FR2711835A1 - Fast-neutron nuclear reactor in which at least one moderator element is incorporated in assemblies of the reactor - Google Patents

Abstract

In a fast-neutron nuclear reactor, it is proposed to introduce a moderator element such as B<11>4C. The neutron spectrum is thus softened, which makes it possible to reduce the vacuum coefficient and increase the Doppler constant. The increase in reactivity which occurs in the event of coolant loss is thus reduced at all points of the core of the reactor. The moderator element can be mixed with the nuclear fuel, or with a material of a different nature, in a homogenous form or placed, in heterogenous form, in specific rods (needles) (14) distributed among the rods (12) containing the nuclear fuel or another material.

Description

FAST NEUTRON NUCLEAR REACTOR IN WHICH
LESS A MODERATOR ELEMENT IS INCORPORATED IN
REACTOR ASSEMBLIES.

DESCRIPTION
The invention relates to a fast neutron nuclear reactor in which a moderator element is incorporated into assemblies of the reactor in a homogeneous or heterogeneous manner so as to modify the neutron spectrum to reduce the void coefficient and increase the Doppler constant. .

In a fast neutron nuclear reactor using sodium as a coolant, the void coefficient corresponds to the change in reactivity due to loss of sodium in a given region of the reactor. This loss of sodium may be due to expansion caused by a rise in temperature.

It can also originate from a partial or total emptying of the region considered.

The vacuum coefficient which prevails at a given location in the core of a fast neutron nuclear reactor is the result of the composition of elementary effects. Among these effects, the two most important are the increase in neutron leakage and the hardening of the neutron spectrum following a loss of sodium in the region under consideration. Increased neutron leakage always has a negative effect on reactivity. In contrast, the hardening of the neutron spectrum has a positive effect on reactivity in fast neutron nuclear reactors using plutonium as fuel. This positive effect is linked to the behavior, at high energies, of the cross sections of actinides.

The relative importance of each of these two effects in the event of sodium loss depends strongly on the region considered of the reactor core. Thus, the vacuum coefficient is positive at the center of the reactor core, where the influence of the hardening of the neutron spectrum is preponderant. On the other hand, this coefficient becomes zero, then negative going towards the periphery of the heart.

Furthermore, the size of the reactor also has an influence on the value of the vacuum coefficient. Indeed, the influence of the increase in neutron leaks is preponderant in small reactors, while the most important elemental effect is by far the hardening of the neutron spectrum in high power reactors. In this case, the introduction of reactivity can vary between 4S and 6S depending on the size and geometry of the reactor core, on the assumption of complete emptying of the fissile zone.

Even though accidental conditions leading to extensive emptying of the reactor core are extremely unlikely, it is therefore desirable to reduce the void coefficient as much as possible, so that the increase in reactivity caused by such emptying is as small as possible.

This reduction in the void coefficient can be achieved by modifying the neutron spectrum aimed at limiting the hardening of this spectrum when sodium is lost (use of moderating materials).

où p1 est la réactivité du coeur à la température absolue
T1 du combustible et p2 est la réactivité du coeur à la température absolue
T2 du combustible.Moreover, the Doppler constant KD is defined by the relation

where p1 is the reactivity of the core at absolute temperature
T1 of the fuel and p2 is the reactivity of the core at absolute temperature
T2 of the fuel.

An increase in temperature in the core has the effect of widening the resonances of the cross sections of the materials therein. This is the Doppler effect. This effect leads to variations in reaction rates in the energy range below about 60 keV. Given that the core of a fast neutron nuclear reactor contains fertile materials and mainly uranium 238, in large quantities, the Doppler effect results in an increase in captures and therefore in a decrease in reactivity which occurs opposes a subsequent increase in fuel temperature. The Dcppler effect therefore constitutes a stabilizing counter reaction.

The Doppler constant is the parameter which characterizes this feedback in fast reactors using fuel consisting of mixed oxides (Pu02-U02).

The effect produced by the feedback due to the Doppler effect is therefore opposed to that of the sodium void coefficient in the event of loss of refrigerant. Therefore, in order to best limit the consequences of a loss of refrigerant, it is desirable to jointly obtain a reduction in the vacuum coefficient and an increase in the Doppler constant. A softening of the neutron spectrum in the reactor core, obtained by the use of moderating materials, makes it possible to simultaneously achieve these two objectives.

Various solutions have already been devised in order to reduce the vacuum coefficient in fast neutron reactors.

A first known solution consists in giving the ratio between the height and the diameter of the reactor core a value much less than one.

Another known solution consists in giving the core of the reactor an annular shape.

Yet another known solution consists in giving the reactor core a heterogeneous structure in the radial and / or axial direction, the different parts being neutronically decoupled.

Finally, another known solution consists in making the core of the reactor in modular form.

All of these known solutions aim to reduce the void coefficient by increasing neutron leaks. The reduction in the vacuum coefficient is therefore not obtained at all points of the core. In addition, since these solutions cause a substantial hardening of the neutron spectrum, the feedback effect due to the Doppler constant is reduced.

It is recalled, moreover, that in order for the radial power distribution in the fissile zone of the core of a nuclear reactor to be as uniform as possible, from the center of this zone to its periphery, different types of cores have been imagined.

For this purpose, it is known to produce the fissile zone by means of at least two types of assemblies.

More precisely, assemblies with high reactivity are placed at the periphery of the fissile zone, while assemblies with lower reactivity are placed at the center of this zone. The difference in reactivity can be obtained either by using assemblies in which the percentage of inert material to fissile material is different, or by using assemblies in which the enrichment of the fuel is different. The latter solution was chosen in particular for the core of the French Super Phénix reactor.

In order to obtain a relatively uniform radial power distribution, it is also known practice to use a single type of fuel assembly and to regularly arrange, in the central part of the core, inert elements making it possible to reduce the reactivity therein. This solution is described in documents FR-A-2,576,704 and FR-A-2,581,232.

Finally, the flattening of the radial power distribution can also be obtained by introducing into the fissile zone fertile materials, either in the form of fertile assemblies, or in the form of pellets of fertile material placed between the pellets of fissile material. in the needles of fuel assemblies. This solution is illustrated in particular by the documents
FR-A-2 023 431, FR-A-2 286 472, FR-A-2 546 656 and EP-AO 097 372.

The subject of the invention is a fast neutron nuclear reactor, the fissile zone of which is produced according to one of the above techniques, or others, but in which the neutron spectrum is softened so as to reduce the sodium void coefficient. and to increase the Doppler constant, so that the increase in reactivity in the event of sodium loss is as low as possible at all points of the core, whatever the size of the reactor.

In accordance with the invention, this result is obtained by means of a fast neutron nuclear reactor whose core consists of juxtaposed assemblies, these assemblies being formed at least in part of fissile assemblies, this reactor being characterized by the fact that at least some of the assemblies contain a moderator element.

For example, the replacement of part of the nuclear fuel by a moderating element, in all the fissile assemblies, makes it possible, by softening the neutron spectrum, to reduce the vacuum coefficient and to increase the Doppler constant everywhere in the heart. In addition, this effect is obtained regardless of the size of the reactor.

In a preferred embodiment of the invention, the moderating element consists of B11 4C. Natural B4C, depleted in B10, can however also be used. In fact, unlike natural boron carbide (atomic percentage in
B10: 19.82% and a fortiori with B4C (enriched in B10), which are neutron absorbing materials, B1 4C is a moderating material.

The use of B11 4C as a moderator in the fissile assemblies of a fast neutron nuclear reactor has many advantages.

Thus, this material has satisfactory physical characteristics, and in particular a high melting point. In addition, it behaves well under irradiation, that is to say that no appreciable dimensional or structural modification is observed. Finally, its compatibility with sodium and with the sheath material of the needles forming the assemblies is good.

The introduction of the moderating element into at least some of the assemblies of the reactor can be carried out either homogeneously or heterogeneously. In the first case, each of the needles of the assemblies concerned contains a mixture of materials comprising several or a single moderating element.

In the second case, the moderating element (s) are placed in specific needles. These needles containing the moderating element are preferably regularly distributed among the other assemblies.

When the invention is applied to fast neutron reactors incinerating actinides, that is to say in which the nuclear fuel comprises a mixed oxide of uranium and / or plutonium and / or minor actinides, a nitride of uranium and / or plutonium and / or minor actinides, or else plutonium and minor actinides,. B11 4C, used in homogeneous mode, also forms an inert matrix for the fuel. Thus, in addition to its effects of reducing the sodium void coefficient and increasing the Doppler constant, the moderating element then improves the incineration performance of the reactor.

To facilitate the transmutation of long-lived radioactive fission products to stable elements, target assemblies also containing a moderating element such as B11 4C are placed in the reactor, in appropriate zones, so as to increase the capture rate of neutrons in fission products.

Various embodiments of the invention will now be described, by way of nonlimiting example, with reference to the accompanying drawings, in which
- Figure 1 is a graph showing neutron spectra, in a fast neutron reactor core studied with and without moderator
- Figure 2 shows, in section along a horizontal plane, three juxtaposed fissile assemblies of the core of a fast neutron nuclear reactor containing moderator; and
FIG. 3 represents the radial distribution of power (in phantom lines) and of neutron flux (in solid lines) inside the assemblies with moderator.

Figure 1 shows the spectrum of neutrons in a high power fast neutron reactor core, in the case of a traditional core without moderator (in phantom lines), and in the case of the same reactor, of which 20% of the fuel has been replaced by B11 4C (solid line).

The curve in figure 1 clearly shows the softening of the neutron spectrum, obtained thanks to the presence of a moderating element in the fissile assemblies. This softening results in a reduction in the vacuum coefficient of the core under study of about 35% and an increase in the Doppler constant of about 40%. These two cumulative effects make it possible to very significantly limit the increase in reactivity induced by a possible loss of sodium.

FIG. 2 represents three juxtaposed fis sile assemblies of the core of a reactor, in accordance with the invention, in the case where a moderating element is distributed heterogeneously in the assemblies.

As illustrated in FIG. 2, in order to distribute the moderating element heterogeneously in the fuel assemblies 10, a bundle of needles formed from two types of fuel is placed inside the casing 11 of each of these assemblies. different needles. First needles, designated by the reference 12 in FIG. 2, conventionally contain nuclear fuel pellets. In the other needles, designated by the reference 14 in FIG. 2, the nuclear fuel pellets are replaced by pellets of moderator element such as B114C. These needles 14 are regularly distributed among the needles 12, so as to install the moderator as regularly as possible inside the assembly.

In the embodiment illustrated more precisely in FIG. 2, the bundle formed by the needles 12 and 14 comprises alternately, starting from any one of the sides of the hexagon formed by the outer casing 11, first rows formed alternating needles 12 and 14 and second rows formed only of needles 12, the first and second rows being arranged alternately.

The particular arrangement illustrated in FIG. 2 makes it possible to avoid the appearance of local power peaks in the reactor core. This characteristic is illustrated by the curves of figure 3 which represent on the abscissa the segment AB of figure 2 (in centimeters) and on the ordinate the evolution of the power (in phantom lines) and of the flow (in continuous line) on this. segment, the corresponding units being given arbitrarily.

On the other hand, it is important to note that the use of a moderating element in inert assemblies would result in the formation of local power peaks in the core, at the interface with the inert assemblies. In addition, this solution would have a very limited effect on the vacuum coefficient and on the Doppler constant. It must therefore be ruled out.

In practice, the moderating element can be introduced into the fissile assemblies, either homogeneously or heterogeneously.

When the moderator element is placed in the nuclear fuel in a homogeneous manner, the decrease in reactivity by irradiation of the fuel increases and the regeneration gain decreases. On the other hand, the size of the core can remain unchanged with respect to the existing reactor.

On the contrary, when the moderator element is introduced heterogeneously into the fissile assemblies, the increase in the drop in reactivity by irradiation of the fuel as well as the decrease in the regeneration gain can be avoided. However, the size of the heart increases.

The principle described above in the case of fissile assemblies placed in the reactor core also applies to target assemblies containing radioactive fission products, optionally placed outside the core. It then improves the transmutation of long-lived radioactive fission products to stable elements. The introduction of a moderating element such as B114C in these target assemblies indeed improves the transmutation process.

The invention relates to a fast neutron nuclear reactor cooled by liquid sodium and the core of which is constituted, in a conventional manner, by juxtaposed assemblies, implanted in a regular network. These assemblies can in particular be arranged vertically inside a reactor vessel filled with liquid sodium, according to well-known technology which does not form part of the invention.

The assemblies constituting the reactor core are formed mainly of fissile assemblies containing the nuclear fuel. As illustrated in FIG. 2, each of the fissile assemblies 10 comprises a metallic outer casing 11, the horizontal section of which is hexagonal. A bundle of needles containing nuclear fuel pellets is placed inside the casing 11.

According to technologies which have been recalled previously, the fissile assemblies can all be identical or comprise at least two different types of assemblies.

If the fissile assemblies are all identical, the relatively uniform nature of the radial power distribution in the core can be obtained either by regularly arranging throughout the central zone of the core, instead of certain fissile assemblies, inert elements. such as empty hexagonal tubes (holes) or steel bars (thin Steel - Na assemblies. Fertile assemblies can also replace some of the fissile assemblies.

In the case where the reactor core comprises at least two types of fissile assemblies, a relatively uniform radial power distribution can be obtained by placing assemblies with high reactivity at the periphery of the fissile zone and assemblies with lower reactivity at the center. of this area. For this, it is possible to use either assemblies in which the percentage of inert material relative to the fissile material is different, or assemblies in which the enrichment of the fuel is different. Pellets of a fertile material can also be placed in the needles of certain fissile assemblies, among the nuclear fuel pellets.

In addition, it must be considered that significant changes in the reactivity and power distribution of the core occur during operation of the reactor. This undesirable effect is usually avoided by placing pilot assemblies in appropriate locations in the heart, which are gradually withdrawn as the reactivity and power distribution in the heart changes.

The invention is equally applicable to all these types of reactors.

Claims (7)

1. Fast neutron nuclear reactor, the core of which consists of juxtaposed assemblies, implanted in a regular network, these assemblies being formed at least in part of fissile assemblies (10), characterized in that at least some of the Reactor assemblies contain a moderating element. 2. Reactor according to claim 1, characterized in that the moderating element consists of B114C. 3. Reactor according to claim 1, characterized in that the moderating element consists of natural B4C, depleted in B10. 4. Reactor according to any one of the preceding claims, characterized in that specific needles (14) containing moderator are introduced inside certain assemblies. 5. Reactor according to any one of claims 1 to 3, characterized in that the moderating element is mixed with another material -e, now in the assemblies. 6. Reactor according to any one of claims 4 and 5, characterized in that the nuclear fuel, being chosen from the group comprising mixed oxides and nitrides of at least one actinide, may or may not contain minor actinides. 7. Reactor according to any one of claims 1 to 6, characterized in that the target assemblies, containing radioactive fission products and a moderating element, are placed in the reactor.