CA1108315A - Nuclear power plant capable of preventing coolant failure accident - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An improved nuclear power plant is disclosed for prevent-ing coolant failure accident. A normal coolant supplying system is connected to a coolant strage tank through a stop valve adapted to be opened when coolant for emergency is to be supplied, whereby the normal coolant supplying system is used also as an emergency coolant supplying system. Major pipings of large diameter included in a cooling system are supported by piping supporting means which allows slight dis-placement of the piping but not a large displacement thereof.

Description

The present invention relates -to a nuclear power plant having a facility to prevent -the coolant failure aCci~ellt.
More particularly, the invention is concerned with a nuclear power plant having a coolant failure accident prevention facility in which, owing to the combination of a mechanism for suppor-ting piping which can diminish the rate of loss of coolant when the piping is bro]cen and a mechanism for lnstantan-taneously pouring a large amount of coolant when the accident has taken place, the reactor core is never exposed, even when the piping of cooling system has been broken.
The present invention will be illustrated by way of ;
the accompanying drawings, in which:
Fig. 1 is a diagram showing a basic system arrangement of a conventional nuclear power plant having a pressure tube type heavy wa-ter reactor;
Fig. 2 is an illustration schematically showing a nuclear power plant embodying the invention, particularly a portion of major pipings whlch are to be supported by piping supporting means~
Fig. 3 is a fragmentary pe~rspective view of an example of the piping supporting means suitably used in the nuclear power plant of the invention; and Fig. 4 is a diagram showing a basic system arrangement of the nuclear power plant as shown in Fig. 2.
The most serious accident assumable with nuclear power plant is an accident due to loss of coolant at~ributable to an instantaneous perfect breakage of the piping of cooling system. ~
An emergency core cooling system is provided against such an ~ ;
accident. This system is adapted to supply the reactor core instantaneously with a large amount of coolant, so as to cool the core down, to thereby prevent the accident from growing more serious.

-2-In the conventional nuclear power plant, it is liable to take place that -the surfaces of breakage of the broken pipe are largely offset from each other to allow the coolant to flow out at a rapid rate. In addition, an emergency power engine and a pump or the like of the emergency core cooling system are started upon detec-t of the accident. Consequently, partly because of the aforesaid rapid rate of flowing out of the coolant, and par~ly because of the time lag of response of the emergency system, the core is inevitably exposed out of the coolant to cause an abrupt temperature rise of fuel : 30 -2a-

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Issembl:ies. Then, as the emergency system is star-ted, tlle core is aga:in dipped in the cvolan-t supplied by the emergency sys-tem, so that -the fuel assemblies may not be damaged. A
serious damage of the fuel assemblies is however inevitable, due to the abrupt temperature ri.se and the subsequent abrupt temperature drop due to the supply of the emergency coolant.
For inst~nce, the basic system :Eor pressure tube type :
heavy water reactor ~as been designed as shown in Fig. 1.

A large number of pressure tubes 2 loaded with fuel assemblies C~/e n Glr, ~
are inserted into a briquet-like ~ar~ ~ tanlc 1 fi1led wi-th C~ ~-v r heavy water ~d~. These pressu:re tubes 2 are connected to a steam drum 4 through respective rise pipes 3. The steam drum 4 in turn is connected to a lower header 10, through a downcomer 5, manifold 6, reciroulat1ng pump 7, recirculating pump discharge pipe 8 and a check valve 9. The~lower header 10 is connec-tefl to the lower ends of respective pressure tubes 2, through inlet tubes 11. The steam drum 4 generates steam which is supplied to a main steam turbine 15~ through a main steam :system 14 having isolation~valves 13a~, 13b dis--: `
posed at -the inside and outside, respectively,of~areactor ves~
sel wall 12. The steam drives the turbine 15, and the turbine in turn dri-ves a main alternator 16 connected thereto. A con-denser 17 is connected to the steam drum 4, via an isolation valve 13c and a check valve 21, by means of a feed water sys-. tem 20 includ1ng a condensate pump 18 and a feed water pump 19.
A condensate storage tank 22 is connected to the steam drum 4 through a hi:gh.-pre.ssure water pouring pump 23 and an i.solation vaIve 13d. The outflow of the condensate storage tank 22 at the same time merges, through a low-pressure water ;....... - 3 -~ ' 3,3 q ~

pouring pump 24, in a :Line coming from a rapid pou~ing tank 25, wi~ich line in turn leads to an isolation valve 13e. The ou~let side of the isolation valve 13e is connected to the steam drum 4 and the lower 1)eader 10, through valves 26 and 27, respec-t:ively~ These valves 26, 27 are selectively opened in accordance with the pOSi tiOIIS of the breakage of pipe.
Namely, it is basically intended to open the valve 27 so as to pour the wa-ter into the lower header 10. ~lowever, when it is judged -that the breakage has taken place in the inlet tubes 11 or in the pipe-connecting nozzle in the lower header 10 and, therefore, the pouring o~ water into the lower header cannot provide a sufficient cooling effect, the valve 27 is closed while the valve 26 is opened insteadly, so that the water may be poured into the steam drum 4. The check valve 9 is provided so that the water poured into the lower header 10 may efEiciently directed toward the reactor, but not directly to the opening of breakage, when the downcomer 5 or the re-circulating pump discharge pipe 8 is broken.
In the above-described design preparing against the coolant failure accident, the acciden-t may be detected at first from a lowering of the water level in the steam drum 4 ;
in the extraordinary condition. Then, an emergency diesel engine is started (it is assumed here that the external power is failed simultaneously with the occurence of the accident), :
and the pouring pumps 23, 24 for emergency are started.
It possibly takes place about 30 seconds or so, until these pumps for emergency are started up to the rated running condition. During this transient period, the fuel in the core comes to be exposed from the cooling water. Consequently, the fuel temperature rises extraordinarily to incur a serious : , ', ; ~ , '' . :
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damclge of tl~e ~uel.

SUMMARY OF THE INVENTION

It is t~erefore an object of the invention to provide an improved nuclear power plant equipped with a facility for preven-ting coolant failure accident, which can preven-t a fuel from being exposed to avoid the damage of fue~ assemblies, even when a breakage of pipings of a cooling system has happen-ed to take place.
It is another object of the invention to provide an improved nuclear power plant which can prevent a breakage of pipings of a cooling system and, even when the breakage of the pipings has happened to take place, coolant leakage can be minimized.
It is a further object of the lnvention to provide an improved nuclear power plant w}-erein an emergency coolant supplying system can certainly operate without any delay to thereby lmprove the reliability of the emergency coolant sup-plying. ~
The present invention is applicable to a nuclear power plant generally comprising a nuclear reactor, a cooling sys-tem which includes a system for normally supplying coolant to the reactor, a system for removing coolant from the reactor and a coolant strage tank, and means for converting thermal energy removed from the reactor by coolant to electric power.
- : , According to the invention, the normal coolant supplying sys-tem is connected to the coolant strage tank through a stop ~ valve adapted to be opened when coolant for emergency is to -: .: ` :
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be suppLied, whereby ~he normal coolant supplying system is used also as an emergency coolant supplying system.
On ~he other hand, major pipings of large diameter included in the cooling system are suppoxted by piping supporting means. This piping supporting means allows slight displacement of the piping hut not a large displacement thereof.
According to the invention, the normal coolant supplying system is operated, even in -the normal running of the plant, by means of a power source independently pro~ided in the plant.

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DF,'rAILED DESCRIPTION or T}IE INVENTION

, The following description will be made in connection with, by way of example~ an embodlment comprising a pressure tube type light water cooled reactor.
As will be seen from Fig. 2 schematically showing the embodiment, a nurnber of pressure tubes 52 loaded with fuel ,~/~ n~rl~
assemblies are inserted to a briquet-shaped'''oa-~a~ tank fnafle ra~or 51 filled with heavy water~m~ at-~P. The pressure tubes 52 are connected to a steam drum 54 through respective rise pipes 53. The liquid side of -the`steam drum 4 is connected to a lower header 60, through a downcomer 55, manifold 56, recir-culation pump 57 and a recirculation pump outlet pipe 58.
The lower header 60 is connected to the pressure tubes 52, through respective inlet tubes 6~ branching therefrom. The steam side of the steam drum 54 leads to a main sbeam system 64 having isolation valves 63a, 63b disposed at the inside and outsidej respectively, of the wall 62 of a reactor vessel.
Feed water from a fee~ water system 70 is introduced into the steam drum 54 through an isolation valve 63c disposed at the outside of the resctor vessel wall 62 and then -through a check valve 71 disposed in the vessel wall 62 and adapted~to func-tion also as an isolation valve.
According to the invention, major pipings having large diameters in the coo1ing system are supported stably by piping supporting means 80 each of which allows a slight movement of the pip1ng but no large d1splacement of the same, so that any ';
; large dlsplacement of the piping in case of breakage of the same may be avoided.
, This arrangement for~prevent1ng the growing of the :' ` ,:
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3~i ~reakage opening, i.e. the arrangemen~ for supporting the pip-ing by above-mentioned supporting means 80, is preferably adopted over the following portions of the piping, as will be seen from Fig. 2. Namely, the first portion of the major ~:
pipings to be supported by this arrangement is the large~
diameter piping of the recirculation system including the down-comer 55, manifold 56, recirculation pump 57, and the recircu-lation pump discharge pipe 58 which leads to th.e lower header.
The second portion to be supported is the portion of the main steam system 64 between the steam drum 54 and the isolation valve 63b outside oÇ the vessel wall ~2. The -third portion :
to be supported is the portion of the feed water system 70 between the outer isolation valve 63c and th~e steam drum 54.
` The portion of -the pipings including the pressure tubes 52, rise pipes 53 and the inlet tubes 61 is devi.ded into a corresponding number of lines to the number of the pressure tubes 52. Therefore, even when one of these lines, each hav-:: ~.:
ing a small diameter, is happened to be broken completely, -:.

~ the rate of the discharge~of the c~oo1ant from the~broke~n part -` ~ is so small that the accident;is:not:so serious.~ It is there-, :
-~ fore not essential to provide th.e above-mentioned piping sup- ~
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: porting means 80 for th.ese lines.

; The piping supporting means 80 comprises, as shown in Fig. 3 by way of example, a combination of a rail member 81 fixed to a wall of apparatus or the like in the cooling system :
and extending in the axial direction of the pipe and a plurality - of annular supporting members 84 each of which having a bore ~- ~ 83 of a diameter slightly larger than the outer diameter:of the pipe 82. Two grooves 85 are formed in the ra1l member 81, ::: : ,~
so as to extend in the longi~udinal direction of the same, : .:
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and receive corr~sporlclirlg :Legs 86 of each annlllar supporting snember or suppc)rt l~ing 84, so that the suppor-t rings 84 are held by the rail member 81 slidably in the longitudinal direc--t-ion of the :Latter. The suppor-ting rings 84 are fi~ed at any desired position on the supporting rail member 81 by means of bolts 87. Thus, the pipe 82 is stably held by inserting it through the bores of the surpport rings 8~, and fixing the support rings at s~itable posit;ions on the rail memher ~1.
The small clearance. between the inner peri.pheraL surface of the bore 83 of the support ring 84 and the outer peripheral ` surface Oe the p.i.pe 8~ allows the small movement of the pipe ' at-tributable to the thermal expansion at the hot (running) condition and the shrinkage at the cold (stopped) condition of the reactor, and also allows the thermal displacement . caused by a discharge of the hot coolant from an opening i formed as a result of the breakage, as well as the thermal .I displacement in the axial direction of the pipe. It will be seen that the supporting means 80 supports the plpe 82 so flexibly that no accident may be derived from the thermal ~ distortion during the normal running of the power plant, nor ; the ~reakage of the pipe is escalted even whén it happened:to .
occur.
Thus, the plping supporting structure as adopted in the nuclear power plant of the invention is effective in prevent-ing the breakage of the piping and, even when the pipes has ~:
happened to be broken, -the diametrical movement Oe the severed pipe sections. In the latter case, the offsetting or stagger-ing of the severed pipe sections is conveniently avoided, so that the rate of the 1055 of coolant is restricted small.
Further, since the support rings 84 are slidable along the rail member 81, the support rings are slided, after the removal of the bolts 87, and collected at a suitable position.

This is ~ ite corlverl-iell~ f`rom the view point of ~ain-tenance and ~ spection, because i-t facili-tates the inspection of the large-diameter pipes during the use, as well as during the repair of the pipes.
Fig. 4 shows a general arrangement of a heavy water reactor power plant in accordance with the present invention.
Most of the major parts of this plant are identical to those of the plant as shown in Fig. 2, and are designated at the same reference numerals.
This plant differs from the conventional plant as shown in Fig. I in that the high-pressure water pouring system and ; the low-pressure water pouring system provided against the coolant failure accident are eliminated, but the feed water system as used in -the normal operation of -the plant also plays the role of these eliminated emergency systems.
The steam is supplied through the main steam system 64 to a turbine 65 which in turn drives an alternator 66. The condensate water in a condenser 67 lS fed back to the steam drum 54 through the feed water system 70 having a condensate pump 68 and a feed water pump 69. The feed water system 70 is connected also to the lower header 60, through a valve 77.
In the normal running of the plant, a valve 76 is opened, while a valve 77 is closed. However, if a breakage of the pipe happens to occur, the state of these valves are switched depend-ing on the positlon of the breakage.
The feed water system 70 is connected further to a con-densate storage tank 72, through a large-diameter pipe having a stop valve 90 which lS normally kept closed.

~ The feed water pump 69 and the condensate pump 68 are .~ :
driven by a separate power source independently installed in '~ - 1 0 --:
-, :: - : , the p]ant, i.e. by an a:lternator 92 driven by a diesel engine ~1. A check valve 59 is provided at the end of ~he recircula-tion pump clischarging pipe 58, as in the conventional plant.
The check valve 59 functions to prevent the water poured :into the header 60 in case of emergency, when the breakage has taken place in the downcomer 55 or the recirculation pump discharging pipe 58, Erom flowing directly to the opening form-ed by the breakage, but rather to direct the water- effectively to the reactor core.
This nuclear power plant is operated in the following manner. The manner of operat:ion during the normal running will not be described, because it is strictly same as that of the conventional plant, except that the condensate pump 68 l and the feed water pump 69 are driven electrically by the separate power source ins-talled in the plant. In the nor~lal running condition, as stated before~ the valve 76 is kept opened, while the valve 77~and the stop valve 90 are kept closed, respectively. ~ -~' In case of a breakage in the piping, the offsetting or .
staggering of the surfaces of breakage of the major piping ha~ing large diameter lS prevented, because these major pipings are prevented from displacing largely, due to the provision of the piping supportlng means 80? even when the breakage has taken place in such maJor pipings, so that the rate of loss of the coolant lS restricted sufflclently small.
As the occurence of~the breakage of the pipe is detected~
for example, from an extraordinary lowerlng of the water level in the stealn drum~54, -the valve ~0 is opened to establish a ~:
direct connection between the suction side of the condensate -pump 68 and the co~ndens~te stora~e tank 72 by the plpe having ~

a large vo:l.ullle, so -tha-t the coollng water is poured at a rapid rate. I`he valves 76 and 77 are s~i-tably opened or closed depending on the position of the breakage, so as to optimi~e the des-tination of the poured water. More specifically, when the breakage has taken place in the inlet tube 61 or in the pipe-connecting nozzle in the lower header 60, -the valve 76 is kept opened, while the valve 77 is closed~ as in the normal running condition, and the pouring of the water is made to the ::
steam drum 54. On the contrary, if the breakage has taken ~
place in other portion -than specified above, th.e ~alve 76 is :
closed while the valve 77 is opened, so that the water is poured to the lower header 60.
Since the feed water pump 69 and the condensate pump 68 of the feed water system 70 are con~inuously driven by the separate power source which runs continuously in the normal state of the plant as well as in case of accident, th.e pour- ~.
ing of cooling water is never i.nterrupted even during the dis-charging of the coolant from the broken part of:the piping.
Consequently, the danger of temporary loss of coolant~ and, accordingly, the exposure of the reactor core is conveniently avoided. ~ :
Needless to say, since the feed water system has to play the double role of feed water supply during the normal runn-ing of the plant and the emergency water pouring system, it `
has to be designed to have sufficient multiplicity, anti-earthquake property, high reliability backed by a superior quality management, and other features required for a protec-: :
tion facility.

. ~ Although the Lnvention has been described ~specifically -~ in connection with~a power generating plant having a boiling . : ':

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light water cooled pressure tube type heavy water ~eactor, this -t;ype of the reactor is not exclusive, and the invention can equally be applied to the power plant having other types of reactor.
The nuclear power plant of -the invention having the abo~e-described features offers the following advantages.
Firstly, since the large displacement of the large-dia-meter major pipings is prevented by the piping supporting means according to the invention, the offsetting or stagger-ing of the surfaces of breakage from each other is restrained, and the rate at which the coolant is lost can be minimized.
Consequently, the undesirable escalation of the accident to a serious loss of coolant can fairly he avoided. At the same time, the level of the maximum pressure increase in the reac-tor vessel due to the breakage of the pipe is conveniently reduced, which in turn facilitates the design of thé vessel considerably.
Secondly, since the pumps of the emergency system are driven by~the separate or lndependent power source~during the running of the plant, the problem derived from the uncer-tainty of the start up of the emergency system can be over-come, and the reliability of the supply of the emergency cool-ant is remarkably improved. At the same time, since the cool-ant can be supplied continuously without any time lag, the ;- excessive lowering of~the ooolant level and, accordingly~ ;~
the dangerous exposure of the core can fairly be,avoided.
As a result, the extraordinary temperature rise in the fuel assemblies and the abrupt cooling down of~the~fuel assembly due to the pouring of the emergency coolant, which in com-bination might cause a serious breakage of the fuel assemblies, ,: - :

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are preventecl.
Thirdly, since the condensate and feed wa-ter system, which are designed to have a sufficien-tly high reliability, function also as the emergency cooling system, -the plant is ~ -prepared against the mere -failure of the external power which might, for otherwise, cause failure of the feed water supply, and.can have an improved reliability in the normal running.
Fourthly, the separate provis.ion of the emergency cool-ing system, which has been ~ecessary in the conventional power plant, as well as -the elec-tric sys-tem, controlling system, vessel-wall pelletrating mechanisms, isolation valves annexed to the separate emergency cooling system are all eliminated, thereby permitting a simp].iication of the construction of the ~ plant. At the same time, the labour of the operators required for periodically checking the safe functloning of the separate . emergency cooling system, which has been essential in the conventional power plant, can be dispensed with advantageously, thereby ensuring~ as a result, an improved reliability of the , -~ management of the nuclear power station.
While the invention has been described wlth respect to a preferred embodlment, it should be apparent to those skilled in the art that numerous~modifications and changes may be made thereto without departing from the spirit and scope of the invention.

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Claims (4)

THE EMBODIMENT OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS.

1. A nuclear power plant capable of preventing coolant failure accident, comprising: a nuclear reactor; a cooling system including a system for normally supplying coolant to the reactor, a system for removing coolant from the reactor, and a coolant storage tank; the normal coolant supplying system being normally operated by means of a power source independently provided in the plant and being connected to the coolant storage tank through a stop valve adapted to be opened when coolant for emergency is to be supplied, whereby the normal coolant supplying system is used also as an emergency coolant supplying system; major pipings of large diameter included in :.
the cooling system being supported by piping supporting means which allows slight displacement of the piping but not a large displacement thereof; and means for converting thermal energy removed from the reactor by coolant to electric power.

2. A plant according to claim 1, wherein the piping supporting means comprises a rail member fixed to a wall of apparatus in the cooling system and extending in the axial direction of the piping, and a plurality of annular supporting members each of which having a piping receiving bore of a diameter slightly larger than the outer diameter of the piping, said annular supporting member being slidably held by the rail member and being fixed at any desired position on the rail member.

3. The plant according to claim 1, including valve means for selecting the destination of the coolant for emergency to be supplied depending on a position of coolant leakage.

4. The plant according to claim 1, wherein said nuclear reactor is a boiling water reactor, and wherein said pipings of large diameter comprises a piping of a steam removing system between a steam drum and an isolation valve at the outside of a reactor vessel wall, a piping of a water supplying system between an isolation valve at the outside of a reactor vessel wall and the steam drum, and a piping interconnecting the steam drum and a lower header of the reactor.