CH667881A5 - NUCLEAR RADIATION ABSORBERS. - Google Patents

Description

DESCRIPTION

The present invention relates to metal absorbers of nuclear radiation. It relates more particularly 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 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 and samarium and dysprosium.

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.

Gadolinium has the highest cross-sectional area of all known absorbers in the thermal neutron spectrum. We can observe 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 zone of epithermal neutrons and neutrons slow (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, nitrue 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 its implementation is delicate because of its oxidation-resistance.

Compared to all the elements mentioned above, the Samarium has extremely interesting neutron-absorbing properties 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 neutron-absorbing elements 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.

Therefore, the applicant aware of the interest of the

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samarium, sought and found ways to combine it with other metallic materials to make them nuclear radiation absorbers having all the qualities mentioned above.

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 metallic matrices concerned. The absorption capacity of an element is defined by its cross section of neutron capture, expressed in BARN. From this effective section a, we can obtain an absorption coefficient | i thanks to the relation:

xi a H = pN- £

p. 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:

MV Cj CT

M-alIiage - pN 2-i ^ j p = density of the alloy

VS; = concentration by weight of element i in the CT alloy; = cross section of element i A; = atomic mass of element i

When 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 let's 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 samarium with a percentage of samarium which can range from 0, 05 to 95% by weight of samarium relative to the total weight of the alloy considered. Below 0.05%, the absorbent effect proves to be too reduced, and above 95% we are in the case of the metal samarium whose oxidizability is high, the technological properties not very interesting, 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% of Sm. With alloys of the Al-Sm family, we will preferentially be in a furnace-1 chette 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 to corrosion, machinability, weldability ...) Similarly, among all the addition elements other than aluminum, copper, magnesium and samarium, other neutrophagous elements such as gadolinium, europium, hafnium, boron (in dispersed phase or not), cadmium, lithium, dysprosium, etc ..., or may be inserted fibers (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.

Table I

Densities of different Al-Sm and Cu-Sm alloys

Alloys

% Weight of Sm

Density

Al-Sm

4

2.8

Al-Sm

10

2.9

Al-Sm

25

3.1

Sm-Al

90

7

Cu-Sm

10

8.7

Cu-Sm

20

8.5

Sm-Cu

85

7.9

Mg-Sm

10

-2.2

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 for are respectively, in W / m ° K (between 0 and 100 ° C): 394.238, 155.10 (approximately). We can immediately see that the samarium compared to the other three elements has a very low conductivity. The thermal conductivity of the final absorbent metallic material will strongly depend on the selected mixture (Al-Sm, Cu-Sm, or Mg-Sm) and possibly on the other addition elements introduced into the alloys for improvement

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r their mechanical, technological or absorption properties. 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 will have to be evacuated. hot parts to cold parts as quickly as possible. It will be noted that the aluminum and copper matrices are very well placed from this point of view.

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. neutrons or other radiation. The solidification interval varies according to the chemical composition and Table II indicates some values of alloys studied.

Table II

Solidification interval of some Al-Sm, Cu-Sm, Mg-Sm alloys (Weight percentages)

Alloy

Beginning of solidification

° C

End of solidification ° C

Al-Sm 4

651

633

Al-Sm 10

640

633

Al-Si 7 Sm 3

615

575

Al-Sm 25

800

633

Cu-Sm 10

1000

882

Cu-Sm 20

Compos. eutectic

882

Cu-Sm 48

Compos. eutectic

818

Cu-Sm 85

Compos. eutectic

597

Cu-Sm 95

1200

597

Mg-Sm 15

-620

-570

The atomic masses of samarium (150.33 g) and copper (63.5 g) being high, the radiations y and X 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 matrices , copper and magnesium used. For aluminum, for example, aluminum silicon (7 to 10% Si) and aluminum magnesium matrices 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 samarium copper alloys with a samarium content of less than 20% by weight, the corrosion resistance is practically unaffected by the presence of the samarium. This corrosion resistance can be further improved by the addition of chromium, nickel, aluminum, tin, etc. With regard to samarium magnesium 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 samarium copper 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 according to the mechanical characteristics but also according to the thermal conductivity, 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.

Table 3

Mechanical tensile properties of a cast Al-Sm 10 alloy in the raw molding state

Test temperature (1 hour hold at T °)

Rm MPa

Rp 0.2 MPA

AT%

HB

20 ° C

140

60

13

40

250 ° C

60

40

25

350 ° C

50

35

35

Table 4

Mechanical tensile properties of a rolled Al-Sm 10 alloy, raw hot-rolled state

Test temperature Long direction Long cross direction

(maintenance of Rm Rp 0.2 A% Rm Rp 0.2 A% HB

IhàT) MPA MPA MPA MPA

20 ° C

120

100

10 120 100 6

42

250 ° C

60

40

25

350 ° C

50

35

35

Table 5

Mechanical tensile properties of an Al-Si alloy 7 Mg 0.3 Sm 2 mold

Delivery condition T ° Rm time RpO, 2A% HB

maintenance test MPA MPA at T °

before the test (h)

Gross of

20 ° C

-

150

80

2

55

molding

Annealed condition

6 'to 2 p.m.

20 JC

-

100

50

4

45

520 ° C

Quenched state and

returned

20 ° C

-

230

180

2

80

Quenched state and

1

190

160

4

returned

200 ° C

1000

90

50

7

Quenched state and

1

110

100

6

returned

250 ° C

1000

55

40

10

Quenched state and

1

52

45

15

returned

315 ° C

1000

35

25

18

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Table 6

Mechanical properties of a Cu-Sm 5 Cr 0.5 alloy, molded or wrought

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

(not hardened)

430

300

10

The case of magnesium-samarium alloys is a bit special, in fact, copper and aluminum do not dissolve samarium in the solid state. On the other hand, magnesium can dissolve up to 12% of samarium at around 550 ° C and this solidity is only 2 or 3% at room temperature; this feature shows a possibility of structural hardening by quenching and tempering on these binary alloys.

5 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 the techniques commonly used in practice for this type of matrix metallic are suitable.

io As examples of application, we can cite: transport and storage baskets for nuclear waste, pool racks for the storage of fuel elements from nuclear reactors, shielding of decontamination installations, anti-shelters -atomic and nuclear protections in

15 in general, nuclear reactor components, the shielding of control devices using radiation or radioactive sources, the shielding of electronic boxes, etc.

Claims (9)

667,881 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 samarium 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. 2 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 claim 1, 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 additional metallic elements 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 y and X radiation.