Light water nuclear reactor (LWR), in particular pressurized water reactor (PWR) or boiling water reactor (BWR), with a ground-based cold source and incorporating an autonomous system for the evacuation of residual power (ERP).

Abstract

Light water reactor (LWR), particularly pressurized water reactor (PWR) or boiling water reactor (BWR), with a ground-based cold source and incorporating an autonomous waste heat evacuation system (WHR). The invention essentially consists of installing an ORC machine and an additional water reservoir, separate from the pool, the energy stored in the pool serving as the heat source for the ORC evaporator, the additional water reservoir directly supplying the ORC condenser via a dedicated pump to constitute the cold source for the ORC condenser. Figure for the abstract: Fig. 3

Description

Title of the invention: Light water nuclear reactor (REL), in particular pressurized water reactor (PWR) or boiling water reactor (BWR), with a ground-based cold source and integrating an autonomous residual power evacuation system (EPUR). Technical field

[0001] The present invention relates to the field of nuclear reactors, in particular pressurized water and boiling water reactors.

[0002] More particularly, the invention relates to an improvement in the residual power evacuation function of these nuclear reactors, in an accident situation. It aims to integrate an autonomous residual power evacuation system (EPUR), into the architecture of safeguard systems for advanced light water reactors (REL).

[0003] The invention thus aims to overcome a major drawback of passive safety condensers, or passive wall-mounted condensers according to the state of the art, which lies in the need to have very large volumes of water at height, which complicates and complicates the civil engineering of a nuclear installation, which is a major constraint, particularly with regard to the problem of earthquakes, and increases the cost.

[0004] The second advantage of the invention consists in obtaining better overall performance of this type of system due to the forced convection of the circuit containing the safety condenser, with also a more compact exchanger due to better heat exchange performance, and therefore a smaller overall volume of the system.

[0005] It is recalled here that the residual power of a nuclear reactor ("decay heat" in English) is the heat produced by the core after the nuclear chain reaction has stopped and constituted by the disintegration energy of the fission products.

[0006] Although described with reference to a pressurized water nuclear reactor, the invention applies to a boiling water nuclear reactor, or any light water nuclear reactor (LWR) whose residual power removal safety means, as currently envisaged, require the availability of large quantities of water at height as a cold source. Prior art

[0007] A pressurized water nuclear reactor (PWR) comprises three cycles (fluid circuits) whose general principle of normal operation is as follows.

[0008] The high-pressure water in a primary circuit draws the energy supplied, in the form of heat, by the fission of uranium nuclei and, where appropriate, plutonium, in the core of the reactor.

[0009] Then, this water under high pressure and high temperature, typically 155 bars and 300 °C, enters a steam generator (SSG) and transmits its energy to a secondary circuit, also using pressurized water as a heat transfer fluid. This water in the form of steam, at high pressure, typically around 70 bars, is then expanded via an expansion device transforming the variation in enthalpy of the fluid into mechanical then electrical work in the presence of an electric generator.

[0010] The water from the secondary circuit is then condensed via a condenser using a third cycle, the cooling cycle, as a cold source.

[0011] Unlike a PWR, a boiling water reactor (BWR) does not have a steam generator: it comprises a single circuit of water and steam produced after evaporation in the vessel. The cooling water is partially vaporized in the core. This water circulates under pressure, but at a lower pressure than that of a PWR, typically 70 to 80 bars.

[0012] Reference may be made to [Fig.2] of publication [1] which illustrates the general configuration of a BWR. The water extracted from the condenser is pumped at the pressure of the reactor vessel via main pumps and admitted into the latter at the periphery of the core. It is then mixed and reheated by a large flow of saturated water originating from the separation of the steam-water emulsion produced in the core. At the outlet of the core, the water-steam mixture is separated by gravity and by centrifugation. The steam produced is directed to steam collectors and turbines downstream, the saturated water is recirculated to be mixed with the cooler water. The water mixture descends along the vessel wall where it is taken up through primary loops external to the vessel by primary pumps to be directed into the core and then passes through the core where the heat produced is extracted causing saturation reheating and evaporation.

[0013] A REB includes safety condensers also called "Isolation Condenser" in English: they constitute the last resort for emergency cooling of the reactor core. A schematic illustration of the arrangement of an "Isolation Condenser" is given in [Fig.4] of the publication [1].

[0014] However, although the operation of light water reactors (LWRs) is known, controlled and reliable, the history of nuclear power, notably with the Fukushima-Daiichi accident in 2011, has shown weaknesses in the management of power plants in the event of extreme accident situations with prolonged loss of voltage from the electrical network, aggravated by the loss of internal electrical production means and the cold source.

[0015] This accidental situation stems in particular from a failure to evacuate the residual power from the reactors. These accidental sequences have also been reported lands for fuel cooling pools during the Fukushima accident in 2011.

[0016] The phenomenon of residual power of the reactor core is expressed as follows.

[0017] When the nuclear reaction stops, the fission products in the process of disintegrating continue to produce heat until they reach a stable state.

[0018] One second after the reactor is shut down, this heat represents 7% of the reactor's nominal thermal power.

[0019] Then, it decreases with time as shown in [Fig.l] from publication [2].

[0020] For example, 72 hours after the reactor has been shut down, it still represents 0.5% of the nominal thermal power. It is therefore essential to evacuate this heat to avoid any risk of degradation or even melting of the core fuel.

[0021] For example, the REP nuclear reactor known as VVER TOI has a nominal electrical power of 1300 MWe and a nominal thermal power of approximately 3200 MWth. 72 hours after its shutdown, this reactor still produces a residual thermal power of approximately 20 MWth.

[0022] Generally speaking, for the evacuation of residual power, improvement of passivity and diversification of systems are constantly sought to ensure better overall reliability. The objective is to preserve the integrity of the structures, namely the first (fuel assembly cladding), second containment barrier (primary circuit), and third barrier (containment vessel), even in the event of a generalized lack of electrical voltage over a long period, which corresponds to a Fukushima-type scenario.

[0023] More particularly, since the Fukushima accident, much research has focused on technologies for passively removing residual power over periods of several dozen hours.

[0024] The requirements for the new solutions primarily concern an improvement in performance and reliability, as well as the greatest possible operating autonomy, at least 72 hours, before any human intervention and external material resources.

[0025] More critically, in the context of the invention, an accidental situation with prolonged interruption, typically of several days, of electrical power supplies of any origin, excluding battery power, is considered. Such a situation is known by the English term "Station BlackOut" (acronym SBO).

[0026] One of the effective means to extract the residual power from the core of a PWR in an accident situation without active means requiring electricity is to cool the core of the reactor via a passive system by transmitting its calories either to the atmosphere via an air exchanger, or to a reserve (pool) of water placed at a height, in order to ensure natural convection. Such a system is referred to as "passive residual heat removal" (PRHR).

[0027] A PRHR has the same overall structure, whether for air or water cooling: a cooling circuit is arranged at the outlet of the steam generator (SG) of the PWR reactor. Thus, instead of sending the steam from the secondary circuit into the turbine, the steam is sent into a parallel circuit where it is cooled and condensed either by an air condenser or by a water condenser.

[0028] A first natural circulation loop allows the calories to be transferred from the core to the steam generator, then a second loop from the steam generator to a condenser. Thus the evacuation of the residual power emitted by the reactor core is ensured by means of the steam generator and two natural circulation loops which are therefore passive.

[0029] An example of an air condenser as a PRHR already implemented is that of the REP VVER TOI reactor, the thermal and electrical powers of which have been mentioned above: the air condenser is in the form of a single-tube exchanger with circular fins generally shaped like a serpentine.

[0030] The advantages of an air condenser, such as this one, lie in the fact that air is an inexhaustible cold source (in an open environment) and naturally present. This air condenser technology is therefore completely independent of the cooling time, and there is no phenomenon of progressive loss of cold source.

[0031] The major drawback of this technology is the volume of the air exchangers. Indeed, since the heat exchange coefficients with air are low, the volumes and surfaces required for the air exchangers are very large, and the thermal extraction performances are very dependent on weather conditions.

[0032] For example, the PRHR of the VVER TOI is sized to be able to withdraw a residual power equal to 2% of the nominal thermal power of the reactor, i.e. a power of 64 MWth. To achieve such a power, it was necessary to install a number of 16 units with an exchange surface equivalent to several thousand square meters, located in the upper part of the nuclear installation.

[0033] As mentioned above, the removal of the residual power necessary to ensure the cooling of the core of a PWR reactor can be ensured by a water condenser, as illustrated schematically in [Fig.2]. The reactor core 1 is connected to a steam generator 2 (SG) and the removal of the residual power is ensured by a closed passive natural circulation loop 3 which includes the SG to a water condenser 4 immersed in a water reserve or pool 5 placed at a height. Thus, this loop 3 makes it possible to transmit the calories from the SG to the water reserve 5. During the removal of the residual power, this reserve 5 rises in temperature until the water boils. The water evaporates into the air at atmospheric pressure with certain kinetics.

[0034] Many current projects use water as a cold source for the PRHR, including:

[0035] - the AP-600 and AP-1000 project of the American company Westinghouse;

[0036] - the Hualong-1 project of the Chinese companies China General Nuclear Power Cor poration (CGNPC) and the China National Nuclear Company (CNNC);

[0037] - the VVER1200 project of the Russian company ROSATOM, in the version of passive safety condensers with cold source water.

[0038] This residual power evacuation system has major drawbacks.

[0039] First of all, the presence of a water source at height complicates and burdens civil engineering due to the need for this cold safety source to keep a structure intact in the event of an extreme attack such as a major earthquake or collision by an aircraft.

[0040] Furthermore, due to the evaporation effect of the water, the cooling time of the steam generator is directly linked to the volume of the pool: the greater the volume of water, the longer the cooling time. For example, the HPR1000 reactor, with a nominal thermal power of 3060 MW, includes a PRHR whose dimensioning has been designed to allow cooling for 72 hours, which implies a pool volume of 2300 m3: [3].

[0041] Thus, the problem with this system lies in a necessary compromise between the civil engineering constraints of the swimming pool and the cooling duration ensured, typically 72 hours minimum.

[0042] An interesting configuration for emergency cooling of a steam generator (SG-ECS, an English acronym for "Steam Generator Emergency Cooling System") has already been proposed for the VVER TOI reactor project, notably in the publication [4]. This configuration is illustrated schematically in [Fig.3]. Pool 5 is arranged on the ground and the SG-ECS comprises in series three closed-loop cooling circuits:

[0043] - the first comprises the steam generator 2, a first water pump 30 (steam condensate of the secondary circuit), and a fluid circuit of a first liquid / liquid exchanger 31; - the second comprises the other fluid circuit of the first exchanger 31, a second water pump 32 and a fluid circuit of a second liquid / liquid exchanger 33, - the third includes the other fluid circuit of the second exchanger 33, a third water pump 34 and the pool 5.

[0044] This safety system is effective and autonomous but not passive: the circulation of the secondary fluid is carried out in a non-gravity manner, from the steam generator 2, to the intermediate exchanger 31. The heat transfer between the steam generator 2 and the cold source on the ground is carried out thanks to the pumps 30, 32, 34, and thanks to the intermediate exchangers 31 and 33. Thus, this system requires a fairly large external energy supply to power the three pumps used 30, 32, 34. This energy supply to electrically power the pumps 30, 32, 34 is carried out by emergency internal combustion engines or possibly gas turbines.

[0045] Exactly the same problem exists for the evacuation of residual power from a boiling water reactor (BWR), which notably led to the meltdown of the core of several units at the Fukushima Daiichi power plant. In this case, it is no longer steam from the secondary circuit of a steam generator (SG), but directly from the reactor vessel that must be cooled and condensed to evacuate the residual power from the core. The cold source necessary for the condensation of the primary steam and its cooling must also be high above the reactor core, and of large volume. The design of this cold source, in the case of the boiling water reactors involved in the Fukushima disaster, did not allow for passive operating autonomies of 72 hours to be achieved, as is currently the case for the majority of reactors.

[0046] Depending on the accident procedures implemented, this may also be the cooling and depressurization system of the containment enclosure of the pressurized or boiling water reactor, which will then serve as the ultimate means of evacuating residual power, in particular in the event of a primary circuit being deliberately opened (so-called "forced-open" configuration in the ultimate scenario) or not (situation of loss of primary coolant following a primary breach type accident). We then speak of the ultimate cold source as that dedicated to the evacuation of the residual power associated with such a cooling means.

[0047] In the two situations mentioned above, these two types of passive safety condensers can only operate over a long period if a sufficient quantity of cold water source can collect the thermal power necessary for cooling the reactor core.

[0048] As for the application concerning pressurized water reactors, this cold source must be located at a height relative to the assembly formed by the reactor vessel and its containment enclosure, in order to establish a natural circulation allowing the evacuation of thermal power from the core of the reactor or the center of the containment enclosure.

[0049] Generally speaking, the natural circulation of a single-phase or two-phase fluid is possible as long as the cold source increasing the density of the fluid is located at a higher altimetry than the hot source lowering the density of this same fluid. Otherwise, there is thermal stratification and blocking of the natural circulation.

[0050] Thus, an autonomous cold source supply device would make it possible to considerably extend the operating autonomy of this type of safety system, compared to an operation of a few hours due to the constraints of restrictions on the volume of water at height.

[0051] By way of illustration, [Fig.2] of publication [5] gives an idea of ​​the necessary volumes of cold source in height dedicated to the operation of the passive containment cooling system ("Passive Containment Cooling System" in English with the acronym PCCS) on the one hand dedicated to the ultimate evacuation, and of the water condenser ("Isolation Condenser") on the other hand dedicated to the safety evacuation.

[0052] The use of an Organic Rankine Cycle (ORC) machine has already been considered in addition to a cooling system for a pressurized water reactor (PWR) in an accident situation.

[0053] As explained previously, the problem with water PRHRs lies in the relationship between pool volume and cooling time.

[0054] Also, a solution to this problem is to remove part of the energy accumulated in the pool via an exchanger. This exchanger is then used as an evaporator of an ORC. The condenser of the ORC circuit is an air condenser (air condenser).

[0055] This solution makes it possible to use the power produced by the PORC turbine by the turbine-generator coupling, to power the PORC pump, which creates an autonomous system making it possible to evacuate part of the heat stored in the pool.

[0056] Such an ORC circuit therefore makes it possible to recover part of the energy stored in the form of heat in the swimming pool, and to evacuate / recover it in a dedicated circuit, and therefore to limit the quantity of water evaporated by the swimming pool, and thus to extend the cooling time of the swimming pool.

[0057] Thus, patent application WO2012 / 145406 proposed such a solution but for a different field of application. Indeed, the calories supplied to the pool come from spent nuclear fuels still producing heat. Thus, this technology, applied to a PWR reactor, can overcome certain problems previously explained. Indeed, part of the calories stored in the pool can be removed by an ORC circuit, which makes it possible to extend the time taken to evacuate the residual power from the core for a given pool volume.

[0058] However, although it makes it possible to improve the ratio between cooling time and pool volume, the effectiveness of this technology is dependent on the volume of the exchangers that allow the power to be evacuated to the ultimate cold source (air). Indeed, for this system to be truly functional over the entire cooling period of a REL reactor, the power extracted by the exchanger from the pool cooling cycle would have to be of the same order of magnitude as the power exchanged between the pool and the reactor.

[0059] However, as explained previously, in the case of the VVER TOI reactor, the residual power of the reactor is of the order of several tens of MW.

[0060] Thus, setting up an ORC cycle as proposed by the aforementioned application, removing all, or at least a large part, of the power exchanged between the pool and the reactor would require colossal installation volumes, especially for the final air exchanger.

[0061] In other words, although it makes it possible to extract the residual power from the reactor core for a given time longer than without an ORC cycle, the system proposed in patent application WO2012 / 145406 remains of very limited real use and really effective for residual powers of around a hundred kW.

[0062] Patent application WO2013 / 019589 proposes a similar solution, namely cooling spent nuclear fuels by immersing them in a water reserve and using the calories from this water reserve to operate an ORC cycle or a Stirling cycle. This patent application also proposes adding a thermoelectric module which will use the heat produced by the spent fuels to transform it into electricity.

[0063] The originality of the solution according to WO2013 / 019589 lies in the use of the electricity produced by these different systems, in addition to the calories removed from the swimming pool, by implementing two water pumps, one of which directs the water from the reserve (swimming pool) to the level of a fan, placed high up, to cool it and the other which pumps water from another water reserve to compensate for the evaporation of the water from the swimming pool.

[0064] Thus, thanks to these water pumps, there is no longer a direct link between the cooling time and the volume of the pool since a dedicated pump allows a constant supply of water to the pool.

[0065] However, the solution according to WO2013 / 019589 has several drawbacks.

[0066] Firstly, the exchangers of the cold source of the Stirling cycle or the ORC are air exchangers, and therefore as explained previously, these exchangers can be of a very large volume and necessarily located in the upper part.

[0067] Furthermore, air exchangers have the characteristic of being highly dependent on the outside temperature and therefore its variability. To ensure their reliability, it is therefore necessary for the system to be able to adapt to temperature variations in the geographical area of ​​the power plant.

[0068] There is therefore a need to improve the residual power removal systems (EPUR) of light water nuclear reactors (REL), in particular pressurized (REP) or boiling water (REB), requiring residual power removal devices over long periods, in order to overcome the aforementioned drawbacks by using an ORC machine (cycle). Statement of the invention

[0069] To this end, the invention relates, in one of its aspects, to a light water nuclear reactor (LWR), in particular a pressurized water reactor (PWR) or a boiling water reactor (BWR), comprising:

[0070] - a reactor core;

[0071] - a cooling circuit comprising a heat exchange means, connected in closed loop to a means of drawing steam from the primary or secondary circuit of the reactor and a first pump for supplying water to the means of the primary or secondary circuit from the heat exchange means;

[0072] - a system for evacuating at least part of the residual power of the core reactor, the system comprising:

[0073] a first water reserve or pool, arranged below the steam generator, in which the heat exchange means is immersed so that the water contained in the latter cools the steam coming from the collection means,

[0074] an organic Rankine cycle (ORC) machine comprising:

[0075] - an expander,

[0076] - a condenser,

[0077] - a second pump,

[0078] - an evaporator arranged in contact with the swimming pool so that the latter constitutes the hot source of PORK,

[0079] - a fluid circuit in which a working fluid circulates in a closed loop, the fluid circuit connecting the expander to the condenser, the condenser to the second pump, the second pump to the evaporator, and the evaporator to the expander;

[0080] a second water reserve, separate from the swimming pool, and a third pump connected to the second water reserve and to the condenser of the ORC to supply the latter with water, as a cold source of the ORC.

[0081] For a pressurized water nuclear reactor (PWR), according to a first embodiment, the PWR reactor comprises a cooling circuit comprising a steam generator and a water condenser immersed in the pool and connected in a closed loop to the steam generator.

[0082] For a pressurized water nuclear reactor (PWR), according to a second embodiment, the means for drawing off the residual power present in the circuit The primary circuit is a liquid / liquid exchanger, and the heat exchange medium is a water exchanger immersed in the pool, so that the water contained in the latter cools the water in the primary circuit circulating in the liquid / liquid exchanger.

[0083] For a boiling water nuclear reactor (BWR), according to a first embodiment, the BWR reactor comprises a cooling circuit comprising:

[0084] - a primary steam sampling on the supply line to the turbine of the reactor;

[0085] - a water condenser immersed in the pool and connected in a closed loop to the meadow steam lifting.

[0086] For a pressurized water nuclear reactor (PWR) or a boiling water nuclear reactor (BWR), according to another embodiment, the means for removing the residual power from the reactor core may be a system for depressurizing the steam present in the containment enclosure and the heat exchange means may be a water exchanger immersed in the pool or a direct withdrawal of water from the pool on the one hand, and a condenser in the containment wall in direct contact with the steam present in the containment enclosure of the reactor on the other hand.

[0087] Advantageously, the first water reserve or pool is arranged on or in the ground.

[0088] Advantageously again, the second water reserve is arranged in a part lower than the swimming pool, advantageously on or in the ground.

[0089] The PORC evaporator can be immersed in the pool or removed from it.

[0090] Preferably, the submerged evaporator is a tubular exchanger or a plate exchanger.

[0091] According to an advantageous embodiment, the reactor further comprises a refrigeration cycle comprising:

[0092] - a compressor;

[0093] - a condenser connected to the second pump to supply the latter with water;

[0094] - a relaxation organ;

[0095] - an air evaporator;

[0096] - a fluid circuit in which a working fluid circulates in a closed loop, the fluid circuit connecting the compressor to the condenser, the condenser to the expansion member, the expansion member to the air evaporator, and the air evaporator to the compressor.

[0097] Advantageously, the condenser of the refrigeration cycle is the condenser of the ORC.

[0098] Advantageously again, the working fluid of the refrigeration cycle is that of the ORC.

[0099] According to an advantageous embodiment variant, the shaft of the ORC expander is coupled to the compressor shaft of the refrigeration cycle.

[0100] Preferably, the reactor comprises batteries adapted for the electrical start-up of the second pump intended to supply the condenser with a cold source. ORC, electrical components of the ORC, and where applicable the refrigeration cycle. Preferably, the reactor includes batteries intended for the operation of the first pump in order to ensure the residual power evacuation function during the period preceding the start-up of the ORC cycle.

[0101] Thus, the invention firstly implements a safety condenser system using as a cold source, a water reserve or pool in which the condenser is immersed and which is arranged below the reactor core, preferably on the ground.

[0102] By "arranged on the ground", it is necessary to understand here and within the framework of the invention an in-ground or "above-ground" swimming pool which rests on the ground.

[0103] This pool allows the residual power to be evacuated from the reactor core.

[0104] However, as explained in the preamble, this architecture is dependent on the volume of the swimming pool: the cooling time of the swimming pool is proportional (or directly linked) to its volume and is therefore limited.

[0105] To overcome this, the invention essentially consists of installing an ORC machine and an additional water reserve, separate from the pool, the energy stored in the pool being the hot source for the PORC evaporator, the additional water reserve directly feeding the ORC condenser by a dedicated pump to constitute the cold source for the ORC condenser.

[0106] In this way, water losses through evaporation from the swimming pool are compensated by bringing water from the additional reserve, advantageously in a lower part, preferably at ground level.

[0107] A major advantage of arranging the cold source from the ground is the very significant simplification of the civil engineering dedicated to the support and protection of this volume of safety cold source in the upper part of the nuclear installation, and the reduction in construction and maintenance costs, and the resulting earthquake resistance studies.

[0108] Having the pool on the ground allows it to have a large volume because there is no significant civil engineering to be planned and therefore to have a significant cold source for the ORC machine. Indeed, the positioning of the pool on the ground allows a direct supply of cold source without having to implement a pumping system which brings water to a height.

[0109] The use of the first pump to convey the secondary condensed water from the ground pool causes an increase in the electrical power to be produced in a configuration where the pool would be at height. However, the presence of this significant cold source on the ground, coupled with an equally significant hot source, makes it possible to produce electrical power that is more than sufficient for these situations.

[0110] Although not passive in the exact sense of the term due to the presence of the cir pumps water circulation and the expander (turbine), the closed cycle secondary safety cooling according to the invention has the major advantage of being significantly more efficient than a passive system using a natural two-phase flow from the steam generator (REP) or the steam collection line (REB).

[0111] Furthermore, the system according to the invention can easily be adjusted by controlling the power of the first pump, which returns the secondary condensates to the steam generator (REP) or the primary condensates to the reactor vessel (REB). This thus allows an operator of a nuclear installation which would integrate a system according to the invention to be able to control the cooling gradient per hour of the reactor, which is desirable for incident phases likely to recur several times in the life of the nuclear unit, or even for normal cooling phases of the reactor.

[0112] The residual power evacuation system according to the invention differs from systems according to the state of the art, in particular by the following aspects:

[0113] - unlike existing systems which use air as a cold source, the water conducted from the lower part from the additional water reserve serves as a cold source for the PORC condenser; - the possibility of having a very substantial cold source due to the possible positioning of the swimming pool and the additional water reserve on the ground; - this water brought from the additional water reserve can also advantageously serve as a cold source for the condenser of a refrigeration cycle which aims to produce cooling power, for example, to cool the ORC expander and therefore ensure better autonomy and reliability of the system or to cool other safety equipment, for example an electrical or control cabinet, a control room, etc.

[0114] A major advantage of a system configuration according to the invention, compared to the geometries of existing systems, is therefore the use of the water conveyed to supply the evaporation pool, as a cold source for the condenser of the ORC cycle and advantageously of a refrigeration cycle.

[0115] Thus, a configuration of a system according to the invention allows the implementation of a plate water exchanger as an evaporator of the ORC, which must certainly be remote from the pool but whose volume is much smaller than air condensers of equivalent power. As an indicative example, a plate water exchanger has a convection exchange coefficient improved by a factor of 50 to 100 compared to a condenser whose fluid is air.

[0116] The use of a water exchanger makes it possible to lower the condensation pressure of the ORC fluid in the ORC cycle and therefore, the efficiency is increased.

[0117] Using pumped water as a cold source for the ORC cycle and advantageously for the refrigeration cycle, which is combined, also makes it possible to greatly increase the reliability of the system: reducing the volumes of the exchangers makes them less vulnerable to external attacks, whether natural or malicious.

[0118] In addition, water / water plate condensers are well-known state-of-the-art exchangers, possessing great reliability (an essential criterion in the nuclear field).

[0119] Also, the fact that the cold source of all the cold source exchangers (reactor cooling cycle, ORC cycle, refrigeration cycle) is water, avoids the use of an additional cold source, air in the state of the art.

[0120] As explained previously, an air exchanger is extremely dependent on the ambient air temperature. Thus, using water from a reserve placed in the lower part, as a cold source for the ORC cycle, makes it possible to be less dependent on the outside temperature and therefore on its variations.

[0121] In fact, with the invention, there is no longer any power limitation due to the cold source of the cycle. Indeed, the size of the water exchanger and the temperature conditions on the cold source side are not as limiting as with an air condenser according to the state of the art, the size of which we will seek to minimize, not to mention the air temperature mentioned above.

[0122] The invention therefore makes it possible to produce significant electrical power, and therefore the possibility of cooling and condensing large quantities of secondary steam from a REP or primary steam from a REB for small installation volumes.

[0123] The addition of a refrigeration cycle to the ORC cycle according to the invention makes it possible to cool the expander of the ORC cycle, as well as other components, for example power electronics to be cooled and therefore to increase the autonomy and reliability of the system. A single condenser can advantageously be common to the ORC cycle and the refrigeration cycle, which is possible with working fluid flows which are either in series or in fluid parallel.

[0124] The surplus electricity from the ORC cycle according to the invention can cover not only the needs described above but also other electrical safety needs of the installation, such as the electrical supply of control or measuring devices, cooling, etc.

[0125] The system according to the invention involves the implementation of batteries necessary for starting the system. These batteries are used in particular for starting the pump dedicated to the return of primary or secondary condensates to the steam generator (REP) or the steam collection (REB). The energy accumulated in these batteries can be very limited, multiple redundant groups ensure high reliability.

[0126] Ultimately, a nuclear reactor with a system according to the invention has many advantages among which we can cite:

[0127] - significant improvement of the existing cooling system of a nuclear reactor, in particular that of a PWR comprising a steam generator, a closed loop natural convection circuit and a cooling pool; - reliable and autonomous system; - possibility of having a nuclear installation with low volumes in the upper part, which alleviates civil engineering and cost constraints; - possibility of controlling the reactor cooling gradient by modifying the speed of the secondary condensate return pump of the REP or primary condensate return pump.

[0128] Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0129] [Fig.l] [Fig.l] illustrates in the form of a curve the temporal decrease of the residual power of a nuclear reactor according to the state of the art known under the designation VVER TOI.

[0130] [Fig.2] [Fig.2] is a schematic view of a passive system for removing residual heat from a PWR type nuclear reactor core according to the state of the art.

[0131] [Fig.3] [Fig.3] is a schematic view of a PWR type nuclear reactor with ground water pool and passive system for removing residual heat from a reactor core according to the state of the art.

[0132] [Fig.4] [Fig.4] is a schematic view of a PWR type nuclear reactor with ground water pool and passive system for removing residual heat from a reactor core according to the invention.

[0133] [Fig.5] [Fig.5] is a schematic view illustrating an embodiment of the invention further comprising a refrigeration cycle.

[0134] [Fig.6] [Fig.6] is an entropy diagram Ts of the ORC cycle and the refrigeration cycle of a system as according to [Fig.5].

[0135] [Fig.7] [Fig.7] is a schematic view illustrating a first variant of a system according to the invention.

[0136] [Fig.8] [Fig.8] is a view illustrating a second variant of a system according to the invention.

[0137] [Fig.9] [Fig.9] is a schematic view illustrating another embodiment according to the invention with a system for depressurizing the steam present in the containment enclosure of a REB or PWR reactor.

[0138] [Fig. 10] [Fig. 10] is a schematic view illustrating a first variant of a heat exchange means according to the invention for a REB or PWR reactor.

[0139] [Fig. 11] [Fig. 11] is a schematic view illustrating a first variant of a heat exchange means according to the invention for a REB or PWR reactor. Detailed description

[0140] Throughout the present application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a cooling pool of a nuclear reactor filled with water, as it is in a horizontal operating configuration and arranged on the ground, that is to say buried or "above ground" resting against the ground.

[0141] Figures 1 to 3 have already been detailed in the preamble, they will therefore not be commented on below.

[0142] For the sake of clarity, the same element according to the invention and according to the state of the art and designated by the same numerical reference in all of Figures 1 to 8.

[0143] It is specified that in figures 4 to 8 relating to the invention only part of the cooling system of a PWR nuclear reactor core is represented, namely the steam generator connected in a closed loop to a water exchanger immersed in the cooling pool.

[0144] It is also specified that the dotted lines designate the electrical supply lines of the various electrical components while those in solid lines designate the fluid lines.

[0145] [Fig.4] illustrates an autonomous system for evacuating at least part of the residual power of a PWR reactor according to the invention.

[0146] The system firstly comprises the cooling pool 5 arranged on the ground, a water condenser 4 immersed in the pool so that the water contained in the latter cools the steam coming from the secondary circuit of the reactor, and a first pump 30 for supplying water to the steam generator from the water condenser.

[0147] It also includes an organic Rankine cycle (ORC) machine 6 comprising:

[0148] - an expander 60;

[0149] - a condenser 61,

[0150] - a second pump, called ORC pump, which is a working fluid pump 62;

[0151] - an evaporator 63 arranged relative to the pool 5 so that the latter constitutes the hot source of the ORC;

[0152] - a fluid circuit 64 in which a working fluid circulates in a closed loop.

[0153] As illustrated, according to the invention, the fluid circuit 64 connects the expander 60 to the condenser 61, the condenser 61 to the pump 62, the pump 62 to the evaporator 63, and evaporator 63 to expander 60.

[0154] A second water reserve forming a general pool 7 contains the entire cold source dedicated to cooling the reactor, and supplies pool 5 dedicated to the operation of the ORC cycle and containing the safety condenser 4 and the ORC evaporator 63.

[0155] The water from the pool 7 serves as a cold source for the condenser exchanger 61.

[0156] The water from the pool 7 is slightly heated by the condenser 61 before to be injected into the pool 5 by means of a third pump which is a water pump 8. This pump 8 supplies a dedicated fluid line 65 to compensate for the evaporation of the pool 5 receiving the residual reactor power.

[0157] The expander 60 may typically be a turbine, a spiral, screw, piston, etc. expander.

[0158] The condenser 61 is typically a plate condenser.

[0159] The pump 62 is typically a centrifugal pump, or a diaphragm pump, screw pump, etc.

[0160] The machine 6 may comprise a buffer tank 66, that is to say a reserve of quantity of working fluid which in particular allows adequate operation of the ORC in variable mode. As illustrated in [Fig.4], this buffer tank 66 can be arranged upstream of the pump 62.

[0161] In the mode illustrated in [Fig.4], the evaporator 63 is a tubular evaporator immersed vertically in the pool 5.

[0162] In the advantageous embodiment of [Fig.5], a refrigeration cycle 9 is further provided comprising:

[0163] - a compressor 90;

[0164] - a condenser 61, which is that of the ORC, connected to the water pump 8 to supply in water the latter;

[0165] - a trigger member 92;

[0166] - an air evaporator 93,

[0167] - a fluid circuit 94 in which a working fluid circulates in a closed loop.

[0168] The fluid circuit 94 connects the compressor 90 to the condenser 61, the condenser of the ORC 61 to the expansion member 92, the expansion member to the air evaporator 93, and the air evaporator 93 to the compressor 90.

[0169] The expansion member 92 may be a valve or preferably a turbine, an ejector,

[0170] Like the ORC cycle 6, the refrigeration cycle 9 can also include a buffer tank forming a reserve of working fluid in this cycle.

[0171] Batteries 10 may be provided for the electrical starting of the various pumps 30, 62, 8, the electrical components of the ORC and, where appropriate, the refrigeration cycle 9. More precisely, the batteries may initially be used for supply pump 30 of the cooling circuit, then in a second stage, when the water reserve 5 is boiling, to allow the ORC to start, that is to say the start of pump 62 and the filling pump 8.

[0172] An example of dimensioning, following an accident situation in the case of a PWR reactor with a power of 3200 MWth, is provided below.

[0173] The working fluid of the ORC cycle is an organic fluid whose evaporation temperature is lower than that of boiling water by approximately 100°C under atmospheric pressure. Examples include Novec649, HFE7000, HFE7100, etc. Many other organic fluids can be considered, such as alkanes, HFC, HFO, HFCO, HFE, as well as some other fluids (NH3, CO2) and all their mixtures.

[0174] The fluid used in the sizing simulation is HFE7100 and it is advantageously used in both the ORC cycle 6 and the refrigeration cycle 9.

[0175] In this example, temperature or water level sensors of the pool 5 make it possible to detect the state of complete saturation of the pool 5 and the start of loss of liquid level by boiling. The indicated value of 50m3 corresponds to a typical 5-minute operating time of a condenser discharging 60 MW from the steam generator. At that moment, the pump 8 injects a flow rate corresponding exactly, by dimensioning, to the replacement of the evaporated water in the pool 5, evaporation produced by the 60MW exchanged.

[0176] The pump 30 is regulated in flow to produce the 60 MW of heat exchange of the condenser 4, and the pumps 8 and 30 are thus connected by the power transfer function of the condenser 4, knowing that the boiling of one kilo of water from the swimming pool 5 requires approximately 2.25 MW of thermal power deposited by the condenser 4.

[0177] The sizing relating to the swimming pool 5 and water reserve 7 is summarized in table 1 below.

[0178] [Tableauxl] Dimension Value Total volume of the pool 5 50 m3 Water reserve 7 depending on the maximum autonomous operating autonomy of the reactor Height difference between pool 5 and the steam generator 2 30 m

[0179] Information relating to the operating time of the pool is summarized in Table 2 below.

[0180] [Tables2] O peration Duration (min) Pool 5 enters saturation under an operating regime of 60MW condenser T0 +5 Start of the ORC T0 +5 + a few seconds

[0181] The flow rates are given in the following table 3:

[0182] [Tables3] Flow rate Value (kg / s) ORC working fluid flow rate 3.5 Pumped water flow rate (cold source) 30 ORC dedicated refrigeration cycle working fluid flow rate 0.2

[0183] We now indicate the correspondence between the flow rates of pumps 8 and 30.

[0184] In this sizing example, the condenser 61 without subcooling condensates, is controlled in power at 60 MWTh by the control command of the installation which orders a cooling of reactor of x degrees / h under a steam pressure GV of 60 bar.

[0185] The flow rate of the pump 30 is equal to the ratio between the power and the latent heat at saturation under 60 bar, i.e. equal to 60 MW / l, 57 MJ / kg or 38.2 kg / s. This gives a volume flow rate of the pump 30 of 180 m3 / h.

[0186] Considering a pump discharge overpressure of 3 bar for the condensate rise and the pressure losses in the circuit, this gives a hydraulic power of 15kW, or with a pump efficiency equal to 0.7, an electrical supply power of 22 kWe. The battery associated with the operation of pump 30 must therefore be able to supply 22kWe for at least 5 minutes, before the ORC takes over.

[0187] The flow rate of pump 8 associated with this operating point is deduced directly by the relationship: the flow rate of pumped water is equal to the ratio between the power and the latent heat at atmospheric pressure, i.e. equal to 60MW / 2.25MJ / kg, or 27 kg / s. The associated volume flow rate of pump 8 is therefore approximately 100 m3 / h. This pump needs to be powered by battery for starting the ORC (cold source supply).

[0188] The external temperatures are given in the following table 4:

[0189]

[0190]

[0191]

[0192]

[0193]

[0194]

[0195]

[0196]

[0197]

[0198]

[0199] [Tables 4] Temperature Value (°C) Average hot source temperature 100 Cold source temperature 30 Cold source temperature (ORC outlet) 34 The internal pressures are given in the following table 5: [Tables5] Pressure Value (bar) High cycle pressure ORC 6 2.4 Low cycle pressure ORC 6 0.45 The powers of the exchangers are given in the following table 6: [Tables] Power Value (kw) Condenser power ORC (61) 600 Evaporator power ORC (63) 550 The electrical powers are given in the following table 7: [Tables 7] Electrical power Value (kw) ORC pump power (62) 1.4 Water pump power (8) 0.6 Water pump power (30) 33 Electric turbine power (60) 35 Thus, under all these operating conditions, the volume of the exchangers to be sized is summarized in the following table 8: [Tables 8] Volumes Value (m3) Condenser volume ORC (HFE7100 / Water) 0.02 Evaporator volume ORC (HFE7100 / Water) 3.66 The Ts diagram of the ORC and refrigeration cycle is shown in [Fig.6]. One of the possible variants of the configuration according to [Fig.5] consists in coupling the shaft 11 of the turbine 60 of the ORC 6 cycle and the shaft of the compressor of the fri cycle This configuration shown in [Fig.7] eliminates the need to power the refrigeration cycle compressor electrically and thus saves energy (electromechanical conversions).

[0200] A second variant of the system consists of sharing more components between the ORC and refrigeration cycles: the working fluid, part of the piping, the condenser 61 as already illustrated.

[0201] Another possible variant is not to use a submerged tubular evaporator as shown in [Fig.4] but a remote evaporator, for example of the plate type. To do this, it is necessary to convey the water from the reserve into a pipe, via a pump 14 as shown in [Fig.8]. This configuration allows a reduction in the volume of the hot exchanger, a reduction in the work of installing the exchanger on the swimming pool or even the possibility, as in the previous configuration, of operating the ORC thanks to an additional hot source. It should be noted that by returning the water to the outlet of the evaporator by mixing it with that coming from the ORC condenser, there is only one need for a single tapping on the swimming pool instead of two in the other configurations and variants.

[0202] The invention is not limited to the examples which have just been described; it is possible in particular to combine characteristics of the illustrated examples within non-illustrated variants.

[0203] Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0204] The EPUR system which has just been described in relation to a pressurized water nuclear reactor can be implemented in a boiling water nuclear reactor (BWR).

[0205] Generally speaking, the invention applies to any pool 5 which may constitute the cold source intended to cool a PWR reactor core or a REB reactor core or to cool and / or depressurize the primary containment enclosure of a PWR reactor or a REB reactor.

[0206] Thus, if in the examples illustrated, the means of evacuating the residual power from the reactor core passes through the steam generator, this means can just as easily be a condenser installed in the containment enclosure whether for a PWR reactor, or for a REB reactor.

[0207] For example, for a PWR reactor, we can refer to the configuration of the ambient condenser panels of the HPR1000 project (“Passive containment heat removal”) or to the publication [6] which describes an optimized condenser mounted against the containment wall (“Passive containment cooling System”). For a REB reactor, we can refer to the configuration of the advanced KERENA reactor of the cooling condensers (“Containment cooling condensates”) of the enclosure.

[0208] More generally, for a PWR or a BWR, the means for removing the residual power from the reactor core may be a system for depressurizing the steam present in the containment enclosure ([Fig.9]), and the heat exchange means may be a water exchanger 4 immersed in the pool 5 (closed loop configuration of [Fig. 11], taken from reference [6]) or a direct withdrawal of water from the pool 5 (closed loop configuration of [Fig.10], taken from reference [6])) on the one hand, and a containment wall condenser 11 in direct contact with the steam present in the containment enclosure 100 of the reactor on the other hand.

[0209] Pool 5 may be the power source for a spray ramp of a spray circuit in the containment (EAS) which, in the event of an accident leading to a significant increase in pressure in the reactor building, makes it possible to reduce this pressure and thus preserve the integrity of the containment. For a PWR reactor, reference may be made to the configuration of spray ramps internal to the primary containment of the HPR 1000 project or external to the primary containment of the AP1000 project.

[0210] Pool 5 may be an overpressure pool of a REB reactor, for example a toroidal-shaped steel pool in a Mark I type reactor, the water vapor accidentally coming from the reactor core is condensed.

[0211] Pool 5 can also be a pool of the safety injection circuit of a PWR reactor, like that of the HPR-1000 project, with the acronym IRWST (“In containment Refueling Water System Tank”). List of cited references

[0212] [1]: https: / / www-pub.iaea.org / MTCD / Publications / PDF / TE-1785_web.pdf.

[0213] [2]: S. Kadalev et al, 2014, Annals of Nuclear Energy, vol. 72, p. 182-188;

[0214] [3]: DC Sun, Y. Li, Z. Xi, YF Zan, PZ Li, WB Zhuo, "Experimental evaluation of safety performance of emergency passive residual heat removal System in HPR1000”, Nuclear Engineering and Design, Volume 318, 2017, Pages 54-60, ISSN 0029-5493, https: / / doi.Org / 10.1016 / j.nucengdes.2017.04.003.

[0215] [4]: ​​Mikhail Maltsev, 2015, “Additional information on VVER Gen 111 modem technology” https: / / www.oecd-nea.org / upload / docs / application / pdf / 2020-07 / ii-la-maltesev.pdf

[0216] [5]: David Hinds and Chris Maslak, “Next-génération nuclear energy: The ESBWR” Nuclear News. January 2006.

[0217] [6]: https: / / www-pub.iaea.org / MTCD / Publications / PDF / te_1624_web.pdf

Claims

1.

2.

3. Claims Light water nuclear reactor (LWR), in particular a pressurized water reactor (PWR) or a boiling water reactor (BWR), comprising: - a reactor core (1); - a cooling circuit comprising a heat exchange means connected in a closed loop to a means for drawing steam from the primary or secondary circuit of the reactor and a first pump (30) for supplying water to the means of the primary or secondary circuit from the heat exchange means; - a system for evacuating at least part of the residual power from the reactor core, the system comprising: a first water reserve or pool (5), arranged below the steam generator, in which the heat exchange means is immersed so that the water contained in the latter cools the steam coming from the collection means; an organic Rankine cycle (ORC) machine (6) comprising: - an expander (60); - a condenser (61), - a second pump (62); - an evaporator (63) arranged in contact with the pool so that that the latter constitutes the hot source of PORK; - a fluid circuit (64) in which a working fluid circulates in a closed loop, the fluid circuit (64) connecting the expander (60) to the condenser (61), the condenser (61) to the second pump (62), the second pump (62) to the evaporator (63), and the evaporator (63) to the expander (60); a second water reserve (7), separate from the pool, and a third pump (8) connected to the second water reserve and to the condenser of the ORC to supply the latter with water, as a cold source of the ORC. Pressurized water nuclear reactor according to claim 1 comprising a cooling circuit comprising a steam generator and a water condenser immersed in the pool and connected in a closed loop to the steam generator. Pressurized water nuclear reactor according to claim 1, the means for drawing off the residual power present in the circuit primary being a liquid / liquid exchanger, and the heat exchange means being a water exchanger immersed in the pool, so that the water contained in the latter cools the water in the primary circuit circulating in the liquid / liquid exchanger.

4. Boiling water nuclear reactor (BWR) according to claim 1, comprising a cooling circuit comprising: - a primary steam extraction on the supply line to the reactor turbine; - a water condenser immersed in the pool and connected in a closed loop to the steam extraction.

5. Pressurized water (PWR) or boiling water (BWR) nuclear reactor, according to claim 1, the means for removing the residual power from the reactor core being a system for depressurizing the steam present in the containment enclosure and the heat exchange means being a water exchanger (4) immersed in the pool or a direct withdrawal of water from the pool (5) on the one hand, and a containment wall condenser (11) in direct contact with the steam present in the containment enclosure (100) of the reactor on the other hand.

6. Nuclear reactor according to one of the preceding claims, the first water reserve or pool being arranged on or in the ground.

7. Nuclear reactor according to one of the preceding claims, the second water reserve being arranged on or in the ground.

8. Nuclear reactor according to one of the preceding claims, the evaporator (63) being immersed in the pool or removed from it.

9. Nuclear reactor according to claim 8, the submerged evaporator being a tubular exchanger.

10. Nuclear reactor according to claim 8, the submerged evaporator being a plate exchanger.

11. Nuclear reactor according to one of the preceding claims, further comprising a refrigeration cycle (9) comprising: - a compressor (90); - a condenser (61) connected to the third pump (8) to supply the latter with water; - an expansion member (92); - an air evaporator (93), - a fluid circuit (94) in which a working fluid circulates in a closed loop, the fluid circuit (94) connecting the compressor (90) to the

12.

13.

14.

15.

16. condenser (61), the condenser (61) to the expansion member (62), the expansion member to the air evaporator (93), and the air evaporator (93) to the compressor (90). Nuclear reactor according to claim 11, the condenser (61) of the refrigeration cycle being the condenser of the ORC. Nuclear reactor according to claim 11 or 12, the working fluid of the refrigeration cycle being that of the ORC. Nuclear reactor according to one of claims 11 to 13, the shaft (11) of the ORC expander being coupled to the shaft of the compressor of the refrigeration cycle. Nuclear reactor according to one of the preceding claims, comprising an injector arranged in a lower part of the system and connected to the third pump arranged in an upper part of the system, the injector being adapted to prime the third pump. Nuclear reactor according to one of the preceding claims, comprising batteries (10) adapted for the electrical start-up of the pumps, the electrical components of the ORC and, where appropriate, the refrigeration cycle.

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