FR2745658A1 - Neutron absorber of boron carbide and hexagonal boron nitride - Google Patents

Abstract

A novel neutron absorber material is obtained by hot pressing a powder mixture of boron carbide, hexagonal boron nitride and impurities until anisotropic mechanical strength and thermal conductivity properties are achieved. Preferably, the powder mixture is filled into a pellet-forming mould and then hot pressed in the direction in which the property anisotropy is greater in the radial direction than in the axial direction of the resulting pellet. Also claimed is a nuclear reactor control rod, obtained by pressing pellets, produced as above, into a fuel tube. Further claimed is a process for producing a neutron absorber material by: (a) mixing boron carbide powder of\-3 microns average particle size with hexagonal boron nitride power of \-10 microns average particle size; and (b) moulding the powder mixture by pressing in a cylindrical mould and then hot pressing at 1700 deg C and 5-50 MPa pressure.

Description

 The present invention relates to a neutron absorbing material for a control bar of a nuclear reactor whose safety in an irradiated environment of neutrons has been proven.

10 he
Boron (B) includes isotopes B and B. Boron
10% natural contains 19.8% B. While boron B does not absorb
10 not really electrons, boron B, has a very large neutron absorption cross section. A boron compound, boron carbonate (B4C) contains a very large amount of boron per unit volume; it is stable at high temperatures and its method of industrial manufacture is confirmed. Boron B has a very large absorption cross section in a thermal environment of neutrons; moreover it has a considerable absorption performance in a zone of fast neutrons. The boron carbonate obtained by increasing the concentration of natural B borate from 19.8 W up to approximately 90 Co, is widely used as a neutron absorbing material for nuclear reactor control rods.

In the case of fast reactors, the control bar generally has a structure with pellets of boron carbonate used as a neutron absorber, placed in fuel-sheathing tubes, made of stainless steel. The boron carbonate pellets are generally manufactured by hot pressing. The boron carbonate pellets are very hard, they have a low consistency and a low resistance to thermal shock. In addition, the neutron absorption of B is primary
10 provided by the reaction B (n, ct). Since more helium (He) accumulates than neutrons in boron carbonate, there is a fundamental problem due to the swelling of the boron carbonate pellets. The mechanical interactions between the boron carbonate pellets and the fuel tube decrease the operating time of the control rods. In addition, boron carbonate pellets produce large amounts of heat when absorbing neutrons, which causes an intense break in the boron carbonate pellets due to thermal stresses. These broken pellets create and accelerate the mechanical interaction between the boron carbonate pellets and the fuel tube which decreases the originally planned operating time of the control rods.

To overcome such problems, the following techniques have been developed (1) Hei 4-78159 discloses a method of controlling
the size of the grains in the sintering step using a
specific proportion of B / C atoms and the primary size of
grains.

(2) The Japanese published document Sho 59-150 369, describes a
thode for disposing a nitrate damping section of
boron in a fuel tube.

(3) The Japanese published document Sho 59-57 195 describes a method
to incorporate powdered glass or powder
boron to prevent boron being sintered in the
fuel tube.

(4) The document Proc. llth Int. Symp. Boron, Borides and Rela
ted Compounds, Tsukuba, 1993 IJAP Series 10 (1994) p. 216-219,
D. Gosset et al., Describes a method to increase the
thermal shock resistance of a carbonate carbonate pellet
boron by adding boron nitrate in a percentage by
volume of 30 W at least.

However, the conventional techniques mentioned above have the following disadvantages
In the state of the art according to 159, the swelling is reduced and the possibility of seeing a damaged fuel tube is lower than in the case of a conventional pellet. However, the problem of slicing the calcium carbonate pellet due to thermal stress remains.

 In the state of the art according to document 369, a control bar is obtained causing slight damage to the fuel tubes even if these tubes are broken by the swelling of the boron carbonate pellets or bursting due to thermal shocks. But that complicates the structure of the control bar.

 In the state of the art according to document 195 the damage due to thermal shock to a pellet or tube is reduced by using boron carbonate powder. However, the density of the absorber material is reduced compared to a sintered pellet. In addition, even if a small crack appears in the fuel tube, the neutron absorbing material powder spreads out of the tube and cancels the absorption performance.

 In the prior art according to D. Gosset, the thermal shock resistance of a neutron absorbing material is increased and the degree of damage to materials by thermal stresses is decreased. In addition, because the damage to the neutron absorbing material decreases, the damage to the tube is decreased. However, swelling of the neutron absorbing material can damage the fuel tube.

 The present invention proposes to solve the problems mentioned above and aims to achieve a neutron absorbing material that does not reduce the operating time of a control bar of a nuclear reactor because of the swelling of the reactor. Neutron absorbing material or thermal stress fracture, including an economical neutron absorbing material, which does not easily damage the fuel tubes due to swelling and breaking of the material as it absorbs neutrons, and thus an absorption material of superior quality, stable to neutron irradiation.

 For this purpose, the invention relates to a neutron absorbing material, characterized in that it is obtained by hot pressing a mixed powder comprising boron carbonate, hexagonal boron nitrate and unavoidable impurities until that the anisotropic properties of mechanical strength and thermal conductivity appear.

According to other features of the invention - a mold is filled to form pellets with powder
mixture comprising boron carbonate, boron nitrate
hexagonal and unavoidable impurities and hot compress the
powder mixed in the direction according to which the anisotro
mechanical resistance and thermal conductivity are
higher in the radial direction of the resulting pellet
only in its axial direction - the anisotropies of mechanical strength and conductivity
are greater than or equal to 1.5 - the proportion of mixing between boron carbonate (B4C) and
hexagonal boron nitrate (BN) is 6: 4.

the proportion of mixing between the boron carbonate (B4C) and the
hexagonal boron nitrate (BN) is 4: 6.

 The invention also relates to the control rods obtained and in particular a reactor control rod comprising cylindrical neutron absorbing pellets whose anisotropies of mechanical strength and thermal conductivity of a neutron absorbing material containing at least boron are 1 , And a fuel tube containing the pellets.

The invention also relates to a method for producing a hot-pressed neutron absorbing material characterized in that - a boron carbonate powder having an average size of
grain of 3 ssm at least, is mixed with a nitrate powder
of hexagonal boron having an average grain size of 10 μm at
less, - the mixed powder is first shaped by compression
in a cylindrical mold then the mixed powder formed first
is hot-pressed at a temperature of 1700 to 2000C and
a pressure of 5 to 50 MPa.

According to other features of the invention - the proportion of mixing between boron carbonate (B4C) and
hexagonal boron nitrate (BN) is 6: 4.

the proportion of mixing between the boron carbonate (B4C) and the
hexagonal boron nitrate (BN) is 4: 6.

 The present invention makes it possible to reduce the swelling and rupture of the neutron absorbing material due to thermal stresses. The process provides a material that reduces the mechanical interaction between the fuel tube and the neutron absorbing material while satisfying the safety and cost-effectiveness of nuclear reactor control rods.

1. When considering the use of a control bar of a reaction
nuclear power, it is possible to control the damage on
a fuel tube even if there is swelling of the ab material
sorbent the cylindrical neutrons, relieving the swelling of the
material in the radial direction.

2. The swelling of a neutron absorbing material, cylindrical
in its radial direction can be controlled by choosing a
greater mechanical strength in the radial direction
only in the axial direction.

3. Thermal stresses produce in an absorbing material
bant the cylindrical neutrons is reduced by increasing the con
thermal ductivity of the material in its radial direction.

4. By increasing the strength of an absorbent material
neutrons in its radial direction, the material undergoes little
damage in this direction; the risk of damaging the
fuel tube is reduced even if the material is broken
in its axial direction because of swelling or con
thermal stresses.

5. Hexagonal boron nitrate has anisotro characteristics
pes, thermal and mechanical, marked, and like graphite
is superior for thermal shock resistance and
chemical stability. Hexagonal boron nitrate has a very
high thermal conductivity in the surface of the layers by
comparison with the surface between the layers. When making
that a cylindrical pellet made of material absorbing
neutrons, it is possible to have a strong resistance and a
high thermal conductivity to the radial direction of the pitch
by controlling the organization of hexagonal boron nitrate
so that the base plane is perpendicular to the axis
of the cylinder.

 The elements mentioned above will be described in more detail below with the aid of the drawings.

Drawings - Figures 1 (A) and 1 (B) are illustrations to explain
the theory of the present invention, - Figure 2 explains a test method for the resistor
and thermal conductivity - Figure 3 shows a sample for the thermal conductivity
that and a thermal conductivity test, - Figure 4 shows a sample and a test method for
the evaluation of the fracture due to thermal stresses; FIG. 5 shows a pellet according to the present invention.

According to Figure 1 (A), the crystallized form of hexagonal boron nitrate has a multilayer structure. When the hexagonal boron nitrate powder is hot-pressed, the hexagonal layers are arranged in the vertical direction relative to the pressing direction, as shown in FIG. 1 (B). In this case, the hexagonal boron nitrate multilayer structure has a higher thermal conductivity in the horizontal direction (direction (a) in Figure 1 (A)). The thermal conductivity is lower in the vertical direction (direction (b) in Fig. 1 (A)). So that its thermal conductivity is in the vertical direction {direction (b) in Fig. 1 (A)). In addition, the boron nitrate expands or comes into easy contact in the vertical direction (direction (b) in Figure 1 (A)) but expands or comes into little contact in the horizontal direction (direction (a) in Figure 1 (A)
When hot pressing C of hexagonal boron nitrate and boron carbonate until adequate anisotropy appears, the mixed powder has another thermal conductivity in the horizontal direction (direction (a) in FIG. B)) towards the hot compression direction and expands or contracts very little in this direction. This results in a neutron absorbing material whose thermal conductivity is lower in the vertical direction (direction (b) in Fig. 1 (B)) parallel to the hot compression direction and the material expands or contracts significantly in the hot compression direction.

When making a neutron absorbing chip with the above material, so that the hot compression direction coincides with the direction of the axis, the pellet has a further thermal conductivity and a high strength in the direction radial. This type of pellet undergoes few longitudinal breaks and has little effect on the sheath tube; so that the duration of use of a control bar using such pellets is not reduced.

 The present invention provides the neutron absorbing material, the absorbent pellets and a control bar for a nuclear reactor.

 The present invention provides a neutron absorbing material by hot pressing a mixture of boron carbonate powder, hexagonal boron nitrate and unavoidable impurities in a mold to form pellets and then hot pressing the mixed powder according to the invention. the direction in which the anisotropy occurs, the thermal conductivity of the pellet obtained being higher in its radial direction than in its axial direction, until the anisotropy appears.

 The present invention also provides a nuclear reactor control rod obtained by placing the above pellets in a fuel tube.

 As described above, a neutron absorbing material or the like according to the present invention comprises sintered pellets made of the powder mixture comprising boron carbonate and hexagonal boron nitrate; it is anisotropic while respecting the mechanical resistance and the thermal conductivity. It is preferable that the degree of anisotropy be 1.5 or more. Anisotropy of the mechanical strength of less than 1.5 is undesirable because only slight swelling is controlled in the direction of high strength. In addition, an anisotropy of thermal conductivity of less than 1.5 is undesirable because thermal stresses increase at the same time in the neutron absorber.

 It is preferred that the boron carbonate used to make a composite pellet according to the present invention has an average primary grain size of less than or equal to 5 clam. When the average size of the crystal grains exceeds 5 μm, the grain-binding area per unit volume decreases and this results in an increase in the He holding time in the crystal grains. This is not advantageous because the amount of He increases, which causes a strong swelling thereafter.

 Since hexagonal boron nitrate can be used in general, the thermal conductivity depends on the crystallinity of the hexagonal boron nitrate. For this, it is better to make an evaluation using the value G.I.

defined in the following expression (1)
GI = (I (100) + I (101)) / (I (102)) (1) wherein I is an integrated intensity, measured by X-ray diffraction. It is preferable not to use a low crystallinity powder with a high GI value because the thermal conductivity of the sintered composite pellet decreases.

 It is preferable not to use powder having low grain size hexagonal boron nitrate because it is difficult to have the anisotropy of mechanical strength and thermal conductivity desired for a sintered composite pellet.

 The above boron carbonate and boron nitrate are mixed according to a general mixing method, such as a dry blending method or a wet blending method using a solvent alcohol to uniformly disperse these materials.

It is possible to use a non-specialized mixer such as a ball mill or a ribbon blender.

 By respecting the proportions of the mixture between the boron carbonate and the boron nitrate, the absorption performance deteriorates when the amount of boron carbonate is very low. A very large amount of boron carbonate is undesirable because it reduces the anisotropy of mechanical strength and thermal conductivity. For this purpose it is necessary to correctly select a proportion of mixture so that the anisotropies of mechanical strength and thermal conductivity are equal to 1.5 or more and that the neutron absorption performance is not too deteriorated.

 The mixed powder is placed in a graphite mold set at a predetermined preliminary forming pressure as necessary; then hot pressing in a hot compression, high frequency induction furnace. For hot compression conditions, a general temperature (1700 to 2000C, approximate pressure 5 to 50 MPa) is used. Extremely low temperature or pressure is undesirable because it takes a long time to obtain a dense pellet. An extremely high temperature is also not desirable because the mixed powder readily reacts with the graphite used for hot pressing. In addition, extremely high sintering pressure is undesirable because the deterioration frequency of the graphite molds used for hot pressing greatly increases. However, it is possible to use normal sintering compression for shaping in known equipment (CIP Technique) and then sinter in an inert atmosphere instead of high pressure sintering.

 In the case of a neutron absorbing material pellet made of a boron carbonate / hexagonal boron nitrate compound, the rate of deterioration of a tube due to swelling is greatly reduced in a neutron irradiation environment. . In addition, it is possible to prevent the rupture of the absorbent material because of thermal stresses or swelling. It is also possible to greatly reduce the frequency of deterioration of a tube. As the life of a nuclear reactor control bar is increased compared to a conventional material, it improves safety and efficiency.

Description
The present invention will be described in more detail in accordance with the embodiment. (Embodiment 1 to 5 and Comparative Examples 1 and 2)
The boron carbonate powder (average grain size 3 Mm measured with the microtrace method, molar ratio B / C of 4 and enrichment 10B of 19.8 W of natural) and hexagonal boron nitrate powder (GI = 1 , 1, average grain size 10 μm measured by the microtrace method) were mixed for 2 hours according to the proportions of Table 1, in a ribbon blender to be uniformly dispersed. The resulting mixed powder was pressed into a graphite mold and first formed at a preliminary formation pressure of 10 MPa and then placed in a hot press, high frequency induction hot press furnace under the conditions (the maximum time for maintaining the temperature and the pressure was 1 hour respectively) of Table 1 to obtain a pellet having a diameter of 50 mm and a height of 40 mm.

 The x-ray diffraction of the pellets obtained reveals that there is boron carbonate and boron nitrate. In addition, a chemical analysis of the pellet confirms that the proportion of mixing between the boron carbonate and the boron nitrate coincides with the case of the mixed material.

[Conditions of Comparative Example 3]
The preparation and evaluation were carried out as in the previous case except that boron nitrate powder was used with a GI of 3.6 and a mean grain size of 4 μm, measured by the microtrace method.

[Evaluation of physical properties]
The following physical properties were evaluated for the resulting composite pellet.

(1) Mechanical resistance
samples 10 and 20 of 3x4x30 mm were chosen from
pellets obtained as shown in FIG.
the 3-point bending resistance of the workpiece in the
directions parallel dl and perpendicular d2 to the direction
hot pressing C, at room temperature.

(2) Thermal conductivity
samples 30, 40 with a diameter of 10 mm and a thick
1.5 mm were chosen from pellets obtained
as shown in Figure 3, to measure conductivity
thermal samples in parallel directions dl and
perpendicular d2 to the hot compression direction C and
at room temperature by the laser beam method.

(3) Evaluation of the breaking properties due to the constraints
thermal
a sample 50 with a diameter of 12.5 mm and a length
25 mm was chosen from pellets obtained as the
shows Figure 4, to evaluate the frequency of 10 cycles
for which a crack appears in a repeating piece
both a heating and cooling cycle
the sample is heated at 4000C for 10 min. and cooled
in water and then dried in an atmosphere at 900C during
2 hours and 1200C for 1 hour, then heated again
a heat shock is then applied.

Advantages of the invention
Table 1 shows the evaluation results of a neutron absorbing material according to the present invention as well as the conditions of preparation.

 As shown in Table 1 and Figure 5, the sintered bodies have a higher resistance to thermal shock and better resistance to fractures due to thermal stresses, compared to reference examples. In addition, in the case of pellets according to the invention, the anisotropies of mechanical strength and thermal conductivity are 1.5 or more. For this, when forming a pellet according to the invention, by sintering, so that the compression direction coincides with the axial direction of the pellet; the pellet has a higher thermal conductivity in its radial direction a and thus it expands in its axial direction but hardly in the radial direction. When this pellet absorbs neutrons, it expands in its axial direction d due to swelling but little in its radial direction a. This is why pellets do not subject mechanical stress to the tube that contains them. In addition, since they preferentially conduct the heat produced when they absorb the neutrons in its radial direction, small temperature differences appear in the radial direction at pellets. This is why the pellet rarely breaks. When a temperature difference appears in the axial direction b of the pellet, it breaks. As the pellets are superimposed on each other in their axial direction, a horizontal break influences less a control bar than a vertical break.

Thus, the pellets according to the invention do not actually affect a tube and do not reduce the duration of use of the control bar in a reactor.

As described above, it is clear that the boron carbonate / hexagonal boron nitrate composite pellets obtained by the present invention (neutron absorbing material according to the present invention) can greatly reduce the mechanical interaction with a fuel tube due to a rupture of the neutron absorbing material caused by swelling or thermal stress in an environment irradiated with neutrons. It has been found that the neutron absorbing material is capable of improving the safety and cost-effectiveness of a control bar compared to conventional materials. TABLE I

COMPOSITION <SEP> CONDITIONS <SEP> PROPERTY <SEP> PHYSICAL <SEP> DE <SEP> LA <SEP> PASTILLE
<tb><SEP>% in <SEP> weight <SEP> DE <SEP> FRITTAGE
<tb> B4C <SEP> BN <SEP> Temperature <SEP> Pressure <SEP> RESISTANCE <SEP> A <SEP><SEP> VOLTAGE <SEP> CONDUCTIVITY <SEP> THERMAL <SEP> FREQUENCY <SEP> FROM
<tb> ture <SEP> in <SEP> MPa <SEP> MPa <SEP> W / mk <SEP> RUFIURE <SEP> DUE <SEP> AUX
<tb> C <SEP> DIRECTION <SEP> DIRECTION <SEP> ANISOTRO <SEP> DIRECTION <SEP> DIRECTION <SEP> ANISOTRO <SEP> CONSTRAINTS
<tb> PARALLEL <SEP> PERPENDICU <SEP> PIE <SEP> PERPENDICU <SEP> PARALLEL <SEP> PIE <SEP> THERMAL
<tb> LAIRE <SEP> LAIRE <SEP> (Number <SEP> of <SEP> times)
<tb> 1 <SEP> 80 <SEP> 20 <SEP> 2000 <SEP> 40 <SEP> 103 <SEP> 40 <SEP> 2.6 <SEP> 34 <SEP> 22 <SEP> 1.5 <SEP > 9
<tb> 2 <SEP> 60 <SEP> 40 <SEP> 2000 <SEP> 40 <SEP> 88 <SEP> 32 <SEP> 2.8 <SEP> 40 <SEP> 23 <SEP> 1.7 <SEP >> 10
<tb> 3 <SEP> 40 <SEP> 60 <SEP> 2000 <SEP> 40 <SEP> 66 <SEP> 18 <SEP> 3.7 <SEP> 45 <SEP> 23 <SEP> 2.0 <SEP >> 10
<tb> 4 <SEP> 40 <SEP> 60 <SEP> 1800 <SEP> 40 <SEP> 47 <SEP> 13 <SEP> 3.6 <SEP> 35 <SEP> 18 <SEP> 1.9 <SEP > 9
<tb> 5 <SEP> 40 <SEP> 60 <SEP> 2000 <SEP> 20 <SEP> 52 <SEP> 28 <SEP> 1.9 <SEP> 39 <SEP> 24 <SEP> 1.6 <SEP >> 10
<tb> 1 <SEP> 100 <SEP> 0 <SEP> 2000 <SEP> 40 <SEP> 435 <SEP> 441 <SEP> 1.0 <SEP> 22 <SEP> 21 <SEP> 1.1 <SEP > 3
<tb> 2 <SEP> 0 <SEP> 100 <SEP> 2000 <SEP> 40 <SEP> - <SEP> 3 <SEP> 40 <SEP> 60 <SEP> 2000 <SEP> 40 <SEP> 151 <SEP > 115 <SEP> 1.3 <SEP> 28 <SEP> 23 <SEP> 1.2 <SEP> 6
<tb> Note. a symbol - in the table means that the physical properties could not be evaluated because a hot compressed pellet broke after the hot compression.

Claims (7)

10) Neutron absorbing material, characterized in that it is obtained by hot pressing a mixed powder comprising boron carbonate, hexagonal boron nitrate and unavoidable impurities until the anisotropic properties of mechanical strength and thermal conductivity appear. 20) Neutron absorbing material according to claim 1, characterized in that a mold is filled into pellets with mixed powder comprising boron carbonate, hexagonal boron nitrate and unavoidable impurities and hot pressing is carried out. powder mixed in the direction in which the anisotropies of mechanical strength and thermal conductivity are higher in the radial direction of the pellet obtained than in its axial direction. 30) Material according to one of claims 1 and 2, characterized in that the anisotropies of mechanical strength and thermal conductivity are greater than or equal to 1.5.  4) Material according to one of claims 1 to 3, characterized in that the proportion of mixing between boron carbonate (B4C) and hexagonal boron nitrate (BN) is 6: 4. 50) Material according to one of claims 1 to 3, characterized in that the proportion of mixing between boron carbonate (B4C) and hexagonal boron nitrate (BN) is 4: 6. 60) control bar for a nuclear reactor, characterized in that it is obtained by compressing the pellets according to claim 2 in a fuel tube. 70) Control rod for nuclear reactor according to claim 6, characterized in that it comprises cylindrical pellets absorbing neutrons whose anisotropies of mechanical strength and thermal conductivity of a neutron absorbing material containing at least boron are 1.5 and a fuel tube containing the pellets.  8) Process for producing a hot-pressed neutron absorbing material according to one of claims 1 to 5, characterized in that - a boron carbonate powder having an average grain size of at least 3 μm is mixed with a hexagonal boron nitrate powder having an average grain size of at least 10 μm; the mixed powder is first shaped by compression in a cylindrical mold and then the first mixed powder is hot-pressed; at a temperature of 1700-2000C and a pressure of 5-50 MPa.  9) A method for producing a neutron absorbing material according to claim 8, characterized in that the mixing ratio between boron carbonate (B4C) and hexagonal boron nitrate (BN) is 6: 4. 100) A method for producing a neutron absorbing material according to claim 8, characterized in that the mixing ratio between boron carbonate (B4C) and hexagonal boron nitrate (BN) is 4: 6.