Light water nuclear reactor (LWR), in particular pressurized water reactor (PWR) or boiling water reactor (BWR), incorporating a passive and autonomous system for the evacuation of residual power (ERP).

Abstract

Light water reactor (LWR), particularly pressurized water reactor (PWR) or boiling water reactor (BWR), incorporating a passive and autonomous residual heat evacuation (RHE) system. The invention essentially consists of installing an ORC machine and an additional water reservoir, separate from the pool. The energy stored in the pool serves as the heat source for the ORC evaporator, while the additional water reservoir directly supplies the ORC condenser via a dedicated pump, thus constituting 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), integrating a passive and 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 a passive and 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 passive cooling by natural convection 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 extracts the energy supplied, in the form of heat, by the fission of uranium nuclei, and where appropriate plutonium, in the reactor core.

[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 water in the primary circuit 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 arises in particular from a failure to evacuate the residual power from the reactors. These accident sequences were also encountered for the fuel cooling pools during the accident at Fukushima-Daüchi 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-Daüchi 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 or to the atmosphere. via an air exchanger, or to a water reserve (pool) placed at a height, in order to ensure natural convection. Such a system is referred to by the Anglo-Saxon term "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 a 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] Exactly the same problem exists for the evacuation of residual power from a boiling water reactor (BWR), which notably led to the core meltdown 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 steam from the reactor vessel that must then 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.

[0043] Depending on the accident procedures put in place, 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 a means of evacuating residual power, in particular in the event of a primary circuit being deliberately opened. (so-called "full-open" configuration in the ultimate scenario) or not (situation of loss of primary refrigerant following a primary breach type accident). We then speak of an ultimate cold source dedicated to the evacuation of the residual power associated with such a cooling means.

[0044] 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.

[0045] 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.

[0046] Generally speaking, the natural circulation of a single-phase or two-phase fluid is possible when the cold source increasing the density of the fluid is located at a higher altitude than the hot source lowering the density of the same fluid. Otherwise, there is thermal stratification and blocking of the natural circulation.

[0047] 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.

[0048] By way of illustration, [Fig.2] of publication [4] 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.

[0049] 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.

[0050] As explained previously, the problem with water PRHRs lies in the relationship between pool volume and cooling time.

[0051] 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).

[0052] 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 achieves a autonomous system allowing part of the heat stored in the pool to be evacuated.

[0053] 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.

[0054] 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.

[0055] However, although it makes it possible to improve the ratio between cooling time and pool volume, the efficiency of this technology is dependent on the volume of the exchangers which 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.

[0056] 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.

[0057] Thus, implementing 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.

[0058] 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.

[0059] 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.

[0060] 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 pool, by implementing two water pumps, one of which directs the water from the reserve (pool) to 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 pool.

[0061] 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.

[0062] However, the solution according to WO2013 / 019589 has several drawbacks.

[0063] 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.

[0064] Furthermore, air exchangers have the characteristic of being highly dependent on the outside temperature and therefore on 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.

[0065] There is therefore a need to improve the residual power evacuation systems (EPUR) of light water nuclear reactors (REL), in particular pressurized (REP) or boiling water (REB), in order to overcome the aforementioned drawbacks by using an ORC machine (cycle). Statement of the invention

[0066] 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 boiling water reactor (BWR), comprising:

[0067] - a reactor core;

[0068] - a system for evacuating at least part of the residual power of the core reactor, the system comprising:

[0069] a first water reserve or pool arranged above the reactor core; a heat exchange means immersed in the pool so that the water contained in the latter cools the steam coming from a means of drawing steam from the primary or secondary circuit of the reactor;

[0070] an organic Rankine cycle (ORC) machine comprising:

[0071] - an expander

[0072] - a first pump

[0073] - a condenser,

[0074] - an evaporator arranged in contact with the pool so that the latter constitutes the hot source of PORK;

[0075] - 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 first pump, the first pump to the evaporator, and the evaporator to the expander;

[0076] a second water reserve, separate from the swimming pool, and a second 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.

[0077] 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.

[0078] For a pressurized water nuclear reactor (PWR), according to a second embodiment, the means for drawing off the residual power present in the primary circuit is a liquid / liquid exchanger, and the heat exchange means is a water exchanger immersed in the pool, so that the water contained in the latter cools the water of the primary circuit circulating in the liquid / liquid exchanger.

[0079] For a boiling water nuclear reactor (BWR), according to a first embodiment, the BWR reactor comprises a cooling circuit comprising:

[0080] - a primary steam sampling on the supply line to the turbine of the reactor;

[0081] - a water condenser immersed in the pool and connected in a closed loop to the meadow steam lifting.

[0082] 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.

[0083] Advantageously, the second water reserve is arranged in a part lower than the swimming pool, advantageously on or in the ground.

[0084] The ORC evaporator can be immersed in the pool or removed from it.

[0085] Preferably, the submerged evaporator is a tubular exchanger or a heat exchanger. plates.

[0086] According to an advantageous embodiment, the reactor further comprises a refrigeration cycle comprising:

[0087] - a compressor;

[0088] - a condenser connected to the second pump, to supply the latter with water;

[0089] - a relaxation organ;

[0090] - an air evaporator;

[0091] - 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.

[0092] Advantageously, the condenser of the refrigeration cycle is the condenser of the ORC.

[0093] Advantageously, the working fluid of the refrigeration cycle is that of the ORC.

[0094] According to an advantageous embodiment variant, the shaft of the ORC expander is coupled to the compressor shaft of the refrigeration cycle.

[0095] According to another advantageous embodiment variant, the ORC machine and, where appropriate, the refrigeration cycle is / are arranged in a lower part of the system, below the pool.

[0096] According to yet another variant, the reactor may comprise an injector arranged in a lower part of the system and connected to the second pump arranged in an upper part of the system, the injector being adapted to prime this second pump.

[0097] Preferably, the reactor comprises batteries adapted for the electrical start of the first pump, electrical components of the ORC and, where appropriate, of the refrigeration cycle as well as of the second pump.

[0098] Thus, the invention firstly implements a passive safety condenser system using as a cold source, a water reserve or pool in which the passive condenser is immersed and which is arranged at height (above the reactor core). This pool makes it possible to evacuate the residual power from the reactor core.

[0099] 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.

[0100] To overcome this, the invention essentially consists of installing an ORC machine and an additional water reserve, separate from the swimming pool, the energy stored in the swimming pool being the hot source for the evaporator of the ORC, the additional water reserve directly feeding the condenser of the ORC by a dedicated pump to constitute the cold source for the condenser of the ORC.

[0101] In this way, water losses through evaporation from the swimming pool are compensated by supplying water from the additional reserve, advantageously in a lower part, preferably at ground level, compared to the swimming pool in the upper part.

[0102] 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.

[0103] 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:

[0104] - 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 ORC condenser - 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.

[0105] 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 in the upper part, to supply the evaporation pool, as a cold source for the condenser of the ORC cycle and advantageously of a refrigeration cycle.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] In addition, water / water plate condensers are very well-known state-of-the-art exchangers, possessing great reliability (an essential criterion in the nuclear field).

[0110] 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.

[0111] 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.

[0112] 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.

[0113] The invention therefore makes it possible to produce significant electrical power, and therefore the possibility of conducting large quantities of water from the lower part to the upper part for low installation volumes.

[0114] 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.

[0115] 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.

[0116] The system according to the invention involves the implementation of batteries necessary for starting the system. Indeed, the cold source of the ORC being the water from the additional reserve, it is necessary to start the water pump which will draw from this additional reserve to start the system. The energy accumulated in these batteries can be very limited, multiple redundant groups make it possible to ensure high reliability.

[0117] Ultimately, a nuclear reactor with a system according to the invention has numerous advantages, including:

[0118] 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;

[0119] - reliable and autonomous system; - possibility of having a nuclear installation with low volumes in the upper part, which alleviates civil engineering and cost constraints; - maintaining the passive design of the reactor's residual power removal by using a passive condenser and its high-level cold source. The removal of residual power remains controlled by the progressive evaporation of the cold source and does not pass entirely through an active system such as an ORC circuit. Only the cold source autonomy depends on the operation of this active but autonomous system. Thus, there is no loss of reliability of the residual power removal function, as in the hypothesis where the residual power would have to pass entirely through an active circuit of the ORC type: [5].

[0120] 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

[0121] [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.

[0122] [Fig.2] [Fig.2] is a schematic view of a passive evacuation system of the residual heat from a PWR nuclear reactor core according to the state of the art.

[0123] [Fig.3] [Fig.3] is a schematic view of a passive and evacuation system of the residual heat from a PWR nuclear reactor core according to one embodiment of the invention.

[0124] [Fig.4] [Fig.4] is an entropy diagram Ts of the ORC cycle and the fri cycle gorific of a system as according to [Fig.3].

[0125] [Fig.5] [Fig.5] is a schematic view illustrating a first variant of a system according to the invention.

[0126] [Fig.6] [Fig.6] is a schematic view illustrating a second variant of a system according to the invention.

[0127] [Fig.7] [Fig.7] is a schematic view illustrating a third variant of a system according to the invention.

[0128] [Fig.8] [Fig.8] 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.

[0129] [Fig.9] [Fig.9] is a schematic view illustrating a first variant of a heat exchange means according to the invention for a REB or PWR reactor.

[0130] [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. Detailed description

[0131] Throughout the present application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a water-filled cooling pool of a nuclear reactor, as it is in a horizontal operating configuration and arranged above the reactor core.

[0132] Figures 1 and 2 have already been detailed in the preamble, they will therefore not be commented on below.

[0133] 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 10.

[0134] It is specified that in figures 3 to 7 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 submerged water exchanger. in the cooling pool.

[0135] 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.

[0136] [Fig.3] illustrates an autonomous system for evacuating at least part of the residual power of a PWR reactor according to the invention.

[0137] The system firstly comprises the cooling pool 5 arranged above the reactor core, 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.

[0138] It also includes an organic Rankine cycle (ORC) machine 6 comprising:

[0139] - an expander 60;

[0140] - a condenser 61,

[0141] - a first pump, with working fluid 62;

[0142] - an evaporator 63 arranged relative to the pool 5 so that the latter constitutes the hot source of the ORC;

[0143] - a fluid circuit 64 in which a working fluid circulates in a closed loop.

[0144] As illustrated, according to the invention, the fluid circuit 64 connects the expander 60 to the condenser 61, the condenser 61 to the first pump, called the ORC pump 62, the ORC pump 62 to the evaporator 63, and the evaporator 63 to the expander 60.

[0145] 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.

[0146] The water from the pool 7 serves as a cold source for the exchanger condenser 61. The water from the pool 7 is slightly heated by the condenser 60 before being injected into the pool 5 by means of a second pump which is a water supply 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.

[0147] The expander 60 may typically be a turbine, a spiral, screw, piston, etc. expander.

[0148] The condenser 61 is typically a plate condenser.

[0149] The ORC 62 pump is typically a centrifugal pump, or diaphragm pump, screw pump, etc.

[0150] 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 regime. As illustrated in [Fig.3], this buffer tank 66 can be arranged upstream of the ORC 62 pump.

[0151] In the mode illustrated in [Fig.3], the evaporator 63 is a tubular evaporator immersed vertically in the pool 5.

[0152] The system also comprises a second water reserve 7, separate from the swimming pool, and a water pump 8 connected to the second water reserve and to the condenser 61 of the ORC to supply the latter with water, as a cold source of the ORC.

[0153] In the advantageous embodiment of [Fig.3], a refrigeration cycle 9 is further provided comprising:

[0154] - a compressor 90;

[0155] - a condenser 61, which is that of the ORC, connected to the water pump 8, to supply in water the latter;

[0156] - a trigger member 92;

[0157] - an air evaporator 93,

[0158] - a fluid circuit 94 in which a working fluid circulates in a closed loop.

[0159] 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.

[0160] The expansion member 92 may be a valve or preferably a turbine, an ejector,

[0161] Like the ORC cycle 6, the refrigeration cycle 9 can also include a buffer tank forming a reserve of working fluid in this cycle.

[0162] Batteries 10 may be provided for the electrical start-up of the various pumps 62, 8, the electrical components of the ORC and, where appropriate, the refrigeration cycle 9. More precisely, the batteries may be used for the function of starting the ORC cycle, i.e. starting the pump 62 of the ORC, and activating the submerged pump 8 which brings the cold source to the pool 5, thus making it possible to supply the cold source to the ORC cycle (condenser exchanger).

[0163] An example of dimensioning, following an accident situation in the case of a PWR reactor with a power of 3200 MWth, is provided below.

[0164] The working fluid of the ORC cycle is an organic fluid whose evaporation temperature is lower than that of boiling water, approximately 100°C under atmospheric pressure. Examples include Novec649, HFE7000, HFE7100, etc.

[0165] 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.

[0166] The fluid used in the sizing simulation is HFE7100 and it is advantageously used in both ORC cycle 6 and refrigeration cycle 9.

[0167] 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.

[0168] We wait for the pool 5 to empty by half before starting the filling pump 8. For reliability purposes, the flow rate of the pump 8 is fixed at the flow rate of loss by evaporation of the pool, at the time of its start-up.

[0169] However, knowing that the residual power of the reactor core decreases over time, from the moment pump 8 starts, pool 5 gains water inventory.

[0170] The sizing relating to the swimming pool is summarized in Table 1 below.

[0171] [Tableauxl] Dimension Value Total volume of the pool 5 1000 m3 Height of the pool 5 10 m Difference in height between pool 5 and water reserve 7 50 m

[0172] Information relating to the operating time of the pool is summarized in Table 2 below.

[0173] [Tables2] Operation Duration (h) Pool 5 enters saturation 1.5 Pool 5 is half empty / ORC 6 starts 20 Pool 5 refilled 90

[0174] The flow rates are given in the following table 3:

[0175] [Tables3] Flow Rate Value (kg / s) Flow rate of PORC working fluid 1 Pumped water flow rate (cold source) 6 Flow rate of refrigeration cycle working fluid 0.02

[0176] The external temperatures are given in the following table 4:

[0177] [Tables4] Temperature Value (°C) Average hot source temperature 100 Cold source temperature 30 Cold source temperature (refrigeration cycle outlet) 30 Cold source temperature (ORC outlet) 38

[0178] The internal pressures are given in the following table 5:

[0179] [Tables5] Pressure Value (bar) High cycle pressure ORC 6 2.7 Low cycle pressure ORC 6 0.5 High refrigeration cycle pressure 9 2.0 Low refrigeration cycle pressure 9 0.1

[0180] The powers of the exchangers are given in the following table 6:

[0181] [Tableauxô] Power Value (kw) ORC condenser power (61) 170 ORC evaporator power (63) 180 Refrigerating condenser power (61) 3 Refrigerating evaporator power (93) 2

[0182] The electrical powers are given in the following table 7:

[0183] [Tables7] Electrical power Value (kw) ORC pump power (62) 0.5 Water pump power (8) 7.5 Compressor power (90) 0.7 Electric turbine power (60) 8.6

[0184] Thus, under all these operating conditions, the volume of the exchangers to be sized is summarized in the following table 8:

[0185] [Tables8] Volumes Value (m3) ORC condenser volume (HFE7100 / Water) 0.01 Refrigeration cycle condenser volume (HFE7100 / Water) 0.0005 ORC evaporator volume (HFE7100 / Water) 0.5

[0186] The Ts diagram of the ORC and refrigeration cycle is shown in [Fig.4].

[0187] One of the possible variants of the configuration according to [Fig.3] consists of coupling the shaft 11 of the turbine 60 of the ORC cycle 6 and the shaft of the compressor of the refrigeration cycle. This configuration shown in [Fig.5] makes it possible not to need to supply the compressor of the refrigeration cycle electrically and thus saves energy (electromechanical conversions).

[0188] 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.

[0189] Another variant of this system consists of placing the ORC and refrigeration cycle in the lower part, thanks to the presence of an intermediate circuit, which makes it possible to couple on the same shaft the turbine of the ORC cycle as well as the pump 8 which makes it possible to convey the water from the lower part to the upper part. This arrangement allows for better reliability of the system, in fact the transmission of power between the turbine and the water pump 8 is purely mechanical: there is no conversion of mechanical energy into electrical energy. However, this arrangement requires the ORC to be in the lower part which makes it vulnerable to many accidental situations: flooding, etc. Furthermore, the calories from the swimming pool must be conveyed in the lower part. This particular arrangement also allows easy access for maintenance and monitoring by operating personnel.

[0190] As shown in [Fig.6], it is also possible to place the ORC in the lower part, thanks to an intermediate circuit, without coupling the water pump 8 with the turbine 60 of the ORC. This intermediate circuit then comprises an additional evaporator 67 supplied by means of a third pump 68. The advantage of this configuration will be the possibility of operating the ORC via an additional hot source 12 and valves 13 to allow maintenance / testing work on the system which increases its reliability.

[0191] Another possible variant is not to use a submerged tubular evaporator as shown in [Fig.3] but a remote evaporator, for example of the plate type. To do this, it is necessary to route the water from the reserve into a pipe, via a pump 14 as shown in [Fig.7]. This configuration allows a reduction in the volume of the hot exchanger, a reduction in the work of installing the exchanger on the 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 at the outlet of the evaporator by mixing it with that coming from the ORC condenser, only one tapping on the pool is needed instead of two in the other configurations and variants.

[0192] Another possible variant of this technology is to place a condensation injector in the lower part of the installation. Thus, it would be possible to position the water pump 8 in the upper part of the structure by activating the pumping movement thanks to the injector in the lower part. This configuration would make it possible to have the entire ORC cycle and the pump 8 in the upper part (and therefore more secure from external attacks, floods, etc.). This injector would make it possible to prime the system: by being powered by a low-capacity thermal energy reserve, this injector would send a sufficient quantity of water into the inlet pipe of the pump 8 for its priming.

[0193] The invention is not limited to the examples which have just been described; in particular, it is possible to combine characteristics of the illustrated examples within non-illustrated variants.

[0194] Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0195] 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).

[0196] 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.

[0197] 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.

[0198] 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.

[0199] 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.8]), and the heat exchange means may be a water exchanger 4 immersed in the pool 5 (closed loop configuration of [Fig. 10], taken from reference [7]) or a direct withdrawal of water from the pool 5 (closed loop configuration of [Fig.9], taken from reference [7])) 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.

[0200] 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. List of cited references

[0201] [1]: https : / / w ww-pub.iaea.org / MTCD / Publications / PDF / TE-1785_web.pdf.

[0202] [2]: S. Kadalev et al, 2014, Annals of Nuclear Energy, vol. 72, p. 182-188.

[0203] [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.

[0204] [4]: David Hinds and Chris Maslak, “Next-génération nuclear energy: The ESBWR” Nuclear News. January 2006.

[0205] [5]: Hofer, Buck, Starflinger, « Operational Analysis ofa self-propelling Heat Removal System using supercritical CO2 with athlet », The 4th European sCO2 Conférence for Energy Systems March 23-24, 2021, Online Conférences CO2, 2021-sCO2.eu-157.[6]: Huiun Ha et al .''Optimal design of passive containment cooling System for innovative PWR” Nuclear Engineering and Technology 49 (2017) p. 941-952.

[0206] [7]: https: / / www-pub.iaea.org / MTCD / Publications / PDF / te_164_web.pdf

Claims

Claims

1. Light water nuclear reactor (LWR), in particular a pressurized water reactor (PWR) or boiling water reactor (BWR), comprising: - a reactor core (1); - a system for removing at least part of the residual power from the reactor core, the system comprising: a first water reserve or pool arranged above the reactor core, in which a heat exchange means immersed in the pool so that the water contained in the latter cools the steam coming from a means for drawing steam from the primary or secondary circuit of the reactor; an organic Rankine cycle (ORC) machine (6) comprising: - an expander (60); - a condenser (61), - a first pump (62); - an evaporator (63) arranged in contact with the pool so that the latter constitutes the PORC hot source;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 first pump (62), the first pump to the evaporator (63), and the evaporator (63) to the expander (60); a second water reserve (7), separate from the pool, and a second 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.;

2. A pressurized water nuclear reactor according to claim 1, comprising a cooling circuit comprising a steam generator and a water condenser submerged in the pool and connected in a closed loop to the steam generator.

3. Pressurized water nuclear reactor according to claim 1, the means for drawing off the residual power present in the primary circuit 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 of 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 evacuating the residual power from the core of the reactor being a system for depressurizing the steam present in the containment enclosure and the heat exchange means may be a water exchanger (4) immersed in the pool or a direct withdrawal of water from the pool (5) on the one hand, and a condenser in the containment wall (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 second water reserve being arranged in a part lower than the pool, advantageously on or in the ground.

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

8. Nuclear reactor according to claim 6, the submerged evaporator being a tubular exchanger.

9. Nuclear reactor according to claim 6, the submerged evaporator being a plate exchanger.

10. 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 second 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 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).

11. Nuclear reactor according to claim 9, the condenser (61) of the refrigeration cycle being the condenser of the ORC.

12. Nuclear reactor according to claim 9 or 10, the working fluid of the refrigeration cycle being that of the ORC.

13. Nuclear reactor according to one of claims 9 to 11, the shaft (11) of the ORC expander being coupled to the shaft of the compressor of the refrigeration cycle.

14. Nuclear reactor according to one of the preceding claims, the ORC machine and where appropriate the refrigeration cycle being arranged in a lower part of the system, below the pool.

15. Nuclear reactor according to one of the preceding claims, comprising an injector arranged in a lower part of the system and connected to the second pump arranged in an upper part of the system, the injector being adapted to prime the second pump.

16. Nuclear reactor according to one of the preceding claims, comprising batteries adapted for the electrical start of the first pump, the electrical components of the ORC and, where appropriate, the refrigeration cycle as well as the second pump.

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