CH667882A5 - NUCLEAR RADIATION ABSORBERS. - Google Patents

Description

DESCRIPTION

The present invention relates to metal absorbers of nuclear radiation. It more particularly relates to metallic nuclear radiation absorbers containing metallic gadolinium in the form of metallic alloys chosen from at least one of the families of copper-gadolinium and magnesium-gadolinium 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.

The importance of nuclear power programs in the world and the development of nuclear techniques require solutions to protect against nuclear radiation (reactor periphery, transport and storage of radioactive waste, nuclear machines, etc.) It is therefore of primary importance and essential 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 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 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.

Cadmium has the disadvantage of being a very toxic product for the human organism and its use is strictly prohibited in many countries. In addition, its melting point (321 ° C) and its boiling point (761 ° C) are very low, and its resistance to corrosion in an aqueous medium is very poor.

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

The most widely used and most well-known absorptive material in terms of criticality is without any test the 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 implement.

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 has interesting neutron-absorbing properties, intermediate between boron and gadolinium for thermal neutrons, superior to boron and gadolinium for intermediate and fast neutrons. However, two zones of weak absorption remain with respect to boron, the first between 1 and 5 eV and the second between 30 and 40 eV.

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 ~ 'to 10 ~ 3 eV, its effective capture section is approximately 100 times higher than that of boron. On the other hand, in the zone of epithermal neutrons and slow neutrons, gadolinium has worse absorption properties than boron but this weakness in the neutron spectrum can be compensated by an increase in the weight percentage of gadolinium in the alloy used.

Gadolinium oxide has already been used for many years in various nuclear installations where, when mixed with fuel, it acts as a moderator. However, its application to the manufacture of radiation absorbers poses problems. Indeed, the oxide, generally available in powder form, must be mixed with other products using very complex technologies, and the poor mechanical properties make its application during the production of barbers of complicated shape, to the both delicate and expensive. In addition, this oxide has poor conductivity.

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thermal bility and its absorption capacity is relatively reduced compared to that of elementary gadolinium.

This is why the applicant, aware of the advantage of using gadolinium as a neutron-absorbing element, has sought and found a way to make it an attractive absorber of nuclear radiation by combining it with other metallic materials.

These new absorbers are characterized by the fact that they essentially constitute two families of metallic alloys, a family having copper as base metal and another family having magnesium as base metal. These two families of alloys present complementary interests:

the Mg-Gd mixtures will be very light but on the other hand will have weak mechanical properties especially at high temperatures. Cu-Gd mixtures, of much higher density, will have high mechanical properties when cold and at high temperatures (up to 500 ° C) and excellent thermal conductivity. In these two families, the absorption properties of nuclear radiation are given by the relative mass of gadolinium present in the matrices concerned. The absorption capacity of an element is defined by its cross section of neutron capture, expressed in BARN. From this cross section a, we can obtain an absorption coefficient [x thanks to the relation:

^ PN "Ä

(i is expressed in cm "1

P is the density of the material in g / cm3 A is the atomic mass in g a is the cross section of capture 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:

xtv Cia

M-; iiliage P -N ^ j p = density of the alloy Ci = weight concentration of the element i in the alloy ai = cross section of the element i Ai = atomic mass of the element i

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 Cu-Gd and Mg-Gd alloys which are the subject of this patent, their absorption coefficient will be directly a function of the percentage by weight of gadolinium.

These Cu-Gd and Mg-Cd alloys are essentially characterized by the fact that they contain as main elements magnesium or copper associated with gadolinium with a weight percentage of gadolinium which can range from 0.05% to 95% relative to the weight total of the alloy considered. Below 0.05%, the absorbent effect proves to be too reduced, and above 95% we fall in the case of gadolinium metal whose oxidizability is high, the technological properties not very interesting with a low thermal conductivity , a difficult implementation and a very high cost.

Preferably, the alloys of the Cu-Gd family will range from 0.05% to 30% Gd, or from 75% to 95% Gd, and with the Mg-Gd family, from 0.05% to 55% Gd (all these percentages are percentages by weight).

These ranges, without being exclusive, present the best compromises of technological properties and the gadolinium content will be calculated according to the level of absorption sought.

The 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 to corrosion, machinability, weldability ...) Likewise, among all the elements of addition other than gadolinium, copper and magnesium, other neutrophagous elements can be added such as samarium, europium, hafnium, boron , cadmium, dysprosium, etc ... or can be inserted fibers (alumina, Si carbide, boron, carbon ...).

Gadolinium copper or gadolinium magnesium alloys have a 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 rolling. or cold, extrusion, forging, vacuum forming ...

These alloys give perfectly homogeneous structures with very regular effective cross-sections. The density of the mixtures will be variable depending on the proportions of gadolinium introduced into copper or magnesium. As an indication, Table I gives density values for different compositions.

Table I

Density, of different Cu-Gd and Mg-Gd alloys

Alloy

% by weight of Gd

Density

Cu-Gd

10

8.8

Cu-Gd

20

8.7

Cu-Gd

85

8.1

Mg-Gd

5

1.9

Mg-Gd

20

2.5

If the density of Cu-Gd alloys varies little with increasing additions of Gd (the two pure metals have very similar densities), it is not the same for the magnesium matrix whose density increases very appreciably as as we add gadolinium.

The thermal conductivity will vary greatly depending on the alloys ultimately chosen for the manufacture of the absorbers: the values for pure copper, pure magnesium and gadolinium are respectively, in W / m ° K (between 0 and 150 ° C): 394 , 155 and 8.8. There is a very large difference in thermal conductivity between the three metals. The thermal conductivity of the final absorbent metallic material will depend on the mixture retained (Cu-Gd or Mg-Gd) and possibly on the other addition elements introduced into the alloys for improve their mechanical, technological or absorption properties. 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 copper matrix is from this point of view the best placed.

In general, the starting points of melting of Cu-Gd and Mg-Gd alloys are high, which gives them very good stability at high temperature, and which allows them to withstand without problem the heating caused by absorption. neutrons or other radiation. The interval

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of solidification varies according to the chemical composition and Table II indicates some values of alloys studied.

Table II

Solidification intervals of some Cu-Gd and Mg-Gd alloys (Weight percentages)

Alloy

Beginning of solidification ° C

End of solidification

° C

Cu-Gd 10

1000

875

Cu-Gd 20

Composition

875

eutectic

Cu-Gd 80

740

668

Cu-Gd 84

Composition

668

eutectic

Mg-Gd 15

-625

-575

The atomic masses of gadolinium (157.25 g) and copper (63.54 g) being high, the radiations y and X will be strongly absorbed by these two elements, while the effect of magnesium is much weaker. The corrosion resistance of copper, in general, is little or not affected by the presence of gadolinium for contents less than 20% by weight, and the corrosion properties will essentially depend on the nature of the copper matrices used. For example, the addition of nickel, chromium, aluminum, tin ... will improve the corrosion properties. With regard to gadolinium magnesium alloys, the corrosion resistance of magnesium matrices is generally low, and the use of gadolinium magnesium alloys will preferably be reserved for uses in a non-corrosive medium.

Above 250 ° C, the oxidation behavior of binary Cu-Gd alloys can cause problems; in fact, the copper begins to oxidize from 250 ° C and the copper oxide is soluble in the copper. It will therefore be advisable to add to the matrix addition elements such as chromium, nickel, aluminum ... An addition of 0.5% chromium, for example, removes the sensitivity to oxidation of the copper up to 700 ° C.

At low temperatures, the families of Cu-Gd and Mg-Gd alloys 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 Cu-Gd, s Mg-Gd alloys and their additional addition elements will be made. The right compromise will have to be found not only as a function of the mechanical characteristics but also as a function of the thermal conductivity of the weight, of the nuclear characteristics, of the possibilities of implementation. As an example, Table III gives the typical results of mechanical tests on a Cu-Gd5-Cr 0.5 alloy.

Table III

Mechanical properties of a Cu-Gd5 - Cr alloy 0.5 mold i5 or wrought at 20 ° C

Alloy state Rm MPA Rp 0.2 MPA A%

Molded state T4

230

100

25

Molded state T6

370

280

15

Corrected state T4

230

100

25

Corrected state T6

430

300

10

(not hardened)

25 The case of Mg-Gd alloys is somewhat special; copper does not dissolve gadolinium in the solid state. On the other hand, magnesium can dissolve up to 11% of gadolinium by weight around 550 ° C, and this solidity is no more than 2 or 3% at room temperature: this particularity shows a possibility of hardening structural by quenching and tempering on these binary alloys.

The machining and welding of Cu-Gd and Mg-Gd alloys, combined or not with other conventional elements, do not pose any particular problems and all techniques commonly used in practice for this type of metal matrix are suitable. .

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

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Claims (8)

667,882 1. Metallic nuclear radiation absorbers containing metallic gadolinium in the form of a metallic alloy chosen from at least one of the families of copper-gadolinium and magnesium-gadolinium alloys, respectively, each of said families of alloys containing 0.05 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-gadolinium alloys contains from 0.05 to 30% or from 75 to 95% by weight of gadolinium relative to the total weight of the alloy. 2 3. Metallic absorbers according to claim 1, characterized in that the family of magnesium-gadolinium alloys contains from 0.05 to 55% by weight of gadolinium relative to the total weight of the alloy. 4. Metal absorbers according to one of claims 1 to 3, characterized in that the metal alloys contain one or more additional metallic elements intended to reinforce or improve the mechanical, physical or technological properties of the absorbers. 5. Metal absorbers according to one of claims 1 to 4, characterized in that the metal alloys contain one or more additional neutron-absorbing metallic elements. 6. Metal absorbers according to one of claims 1 to 5, characterized in that the metal alloys contain fibers, such as alumina fibers, silicon carbide, boron or carbon for example. 7. Metal absorbers according to one of claims 1 to 6, characterized in that the metal alloys contain one or more additional metallic elements intended to reinforce or improve the corrosion resistance of the absorbers. 8. Use of metallic absorbers according to one of claims 1 to 7 for the absorption of nuclear radiation, in particular neutrons and y and X radiation.