Boiling reactor

Boiling reactor comprising a reactor core (1) and a pressure vessel (2) enclosing said reactor core and being provided with at least one conduit for discharged steam and at least one conduit for feed water and during normal operation being filled with water up to a certain normal level (11), the steam pressure in the pressure vessel having a substantially constant value of at least 5 MPa. The pressure vessel (2) is surrounded by a water-filled pool (3) with a water volume above the reactor core (1) which is considerably greater than the water volume within the pressure vessel (2). The pressure vessel (2) is a concrete vessel having an internal thermal insulation (9). The reactor comprises emergency cooling pipes (19, 20) with valves (19', 20') connecting the reactor vessel (2) to the pool (3). The valves (19', 20') are normally closed. The pipes (19, 20) are positioned at different levels. The emergency cooling valves (19', 20') are adapted to be controlled in dependence on the water level (11) in the reactor vessel (2) in such a way that they are opened when the water level is below a certain minimum level. When the valves (19', 20') are opened, steam is flowing out of the reactor vessel (2) through the upper emergency cooling pipe (19), whereas water is flowing from the pool (3) into the reactor vessel (2) through the lower emergency cooling pipes (20) as soon as the difference in static pressure in the pool between lower emergency cooling pipe (20) and upper emergency cooling pipe (19) is greater than the pressure drop of the steam flux passing through the latter. Emergency cooling is achieved entirely without the use of pumps. (FIG. 1.)

The present invention relates to a boiling reactor according to the 
preamble to claim 1. More particularly, the invention relates to a boiling 
reactor having extremely high security against dryout. Reactors for pure 
heat production with extremely high security against dryout are disclosed 
in Swedish Pat. No. 7506607-6 and Canadian Pat. Nos. 1,066,435 and 
1,070,860. 
The above-mentioned known reactors are primarily intended for district 
heating plants and may only operate at relatively low temperatures, that 
is, in the temperature range 100.degree.-120.degree. C. 
A reactor according to the invention, however, is primarily intended to 
operate a steam turbine and to produce steam at an operating pressure of 
at least 5.5 MPa. 
What characterises the invention will become clear from the appended claims 
.

In the drawings, 1 designates the reactor core of a boiling reactor and 2 a 
concrete pressure vessel which is dimensioned for a pressure of at least 5 
MPa. The cover of the pressure vessel is designated 2'. The pressure 
vessel 2 is arranged in a pool space 3 formed by concrete walls 4. The 
wall of the pressure vessel 2 need not be in contact with the water of the 
pool. The main thing is that water is present in a sufficient amount in a 
pool space located above the top of the core. The pool is filled with 
water to a level indicated by the level symbol 5. The water-filled portion 
of the pool 3 is arranged in a hollow in the ground, for example in rock, 
and surrounded, for example, by a dense layer of clay 6, the thickness of 
which is considerably greater than the thickness of the walls 4. The pool 
space 3 is preferably provided with a filling pipe which is in 
communication with some large water reservoir. In FIG. 1 such a filling 
pipe is designated 7 and provided with a valve 8 controlled by a level 
monitor (not shown). The pressure vessel 2 is made of prestressed 
concrete. Internally it has a relatively thick heat insulating layer 9 and 
inside this a thin metallic lining 10. During normal operation, the 
pressure vessel 2 is water-filled up to a level indicated by the level 
symbol 11. The water is driven through the core 1 as indicated by arrows 
by means of circulating pumps (not shown). There may also be 
self-circulation. The steam developed in the core passes through the steam 
separator 12 and a steam dryer 13 and thereafter leaves the pressure 
vessel 2 through a plurality of steam pipes 14 connected to a turbine, 
each of said steam pipes being provided with a steam valve 15. The feed 
water enters into the pressure vessel through a plurality of feed water 
pipes 16, each of which is provided with a feed water valve 17. Control 
rods (not shown in the drawing) provided with hydraulic drives are 
arranged in a water-filled space 18 above the core. The portion of the 
water-filled space of the pool 3 that is located above the highest point 
of the reactor core 1, contains an amount of water which is at least as 
great as the water volume of the reactor vessel during normal operation, 
preferably more than three times this water volume. 
The cylindrical wall of the pressure vessel 2 is provided with at least one 
upper emergency cooling pipe 19 and with at least one lower emergency 
cooling pipe 20, which open out into the pool 3 via individual valves 19' 
and 20', respectively, which are closed during normal reactor operation. 
The valves 19' and 20' are operable by means of separate hydraulic 
differential control devices 23 and 24, respectively, in such a way that 
the valves are closed for as long as a pressure difference--supplied to 
each of the control devices--lies above a certain value, but fully opened 
when the pressure difference falls below said value. The pressure 
difference supplied to the control devices 23 and 24 is created by a 
sensing pump 22 arranged outside the pressure vessel 2, the input side of 
said pump 22 being connected to a sensing pipe 21 which opens out into the 
pressure vessel 2 at a level located below the normal level 11, but not 
below the lowest permissible water level in the pressure vessel 2. 
The emergency cooling pipe 19 is intended for steam which is flowing out, 
whereas the emergency cooling pipe 20 is intended for pool water flowing 
into the pressure vessel. In the event of damage, such an amount of steam 
shall be able to flow out through pipe 19 and valve 19' that the steam 
pressure in the pressure vessel falls from the normal operating pressure 
to a pressure which differs only to a minor extent, for example with a 
water column less than three meters, from the hydrostatic pressure at the 
place in the pool 3 where the emergency cooling pipe 19 passes through the 
wall of the pressure vessel. If this hydrostatic pressure is compared with 
the pressure at the position in the pool where the emergency cooling pipe 
20 passes through the wall of the pressure vessel, it is seen that water 
flows into the pressure vessel through the pipe 20 when the difference 
between the first-mentioned and the last-mentioned pressure exceeds the 
pressure drop of the steam through pipe 19 and valve 19'. 
The principle of operation of the hydraulic differential devices 23 and 24 
will be clear from FIG. 3. 
During normal operation the sensing pump 22 runs continuously. The 
differential devices 23 and 24 are supplied by way of two concentric 
conduits 25 and 26 branching to each of them, with a first pressure 
P.sub.r which is the pressure in the reactor pressure vessel 2 at the 
position where the sensing pipe 21 opens out into the pressure vessel, and 
with a second pressure P.sub.t which is the sum of P.sub.r and a pressure 
.DELTA.P which is achieved by means of the sensing pump 22. The pressure 
P.sub.r is supplied to the two pressure differential devices 23 and 24 via 
conduit 26 and the pressure P.sub.t is supplied via conduit 25. Each of 
the devices 23 and 24 consists of a closed, circular, cylindrical pressure 
container 27 and a coaxially arranged bellows 28 arranged in said pressure 
container 27, said bellows 28 being flexible in the axial direction. One 
of the end walls of the bellows 28 is mechanically connected to a plane, 
circular inner surface of the container 27 with the aid of a plurality of 
fixing blocks 29, whereas the other end wall of the bellows is provided 
with an operating rod 30 which is passed pressure-tightly through the wall 
of the pressure container by means of a sealing bellows 31 welded to the 
wall of the pressure container and to the operating rod. In each of the 
differential pressure devices 23 and 24, the pressure container 27 is 
hydraulically connected to the conduit 25 and the bellows 28 is 
hydraulically connected to the conduit 26. During normal reactor 
operation, each bellows 28 has an internal pressure P.sub.r, which is 
equal to the pressure in the reactor pressure vessel 2 at the position 
where the sensing conduit 21 opens out into the pressure vessel, whereas 
the external pressure P.sub.t of each of the bellows 28 is equal to 
P.sub.r +.DELTA.P. This means that each bellows 28 is subjected to an 
axial force which is proportional to P.sub.t -P.sub.r, that is, to the 
pump pressure .DELTA.P. During normal reactor operation, the pump pressure 
.DELTA.P is constant and its value is in the range of 2-30%, preferably 
3-15% of the steam pressure in the pressure vessel 2. The above-mentioned 
force which is proportional to the pump pressure is for the main part 
taken up by the valve 19' and 20', respectively, in such a way that the 
valve is held closed. Each operating rod 30 is connected to the 
corresponding valve by means of a mechanical coupling 19" and 20", 
respectively. 
When the pressure difference .DELTA.P falls below a certain value, each of 
the bellows 28 is extended to such a degree that the valves 19' and 20' 
open. As opposed to valve 19', valve 20' opens with a certain delay. This 
is accomplished by means of a delay device consisting of an auxiliary 
pressure container 32 connected to the container 27 in combination with a 
throttle device 34 arranged in the thickest of the two pressure pipes 
connected to the difference device 24. The auxiliary pressure container 32 
contains an air cushion 33. 
If a tube rupture should occur in a steam conduit 14 or a feed water pipe 
16, the water level in the pressure vessel 2 may drop considerably below 
the normal level 11. If the water level drops below a certain permissible 
level, that is, so low that the suction opening of the sensing conduit 21 
no longer emerges into the water, a significant reduction of the output 
pressure of the sensing pump 22, that is of the pressure difference 
.DELTA.P, will occur, resulting in the emergency valve 19' of the upper 
emergency cooling pipe 19 opening and in steam being blown into the pool 
water and being condensed therein. After a short time interval, for 
example 3-30 minutes, preferably 5-15 minutes, such an amount of steam has 
blown out that the pressure in the pressure vessel 2 has dropped from a 
normal value of at least 5 MPa to a value determined by the water level 5 
in pool 3, and at this time also the delayed emergency cooling valve 20' 
opens causing pool water to flow into the pressure vessel 2 through the 
lower emergency cooling pipe 21, as indicated on FIG. 2. 
Because conduit 26 is surrounded by conduit 25, the former is well 
protected against tube rupture. If a tube rupture should occur in conduit 
25, this would result in the emergency cooling valves opening. 
A rapid blow-out, that is a blow-out involving a relatively large amount of 
steam per time unit, easily results in the water present in the pressure 
vessel being mixed with steam to such an extent that there will hardly be 
any well-defined limit between water and steam. When describing the manner 
of operation of the emergency cooling system in a reactor according to the 
invention, it is consequently not sufficient to establish that the 
pressure of the sensing pump is considerably reduced when the water level 
drops below a certain limit. If the water present in the pressure vessel 
becomes mixed with steam to a substantial extent, this is--in a reactor 
according to the invention--a criterion that the described emergency 
cooling system should be actuated. An actuation will take place when the 
steam content of the water near the inlet opening of the sensing pipe 21 
rises beyond a certain value at which the output pressure of the sensing 
pump drops below the minimum pressure required for keeping the valves 19' 
and 20' shut. 
When the emergency cooling valves have opened and water streams into the 
pressure vessel and steam flows out of it in the manner described above, 
the core will become cooled in a reliable manner by natural circulation 
between the core and the pool. The reactor vessel is made so large that, 
in all cases that may occur, the core remains covered by water during the 
whole blow down process and also after the blow down. 
Drainage of the core, which is a condition for a core melt-down, can now 
only take place when the water of the pool has boiled away. The pool is 
made so large that its water would last for several weeks. In addition, 
water can be supplied through conduit 7. The reactor vessel 2 is 
preferably provided with a plurality of upper emergency cooling pipes 19 
and with a plurality of lower emergency cooling pipes 20, each emergency 
cooling pipe being provided with an emergency cooling valve and an 
associated differential pressure device. 
A reactor according to the invention, for example that disclosed in FIGS. 
1-3, is preferably made with a plurality of upper emergency cooling pipes 
19, which are each furnished with a controlled valve 19' and a 
corresponding differential control device 23. It is then advantageous to 
furnish some of the differential control devices 23 with individual delay 
devices, having different delay times which are all smaller than the delay 
time of the differential control device 24. This may, for example, be 
accomplished by means of the delay device disclosed in FIG. 3. A signal 
from a level transducer, for example the sensing pump 22, will then give a 
practically undelayed response from one of the valves 19' and actuate 
other valves 19' with intervals which are greater than one minute, 
preferably greater than two minutes. Thus, too violent a boiling in the 
pressure vessel is avoided. 
Instead of the arrangement shown in FIG. 3, a slightly deviating version 
may be used, in which the sensing pump 22 and the conduits 21, 25 and 26 
are arranged with respect to each other and the reactor vessel 2 as shown 
in FIG. 3, but in which a plurality of emergency cooling valves 36, 
mutually equally constructed according to FIG. 4, are arranged to be 
controlled by means of a common compressed air source, which is designated 
35 in FIG. 4. The compressed air source 35 may suitably be connected to a 
number of upper and to a number of lower emergency cooling valves, the 
latter being connected to the compressed air source 35 via a common delay 
device, which is constructed according to the same principle as that shown 
in FIG. 3. Valve 36 has a valve housing 37 with two bored holes 38 and 39, 
of which bore 38 opens out into the pool water whereas bore 39 is intended 
to be connected to an emergency cooling pipe. A valve disc 40 is connected 
via an operating rod 41 to an operating piston 42, which is arranged in an 
operating cylinder 43. A spring 44 is arranged to exert a compressive 
force on the lower side of piston 42. During normal reactor operation, 
spring 44 is compressed and the valve is closed by compressed air from 
source 35 providing a holding pressure on the upper side of operating 
piston 42. If the pressure .DELTA.P of the sensing pump 22 drops below a 
certain value, the differential pressure device 23 opens an emptying valve 
(not shown) associated with the compressed air source 35, which results in 
the holding pressure disappearing and in all emergency cooling valves, 
connected to source 35, being opened. 
The pressure cylinder space present below the operating piston 42 is 
provided with a connection 45 for operating pressure during normal blow 
down and for testing during operation. 
The above-described emergency cooling principle results in a temperature 
reduction, in the space surrounded by the pressure vessel, which is 
considerable and which may take place in a very short time. Since the 
pressure vessel is a concrete vessel which is provided with internal heat 
insulation, similarly to known concrete vessels, the pressure vessel is 
capable of enduring the rapid temperature reduction. 
With the embodiment illustrated by FIG. 5, a plurality of emergency cooling 
pipes of the kind designated 19 in FIG. 1 and a plurality of emergency 
pipes of the kind designated 20 in FIG. 1 are furnished with individual 
valve assemblies, each valve assembly comprising an emergency cooling 
valve and a corresponding control means disposed in a valve housing 50. 
One of these valve assemblies is disclosed in FIG. 5. All valve assemblies 
are preferably of one and the same design, except for the fact that some 
of them are furnished with delay means. 
The valve housing 50 has a high pressure channel 51 connected to an 
emergency pipe and a low pressure channel 52 connected to the water pool 
3. The inner end of the high pressure channel 51 is furnished with an 
annular valve seat 53 and closed during normal reactor operation by means 
of a movable valve member 54. The valve member 54 is mechanically 
connected to a cylindrical actuating member 55. The valve housing 50 has a 
projecting cylindrical valve housing compartment 56 furnished with an end 
wall 57 at its axially outer end. The cylinder axis of the valve housing 
compartment 56 is directed towards the central point of the annular valve 
seat 53. The actuating member 55 is journalled coaxially and axially 
movable in the housing compartment 56 by means of two, non-tightening 
sliding rings 58 and 58'. A tube-like bellows 59 is disposed coaxially in 
the housing compartment 56. The axially inner end of the bellows 59 is 
arranged in fluid-tight mechanical connection with the actuating member 
55, whereas the axially outer end of the bellows is arranged in 
fluid-tight mechanical connection with the end wall 57. The outlet of the 
sensing pump 22 is hydraulically connected to the interior of the bellows 
through an inlet opening made in the end wall 57. 
In the same way as above, the pressure of the pressure vessel 2 is 
designated P.sub.r, and the additional pressure produced by the sensing 
pump 22 is called .DELTA.P. The area subjected to high pressure is 
substantially greater with the movable valve member 54 than with the 
actuating member 55. During normal reactor operation, however, the 
additional pressure .DELTA.P is so great that the movable valve member 54 
is retained in its blocking position. 
As soon as the pressure .DELTA.P drops below a certain value, the movable 
valve member 53 is moved out of its blocking position. In cases where a 
delayed action is desired, the valve assembly described above may be 
furnished with delay means. If, for example, the delay means disclosed in 
FIG. 3 are used, the pressure container 32 may be connected to a branch 
extending from the connection pipe between the sensing pump 22 and the 
bellows 59, and the throttle device 34 may be inserted in this connection 
pipe between the branch and the sensing pump 22. 
In addition to the designs described with reference to the drawings, there 
are other feasible embodiments of a reactor according to the invention. 
For example, instead of the pool 3, a water reservoir not surrounding the 
pressure cylinder can be used. Further, instead of the sensing pump 22, it 
is possible to use a conventional level transducer.