NO148745B - ANALOGY PROCEDURE FOR THE PREPARATION OF PHARMACOLOGICALLY ACTIVE DIBENZO (B, D) PYRANDER DERIVATIVES - Google Patents

Description

Nuclear reactor.

The present invention relates to core

reactors with heterogeneous core devices

net fuel which is fissionable by reaction with moderated neutrons and which is calculated

net on being cooled by a fluid which also

acts as a moderator for the core.

Many reactors are known

water-cooled moderator, and these are often referred to as pressurized water reactors and boiling water reactors. In such reactors, water flows

naturally the pressurized water or the boiling water up through the core in one path, but it is also known to pass pressurized water so that it acts as a coolant and moderator, first down through an outer core zone and then

after up through the central residual

the area of the nucleus.

Such reactors as well as nuclear reactors

Tors are generally controlled by neutron-absorbing control rods. However, since the use of control rods entails a number of disadvantages, other control methods for reactors have been tried. An alternative me-

method for controlling a reactor is proposed in German specification no. 1 020 127, where the nuclear reactor is controlled by varying the concentration of gaseous fluid in the core when the gaseous fluid contains

holds a limited fraction of the hydrogen atoms necessary for the desired

bored moderation.

According to the layout document, floating

the gaseous fluid through the core in special passages which are part of the flow system and are separated from the normal cooling system for the fuel, and they are also

separated from the core's normal moderator-an-

arrangement.

f

In the present invention, one has

went a different way, and the invention is known

characterized by the fact that the reactor is designed so that the gaseous fluid in a first path in the core is carried as a cooling fluid over a number of fuel elements, in a second path

between the fuel elements isolated from and without direct heat transfer with them,

and in a third path fuel elements are fed as cooling fluid for the remaining part.

These measures, according to up-

the invention, leads at the same time to a number of

parts. Thus, the gaseous fluid in contact with the fuel elements will change its density immediately upon a change in the energy output from the fuel elements,

whereby unwanted changes in the heat release rates are counteracted, because the resulting change in mode

the density of the radiator or coolant affects

ker the reactivity of the reactor. The counter-effect of desired changes in the rate of heat release from the fuel elements increases steadily over a period, while the gas

formed fluid from the first path replaces the gaseous fluid in the second path.

As the second moderating path receives gaseous fluid that has been heated by only some of the fuel elements,

the gaseous fluid in the second moderating path will be at a lower temperature

temperature than the temperature at which <!>the gaseous fluid leaves the core.

For this reason, the gaseous fluid will

um have greater density and the core can be smaller than if the gaseous fluid had cooled all fuel elements before flow-

as it entered the moderating lane. In the

other moderating path, the gaseous fluid is not particularly heated, and in this path, for this reason, it tends to produce a uniform moderating effect over the entire length of the core. The gaseous fluid in the second moderating path flows between the fuel elements which may lie in grooved tubes which relatively closely surround the fuel elements, in order to obtain a suitably high flow rate for the gaseous fluid for cooling the fuel elements.

In contrast to a number of known proposals where the cooling fluid in the first path flows through a central zone or an outer zone of a reactor core, and in its second path flows through the outer or the central zone of the core, the gaseous fluid in a reactor carried out according to the invention, first flow through lined pipes or sleeve pipes which are uniformly distributed and lead into the second path, while the third path may comprise sleeve pipes which are also evenly distributed and penetrate into the second path.

In the following, the invention is described with reference to the drawings, where: Fig. 1 schematically shows a nuclear reactor with a moderator cooling device. Fig. 2 is a section through a pressure vessel with a reactor core in the central middle part. Fig. 3 is a section along the line III-III in fig. 2 and shows the mutual location of the fuel elements' tube plates. Fig. 4 is a ground plan on a larger scale of the plates in fig. 3 and shows the heads of the fuel elements. Fig. 5 shows on a larger scale some of the parts in fig. 1 in more detail. Fig. 6 is a cross-section along the line VI-VI in fig. 5 and shows fuel elements and tube plates with spacers. Fig. 7 is a longitudinal section on a larger scale of a small piece of fuel element inside a sleeve tube for a third path to gaseous fluid. Fig. 8 is also a longitudinal section on a larger scale of a small piece of fuel element inside a sleeve tube for a first path to gaseous fluid. Fig. 9 is a further longitudinal section on a larger scale of a small piece of fuel element in a third track sleeve tube and shows a modified design. Fig. 10, 11 and 12 are graphical curves to which reference is made in the following to facilitate understanding of the description. Fig. 13 corresponds to fig. 1, but is equipped with additional pipelines for gaseous

fluid to further facilitate the control of the device or system.

As shown in fig. 1 is a heterogeneous nuclear reactor 20 placed in a pressure vessel 22 and so arranged that it can be moderated and cooled by means of a gaseous fluid which is driven by a pump 40 through a circuit comprising heat exchangers located outside the pressure vessel and arranged to remove it from the gaseous fluid heat the fluid gets when cooling the reactor. The fluid is caused to flow successively through the reactor core in a first channel 24, a second channel 26 and a third channel 28.

The reactor core comprises an ordered group of fuel elements which —• taking into account the device's geometry,

neutron moderation and other necessary

factors — provide fission material in sufficient quantity to ensure continuity

of the fission-type chain reaction necessary for reactor operation. The arrangement of the gaseous fluid flow in the reactor is such that the gas flow in its first path

through the core cools part of the fuel elements and in its third path through the core cools the remaining elements, while in its second path it passes through the core without touching any of the fuel elements. The gaseous fluid naturally acts in all directions as a mod-

rator.

In the second path, the gas flows mostly upwards into the core space which is occupied with the group of vertical fuel elements. It is supplied to the reactor through an inlet line 30 and passes in the first path through the core downwards in parallel streams in sleeve tubes around fuel elements which only comprise a part of the total number of fuel elements. The casing tubes separate the gaseous fluid in the first path from the gaseous fluid in the second path, but the gases leave the casing tubes at their lower ends to enter the lower end of the second path. From the upper end of the second path, the gas enters the upper ends of the sleeve tubes around the remaining fuel elements and flows in the third path downwards in parallel flows through the sleeve tubes which separate the gas in the third path from the gas in the second path. The gas then leaves the reactor through the outlet line 32.

The heated gas fluid then flows through heat exchangers 34 and 36 and emits indirect heat to a secondary heat transfer fluid which is led to a point of consumption not shown. The heat exchangers can work in series or, as shown, in parallel. One heat exchanger, 34, can serve as a steam superheater and the other, 36, as a re-superheater or refresher for steam. When the gaseous moderator-coolant fluid leaves the heat exchangers 34 and 36, it flows through a line 38 to the inlet of a pump 40 which, through a line 42, delivers the fluid to a third heat exchanger 46 which can serve to develop the steam that is heated in the heat exchangers 34 and 36. After leaving the heat exchanger 46, the fluid flows back to the reactor through the inlet line 30.

An opening 48 is arranged in the pump's outlet line 42 with such a flow-through area that an insignificant pressure difference occurs between its two sides. Before and after this opening 48 form two lines 50 resp. 52 connections with a chamber 54 • and through these lines the fluid flows at a reduced speed parallel to the flow through the opening 48.

The reactor plant is equipped with a line 56 for supplying the gaseous moderator coolant fluid. It is provided with a suitable regulator or measuring device 58 and is connected with the line 38 to the pump 40 inlet. There is also arranged an exhaust line 60 provided with a regulating device 62 which is in connection with the outlet line 32. The regulators 58 and 62 can be of any known type suitable for the purpose and can include known regulator valves or pumps and the primary requirement is that they must be capable of introducing the precise predetermined amounts of gaseous moderator-refrigerant sub-fluid into or out of the fluid circuit necessary to effect a precisely controlled variation of the total amount or content of gaseous fluid circulating in the circuit. A safety valve 63 is also provided which enables gaseous fluid to flow out of the circuit through line 60 without passing through the regulator 62. The safety valve can be either automatically or manually operated and serves to release fluid out of the circuit and into a container with lower pressure, not shown, so that the pressure in the circuit can be reduced quickly.

The gaseous moderator-refrigerant sub-fluid may be any of a plurality of fluids which provide neutron moderation by means of hydrogen atoms, for example gaseous hydrocarbons, pure hydrogen gas, water vapor or a mixture of any of these gases with inert gases. For reasons discussed later, water vapor at supercritical pressure is the gaseous moderator refrigerant sub-fluid in question here.

It is known that in a nuclear reactor, when the material undergoes fission, neutrons in different energy ranges will be present in the core. The proportions of the total neutrons with energy in different areas can vary with the average energies in these areas according to those in fig. 10 showed lines where the abscissa indicates neutron energies that decrease from left to right. Neutrons produced by fission of an atom have a high energy level and as they travel inside the nucleus they are progressively moderated or slowed down by the various materials present in the nucleus. When they are slowed down, they pass through an area of resonance reaction energy, where fertile material in the core can capture neutrons and in this way produce additional fission material of fertile material. At lower neutron energies, i.e. after the resonance range, lies the thermal energy range which contains the neutrons responsible for the largest part of fissions. As you will understand, a part of the neutrons is lost through unproductive absorption in the core materials, through the possible formation of toxic substances and through leakage. It has been shown that the ratio between the amount of neutrons in the area for resonance reactions and in the thermal reaction area can be changed by varying the moderation, i.e. the rate of deceleration, to which the neutrons are subjected. If the neutrons are thus subjected to a relatively strong moderation, the relative amount of neutrons within the different energy ranges will roughly correspond to what is shown with the dashed line 65 in fig. 10, from which it appears that the amount of neutrons in the thermal energy range is relatively high, while it is relatively low in the region of resonance reaction. If the moderation, on the other hand, were to be relatively low, the amount of neutrons in the different energy ranges would approximately correspond to what is shown by the solid line 64, which shows fewer neutrons in the thermal energy range than in the above-mentioned case and more neutrons in the region of resonance reaction than in the aforementioned case.

During a reactor's start-up period, the amount of fuel in the core is normally relatively high, while the amount of toxic substances in the core is relatively low. Due to the high utilization of thermal neutrons for fission and the low absorption of thermal neutrons by poisons, the nuclear reaction can be maintained with a smaller relative amount of thermal neutrons than in the following, i.e. the neutron moderation can be less and if this happens the transformation of fertile material to fissionable material favored. But after extended periods of operation, the amount of fuel in the core will have decreased considerably with a corresponding increase in fission, which produces a content of neutron poisons in the core. In that case, more neutron moderation is required because a greater relative amount of thermal neutrons is required to sustain the fission reaction. It follows from this that the relative number of neutrons available in the area for resonance reactions, to react with the fertile material, is reduced.

It thus turns out that the core can be kept reactive during its lifetime by adjusting the degree of moderation to the conditions, without using the hitherto usual control rods that absorb neutrons unproductively and, on the other hand, with an extensive transformation of fertile material into fissionable material , which represents a productive use of neutrons. In order to be able to change the output at any time, the reactor can thus be controlled by suitably varying the degree of moderation.

The one in fig. 1 shown and previously described reactor can be operated and its performance regulated by proper control of the gaseous fluid in the circuit. It may turn out that with certain hydrogen-carrying fluids, for example pure hydrogen, the pressure required, in a reactor of reasonable dimensions, to achieve the necessary density in the core to provide moderation to sustain a nuclear reaction, will exceed the constructive limits . Furthermore, the use of pure hydrogen will not provide the heat-transferring properties that enable the construction of a power-producing reactor with the required degree of efficiency. However, it has been shown that steam at high pressure and high temperature meets the stated requirements. Steam with a temperature of approx. 370°C and a pressure of approx. 240 kg/cms will ensure a usable high concentration of hydrogen atoms inside the reactor core, while at the same time it has the required properties for heat transfer and flow. By varying the density of the steam in the reactor core, for example by changing its temperature or its pressure or by diluting the steam with a non-moderating gaseous fluid, the neutron moderation in the core and thus the reactor's reactivity can be varied under control.

The safety valve 63 ensures that the reactor is stopped when the conditions require it and otherwise when required, for example for safety in an emergency. As a reactor will not work without sufficient moderation to enable the continuity of the chain reaction, the reactor will quickly become subcritical and stop by itself if the concentration of the moderator inside the core can suddenly drop. Such a lowering of the concentration can be achieved by opening the safety valve 63, whereby the moderator-coolant fluid flows out of the reactor circuit to a lower-pressure container, not shown. As the reactor works with a gaseous fluid, the density of this in the core will be greatly reduced, but if the moderator coolant is a liquid, e.g. pressurized water, the pressure change obtained by opening a safety valve will not be sufficient to provide effective regulation, as water in its liquid state is almost incompressible. Such regulation is also not possible in a reactor with boiling water, as the pressure change caused by blowing off will not lower the density of the entire fluid, but only instantly vaporize a fraction of it, while liquid remains in the core which causes neutron moderation so the nuclear reaction continues.

When operation is to begin, the reactor plant is first brought to an equilibrium operating temperature by circulating a small amount of gaseous moderator-coolant fluid through the plant while heat is continuously added from an external heat source not shown. When the reactor plant has been brought. up to operating temperature for start, the extra fluid is introduced into the circuit through the inlet's regulating device 58 and the line 58, whereby the density of the moderator-coolant fluid increases until the reactor's critical state is reached and a self-sustaining chain reaction of the fission type is started. By appropriately regulating the amount of fluid in the circuit, by necessary use of the regulating devices for both inlet and outlet, the reactor's power is then increased to the required level and the external heat source is switched off.

The heat exchangers 34, 36 and 46 in the plant have meanwhile begun to function and the primary or moderator-refrigerant sub-fluid emits heat generated in the reactor to a secondary fluid for conversion into useful power, e.g. mechanical energy or otherwise.

As can be seen from fig. 11 will for a reactor — which requires a density (o) of moderator coolant vapor of approx. 160 kg/m"> and which works within a temperature range between saturation temperature and 405°C

— the pressure in the moderator refrigerant (P) lies within a pressure range, AP, which extends from approx. 200 kg/cm2 abs. to 260 kg/cm2

abs. on the condition that the enthalpy (h) is kept within a corresponding range, Ah, from approx. 585,000 to 625,000 cal/kg. These conditions are generally denoted by W. As the reactor ages, the moderation necessary to maintain the chain reaction increases and the steam mass in the circuit is increased to the necessary extent, so that the steam density at the end of the core's life will be approximately 320 kg/cms. At this point the steam, while being kept within the same temperature range, will operate over a pressure range AP', between 225 to 310 kg/cms abs. with a corresponding enthalpy range of 500,000 to 533,375 cal/kg as indicated by Y. The values mentioned above serve only as examples, it being noted that the required vapor density will depend on the particular construction of the reactor in question . As far as the temperatures are concerned, there will be an upper temperature limit based on the temperature limits for the different parts of the facility. The lower temperature limit is specified as the saturation temperature, because the moderator refrigerant must be a gas and not a liquid.

It may be appropriate to keep the steam inside the reactor in the supercritical temperature range, because the steam when it works with approx. 245 kg/cm2 abs. in the temperature range between 305 and 565°C can absorb and transfer to the heat exchangers approx. 389,200 cal/kg, in contrast to steam's transfer capacity of 222,400 cal/kg, when in the same temperature range it works with only 200 kg/cm2. As a result of the higher heat transfer capacity per kg of steam, the plant's various parts can be made smaller for the same energy output.

When working with a gaseous fluid that has a large heat transfer and heat transfer capacity throughout the reactor's lifetime, it may be necessary to add a compatible gaseous fluid with little or no moderating effect to the steam in order to obtain a satisfactory lifetime of the fuel elements. Such a diluent can be any gas which is compatible with the water vapor and which produces the required thermodynamic effects, e.g. steam of heavy water, D2o, which has a moderator effect considerably less than ordinary water. By diluting the steam, a satisfactory lifetime of the fuel elements can be achieved by varying the steam content, even if the pressure in the plant is kept between 250 and 275 kg/cms abs. For comparison, it should be noted that if the density of the steam were to be varied throughout the entire lifetime of the core by just

vary its pressure and temperature and not

dilution, it would be necessary to

use a larger pressure range, e.g. between 200 and 275 kg/cm2 abs., in order to achieve a satisfactory lifetime of the fuel elements and the project had to take into account the aforementioned lower thermal conductivity at the lower pressure of the circulating gaseous fluid. If the heat transfer capacity of the gaseous fluid is high, it will easily be possible to keep the construction material in the fuel elements in the first path at a temperature low enough to allow the use of material with a smaller neutron-absorbing cross-section than in stainless steel which otherwise would be required.

The reactor's moderator includes all gaseous fluids in the core. Since the core space between the fuel elements, in which the moderator must be present in an appropriate amount, is not coated by the gas in a single path or channel, but where the space is divided so that the gas flows through it in three successive paths, a certain amount of flowing gas through the core requires higher average current speeds through the current path than would be the case through a single channel. Suitable higher speeds of the flow of the gaseous fluid through the sleeves which surround the fuel elements to cool them can easily be achieved by reshaping the flow cross-section of the paths.

It should be noted that the volume of the gas in the first and third path will be small in relation to its volume in the second path, for example the ratio between the volumes in the first, second and third path may be 10.6 : 77.0 : 12.4. When planning, the gas conditions that will prevail in the second path must therefore first be determined and then the additional moderation effects of the gas with the higher average density in the first path and with the lower average density in the third path must be calculated.

If during operation — with all other conditions constant — an increase in heat release should occur in the reactor, the temperatures of the gas touching the fuel elements will immediately rise and the average temperature of the gas in the other path will begin to rise. Even if the gas circuit has a constant volume, these temperature rises will temporarily reduce the average gas density in the reactor and since lower gas densities in this correspond to reduced reactivity, the reactor will temporarily become subcritical so that heat release will begin to fall. If a decrease in the heat release in the reactor occurs in a similar way, the reactor will temporarily become supercritical and the heat release will begin to rise. The plant thus strives to remain easily controllable, as the control, as explained, is exercised by varying the content or the total amount of gas fluid circulating in the circuit, depending on the circumstances. The chamber 54 increases the plant's volume outside the reactor and therefore enables temperature rises and falls to have a greater influence on the average density of the gas fluid in the reactor.

If the need for the performance of the plant rises during operation, then the temperature of the gas fluid, which flows into the reactor through the inlet line 30, will drop and even though the gas circuit has a constant volume, the density of the gas will increase and thus the reactivity of the core will rise, which is the first effect that is required for the reactor's energy output to be able to rise to a certain new constant value. If the need for performance similarly decreases, the reactivity of the core will decrease, which is the first effect required for the reactor's performance to fall to a certain new, constant value. The plant is therefore adapted to rapid successive changes in the load. Control to restore the core's critical state and then maintain it at the new energy level is accomplished by varying, as required, the total amount of gas fluid circulating in the circuit. The chamber 54 thus increases the plant's volume outside the heat exchangers and therefore reduces pressure rises or falls in the reactor caused by sudden rises or falls in the load.

If the paths of the gas fluid were arranged in such a way that the fluid cooled all fuel elements before it flowed into the moderation path 26 in which it does not touch fuel elements, then the neutron moderation that takes place in this path 26 and must be considered as the largest part of the total moderation, would have take place using a gas fluid that has the lowest density in the circuit. This will be insufficient or even completely impossible. The main moderation by means of low-density steam, which would then prevail in the moderation path 26, will presumably be impossible in a reactor of practical size. If, on the other hand, the paths were so arranged that the gas fluid flowed through the moderating path 26 before cooling any of the fuel elements, then the change in reactivity due to a change in the core's heat output would be limited to that resulting from changes in density of the gas fluid that has immediate contact with the fuel elements and this would generally be a serious limitation. Due to what has been discussed above, in the reactor according to the invention some, e.g. a quarter, of the fuel elements cooled in the first path 24 before the gas enters the second or moderation path 26, while the cooling of the rest of the elements takes place in the third path 28 after the gas leaves the path 26.

Although the chamber 54 increases the content of gas fluid in the plant with the above advantages, it has no effect on constant conditions, in which the gas fluid in the chamber has the same temperature as in the line 42. The use of the opening 48 in parallel with the chamber 54 ensures that at a change in the temperature of the gas fluid leaving the pump 40 then the temperature of the gas fluid flowing into the heat exchanger 46 will change in the same direction without being completely delayed until the gas fluid with the new temperature reaches the outlet from the said chamber after having flowed through the entire chamber.

In that in fig. 1, the pump 40 for the moderator-coolant in the direction of flow is located between the parallel connected heat exchangers 34, 36 and the heat exchanger 46 and not between this heat exchanger and the reactor inlet line 30, as with the latter device there could be a risk of condensation of the steam under large transient state changes. If, however, sufficiently high pressures are used, the pump can be safely placed between the last heat exchanger 46 and the reactor inlet 30, with a consequent material saving in the plant's pumping power requirement as a result of the increased density at this location in the moderator coolant.

After a modification, at least part of the moderator coolant that is heated in the reactor can be used to operate devices, e.g. a turbine for the direct production of mechanical energy.

Fig. 2 to 8 show a preferred embodiment of a reactor for the flow of gas fluid in three paths. The pressure vessel 22, in which the reactor is placed, comprises a standing elongated cylindrical jacket 66, the lower end of which is closed with a hemispherical bottom cover 68 with the flange 70. At the upper end, the jacket 66 has a

flange 72. A top cap 74 has a hemispherical part and a flange 78 and serves to close the upper end of the pressure vessel. The flange 78 corresponds to the flange 72 and is attached to this by means of a plurality of stud screws 80. An inlet connection 82 is formed in the upper part of the cylinder head and is arranged to be connected to the inlet line 30 (Fig. 1) for gas

the fluid circuit. The connection between the cylinder head 74 and the flange 72 is provided with a pressure-tight seal 84, for example of the well-known half-toroidal type. The mantle can be equipped on the outside with a plurality of peripheral reinforcing bands 86 so that it can withstand a high internal pressure.

The inside of the mantle 66, with the exception of the flange part and the inside of the bottom lid 68, which is known per se, is provided with a coating of thermally shielding material, e.g. ZrH2 and steel or other shield material. Additional thermal shielding material is placed at each end of the mantle and will be explained in more detail later. A layer of neutron moderating material 90, for example of graphite, of roughly the same length as the mantle 66 rests against the inside of the thermal shield material 88 and is supported by an annular support flange 92 attached to the inside of the pressure vessel.

As shown in fig. 2, the pressure chamber is divided into three vertically overlapping sections. respectively an upper inlet chamber 94, a central core chamber 95 and a lower one

leaching chamber 96. which chamber is separated by an upper and a lower tube plate 97 resn. 98. The intake chamber 94 is formed in

the upper part of the pressure vessel by the heel of the upper tube plate 97 which is supported by a skirt, the upper end of which has a projecting flange 109 fitted in a peripheral groove 104 in the casing flange 72 at the connection between this and the tono lid 74. A semi-toroidal sealing machine 106 is placed between the skid plate 102 and the casing flange 72 to prevent leakage between these flanges. As shown in fig. 5, there is arranged a secondary pipe connection HO, which at the heel of bolts 108 with Rvptandspieces 109 is carried by resp. is separated from the upper tube sheet 97. A plurality of tubes 112 with internal spacing extends around the two upper tube sheets 97 and 110 and through the central core chamber 95. The upper ends of the tubes 112 are fixed in the upper tube sheet 97 and have elidenasnine in the secondary upper tube 110 and their lower ends open into the lower portion of the core chamber. Another plurality of tubes 114 with internal support ends above between the upper tubes 97 ø 110 ø has a sliding groove through the secondary upper tube ø ø and extends see through the core chamber 95 ø and further through the lower tube ø

98. to which their lower ends are attached. IN

instead of the pipes 114 being movable in relation to the secondary upper pipe plate 110, their upper ends can be attached to this plate which must then be supported by the bolts 108 without the spacers 109 so that the differential

movement between the two plates must be possible.

The lower pipe plate 98 forms the top of a pressure-tight outlet chamber 118 to whose bottom plate 116 is attached a plurality of pipes 120 leading to an internal outlet spout 122. This runs centrally through the pressure vessel's outlet spout 124. This is provided at the top with a flange 126 which rests tightly against the inner side of the lower container flange 70. The lower end of the outlet spout 124 is aligned

to be connected to the outlet line 32 (fig.

1) for the gas fluid circuit. A split stop ring

128 rests against the end surface of the outlet flange 70 to which it is attached by means of screws not shown and it engages in a groove 130 around the outside of the spout 124. A half-toroidal sealing ring 131 is placed between the spout 124 and the flange 70 to ensure a fluid-tight connection.

As shown in fig. 2, a layer of neutron moderating material 132 is placed on the underside of the secondary tube plate 110 and a similar layer 134 is placed on the upper side of the lower tube plate 98. Both of these layers are omitted in fig. 5. These reflective layers in connection with the wall's reflective layer 90 form the core chamber 95 enclosed by reflector material. Thermal shielding material is also placed at the upper and lower ends of the core chamber 95 to protect the ends of the pressure vessel against excessive heat effects. The upper shield material is placed in the inlet chamber 94 and comprises an annular part placed a little above the upper tube plate 97 and a circular part which in turn is placed a little above the annular part so that a flow area appears for the gas fluid from the reactor inlet to the upper pipe plate. The lower screen material 138 is situated in the space between the underside of the outlet box 118 and the inner end of the outlet spout 122. A condensate chamber 140 is mounted in the inlet chamber 94 by means of a flange 141 on the upper side of the skirt flange 102. The condensate chamber has a largely annular shape and lies coaxially in the pressure vessel. It is open at the top towards the inlet chamber 94 and closed at the bottom and its central part forms a channel for the gaseous moderator coolant from the inlet spigot 82 to the upper tube plate 97. A double-conical guide member 142 is located coaxially to the condensate chamber and the inlet spigot 82 and directs the inflowing moderator refrigerant so that it does not flow directly from the stub 82

to the condensate chamber's central channel, but must first pass over the condensate chamber's open top. This chamber serves to collect liquid that may be underneath

special transient conditions may be present in the moderator refrigerant. The guide member 142 and the condensate chamber 140 also serve to cause any temperature changes in the inflowing gas fluid to become more uniform. Devices not shown can be arranged to pass cooling fluid completely through the neutron-reflecting material in order to keep these materials at the required temperature during operation.

The various parts of the reactor are assembled in the pressure vessel as follows: Before the tubes 112 are placed in the reactor, they are attached to the tube plate 97, while the tubes 114 are attached to the tube plate 98 and the secondary tube plate 110 is placed in the appropriate position. The lower pipe plate 98, the outlet box 118 and the associated outlet spout 122 are conveniently supported by the upper pipe plate 97 by means of carrier bolts not shown passing through the pipes. During the placement of the pipes 112 and 114 in relation to the pipe plates 97 and 98, a plurality (described later) spacers 162 are placed, whereof the upper and lower neutron reflector 132 resp. 134 and the lower thermal shield 138 are placed in place. This whole unit is then lowered into the pressure vessel so that it hangs with the flange 102 on the container flange 72, where it engages in the groove 104, whereupon the upper and lower sealing ring 106 resp. 131 are inducted. Finally, the ring 128 is mounted and attaches the outlet spout 124 to the outlet flange 70.

It should be noted that this arrangement allows for differential thermal expansion of the various reactor parts, the lower ends of the tubes 112 being free, while the unit, comprising the tubes 114, the outlet box 118 and the inner outlet nozzle 122, is rigidly connected only to the outer outlet nozzle 124. parts can thus freely expand upwards as the upper ends of the pipes 114 have a sliding fit in the secondary upper pipe plate 110. The upper pipe plate with associated parts hangs, as mentioned, by means of the flange 102, down from the container flange 72 and the pipes 112 can freely extend through the secondary tube plate 110 as they have sufficient clearance for such mutual movement to be possible.

After the aforementioned components of the reactor have been assembled, the reactor's fuel elements are inserted and the upper thermal shield 136, the condensate chamber 140 and the guide member 142 are placed in place whereupon the top cover 74 is attached to the pressure vessel.

The reactor's fuel elements are cylindrical and suspended coaxially in the tubes 112 and 114, which thus form sleeves around the fuel elements. Between the outside of each fuel element and the inside of the corresponding tube is provided an annular space through which the gaseous moderator-refrigerant fluid flows while absorbing heat by direct heat transfer from the fuel elements.

As can be seen from fig. 3 and 4, the fuel elements are arranged with a triangular division, i.e. spaced apart, and collected in rods or bundles with a largely hexagonal cross-section. As shown in fig. 4 and 5, each element rod is equipped with a hexagonal tube plate 148 located above the upper tube plate

97 and the fuel elements 160 extend

downwards through openings in the element plate

148 and hangs down from this by means of

larger caps 150. A majority small openings

152 passes through the element plate 148 and serves to introduce moderator coolant into the tubes 112. A secondary fuel element plate 156 abuts the element plate 148 and can move freely along the fuel elements, so that when the element rod 144 is pulled upwards from the associated tubes 112 and 114, then this secondary element plate 156 will be located at the element rod, thereby ensuring the correct distance between the lower ends of the elements 160. A plurality of spacers 154 are arranged on the underside of each secondary element plate 156 and separate both this and the element plate

148 from the upper side of the upper tube plate 97

so that a current cross-section for the moderator coolant appears. It should be noted that each of the fuel elements in the tubes

114 has an extended upper part 158 to

prevent excessive flow of moderator coolant from the inlet chamber 94 directly into the pipes 114. Each element rod 144 is provided with a central operating button 146.

The gaseous moderator-refrigerant sub-fluid which enters the pressure vessel through the inlet nozzle 82 flows around the guide member 142, through the central channel in the condensate chamber 140, past the upper thermal shield 136 and through the openings 152 in the plates 148 and 156 as well as between the element plates 156 and 97 to flow into the tubes 112 in the upper tube plate 97. The gas fluid then flows through the central core chamber 95 in three paths and in the first of these the gas flows down into the tubes 112 and cools the fuel elements therein. The gas fluid leaves the pipes 12 in the lower part of the core and then flows upwards in the second path in the spaces between the pipes 112 and 114 to the upper part of the core. The gas then flows through channels not shown in the upper reflector 132 and in the secondary tube plate 110 and into the space between the upper tube plates 97 and 110 from where it enters the tubes 114. The gas is now sent in the third path through the core, as it flows downwards through the pipes 114 and cools the fuel elements in these pipes from which it runs out into the outlet box 118 which the gas finally leaves through the connecting pipes 120 and the outlet spout 122.

A cross-section of part of the central core chamber 95 is shown in fig. 6, from which it appears that there are three times as many third-path tubes 114 as first-path tubes 112. It is also seen that the outside of the third-path tubes 114 is coated with insulation 166 which limits the heat transfer between the fuel elements in them and the gas fluid in the second path. Fig. 6 also shows that spacers 162 for the pipes are arranged at a horizontal distance from each other throughout the entire height of the central core chamber 95, see fig. 2. Each spacer consists of two e.g. by means of bolts 164 connected sections around the first-course pipes 112 with abutment against the pipes 114 so that these can move in the longitudinal direction independently of the pipes 112. The spacers are arranged in such a way that flow channels 165 are formed between them with approximately the same flow cross-sections such as the flow cross-section of the annular channels of the first lane and the flow cross-section of the third lane. The spacer 162 thus serves to stiffen the pipes 112 and 114 laterally while enabling the flow of the gas fluid in the second path.

As shown in fig. 7, the fuel element 160 located in a third path tube 114 contains a plurality of pieces or units 167 of fission material arranged end to end inside a corrugated tube 170 (which is the usual shape of tube 114) but separated from this by a plurality of longitudinal strips 168 evenly spaced around the fuel element. The flow cross-section for the gas medium thus becomes the annular space between the fuel element 160 and the corrugated pipe 170. A layer of heat-insulating material 174, e.g. stainless steel wool, is placed on the outside of the corrugated pipe 170 and is enclosed by a casing pipe 172. The insulation material, which is generally denoted by 166 in fig. 6, reduces the heat transfer from the fuel element 160 in the pipe of the third path through the gas fluid flowing in this path to the surrounding gas fluid flowing in another path. Although straight pipes can be used, corrugated ones are preferred as this construction allows the use of relatively thin walls with consequent less neutron loss, while at the same time providing sufficient strength. Fig. 8 shows the fuel element 160 arranged in the same way as in fig. 7 located inside a corrugated pipe 178, which is the usual shape of the pipes 112, but here there is no thermal insulation on the outside of the pipe. The pipes in both the first and third lanes have rough exteriors 180 to increase the heat transfer coefficient between the fuel elements and the gas fluid that cools the elements. Fig. 9 shows a modified version of the pipe device in the third course where the corrugated pipe 170 is provided with an external heat insulation in the form of a multiple layer of stainless steel wool 182 enclosed in an outer jacket 184 which has a series of grooves or knobs 186 to increase stiffness.

The following table contains a number of data regarding the one in fig. 2 to 7 showed and in the reactor described above.

The steam's moderating capabilities at supercritical pressures, of which the use for cooling, moderating and control is discussed, are. such that they enable the use of compact reactor cores capable of giving considerably greater output power per unit core volume in currently available sizes of pressure vessels. The construction of the reactor and pressure vessel is simplified by the fact that the control rods are looped, and that pumps and safety valves are arranged in their place, the sensitivity of which enables precise regulation to produce controlled changes in reactivity by simply varying the steam content of the reactor plant and

-density.

The thermal flux pattern is no longer distorted by control rods.

As the density of the steam decreases as it flows along the fuel elements and a lower steam density corresponds to less reactivity, the maximum steam production will tend to lie closer to the steam inlet end than the outlet end, which is in principle advantageous. In order to assist an advantageous heat release along the respective lengths of the fuel elements, the insulating material between the tubes 170 and the casing tube 172 (Fig. 7) in the third path tube 114 can contain ZrH2 in areas where more moderation is required and A1903 in other areas where this further moderation is not is required.

In the first and third lane pipes 112 resp. 114, in each core area where a greater heat release can take place, the steam will have a lower average density and from the lower ends of the first path's pipe 112 in this area, steam with a lower average density is emptied into the second path in order, in any case to begin with, to flow upwards in the same area. As less density corresponds to less reactivity, there will be a tendency towards automatic so-called "straightening" of the thermal flux pattern in radial terms. This alignment of energy density appears in fig. 12, where the full line 192 indicates the energy distribution in a reactor with uniform moderation density and fuel enrichment throughout the core, while the dashed line 194 indicates the relative energy density in the reactor described above.

Fig. 13 shows a modification of the circuit for the gas fluid. Here, 200 denotes a gas fluid line with a valve 202 which enables control of the flow of the gas fluid from the end of the first path 24 in the reactor to the inlet line 38 for the pump 40, whereby the heat exchangers 34 and 36 can be short-circuited. The line 200 with the valve 202 makes it possible, to a controlled extent, to increase the ratio between the flow through the first path 24 and the flow in the second and third paths, for, if so required, operation with an increased gas density in the second path 26.

If a greater amount of heat is to be emitted in the boiler 46 to cool the temperature of the moderator coolant at the reactor inlet and if the simultaneous heat absorption in the reactor and its outlet temperature are to be kept constant, then the flow rate through the reactor must be less . However, the moderator-coolant flow rates over the fuel elements must be sufficiently large. According to the device shown, it is possible to remove a part of the moderator coolant at the end of the first path 24 via the line 200, past the valve 202 and via the inlet line 38 to the pump 40 and to introduce a corresponding amount of moderator coolant back to the reactor at the beginning of third path 28 through a bypass line 204, a valve 206 and a line 208. As the diverted moderator coolant is mixed with much hotter moderator coolant from the heat exchangers 34 and 36, the gas that is fed back into the reactor via the lines 204 and 208 will also have a higher temperature than the gas that normally flows from the first path and the outlet temperature and outlet velocity from the reactor can be kept constant.

Another line 212 with valve 210 runs from line 204 to the reactor's inlet line 30 and when fluid through line 212 is led past the heat exchanger 46, the reactor's inlet temperature can be raised as needed. A further line 214 with the valve 216 leads from the outlet of the heat exchanger 46 to the beginning of the third path 28 in the reactor and enables, with the valve 206 closed, a regulated increase in the ratio of the flow through the third path 28 and through the first and second lane 24 or 26.

Many variations present themselves with regard to the different types of gaseous moderator-refrigerant-fluids that may be involved. These fluids must have suitable heat transfer properties and for use they must agree with the construction materials that are applicable at the temperatures and pressures in question here. But it is of course necessary that the fluid contains a sufficient amount of hydrogen atoms, either in a free state or in combination, to provide sufficient neutron moderation at practical working temperatures and pressures. One can thus see that hydrocarbon vapor can be used as a moderator - refrigerant fluid. Although the operating pressures for this fluid are generally lower than those required when using steam moderation, the basic theory and operation will be the same. In addition, the use of hydrocarbon gas will enable the utilization of neutron radiation for refining the hydrocarbons and the production of petroleum products and other chemical compounds. Acetylene or benzene vapor can e.g. are used as moderator refrigerants, but they convert to hydrocarbon polymer by means of a chemical nuclear reaction.

Control of a reactor by varying the hydrogen concentration in a gaseous moderator-coolant fluid can take place even if the core is without fertile material. With such a reactor, the neutron economy will be of secondary importance, as the main purpose is to obtain a reactor with a large energy output from a relatively small, compact core. In that case, variation in the moderation will serve to change the reactivity mainly by changing the size of neutron leakage from the reactor core.

Steering or control of a reactor by variation of the hydrogen concentration in a gaseous moderating and cooling fluid can well be carried out, even if the gaseous fluid is not the only moderator. For example, part of the moderation can be carried out by a fixed built-in moderator type, e.g. of graphite or ZrH2. The reactor would then be calculated so that the fixed moderator alone is not capable of moderating sufficiently for the reactor core to become critical. The moderation which is then missing and which is necessary to make the reactor critical, is produced by the hydrogen-containing, gaseous, moderating and cooling fluid which is made to flow in three paths through the core. With this arrangement, it is possible to regulate the reactivity of the reactor and to stop the chain reaction by varying the density of the gaseous fluid which is to both moderate and cool.

Claims (17)

1. Nuclear reactor with fuel arranged heterogeneously in the core which is fissionable by reaction with moderated neutrons and intended to be cooled by a fluid which also acts as a moderator for the core and has devices for conducting a gaseous moderator fluid through the core, to provide neutron moderation with hydrogen atoms whose concentration can be varied for control of the reactor, characterized in that the reactor is arranged so that the gaseous fluid in a first path in the core is passed as a cooling fluid over some fuel elements, in a second path is passed between the fuel elements isolated from and without direct heat transfer with these, and in a third path fuel elements are fed as cooling fluid for the remaining part. 2. Reactor as stated in claim 1, characterized in that the gas fluid in the third path is separated from the gas fluid in the second path by means of thermal insulation. 3. Reactor as stated in claim 1 or 2, characterized in that of the total number of fuel elements in the core, approximately a quarter of the gas fluid is cooled in the first path. 4. Reactor as specified in any of claims 1-3, characterized in that the fuel material comprises a plurality of fuel elements located parallel to each other in the core surrounded by casing tubes, and that the gas fluid in the first and third paths flows in the same direction through the casing tubes and coats the fuel elements in these, while the gas fluid in the second path flows in a direction opposite to the direction of flow in the first and third paths. 5. Reactor as set forth in claim 4, characterized in that the inlet ends of the first path casing are fixed in a tube plate at the core's inlet end, and that the third path's casing at the outlet ends passes through a tube plate at the core's outlet end. 6. Reactor as stated in claim 5, characterized in that the outlet ends of the sleeve pipe of the first path open freely into the second path. 7. Reactor as stated in claim 5 or 6, characterized in that the outlet ends of the third path's sleeve tubes are fixed in the tube plate at the core's outlet end and that their inlet ends are slidably arranged in a third tube plate located at a distance from and fixed to the tube plate at the core's inlet end. 8. Reactor as specified in claim 5, 6 or 7, characterized in that all fuel elements extend through the tube plate at the inlet end of the core and that rods or bundles of fuel elements are connected to their respective tube plates, so that all fuel elements in each group can simultaneously removed from the nucleus. 9. Reactor as set forth in any one of claims 4-8, characterized in that the sleeve tubes are kept at a distance from each other by means of spacers which extend across of the sleeve tubes and is fixed to the sleeve tubes in the first run and is movable in the longitudinal direction and in relation to the sleeve tubes in the third run. 10. Reactor as stated in any one of claims 4-9, characterized in that the sleeve tube of the third path is surrounded by heat-insulating material enclosed in an outer casing tube. 11. Reactor as set forth in any one of claims 4-10, characterized in that the core occupies a central area between an upper inlet area and a lower outlet area in a pressure vessel with a vertical axis and that a condensate chamber is arranged in the upper area and has an upper open annular top over which the gas fluid is forced on its way to the core. 12. Reactor as stated in any one of claims 4-11, characterized in that means are arranged for the rapid discharge of gaseous moderator-coolant fluid from the core for emergency stopping of the reactor. 13. Reactor as stated in any one of claims 4-12, with devices for circulating the gaseous fluid in a closed circuit, including the core and devices for heat exchange with the gaseous fluid, characterized in that the gaseous fluid in the circuit is arranged to be led to a chamber (54, fig. 13) whose inlet and outlet are connected with a nozzle (48) which produces a pressure difference. 14. Reactor as set forth in claim 13, characterized in that a shunt line with a valve is arranged for the by-flow of gas fluid outside the third path and outside parts of the heat removal means. 15. Reactor as stated in any one of claims 12 and 14, characterized in that a line with a valve is arranged for controlled flow of the gaseous fluid outside parts of the heat removal means and outside the first and second paths. 16. Reactor as stated in any one of claims 12-14, characterized in that a line with a valve is arranged for the controlled past flow of gaseous fluid outside the part of the heat removal means which in the flow path of the gas fluid in the circuits lies immediately in front of the first path. 17. Reactor as stated in any one of claims 12-16, characterized in that a line with a valve is arranged for controlled flow of gas fluid past the first path.