GB2039409A - A method for reducing desorption from an alloy of a rare gas formed by transmutation of one element of the alloy and an alloy thus obtained - Google Patents

Abstract

Desorption from an alloy of a rare gas formed by transmutation of one element of the alloy is reduced by forming the alloy in a single phase by increasing the concentration of one element in the alloy with a non- transmutable isotope of the element. The method is primarily applicable to the fabrication of tritiated targets e.g. tritiated titanium and erbium for sealed neutron tubes, the non transmutable isotope used being deuterium.

Description

SPECIFICATION A method for reducing desorption from an alloy of a rare gas formed by transmutation of one element of the alloy and an alloy thus obtained This invention relates to a method for reducing the desorption of a rare gas from an alloy, the rare gas being formed by transmutation of one element of an alloy which can have one or a number of phases according to the concentration ratio of the transmutable element. The invention is further concerned with the alloy thus obtained.

The invention relates to the progressive variation of physical characteristics of certain alloys having several phases in which at least one of the constituent elements undergoes a spontaneous or induced transmutation reaction resulting in the formation of a rare gas which rapidly attains a state of supersaturation within the alloy and causes impairment of the original physical qualities of said alloy as a result of desorption.

The reduction in concentration of the disturbing transmutable element is made practically ineffective by the presence of a number of phases in equilibrium since only the proportion of these phases varies whilst their intrinsic concentration remains constant, which does not produce any local modification of the phenomenon of desorption of the rare gas formed by transmutation.

The aim of the present invention is to produce a homogeneous reduction in concentration of the disturbing element on the scale of the grains.

To this end and in accordance with the invention, the method is distinguished by the fact that the alloy is formed in a single phase by completing the low concentration of the transmutable element with non-transmutable isotopic elements until the close neighbourhood of the minimum high concentration ratio is attained, thus making it possible to obtain said single phase.

The reduction in apparent concentration of the disturbing element thus effectively results in a reduction in concentration at the level of the phase containing the highest proportion of disturbing element and therefore of rare gas.

In a first embodiment of the invention, the method consists in mixing the different isotopes of said element in a homogeneous manner, then in carrying out direct synthesis of the alloy from the different constituent elements at the minimal concentration imposed by the utilization and/or storage temperature of said alloy, a concentration which permits the said single phase to be obtained.

In a second embodiment of the invention, the method consists in forming the alloy initially with a single element at the minimal imposed concentration, then in carrying out isotopic exchange between the element which is present in the alloy and the isotope or isotopes to be introduced in order to obtain the desired concentration ratio to be utilised in the case of the transmutable element.

A more complete understanding of the manner in which the invention can be implemented will be gained from the following description and example of application, reference being made to the accompanying drawings in which: Figure 1 is a phase diagram of a two-phase alloy in accordance with the prior art; Figures 2a and 2b show the same phase diagram according to the present invention with a different scale of abscissae.

Consideration will first be given to the simple and conventional example of an initially binary alloy MXT which can have two phases known as the a phase and they phase according to the concentration T of the disturbing element X with respect to the second element M of the alloy and according to the temperature.

Figure 1 shows a phase diagram of this type in which the temperature is plotted as ordinates and in which the ratio of the number of atoms X to the number of atoms M present in the alloy is plotted as abscissae, this ratio being designated hereinafter as the concentration T. In the case of very low values of t, the alloy has a single a phase.Similarly, in the case of high values of t, the alloy has a single y phase, in the case of intermediate values, the alloy has two phases as designated in the figure by (a + y), which means that the alloy is not homogeneous but constituted by a mixture of zones in the a phase at a low concentration Ip and in they phase at a high concentration To. This alloy which is represented by the point A, for example, and has a concentration tA iS therefore provided locally with zones which are as rich in the disturbing element X as an alloy having a concentration ta as represented by the point Q.The properties of desorption of the rare gases which are obtained by the effect of transmutation of X are dependent on the initial concentration of said element and are therefore closely related in the case of the alloy A and the alloy Q. Thus the presence of a number of phases in equilibrium results in low efficiency of reduction in concentration of the disturbing element, X, that is, as long as the concentration does not fall below the very low ratio tp.

The present invention is therefore concerned with cases in which it is desired to obtain an alloy having an intermediate concentration, that is to say in which the ratio t is within the range of Tp to aa.

The invention permits a reduction, in an effecture manner of the concentration of the disturbing element X in the phase which contains the highest proportion of the disturbing element.

In accordance with the present invention which is described with reference to Figures 2a and 2b, a homogeneous mixture is formed artificially in the single y phase at the minimum concentration T0 but this concentration is in fact the sum of two elementary concentrations: the concentration T of the disturbing element X and the concentration T' (I" ...) of one (or a number) of non-disturbing isotope(s) X'(X"...). In the example of an alloy containing two intimately mixed elements X and X', there will thus be a single-phase alloy B of the type M.XT.X'T, which satisfies the quadrupel condition.

TB = t + I = TO > t > tp In Figure 2a, in which the axis of abscissae indicates the concentration T of the element X, the alloy B is represented by the point B located between the points P and Q. In Figure 2b, in which the axis of abscissae indicates the total concentration T + T' of the elements X and X', the alloy B is represented by the point B located just beyond the point Q.Thus the atoms of the two isotopes X and X' perform the same function in regard to the crystal structure of the alloy which is in they phase by virtue of the fact that the total concentration TB iS equal or just greater than Tq whereas, in regard to transmutation properties, the disturbing element X is diluted to a concentration t < ta in they -phase zones. The total concentration T + t' may also not attain the value ta but the gain in desorption of rare gas will be less marked since they phase will be preponderant but not a single phase. The desorption gain of a rare gas will be optimal for n + I' = To.

In a preferential manner, the two isotopes X and X' are chosen so as to be as closely related as possible in order to ensure greater similarity of their physico-chemical properties and better homogeneity of the alloy with the other constituent element M while also ensuring that the isotope X' is not transmutable or, in the event of its being transmutable, to ensure that it is not liable to interfere with the properties of the alloy which should not undergo any degradation.

The invention is also concerned with methods of fabrication of alloys of this type. Such methods must permit maximum homogeneity of distribution of the different isotopes X, X'... within the structure which it is sought to obtain.

Afirst method of fabrication according to the invention consists in mixing the isotopes X, X'... as homogeneously as possible and in proportions corresponding to the alloy to be obtained in order to ensure that the sum of the different concentrations of isotopes is equal to the minimum concentration imposed by the utilization and/or storage temperature so as to obtain a single-phase alloy. The alloy is then formed by direct synthesis from constituent elements of the M type and of the X type in accordance with the methods which are already known per sue and employed for this purpose.

A second method of fabrication according to the invention consists in forming the alloy MX' by direct synthesis from the element M and from the single element X' at the total concentration desired. An isotopic exchange is then carried out between the element X' which is present in the alloy and the isotope X to be introduced with a view to obtaining a lower concentration ratio in the case of the element X as desired. It is also possible to form the alloy MX first at the total desired concentration and then to carry out reciprocal isotopic exchange. A more complete understanding of the manner in which the invention is carried into effect will be obtained from the following example of application which is given without any limitation being implied.

Example : Long-lifetime tritiated targets for sealed neutron tubes operating in the pulsed mode These targets are employed for the production of neutrons in accordance with the reaction T (d, n) He4 (relevant information can be obtained, for example, from the CEA-BIST review No 178 of February 1973, page 59).

The service life of sealed neutron tubes equipped with tritiated metal targets is often determined by the quantity of helium-3 obtained from natural radio-active disintegration of the tritium of the target which undergoes a transition to the gas phase within the tube. Said service life can be increased by means of a homogeneous reduction of the tritium concentration of the target in accordance with the method described earlier, in which case the spontaneously disturbing element is tritium (T) and its nearest non-disturbing isotope is deuterium (D).

In the particular case of titanium tritide targets, the phase limits at room temperature are as follows: Tp = 0.0014 ta = 1.35 to 1.65 according to the technology adopted.

The compact hexagonal a phase contains practically no tritium; only the face-centered-cubic y-phase (type CaF2) can contain tritium.

For this application, there will therefore be formed a homogeneous thin layer (in which the thickness of the layer is of the same order of magnitude as the depth of penetration of the deuterium ions, which bombard the target in practice) of titanium combined with a low concentration of tritium and with deuterium, the titanium being simultaneously impregnated with tritium at a concentration tT and with deuterium at a concentration TD. If we have TO = 1.50 in the case of a given process, the following alloy will therefore be formed by way of example: Ti.TtT D(1.SO-TT) Similarly, in the case of tritiated and deuterated erbium targets, for example, it would be found necessary to produce the following configuration: Er . TTT. D(1 9O-TT) Since the isotopes of hydrogen are present in gaseous form at room temperature and since diffusion of said isotopes takes place at a high rate in the hydrides, two examples of methods of formation of said thin layers have been carried into effect by the present Applicant: a) Deposition of a metallic layer in an evacuated space followed by composite impregnation with a homogeneous mixture (D + T) in accordance with thermodynamic conditions which depend on the finality of the layer, followed by sealing in an envelope.

b) Reactive deposition in an evacuated space of the hydrided layer in an atmosphere containing either hydrogen (H) or deuterium (D) followed by isotopic exchange with tritium (T) in accordance with thermodynamic conditions which depend on the finality of the layer, followed by sealing in an envelope.

Many types of spontaneous or induced reaction can be included in the scope of the present invention as evidenced by the summary table of disturbing transmutations given hereunder.

TABLE: Examples of transmutation of elements causing the formation of rare gases Nature and Disturbing Rare gases origin isotope Reaction produced Spontaneous: p-emission Radioactive H3 Half-life = He3 decay 12,26 years Induced: lon H3 (d, n) He4 bombardment Li6 (n, a) He4 H3 < He3 Induced: Be9 (n, 2n) He4 Neutron B10 (n, a) He4 Bombardment M925 (n, a) He4, Ne22 U235 Fission Xe, Kr Induced: Irradiation Be9 (y,n) He4 It is wholly apparent to anyone versed in the art that the system is not limited to alloys of the binary type but also extends to alloys of the ternary type, quaternary type and so forth, and that any non-essential modification of the characteristic features of the invention hereinabove described would not constitute any departure from the scope or the spirit of the present invention as defined by the appended claims.

Claims (7)

1. A method for reducing the desorption from an alloy of a rare gas formed by transmutation of one element of the said alloy which can have one or a number of phases according to the concentration ratio of the transmutable element, characterized in that the alloy is formed in a single phase by completing the low concentration of the transmutable element with non-transmutable isotopic elements until the close neighbourhood of the minimum high concentration ratio is attained, thus making it possible to obtain the said single phase.

2. A method in accordance with claim 1 for reducing the desorption from an alloy of a rare gas formed by transmutation of one element of the alloy characterized in that the method consists in forming the said alloy in a single phase by mixing the different isotopes of the said element in a homogeneous manner, then in carrying out direct synthesis of the alloy from the different constituent elements at the minimal high concentration imposed by the utilization and/or storage temperature of the said alloy, said concentration permitting the said single phase to be obtained.

3. A method in accordance with claim 1 for reducing the desorption from an alloy of a rare gas formed by transmutation of one element of the alloy, characterized in that the method consists in forming the alloy initially with a single element at the minimal imposed concentration, then in carrying out isotopic exchange between the element which is present in the alloy and the isotopes to be introduced in order to obtain the desired concentration ratio to be utilised in the case of the transmutable element.

4. An alloy obtained by application of a method in accordance with any one of claims 1 to 3.

5. Tritiated targets for sealed neutron tubes utilizing narrow pulse widths in which the tritium concentration has been reduced in a manner homogeneous in volume and by maintaining the single phase crystalline structure of the target by substitution of deuterium which is the nearest isotope as a result of application of a method in accordance with any one of claims 1 to 3.

6. A method for reducing the desorption of a rare gas from an alloy, substantially as hereinbefore described with reference to the accompanying drawings.

7. An alloy obtained as a result of the practical application of the method aforesaid, substantially as hereinbefore described.