WO2019190367A1

WO2019190367A1 - A safety system of a nuclear reactor for stabilization of ex-vessel core melt during a severe accident

Info

Publication number: WO2019190367A1
Application number: PCT/SE2018/050333
Authority: WO
Inventors: Sevostian BECHTA; Weimin Ma; Andrei KOMLEV; Louis Tamilarasan MANICKAM; Alexander KONOVALENKO; Walter VILLANUEVA; Sean Shahriar ROSHAN GHIAS; Aram KARBOJIAN
Original assignee: Bechta Sevostian; Weimin Ma; Komlev Andrei; Manickam Louis Tamilarasan; Konovalenko Alexander; Villanueva Walter; Roshan Ghias Sean Shahriar; Karbojian Aram
Priority date: 2018-03-28
Filing date: 2018-03-28
Publication date: 2019-10-03

Abstract

There is provided a safety system for a nuclear reactor (1), wherein the nuclear reactor (1) has a reactor pressure vessel (2) and the nuclear reactor is arranged at least partly in and/or above a reactor cavity (3). The safety system comprises a plurality of floatable objects (4) arranged in and/or in connection with the reactor cavity (3) for providing a floating bed of objects under the reactor pressure vessel (2) when the reactor cavity (3) is at least partly water filled.

Description

A safety system of a nuclear reactor for stabilization of ex-vessel core melt during a severe accident

TECHNICAL FIELD

The present invention generally relates to nuclear reactor safety, and more particularly concerns a safety system for a nuclear reactor and an arrangement for core melt stabilization in the event of a severe accident in a nuclear reactor, as well as a corresponding nuclear power plant, and a method for preparation and effective operation of a safety system for a nuclear reactor.

BACKGROUND

During a severe accident in a nuclear power plant (NPP), the reactor core may undergo meltdown resulting in corium formation. Corium is a generic name for the family of technogenic materials formed by melting/solidification of different compounds containing such elements as: U, Pu, Zr, Fe, Cr, Ni, B, C, O and others. In its liquid state, corium represents a single liquid or an immiscible system of oxidic and metallic liquids, which contains mostly semi- and low-volatile fission products having significant radiotoxicity and decay heat. In its solid state, corium represents a large variety of oxidic and metallic phases spatially separated at micro or macro levels.

Corium can relocate from the core region into the reactor lower head, melt-through the reactor pressure vessel, release into the reactor cavity, and damage the reactor containment, which is the last safety barrier of the NPP preventing radioactive contamination of environment. To maintain the containment integrity, different severe accident management strategies have been developed to prevent further melt progression and to stabilize corium in the reactor pressure vessel [1] or in the reactor pit [2, 3], realizing melt solidification and long-term extraction of the decay heat either from the melt inside the vessel or in the reactor pit. The severe accident management concept of Nordic boiling water reactors (BWRs) [4, 5] is based on corium melt fragmentation and quenching in a deep water pool of the flooded reactor cavity. The main residual risks of this concept [2] are determined by the possibilities of a steam explosion and the lack of debris bed coolability. Steam explosion can produce strong shock wave posing an imminent threat to the reactor containment integrity. Re-melting of uncoolable debris bed may result in the late containment bypass caused by the ablation of the steel and concrete liners, as can occur in the Fukushima accidents, and the containment melt-through possibly resulting in the melt attack of the water table.

There is a general demand for improvements within the field of nuclear reactor safety.

SUMMARY

It is a general object to improve the safety of a nuclear power plant and/or to provide improved nuclear reactor safety.

It is a specific object to provide a safety system for a nuclear reactor.

It is another object to provide an arrangement for core melt stabilization in the event of a severe accident in a nuclear reactor.

It is yet another object to provide a nuclear power plant comprising a safety system and/or arrangement for core melt stabilization.

It is also an object to provide a method for preparation and effective operation of a safety system for a nuclear reactor.

These and other objects are met by embodiments of the proposed invention. According to a first aspect, there is provided a safety system for a nuclear reactor, wherein the nuclear reactor has a reactor pressure vessel and the nuclear reactor is arranged at least partly in and/or above a reactor cavity. The safety system comprises a plurality of floatable objects arranged in and/or in connection with the reactor cavity for providing a floating bed of objects under the reactor pressure vessel when the reactor cavity is at least partly water filled.

In this way, it is possible to more effectively manage a severe accident with melt release from the reactor pressure vessel into the cavity.

For example, the proposed invention enables improved fragmentation, solidification and/or cooling of corium melt with reduced risk and/or energy of steam explosion in the event of a severe accident.

According to a second aspect, there is provided an arrangement for core melt stabilization in the event of a severe accident in a nuclear reactor, wherein the nuclear reactor has a reactor pressure vessel and the nuclear reactor is arranged at least partly in and/or above a reactor cavity. The arrangement comprises a plurality of floatable objects arranged in and/or in connection with the reactor cavity for providing a melt- fragmenting floating bed of objects under the reactor pressure vessel when the reactor cavity is at least partly water filled.

According to a third aspect, there is provided a nuclear power plant comprising such a safety system and/or arrangement for core melt stabilization.

According to a fourth aspect, there is provided a method for preparation and effective operation of a safety system for a nuclear reactor, wherein the nuclear reactor has a reactor pressure vessel and the nuclear reactor is arranged at least partly in and/or above a reactor cavity. The method comprises arranging a plurality of floatable objects in and/or in connection with the reactor cavity for providing a floating bed of objects under the reactor pressure vessel when the reactor cavity is at least partly water filled to thereby enable efficient fragmentation, solidification and/or cooling of corium melt with reduced risk and/or energy of steam explosion in the event of a severe accident with melt release from the reactor pressure vessel into the cavity.

Other advantages and improvements will be appreciated when reading the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments, together with further objects and advantages thereof, may best be understood by referring to the following description taken together with the accompanying drawings, in which:

FIG. 1 A is a schematic showing an example of the process of core melt jet fragmentation and debris bed formation with a safety system according to the proposed technology.

FIG. IB is a schematic showing another example of the process of core melt jet fragmentation and debris bed formation with a safety system according to the proposed technology.

FIG. 1C is a schematic showing an example of the process of core melt jet fragmentation and debris bed formation without any safety systems according to the proposed invention.

FIG. 2 is a principal scheme of an experimental facility for study of floating bed influence on melt jet fragmentation.

FIG. 3 A is an experimentally obtained snapshot showing melt jet fragmentation in a water pool having a bed of floatable objects according to the proposed technology. FIG. 3B is an experimentally obtained snapshot showing melt jet fragmentation in a water pool without any floatable objects.

FIG. 4 is a graph illustrating the experimental results and showing a decrease of jet breakup length caused by using a layer of floatable objects.

FIG. 5 is a view of experimental debris bed illustrating captured floatable objects in it.

FIG. 6 is a schematic showing safety system implementation and location of floating bodies in the under-reactor space prior to melt jet delivery at safety system operation.

FIGs. 7 A and B are schematics showing examples of implementation and location of floating bodies (A) prior to safety system operation, e.g. during normal reactor operation, and (B) prior to melt jet delivery at safety system operation with an optional location of floating bodies in the baskets, according to an embodiment.

FIGs. 8A and B are schematics showing other examples of implementation and location of floating and submerged bodies (A) prior to safety system operation and (B) prior to melt jet delivery at safety system operation with an optional bottom layer of submerged bodies, according to an embodiment.

DETAILED DESCRIPTION

Generally, all terms used herein are to be interpreted according to their ordinary meaning in the relevant technical field, unless a different meaning is clearly given and/or is implied from the context in which it is used.

According to a first aspect of the proposed technology, there is provided a safety system for a nuclear reactor, wherein the nuclear reactor has a reactor pressure vessel and the nuclear reactor is arranged at least partly in and/or above a reactor cavity. The safety system comprises a plurality of floatable objects arranged in and/or in connection with the reactor cavity for providing a floating bed of objects under the reactor pressure vessel when the reactor cavity is at least partly water filled.

For example, the proposed technology enables efficient fragmentation, solidification and/or cooling of corium melt with reduced risk and/or energy of steam explosion in the event of a severe accident.

In other words, e.g., with reference to FIGs. 1 A-C, the proposed technology may provide a novel safety system for a nuclear reactor 1 for improvement of core melt fragmentation and/or stabilization in the event of a severe accident in a nuclear power plant.

By way of example, the safety system may be installed in a cavity 3 constructed under the reactor pressure vessel 2 and may serve as a means of stabilizing and terminating a severe NPP accident. In a particular example, the proposed technology employs a plurality of floatable objects 4, also referred to as bodies, in the reactor cavity 3 for providing a floating bed of the objects 4 when the reactor cavity 3 is at least partly water filled. Typically, the floatable objects 4 will cause increased fragmentation of the corium melt jet 5 leading to enhanced coolability and consequently termination of an accident, in which a large release of fission products could occur to the environment (e.g., see FIG 1 A and FIG. IB). The safety system for causing enhanced fragmentation and earlier coolability of a large mass of corium melt should also substantially reduce the risks of the occurrence of a steam explosion and its energetics.

The present invention thus relates to the field of safety systems for a nuclear reactor of a nuclear power plant and is mainly focused on obtaining core melt stabilization, fragmentation, solidification, cooling and/or retention within an at least partially water- filled reactor cavity. Expressed slightly differently, the proposed invention concerns management of a severe accident during its ex-vessel phase.

By way of example, the floating bed may be and/or function as a melt-fragmenting floating bed to thereby enable fragmentation of corium melt in the event of a severe accident in the reactor with core melt release into the cavity.

According to a first set of example embodiments, the reactor cavity is not water filled during normal reactor operation, and the floatable objects are arranged in the reactor cavity in dry conditions, and the system is configured to at least partially fill the reactor cavity with water in the event of a severe accident and thereby enable formation of the floating bed of objects under the reactor pressure vessel.

As an example, at least a part of the floatable objects is arranged in at least one basket and/or other container open from the top, and/or at least part of the floatable objects are arranged at the bottom of the cavity to be water filled.

According to a second set of example embodiments, the reactor cavity is at least partly water filled and the floatable objects are arranged in the reactor cavity in wet conditions to form the floating bed of objects under the reactor pressure vessel.

According to a third set of example embodiments, the floatable objects are arranged in connection with the reactor cavity and transferred into the reactor cavity before or after the reactor cavity has been water filled and/or during the process of filling the reactor cavity with water.

As an example, at least a subset of the floatable objects may be transferred into the reactor cavity together with the water. In general, the nuclear reactor may be, e.g., a light water reactor or heavy water reactor of a nuclear power plant.

By way of example, corium melt in case of an accident may have the form of a coherent melt jet or jets, and the interaction of the melt jet with the floating bed will reduce and/or break the integrity and/or coherence of the melt jet(s).

For example, the interaction may take place before arrival of the melt jet into the water, i.e. in the dry zone of the floating bed, and/or after arrival into the water.

In a non-limiting example, the characteristic size of at least a subset of the floatable objects may be within the range from 0.001 to 0.500 m.

In another non-limiting example, the amount and sizes of the floatable objects are adapted for providing a floating bed having a thickness within the range from 0.001 m up to the water depth in the reactor pit.

As an example, the floatable objects include spherical objects and/or non-spherical objects. For example, the floatable objects may have different sizes.

In a particular example, the spherical objects, for example balls, have different diameters and formed floatable bed includes different mass/volume fractions of the objects in order to reduce the amount of water in the floating bed, i.e., its open porosity.

Optionally, the floating bed is packed with non-spherical objects in order to reduce the risk for channeling of the floating bed by the melt jet due to reduction of the free space between the objects and formation of the floatable bed having higher mechanical strength. The amount of water in the floating bed can be reduced more significantly than in the case of using spherical objects of different sizes. By way of example, the amount and/or sizes of the floatable objects may generally be adapted for providing a dry layer of the floating bed atop the water level to thereby enable fragmentation and cooling of corium melt under dry conditions and/or interaction of corium melt with the material(s) of the dry layer of the floating bed.

In an optional embodiment, at least a part of the floatable objects is adapted for enabling adhesion of corium melt with objects in the dry and/or wet layer to produce debris material comprising at least fragments of floatable objects and melt fragments.

In a particular example, at least a part of the floatable objects comprises an addition of neutron absorbers.

By way of example, at least a part of the floatable objects includes and/or is made of metallic and/or non-metallic materials.

Optionally, at least a part of the floatable objects is made from a material, which is denser than the water, but has a closed porosity resulting in an effective density below that of the water.

In a particular example, at least part of the floatable objects is made as porous or hollow bodies, each having a surface layer that prevents water from penetrating into the body before interaction with melt and allows, when the integrity of surface layer is damaged due to interaction with melt, water to penetrate into the porous or hollow body such that the body loses buoyancy in water, sinks and mixes with the particles in debris bed.

As an example, the porous or hollow bodies include water-soluble surfactants and/or nanoparticles and or other functional additives to decrease steam explosion probability/energy and/or fission product leaching rate by water and/or corrosion rate of structural materials and/or water-soluble neutron absorbers to prevent recriticality in the system. In a particular example, at least a part of the porous or hollow bodies comprises any functional additives as condensed and/or gas phases.

Optionally, the safety system further comprises a plurality of objects having higher density than the density of water to form a pebble layer and/or bed at the bottom of the reactor cavity prior to the melt sinking to the bottom of the reactor cavity (e.g., see FIG. 8B).

For example, the floatable objects, which have higher density in comparison with water, may originally be placed atop of the objects with lower density, e.g., inside one or more baskets or containers (as illustrated in FIG. 8A). In the example of FIG. 8A and FIG. 8B, the Archimedes force of the bodies located below is used for lifting and supplying the more dense subjects out of one or more baskets or containers, when the cavity is being filled with water, into the free volume of the water pool of the reactor pit.

In a particular example, at least part of the higher density objects, each have a hollow space inside that is completely or partially filled by surfactant or functional additions adapted for changing water properties, improving particle and debris cooling, decreasing the risk/energy of steam explosion, attenuating shock wave during propagation phase and decreasing the risk of recriticality in the system.

Optionally, at least a part of the floatable objects is generally made of different functional materials for selective interactions with oxidic and/or metallic parts of the core melt, and/or made from mixed and/or layered and/or composite functional materials.

According to a second aspect, there is provided an arrangement for core melt stabilization in the event of a severe accident in a nuclear reactor, wherein the nuclear reactor has a reactor pressure vessel and the nuclear reactor is arranged at least partly in and/or above a reactor cavity. The arrangement comprises a plurality of floatable objects arranged in and/or in connection with the reactor cavity for providing a melt fragmenting floating bed of objects under the reactor pressure vessel when the reactor cavity is at least partly water filled.

According to a third aspect, there is provided a nuclear power plant comprising a safety system and/or arrangement for core melt stabilization as described herein.

In the following, non-limiting example embodiments will be described, for a better understanding of some aspects of the proposed technology.

For a better understanding of the proposed technology, it may be useful to start with a brief overview and analysis:

The steam explosion energetics and debris bed formation depend upon several mechanisms: including melt jet breakup, melt droplet behavior (e.g., deformation/fragmentation in water), quenching of fragments, and settling (sediment/packing) of debris particles on the floor of the containment. The jet breakup process forms the initial conditions for melt-droplet evolution in the pre-mixture phase of a steam explosion. The sizes of the melt droplets determine the rate of effective heat transfer to water, and consequently the quench rate and void build-up. The quench rate and void build-up are important parameters that govern steam explosion initiation and energetics [6]. For a case without steam explosion, the size distribution and morphology of the debris particles can be directly related to the jet breakup and quench parameters, which are crucial to debris bed coolability, since they determine the size distribution of debris particles [7] and the effective porosity of the debris bed [8] employed in coolability analysis.

Amplification of melt fragmentation in water can result in an enhanced rate of effective heat transfer thereby resulting in rapid solidification of melt fragments and also increased void build-up in the pre-mixture, which are crucial to suppress a potential steam explosion. Further, the well-fragmented and packed debris particles without agglomeration are favorable to achieve debris bed coolability, more readily than otherwise, since water ingression in the bed is easier.

To enhance, e.g., melt fragmentation in water, a novel technique is proposed by the present invention, where floatable objects or bodies are employed in the water pool. On interaction with the floatable bodies, the melt jet can undergo more effective fragmentation since the coherent melt jet (so-called jet breakup length) penetrating the water column can be reduced significantly.

The proposed invention relates to a safety system for a nuclear reactor, especially for improved core melt stabilization, localization and/or retention through core melt fragmentation and/or cooling in an at least partially water filled reactor cavity, by using floatable objects (for example, balls). The objects are chosen in such a way that their effective densities are less than the density of water at corresponding temperature, resulting in a floating bed layer in the upper part of the water pool, exactly under the reactor pressure vessel (FIGs. 1 A and B).

By way of example, the safety system including floatable bodies according to this invention may provide one or more of the following technical effects: - Enhanced melt fragmentation, specifically, reduction in jet breakup length (Lb,-k - described as the distance travelled by a coherent melt jet in water until the start of fragmentation into discrete droplets) in the water pool as a result of the manipulated disturbance implemented in the form of floating bodies. By achieving enhancement in melt fragmentation, the melt surface area exposed to water increases resulting in rapid solidification of melt fragments. The risk of melt agglomeration (commonly referred to as cake which is possible when the melt is insufficiently fragmented and reaches the bottom of the water pool or if the temperature of the settled fragments is high enough that the fragments can sinter with each other) in debris bed 21 (FIGs. 1 A and B) is also considerably lower than in debris bed 22 (FIG. 1C). Agglomerates exceeding certain dimensions cannot be cooled down by water natural circulation and even a single non-coolable cake can provoke further debris re-melting.

- Reduction of steam explosion risk: Melt solidification and void production are the major steam explosion limiting mechanisms [6]. Fine fragmentation of the melt is the prerequisite for steam explosion. Following the extensive fragmentation achieved by the disclosed method, the melt surface area exposed to water increases, resulting in rapid solidification of the melt fragments simultaneously increasing the extent of void produced important for suppression of steam explosion propagation. Rapid solidification of the melt reduces the active liquid melt mass that can participate in a steam explosion. Extensive void production (i) reduces risk of direct contact of melt with water, which is prerequisite of steam explosion, (ii) reduces the mass of water premixed with the melt, which set the steam explosion energy, and (iii) supresses the propagation of a steam explosion due to lack of water and steam compressibility. Reduction of shock wave impulse acting at the water pool boundaries is also provided, thanks to shock wave attenuation in the floatable objects and energy absorption due to mechanical fragmentation of them.

- Prevention of debris bed recriticality, which can contribute to debris bed remelting and containment damage. Neutron multiplication factor in the debris bed is significantly reduced by addition of neutron absorbers, such as Boron, Gadolinium and/or others, into the material in the floating bed. A fraction of floating material will prevent recriticality of pre-fragmented melt premixed with water in the water pool volume, while the fraction of floating bed material, which has interacted with the melt and lost buoyancy, will sediment and mix with corium particles of debris bed 21 (FIGs. 1A and B), preventing recriticality which can happen in debris bed 22 (FIG. 1C) without additional neutron absorbers. Addition of neutron absorbers into floating beds can be important in situations with unborated water injections.

- Better coolability of debris bed 21 (FIGs. 1A and B) in comparison with debris bed 22 (FIG. 1C), due to improvement of the bed’s properties, such as absence of cakes, reduced mass fraction of larger sized debris, and reduced volumetric decay heat in debris bed diluted by the ballast of floating balls or their disintegration products. Initially the layer of floating body is supported by sufficient buoyancy in water. Lack of buoyancy will occur during melt interactions with the particles in the dry area of the floating bed, resulting in simultaneous sediment of the floating bodies and debris.

For example, a part of the described floatable objects of safety system occupy the dry space atop the water pool first interacting with the falling core melt jet (FIG. 1A). The mechanical interaction of the melt jet with the objects of the dry layer breaks the integrity of the jet before its arrival in the water pool. That leads to reduction of the jet and particle velocity, melt temperature prior to the melt contact with water, as well as the breakup length. Jet fragmentation is also improved in the water filled zone of the floating object layer because of jet mechanical interactions with the objects. An analogy with moderation of neutrons having collisions with atoms of light elements, such as H, O, Be and Al, can be used to imagine elastic scattering which can be considered as one of the phenomena affecting floating bed system behavior.

The reduction of the jet breakup length due to implementation of floatable bed compared to the water pool without floatable objects was confirmed by experiments on Wood’s metal jet fragmentation. The experimental setup shown in FIG. 2 comprises a test section 23, a support frame 24, a debris catcher 25 for collection of debris particles 26, multipoint thermocouples 27, a high speed video camera 28, water supply system 29, a nozzle 30 for melt delivery into the water pool 35, temperature controller 31, air supply system 32, melt preparation system 33 and an isolation valve 34. The experiments were conducted with and without floatable objects 36 of polyethylene balls located in, and partially atop, the water pool 35. Temperatures of water and Wood’s metal were kept in the ranges of 10 - 12 °C and 95 - 97 °C, respectively. The key parameter characterizing the influence of the described floatable objects on the jet fragmentation is the dimensionless jet break-up length (IM) or the ratio of jet break-up length and jet diameter L_brkID_jet. Examples of jet fragmentation in the water pool with and without a floating bed are presented in FIGs. 3A and 3B, respectively. Summary of the experimental results obtained with 20 mm balls diameter and 20 mm jet diameter is presented in the FIG. 4. Experimental data are compared with the Epstein and Fauske correlation [9] (equation 1):

where, E₀ is the entrainment coefficient, which is in the range between 0.05 and 0.1, p_j and p_c are the densities of the melt jet and water respectively.

Increase of floatable objects layer (floating bed) height - H_bed (17 in FIG IB) or dimensionless height—

— generally leads to the reduction of the jet breakup length

^Dj

because of mechanical interactions between floatable bodies and molten jet and reduction of the jet channeling effect as well. The result plotted in the area of FIG. 4, which is marked in red background, show decrease of dimensionless melt breakup length ) from ~ 19 to ~0.8 with the increase of dimensionless thickness of floating

H

bed (—— ) from 0 to 4. In this range of thicknesses fragmentation happens not in floating Dj bed but in free water (water pool below the floatable bed). After reaching the critical floating bed height - Hbed-a- (18 in FIG 1 A) or dimensionless height of the floating bed

- which is close to4 in our experiments with Woods metal, jet fragmentation

starts within the floating bed and continue in free water (FIG. 1 A and FIG. 4), i.e. jet breakup length becomes smaller than the floating bed height. Further height increase allows complete melt jet fragmentation and cooldown or even solidification of fragmented particles within the floating bed which is preferable for efficiency of this method and corresponding safety system.

The effect of significant reduction of melt breakup and full fragmentation jet with the floating bed height increase is explicitly important for reactor application, specifically for accident scenarios with large diameter jets, which require very deep water pool for complete melt fragmentation and particle quenching.

Flotation and, in particular the density, of the described bodies should be optimized to provide a specific thickness of dry layer atop the water level. This will provide pre- fragmentation of the melt jet under dry conditions, and interaction of corium melt with the dry material may allow partial sedimentation and mixing of floatable objects with the adsorbed corium fragments in the final debris bed on the pool bottom. The height of the dry layer however should be limited to prevent possible formation of cake in the dry zone.

With illustrative reference once again to FIGs. 1A-C, mechanical interaction of core melt jet with floating bed allows to change debris properties in order to improve its coolability. Intensive cooling of core melt starts after disintegration of melt jet 5 into the relatively hot separated fragments 14 and continues after debris bed 21 formation. As was shown above, the jet breakup length 20 (FIG. IB) is shorter in the case of using described floating bed 4, than jet breakup length 19 without described safety system (FIG. 1C). Consequently, transport length of melt fragments to the water pool bottom is longer and the fragments 14 can be better cooled and as the result debris bed 21 (FIGs. 1 A and B) includes relatively cold jet fragments 15 in comparison with debris bed 22 (FIG. 1C). Longer transport length promotes additional reduction of melt temperature and reduces the risk of debris agglomeration and formation of a cake, which is complicated to cool down.

After interaction of the core melt jet with floating bed, some balls 16 are caught by jet fragments (FIG. 1A-B) and involved into the debris bed formation (FIG. 5). It is achieved by adhesion of dense melt, whose density is in the range of 7500 to 9000 kg/m³, with the balls in the dry zone of a limited height, in which the surface superheat of the melt fragments is sufficient for activation of physicochemical interaction between the melt and the ball material. The resulting debris particles 16 (ball fragments adhered to the melt fragments) have the average density above that of water, and therefore sink to the bottom of the water pool, forming a debris bed 21. The addition of intact and fragmented balls into the debris bed decreases volumetric heat in the debris bed 21 (FIG. 1A and B) in comparison with the debris bed 22 (FIG. 1C). Reduction of volumetric heat is very favorable for the bed coolability. Hence, specific properties of the objects in the floating bed, such as wettability by core melt and surface roughness should be carefully selected to provide reliable capture and holding by core melt jet and its fragments. Other properties, such us heat capacity, fusion heat, thermal conductivity, free energies of different compound formation at interaction of corium melt with functional materials, physicochemical, material and thermodynamic properties of resulting alloy, should be optimized in order to improve debris bed coolability and reduce fission product release in a long term.

Transport of the floating bed material to the water pool bottom can be promoted if the floatable objects will be produced of a functional material - e.g., ceramic one (AI2O3, Fe₂03, etc,), which is more dense than the water, but has closed porosity and apparent density below that of the water. Thin glazed surface layer of the porous balls prevents water from penetrating into the intact balls before their interaction with melt. After the protective surface layer is damaged due to interaction with melt, water penetrates into the open porosity and the floatable objects lose buoyancy in water. Another design option of originally floating objects is hollow body (ball), e.g. made of metal, borosilicate glass, ceramic or other watertight material, filled with a certain mass of dense smaller balls or dense powder, which sinks in water after hollow body wall failure caused by interaction with high temperature melt. Compounds soluble in water, e.g. surfactants to prevent steam explosion or soluble neutron absorbers to prevent recriticality, can be additionally added into the hollow balls.

Molten corium can contain unmixable liquids of oxidic and metallic origins. Different functional materials can be used for improvement of fragmentation of oxidic and metallic melts, suppression of steam explosion and for management of coolability, recriticality, source term and long-term behavior of corium debris bed. Different functional materials can be used in different floatable objects or functional material composites can be developed. The mass ratio between oxidic and metallic corium influences the mass ratio between specific functional materials used in the system.

By way of example, the floatable objects may include spherical bodies of different sizes, but also non-spherical bodies having other shapes in order to reduce the volume (mass) of water in the floating bed and to reduce its channeling by the melt jet. Reduction of open porosity (free space between bodies in the floatable bed) which can be filled by water can be achieved by using, for example, mixture of fractions of spherical bodies having different diameter. By using at least 6 volume (mass) fractions of the spherical objects, the free space of the floatable bed can be reduced by more than 6 times (see Table 1). Table 1. Free space fraction in the mixture of balls having different diameters

Specific shapes can also be selected to affect the rotational degree of freedom of the particle while depositing down to enhance the debris self-levelling on the floor promoted by the water/steam flows.

Interaction between core melt jet and floating bed packed by spherical objects can be characterized by isotropic impulse propagation covering mechanical interaction as well as shock-wave propagation during steam explosion. In case of using non-spherical objects in floating bed, possibility of anisotropic impulse propagation appears. That can lead to the opportunity of impulse transfer management to protect the most vulnerable part of the containment from, for instance, steam explosion.

The jet breakup and melt fragment/droplet formation stages (commonly referred to as the premixing phase in steam explosion research) depict the initial conditions for a potential steam explosion to occur [6]. With enhanced fragmentation manipulated by the floatable objects, extensive void will be produced in replacing the water content in the premixing region. Further, the layer of floatable objects also reduces the volume of water available in the premixing region thereby reducing the risk of steam explosion propagation because of compressibility (provided by the void in the pre-mixture) and attenuation (provided by the floating body). By mitigating propagation, the shock wave will be damped from affecting the containment structures. As described above, destruction as well as intensive cooling of core melt jet can start in the floating bed, before contact with the bulk of water pool. Cooling of the melt fragments in the floating bed having sufficient height can promote formation of solid crust around them, leading to absence of the main condition for steam explosion: direct contact between molten core materials and water. The mechanical properties of the floatable objects should be selected in the ranges to provide their effective destruction in the course of shock wave propagation through the floating bed with significant absorption of energy in the case of steam explosion. In that case, as was mentioned above, floating bodies can be also made of ceramic material (based on the system including AI2O3, for example) with relatively high close porosity, to provide buoyancy on the water pool. Such properties are also favorable for dissipation of a pressure wave, which can trigger steam explosion from an external source.

FIG. 6 is a schematic illustrating an example of implementation and location of floating bodies in the under-reactor space according to an embodiment.

In this example, the safety system comprises a plurality of floatable objects 4 arranged in and/or in connection with the reactor cavity 3 for providing a floating bed of objects 4 under the reactor pressure vessel 2 when the reactor cavity 3 is at least partly water filled.

In an example, the floatable objects or bodies 4 may initially be located in dry conditions of the under-reactor space 3 during regular mode of reactor normal operation. The locations are selected to exclude or minimize possible obstacles for normal operation and equipment service. For example, the bodies can be placed inside baskets, which are open from the top. In case of using two types of bodies - with lower and higher densities than water, the last ones are placed a top. Shape and dimensions of the baskets are determined based on the specific reactor design considering available space and necessary amount of bodies to form sufficiently thick floating surface layer for effective system operation. In specific reactor designs, the bodies can be placed on the cavity bottom.

The floating bodies are made of long-term stable material, such as ceramic or metal, therefore there are no any material releases influencing the containment environment and no floating body material degradation for the whole reactor lifetime. If the material degradation cannot be prevented for the whole reactor lifetime, the floating bodies can be replaced periodically with the new ones.

In a sense, the invention relates to NPP passive safety and typically does not require any operator actions except of the under reactor space flooding, which is already a part of other active or passive systems for severe accident management at different reactors.

During a normal operation mode, the nuclear reactor 1 including reactor pressure vessel 2, elements of active zone 9, water as heat transfer agent 10, etc. has closed outlet 11/inlet 12 water circuit.

In the case of severe accident with core melt 5 release from reactor pressure vessel 2, the described safety system allows to improve core melt fragmentation and stabilization. This can be reached, for example, by the following way. Water starts to fill the underreactor space due to supply water system 13 and forms water pool 6 of under reactor space. The bodies with lower density float up and form the floatable bed 4, i.e. the system becomes ready for melt delivery.

Two examples of the mode of operation after the melt delivery are illustrated in FIG. 1A and FIG. IB. FIGs. 7A and B are schematics showing examples of implementation and location of floating bodies during normal reactor operation and in the case of severe accident, respectively, with an optional location of floating bodies in the baskets, according to an embodiment. FIG. 7A reflects regular operational mode and FIG. 7B shows severe accident mode after core melting but prior to melt release into the reactor cavity.

In this example, the floating objects 4 are initially located in dry conditions of the under- reactor space 3 during regular mode of reactor normal operation. The locations are selected to exclude or minimize possible obstacles for normal operation and equipment service. Such locations are selected at the periphery of the rector pit below the reactor pressure vessel 2. The objects are placed inside one or more baskets 7 or similar containers, which are open from the top. Shape and dimensions of the baskets are determined based on the specific reactor design considering available space and necessary amount of bodies to form sufficiently thick floating surface layer for effective system operation.

In specific reactor designs the bodies can initially be placed on the cavity bottom. This layout is not shown in FIGs. 7 A and B.

In the case of severe accident with core melt release from reactor pressure vessel, the described safety system allows to improve core melt fragmentation and stabilization. This can be reached, for example, in the following way. Water starts to fill the underreactor space due to supply water system 13, covers the baskets 7 tops and forms water pool 6 of under reactor space.

The bodies with lower density float up and form the floatable bed 4, i.e. the system becomes ready for melt delivery.

FIGs. 8A and B are schematics showing other examples of implementation and location of floating bodies during normal reactor operation and in the case of severe accident, respectively, with an optional bottom layer, according to an embodiment. FIG. 8A reflects regular operational mode and FIG. 8B shows severe accident mode after core melting but prior to melt release into the reactor cavity.

In this example, the floating objects 4 are initially located in dry conditions of the under- reactor space 3 during regular mode of reactor normal operation. The locations are selected to exclude or minimize possible obstacles for normal operation and equipment service. Such locations are selected at the periphery of the rector pit below the reactor pressure vessel 2. The described objects are placed inside one or more baskets 7 or containers, which are open from the top but also have penetrations for water. Shape and dimensions of the baskets are determined based on the specific reactor design considering available space and necessary amount of bodies to form sufficiently thick floating surface layer for effective system operation.

In case of severe accident with core melt release from reactor pressure vessel 2 described safety system allows to improve core melt fragmentation and stabilization. This can be reached, for example, by the following way. Water starts to fill the underreactor space due to supply water system 13, covers the baskets tops and forms water pool 6 of under reactor space 3. The bodies with lower density 4 push the bodies with higher density 8 due to Archimedes forces and allow them to sink down. Then bodies with lower density float up from the baskets and form the floatable bed 4 while the bodies with higher density form bottom peddle layer (particulate bed) 8, i.e. the system becomes ready for melt delivery.

The embodiments described above are merely given as examples, and it should be understood that the proposed technology is not limited thereto. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the present scope as defined by the appended claims. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. REFERENCES

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2. Fischer, M., Bechta, S.V., Bezlepkin, V.V., Hamazaki, R., Miassoedov, A. (2016). Core melt stabilization concepts for existing and future LWRs and associated research and development needs. Nuclear Technology 196 (3): 524-537.

3. Khabensky, V.B., Granovsky, V.S., Bechta, S.V., Gusarov, V.V. (2009). Severe accident management concept of the VVER- 1000 and the justification of corium retention in a crucible-type core catcher. Nuclear Engineering and Technology 41 (5): 561-574.

4. Becker, K.M., Engstrom, J. (1990). An arrangement for protecting reactor containment integrity. Patent W01990010936 Al. Also published as EP0478552A1, and US Patent 5,192,494.

5. Frid, W. (2002). Severe Accident Research and Management in Nordic Countries. NKS Report, NKS-71, ISBN 87-7893-127-4.

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9. Epstein, M. and Fauske, H. K., 2001. Applications of the turbulent entrainment assumption to immiscible gas-liquid and liquid-liquid systems, IChemE 79: 453-462.

Claims

1. A safety system for a nuclear reactor ( 1 ), wherein the nuclear reactor has a reactor pressure vessel (2) and the nuclear reactor is arranged at least partly in and/or above a reactor cavity (3),

wherein the safety system comprises a plurality of floatable objects (4) arranged in and/or in connection with the reactor cavity (3) for providing a floating bed of objects (4) under the reactor pressure vessel (2) when the reactor cavity (3) is at least partly water filled.

2. The safety system of claim 1, wherein the floating bed is a melt-fragmenting floating bed to thereby enable fragmentation of corium melt (5) in the event of a severe accident in the reactor with core melt release into the cavity (3).

3. The safety system of claim 1 or 2, wherein the reactor cavity (3) is not water filled during normal reactor operation, and the floatable objects (4) are arranged in the reactor cavity (3) in dry conditions, and the system is configured to at least partially fill the reactor cavity (3) with water (6) in the event of a severe accident and thereby enable formation of the floating bed of objects (4) under the reactor pressure vessel (2).

4. The safety system of claim 3, wherein at least a part of the floatable objects (4) is arranged in at least one basket (7) and/or other container open from the top, and/or at least part of the floatable objects (4) are arranged at the bottom of the cavity (3) to be water filled.

5. The safety system of claim 1 or 2, wherein the reactor cavity (3) is at least partly water filled and the floatable objects (4) are arranged in the reactor cavity (3) in wet conditions to form the floating bed of objects (4) under the reactor pressure vessel (2).

6. The safety system of claim 1 or 2, wherein the floatable objects (4) are arranged in connection with the reactor cavity (3) and transferred into the reactor cavity before or after the reactor cavity has been water filled and/or during the process of filling the reactor cavity with water.

7. The safety system of claim 6, wherein at least a subset of the floatable objects (4) is transferred into the reactor cavity (3) together with the water.

8. The safety system of any of the claims 1 to 7, wherein the nuclear reactor (1) is a light water reactor or heavy water reactor of a nuclear power plant.

9. The safety system of any of the claims 1 to 8, wherein eorium melt (5) in case of an accident will have the form of a coherent melt jet, and the interaction of the melt jet with the floating bed will reduce and/or break the integrity and/or coherence of the melt jet.

10. The safety system of claim 9, wherein the interaction takes place before arrival of the melt jet (5) into the water (6), i.e. in the dry zone of the floating bed (4), and/or after arrival into the water.

11. The safety system of any of the claims 1 to 10, wherein the characteristic size of at least a subset of the floatable objects (4) is within the range from 0.001 to 0.500 m

12. The safety system of any of the claims 1 to 11, wherein the amount and sizes of the floatable objects (4) are adapted for providing a floating bed having a thickness within the range from 0.001 m up to the water depth in the reactor cavity (3).

13. The safety system of any of the claims 1 to 12, wherein the floatable objects (4) include spherical objects and/or non-spherical objects.

14. The safety system of any of the claims 1 to 13, wherein the floatable objects (4) have different sizes.

15. The safety system of claim 13 or 14, wherein the floatable objects (4), for example balls, have different diameters and the formed floatable bed includes different mass and/or volume fractions of the objects in order to reduce the amount of water in the floating bed, i.e., its open porosity.

16. The safety system of claim 14, wherein the floating bed (4) is packed with non- spherical objects in order to reduce the risk for channeling of the floating bed by the melt jet due to reduction of the free space between the objects and formation of the floatable bed having higher mechanical strength.

17. The safety system of any of the claims 1 to 16, wherein the amount and/or sizes of the floatable objects (4) are adapted for providing a dry layer of the floating bed atop the water level to thereby enable fragmentation and cooling of corium melt (5) under dry conditions and/or interaction of corium melt with the material(s) of the dry layer of the floating bed.

18. The safety system of any of the claims 1 to 17, wherein at least a part of the floatable objects (4) is adapted for enabling adhesion of corium melt (5) with objects in the dry and/or wet layer to produce debris material comprising at least fragments of floatable objects (4) and melt fragments.

19. The safety system of any of the claims 1 to 18, wherein at least part of the floatable objects (4) comprises an addition of neutron absorbers.

20. The safety system of any of the claims 1 to 19, wherein at least a part of the floatable objects (4) includes and/or is made of metallic and/or non-metallic materials.

21. The safety system of any of the claims 1 to 20, wherein at least a part of the floatable objects (4) is made from a material, which is more dense than the water, but has a closed porosity resulting in an effective density below that of the water.

22. The safety system of any of the claims 1 to 21 , wherein at least a part of the floatable objects (4) is made as porous or hollow bodies, each having a surface layer that prevents water from penetrating into the body before interaction with melt and allows, when the integrity of surface layer is damaged due to interaction with melt, water to penetrate into the porous or hollow body such that the body loses buoyancy in water, sinks and mixes with the particles in debris bed.

23. The safety system of claim 22, wherein the porous or hollow bodies include water- soluble surfactants and/or nanoparticles and/or other functional additives to decrease steam explosion probability/energy and/or fission product leaching rate by water and/or corrosion rate of structural materials and/or water-soluble neutron absorbers to prevent recriticality in the system.

24. The safety system of claim 23, wherein the porous or hollow bodies include any functional additives as condensed and/or gas phases.

25. The safety system of any of the claims 1 to 24, wherein the safety system further comprises a plurality of objects (8) having higher density than the density of water to form a pebble layer and/or bed at the bottom of the reactor cavity prior to the melt sinking to the bottom of the reactor cavity (3).

26. The safety system of claim 25, where the objects (8), which have higher density in comparison with water, are originally placed atop of the floatable objects (4) with lower density.

27. The safety system of any the claim 26, wherein at least a part of the higher density objects (8), each have a hollow space inside that is completely or partially filled by surfactant or functional additions adapted for changing water properties, improving particle and debris cooling, decreasing the risk/energy of steam explosion, attenuating shock wave during propagation phase and decreasing the risk of recriticality in the system.

28. The safety system of any of the claims 1 to 27, wherein at least a part of the floatable objects (4) is made of different functional materials for selective interactions with oxidic and/or metallic parts of the core melt, and/or made from mixed and/or layered and/or composite functional materials.

29. An arrangement for core melt stabilization in the event of a severe accident in a nuclear reactor (1), wherein the nuclear reactor (1) has a reactor pressure vessel (2) and the nuclear reactor is arranged at least partly in and/or above a reactor cavity (3),

wherein the arrangement comprises a plurality of floatable objects (4) arranged in and/or in connection with the reactor cavity (3) for providing a me It- fragmenting floating bed of objects (4) under the reactor pressure vessel (2) when the reactor cavity

(3) is at least partly water filled.

30. A nuclear power plant comprising a safety system and/or arrangement for core melt stabilization according to any of claims 1 to 29.

31. A method for preparation and effective operation of a safety system for a nuclear reactor (1), wherein the nuclear reactor (1) has a reactor pressure vessel (2) and the nuclear reactor is arranged at least partly in and/or above a reactor cavity (3),

wherein the method comprises arranging a plurality of floatable objects (4) in and/or in connection with the reactor cavity (3) for providing a floating bed of objects

(4) under the reactor pressure vessel (2) when the reactor cavity (3) is at least partly water filled to thereby enable efficient fragmentation, solidification and/or cooling of corium melt with reduced risk and/or energy of steam explosion in the event of a severe accident with melt release from the reactor pressure vessel (2) into the cavity (3).