Nuclear reactor and method of operating a nuclear reactor

Abstract

According to an example aspect of the present invention, there is provided a nuclear reactor (170) comprising a reactor core (172) encased in a pressure vessel (171), a riser barrel (179) defining a primary fluid circulation path for a coolant contained in the pressure vessel (171), and a radial reflector (212) arranged between the riser barrel (179) and the reactor core (172) and comprising at least one heat exchanger formed by a first fluid channel (213) and a second fluid channel (214), wherein the first fluid channel (213) has a first fluid inlet (215) fluidly connected to a riser (190) or to the reactor core (172) and arranged above a first fluid outlet (217) fluidly connected to the reactor core (172), and wherein the second fluid channel (214) has a second fluid inlet (219) fluidly connected to a downcomer (194) through the riser barrel (179) and arranged below a second fluid outlet (220) fluidly connected to the downcomer (194) through the riser barrel (179).

Description

NUCLEAR REACTOR AND METHOD OF OPERATING A NUCLEAR REACTORFIELD

[0001] The present invention relates to a nuclear reactor.

[0002] Additionally, the present invention relates to a method of operating a nuclear reactor.BACKGROUND

[0003] Small modular nuclear reactors based on natural circulation are known. Such nuclear reactors typically comprise a reactor core encased in a pressure vessel, which in turn comprises a lid. The reactor core is located inside the pressure vessel at a lower section of the pressure vessel. The pressure vessel defines an inner volume of the nuclear reactor. An annular inner wall is provided within the pressure vessel to create a riser barrel. The riser barrel envelops the reactor core and extends towards an upper plenum. The purpose of the riser barrel is to define a primary fluid circulation path for a coolant contained in the pressure vessel. The riser barrel forms a riser on top of the reactor core. An annular space is formed between the riser barrel and the pressure vessel as a downcomer. A main heat exchanger is provided between the upper plenum and the downcomer. The downcomer communicates with a lower plenum, which is formed between a lower core support plate and the bottom of the pressure vessel.

[0004] For example, document WO 2010 / 057205 Al discloses a nuclear reactor module including a reactor core and a reactor housing that surrounds the reactor core. The reactor core is configured to direct coolant through the reactor core. A neutron reflector is further located between the reactor core and the reactor housing. The neutron reflector has a plurality of inlet ports facing the reactor core. Additionally, the neutron reflector has a plurality of outlet ports fluidly connected to the inlet ports to direct a portion of the coolant through the neutron reflector in a direction upwards.

[0005] Under certain circumstances the primary circulation path of the coolant can be blocked, for example when the coolant level has dropped below a top of the riser barrel.

[0006] In view of the foregoing, it would be beneficial to provide a nuclear reactor, wherein the reflector cooling function can be extended to cover a part of the emergency reactor core cooling heat transfer chain in case that the primary circulation loop is blocked.SUMMARY OF THE INVENTION

[0007] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0008] According to a first aspect of the present invention, there is provided a nuclear reactor comprising a reactor core encased in a pressure vessel, a riser barrel defining a primary fluid circulation path for a liquid coolant contained in the pressure vessel, and a radial reflector arranged between the riser barrel and the reactor core and comprising at least one heat exchanger formed by a first fluid channel and a second fluid channel, wherein the first fluid channel has a first fluid inlet fluidly connected to a riser or to the reactor core and arranged above a first fluid outlet fluidly connected to the reactor core, and wherein the second fluid channel has a second fluid inlet fluidly connected to a downcomer and arranged below a second fluid outlet fluidly connected to the downcomer through the riser barrel.

[0009] Certain embodiments of the first aspect may include at least one feature from the following bulleted list:• a first portion of the first fluid channel is arranged adjacent to a second portion of the second fluid channel• the first portion of the first fluid channel and the second portion of the second fluid channel are arranged parallel to one another• the first portion of the first fluid channel and the second portion of the second fluid channel are arranged vertically or substantially vertically the nuclear reactor is configured to guide a first portion of the coolant from a riser to the reactor core via the first fluid channel and to simultaneously guide a secondportion of the coolant from a downcomer to the downcomer via the second fluid channel when a coolant level is between a top of the riser barrel and a top of the reflector• the nuclear reactor is configured to guide a first portion of the coolant from the reactor core to the reactor core via the first fluid channel and to simultaneously guide a second portion of the coolant from a downcomer to the downcomer via the second fluid channel when a coolant level is between a top of the riser barrel and at least a part of the first fluid inlet and at least a part of the second fluid outlet• the nuclear reactor is configured to guide a first portion of a gas from the riser or the reactor core to the reactor core via the first fluid channel and to simultaneously guide a second portion of the gas from the downcomer to the downcomer via the second fluid channel when a coolant level is above a bottom of the riser barrel and below at least a part of the second fluid inlet and below at least a part of the first fluid outleta bottom of the riser barrel comprises a lower core support plate having a plurality of openings• the reflector comprises a plurality of vertical or substantially vertical borings through the reflector• the first fluid channel is fluidly connected to the riser through a top of the reflector or arranged in the proximity of the top of the reflector, the first fluid outlet is arranged in the proximity of a bottom of the reflector, the second fluid inlet is arranged in the proximity of the bottom of the reflector and the second fluid outlet is arranged in the proximity of the top of the reflector• the reflector is made of steel or a metal alloy• the first fluid outlet and the second fluid inlet are located at the same level in vertical direction of the nuclear reactor• the nuclear reactor comprises a plurality of heat exchangers, wherein channel lengths of the first and second fluid channels of at least one heat exchanger are identical to or differ from channel lengths of the first and second fluid channels of at least one other heat exchanger

[0010] According to a second aspect of the present invention, there is provided a method of operating a nuclear reactor, the method comprising providing a reactor core encased in a pressure vessel, providing a riser barrel defining a primary fluid circulation path for a liquid coolant contained in the pressure vessel, arranging a radial reflector between the riser barrel and the reactor core, forming a heat exchanger capable of locally decreasing a temperature of at least a part of the reflector by guiding a first portion of a fluid from a riser or the reactor core via a first fluid channel in a direction downwards or substantially downwards to the reactor core as well as simultaneously guiding a second portion of the fluid from a downcomer through the riser barrel via a second fluid channel in a direction upwards or substantially upwards and through the riser barrel to the downcomer.

[0011] Certain embodiments of the second aspect may include at least one feature from the following bulleted list:• guiding of the coolant takes place via the first and second fluid channels when a coolant level is between a top of the riser barrel and a top of the reflector• guiding of the coolant via the first and second fluid channels takes place when a coolant level is between a top of the riser barrel and at least a part of a first fluid inlet of the first fluid channel and at least a part of a second fluid outlet of the second fluid channel, wherein the first fluid inlet and the second fluid outlet are arranged in the proximity of a top of the reflector• guiding of a gas takes place via the first and second fluid channels when a coolant level is above a bottom of the riser barrel and below at least a part of the second fluid inlet and below at least a part of the first fluid outlet• the first fluid channel and the second fluid channel are arranged within the reflector

[0012] Considerable advantages are obtained by certain embodiments of the present invention. A nuclear reactor and a method of operating a nuclear reactor are provided. Small modular reactors based on natural circulation are prone to overheating, if the primary circulation path is blocked, for example due to a reduced amount of coolant inventory. In such a situation, the primary circulation may lead to overheating of the reactor core and eventually to reactor core damage, if sufficient cooling is not provided by other means. According to certain embodiments of the present invention, the radialreflector surrounding the reactor core is constructed in a way that two separate fluid channels are formed inside the reflector. The first fluid channel is connected to a bottom of the reactor core at one end and located above the reactor core at the other end. The second fluid channel is connected to the downcomer through the riser barrel at two different levels in vertical direction of the nuclear reactor. This allows the reflector to serve under certain conditions as a heat exchanger between the reactor core and the downcomer, thus transferring heat towards the pressure vessel walls where the rest of an emergency core cooling system can transfer the heat out of the pressure vessel.

[0013] Condition 1 : When the coolant level has dropped below a top of the riser barrel, the primary fluid circulation path is blocked and the flow circulation from the downcomer to the reactor core is stopped. As a consequence, temperatures in the reactor core start to increase due to loss of flow, and eventually the primary fluid or coolant starts to boil in the core region and riser, if heat losses through the surrounding structures are not sufficient or cooling is not provided by other means. In case of a leak in the primary circuit (loss of coolant accident, LOCA), and assuming the primary inventory is not replenished, the primary fluid level or coolant level continues to decrease, since either water or steam is leaked out of the pressure vessel. According to certain embodiments of the present invention, the reflector can act as a heat exchanger between the reactor core and the downcomer. As long as the coolant level is between a top of the riser barrel and a top of the reflector, guiding a first portion of the coolant from the riser to the reactor core via the first fluid channel and simultaneously guiding a second portion of the coolant from the downcomer to the downcomer via the second fluid channel takes place automatically, thus extending the reflector cooling function to cover part of the emergency core cooling heat transfer chain. Heat can be transferred to the walls of the pressurized vessel where the rest of the emergency core cooling system can transfer the heat out of the pressurized vessel.

[0014] Condition 2: When the coolant level has dropped below the bottom of the active fuel of the nuclear reactor, radiative heat transfer from the reactor core might evaporate the coolant at the bottom of the riser barrel to some degree, wherein the resulting steam flow would provide minimal cooling, but likely the entrance to the core formed by the lower core support plate having a plurality of openings at the bottom of the riser barrel will remain submerged. This effectively prevents a gas circulation path from the downcomer to the reactor core, which would be the worst imaginable situation from the point of cooling down the reactor core. According to certain embodiments of the presentinvention, the reflector can act as a heat exchanger between the reactor core and the downcomer. When the coolant level is above a bottom of the riser barrel and below at least a part of the second fluid inlet and below at least a part of the first fluid outlet, guiding a first portion of a gas from the riser to the reactor core via the first fluid channel and simultaneously guiding a second portion of the gas from the downcomer to the downcomer via the second fluid channel takes place automatically, thus extending the reflector cooling function to cover part of the emergency core cooling heat transfer chain. Heat can be transferred to the walls of the pressurized vessel where the rest of the emergency core cooling system can transfer the heat out of the pressurized vessel.

[0015] Certain embodiments of the present invention are particularly beneficial for extending the reflector cooling function to cover a part of the emergency reactor core cooling heat transfer chain in case that the primary fluid circulation path is blocked. The added cooling function of the reflector comprising at least one heat exchanger formed by a first fluid channel and a second fluid channel is fully passive and has no or minimal adverse effects on the normal operation of the nuclear reactor.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIGURE 1 illustrates a schematic view of an example of a nuclear reactor,

[0017] FIGURE 2 illustrates a schematic view of a cross-section of details of a nuclear reactor in accordance with at least some embodiments of the present invention, wherein a coolant level has dropped to a level between a top of the riser barrel and a top of the reflector, and

[0018] FIGURE 3 illustrates a schematic view of a cross-section of details of a nuclear reactor in accordance with at least some embodiments of the present invention, wherein a coolant level is above a bottom of the riser barrel and below at least a part of the second fluid inlet and below at least a part of the first fluid outlet.EMBODIMENTS

[0019] In FIGURE 1 a schematic view of an example of a nuclear reactor 170 is illustrated. The nuclear reactor 170 comprises a reactor core 172 encased in a pressure vessel 171, which in turn comprises a lid 173. The reactor core 172 is located at a lower section of the pressure vessel 171. The pressure vessel 171 defines an inner volume of the nuclear reactor 170. The reactor core 172 is located inside the pressure vessel 171.

[0020] The nuclear reactor 170 is shown in FIGURE 1 in a simplified manner. The exemplary nuclear reactor 170 is a district heating reactor but the principles disclosed herein are applicable to other reactor types as well. The nuclear reactor 170 features a double-vessel configuration with the pressure vessel 171 contained within a containment vessel 210, which is closed by a lid 211. It is, however, to be noted, that the embodiments herein disclosed are equally applicable to a single-vessel configuration. An annular inner wall is provided within the pressure vessel 171 to create a riser barrel 179. The riser barrel 179 may envelop the reactor core 172 and extend towards the upper plenum 191. The purpose of the riser barrel 179 is to define a fluid circulation path for the primary fluid or coolant contained in the pressure vessel 171. The riser barrel 179 forms a riser 190 on top of the reactor core 172. The riser 190 is the inner volume defined by the riser barrel 179. The space defined between the riser 190, i.e. the top level of the riser barrel 179, and a split seam between the pressure vessel body and lid 173, is an upper plenum 191. The space between the split seam and the lid 173 is a pressurizer 192.

[0021] An annular space is formed between the riser barrel 179 and the pressure vessel 171 as a downcomer 194. A main heat exchanger 193 is provided between the upper plenum 191 and the downcomer 194. The downcomer 194 communicates with a lower plenum 195, which is formed between a lower core support plate 223 having a plurality of openings and the bottom of the pressure vessel 171.

[0022] In FIGURE 1 a primary fluid circulation path for a coolant contained in the pressure vessel 171 is shown by the arrows. Under certain circumstances the primary fluid circulation path of the coolant can, however, be blocked, for example when the coolant level has dropped below a top 226 of the reflector riser barrel 179.

[0023] In FIGURE 2 a schematic view of a cross-section of details of a nuclear reactor 170 in accordance with at least some embodiments of the present invention isillustrated. Only a lower part of the nuclear reactor 170 is shown in FIGURE 2. As described above in connection with FIGURE 1, the nuclear reactor 170 comprises a reactor core 172 encased in a pressure vessel 171 and a riser barrel 179 defining a fluid circulation path for a liquid coolant contained in the pressure vessel 171. The coolant typically comprises water. Additionally, the nuclear reactor 170 comprises a radial reflector 212. The reflector 212 is a neutron reflector. The radial reflector 212 is located between the reactor core 172 and the riser barrel 179. The reflector 212 may be made of steel or a metal alloy, for instance. The radial reflector is arranged within the riser barrel 179 and comprises at least one heat exchanger formed by a first fluid channel 213 and a second fluid channel 214. Two heat exchangers are shown in FIGURE 2. The reflector 212 may comprise any suitable number of heat exchangers 212. The first fluid channel 213 has a first fluid inlet 215 fluidly connected to a riser 190 and arranged above a first fluid outlet 217 fluidly connected to the reactor core 172. In FIGURE 2, the first fluid inlet 215 is fluidly connected to the riser 190 through a top of the reflector 212. Alternatively, the first fluid inlet 215 can also be fluidly connected to the reactor core 172, typically in the proximity of the top 216 of the reflector 212. The first fluid outlet 217 is typically arranged in the proximity of a bottom 218 of the reflector 212. In this connection, the words “in the proximity of the top of the reflector” and “in the proximity of a bottom of the reflector” mean that the first fluid inlet 215 is located closer to the top 216 of the reflector 212 than to the bottom 218 of the reflector 212 and, respectively, that the first fluid outlet 217 is located closer to the bottom 218 of the reflector 212 than to the top 216 of the reflector 212. Typically, the first fluid outlet 217 is located as close as possible to the bottom 218 of the reflector 212. The second fluid channel 214 has a second fluid inlet 219 fluidly connected to a downcomer 194 through the riser barrel 179 and arranged below a second fluid outlet 220 fluidly connected to the downcomer 194 through the riser barrel 179. Typically, the second fluid inlet 219 is arranged in the proximity of the bottom 218 of the reflector 212 and the second fluid outlet 220 is arranged in the proximity of the top 216 of the reflector 212. In this connection, the words “in the proximity of the bottom of the reflector” and “in the proximity of the top of the reflector” mean that the second fluid inlet 219 is located closer to the bottom 218 of the reflector 212 than to the top 216 of the reflector 212 and, respectively, that the second fluid outlet 220 is located closer to the top 216 of the reflector 212 than to the bottom 218 of the reflector 212. In other words, the first fluid inlet 215 and the second fluid outlet 220 are typically, but not necessarily, located in the area of the top 216 of the reflector 212 and the first fluid outlet 217 and the secondfluid inlet 219 are typically, but not necessarily located in the area of the bottom 218 of the reflector 212. Typically, the second fluid inlet 219 is located as close as possible to the bottom 218 of the reflector 212 and the second fluid outlet is located as close as possible to the top 216 of the reflector 212. The first fluid outlet 217 and the second fluid inlet 219 are typically located at the same level in vertical direction of the nuclear reactor 170. However, the reflector 212 may comprise a plurality of heat exchangers according to certain embodiments. In such a case, channel lengths of the first and second fluid channels 213, 214 of at least one heat exchanger can be identical to or differ from channel lengths of the first and second fluid channels 213, 214 of at least one other heat exchanger.

[0024] A first portion 221 of the first fluid channel 213 is arranged adjacent to a second portion 222 of the second fluid channel 214. The distance between the first portion 221 of the first fluid channel 213 and the second portion 222 of the second fluid channel 214 is typically quite small due to the material of the reflector 212. For example, the distance may be less than 3 cm, less than 2 cm or less than 1 cm. Both the first portion of the first fluid channel 213 and the second portion 222 of the second fluid channel 214 are typically straight. Further, the first portion 221 of the first fluid channel 213 and the second portion 222 of the second fluid channel 214 are normally arranged parallel to one another. Additionally, the first portion 221 of the first fluid channel 213 and the second portion 222 of the second fluid channel 214 are typically arranged vertically or substantially vertically. Preferably, the first portion 221 of the first fluid channel 213 and the second portion 222 of the second fluid channel 214 are as long as possible to provide optimum heat transfer. In other words, the first portion 221 of the first fluid channel 213 and the second portion 222 of the second fluid channel 214 typically extend almost through the entire reflector 212.

[0025] Condition 1 : In FIGURE 2, a coolant level 225 has dropped to a level between a top of the riser barrel 179 and a top 216 of the reflector 212. In other words, the primary fluid circulation path for the coolant shown in FIGURE 1 is blocked and the flow circulation from the reactor core 172 to the downcomer 194 is stopped. In such a situation, the nuclear reactor is 170 configured to guide a first portion of the coolant from the riser 190 to the reactor core 172 via the first fluid channel 213, i.e. in a direction downwards or substantially downwards, and to simultaneously guide a second portion of the coolant from the downcomer 194 to the downcomer 194, i.e. in a direction upwards or substantially upwards, via the second fluid channel 214. As a consequence, convective flows are formed inside the first and second cooling channels 213, 214 and the reflector temperature islocally decreased. A CFD analysis has e.g. shown that local reflector temperatures in the area of the second fluid channel 214 can be substantially decreased. The local circulation paths of the coolant are shown only for one heat exchanger by arrows.

[0026] Consequently, the reflector cooling function is extended to cover a part of the emergency reactor core cooling heat transfer chain in case that the primary fluid circulation path is blocked. The added cooling function of the reflector comprising at least one heat exchanger formed by a first fluid channel and a second fluid channel is fully passive and has no or minimal adverse effects on the normal operation of the nuclear reactor.

[0027] According to certain embodiments, the reflector 212 may be higher than the reactor core 172 and the first fluid inlet 215 and the second fluid outlet 220 may arranged in the proximity of a top 216 of the reflector 212. Guiding of the coolant via the first and second fluid channels 213, 214 may then take place when a coolant level is between a top 226 of the riser barrel 179 and at least a part of a first fluid inlet 215 of the first fluid channel 213 and at least a part of a second fluid outlet 220 of the second fluid channel 214.

[0028] In FIGURE 3 a schematic view of a cross-section of details of a nuclear reactor 170 in accordance with at least some embodiments of the present invention is illustrated. Only a lower part of the nuclear reactor 170 is shown in FIGURE 3. The nuclear reactor 170 comprises the features shown and described above in connection with FIGURE 2.

[0029] Condition 2: In FIGURE 3, a coolant level 225 is above a bottom 224 of the riser barrel 179 and below at least a part of the second fluid inlet 219 and below at least a part of the first fluid outlet 217. In other words, the primary fluid circulation path for the coolant shown in FIGURE 1 is blocked and gas circulation from the downcomer 194 to the reactor core 172 is stopped. The entrance to the reactor core 172 formed by the lower core support plate 223 having a plurality of openings at the bottom 224 of the riser barrel 179 is submerged. A gas is present above the coolant level 225. The gas typically comprises steam. In such a situation, the nuclear reactor 170 is configured to guide a first portion of a gas present in the pressure vessel 171 above the coolant level 225 from the riser 190 or the reactor core 172 to the reactor core 172 via the first fluid channel 213, i.e. in a direction downwards or substantially downwards, and to simultaneously guide a second portion of the gas from the downcomer 194 to the downcomer 194, i.e. in a direction upwards or substantially upwards, via the second fluid channel 214. As a consequence, convectiveflows are formed inside the first and second cooling channels 213, 214 and the reflector temperature is locally decreased. The local circulation paths of the gas are shown only for one heat exchanger by arrows.

[0030] Consequently, the reflector cooling function is extended to cover a part of the emergency reactor core cooling heat transfer chain in case that the primary fluid circulation path is blocked. The added cooling function of the reflector comprising at least one heat exchanger formed by a first fluid channel and a second fluid channel is fully passive and has no or minimal adverse effects on the normal operation of the nuclear reactor.

[0031] According to certain embodiments, the reflector 212 may be higher than the reactor core 172 and the first fluid inlet 215 and the second fluid outlet 220 may arranged in the proximity of a top 216 of the reflector 212. Guiding of a gas via the first and second fluid channels 213, 214 may then take place when a coolant level is above a bottom 224 of the riser barrel 179 and below at least a part of the second fluid inlet 219 and below at least a part of the first fluid outlet 217.

[0032] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0033] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0034] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein alongwith alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0035] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0036] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0037] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.INDUSTRIAL APPLICABILITY

[0038] At least some embodiments of the present invention find industrial application in operating a nuclear reactor.REFERENCE SIGNS LIST nuclear reactor pressure vessel reactor core lid of pressure vessel riser barrel riser upper plenum pressurizer main heat exchanger downcomer lower plenum containment vessel lid of containment vessel reflector first fluid channel second fluid channel first fluid inlet top of reflector first fluid outlet bottom of reflector second fluid inlet second fluid outlet221 first portion of first fluid channel222 second portion of second fluid channel223 lower core support plate224 bottom of riser barrel 225 coolant level226 top of riser barrelCITATION LISTPatent LiteratureWO 2010 / 057205 Al

Claims

CLAIMS:

1. A nuclear reactor (170) comprising:- a reactor core (172) encased in a pressure vessel (171),- a riser barrel (179) defining a primary fluid circulation path for a liquid coolant contained in the pressure vessel (171), and- a radial reflector (212) arranged between the riser barrel (179) and the reactor core (172) and comprising at least one heat exchanger formed by a first fluid channel (213) and a second fluid channel (214),- wherein the first fluid channel (213) has a first fluid inlet (215) fluidly connected to a riser (190) or to the reactor core (172) and arranged above a first fluid outlet (217) fluidly connected to the reactor core (172), and- wherein the second fluid channel (214) has a second fluid inlet (219) fluidly connected to a downcomer (194) through the riser barrel (179) and arranged below a second fluid outlet (220) fluidly connected to the downcomer (194) through the riser barrel (179).

2. The nuclear reactor (170) according to claim 1, wherein a first portion (221) of the first fluid channel (213) is arranged adjacent to a second portion (222) of the second fluid channel (214).

3. The nuclear reactor (170) according to claim 2, wherein the first portion (221) of the first fluid channel (213) and the second portion (222) of the second fluid channel (214) are arranged parallel to one another.

4. The nuclear reactor (170) according to claim 2 or 3, wherein the first portion (221) of the first fluid channel (213) and the second portion (222) of the second fluid channel (214) are arranged vertically or substantially vertically.

5. The nuclear reactor (170) according to any one of claims 1 - 4, wherein the nuclear reactor is (170) configured to guide a first portion of the coolant from the riser (190) to the reactor core (172) via the first fluid channel (213) and to simultaneously guide a secondportion of the coolant from the downcomer (194) to the downcomer (194) via the second fluid channel (214) when a coolant level is between a top (226) of the riser barrel (179) and the top (216) of the reflector (212), or the nuclear reactor is (170) configured to guide a first portion of the coolant from the reactor core (172) to the reactor core (172) via the first fluid channel (213) and to simultaneously guide a second portion of the coolant from the downcomer (194) to the downcomer (194) via the second fluid channel (214) when a coolant level is between a top (226) of the riser barrel (179) and at least a part of the first fluid inlet (215) and at least a part of the second fluid outlet (220).

6. The nuclear reactor (170) according to any one of claims 1 - 4, wherein the nuclear reactor is (170) configured to guide a first portion of a gas from the riser (190) or the reactor core (172) to the reactor core (172) via the first fluid channel (213) and to simultaneously guide a second portion of the gas from the downcomer (194) to the downcomer (194) via the second fluid channel (214) when a coolant level is above a bottom (224) of the riser barrel (179) and below at least a part of the second fluid inlet (219) and below at least a part of the first fluid outlet (217).

7. The nuclear reactor (170) according to any one of claims 1 - 6, wherein a bottom (224) of the riser barrel (179) comprises a lower core support plate (223) having a plurality of openings.

8. The nuclear reactor (170) according to any one of claims 1 - 7, wherein the reflector (212) comprises a plurality of vertical or substantially vertical borings through the reflector (212).

9. The nuclear reactor (170) according to any one of claims 1 - 8, wherein the first fluid channel (213) is fluidly connected to the riser (190) through a top (216) of the reflector (212) or arranged in the proximity of the top (216) of the reflector (212), the first fluid outlet is arranged in the proximity of a bottom (218) of the reflector (212), the second fluid inlet (219) is arranged in the proximity of the bottom (218) of the reflector (212) and the second fluid outlet (220) is arranged in the proximity of the top (216) of the reflector (212).

10. The nuclear reactor (170) according to any one of claims 1 - 9, wherein the first fluid outlet (217) and the second fluid inlet (219) are located at the same level in vertical direction of the nuclear reactor (170).

11. A method of operating a nuclear reactor (170), the method comprising:- providing a reactor core (172) encased in a pressure vessel (171),- providing a riser barrel (179) defining a primary fluid circulation path for a liquid coolant contained in the pressure vessel (171),- arranging a radial reflector (212) between the riser barrel (179) and the reactor core (172),- forming a heat exchanger capable of locally decreasing a temperature of at least a part of the reflector (212) by guiding a first portion of a fluid from a riser (190) or the reactor core (172) via a first fluid channel (213) in a direction downwards or substantially downwards to the reactor core (172) as well as simultaneously guiding a second portion of the fluid from a downcomer (194) through the riser barrel (179) via a second channel (214) in a direction upwards or substantially upwards and through the riser barrel (179) to the downcomer (194).

12. The method according to claim 11, wherein guiding of the coolant via the first and second fluid channels (213, 214) takes place when a coolant level is between a top (226) of the riser barrel (179) and a top (216) of the reflector (212) or when a coolant level is between a top (226) of the riser barrel (179) and at least a part of the first fluid inlet (215) and at least a part of the second fluid outlet (220).

13. The method according to claim 11, wherein guiding of the coolant via the first and second fluid channels (213, 214) takes place when a coolant level is between a top (226) of the riser barrel (179) and at least a part of a first fluid inlet (215) of the first fluid channel (213) and at least a part of a second fluid outlet (220) of the second fluid channel (214), wherein the first fluid inlet (215) and the second fluid outlet (220) are arranged in the proximity of a top (216) of the reflector (212).

14. The method according to claim 11, wherein guiding of a gas via the first and second fluid channels (213, 214) takes place when a coolant level is above a bottom (224) of theriser barrel (179) and below at least a part of the second fluid inlet (219) and below at least a part of the first fluid outlet (217).

15. The method according to any one of claims 11 - 14, wherein the first fluid channel (213) and the second fluid channel are arranged within the reflector (212).

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