Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Shaping or treating of multilayered – impregnated – or...
Reexamination Certificate
2001-09-28
2003-12-30
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Shaping or treating of multilayered, impregnated, or...
C264S674000
Reexamination Certificate
active
06669893
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a composite neutron absorbent material and to a process for manufacturing this material.
Neutron absorbent materials are neutron absorbers. They find application in the manufacture of control rods for example used to control the reactivity of nuclear reactors, in particular to control pressurized water nuclear reactors (PWR) and fast neutron reactors (FNR).
Inside the cores of nuclear reactors, neutron absorbent materials are indispensable components of the control rods. The latter form the command, adjustment and stoppage systems of reactor reactivity.
The materials which make up these control rods contain nuclides able to absorb the neutrons in order to reduce the reactor's neutron flow.
These neutron-absorbing materials may for example be used in the two main types of French nuclear reactors: Pressurized Water Reactors and Fast Neutron reactors.
The first choice that a core designer must face is which neutron-absorbent nuclide to use. It must meet the requirements of anti-reactivity related to the energy rating of the core: type of fuel and fuel assembly, desired neutron flow, nuclear station power, etc. Depending upon desired core power, upon the intensity of the neutron flow, it is essential to provide the necessary antireactivity for normal adjustment of this neutron flow or possibly for the emergency stoppage of nuclear fission reactions. During the fission of a heavy nucleus in the core of a nuclear reactor, a few neutrons are released in the free state. If, among these neutrons that are released, some happen to meet a fissile nucleus and cause its fission they in turn generate descendants which themselves may cause the fission of another nucleus and give birth from generation to generation to a chain reaction. It therefore appears important to control the quantity of free neutrons formed in order to prevent the fission reaction from racing out of control and to maintain this fission in a critical state, that is to say in equilibrium.
Therefore the control rods containing the neutron absorbent materials are mobile rods mounted in the core of nuclear reactors such that they can slide between the fuel assemblies, or mounted in a network of fuel pins of an assembly. Control of core fission is made by inserting or withdrawing these rods from the core of the nuclear reactor by sliding them in or out of position.
The absorbent materials may be used to maintain nuclear fission in the critical state, in which case they form piloting rods. They may also be used to ensure quick stoppage of a chain reaction in which case they form safety rods.
Other criteria may be taken into account in the choice of absorbent material. These criteria are:
good mechanical properties, in particular a Young's modulus that is as low as possible, moderate ultimate stress, good resistance to mechanical damage and more particularly good resistance to crack propagation,
a reasonable overall cost (raw material and manufacture),
good chemical and shrinkage resistance to radiation
optionally, chemical compatibility with the cladding (generally in stainless steel) which may be used to shield the absorbent.
A great amount of research had been conducted on boron carbide B
4
C with the sole view to its use as neutron absorber given its high effective neutron capture cross-section. B
4
C absorbent material is used in the form of stacks of sintered cylindrical pellets, made from powders.
Although having substantial chemical inertia, B
4
C oxides easily on and after 600° C. in the presence of oxygen. This compound is also sensitive to water corrosion in the primary PWR medium, in particular when radiated by the neutrons or when subjected to neutron radiation. This is one of the reasons why it is generally inserted in stainless steel cladding.
Also, the lifetime of boron carbide never reaches the theoretical limit fixed by boron exhaustion on account of damage to the material caused by the large quantity of helium and lithium formed by neutron absorption
10
B(n,&agr;), Li. Therefore, under the effect of temperature, one fraction of the helium formed diffuses outside the material while the other accumulates therein, causing swelling and micro-fracturing of the material.
In the particular case of fast neutron reactors, the flow of neutrons (energy greater than 1 MeV) penetrates inside all the absorbent material which causes a volume release of heat whereas the surface of the absorbent is directly cooled by sodium. A substantial radial thermal gradient therefore occurs in the material, which may reach several hundred degrees per centimeter. This thermal gradient involves major tangential heat stresses in the material which cause critical radial cracking and complete fragmentation of the absorbent material.
In the particular case of pressurized water reactors, the flow of neutrons (energy less than a few eV) only penetrates the peripheral part of the absorbent material. The swelling described previously therefore only occurs in a peripheral ring of the absorbent. Differential swelling therefore occurs between the circumference and the core of the absorber pellets which causes major radial stresses, critical tangential cracking superimposing itself upon the micro-cracking previously described, and complete fragmentation of the absorbent material.
The combination of swelling, microfracturing and cracking of the material may, under strong radiation, cause a mechanical interaction between the absorbent material and the steel cladding which may lead to fracture of the cladding which itself is subject to weakening firstly by fast neutron radiation and secondly by the diffusion of a certain amount of boron and carbon derived from the absorbent material.
These two modes of critical cracking are macroscopic crack phenomena with imposed strain.
It is therefore necessary to develop a neutron absorbent material which may be used in these two types of reactors.
This material therefore, in addition to the above-mentioned properties of a low Young's modulus and a low coefficient of thermal expansion, must offer high heat conductivity, toughness, resistance to crack propagation and resistance to mechanical damage.
Document EP-A-0 359 683 describes a neutron absorbent pellet and its process of manufacture. It describes a scarcely absorbent element obtained by moulding or sintering a mixture of ceramic powders (B
4
C, HfO
2
, Eu
2
O
3
) and metal (Hf, Eu, Ni, Cr).
Document WO-A-94/28556 describes a neutron absorbent material and its method of preparation. The material described contains boron carbide and may contain hafnium, in particular hafnium diboride. The hafnium diboride represents no more than 40% by volume, preferably from 20 to 30% by volume.
DISCLOSURE OF THE INVENTION
The purpose of the present invention is precisely to remedy the above-mentioned disadvantages and to provide a neutron absorbent material having all the required properties, in particular for its use in control rods for a nuclear reactor.
The neutron absorbent material of the present invention is characterized in that it contains boron carbide and hafnium, in particular it may contain boron carbide and hafnium diboride.
According to the invention, the boron may account for at least approximately 65% by atoms of the material, for example approximately 72% by atoms of the material.
According to the invention, the hafnium may account for up to approximately 18% by atoms of the material, for example approximately 10% by atoms of the material.
According to the invention, the boron carbide may be in the form of particles having a diameter of up to approximately 50 &mgr;m.
According to the invention, the hafnium may be in the form of agglomerates of hafnium boride whose size preferably ranges up to approximately 500 &mgr;m, for example up to approximately 250 &mgr;m.
The material of the present invention may have a density of approximately 2870 to 6800 kg/m
3
, for example of approximately 3220 to 5770 kg/m
3
, for example of 5165 kg/m
3
or 5060 kg/m
3
.
The material of the present invention may also co
Bry Philippe
Deschanels Xavier
Provot Bruno
Commissariat a l'Energie Atomique
Fiorilla Christopher A.
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