Compositions – X-ray or neutron shield
Reexamination Certificate
2000-09-21
2002-01-01
Tucker, Philip (Department: 1712)
Compositions
X-ray or neutron shield
Reexamination Certificate
active
06334963
ABSTRACT:
DESCRIPTION
1. Technological Field of the Invention
The invention relates to a neutron absorbent composite material and a method of manufacturing this material.
Neutron absorbent materials are neutron absorbers. They find application for example, in the manufacture of control rods which are used for the control of the reactivity of nuclear reactors, notably for the control of pressurized water nuclear reactors (PWRs).
In effect, when fission of a heavy nucleus occurs in the core of a nuclear reactor, neutrons are emitted in the free state. The neutron is a projectile capable of causing fission of heavy nuclei. If among the emitted neutrons, certain of them have the opportunity of colliding with a fissile nucleus and causing its fission, they will in their turn generate descendants which themselves can cause the fission of another nucleus and give rise to further generations in a chain reaction. It is therefore clearly important to control the quantity of free neutrons formed so as to prevent the fission reaction accelerating and to keep the fission in a critical state, that is to say, in equilibrium.
Hence, control rods comprising neutron absorbent materials are movable rods mounted in the core of nuclear reactors in such a way that they are able to slide between the fuel assemblies or within the network formed by an assembly of fuel rods. Control of fission in the core is by inserting or withdrawing these rods from the core of the nuclear reactor by sliding them in and out.
Absorbent materials can be used to maintain the nuclear fission in the critical state, in which case they constitute control rods. They can be used to ensure the rapid halting of the chain reaction, in which case they then constitute safety shut-off rods.
In order to be efficient in the control of nuclear reactors, the neutron absorbent material must meet the following selection criteria it must have a high effective neutron absorption cross section, good mechanical characteristics, good chemical resistance and good dimensional stability at temperature and under irradiation.
In certain cases, the neutron absorbent material must be covered with a sheath, generally made of stainless steel. It must be chemically compatible with this sheath.
In addition, the costs of the raw materials and the cost of manufacturing the neutron absorbent material must be kept reasonable.
2. Prior Art
At the present time, the neutron absorbent materials most widely used in the control rods for pressurized light water nuclear reactors (PWRs) are boron carbide (B
4
C), and a metal alloy of silver, indium and cadmium (SIC).
These materials have the advantage of having an effective neutron absorption cross section that meets the selection criteria for neutron absorbent materials.
The B
4
C absorbent material is used in the form of stacks of sintered cylindrical pellets produced from powders.
Although it is highly chemically inert, B
4
C oxidizes starting from 600° C. in the presence of oxygen. This compound is also sensitive to corrosion by the water in the primary PWR medium, notably when it has been irradiated by neutrons. This is one of the reasons why it is inserted into sheaths that are generally made of stainless steel.
In addition, the life of the boron carbide never reaches the theoretical limit fixed by the depletion of the boron because of damage to the material caused by the large quantity of helium and lithium formed by the absorption of neutrons. In effect, under the effect of the temperature, a part of the helium formed diffuses out of the material while the other part accumulates in it, causing swelling and micro-fractures in the material.
The combination of the swelling and the micro-fractures can cause, under strong irradiation, a mechanical interaction between the absorbent material and the steel sheath which can lead to rupture of the sheath, which is itself weakened by the fast neutron irradiation on the one hand and by the diffusion of a certain quantity of boron and carbide from the absorbent material on the other hand.
SIC absorbent materials comprise by mass about 80% silver, 15% indium and 5% cadmium. These SIC materials are used in cylindrical sheaths made of stainless steel since they have poor resistance to corrosion at the operating temperatures of nuclear reactors, in water that may incidentally contain oxygen.
SIC has good physical and chemical properties under irradiation and the modifications to which this material is subject in the course of the neutron absorption are considered to be acceptable for the control rods of present day PWRS. However, the very low melting point of this material and the cost of the silver which it contains are disadvantages that cannot be ignored for the use of this material for the control of nuclear reactors.
The materials B
4
C and SIC do not therefore meet the selection criteria described previously, to a satisfactory extent.
DESCRIPTION OF THE INVENTION
The precise aim of this invention is to provide a neutron absorbent material which enables one to resolve the problems described above, as well as a method of manufacturing said material.
According to the invention, the neutron absorbent material is a composite material comprising hafnium diboride and hafnium dioxide.
According to the invention, hafnium diboride can represent preferably at least 80% by volume of the material, more preferably about 90% by volume of the material.
According to the invention, hafnium dioxide can represent preferably up to 20% by volume of the material, more preferably up to 10% by volume of the material.
According to the invention, the hafnium diboride can be in the form of particles in the composite material, said particles preferably having a diameter ranging up to about 50 &mgr;m.
According to the invention, the hafnium dioxide an be in the form of particles in the composite material, said particles preferably having a diameter ranging up to about 20 &mgr;m, more preferably ranging up to about 10 &mgr;m.
According to the invention, the composite material comprising hafnium diboride and hafnium dioxide of the invention, can have a density of about 10000 to 11000 kg/m
3
, preferably about 10550 to 10630 kg/m
3
, and more preferably about 10590 kg/m
3
.
The neutron absorbent material conforming to the invention has the advantage of greater resistance to corrosion by the water of the primary medium in the PWR, that is to say that it contains a maximum content of 2500 ppm dissolved boron and 2.5 ppm dissolved lithium, at a temperature of about 345° C. and at a pressure of about 155 bars, this being translated as a quasi-zero dissolution of boron in the water.
Another advantage of the material according to the invention is that it keeps its integrity after a corrosion test lasting 1000 hours at a temperature of 345° C. and at a pressure of 15.5×10
6
Pa in water that is representative of that found in the primary medium of a PWR.
Another advantage of the material according to the invention has been revealed by tests carried out on a pure HfB
2
material at a temperature of 345° C. and at a pressure of 15.5×10
6
Pa in water that is representative of that found in the primary medium of a PWR. These tests have shown fracturing of this material caused by the formation of corrosion pits rich in oxygen called the oxide phase, within the mass of the pellet. In effect, these pits have generated internal stresses because of the density difference between the oxide phase and the boride phase, which have caused fracturing of the pellets.
In the case of the composite material according to the invention, corrosion pits are also formed but they are of much reduced size, since their development has been blocked by the presence of hafnium dioxide which has limited their propagation.
This result translates itself into increased toughness of the composite material of this invention which is greater than that of pure HfB
2
.
The neutron absorbent composite material according to the invention can be described as comprising a homogeneous matrix of hafnium diboride (HfB
2
) in which fine particles of
Bry Philippe
Deschanels Xavier
Koci Jean-Pierre
Provot Bruno
Burns Doane , Swecker, Mathis LLP
Commisariat a l'Energie Atomique
Tucker Philip
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