄C (boron carbide) and/or Gd₂0₃ (gadolinia). B₄C is preferred. In one example, the B₄C is prepared using exclusively ¹⁰B i.e. ¹⁰B₄C.

In one example, the first burnable poison rod comprises B₄C in a range from 0.01 wt.% to 2.0 wt. %, preferably in a range from 0.05 wt.% to 1.0 wt.%, more preferably in a range from 0.1 wt.% to 0.5 wt.%, most preferably in a range from 0.2 wt.% to 0.4 wt.%, for example 0.3 wt.% by weight of the rod. In one example, the first burnable poison rod comprises Gd₂0₃ in a range from 0.1 wt.% to 20 wt. %, preferably in a range from 0.5 wt.% to 10 wt.%, more preferably in a range from 1 .0 wt.% to 9.0 wt.%, most preferably in a range from 2.0 wt.% to 8.0 wt.%, for example 7.0 wt.% by weight of the rod.

In one example, the second set of burnable poison rods includes N burnable poison rods, wherein N is a natural number in a range from 1 to 1 ,000, for example 2 or 14 per fuel block. In one example, each of the N burnable poison rods is as described with respect to the first burnable poison rod.

Moveable moderator

The HTR comprises the moveable moderator. The HTR comprises the passageway arranged to receive the moderator therein in the first position and in the second position. It should be understood that the moveable moderator and the passageway thus have corresponding shapes.

In one example, the moderator comprises and/or is graphite and/or a metal hydride, for example yttrium hydride. Graphite is preferred. In one example, the moderator comprises graphite in a range from 70 wt.% to 100 wt.%, preferably in a range from 90 wt.% to 100 wt.%, more preferably in a range from 95 wt.% to 100 wt.% by weight percent of the moderator. In one example, the moderator comprises graphite in an amount of at least 70 wt.%, at least 75 wt.%, at least 80 wt.%, at least 85 wt.%, at least 90 wt.%, at least 95 wt.%, at least 97.5 wt.%, at least 99 wt.% or at least 99.5 wt.% by weight percent of the moderator. In one example, the moderator comprises graphite in an amount of at most 75 wt.%, at most 80 wt.%, at most 85 wt.%, at most 90 wt.%, at most 95 wt.%, at most 97.5 wt.%, at most 99 wt.%, at most 99.5 wt.% or at most 99.9 wt.% by weight percent of the moderator. In one example, the moderator comprises a monolithic (i.e. a singular) moderator, for example comprising, comprising substantially, comprising essentially and/or consisting of graphite, for example in a range and/or an amount as described above. In this way, control, actuation and/or movement of the moderator is facilitated and/or simplified. In this way, a symmetry of a flux profile of the HTR may be controlled and/or maintained.

In one example, the moderator comprises a cross-sectional shape, for example a transverse cross-sectional shape, having a relatively high order n of rotational symmetry, for example where n is at least 5, 6, 8, 10, 12, 16, 20, 24, 36 or more. In one example, the moderator comprises a circular cross-sectional shape. In one example, the moderator comprises and/or is a cylinder.

In one example, the moderator comprises a set of moderators such as coaxial toroidal (i.e. nested) moderators, for example a set of coaxial toroidal graphite moderators. In this way, control of reactivity may be improved.

In one example, the moderator is arranged to be moved axially in the passageway. In one example, the moderator is arranged to be moved slidably in the passageway.

Passageway

The HTR comprises the passageway arranged to receive the moderator therein, for example having dimensions to receive the moderator therein.

In one example, the passageway comprises and/or is an axial (also known as central) passageway. In this way, a symmetry of a flux profile of the HTR may be controlled and/or maintained.

In one example, the passageway extends through a length of at least 70 wt.%, at least 75 wt.%, at least 80 wt.%, at least 85 wt.%, at least 90 wt.%, at least 95 wt.%, at least 97.5 wt.%, at least 99 wt.% or at least 99.5 wt.% by length of the set of fuel columns. In one example, the passageway protrudes beyond the set of fuel columns, for example beyond a region comprising the set of fuel columns.

In one example, the passageway comprises a cylindrical passageway and the moderator comprises a cylindrical moderator, receiveable therein.

In one example, the passageway has a diameter in a range from 10 cm to 100 cm, preferably in a range from 15 cm to 50 cm, more preferably in a range from 20 cm to 40 cm. In one example, the passageway has a diameter as a proportion of a diameter of the HTR in a range from 2 % to 40%, preferably in a range from 5% to 30%, more preferably in a range from 10% to 20%.

In one example, the HTR comprises a coolant, for example He, and the passageway is arranged to receive the coolant. Reflector

The HTR comprises the first reflector (also known as a side reflector) arranged to surround the first set of fuel columns, the second set of burnable poison rods and the passageway. It should be understood that the first reflector is a static reflector. In one example, the first reflector is arranged to at least partly surround the surround the first set of fuel columns, the second set of burnable poison rods and the passageway. In one example, the first reflector is arranged to fully (i.e. completely) surround the surround the first set of fuel columns, the second set of burnable poison rods and the passageway.

It should be understood that the first reflector is a neutron reflector, that elastically scatters neutrons (c.f. specular reflection). A neutron reflector can make an otherwise subcritical mass of fissile fuel critical, or increase the amount of nuclear fission that a critical or supercritical mass will undergo. Generally, a neutron reflector may reduce non-uniformity of a power distribution in peripheral fuel assemblies, reduce neutron leakage and/or reduces a coolant flow bypass of the core. By reducing neutron leakage, the neutron reflector increases reactivity of the core and reduces the amount of fuel necessary to maintain the reactor critical for a long period. In one example, the first reflector comprises graphite, Be, BeO (beryllium oxide), steel, WC (tungsten carbide) and/or a mixture thereof. In this way, the overall full power lifecycle may be improved.

In one example, the HTR comprises a second reflector comprising the passageway. It should be understood that the second reflector is a static reflector.

In one example, the second reflector is a central reflector, comprising the passageway. In one example, the second reflector is a toroidal reflector. In one example, the first set of fuel columns and/or the second set of burnable poison rods is arranged to surround the passageway.

In one example, the second reflector comprises graphite, Be, BeO (beryllium oxide), steel, WC (tungsten carbide) and/or a mixture thereof. In this way, the overall full power lifecycle may be improved.

First configuration and second configuration

The HTR is arrangeable in: the first configuration, wherein the moderator is received in the passageway in the first position thereby providing the first moderation of the neutrons produced by the fissile fuel; and the second configuration, wherein the moderator is received in the passageway in the second position thereby providing the second moderation of the neutrons produced by the fissile fuel.

In one example, the first position corresponds with the moderator removed, for example completely removed, from a region comprising the set of fuel columns (i.e. moderator out), whereby the first moderation is relatively lower and, in turn, the criticality is relatively lower, for example at a start of operation of the HTR.

In one example, the second position corresponds with the moderator within, for example a least partly within or fully within, a region comprising the set of fuel columns (i.e. moderator in), whereby the second moderation is relatively higher and, in turn, the criticality is relatively higher, for example at during operation of the HTR, when at least some of the fissile fuel has been depleted and criticality may be increased.

Change in criticality

The change in criticality, in use, due to depletion of the fissile fuel and/or of the burnable poison rods is attenuated (i.e. an effect thereof reduced).

That is, the moveable moderator provides control of interplay between depletion of the fissile fuel and of the burnable poison. In the beginning of life (i.e. at start of operation), the criticality is controlled by reduced moderation (i.e. moderator out) and the burnable poison. If the criticality becomes too low during operation, for example due to depletion of the fissile material, the criticality may be increased by inserting the moderator (i.e. moderator in). When the burnable poison is burnt out (i.e. depleted) during operation, the criticality increases again, and the moderation may be reduced (i.e. moderator out). When the fissile fuel is depleted during operation, for example towards the end of life of the core, moderation is again increased (i.e. moderator in) to gain criticality for a longer life of the core.

In one example, the first reflector, the first set of fuel columns, the second set of burnable poison rods and the passageway are arranged coaxially.

Actuator

In one example, the HTR comprises an actuator arranged to move the HTR from the first configuration to the second configuration by moving the moderator, in use, from the first position to the second position. In one example, the actuator is arranged to move the HTR from the first configuration to the second configuration by moving the moderator, in use, upwards from the first position to the second position.

In one example, the actuator is arranged to move the HTR from the second configuration to the first configuration responsive to a fault condition, for example by moving the moderator, in use, downwards from the second position to the first position, for example due to gravity. In this way, the operation of the moderator actuator may be fail safe, thereby enhancing safety and/or reducing risk, since moderation is thereby reduced.

In one example, the actuator comprises a screw, a piston and/or a winch.

Controller

In one example, the HTR comprises a controller arranged to control moderation within 5%, preferably within 4%, more preferably within 3%, most preferably within 2% of a predetermined criticality of the HTR, by controlling movement of the moderator from the first configuration to the second configuration or vice versa. In this way, control of operation of the HTR may be improved.

In one example, the controller is arranged to control the moderation responsive to the depletion of the fissile material, in use, of the burnable poison rods. In this way, the moderation may be controlled dynamically, for example in a feedback loop.

Control rods

In one example, the HTR comprises a third set of control rods including P control rods, wherein P is a natural number in a range from 1 to 24, preferably in a range from 2 to 12, for example 4 or 6.

Generally, control rods are used in nuclear reactors to control, at least in part, a fission rate of the fissile fuel. Control rods typically comprise elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning. Since these elements have different capture cross sections for neutrons of varying energies, a composition of the control rods is designed for the reactor's neutron spectrum.

In one example, a control rod of the third set of control rods comprises silver, indium, boron, cobalt, hafnium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, an alloy such as high-boron steel or silver-indium-cadmium alloy, a compound such as boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium nitrate, gadolinium titanate, dysprosium titanate, composite such as boron carbide - europium hexaboride composite, or a mixture thereof. Boron is preferred.

Method

The second aspect provides a method of controlling moderation in a HTR according to the first aspect, the method comprising:

In one example, the method comprises:

moving the moderator upwards from the first position to the second position.

In one example, the method comprises:

controlling the moderation within 5%, preferably within 4%, more preferably within 3%, most preferably within 2% of a predetermined criticality of the HTR.

In one example, the method comprises:

controlling the moderation responsive to the depletion of the burnable poison rods.

In one example, the method comprises:

removing the moderator from the passageway responsive to a fault condition.

The method may include any of the steps described with respect to the first aspect.

Definitions

Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The term“consisting essentially of or“consists essentially of” means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention, such as colourants, and the like. The term “consisting of” or “consists of means including the components specified but excluding other components.

Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising” may also be taken to include the meaning“consists essentially of” or“consisting essentially of, and also may also be taken to include the meaning“consists of or“consisting of.

The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention, as set out herein are also applicable to all other aspects or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each aspect or exemplary embodiment of the invention as interchangeable and combinable between different aspects and exemplary embodiments.

Brief description of the drawings

For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

Figure 1 schematically depicts a transverse cross-section of a conventional high temperature engineering test reactor (HTTR);

Figure 2A schematically depicts a transverse cross-section of a conventional U-Battery micro modular reactor (MMR) and Figure 2B schematically depicts a transverse cross-section of a modified U-Battery micro modular reactor (MMR);

Figure 3 schematically depicts a high temperature reactor, HTR, according to an exemplary embodiment. Figure 3A schematically depicts a transverse cross-section of the HTR and Figure 3B schematically depicts an axial cross-section of the HTR;

Figure 4 schematically depicts a transverse cross-section of a simulation of the HTR of Figure 3;

Figure 5 depicts a graph of simulated criticality of the HTR of Figure 3 as a function of time; Figure 6 depicts a graph of simulated baselined criticality of the HTR of Figure 3 as a function of time and position of the moderator in the first and the second position;

Figure 7 schematically depicts a transverse cross-section of the HTR fuel assembly/block of Figure 3, in more detail;

Figure 8 depicts a graph of simulated atomic density of ²³⁹Puof the HTR of Figure 3 as a function of time;

Figure 9 depicts graphs of power distribution of the HTR of Figure 3, in use. Figure 9A depicts a graph of the power distribution of the HTR of Figure 3 in a first configuration and Figure 9B depicts a graph of the power distribution of the HTR of Figure 3 in a second configuration;

Figure 10 depicts a graph of a change in power distribution between Figures 9A and 9B;

Figure 11 depicts a graph of axial power distribution in the HTR for the moderator in the first and the second position of Figure 3, in use;

Figure 12 schematically depicts an axial cross-section of a control rod according to an exemplary embodiment;

Figure 13 schematically depicts a transverse cross-section of the HTR of Figure 3, in more detail, including the control rod of Figure 13;

Figure 14 depicts a graph of simulated criticality as a function of time for the HTR of Figure 3, arranged in the second configuration and close to endo of life of the core stepwise moved to the first position; and

Figure 15 schematically depicts a method of controlling moderation in a HTR according to an exemplary embodiment.

Detailed Description of the Drawings

Example 1

Figure 2A schematically depicts a transverse cross-section of a conventional U-Battery micro modular reactor (MMR) and Figure 2B schematically depicts a transverse cross-section of a modified U-Battery micro modular reactor (MMR). The design chosen to test the new approach is the U-Battery 9, a prismatic HTR SMR based on the prismatic core design, as shown in Figure 2A, being a 10 MWth design version. The side reflector has been switched to a graphite over a beryllium core due to ease of deployment, as shown in Figure 2B. The final design dimensions are detailed in Tables 2 to 4 (below).

Example 1: methodology

Due to a small size of the MMR, the peak fluxes as well as the resulting power distribution are all heavily dependent on the central reflector (CenRef) of the annular core design. The central reflector block is in the investigated case made solely of graphite, to aid the neutron moderation, this aspect can be utilised by removing the moderation to reduce the criticality by spectral hardening. A similar approach is used during the operation in the boiling water reactor, where the reduced coolant flow allows for reduced moderation due to additional void content and thus changes the spectrum to reduce the criticality.

Figure 3 schematically depicts a high temperature reactor, HTR, 100 according to an exemplary embodiment. Figure 3A schematically depicts a transverse cross-section of the HTR 100 and Figure 3B schematically depicts an axial cross-section of the HTR 100.

Particularly, the HTR 100 comprises a first set of fuel columns 110, including a first fuel column 110A, comprising fissile fuel 120. The HTR 100 comprises a second set of burnable poison rods 130, including a first burnable poison rod 130A. The HTR 100 comprises a moveable moderator 140. The HTR 100 comprises a passageway 150 arranged to receive the moderator 140 therein. The HTR 100 comprises a first reflector 160 arranged to surround the first set of fuel columns 110, the second set of burnable poison rods 130 and the passageway 150. The HTR 100 is arrangeable in a first configuration C1 , wherein the moderator 140 is received in the passageway 150 in a first position P1 thereby providing a first moderation M1 of neutrons produced by the fissile fuel 120. The HTR 100 is arrangeable in a second configuration C2, wherein the moderator 140 is received in the passageway 150 in a second position P2 thereby providing a second moderation M2 of the neutrons produced by the fissile fuel 120, whereby a change in criticality, in use, due to depletion of the fissile fuel and/or of the burnable poison rods 130 is attenuated.

In this example, the HTR 100 comprises a MMR as described herein. For example, the HTR 100 may comprise and/or be an adapted U-Battery. The HTR 100 also comprises a top plenum 1 , a top reflector 2, a bottom reflector 3, a bottom plenum 4, thermal insulation 5, a barrel 6 and a reactor pressure vessel (RPV) 7. In this example, the fissile fuel 120 comprises and/or is ²³⁵U, having an initial enrichment in a range from 15% to 100%, for example 17% or 20%. In this example, a mass of the fissile fuel 120 is in a range from 150 kg to 1 ,500 kg, for example 208 kg or 1 ,040 kg. In this example, the first fuel column 1 10A of the first set of fuel columns 1 10 comprises TRISO fuel comprising the fissile fuel 120. In this example, the TRISO fuel is arranged in a set of TRISO fuel compacts, and a set of TRSO fuel compacts comprise the fuel block. In this example, a TRISO fuel compact comprises a cylinder having a diameter of 10 mm and a length of 39 mm. In this example, the fuel columns 1 10 of the first set of fuel columns 1 10 are arranged in fuel blocks, for example in 6 x 4 or 30 x 4 fuel blocks. In this example, the first set of fuel columns 1 10 includes M fuel columns 1 10, wherein M is 216. In this example, each of the M fuel columns 1 10 is as described with respect to the first fuel column 1 10A. In this example, fuel columns 1 10 of the first set of fuel columns 1 10 are mutually spaced apart. In this example, the first set of fuel columns 1 10 are arranged in an array, for example a regular array. In this example, the first set of fuel columns 1 10 are arranged symmetrically. In this example, the first set of fuel columns 1 10 is arranged to surround the passageway 150.

In this example, the first burnable poison rod 130A comprises B₄C in a range from 0.2 wt.% to 1 wt.%, for example 0.3 wt.% by weight of the rod. In this example, the second set of burnable poison rods 130 includes N burnable poison rods 130, wherein N is 2 or 14 per fuel block. In this example, each of the N burnable poison rods 130 is as described with respect to the first burnable poison rod 130A.

In this example, the moderator 140 is graphite. In this example, the moderator 140 comprises a monolithic (i.e. a singular) moderator 140 consisting of graphite and assembled from graphite blocks.

In this example, the passageway 150 comprises and/or is an axial (also known as central) passageway 150. In this example, the passageway 150 extends completely through a length of the set of fuel columns 1 10. In this example, the passageway 150 protrudes beyond the set of fuel columns 1 10, for example beyond a region comprising the set of fuel columns 1 10. In this example, the passageway 150 comprises a cylindrical passageway 150.

In this example, the HTR comprises a coolant 180, for example He. The passageway 150 is arranged to receive a part of the coolant while other part of the He streams through the coolant channels within the fuel blocks.

In this example, the first reflector 160 is arranged to fully (i.e. completely) surround the first set of fuel columns 1 10, the second set of burnable poison rods 130 and the passageway 150. In this example, the first reflector 160 comprises graphite. In this example, the HTR comprises a second reflector 170 comprising the passageway 150. In this example, the second reflector 170 is a central reflector, comprising the passageway 150. In this example, the second reflector 170 is a toroidal reflector. In this example, the first set of fuel columns 110 and/or the second set of burnable poison rods 130 is arranged to surround the passageway 150. In this example, the second reflector 170 consists of graphite.

In this example, the first position P1 corresponds with the moderator 140 removed, for example completely removed, from a region comprising the set of fuel columns 110 (i.e. moderator 140 out), whereby the first moderation M1 is relatively lower and, in turn, the criticality is relatively lower, for example at a start of operation of the HTR.

In this example, the second position P2 corresponds with the moderator 140 within, for example a least partly within or fully within, a region comprising the set of fuel columns 110 (i.e. moderator 140 in), whereby the second moderation M2 is relatively higher and, in turn, the criticality is relatively higher, for example at during operation of the HTR, when at least some of the fissile fuel 120 has been depleted and criticality may be increased.

In this example, the first reflector 160, the first set of fuel columns 110, the second set of burnable poison rods 130 and the passageway 150 are arranged coaxially.

In this example, the HTR comprises an actuator 190 (not shown) arranged to move the HTR from the first configuration C1 to the second configuration C2 by moving the moderator 140, in use, from the first position P1 to the second position P2. In this example, the actuator 190 is arranged to move the HTR from the first configuration C1 to the second configuration C2 by moving the moderator 140, in use, upwards from the first position P1 to the second position P2. In this example, the actuator 190 is arranged to move the HTR from the second configuration C2 to the first configuration C1 responsive to a fault condition, for example by moving the moderator 140, in use, downwards from the second position P2 to the first position P1 , for example due to gravity.

In this example, the HTR comprises a controller 10 arranged to control moderation within 5%, preferably within 4%, more preferably within 3%, most preferably within 2% of a predetermined criticality of the HTR, by controlling movement of the moderator 140 from the moderator 140 from the first configuration C1 to the second configuration C2. In this example, the controller 10 is arranged to control the moderation responsive to the depletion, in use, of the burnable poison rods 130. In this example, the HTR comprises a third set of control rods 200 including P control rods, wherein P is a natural number in a range from 1 to 24, preferably in a range from 2 to 12, for example 4 or 6. In this example, a control rod of the third set of control rods 200 comprises boron.

In more detail, graphite undergoes an altering process which physically changes its conditions during irradiation. This implies that graphite needs to be handled carefully over time, so the concept of moving the whole hexagonal block seems to be unrealistic due to the difficulties of mechanical deformation of the material and the structural integrity of the core itself. Hence, to maintain structural integrity over time, the outside structure of the hexagonal graphite block is maintained and only a 26 cm diameter column (i.e. the passageway 150) is cut out of the centre of the central hexagon as depicted in Figure 3. This first approach can be optimised as soon as the first structural integrity evaluation is made available.

We believe in the case of such a small core that the removal of a singular volume of graphite is more beneficial than multiple moderator rods. This is due to the simplicity of the control system and the benefits of maintaining a symmetrical flux profile. It should be noted that for additional benefits, the central reflector is not limited to graphite and could be made of more advanced moderating materials such as yttrium hydride.

Figure 3 shows the central reflector removed in the centre of the core and replaced with helium coolant.

When considering the task of reactivity control, the requirement for exceptional safety critical systems is paramount. The IAEA safety guide contains regulations that require control rods to be fitted with an interlock to allow the system to be not triggered in unwanted situations. While moving the central reflector block, a similar amount of safety confidence will be required, as the insertion at the wrong time could cause the core to become supercritical. In addition, a failsafe design system is preferred and hence the central reflector is inserted from the bottom of the core upwards, this eliminates the risk of the central reflector of entering the core under accidental scenarios.

The control of a central column must be qualified to a similar degree as a control rod drive to pass regulatory standards. This could be achieved in a similar affect as a mechanical jack, where each rotation of the jack is limited to a single turn for the operators input to be limited and controlled. This would provide a high safety factor and remove the potential hazard of unwanted criticality insertions. There are several important aspects which must be investigated, when changing the system in this manner to create a deeper understanding of the relevant effects. The first is the overall benefit in reactivity control which can be gained from the procedure by producing a fuel lifecycle criticality experiment. This experiment takes place by using an un-poisoned version of the MMR and the three CenRef material (i.e. the moderator 140) positional arrangements conditions shown Table 1. During the removal of the CenRef the gap left is modelled using helium to represent the coolant in the system.

Table 1 - CenRef positions considered.

When the point of 1.02 criticality is reached due to fuel and/or burnable poison burnup, the central reflector is the directly re-inserted to determine the long-term behaviour of the core and maximum lifetime. The second test is to determine the effect on the power distribution due to the removal of the central reflector and of the re-insertion. The central compacts are expected to deliver highest power, so removing the moderation will have most probably a positive influence on the overall power distribution in the core the effect will be changed when the moderator is re-inserted. The changes could lead to additional power peaking which could cause overheating.

Figure 4 schematically depicts a transverse cross-section of a simulation of the HTR 100 of Figure 3. Particularly, Figure 4 shows a Serpent model of MMR with the central reflector removed. All neutronic results presented are simulated through a Monte Carlo simulation routine using the Serpent 2.1 .27 23, using data libraries JEFF 3.11 as shown in Figure 4. The simulations are performed using a 100k neutron population with 25 inactive cycles and 25 active cycles to allow for suitable flux convergence. The burnup procedure was produced using Serpent’s CRAM 24 burnup procedure with a maximum burnup step of 31 days to reduce errors during the burnup steps.

Example 1: results and discussion

The initial test determines the overall criticality changes of the system over the period of the possible reactor operation. This experiment models the total criticality without poisons with the column in, out and half out. At the point in when the criticality is at 1.02, the column is reinserted to determine the overall lifetime achievable.

Figure 5 depicts a graph of simulated criticality of the HTR of Figure 3 as a function of time. Particularly, Figure 5 shows criticality with CenRef column at varying positions, with criticality 1.02 marked.

Figure 6 depicts a graph of simulated baselined criticality of the HTR of Figure 3 as a function of time. Particularly, Figure 6 shows delta from base of CenRef removal (Reference - Half removed) & (Reference-Fully removed).

Figure 5 and 6 show the initial criticality is dropped by 0.032 and 0.014 with the CenRef withdrawn fully and half with respect to the base model. From the consideration of HTR operation, this is the point at which the reactivity control mechanism would have been activated to accommodate this criticality drop to limit the excess reactivity in the core. There is still a significant distance until reaching unity, which would have to be accommodated in a real design by a combination using FBPs and control rods. However, the applied method of withdrawal of a part of the CenRef has shown a clear drop-in criticality at initial start-up. At day 920 and 1085 the two withdrawn pieces of the central reflector must be reinserted into the centre to keep the criticality above the considered limit level of 1.02. Both models show after reinsertion of the central moderator a slightly higher criticality than the reference case.

The slightly increased criticality is likely due to an increased build-up on ²³⁹Pu. This additional fissile fuel content allows a slight increase (~31) in full power days compared to the base model.

Figure 7 schematically depicts a transverse cross-section of the HTR of Figure 3, in more detail. Particularly, Figure 7 shows a fuel compact monitored for the ²³⁹Pu build up analysis.

To analyse the ²³⁹Pu build up dependent on the position on the central moderator piece, a single 10 cm section at the centre of the axial fuel height was monitored to determine the overall ²³⁹Pu content over the burnup of the core. The fuel compact is identified in Figure 7.

Figure 8 depicts a graph of simulated atomic density of the HTR 100 of Figure 3 as a function of time and the moderation. Particularly, Figure 8 shows ²³⁹Pu build up over time with the different central reflector arrangements in the monitored central pin.

At the beginning of the life of the core there is zero content in Pu since Pu is not a content in the fresh U0₂ fuel. Plutonium is bred time as neutrons are captured in the ²³⁸U atom which

is transmuted via Neptunium into The development of the atomic densities of the Pu is recorded over burnup and compared between the three systems in Figure 8.

The positioning of the observed piece of fuel used to monitor the material composition is important as the system with half the reflector in is still partly benefitting from a large thermal spectrum across it since it is close to the upper surface of the movable moderator block. This is shown by the similarities to the base case in the system. When inserting the central reflector again, the half in system provided minimal variation due to the position. In the case of the fully removed central reflector the re-insertion provides an immediate bend in the curve of the ²³⁹Pu concentration build up. Once the central reflector is re-inserted more ²³⁹Pu is consumed than new build. This is an effect of the higher capture cross section of ²³⁹Pu within the thermal energy range. This result reaffirms the conclusion that the additional criticality is granted from the breeding of additional fissile fuel and answers the hypothesis that the stored neutrons are capable of being re-deployed at a later stage.

Figure 9 depicts graphs of power distribution of the HTR of Figure 3, in use. Figure 9A depicts a graph of the power distribution of the HTR of Figure 3 in a first configuration and Figure 9B depicts a graph of the power distribution of the HTR of Figure 3 in a second configuration.. Particularly, Figure 9A shows a power distribution with the central reflector fully removed and Figure 9B shows a power distribution with the central reflector fully inserted (undisturbed reference vase).

The second step is to investigate the power distribution in the compacts of the fuel assembly at the start-up of the reactor to see the effect of the absence of a major piece of the central reflector on the power distribution in the fuel assembly. The following figures represent the powers production in the fuel compacts based on the reaction rates across one half of the eastern fuel block with withdrawn moderator piece and the reference undisturbed case as shown in Figures 9A and 9B.

Both Figures 9A and 9B show a pronounced power distribution with a reduced power production in the centre of the fuel assembly and a clear increase of the power production in the compacts close to the central and the outer reflector. This characteristic power distribution in a HTR fuel block can be explained with the self-shielding of the fuel against the thermal neutron flux which is created in the pure moderator regions. Highly thermalized neutrons are created in the graphite reflector which has an extremely low absorption cross section for neutrons. As soon as the thermal neutrons are re-entering the fuel block, there is a high probability to cause fission reactions, thus the thermal neutrons have a low probability to reach the centre of the fuel assembly and cause fissions there. A close comparison of the power production in the innermost row of Figures 9A and 9B indicates that the removal of a part of the central moderator reduces the power production in the part of the fuel assembly close to the centre of the core. The detailed analysis of the deviation of the power production between the reference case and the case with extracted central moderator piece, given in Figure 10 shows a general power shift away from the centre of the core as a part of the moderator is removed. This is important due to the highest power pin being in the centre of the core which leads to a significant load reduction as the power is shifted across the core to the fuel pins close to the side reflector. In addition, the control of the reactivity must be assisted using burnable poisons and control rods. In most of the proposed fuel block designs for block type HTRs, the burnable poisons are in the corner compacts of the hexagons, since these are the positions which are exposed to the highest thermal neutron flux due to the location close to the reflectors while the control rods are in the side reflector.

Figure 10 depicts a graph of a change in power distribution between Figures 9A and 9B. Particularly, Figure 10 shows a change in power (Central reflector out- central reflector in).

Figure 10 emphasises the drop-in power on the central pins, with an overall power drop of ~30% during the full removal of the central reflector and a power increase of ~7.5% in the pins close to the side reflector.

Figure 11 depicts a graph of axial power distribution in the HTR of Figure 3, in use. Particularly, Figure 11 shows axial power distribution in the monitored central pin.

Axial power distribution is shown in Figure 11 for the operational point just before and after the insertion of the central reflector piece. It is obvious that the total power is significantly reduced in the central pins. The removal of the central reflector completely changes the power production in the pin in Figure 7. Only the top and bottom of the pins produce identical power which could be explained by the effect of the axial reflectors.

Example 1: summary

Example 1 has investigated a new type of reactivity control mechanism for a SMR HTR. The concept touches on the ability to preserve some neutrons during high reactivity periods, for them to be utilised at a period later in the fuel cycle. This is achieved by breeding ²³⁹Pu at a higher rate than would be normally active in the fuel cycle by reducing the neutron moderation. This hypothesis was demonstrated that ~30% higher ²³⁹Pu production was witnessed at the point before the moderator is re-inserted. The ²³⁹Pu concentration depleted once the central reflector was reintroduced and this provided a large reactivity increase. This additional reactivity provided an increase of full power days of the reactor by approximately 31 days which corresponds to a 2.5% longer core lifecycle. It should be noted, that the presented results demonstrate the concept of the moveable moderator reactivity control. The design of the moveable moderator can be further optimised, such as introducing the moveable moderator into the graphite reflector regions, concentric rings of graphite in the central reflector to allow for more precise reactivity control. The drawback comes with adding complexity into the control design, with the method demonstrated here focusing on the most simplistic method available.

The second test tried to identify any possible penalties with the fuel power loading within the core. In the case with the reflector removed, the overall power dropped by 30% within the highest loading pins which must be recognised as a positive effect. The power was shifted gradually across the core to the pins by the side reflector, which do not suffer from such high power as well as high burnups. Thus, we have identified a significant advantage when considering the power and resulting burnup distribution as the pins with the highest loading have had their power significantly reduced without the aid of neutron poisons.

Particularly, Example 1 is a novel approach applied to reduce the overall dependence on fixed burnable poisons and control rods during high reactivity periods within a high temperature graphite moderated reactor. To reduce the excess activity, we aim to harden the flux spectrum across the core by removing part of the central moderation column, thus breeding more plutonium, in a later period the flux spectrum is softened again to utilise this plutonium again. This provides a neutron storage effect within the ²³⁸U and the resulting breeding of Plutonium.

Due to the small size and the annular design of the high temperature reactor, the central reflector is key to the thermalization process. By removing a large proportion of the central reflector, the fuel within the proximity of the central reflector are less likely to receive neutrons within the thermal energy range. In addition to this, the fuel at the extremities of the core have a higher chance of fission due to the higher number of neutrons reaching them. This works as a method of balancing the power distribution between the central and outside fuel pins.

During points of low reactivity, such as the end of the fuel cycle, the central reflector can be reinserted and the additionally bred plutonium and U²³⁵ at the centre of the core will encounter a higher probability of fission due to more thermal neutrons within this region.

By removing the central reflector, this provided a 320 pern reactivity drop for the duration of the fuel cycle. The plutonium buildup provided additional fissile fuel up until the central reflector was reinserted. The described method created a two-fold benefit. The overall full power days within the core was increased by ~31 days due to the additional fissile fuel within the core and secondly the highest loaded power pins saw a 30% power reduction during the removal of the central reflector column. In conclusion, the overall effect of changing the moderation within the centre of the core provides a two-fold benefit, initially with longer fuel life cycles and secondly with a better power distribution.

Example 1: supplementary description

The original U-Battery report stated that a five year fuel lifecycle could be achieved in the core configuration in Figure 2. The initial aims investigated replicating these results by using the data provided in the U-Battery manual, these values are represented in Tables 2 to 4.

Table 2: Left, Radial dimensions, Right Axial dimensions.

Table 3: Triso layers dimensions.

Table 4: Material compositions.

Mass Temperature Density

Part Material Composition

fraction (k) g/cm

Example 2

Example 2 is based on the MMR concept and the parallel application of the movable moderator and burnable poisons.

Table 5: Control rod description

Due to the small size of the core, the control rod positioning becomes difficult to keep the symmetry. We propose six control rods placed at the exterior of the active core within the side reflector as depicted by Figure 2B.

The control rods design in this example are not varied. All control rods are moved simultaneously to preserve a symmetrical power profile across the radial core. A full breakdown on the geometrical description can be found in the appendices.

All neutronic results presented are simulated through a Monte Carlo simulation routine using the Serpent 2.1.27 [12], using data libraries JEFF 3.11.

Example 2: methodology

FBPs are designed to provide sufficient reactivity repression for the start of the lifecycle of the fuel.. The aim is to reduce the criticality of the core to a reasonable level, which can then be controlled in conjunction with traditional control rod mechanisms and the movable moderator. In this case an arbitrary value of criticality of 1.02 is chosen as a safety margin. With the inclusion of poisons the aim was to reduce the initial criticality to ~1.05.

Example 2: Design 1

Design 1 aims to use a thin coating of Gd₂0₃ on compacts across the centre of the fuel columns. Figure 12 schematically depicts an axial cross-section of a control rod 200 according to an exemplary embodiment. Particularly, Figure 12 shows a FBP sleeve.

The correct criticality range was obtained from the configuration in Figure 13.

Figure 13 schematically depicts a transverse cross-section of the HTR of Figure 3, in more detail, including the control rod of Figure 13. Particularly, Figure 13 shows Design 1 poison column locations.

Example 2: results and discussion

When considering the remaining activity, this is still significantly large to be held down by the form of control rods, due to the flux suppression to the bottom of the core which will lead to an unacceptable power distribution. To accommodate this, the movable moderator is used. Due to the small size of the core the central reflector has a significant effect on the thermalisation of neutrons. By manipulating the position of the central reflector, the thermal neutrons within the system should dramatically reduce, thus significantly reducing the criticality of the system.

For this case the variable part central reflector has a 26 cm diameter cylinder cut from the centre as shown in Figure 3 and as described above with reference to Example 1. This cylinder can then be manoeuvred to change the moderation within the centre of the core.

The control rod manipulation would not be limited to the 1.02 criticality, as with the poisons as this method of control is variable, implying that the central reflector manoeuvring could be used in conjunction with the control rods. Due to the change in moderation not having a significant negative effect in the neutron economy, it would be preferred to move the central reflector than the control rods, however, use of the central reflector should require user input.

Figure 14 depicts a graph of simulated criticality as a function of time for the HTR 100 of Figure 3, arranged in the second configuration and having poison rods arranged according to Figure 13. Particularly, Figure 14 is the new criticality curve with the central reflector completely removed up until point P1 and shows criticality using the central reflector as an effective reactivity control device.

From Figure 14, it is noted that the initial criticality, even within the first day, drops below that of 1.02, this is due to the build up of xenon during the fission process. At this stage the control rods would be used to aid start-up and keep the criticality to unity. The lowest point at which the poisons drop keeps the core above unity due to the central reflectors removal, this is at day 186, where the criticality once again starts to rise due to the burnup of the burnable poison. The second rise is now reduced significantly to 1.033 at ~700 days, this implies that the control rods still need to take a slightly higher amount of reactivity from the core at this point than at the begin of life. Denoted by points P1 and P2 respectively are the positions where the central reflector has been re-inserted each time by 50% of its total height. This 50% figure was chosen to reduce movement and thus any risk associated with it, this also provided the 0.02 reactivity which was thought to be easily controlled by the control rods. The total lifetime is the same as that of the base model without any control concept, which shows the small lifetime increase has effectively cancelled out the lifetime lost by the inclusion of FBPs.

Example 2: Summary

Example 2 describes combined application of several methods of reactivity control within a small modular HTR. The overall goal aimed to exhibit how different control methods are used and interplay to manipulate the reactivity at different stages of a single fuel cycle, while trying to keep the minimum dependency on the control rods. This brought forward the application of an optimized burnable poison scheme, which showed a still too large rising peak in criticality (~1 .07) to overcome by the control rods alone. Following this, the invention of movable moderator by manipulating the central reflector leading to spectrum hardening to reduce the probability of fission has been used. This provided significant additional reactivity control within the system. The central reflectors breeding method cancelled out the fuel lifetime penalty which had been introduced by the inclusion of the FBPs, bringing the overall lifetime of the reactor to 1364 days, the same as without any poisons included.

The removal of the central reflector allows the operator to balance the power distribution across the core well radially, this is due to the highest power pins being swapped between the inside to outside of the fuel block.

Example 2: supplementary description

The core geometry was originally based on the U-Battery, however, studies regarding the effectiveness determined that there was no significant benefit of using graphite as a moderator. Thus, the new core geometry is listed below. Table 7: Material compositions for the revised conceptual U-Battery design.

Table 8: Radial dimensions of the new conceptual design.

Table 9: Axial dimensions of the new conceptual design.

Method

Figure 15 schematically depicts a method of controlling moderation in a HTR, for example the HTR 100 as described above, according to an exemplary embodiment.

At S3301 , the HTR is moved from the first configuration to the second configuration by changing a position of the moderator received in the passageway from the first position to the second position, whereby a change in criticality due to depletion of the fissile fuel and/or of the burnable poison rods is attenuated.

The method may include any of the steps described herein.

In one example, the method comprises moving the moderator upwards from the first position to the second position. In one example, the method comprises controlling the moderation within 5%, preferably within 4%, more preferably within 3%, most preferably within 2% of a predetermined criticality of the HTR. In one example, the method comprises controlling the moderation responsive to the depletion of the burnable poison rods. In one example, the method comprises removing the moderator from the passageway responsive to a fault condition.

Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above.

In summary, the invention provides a HTR arrangeable in a first configuration, wherein a moderator is received in a passageway in a first position thereby providing a first moderation of neutrons, for example a state or setup thereof, produced by fissile fuel and a second configuration, wherein the moderator is received in the passageway in a second position thereby providing a second moderation of the neutrons produced by the fissile fuel, whereby a change in criticality, in use, due to depletion of the fissile fuel and/or of the burnable poison rods is attenuated. The change in criticality, in use, due to depletion of the fissile fuel and/or of the burnable poison rods is attenuated (i.e. an effect thereof reduced). That is, the moveable moderator provides control of interplay between depletion of the fissile fuel and of the burnable poison. In the beginning of life (i.e. at start of operation), the criticality is controlled by reduced moderation (i.e. moderator out) and the burnable poison. If the criticality becomes too low during operation, for example due to depletion of the fissile material, the criticality may be increased by inserting the moderator (i.e. moderator in). When the burnable poison is burnt out (i.e. depleted) during operation, the criticality increases again, and the moderation may be reduced (i.e. moderator out). When the fissile fuel is depleted during operation, moderation is again increased (i.e. moderator in) to gain criticality for a longer life of the core.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A high temperature reactor, HTR, comprising:

a second set of burnable poison rods, including a first burnable poison rod;

a moveable moderator; and

a passageway arranged to receive the moderator therein; and

wherein the HTR is arrangeable in:

2. The HTR according to claim 1 , wherein the first set of fuel columns is arranged to surround the passageway.

3. The HTR according to claim 2, wherein the passageway is arranged axially.

4. The HTR according to claim 3, wherein the first reflector, the first set of fuel columns, the second set of burnable poison rods and the passageway are arranged coaxially.

5. The HTR according to any previous claim, comprising a second reflector comprising the passageway.

6. The HTR according to any previous claim, comprising an actuator arranged to move the HTR from the first configuration to the second configuration by moving the moderator, in use, from the first position to the second position.

7. The HTR according to claim 6, wherein the actuator is arranged to move the HTR from the first configuration to the second configuration by moving the moderator, in use, upwards from the first position to the second position.

8. The HTR according to any previous claim, wherein the moderator comprises and/or is graphite or a metal hydride.

9. The HTR according to any previous claim, comprising a controller arranged to control moderation within 5%, preferably within 4%, more preferably within 3%, most preferably within 2% of a predetermined criticality of the HTR, by controlling movement of the moderator from the moderator from the first configuration to the second configuration.

10. The HTR according to claim 9, wherein the controller is arranged to control the moderation responsive to the depletion, in use, of the burnable poison rods.

11. A method of controlling moderation in a HTR according to any previous claim, the method comprising:

12. The method according to claim 11 , comprising:

moving the moderator upwards from the first position to the second position.

13. The method according to any of claims 11 to 12, comprising:

14. The method according to any of claims 11 to 13, comprising:

15. The method according to any of claims 11 to 14, comprising:

removing the moderator from the passageway responsive to a fault condition.