EP0258178A1 - Nuclear radiation absorbers - Google Patents

Abstract

Metal absorbers of nuclear radiations contain metallic samarium in the form of a metal alloy chosen from at least one of the following classes of alloys: copper-samarium, aluminium-samarium and magnesium-samarium, respectively, each of the said classes of alloys containing from 0.05 to 95% by weight of samarium relative to the total weight of the alloy. They may additionally contain additional elements such as neutron- absorbing elements, elements reinforcing the physical, mechanical or technological properties, fibres or anticorrosion elements. They may be used especially for absorbing neutrons and gamma and X radiations.

Description

The present invention relates to metal absorbers of nuclear radiation. It more particularly relates to metallic nuclear radiation absorbers containing metallic samarium in the form of metallic alloy chosen from at least one of the families of copper-samarium, aluminum-samarium and magnesium-samarium alloys, respectively, each of said families of alloys containing from 0.05 to 95% by weight of samarium relative to the total weight of the alloy.

The importance of nuclear power programs in the world and the development of nuclear techniques require solutions to protect against nuclear radiation (periphery of reactors, transport and storage of radioactive waste, nuclear machines, etc.). It is therefore of primary importance and necessity to design and manufacture efficient and competitive radiation absorbers.

The absorption materials must meet the following criteria:
- firstly, having specific nuclear properties: large cross section of neutron capture, low emission of secondary radiation, good stability over time with respect to radiation;
- have a high melting point to withstand the heating generated by the absorption of radiation, and in particular neutron fluxes;
- be a good conductor of heat to facilitate cooling to the outside;
- not too high residual heat (released as radiation after stopping);
- sufficiently high mechanical resistance;
- resistance to corrosion with respect to the refrigerant, or in the working atmosphere;
- have good stability with respect to heat and radiation;
- competitive cost, both in terms of raw material and in implementation.

All the elements absorb more or less nuclear radiation, but those which have the most striking neutron-absorbing properties are: cadmium, boron, europium, hafnium, gadolinium, samarium and dysprosium.

Europium and dysprosium, although having a large effective cross-section, give rise to very limited applications, given their very high price.

Gadolinium has the highest cross-sectional area of all known absorbers in the thermal neutron spectrum. It can be observed that for example for neutrons of initial energy from 10⁻¹ to 10⁻³ Electronvolts, its effective capture section is approximately 100 times higher than that of boron. Unfortunately, in the area of epithermal neutrons and slow neutrons (energy from 0.3 to 10² Electronvolts, the absorption properties are very reduced compared to boron.

The most widespread and most well-known absorptive material in terms of criticality is undoubtedly possible boron, which is used in different forms: elemental boron, borides, boron carbide, boric acid, oxide, nitride, etc. and many patents have been filed.

The use of boron-based materials is delicate: elemental boron has poor mechanical properties, it is highly oxidizable at high temperature and its corrosion resistance is poor; it must then be inserted in the form of chemical compounds defined in various matrices, and these composite materials pose problems of homogeneity and are difficult to use.

Hafnium has much lower absorption properties than boron for thermal and epithermal neutrons, its cost is high and it is difficult to use because of its oxidability.

The samarium, compared to all the elements mentioned above, has extreme neutron-absorbing properties interesting, intermediate between boron and gadolinium for thermal neutrons, superior to boron and gadolinium for intermediate and fast neutrons; two resonance zones give only two weaknesses for the samarium compared to boron, the first between 1 and 5 eV of neutron energy, and the second between 30 and 40 eV, but these weaknesses can be compensated by the quantity of elements neutrophages introduced into the final alloy. Compared to gadolinium, it is perfectly clear that the samarium is more interesting overall on the whole spectrum of energy of neutrons.

This is why the applicant, aware of the interest of the samarium, has sought and found means of combining it with other metallic materials in order to make it absorbers of nuclear radiation having all the qualities mentioned above.

, on peut obtenir un coefficient d'absorption µ grâce à la relation:
µ = PN

µ est exprimé en cm⁻¹
P est la masse volumique du matériau en g/cm3
A est la masse atomique en g
est la section efficace de capture en cm2
N est le nombre d'Avogadro.These new absorbers are characterized by the fact that they essentially constitute three families of alloys, one family having as base metal aluminum, another family having as base metal copper, and a third having as base metal the magnesium. These three families of new alloys generally present complementary interests. Indeed, aluminum is very light but has fairly low mechanical properties above 300 ° C. In comparison, copper is heavier, but has a higher thermal conductivity than aluminum (which is already excellent) and gives high mechanical properties up to 500 ° C. Magnesium will give rise to the lightest alloys, but its resistance to corrosion is low, and its thermal conductivity lower than that of aluminum. In these three families, the absorption properties of nuclear radiation are given by the relative mass of samarium present in the metal matrices concerned. The absorption capacity of an element is defined by its cross section of neutron capture, expressed in BARN. From this effective section

, we can obtain an absorption coefficient µ thanks to the relation:
µ = PN

µ is expressed in cm⁻¹
P is the density of the material in g / cm3
A is the atomic mass in g
is the cross section of the catch in cm2
N is the Avogadro number.

To calculate the absorption coefficient of an alloy, you must take into account all the addition elements present, and then use the formula:

By considering a given addition element i, the absorption coefficient of the alloy is directly a function of the weight percentage of this element in the alloy. Thus, for all the Al-Sm, Cu-Sm and Mg-Sm alloys which are the subject of this patent, their absorption coefficient will be directly a function of the percentage by weight of samarium.

But we come to the families of alloys Al-Sm, Cu-Sm and Mg-Sm, therefore of alloys comprising as main elements aluminum, copper and magnesium, associated with the samarium which can range from 0.05% to 95% by weight of samarium compared to the total weight of the alloy considered. Below 0.05%, the absorbent effect proves to be too reduced, and above 95% falls in the case of the metal samarium whose oxidizability is high, the technological properties not very attractive, a high price and difficult implementation.

Preferably, the alloys of the Cu-Sm family will be situated in a range of 0.05% to 50% of Sm, or in a range of 70% to 90% Sm. With the alloys of the Al-Sm family, it will preferably be situated in a range of 0.05% to 25% by weight of Sm, and for the Mg-Sm family, in a range of 0.05 to 55%.

These forks, without being exclusive, present the best compromises of technological properties and the samarium content will be calculated according to the radiation flux to be absorbed.

The aluminum, copper and magnesium used can be pure, or alloyed with any other addition element which will make it possible to reinforce the mechanical properties of the absorbers or to modify their technological properties (ease of implementation, resistance corrosion, machinability, weldability ...). Similarly, among all the addition elements other than aluminum, copper, magnesium and samarium, other neutron-absorbing elements can be added such as gadolinium, europium, hafnium, boron (in phase dispersed or not), cadmium, lithium, dysprosium, etc. where fibers can be inserted (alumina, silicon carbide, boron, carbon ...).

The aluminum-samarium, or copper-samarium, or magnesium-samarium alloys exhibit very good ease of implementation by at least one of the manufacturing processes chosen from molding, whether in sand, in shell, under high or low pressure, hot or cold rolling, extrusion, forging, vacuum forming ...

These alloys which are the subject of this patent give perfectly homogeneous structures with very regular neutron capture cross sections. The density of the mixtures will be variable depending on the proportions of samarium introduced into aluminum, copper or magnesium. As an indication, Table I gives density values for different compositions.

We can see that for aluminum with alloys up to about 25% by weight of samarium, the density remains low and will therefore allow the manufacture of very light radiation absorbers. On the other hand, with Cu-Sm alloys, the density of the two metals being more similar (8.92 for copper and 7.52 for samarium), the density values are relatively little affected by the samarium content. The Mg-Sm alloys obviously have the lowest densities.

Regarding the thermal conductivity, it will be very variable depending on the alloys ultimately chosen for the manufacture of the absorbers: the values for pure copper, pure aluminum, pure magnesium and samarium are respectively, in W / m ° K (between 0 and 100 ° C): 394, 238, 155 and 10 (approximately). We can immediately see that the samarium in relation to the other three elements has a conduc very low reliability. The thermal conductivity of the final absorbent metallic material will strongly depend on the mixture selected (Al-Sm, Cu-Sm or Mg-Sm) and possibly on other addition elements introduced into the alloys to improve their mechanical, technological or absorption. For example, an Al-Sm alloy with 10% Sm will have a thermal conductivity of 150 W / m ° K, an Al-Si-Sm alloy with 7% silicon and 2% samarium, the same thing , finally a Cu-Sm alloy with 4% of Sm will show a thermal conductivity of 250 W / m ° K approximately. This notion of thermal conductivity is important and will strongly influence the choice of the optimal composition sought for the absorbent material, because any absorption of radiation (and especially neutron capture) is accompanied by a release of heat which must be removed from the hot parts to cold parts as quickly as possible. It will be noted that the aluminum and copper matrices are from this point of view very well placed.

In general, the starting points of melting of the alloys Al-Sm, Cu-Sm, Mg-Sm are high, which gives them very good stability at high temperature, and which allows them to withstand without problem the heating caused by absorption of neutrons or other radiation. The solidification interval varies according to the chemical composition and Table II indicates some values of alloys studied.

The atomic masses of samarium (150.33 g) and copper (63.5 g) being high, the γ and χ radiations will be strongly absorbed by these two elements, while the effect of aluminum and magnesium is much weaker.

Corrosion resistance, in general, is not or little affected by the presence of samarium for contents less than 25% by weight, and the corrosion properties will essentially depend on the nature of the aluminum, copper matrices and magnesium used. For aluminum for example, aluminum-silicon matrices (7 to 10% of Si) and aluminum-magnesium will exhibit good corrosion resistance against atmospheric agents, against demineralized water at 50 ° C or in a marine atmosphere. This behavior could be further improved by appropriate surface treatments (anodization, alodine, paints, plastic coatings ...). For copper-samarium alloys having a samarium content of less than 20% by weight, the corrosion resistance is practically not affected by the presence of the samarium. This corrosion resistance can be further improved by additions of chromium, nickel, aluminum, tin ...

As regards magnesium-samarium alloys, the corrosion resistance will generally be low, and the use of these will be reserved for applications in a non-corrosive environment.

At high temperature, the resistance to oxidation of Al-Sm alloys is remarkable, of the same order of magnitude as that of conventional aluminum alloys. The use of such materials at high temperature will therefore not pose a problem of resistance over time. On the other hand, binary copper-samarium alloys can be problematic, because copper oxidizes from 250 ° C and copper oxide is soluble in copper. For high temperatures, it is therefore necessary to use an additional addition element which will give the matrix its oxidation resistance properties. It will be for example nickel, chromium, aluminum ...

At low temperatures, it should be noted that all the Al-Sm, Cu-Sm and Mg-Sm families show no sign of embrittlement.

Radiation absorbers must have high mechanical properties and be as stable as possible at high temperatures. To do this, and depending on the specifications imposed, a judicious choice of the Al-Sm, Cu-Sm and Mg-Sm alloys and their additional addition elements will be made. The right compromise will have to be found not only based on mechanical characteristics nics, but also depending on the thermal conductivity of the weight, the nuclear characteristics, the possibilities of implementation. As an example, we will see in the following tables the results of mechanical tests on different Al-Sm and Cu-Sm alloys.

The case of magnesium-samarium alloys is somewhat special; copper and aluminum do not dissolve samarias in the solid state. On the other hand, magnesium can dissolve up to 12% of samarium at around 550 ° C, and this solidity is no more than 2 or 3% at room temperature: this characteristic shows a possibility of structural hardening by quenching and tempering on these binary alloys.

The machining and welding of Al-Sm, Cu-Sm and Mg-Sm alloys, whether or not alloyed with other conventional elements, do not pose any particular problems and all techniques commonly used in practice for this type of metal matrix agree.

Examples of applications include: baskets for transporting and storing nuclear waste, pool racks for storing fuel elements from nuclear reactors, shielding decontamination facilities, atomic and nuclear protections in general, nuclear reactor components, shielding of control devices using radiation or radioactive sources, shielding of electronic boxes, etc.

Preparation of an alloy absorbing nuclear radiation Cu-Sm 17-Cr 0.4

1922 grams of metallic samarium in pieces, 9294 grams of pure copper in the form of ingots and 56 grams of pure chromium are placed in a graphite crucible. The crucible is then introduced into an electrically heated or induction heated oven: the metals can be melted under vacuum or under an inert atmosphere.

The metals placed in the crucible are heated for 1 hour at 1200 ° C, then the resulting mixture is maintained for 1 hour at 1100 ° C in order to obtain a perfectly homogeneous liquid mass. The oven is then opened, the top of the crucible stripped of its encrustations and its contents poured into a mold such as an ingot mold, which can be cooled with water.

Preparation of an Al-Sm 12 nuclear radiation absorber alloy

3740 grams of pure aluminum in pieces and 510 grams of metallic samarium in pieces are placed in a graphite crucible. The melting of metals can be carried out under vacuum or under an inert atmosphere, once the crucible is introduced into an electrically heated or induction heated oven.

Before starting the heating, it is advisable to rid the pieces of samarium of any trace of humidity, because there would be risk of explosion on contact with molten aluminum. The metal mass is first brought to 660 ° C to melt the aluminum, then brought to 1100 ° C for about 1 hour. The samarium gradually dissolves in liquid aluminum. After complete dissolution of the samarium, the temperature is reduced to 800 ° C, the oven is opened, the oxides floating on the surface of the liquid eliminated and the contents of the crucible poured into a mold, such as a metal mold, a sand mold, a ceramic mold or an ingot mold.

In the two cases above, once the preform obtained by molding or ingot ingot, the alloy obtained can be put into its final form using the usual transformation techniques, such as machining, forging, lamination or extrusion.

Claims (9)

1. Metallic nuclear radiation absorbers containing metallic samarium in the form of metallic alloy chosen from at least one of the families of copper-samarium, aluminum-samarium and magnesium-samarium alloys, respectively, each of said families of alloys containing from 0.05 to 95% by weight of gadolinium relative to the total weight of the alloy. 2. Metallic absorbers according to claim 1, characterized in that the family of copper-samarium alloys contains from 0.05 to 50% or from 70 to 90% by weight of samarium relative to the total weight of the alloy. 3. Metal absorbers according to claim 1, characterized in that the family of aluminum-samarium alloys contains from 0.05 to 25% by weight of samarium relative to the total weight of the alloy. 4. Metal absorbers according to one of claims 1 to 3, characterized in that the family of magnesium-samarium alloys contains from 0.05 to 55% by weight of samarium relative to the total weight of the alloy. 5. Metallic absorbers according to one of claims 1 to 4, characterized in that the metallic alloys contain one or more additional metallic elements intended to reinforce or improve the mechanical, physical or technological properties of the absorbers. 6. Metal absorbers according to one of claims 1 to 5, characterized in that the metal alloys contain one or more additional neutron-absorbing metallic elements. 7. Metal absorbers according to one of claims 1 to 6, characterized in that the metal alloys contain fibers, such as alumina, silicon carbide, boron or carbon fibers for example. 8. Metal absorbers according to one of claims 1 to 7, characterized in that the metal alloys contain one or more metallic elements additional intended to reinforce or improve the corrosion resistance of the absorbers. 9. Use of metallic absorbers according to one of claims 1 to 8 for the absorption of nuclear radiation, in particular neutrons and γ and X rays.