WO2011128258A1

WO2011128258A1 - Liquid-metal-cooled fast-neutron reactor core having a low draining effect

Info

Publication number: WO2011128258A1
Application number: PCT/EP2011/055497
Authority: WO
Inventors: Frédéric VARAINE; Pierre Sciora; Laurent Buiron
Original assignee: Commissariat à l'énergie atomique et aux énergies alternatives; Electricite De France; Areva Np
Priority date: 2010-04-12
Filing date: 2011-04-08
Publication date: 2011-10-20
Also published as: FR2958781B1; FR2958781A1

Abstract

The invention relates to a liquid-metal-cooled fast-neutron reactor core having a low draining effect. The core comprises, from top to bottom, a neutron-absorbing upper zone (8), a liquid-metal plenum zone (10), an upper fertile zone (12), an intermediate fertile zone (14) and a lower fissile zone (16). The horizontal mid-plane (P1) of the intermediate fertile zone, of height HFE*, lies beneath the horizontal mid-plane (P2) of the assembly formed by the lower and upper fissile zones and the intermediate fertile zone, the ratio of HFE* to the height of this assembly varying from 0.25 to 0.40.

Description

FAST NEUTRON REACTOR HEART, COOLED BY A LIQUID METAL, WITH LOW DRAIN EFFECT

DESCRIPTION

TECHNICAL AREA

The present invention relates to a fast neutron reactor core, cooled by a liquid metal, with a low draining effect.

It applies in particular to such a reactor in which the coolant (English, coolant) is liquid sodium, but is not limited to the use of this liquid metal as a cooling fluid of the heart: in the the coolant may be another liquid metal.

The present invention aims to improve the safety of the core of a liquid metal cooled fast neutron reactor in a power range of 50 to 5000 MWth, by adapting the design of the core.

The present invention consists in implementing a core design such that the physical laws, which govern the nuclear reactions during accidental transients leading to the loss of the coolant, naturally tend to smother any potential runaway of the reactor.

The aim of the invention is to obtain a negative or near zero void effect in a fast neutron reactor, in particular when the coolant is sodium.

This effect is positive in fast neutron reactors, cooled by liquid sodium, large size (typically greater than 800MWe), which have been designed to date. During some accidental transients, leading to the emptying of sodium in the heart, there is a significant increase in the reactivity of the heart. This increase can lead to the destruction of the core by local or generalized fusion of fuel assemblies.

Remember that the drain effect acts on the reactivity of the reactor and is expressed in dollars. It is attributable to the loss of coolant.

More specifically, it defines a void coefficient (in English, void coefficient) (expressed in dollars) which reflects the variation of the multiplication factor of the reactor when the coolant is no longer present in the core. If this coefficient is positive, the emptying will result in an increase in reactivity and power. If it is negative, this effect will tend to stop the chain reaction.

Recall also that the dollar is a unit of responsiveness. A dollar ($) corresponds to an increase in reactivity counted in relation to the proportion of delayed neutron fractions.

STATE OF THE PRIOR ART

The problem of reducing the emptying effect is not currently solved for cores with a high unit power (greater than 800MWe), especially those intended for industrial sectors. Control of this effect involves setting up active systems to ensure acceptable reactor behavior during accidental transients.

Various designs of the core of a fast neutron reactor have already been proposed to reduce the draining effect.

We will refer to the following article:

A.N. Chebeskov, "Evaluation of Sodium Void Reactivity on the BN-800 Fast Reactor Design", Physor 1996, vol. 2, C-49.

The design described in this article is associated with the BN-800 reactor. According to this design, an internal fertile plate (between two zones of fissile material) and an associated sodium plenum (in English, sodium plenum) are provided. The goal is essentially the passage of the emptying effect under the value of $ 0.

This leads to "pancake" type hearts often having very large radii.

An international simulation comparison of such a concept under the aegis of the IAEA ("Evaluation of benchmark calculations on a fast power reactor core with near zero sodium void effect") has shown an overestimation of the announced gain on the effect of drain. See IAEA-TECDOC-731, January 1994

See also the following article: T. Takeda et al., "Optimization of

Internal Blanket Configuration of Large Fast Reactor, Short Note, Journal of Nuclear Science and Technology, 30, pp. 481-484, 1983.

This is a parametric study of an internal fertile plate with varying heights and radius. We seek to have a low draining effect, while maintaining the coupling of two fissile axial zones.

The conclusions drawn in this article are partial because the parametric study is limited: the core geometry studied is fixed at lm, which is different from the present invention.

In addition, the proposed design has no "crucible", that is to say no difference in height between the inner portion and the outer portion of the core, contrary to what is proposed in a preferred embodiment of this invention.

Finally, the presence of a "gas plenum" zone, at the top of the needles in the plenum, suggests a plenum design, with a large amount of steel. This design is different from that used in the present invention.

See also the following article:

T. Yokoyama et al. "A study on reactivity insertion controlled LMR cores with metallic fuel," Progress in Nuclear Energy, vol. 47, pp. 251-259, 2005.

This is a study of a metal fuel core. The crucible form is used there without internal fertile plate, and the emptying effect remains important. Document FR 2 679 062 also discloses a fast breeder reactor core and a fuel assembly used in such a core.

STATEMENT OF THE INVENTION

The present invention proposes a new design of the core of a fast neutron reactor, cooled by a liquid metal, to significantly reduce the emptying effect, or even to make it negative.

The effects of old designs, such as EFR (European Fast Reactor) or SUPERPHENIX, led to emptying effects in the range of $ 5- $ 7.

The present invention makes it possible to obtain a zero emptying effect, or even to make this effect negative.

To do this, the invention combines several characteristics; and this combination has physical effects on the axial redistribution of the neutron flux, which amplifies the gains.

The invention uses an intermediate fertile zone, for example natural uranium, depleted or reprocessed, which is between two fissile zones and whose median plane is placed under the median plane of all three zones.

This intermediate fertile zone may concern only the central part of the heart or the whole of it. And the height of fertile material is between 25% to 40% of the total height (fissile material plus fertile material). In addition, the invention uses a plenum containing the coolant, for example a sodium plenum in the case of a reactor cooled by liquid sodium. Above the plenum is a neutron absorbing material.

In addition, the core object of the invention may optionally comprise an outer portion and an inner portion whose height may be smaller than that of the outer portion, which gives it a crucible shape.

The invention makes it possible to reduce the emptying effect of a fast neutron reactor cooled by a liquid metal, for example sodium, even for a unitary high-power reactor.

This improves the safety of the reactor, and is likely to reduce the number of active systems that are associated with the reactor, to ensure the safety of its operation, and thus reduce the cost of the reactor.

In addition, the presence of the intermediate zone in fertile material leads to an improvement in the behavior of the fuel pellets (in English, fuel pellets) vis-à-vis the sheath (in English, cladding), and conversely, in the assemblies fuel that the heart has, during the nominal operation of the latter. Indeed, this intermediate zone is likely to limit corrosion at the fissile / fertile axial interface, and also to reduce the axial form factor.

And the design proposed in the invention allows a unique content of fissile material in fuel assemblies, while known designs require two to three different grades. This simplifies the manufacturing process of the fissile material.

In addition, the invention applies to any fast neutron reactor core, whatever

- the type of fuel (uranium, plutonium or other),

the type of support matrix (oxide matrix, metal matrix, carbide matrix, inert matrix, or other),

- the geometry of the fuel assemblies, and

- the type of coolant metal (sodium, lead, or other).

Specifically, the present invention relates to a fast neutron reactor core, cooled by a liquid metal, comprising, going up and down the heart:

an upper zone of fissile material,

an intermediate zone of fertile material, and

a lower zone of fissile material, in which the horizontal median plane of the intermediate zone of fertile material is below the horizontal median plane of the assembly formed by the upper zone of fissile material, the intermediate zone of fertile material and the zone lower of fissile material, and wherein the ratio R of the height of the intermediate zone of fertile material to the height of the assembly formed by the upper zone of fissile material, the intermediate zone of fertile material and the lower zone of fissile material is in the range of 0.25 to 0.40 (the heights being counted vertically),

characterized in that it further comprises an upper absorbent zone, made of a neutron-absorbing material, and, beneath the upper absorbent zone, a liquid metal plenum zone, the upper zone of fissile material being below the plenum zone of liquid metal.

Preferably, the ratio R is in the range of 0.25 to 0.35.

Advantageously, the fast neutron reactor core, object of the invention, further comprises a lower zone of fertile material, located under the lower zone of fissile material. This lower zone of fertile material makes it possible to respect the regeneration criteria of the core, that is to say to have a production of fissile material equal to the consumption of fissile material (isogenation).

According to a particular embodiment of the invention, the core comprises, radially, an internal zone, of height HZ1 (height of the assembly formed by the upper zone of fissile material, the intermediate zone of fertile material and the lower zone of fissile material), and an outer zone, of height HZ2 (height of the assembly formed by the upper zone of fissile material, the intermediate zone of fertile material and the lower zone of fissile material); and the intermediate zone of fertile material extends radially at least in the inner zone, while the liquid metal plenum zone and the inferior zone of fertile material extend radially in the inner zone and the outer zone.

Preferably, the HZ1 / HZ2 ratio is in the range of 0.6 to 1. This ratio may be 1 (HZ1 = HZ2). But it can also be less than 1 (HZKHZ2) in which case the heart has a "crucible" type geometry.

Preferably, the ratio HFI> / HZ1 is in the range of 0.3 to 0.5, where HFI> represents the height of the upper zone of fissile material of the inner zone (radially).

According to a particular embodiment of the core object of the invention, the inner zone of the heart forms a volume which is delimited by a first cylinder with polygonal base whose generator is vertical, and the external zone of the heart forms another volume which is delimited internally by this first cylinder and externally by a second polygonal base cylinder whose generator is parallel to that of the first cylinder.

Preferably, the ratio RZ1 / RC is greater than or equal to 0.75, where RZ1 represents the mean radius of the inner zone and RC the average outer radius of the outer zone.

The average radius of the inner zone is the radius of the disk whose area is equal to that which is delimited by the base (also called director) of the first cylinder. The average outer radius of the outer zone is the radius of the disk whose area is equal to that delimited by the base of the second cylinder.

Preferably, the height of the liquid metal plenum zone in the inner zone is at least 20 cm.

Preferably, the height of the liquid metal plenum zone in the outer zone is at least 20 cm.

According to a particular embodiment of the invention, the intermediate zone of fertile material further extends into the outer zone.

Let's go back to the documents Takeda et al. and FR 2 679 062 which have been mentioned above.

The article by T. TAKEDA et al. does not disclose or suggest two essential characteristics of the reactor core, object of the present invention, namely the upper absorbent zone, made of a neutron-absorbing material, and in this zone, the zone of liquid metal plenum which is above the upper zone of fissile material and therefore below the fuel elements.

The present invention therefore satisfies two conditions, namely the presence of a neutron-absorbing material at the top of the assembly and the use of fuel elements that do not pass through the liquid metal plenum. These two conditions are necessary to obtain a fast neutron reactor core, cooled by a liquid metal, in which the draining effect is very significantly reduced or the latter is made negative. It should be noted that the upper part of the configuration proposed in Figure 1 of the article by Takeda et al. consists of a "gas plenum" which is in no way a neutron-absorbing material.

A "gas plenum" is a fuel element and its function is to retain the fission gases. This results in the presence of the fuel element up to the dimension 270 shown in Figure 1 of Takeda et al. It follows therefore that the area called "Na region", below, is crossed by the fuel element which must be continuous so that the region called "gas plenum" can fulfill its role.

In the document FR 2 679 062 are presented geometrical configurations of heart which differ fundamentally from the present invention. Indeed, in any case, these configurations do not include a neutron-absorbing material at their upper part. However, the presence of a neutron-absorbing material at the upper part of the core is an essential feature of the present invention and contributes to its unique performance in terms of lowering the coolant drain effect.

It should be noted that the structures shown in FIGS. 3, 4 and 5 of the document do not include any material above the sodium reflector zone, and that those shown in FIGS. 13 and 14 of the document contain, at their upper fertile material that is by no means a neutron-absorbing material.

Recall that a neutron-absorbing material is a material whose physical characteristics give it the ability to capture neutrons, and that a fertile material is a material whose physical characteristics give it the capacity to produce a fissile material under neutron flux. Although a fertile material captures neutrons, it produces fissile elements that produce neutrons.

The present invention would not work with such a fertile material.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given below, purely by way of indication and in no way limiting, with reference to the appended drawings in which:

FIG. 1 is a diagrammatic and partial radial sectional view of a particular embodiment of the fast neutron reactor core, object of the invention;

FIG. 2 is a schematic and partial radial sectional view of a known fast neutron reactor core, and

FIGS. 3 and 4 are diagrammatic and partial radial sectional views of other modes of particular embodiments of the fast neutron reactor core, object of the invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS FIG. 1 is a diagrammatic and partial radial sectional view of an example of the fast neutron reactor core which is the subject of the invention. The core 2 of Figure 1 is cooled by a liquid metal which is sodium in the example.

The heart 2 forms a volume which is delimited by a cylinder with polygonal base, or external cylinder, whose generator is vertical and whose average radius is noted RC.

For the modeling of the core 2, one can use a simplified geometry of cylindrical type, circular base. But this heart is actually made of crowns, or volumes, delimited by cylinders with polygonal bases with, in the center of the heart, a cylindrical portion with a polygonal base.

The heights that will be discussed later are counted along a vertical axis z which is parallel to the generatrix mentioned above; and the rays, in particular the radius RC, are counted along a horizontal axis r (which is perpendicular to the z axis).

The core 2 comprises an inner zone 4, of height HZ1, and an outer zone 6, of height HZ2.

The inner zone 4 forms a volume delimited by a cylinder with a polygonal base, or an internal cylinder, whose generator is parallel to the generatrix mentioned above and whose average radius is RZl.

And the outer zone 6 forms another volume, or crown, which is delimited by the inner and outer cylinders, the generatrix of which is parallel to the generatrix mentioned above and whose average internal radius and the mean outer radius respectively are RZ1 and RC.

It is specified that the polygonal bases (also known as "guidelines") mentioned above can be polygons, for example hexagons. These polygons, especially these hexagons, may be regular but not necessarily.

In the example of FIG. 1, the base of each of the cylinders considered forms a regular hexagon and one can then speak of the axis common to these cylinders, namely the z axis in FIG.

According to the invention, the heart 2 comprises, going from the top to the bottom of the heart:

an upper absorbent zone 8, made of a neutron-absorbing material,

a zone of sodium plenum 10,

an upper zone of fissile material

12

an intermediate zone of fertile material 14,

a lower zone of fissile material a lower zone of fertile material 18, which is optional.

In addition, the horizontal median plane P1 of the intermediate zone of fertile material is situated below the horizontal median plane P2 of the assembly formed by the upper zone of fissile material 12, the intermediate zone of fertile material 14 and the lower zone of fissile material 16.

In the example shown, the upper zone of fissile material 12, the intermediate zone of fertile material 14 and the lower zone of fissile material 16 extend only into inner zone 4 of core 2; but the lower zone of fertile material 18 and the sodium plenum zone 10 extend in the inner zone 4 and the outer zone 6; and the outer zone 6 is made only of fissile material.

Different fissile materials may be used in zones 12, 16 and 6. However, it is also possible to use the same fissile material (for example uranium or uranium-plutonium mixed oxide) for these three zones, as allowed by the invention.

And, as a fertile material, one can for example use uranium 238.

In addition, the heart 2 is provided

a lower zone 20, made of a neutron reflective material, more simply referred to as reflective material, under the lower zone of fertile material 18, and - A peripheral zone 22, also made of a reflective material, which surrounds the core 2 and is connected to the zone 8 of neutron-absorbing material and the zone 20 of reflective material, as seen.

For example, zone 8 is made partly of boron carbide constituting the neutron-absorbing material, zone 20 is made of steel blocks, traversed by sodium, and zone 22 is made of steel assemblies.

In FIG. 1, only the elements of the core 2 which allow the understanding of the invention have been represented. For example, the locations of the reactor control rods are not shown.

Moreover, as we know, the core 2 results from a set of vertical fuel elements, namely the fuel pins (in English, fuel pins), which are not shown in FIG.

The nature of a needle depends on the location thereof and therefore the coordinate r. In addition, each needle comprises a stack (vertical) of elements in a sheath, and the nature of an element depends on its position, counted along the z axis.

Among these elements are pellets; and, depending on its position (counted along the Z axis), a pellet may be made of a fissile material or a fertile material.

Only the overall geometry of the core 2 is shown in FIG. 1. Areas of fissile material or fertile material discussed higher are precisely pellets assemblies that have been used.

The following conditions are respected in the core 2, whatever the power of the latter:

the ratio R of the height HFE * of the intermediate zone of fertile material 14 to the height HFI> + HFE * + HFK of the assembly formed by the upper zone of fissile material 12, of height HFI>, the intermediate zone of material 14 and the lower zone of fissile material 16, of height HFK, is in the range of 0.25 to 0.40 (thus the height of the "internal fertile plate", that is to say the zone intermediate of fertile material 14, depends on the height of the "internal fissile zone", that is to say all areas 12 and 16);

the ratio HZ1 / HZ2 is in the range from 0.6 to 1 (thus the height of the inner zone depends on the height of the outer zone);

- The ratio HFI> / HZ1 is in the range of 0.3 to 0.5 (so is a condition imposed on the position of the internal fertile plate within the inner zone);

the ratio RZ1 / RC is greater than or equal to 0.75 (thus the radius of the inner zone depends on the total radius of the heart);

the height of the sodium plenum zone 10 in the inner zone 4 is greater than 20 cm; and the height of the sodium plenum zone in the outer zone 6 is also greater than 20 cm (thus are conditions imposed on the minimum plenum height above the fissile zone)?

The core 2 is optimized to allow satisfactory reactor control. Indeed, the addition of an internal fertile plate (zone 14) tends to uncouple the two axial zones of fuel (zones 12 and 16) when its thickness becomes too great.

It has been verified that, in the proposed design margins, the efficiency curve of the control rods is compatible with the control of the core. Previous publications study this design point in a simplified way.

The interest of the core object of the invention, in particular the core 2, is based primarily on an amplifying effect, resulting from the combination of individual characteristics that reduce the draining effect, characteristics that had been presented in isolation in the prior art.

The heart, object of the invention, in particular the heart 2, brings them together in a very precise arrangement. This makes it possible to correlate their respective advantages in a consistent and amplified way.

The effect obtained (the gain on the emptying effect) is nearly twice the expected effect by summing up the different effects taken individually.

In particular, the "crucible" type geometry (HZ1 / HZ2 <1, that is to say HZKHZ2) meets the desired objectives, namely the reduction of the draining effect, the possibility of controlling the heart and the optimization of the diameter of the heart. The gains obtained were evaluated for the most penalizing situation (end of cycle). In addition to the direct gain on the drain effect, namely a change from $ 5 to $ -0.5, other gains have been highlighted:

gains during accidental spikes:

• gains in sodium temperatures during primary rate loss accidents,

• gains in heart choking temperatures, and

• margins on the times before the loss of the integrity of the structures, during transients of cold loss type (ULOHS for Unprotected Loss of Heat Sink, ULOSSP for Unprotected Loss of Station Service Power),

gains in fuel behavior due to a decreasing maximum linear power history,

potential gains in the corrosion of needle sheaths, and

the possibility of having a single fissile material content for the whole heart, which can compensate for the slight complexity of the manufacturing process, linked to the presence of the internal fertile plate (zone 14).

Finally, it can be noted that a core according to the invention is part of the criteria that is targeted for the fourth generation of reactors.

In order to confirm the results mentioned above, several studies have been conducted: Several calculations of the core and the emptying effect were carried out independently, with the ERANOS calculation code which uses different calculation schemes, and confirmed the results (cross-checking).

Calculations were made to ensure that the geometric effects were well predicted by ERANOS, which is a deterministic code: the absolute deviations on the emptying effect are of the order of $ 0.3 and do not reset cause the invention.

In addition, several integral experiments had been carried out in the past on the different individual characteristics that can be used in a core according to the invention.

Two of these experiments focused on axially heterogeneous cores (z-axis composition of different needles) while another experiment investigated the emptying effect in assemblies with a plenum.

And an irradiation experiment showed that the internal fertile plate (zone 14) limited the corrosion of the sheaths, which makes it possible to increase the rate of combustion.

The geometric optimization of the different heights and rays ratios, mentioned above, makes it possible to maximize the level and the flux gradient at the interface of the upper fissile medium, the internal zone and the plenum.

With this arrangement, the flow level in this interface is amplified by about 30% compared to a homogeneous core of the same power. Leaks, in the case of a situation of emptying, are therefore increased very substantially.

Axial leakage to the plenum, and thus the draining effect, is directly dependent on the thickness of the fertile plate, its axial position, its radial length, and the height of the crucible.

In order to quantify the gains, a comparison was made between a core of 2400 MWth, of standard configuration (homogeneous core), and a core of the same power, in accordance with the invention.

The homogeneous core 24, of standard configuration, is schematically and partially shown in Figure 2. It has a cylindrical symmetry about the z axis.

It has internal 26 and outer 28 fissile zones between two fertile zones 30 and 32. The zone 26 and the zone 28 are made of different fissile materials from each other.

In addition, as can be seen, a zone 33 of neutron-absorbing material is provided above the zone 30, a zone 34 made of reflective material is provided below the zone 32, and the entire core is surrounded by reflective material 35.

Three-dimensional modeling has been performed; in this one, each assembly was based on a network of hexagonal needles. The coolant was sodium, and the fuel used plutonium oxide and uranium (in needles provided with a metal sheath). The code of The calculation used was ERANOS, the current reference code for sodium cooled fast reactors.

The table below shows the emptying values for both cores; it uses the following parameters:

Neutrophage zone 8 in B ₄ C 70 cm thick; HFI> = 32.5 cm; HFE * = 25 cm; HFK = 22.5 cm; HFE <= 30 cm; HZ1 = 80 cm; RZ1 = 1.9 m; RC = 2.3 m; HZ2 = 1 m.

The gain on this configuration is greater than $ 5 and entails, for the heart of the invention, a negative overall drain effect, entirely consistent with the expected results.

All the verifications for this specific case were made by neutron calculations, transient calculations and thermal calculations for the fuel.

These calculations made it possible to check the good behavior of the heart and to evaluate the potential gains.

Figure 3 is a schematic and partial view of another particular embodiment of the invention. As can be seen, the example of FIG. 3 simply differs from that of FIG. 1 in that, in the heart of FIG. 3, the intermediate zone of fertile material 14 also extends into the outer zone 6 of the heart 2 and thus through a fissile material.

Figure 4 is a schematic and partial view of another particular embodiment of the invention.

As can be seen, the example of FIG. 4 differs simply from that of FIG. 3 in that, in the core of FIG. 4, HZ1 and HZ2 are equal, so that HP1 and HP2 are also equal.

According to another particular embodiment not shown, HZ1 and HZ2 are equal but the intermediate zone of fertile material extends only in the inner zone 4 of the core 2.

Claims

1. Heart (2) fast neutron reactor, cooled by a liquid metal, comprising, going from top to bottom of the heart (2):

an upper zone of fissile material

(12),

an intermediate zone of fertile material (14), and

a lower zone of fissile material

(16),

wherein the horizontal median plane (PI) of the intermediate zone of fertile material (14) is located below the horizontal median plane (P2) of the assembly formed by the upper zone of fissile material (12), the intermediate zone of fertile material (14) and the lower zone of fissile material (16), and wherein the ratio R of the height of the intermediate zone of fertile material (14) to the height of the assembly formed by the upper zone of fissile material (12), the intermediate zone of fertile material (14) and the lower zone of fissile material (16) is in the range of 0.25 to 0.40,

characterized in that it further comprises an upper absorbent zone (8), made of a neutron absorbing material, and, beneath the upper absorbing zone, a liquid metal plenum zone (10), the upper zone of fissile material being under the liquid metal plenum zone.

The heart according to claim 1, wherein the ratio R is in the range of 0.25 to 0.35.

3. Heart according to any one of claims 1 and 2, further comprising a lower zone of fertile material (18), located under the lower zone of fissile material (16).

4. Heart according to any one of claims 1 to 3, comprising, radially, an inner zone (4), of height HZ1, and an outer zone (6), of height HZ2, in which the intermediate zone of fertile material ( 14) extends radially at least in the inner zone (4) while the liquid metal plenum zone (10) and the lower zone of fertile material (18) extend radially in the inner zone (4) and the external area (6).

Heart according to claim 4, wherein the HZ1 / HZ2 ratio is in the range of 0.6 to 1.

Heart according to any one of claims 4 and 5, wherein the ratio HZ1 / HZ2 is equal to 1.

Heart according to any one of claims 4 and 5, wherein the ratio HZ1 / HZ2 is less than 1.

Heart according to any one of claims 4 to 7, wherein the ratio HFI> / HZ1 is in the range of 0.3 to 0.5, where HFI> represents the height of the upper zone of fissile material (12) of the inner zone.

Heart according to any one of claims 4 to 8, wherein the inner zone (4) of the heart (2) forms a volume which is delimited by a first cylinder with a polygonal base whose generator is vertical, and the external zone. (6) of the core (2) forms another volume which is delimited, internally, by the first cylinder and externally by a second polygonal base cylinder whose generator is parallel to that of the first cylinder.

Heart according to claim 9, wherein the ratio RZ1 / RC is greater than or equal to 0.75, where RZ1 represents the mean radius of the internal zone (4) and RC the mean external radius of the external zone (6). .

The heart according to any one of claims 4 to 10, wherein the height of the liquid metal plenum zone (10) in the inner zone (4) is at least 20 cm.

Heart according to any one of claims 4 to 10, wherein the height of the liquid metal plenum zone (10) in the outer zone (6) is at least 20 cm.

The heart according to any one of claims 4 to 12, wherein the intermediate zone of fertile material (14) further extends into the outer zone (6).