Stock material or miscellaneous articles – All metal or with adjacent metals – Having metal particles
Patent
1995-02-07
1996-12-31
Walsh, Donald P.
Stock material or miscellaneous articles
All metal or with adjacent metals
Having metal particles
428546, 428551, 419 12, 419 14, 419 16, 419 32, 419 48, 419 49, 419 51, B22F 312
Patent
active
055903932
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a neutron-absorbing material and to its production process.
A material of this type can in particular be used in nuclear reactors such as pressurized water reactors and fast neutron reactors.
At present, the most widely used neutron-absorbing material in nuclear reactors is boron carbide, e.g. B.sub.4 C, because it is an inexpensive material, whose effectiveness can be modulated by acting both on the .sup.10 B isotope content of the boron and on the density and which also has good chemical inertia properties and a highly refractory character. Generally, it is used in the form of cylindrical pellets stacked in metal sheaths in order to form the control rods of the reactors.
However, it suffers from the disadvantage of having a poor behaviour under irradiation due to poor thermomechanical properties (low thermal conductivity and fragile nature), which limit its life. Thus, the emission of heat due to neutron (n, alpha) captures is sufficient to induce its fracturing, because the very high thermal gradients (up to 1000.degree. C./cm in a fast neutron nuclear reactor) resulting therefrom lead to stresses occurring which exceed the strength of the material. Moreover, the generation of large quantities of helium brings about a significant swelling of the material and a microscopic cracking, which can in the long term bring about a complete disintegration of the pellets.
Research has also been undertaken with a view to improving the behaviour under irradiation of boron carbide, whilst retaining the greatest possible neutron absorption efficiency.
The present invention is directed at a neutron-absorbing material based on boron carbide having such improved properties. The neutron-absorbing material according to the invention is a composite material comprising a homogeneous boron carbide matrix in which are homogeneously dispersed calibrated clusters of at least one boride chosen from among HfB.sub.2, TiB.sub.2 and ZrB.sub.2.
Therefore this material is a composite material of the ceramic/ceramic type, in which the reinforcement is ensured by the calibrated clusters of the boride HfB.sub.2, TiB.sub.2 and/or ZrB.sub.2. The reinforcement due to these calibrated clusters is obtained as a result of the existence of residual stress fields in the material, which lead to a deflection of the cracks in the vicinity of the smallest calibrated clusters and to high microcrack concentrations in the largest calibrated clusters, which prevent the free propagation of the cracks.
In order to obtain these results, it is consequently important for the calibrated clusters to have the appropriate dimensions. In general, they are pseudospherical clusters with dimensions in the range 100 to 500 .mu.m. It is also advantageous that these clusters have a residual porosity forming microcracks.
In order to obtain these results, the material must contain a sufficient boride quantity, but said quantity must not be excessive so as not to harm the neutron absorption efficiency of said material, the density of absorbing nuclei being lower in diborides than in boron carbide.
In general, the material incorporates at the most 40 vol % in all of boride or borides chosen from among HfB.sub.2, TiB.sub.2 and ZrB.sub.2, preferably 20 to 30 vol. % of boride or borides.
Among the borides which can be used, hafnium diboride HfB.sub.2 is particularly interesting, because it has the following remarkable properties: effective than boron in the epithermal neutron range, density, carbon, which makes it possible to produce stable mixtures with the boron carbide and also use the same shaping processes as those used for the boron carbide, particularly sintering under a uniaxial load at high temperatures in a graphite matrix and which makes it possible to produce residual stresses in compression in the boron carbide matrix.
A composite material of the boron carbide-titanium diboride type was described by Nishiyama et al in Trans JSCM, vol. 11, No. 2, 1985, pp. 53-62, but in this case the sought objective was to improve the hardness of t
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Decroix Guy M.
Gosset Dominique
Kryger Bernard
Commissariat a l''Energie Atomique
Greaves John N.
Walsh Donald P.
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