DE2828975A1 - Pebble bed reactor having hollow absorber rods - with gas cooling by central passage and annular outer passage between coaxial shells - Google Patents

Abstract

Gas-cooled, high-temp. reactor of pebble-bed type has downward coolant flow and absorber rods introduced directly into the bed from above. Each absorber rod consists of three concentric tubes the space between the two inner tubes is filled with absorber material, while the annular space between the central and outer tubes is empty; gas blows downwards both through this annular space and down the central passage of the absorber rod. Used for a pebble-bed reactor, where the absorber rods are not guided by any surrounding duct as they enter the core. The core can be operated at a higher temp., e.g. above the previous limit of 750 degrees C, permitting higher output rating.

Description

Gas-cooled high temperature reactor with a

Bulk of spherical fuel elements and directly into the bulk retractable absorber rods The invention relates to a gas-cooled high-temperature reactor with a bed of spherical fuel elements, which from top to bottom of the Cooling gas is flowed through, and with absorber rods that can be inserted directly into the bed, the two coaxially arranged steel ducts and one gas-tight from the two ducts enclosed absorber part exist, with the central Inside the absorber rods a cooling gas flow is passed.

It is known to control or switch off so-called pebble bed reactors with the help of absorber rods, which without a special guide such as pipes, Noses or the like. Be retracted directly into the fuel assembly bed.

As described in DE-OS 12 63 939, such absorber rods exist of an outer steel casing, the neutron absorbing material and a inner support tube, which has the support function and the power transmission during rod movements takes over in the fuel element bed. A is used to cool the absorber rods Partial flow of the cooling gas from the area above the gap zone through the interior the support tube, which exits through openings in the rod tips.

The outer covering of each absorber rod is directly from the through the fuel element bed wets the flowing cooling gas and is thus a higher one Exposed to temperature than the support tube. As a result of the heat production in the absorber rods due to neutron and t capture, the temperature of the outer shell is basically above that of the surrounding cooling gas in the fuel element bed. The use of the known rod construction is therefore used by the temperature limits of the Cladding tube material on nuclear reactors with moderately high core outlet temperatures (approx. 7500C).

When using the known absorber rods in nuclear reactors with higher Core outlet temperatures of the cooling gas (approx. 8500 to 9500C) should be the permissible Rod entry depths are reduced, which results in considerable restrictions the implementation of procedurally simple shutdown concepts would result.

In the case of nuclear reactors with block-shaped fuel assemblies, it is known to cool the outer surface of the absorber rods with the help of the circulating coolant. In these nuclear reactors, the absorber rods can be moved in vertical directions Reactor core penetrating guide tubes arranged, which are permanently installed. The guide tubes are always fluidly parallel to those in the reactor core located cooling channels and are switched in their entire length by the coolant flows through. In DE-AS 11 21 240 and 12 48 823 such absorber rod cooling devices are described.

From DE-OS 18 03 067 is another arrangement for cooling Known control elements, which also moves in fixed guide tubes will. The guide tubes have a hexagonal cross section on, and a bundle of absorber rods is installed in each guide tube, the is surrounded by a casing with a likewise hexagonal cross-section.

The gaseous or vaporous coolant is first in a gap guided upwards between the casing and the guide tube, enters the Bundle of absorber rods and then flows between the absorber rods of the bundle downward.

The prior art also includes the cooling of a control group for a nuclear reactor, one made of fuel elements and one made of absorber elements formed section are arranged one below the other in a housing that is fixed in a is slidable sleeve installed in the crevice zone. The coolant flows through one after the other the section of the fuel and the absorber elements as well as the annular ones Gap between the movable housing and the stationary sleeve, as in DE-PS 24 23 027 shown. To increase the thermal efficiency of the nuclear reactor, becomes part of the coolant flow after passing through the section of the fuel elements derived from the movable housing and the annular gap around this housing fed.

The invention is based on this prior art, wherein it is based on the task of creating a gas-cooled high-temperature reactor of the initially introduced design described so that the thermal load on the absorber rods is considerably reduced to the use of these rods also in nuclear reactors higher Enable performance.

According to the invention the object is achieved in that to each absorber rod an additional cladding tube made of steel is arranged, with between an annular gap is provided for the additional (outer) cladding tube and the middle cladding tube is, and that a cooling gas stream is also passed through this annular gap.

As with the well-known absorber rod for pebble bed reactors, this is Absorber material enclosed in a gastight manner by two ducts, the inner one of which Cladding tube cooled with the aid of the cooling gas flowing through the central interior will. The cladding tube surrounding the absorber part outside - in the following middle cladding tube called - is cooled according to the invention by the cooling gas flow in the annular gap between this cladding tube and the additional cladding tube - the outer cladding tube - after is guided below. Both streams of cooling gas flow through the absorber rod on the ground the driving pressure difference of the downward main cooling gas flow im Reactor core. The heat generated in the absorber part of the rod becomes radial to the two Derived cooling gas flows and causes a slight heating of these gas flows their passage through the staff.

It is advantageous to use part of the for cooling the absorber rods To use cooling gas stream, which is passed through the fuel bed. This partial flow is branched off above the fuel element bed and both through the central interior of the absorber rods as well as through the ring it splits Rods led. The partial flow is therefore taken from an area in which relatively low Another possibility is to supply the gas To supply cooling of the absorber rods from the outside, d. H. to be taken from a gas storage tank.

This gas is also fed into the central interiors and the top Annular gaps of the absorber rods initiated.

The flow of cooling gas flowing through the annular gap also cools that outer Cladding tube, the temperature of which is thus between the gas temperature in the annular gap and the temperature of the cooling gas flowing through the fuel assembly bed. the The temperature of the outer cladding tube is therefore basically lower than that of the cooling gas in the reactor core in the vicinity of the absorber rod. With suitable Parameter selection can thus be sufficiently low material temperatures of the inner and also reach the outer cladding tube. It is therefore possible according to the invention trained absorber rods even in high-temperature reactors with high cooling gas outlet temperatures to be used, for example in nuclear reactors coupled with gas turbine engines. In such nuclear reactor systems with a closed cooling gas circuit (single circuit systems) the gas temperature in the reactor core reaches values of 750 at a depth of 1.50 m up to 8000C, since the cooling gas enters the reactor core at a temperature that is around 2000C higher than the cooling gas inlet temperature in so-called two-circuit systems. The specified values apply in the event that with such a single-circuit system a charging process is used in which the fuel elements after one time Pass through the reactor core have reached the desired final burn-up state (which, due to the resulting power density distribution, leads to a steep Increase in cooling gas temperature in the upper third of the reactor core).

Appropriately, are between the outer and the middle cladding tube each absorber rod arranged radial guide ribs, their size and number in certain limits are variable.

They also serve to improve the heat transfer from outer or middle cladding tube or both cladding tubes to the cooling gas. Depending on the intended Use and mode of operation of the absorber rods has in connection with the width of the annular gap to optimize the size and number of guide ribs, since a change in the geometry has a strong influence on the flow conditions in the annular gap and thus also on the heat transfer conditions.

The support function can be shared by several for each absorber rod of the three cladding tubes or even one cladding tube alone, whatever depending on the requirements and the overriding structural aspects directs. However, the outer cladding tube preferably has no load-bearing function, it only separates the partial flow of cooling gas that is used for cooling the absorber rod serves, from the main cooling gas flow. The cladding tube, which takes on the supporting function, has appropriate dimensions for this purpose.

As already mentioned, the absorber rods designed according to the invention can can also be used in gas turbine systems with a closed gas cycle, the have a high temperature reactor as a heat source. Such a gas turbine plant is known for example from DE-OS 26 39 877. It consists of the nuclear reactor, housed in the liner-lined cavern of a pressure vessel is, at least one turbine with HP and possibly LP compressor and one Number of recuperators, pre-coolers and, if necessary, intercoolers. The mentioned Circuit components are installed in the same pressure vessel as the nuclear reactor. In this system, the liner of the cavern containing the nuclear reactor is supplied with circulating gas low temperature - hereinafter referred to as cold gas - cooled, with the entire gas emerging from the HP compressor before it enters the recuperators is bypassed the surface of the liner. The temperature of this "cold gas" is approx. 110 ° C.

According to an advantageous further development of the invention, at such a system a part of the cold gas through the central interiors and out the annular gaps of the absorber rods.

The cold gas is in a known manner after exiting the HP compressor in a gas line that is coaxial with the bottom side of the nuclear reactor exiting hot gas line runs, guided into the reactor cavern and flows into an annulus between the thermal side shield and the liner to the top. Thereon it gets into one located between the thermal ceiling shield and the liner Space from which the cold gas is fed to the individual absorber rods.

The absorber rods advantageously have their upper part around a number of slots for the cooling gas inlet are distributed around the circumference, and on Further slots for the cooling gas outlet are provided at its lower end. Are the absorber rods are conically pointed at the bottom, so are the slots for the The cooling gas exits from the central interior directly at the tip of the rod.

If cold gas is present in the high-temperature reactor according to the invention about 1100C is available for cooling the absorber rods, so it is advantageous a device for controlling the cold gas inlet as a function of each absorber rod provided by the penetration depth of the absorber rod in the fuel element bed. This measure pursues the purpose, which is due to the deterioration in the degree of recuperator exchange unavoidable (even if only minor) losses in efficiency as low as possible to keep. According to the invention, therefore, cold gas is only used when necessary, i. H. if the absorber rod tips are below a constant depth in the fuel element bed are introduced into the absorber rods. This creates a constant bypass flow of the cold gas avoided in the reactor core.

The control device can be designed as a slide control, in which the absorber rod itself takes over the function of the slide. By means of a appropriately attached seal ensures that the cold gas flow only from one can enter the absorber rod at a certain depth.

Another seal prevents the constant inflow of cold gas outside along the absorber rods in the reactor core The control device can also consist of an adjustable cooling gas throttle. This is expedient within the bounded by the cavern liner and the thermal ceiling shield Space around the respective absorber rod.

In the drawing, a high-temperature reactor is shown as an exemplary embodiment with a closed gas circuit and a gas turbine coupled to the reactor, schematically shown. This is a 1-loop system with intermediate cooling and two hot gas lines; the circuit of the heat-exchanging apparatus is also executed in two strands. The figures show in detail: FIG. 1 a horizontal section along the line I - I of FIG. 2, FIG. 2 shows a vertical section along the line II - II of FIG. 1, FIG. 3 shows an individual absorber rod in vertical section, FIG. 4 a section of another absorber rod, also in vertical section, enlarged Fig. 5 is a horizontal section along the line V - V of Fig. 4, Fig. 6 shows an absorber rod with a control device for the cooling gas inlet, FIG. 7 an absorber rod with another control device for the cooling gas inlet, 8 shows a further variant of this control device.

Figures 1 and 2 show a prestressed concrete pressure vessel 1, which is cylindrical and centrally located inside a (not shown) as well cylindrical containment made of reinforced concrete is arranged. Within the Prestressed concrete pressure vessel 1 are a high temperature reactor 2 and the other components the primary or cooling gas circuit housed. These include a gas turbine set as well as two recuperators, pre-coolers and intercoolers.

The high-temperature reactor 2, which is installed in a cavern 3, is designed as a graphite-moderated, helium-cooled reactor, its fuel elements are spherical.

A hot gas collecting space is located below the bottom of the reactor core 4 for receiving the heated gas emerging from the core. Above the reactor core a hot gas collecting chamber 5 is provided, which flows back from the main circuit Absorbs gas before it is fed back to the reactor core. The reactor core is of a cylindrical thermal shield 6 and a thermal Surrounding the ceiling sign 6 a.

The cavern 3 has a liner 7 for sealing, which is on the reactor side does not have a thermal protection device such as insulation or cooling system. Between the thermal shield 6 and the liner 7 is an annular space 8. Another Room 8 a is between the thermal ceiling shield 6 a and the Liner 7 provided.

Armored pipes extend through the ceiling of the prestressed concrete container 1 30, in which absorber rods 31 are movably arranged, which directly into the bed the spherical fuel assemblies are retracted. Through an addition tube 32 can the fuel elements are entered into the high-temperature reactor 2. The distance the fuel elements from the core takes place through a flue pipe 33. In Figures 3, 4 and 5, the absorber rods 31 are shown in more detail.

By two laterally attached to the bottom of the high-temperature reactor 2 Outlet nozzles 9 and the same number of inlet nozzles 10 attached laterally at the top is the high-temperature reactor 2 with the other components of the main circuit tied together.

A horizontal tunnel is located vertically below the high temperature reactor 2 11 worked in the prestressed concrete pressure vessel 1, in which one in separate housings single-shaft gas turbine 12, a LP compressor 13 and a HP compressor 14 installed are. The compressors sit on a common shaft with the gas turbine. A Generator (not shown) installed in the containment is coupled to the gas turbine 12.

Two vertical gas ducts 15 extend laterally next to of the gas turbine 12 up to the level of the bottom of the reactor core, and in each A hot gas line 16 is installed in this gas duct. Any hot gas line 16 is with one of the reactor outlet connection 9 and with a turbine inlet nozzle tied together. In the upper part of the prestressed concrete pressure vessel 1 there are two more vertical gas guiding lugs 17, each with one of the gas guiding lugs 15 cursing. In each one Waringasleitung 18 is arranged, and each hot gas line 18 is connected to one of the two reactor inlet nozzles 10.

There are six vertical pods on a pitch circle around the reactor cavern 3 19 are provided, which are closed with rupture-proof lids 20. The pods 19 serve to accommodate the heat-exchanging apparatus, in a symmetrical arrangement to the turbine tunnel 11 two recuperators 21, two precoolers 22 and two intercoolers 23 are housed. All heat exchanging apparatus are in the same amount as the reactor cavern 3 installed. In addition to the pods 19, there are three more vertical ones Pods 29 are available, which are used to accommodate a residual heat removal system.

At the level of the turbine tunnel are in the prestressed concrete pressure vessel 1 several horizontal gas ducts are provided, which the heat exchanging apparatus of a Strand with each other or

connect to the gas turbine set. The gas line between the recuperator and precooler of each strand serves a gas guide 24 while the connection between the two recuperators 21 and the turbine outlet nozzle each by one Gas guide 25 is made. From the pre-coolers 22 to the two inlet nozzles of the LP compressor 13, the gas is passed through a gas duct 26, and between the LP compressor outputs and the two intercoolers 23 is in each case a gas duct 27 is provided. There are still on a slightly deeper level two gas ducts 28, which the two intercoolers 23 with the Connect the inputs of the HP compressor.

The gas is transferred from the HP compressor 14 to the two recuperators 21 on a large part of its flow path through the vertical gas guide tunnels 15 and 17, with -es on the outside of the hot gas lines 16 and the hot gas lines 18 flows along, which are designed as coaxial gas guides.

On its way from the gas duct 15 to the gas duct 17 the gas, which has a temperature of approx. 1100C, is coaxial with the reactor outlet nozzle 9 in die'Reaktorkaverne 3 and occurs here in the annular space 8 between the thermal shield 6 and the liner 7 a. In this annulus it flows upwards in the space 8 a, where there is the liner 7 in the area of the annular space 8 and the space 8 a cools.

According to the invention, a part of the cold gas is in the absorber rods 31 initiated, which it from top to bottom in a central interior 40 and a Flows through annular gap 41, as shown in FIG. 3. The cold gas then steps on it at the lower end of the absorber rods 31 and mixes with the main cooling gas flow, which flows through the bulk of the spherical fuel elements.

Fig. 3 shows a single absorber rod 31, which is cylindrical is formed and has a frustoconical tip 34. This has one of the Recess 35 adapted to the shape of the fuel assemblies. The absorber rod 31 consists of three steel ducts arranged coaxially to one another, the inner one of which Cladding tube 36 and the middle cladding tube 37 enclose an absorber part 39 (B4C) in a gas-tight manner.

The inner jacket tube 36 represents the support tube that the Carrying function and takes over the power transmission during rod movements. It has a corresponding Wall thickness. A free space 40 is provided in its interior. Between the middle one Cladding tube 37 and the outer cladding tube 38 is an annular gap 41. On the middle Cooling fins 42 are attached to the cladding tube 37 and extend into the annular gap 41.

In the upper part of the absorber rod 31 are several slots for the Entry of the cold cooling gas present (not shown here.) The tip of the rod 34 has outlet openings 43 which are in communication with the interior 40. Further slots 44 for the cold gas outlet are in the lower end of the outer one Cladding tube 38 is provided.

Part of the cold gas flowing through the space 8 a (see FIG. 2) is branched off and introduced into each of the absorber rods 31. In Fig. 3 is the flow path of the cold gas through the absorber rod 31 is indicated by arrows. The cold gas flowing through the interior 40 cools the inner cladding tube 36, while the middle cladding tube 37 and the outer cladding tube 38 from the one through the annular gap 41 flowing cold gas are cooled.

Figures 4 and 5 show sections of a variant of the absorber rod 31. This absorber rod also has three cladding tubes arranged coaxially to one another on; however, the support function is assumed here by the outer cladding tube 47, which a greater wall thickness compared to the cladding tube of the absorber rod just described owns. The inner cladding tube 45 and the middle cladding tube 46 surround the gas-tight absorber part 48 consisting of B4C. The one penetrating into the absorber rod at the top Cold gas Here, too, flows through a free interior space 49 and one between the middle and the middle Annular gap 50 located in the outer cladding tube. In the annular gap 50 there are guide ribs 51 arranged, which keeps the distance between the two cladding tubes 46 and 47 constant hold and improve heat transfer.

The heat generated in the absorber part 48 is generated by the two cooling gas flows radially inwards and outwards, whereby the cooling gas flows slightly Warm up. The temperature profile through the absorber rod wall is shown schematically in FIG. 4 shown. As can be seen from this, the temperature of the outer cladding tube is 47 between that of the cooling gas in the reactor core and that of the cooling gas in the annular gap 50; d. H. it basically takes a value lower than is the core temperature in the vicinity of the absorber rod 31.

Figures 6, 7 and 8 show three design variants of a control device 52 for regulating the entry of salt gas into an absorber rod 31 as a function of its depth into the bed 53 of the spherical fuel elements 54. The Absorber rod 31 is movably arranged in the armored tube 30, which is in the ceiling of the Prestressed concrete pressure vessel 1 is installed, as described in FIGS. 1 and 2.

The cold gas is in the space 8 a between the liner 7 and the thermal Ceiling sign 6 a. Between the thermal ceiling shield 6 a and the ceiling reflector 55 is the hot gas collecting space 5, from which cooling gas from 4680C does not pass The openings shown in the ceiling reflector 55 enter the fuel element fill 53. The absorber rod 31 has in its upper part to its circumference distributes a series of entry slots 56 for the cold gas. The exit slots 43, 44 are provided in the rod tip 34. The absorber rod 31 is by means of a Drive 57 moved, which is arranged within a drive cylinder 58. Of the Drive 57 works pneumatically.

In the absorber rod 31 shown in FIGS. 6 and 7, the control device is 52- designed as a slide control, in which the rod itself does the function of the slide takes over. On an extension of the drive cylinder 58 is a Seal 59 is provided, which causes the cold gas only from a certain depth of the rod into the fuel element bed 53 into the inlet slots 56 of the absorber rod 31 can flow in. The position of the seal 59 is determined by the entry slots 56 determined that at the highest rod position not within the drive cylinder 58 may lie.

Within the space 8 a, from which the cold gas is derived, is a cylinder 60 is arranged around the absorber rod 31, with slots 61 for the Cold gas passage is provided. A constant bypass of the cold gas in the fuel element bed 53 is to block off the space enclosed by the cylinder 60 to the bed 53 and to the hot gas collecting space 5 required.

In the embodiment shown in FIG. 6, this is the case Purpose once a seal 62 is provided within the ceiling reflector 55, and on the other hand, there is another one around the absorber rod in the hot gas collecting space 31 arranged Cylinder 63.

7 shows another solution for preventing a cold gas bypass into the fuel element bed 53. Here the drive cylinder has an extension 64, which is continued into the ceiling reflector 55, where it is tight at the breakthrough for the absorber rod located in the thermal side plate 6 a 31 is present. A seal 65 is arranged on the inside at the lower end of the extension 64.

In the absorber rod 31 shown in FIG. 8, there is the control device 52 from an adjustable cooling gas throttle 66, which is located within the absorber rod 31 surrounding cylinder 60 is installed. Their actuation takes place in dependence from the penetration depth of the absorber rod 31. So only then occurs cold gas in the by the cylinder 60 enclosed space when the absorber rod tip 34 a has reached the specified depth. A seal towards the fuel element fill 53 is therefore not required here. The hot gas plenum 5 is through a to the Absorber rod 31 arranged cylinder 63 blocked against the cold gas. In Fig. 8, the cooling gas throttle 66 is open on the left side of the absorber rod 31 State shown; to the right of the absorber rod 31 is the closed cooling gas throttle 66 shown.

Claims (11)

Claims ============================== Gas-cooled high temperature reactor with a bed of spherical fuel elements, which from top to bottom of the Cooling gas is flowed through, and with absorber rods that can be inserted directly into the bed, the two coaxially arranged steel ducts and one gas-tight from the two ducts enclosed absorber part exist, with the central Inside the absorber rods a flow of cooling gas is passed, characterized in that that an additional cladding tube (38, 47) made of steel is arranged around each absorber rod (31) is, between the additional (outer) cladding tube (38, 47) and the middle Cladding tube (37, 46) an annular gap (41, 50) is provided, and that through this annular gap (41, 50) a stream of cooling gas is also passed. 2. Gas-cooled high-temperature reactor according to claim 1, characterized in that that part of the stream of cooling gas to be introduced into the fuel element bed (53) branched off above the bed (53) and both through the central interior (40, 49) of the absorber rods (31) and through the annular gap (41, 50) of these rods to be led. 3. Gas-cooled high-temperature reactor according to claim 1, characterized in that that the passed through the central interior and the annular gap of the absorber rods Cooling gas flow is taken from a gas storage tank. 4. Gas-cooled high-temperature reactor according to claim 1, 2 or 3 thereby characterized in that between the outer (47) and the middle cladding tube (46) radial Guide ribs (51) are arranged. 5. Gas-cooled high temperature reactor according to claim 1, 2 or 3, characterized characterized in that several of the three casing tubes of each absorber rod (31) together take over the support function for this absorber rod. 6. Gas-cooled high-temperature reactor according to claim 1, 2 or .3, characterized in that one of the three cladding tubes (36, 47) of each absorber rod (31) alone takes on the support function for this absorber rod. 7. Gas-cooled high temperature reactor according to claim 2, which is in a housed with a liner lined pressure vessel cavern and with at least is coupled to a gas turbine set installed in the same pressure vessel, wherein the liner of the pressure vessel cavern with low-temperature cycle gas (cold gas) is cooled, characterized in that part of the cold gas through the central Interiors (40, 49) and the annular gaps (41, 50) of the absorber rods (31) will. 8. Gas-cooled high temperature reactor according to claim 1, 2 or 7, characterized characterized in that the absorber rods (31) are distributed around the circumference in their upper part have a number of slots (56) for the cooling gas inlet and that at the lower At the end of the absorber rods (31) further slots (43, 44) are provided for the cooling gas outlet are. 9. Gas-cooled high-temperature reactor according to claim 7, characterized in that that on each absorber rod (31) a device (52) for controlling the cold gas inlet depending on the penetration depth of the absorber rod (31) into the fuel element bed (53) is provided. 10. Gas-cooled high-temperature reactor according to claim 9, characterized in that that the control device (52) is designed as a slide control in which the Absorber rod (31) takes over the function of the slide. 11. Gas-cooled high-temperature reactor according to claim 9, characterized in that that the control device (52) consists of an adjustable throttle (66).