Nuclear power plant with a gas cooled high temperature reactor

A nuclear power plant with a gas cooled high temperature reactor, installed in a prestressed concrete pressure vessel with the operational and decay heat removal systems. The prestressed concrete pressure vessel has a thermal protection system, with a thermal insulating layer and a liner cooling system. The liner cooling system, which is includes water carrying cooling pipes, which along with intermediate heat exchangers and cooling water pumps make up a closed intermediate cooling loop used for removal of the decay heat in case a failure of the decay heat removal systems. The elements of the invention assure an adequate water flow for the removal of decay heat in the liner cooling system in any situation, i.e. such that the decay heat may also be removed by natural convection. These element insure a sufficient driving pressure differences and minimize pressure losses in the intermediate cooling loop.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a nuclear power plant with a gas cooled high 
temperature reactor, and more particularly to a reactor with a spherical 
fuel element core and a prestressed concrete pressure vessel surrounding 
the high temperature reactor. The reactor cavity is clad with a liner and 
contains a plurality of heat exchangers, preferably steam generators for 
operational heat removal. At least two auxiliary heat exchangers for decay 
heat removal are also arranged in the reactor cavity. The prestressed 
concrete pressure vessel has a thermal protection system including a 
thermal insulating layer and a liner cooling system comprising a plurality 
of cooling pipes through which water flows and which, together with 
intermediate heat exchangers and cooling water pumps, a closed 
intermediate cooling loop used for the removal of the decay heat in case 
of an auxiliary heat exchanger failure. 
2. Description of the Related Technology 
U.S. Pat. No. 4,554,129 shows a gas cooled nuclear reactor installed in the 
cavity of a prestressed concrete pressure vessel. The heat generated in 
the reactor core is transferred to several heat exchangers located in the 
cavity above the nuclear reactor. No auxiliary heat exchangers are 
provided for the removal of the decay heat. Decay heat is removed by a 
liner cooling system made up of cooling pipes welded to the cavity liner 
inlet and return lines connected to the cooling pipes and a plurality of 
circulating pumps. Heat is transported from the reactor core to the liner 
cooling system by central pipes provided inside the heat exchanger and by 
shut-off valves closed in normal operation in a annular space bordering on 
the liner. A natural downward directed flow of heat is established in the 
annular space, so that all areas of the liner are exposed to the cooling 
gas. 
DE-OS No. 33 35 268 shows a high temperature reactor with spherical fuel 
elements. Steam generators and blowers for operational removal of heat and 
auxiliary cooling systems for the decay heat removal in case of accidents 
are located together with the other components of the primary loop in the 
cavity of the prestressed concrete pressure vessel. The cavity is provided 
with a liner exhibiting a cooling system. This liner cooling system may be 
used for the removal of the decay heat, should the auxiliary cooling 
systems fail. It has also been proposed to conduct the decay heat to the 
liner cooling system by natural convection without coolant loss in case of 
accidents. 
DE-OS No. 31 21 377 shows a liner cooling system for a prestressed concrete 
pressure vessel intended to house a nuclear reactor and containing an 
interior thermal insulating layer. The liner cooling system is redundant, 
i.e. it consists of several mutually independent water circulating loops. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a nuclear power plant with an 
adequate mass flow of water in the liner cooling system for removal of 
decay heat in any situation. 
According to the invention, this object is attained by the following 
characteristics: 
(a) Arranging intermediate heat exchangers above the upper edge of the 
prestressed concrete pressure vessel 
(b) Dividing the liner cooling system into several cooling pipe zones 
according to vertical or height positions wherein at least one cooling 
pipe zone in an upper zone range (by height) is always connected to a zone 
in a lower location; 
(c) Arranging a supplemental pump connected to an emergency power system in 
parallel with each primary cooling water pump, where the supplemental 
pumps may have a significantly lower capacity; 
(d) Providing every cooling water pump with a bypass line with an actively 
or passively controlled check valve, where the check valve opens if the 
cooling water pump is inoperative and; 
(e) Providing an installation for removal of the decay heat by natural 
convection in case of a failure of the intermediate heat exchangers 
connected to the liner cooling system. 
The natural convection decay heat removal system includes: 
a water reservoir with a nitrogen cushion connected to the forward or feed 
line of the liner cooling system, 
a vertical boiling tube, placed in the intermediate loop, connected to the 
liner cooling system return line and located geodesically higher than the 
liner cooling system, 
a water separator located in the intermediate cooling loop, connected to 
the vertical boiling tube and including a nitrogen cushion, and 
a safety blow-off valve connected to the water separator. 
Adequate flow through the liner cooling system requires an appropriate 
pressure forcing the water through the system. The amount of pressure is 
referred to as a "driving pressure difference" or .DELTA.P.sub.tr. 
The decisive factor for the presence of an adequately high water mass flow 
in the liner cooling system without the use of active aggregates, such as 
pumps or blowers, is a sufficiently large driving pressure difference 
.DELTA.P.sub.tr in the intermediate cooling loop. The driving pressure 
difference is proportional to the product .DELTA..rho..g.H 
(.DELTA..rho.=difference in density, H=height). According to invention, 
the necessary driving pressure difference .DELTA.P.sub.tr is provided by 
acting on the factor H, i.e. by the geometric layout of the intermediate 
heat exchangers, and by an appropriate interconnection of cooling pipe 
zones of differing heights. The latter measure is required, as the reactor 
protection building enclosing the prestressed concrete pressure vessel 
only has a limited height available for the intermediate heat exchangers. 
According to the invention, in case the cooling water pumps are not 
operating due to a power failure, the necessary driving pressure 
difference .DELTA.P.sub.tr is assured by supplemental pumps connected in 
parallel to the cooling water pumps and operated with emergency power. No 
problems arise from the connection to the emergency power system as the 
supplemental pumps have capacities lower by one to two orders of magnitude 
than the normal operation cooling water pumps in view of a permissibly 
higher cooling water heating range and correspondingly lower water flow 
rates. 
The reduction of pressure losses in the intermediate cooling loops leads to 
an increased water flow rate. High pressure losses can occur in case of a 
failure of a the cooling water pumps; that is exactly a situation where an 
adequate flow of water is absolutely necessary since heat may be removed 
from the liner cooling system only by natural convection. High pressure 
losses of inactive cooling water pumps are reduced to a minimum by the 
proposed bypass lines with check valves. 
In case the intermediate heat exchangers of the intermediate cooling loop 
fail, a device connected to the liner cooling system is activated, whereby 
the decay heat is removed by the heat of evaporation contained in a water 
reservoir through a safety blow-off valve. 
The advantage of the invention lies in that decay heat can be removed 
safely even in case of a failure of the auxiliary heat exchangers and the 
intermediate heat exchangers and also in the case of a power failure, so 
that any impermissible heating of the primary loop components is prevented 
and the risk of the release of activities (radioactivity) reduced. 
The higher density differences of cooling water present in the liner 
cooling system may be utilized advantageously to increase the water flow 
rate by natural convection in the rare case of accidents, as a result of 
the high, acceptable, concrete and liner temperatures during removal of 
decay heat. 
An increase in temperature of 200.degree.-300.degree. C. over normal 
operating temperature due the thermal capacity. The temperature rise will 
result in a considerable rise in density difference .DELTA.P.sub.tr of the 
cooling water above the normal operation level. The driving pressure 
difference .DELTA.P.sub.tr is also raised in the process. In a single 
phase heat removal process in the liner cooling system the temperature of 
the cooling water may be raised to slightly below the boiling point. A 
further increase of .DELTA..rho. may be obtained by two-phase heat removal 
(bubble boiling) in the liner cooling system. 
An increase in the flow of water in the liner cooling system may be 
achieved by reducing pressure losses in the intermediate cooling loop. 
Pressure losses in the operation of decay heat removal is defined by: 
##EQU1## 
wherein N=nominal operation 
NWA=decay heat removal operation 
.DELTA..delta.=heating range of the cooling water 
Q=volume of heat removed by the liner cooling system 
.DELTA.P.sub.P =pressure loss of the inactive cooling water pumps. 
It follows from this relationship that it is advantageous to choose a high 
heating range for the cooling water (to the boiling temperature or 
slightly thereunder). The ratio of the volumes of heat removed in nominal 
operation and in a decay heat removal operations are determined by the 
temperatures generated. 
The nominal pressure losses .DELTA.P.sub.N may be reduced by the 
appropriate layout of the liner cooling system and choice of the 
components of the intermediate cooling loop. The following measures may be 
effected to reduce nominal pressure loss extensive equalization of 
pressure losses in the individual cooling pipes, in particular reduction 
of peak values, or selection of intermediate heat exchangers with low 
pressure losses on the liner cooling system side. 
The composition of different cooling pipe zones of the liner cooling system 
may be established advantageously by direct coupling during the layout of 
the liner cooling system. Alternatively, the cooling pipe zones may be 
connected to each other in the decay heat removal operation by short 
circuiting the forward and return lines of the zones by externally 
accessible valves. The valves may be manually actuated. This so-called 
external coupling is taken into consideration in the layout of the liner 
cooling system. 
Check valves located in the cooling water pump bypass lines may be actuated 
in a number of ways: actively, controlled by the speed of the cooling 
water pump involved (for example, the check valve concerned remains closed 
at pump speed of equal to or higher than 100 rpm and opens if the velocity 
drops below that rate), or passively by the pressure difference applied to 
the cooling water pump involved. In the latter case the check valve 
concerned, which in normal operation is closed, may be opened by its own 
weight or the release of a spring. 
According to a further embodiment of the invention the decay heat removal 
capacity of the liner cooling system by natural and forced convection must 
be optimized against the thermal resistance of the thermal insulating 
layer so that the maximum permissible liner, fuel element and reactor 
installation temperature is not exceeded. 
Heat resistance corresponding to insulation thickness of the thermal 
insulating layer decisively affects a rise in temperature of structural 
parts located within the prestressed concrete pressure vessel in case of 
failure of decay heat removal by the auxiliary heat exchangers and the 
liner cooling system. The aforementioned component temperatures are 
calculated as a function of the thermal resistivity of the thermal 
insulating layer during layout of the thermal insulating layer for the 
case of a "failure of the decay heat removal installation". The optimum is 
achieved when the same safety margin from the maximally permissible 
limiting temperature is established for all of the structural components. 
The two extreme cases described below serve to demonstrate this condition 
in case of failure of the auxiliary heat exchangers and the removal of the 
decay heat through natural convection in the liner cooling system (this is 
also valid for forced convection, decay heat removal by the liner cooling 
system. 
If the insulating layer thickness is inadequate: 
Excessive heat is transported to the liner and in the liner cooling system 
and leads to exceeding the maximum permissible liner and concrete 
temperatures, however, in view of the effective removal of heat from the 
primary loop through the liner cooling system the temperature of the fuel 
elements and of the reactor installations remain far below its limiting 
value. 
If the thickness of the thermal insulating layer is too large: 
Inadequate decay heat removal from the primary loop results in fuel element 
and the reactor installation temperatures exceeding their failure limits 
due to an excessive cooling gas temperature while the liner temperature 
remains far below the permissible limiting value (and the heat removal 
capacity of the liner cooling system remains unutilized). 
According to a feature of the invention an optimization is effected for the 
liner cooling system alone (i.e. not for the entire thermal protection 
system) relative to the maximum removal of decay heat by natural 
convection and of a minimalization of the liner temperature respectively. 
The parameters used in for this optimization are the cooling pipe diameter 
and the cooling pipe spacing. The flow of cooling water and thus the 
amount of heat that may be removed in a natural convection liner cooling 
system depends on the choice of the cooling pipe diameter and the existing 
cooling pipe spacing. The flow of cooling water in a liner cooling system 
(flow per m.sup.2 of cooling water) is predetermined by the heating range. 
In order to increase the natural convection flow of the cooling pipe cross 
section must be enlarged proportionally to the spacing of the cooling 
pipes. This measure is limited by the maximum temperature of the liner 
which rises with increasing cooling pipe spacing. A variation of the 
cooling pipe cross section and the cooling pipe spacing leads to 
optimalization of minimizing the liner temperature and to maximizing the 
cooling water flow and liner cooling system natural convection heat 
removal capacity resulting in a reduction of the residual risk in case of 
a decay heat removal failure. 
This optimalization leads to a reduction in liner cooling pipe spacing 
(while maintaining cooling pipe diameter) or an increase in cooling pipe 
diameter and a correspondingly larger cooling pipe spacing or a 
combination of the two measures. 
Advantageously, the installation for the removal of decay heat, in case of 
a failure of the intermediate heat exchangers, the pressure of the 
nitrogen cushion in the water separator and the actuating pressure of the 
safety blow-off valve may be correlated in a manner such that heat removal 
from the intermediate cooling loop to a predetermined cooling water 
temperature is effected initially in a single phase range and passes into 
a two-phase range only when the saturation pressure in the water separator 
attains the actuating pressure the safety blow-off valve. Additionally the 
water reservoir may have an open configuration and be located geodesically 
high enough so that upon a rise of the pressure in the intermediate 
cooling loop to the actuating pressure of the safety blow-off valve a 
blow-off through the open water reservoir is prevented. Alternatively the 
water container may have a closed configuration and is pressure connects 
by a connecting line to the water separator and that its geodesic height 
is determined so that the same water level may be established in the water 
separator and the water reservoir.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a reactor protection building 1 enclosing a prestressed 
concrete pressure vessel 2. The pressure vessel defines a reactor cavity 3 
clad with a metal liner 4, containing a gas cooled high temperature 
reactor 5. The core 6 of the reactor contains spherical fuel elements. The 
reactor cavity 3 also contains several steam generators 7 for operational 
heat removal, and at least two auxiliary heat exchangers 9 for removal of 
decay heat. The cooling gas, which flows downward through the reactor core 
is circulated in normal operation by blowers 8. Blowers 10 circulate the 
cooling gas in the decay heat removal mode. 
A shutdown system 11 is provided for control and shutdown of the high 
temperature reactor 5. The shutdown may include a plurality of absorber 
rods insertable into the reactor core 6. 
A thermal protection system made up of a thermal insulating layer 12 and a 
liner cooling system 13 is arranged on the inside of the prestressed 
concrete pressure vessel 2. The liner cooling system contains a plurality 
of cooling pipes 14. Water flows through the cooling pipes which are part 
of a closed intermediate cooling loop 15 (shown in subsequent figures). 
The liner cooling system 13 and the intermediate cooling loop 15 are laid 
out in a manner and have sufficient capacity such that they are capable of 
removing all of th decay heat in case of a failure of the auxiliary heat 
exchanger 9. 
FIG. 2 shows the prestressed concrete pressure vessel 2 with the (highly 
schematically drawn) liner cooling system 13 and the intermediate cooling 
loop 15, connected by a forward or feed line V and a return line R connect 
the intermediate cooling loop 15 to the liner cooling system 13. The 
intermediate loop 15 further includes a plurality of intermediate heat 
exchangers 16 each with a corresponding cooling water pump 17 (only one of 
each is shown in the figures). The intermediate heat exchanger 16 is 
located in a position elevated above the upper edge of the prestressed 
concrete pressure vessel 2 in order to have available a sufficiently large 
driving pressure difference. The driving pressure is a function of the 
height H. 
A bypass line 18 is provided for every cooling water pump 17. A check valve 
19 is located in the bypass line. The check valve may be actively 
controlled by the speed of the pump. The valve may be set so that it opens 
at a number of rotational speed equal to or less than 100 rpm. 
A supplemental pump 20 may be connected in parallel with every cooling 
water pump 17 in order to assure an adequate driving pressure difference 
even in case of a power failure (resulting in the deactivation of the 
cooling water pump 17). The supplemental pump may have a significantly 
lower capacity than the main pump and is connected to the emergency power 
system. 
The liner cooling system 13 is divided into several cooling pipe zones of 
differing elevations available for the creation of a driving pressure 
difference within the protective reactor building. At least one cooling 
pipe zone in a lower height location is connected to at least one zone in 
a higher location. Each of FIG. 3 and 4 shows an alternative of the 
interconnection of different cooling pipe zones. 
The liner cooling system is divided into individual zones. In order to 
facilitate natural convection through the various zones it is important 
that zones of differing elevations be connected serially rather than in 
parallel. As can be seen in FIG. 2 the height differential between a roof 
reflector zone and the heat exchanger may be minimal due to required 
layout within the pressure vessel. By connecting the roof reflector zone 
to a zone of lower elevation, such as a side reflector zone, the effective 
height differential is greatly increased (to H) thereby increasing the 
driving pressure to the roof zone and facilitating natural convection when 
needed. 
Driving pressure and thus natural convection is further facilitated by a 
vertical "hot-strand". If the height of the "hot-strand" is insufficient 
there will not be an adequate driving pressure. For this reason at least 
the horizontal roof reflector zones are serially coupled to lateral wall 
zones thus establishing a sufficient "hot-strand" to drive natural 
convection. 
During normal operation of the facility the parallel zone layout enhances 
performance by favorable impacting design considerations, i.e. redundancy, 
reliability, lowering pressure differential etc. 
FIG. 3 shows direct coupling of individual cooling pipe zones, i.e. the 
interconnection of the cooling pipe zones has been effected in the course 
of the layout of the liner cooling system 13. As seen in FIG. 3, the liner 
cooling system 13 is divided into six zones I . . . VI, of which zones IV 
and V impact the roof with a lower .DELTA.P.sub.tr, the cooling pipe zones 
IV and are coupled with the cooling pipe zones II and III. 
In this embodiment the check valve 19' of the intermediate cooling loop 15 
is controlled passively by the pressure difference applied to the cooling 
water pump 17. For the purpose, the check valve 19' is moved into its 
closed position upon the start-up of the cooling water pump 17 by a single 
electromagnetic impulse (for example the starting current) and remains 
closed due to the aforementioned pressure difference in normal operation. 
In case of a failure of the cooling water pump 17, the pressure difference 
is reduced approximately to 0, and the check valve 19' is opened without 
any additional energy by gravitational force due to its own weight only. 
FIG. 4 shows an embodiment where cooling pipe zones I . . . VI are 
interconnected by valves 22 and 22' accessible from the outside. A 
passively controlled check valve 19" is provided in the intermediate 
cooling loop 15. This check valve may be closed by an electromagnetic 
impulse and remains in this position under the effect of the pressure 
difference applied to the cooling water pump 17. However, the check valve 
19" is opened here in case of the failure of the cooling water pump 17 not 
by gravity, but by a tension spring 21, which is stressed when the valve 
is close and is released when the pressure difference is eliminated. 
In the embodiment shown in FIG. 4 the liner cooling system 13 is laid out 
in the planning stage so that it is possible to short-circuit the feed 
lines V1, V2 . . . V5 and the return lines R1, R2 . . . R5 of the cooling 
pipe zones concerned in case of an accident, by means of the valves 22, 
22'. Thus for example the cooling pipe zones II and V or III and IV, which 
in normal operation are supplied individually, may be interconnected by 
valve actuation in the following manner: 
interconnection of cooling pipe zones II and V by connecting the return R2 
and the forward line V5 and 
interconnection of the cooling pipe zone III and IV by connecting R3 with 
V4. 
During normal operation all valves 22 are open and valves 22' are closed. 
In order to establish natural convection valves 22 are closed and 22' are 
opened thereby establishing serial connection between zones II and V, and 
zones II and IV. 
FIG. 5 shows an installation for decay heat removal by natural convection, 
in addition to the liner cooling system 13 with the intermediate loop 15. 
The natural convection installation is actuated when the intermediate heat 
exchangers fail. The installation is made up of a water reservoir 23 with 
a nitrogen cushion, connected with the forward line V of the liner cooling 
system 13, a vertical boiling tube 24 located in the intermediate cooling 
loop 15 and being placed geodesically higher than the liner cooling system 
13 and connected to the return line R of the liner cooling system 13, a 
water separator 25 with a nitrogen cushion, also located in the 
intermediate cooling loop 15 and connected to the vertical boiling tube 
24, and a safety blow-off valve 26 mounted on the water separator 25. The 
nitrogen cushion in the water reservoir 23 and the water separator 25 has 
a pressure of-.gtoreq.1.5 bar. 
An embodiment of the water or steam separator 25 is illustrated in FIG. 6. 
The steam separator is made up of a horizontally aligned water-steam drum 
40. One or more vertical boiler tubes 24 are connected to the bottom of 
the drum. The tube diameter is large enough to remove the quantity of 
steam generated in the liner cooling system. Baffles or diverting plates 
41 may be built into the drum to prevent escape of water through steam 
outlet 42. In order to maintain appropriate pressure in the steam 
separator (and the water reservoir 23) a volume of nitrogen, referred to 
as a cushion is present in the freespace above the water surface. 
The installation operates in the following manner: 
The safety blow-off valve 26 is set for example at 2 bar. In normal 
operation the nitrogen cushion of .gtoreq.1.5 bar and the setting of the 
safety blow-off valve 26 establish a pressure of between 1.5 bar and 2 bar 
at the uppermost point of the intermediate cooling loop 15 (water 
separator 25). The water separator 25 prevents the release of a water and 
steam mixture by the blow-off valve 26 upon the occurrence of bubble 
boiling and thus the rapid emptying of the intermediate cooling loop 15 
without the complete utilization of the heat of evaporation. 
Natural convection in the intermediate cooling loop initially takes place 
in the single phase zone. Up to a temperature of approx. 111.degree. C. 
and a saturation pressure of 1.5 bar, the pressure in the water supply of 
the loop 15 is controlled by th nitrogen cushion at 1.5 bar. If the 
cooling water outlet temperature continues to rise, the corresponding 
saturation pressure determines the pressure in the water separator 25 (a 
check valve 28 provided on the water separator 25 for the N.sub.2 supply, 
closes). At a water outlet temperature of 120.degree. C.=2.0 bar 
saturation pressure boiling begins on the surface in the water separator 
25. At a continued rise of the water outlet temperature to approx. 
130.degree. C., the onset of boiling is shifted to the lower end of the 
vertical boiling tube 24, which has a height of about 5 to 10 m. If the 
amount of heat supplied by the liner cooling system 13 is larger than the 
amount of heat discharged in the boiling tube 24 by evaporation, the onset 
of boiling is also displaced to the geodesically lower areas of the liner 
cooling system 13. 
To increase the evaporation energy contained in the intermediate cooling 
loop 15, the water reservoir 23 is provided. The latter may have an open 
or closed configuration. 
FIG. 5 shows a closed water reservoir 23. In this case the reservoir must 
be brought to the pressure of the water separator 25 by a connecting line 
27. The geodesic height of the water reservoir 23 must be such that the 
water levels in the water separator 25 and the water reservoir 23 may 
establish themselves at the same height. In the downward direction, an 
extension of the water reservoir 23 to the lowest point of the liner 
cooling system 13 is useful and appropriate. A battery of standing pipes 
is used conveniently as the water reservoir. While FIG. 5 illustrates a 
vertical cylinder for a reservoir 23, any configuration may be used as 
long as the water level in the reservoir 23 is the same as that in the 
water or steam separator 25. 
If the water reservoir has an open configuration, it must be at a geodesic 
height such that in case of a rise of the pressure in the intermediate 
cooling loop 15 to the actuating pressure of the safety blow-off valve 26, 
the blowing of the intermediate cooling loop 15 though the open water 
reservoir is prevented (not shown). With an actuating pressure of the 
safety blow-off valve 26 of for example 2 bar, the water level in the 
water reservoir must therefore be at least 10 m above the water level of 
the water separator 25. If these requirements relative to the geodesic 
height of a water reservoir cannot be satisfied (for example because 
reactor protective building is too low), a closed water reservoir must be 
used.