EP0400122A1

EP0400122A1 - METHOD OF UTILIZING THE (n, gamma) REACTION OF THERMAL NEUTRONS

Info

Publication number: EP0400122A1
Application number: EP19890912826
Authority: EP
Inventors: Péter Teleki
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-11-28
Filing date: 1989-11-20
Publication date: 1990-12-05
Also published as: AU4528389A; CA2003671A1; WO1990006583A1

Abstract

Selon un procédé d'utilisation de la réaction (n, gamma) de neutrons thermiques, on place une cible (2) devant une source (1) de neutrons thermiques. La cible (2) comprend une surface frontale tournée vers la source (1) de neutrons thermiques et une face postérieure située derrière la face frontale. On prépare la cible (2) avec un corps (4) en métal basique composé de 70Yb et/ou de 74W. On produit avec les neutrons thermiques un mélange métallique qui contient les métaux basiques et au moins une des paires formées par les métaux 71Lu + 72Hf et 75Re + 76Os, puis on stocke le mélange métallique afin d'en réduire l'activité nucléaire.According to a method of using the reaction (n, gamma) of thermal neutrons, a target (2) is placed in front of a source (1) of thermal neutrons. The target (2) comprises a front surface facing the source (1) of thermal neutrons and a posterior face situated behind the front face. The target (2) is prepared with a basic metal body (4) composed of 70Yb and / or 74W. A metallic mixture is produced with thermal neutrons which contains the basic metals and at least one of the pairs formed by the metals 71Lu + 72Hf and 75Re + 76Os, then the metallic mixture is stored in order to reduce its nuclear activity.

Description

METHOD OF UTILIZING THE (n, gamma) REACTION

OF THERMAL NEUTRONS

FIELD OF THE INVENTION

The present invention refers to a method of utilizing the (n, gamma) reaction of thermal neutrons, wherein a target is arranged before a source of thermal neutrons. The method of the invention results in possibility of making use of the thermal neutron flux of a nuclear reactor, with disregard to the kind of the reactor, whereby the economy of operating of the different reactors can be highly improved. The proposed method can be realised with reactors of diverse kinds, e.g. with experimental reactors, energetic or boiler reactors etc.

BACKGROUND OF THE INVENTION

It is known from the literature that the (n, gamma) reaction can be applied for producing some isotopes. For example, the reaction

is the basis of generating the very important isotope of cobalt having mass numer 60 which is widely used in the medicine and industry. In this process the end product is a substance showing high level of radioactivity (gamma-activity). This process may not be ralised without special security measures.

The theory of atomic nuclei recites lots of theoretical and practical reactions for transforming chemical elements, i.e. atomic nuclei. In the handbooks e.g. the process can be found for producing gold, wherein the half-period of decay of the intermediate platinum isotope is relatively short, about 20 hours. This way of producing gold is very expensive and inconvenient: the substance at the beginning of the process is twice so expensive than the gold received. Another disadvantage of this process is that the platinum isotope with mass number 196 amounts about 25.3 % of the whole platinum mass and therefore a separate process is necessary for yielding the gold.

SUMMARY OF THE INVENTION

The object of the present invention is to make use of the thermal neutron flux of a reactor for producing non radioactive materials, wherein no special security measures are to be taken.

The invention is based on the recognition that ytterbium and tungsten can be transformed into a mixture of different elements showing no or very low level radioactivity by means of the thermal neutrons generated in each radioactive reactor.

Hence, the invention proposes a method of utilizing the (n, gamma) reaction of thermal neutrons of a reactor, the method comprising the step of arranging a target directed with its front surface to a source of thermal neutrons, especially a reactor, wherein according to the invention the target is consisted of ₇₀Yb and/or ₇₄W. It is especially advantageous to apply before the target a plate shaped body for slowing down the quick and/or the reactor neutrons, consisted of ₄₁Nb for slowing down the reactorneutrons and/or ₅₉Pr for slowing down the quick neutrons. Of course, this moderator of neutrons can be made also of beryllium. A beryllium plated can be applied also for covering the rear side of the target - this ensures reflection of the neutrons back to the target. By the means of the method proposed by the invention about 30 % of the amount of ytterbium can be transformed into lutetion and the same amount of tungsten into rhenium. Above that about 20 % of tungsten transform into osmium. The metals received, i.e. lutetium, rhenium and osmium are much more expensive than the input metal of the process and can be separated therefrom by simple thermal processing because of considerable differences in the respective melting points.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further disclosed in more detail by way of example and with reference to the attached drawings. In the drawings

FIG. 1 shows the cross-section of a target applied in realising the present invention.

DETAILLED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the vicinity of a reactor 1 limited by a wall 7 a target 2 is arranged in an appropriate place. The target 2 consists of a front layer 3 forming a moderating body, a metal plate 4 including ytterbium and/or tungsten to be transformed and a rear reflecting layer 5. The front layer 3 is made of ₄₁Nb and/or ₅₉Pr. If necessary, ₄Be can be applied to. The mentioned metals slow down the flux of the neutrons leaving the interior of the reactor 2. The reflecting layer 5 covering the rear surface of the metal plate 4 reflects the neutrons back to the metal plate 4. The target 2 is arranged to be irradiated by a thermal neutron flux 6 and the front layer 3 receives the neutrons before entering the metal plate 4.

The neutron flux 6 can be directed to the target 2 through the wall 7 of the raactor 1 in a known way, e.g. by the means of a window prepared in the wall 7.

As mentioned, the metal plate 4 is made of ytterbium and/or tungsten. The irradiation of this plate carried out by the thermal neutrons generated by the reactor 1 or produced by the front layer in a (n, 2n) reaction should result in an alloy like mixture consisting of the following metals (the composition is given with approxinate data):

a) on the basis of ytterbium:

37 112 101.4 127

₇₀Yb + ₇₁Lu + ₇₂Hf (+ ₆₉Tm)

60 % 30 % 10 % 0.1 %

b) on the basis of tungsten:

19.2 86 15.3 21

74^{w +} ₇₅Re + ₇₆Os (+ ₇₃Ta)

50 % 30 % 20 % 0.1 %

The line over the signs of the elements give the value of the cross-section for the process expressed in barns.

When taking ytterbium, the metal includes the following isotopes:

From this table it follows that about 55.3 % of all (n, gamma) reactions do not result in any change of the atomic number. These reactions are:

The following reactions result in transformation of elements:

The percentage values means the proportion of the given stable isotope in the metal mentioned.

The process of stabilization of is characteris

ed by the half-period 6.75 days.

When taking tungsten, the metal includes the following isotopes: v

From this table it follows that about 40.8 % of all (n, gamma) reactions do not result in any change of the atomic number. These reactions are:

The following reactions result in transformation of elements :

In normal circumstances, 7 e is transformed into

characterised by half-period about 5.10¹⁰ years by weak e- radiation. In a (n, gamma) reaction, however another process dominates:

The half-period of rhenium is 18 hours, the isomeric osmium nucleus shows half-period 26 days. In these conditions the rhenium Re can be also activated and in decay processes (e-, gamma, K) it can be transformed partly into tungsten, partly into osmium: a dominate part, however, remains unchanged in form of rhenium.

In both series of reactions, the gamma radiation coming into being is a low energy, low intensity weak radiation.

The metallic mixtures prepared by the invention require at least 1/2 year storage before further processing. During this time the radiation level of the mixture falls under a maximum level allowed by the rules.

When considering the basic metal and the metallic components produced bv the method of the invention it can be stated that they are capable of bearing high thermal load and the alloy received in the process is stable. The melting points of the metals in the mixtures mentioned are the following: ₇₀Yb + ₇₁Lu + ₇₂Hf + ₆₉Tm

824 ºc 1652 ºC 2222 ºC 1545 °C

₇₄W + ₇₅Re + _{7 6}Os + ₇₃Ta

3410 ºC 3180 ºC 2700 °C 2996 °C

In realising the method of the invention it is advantageous to arrange the target 2 in the proximity of the active zone of the reactor, but under the condition that the target can not be the object of radiation comprising charged particles and fission products. If these factors are excluded the only disturbing effects follow from the gamma radiation of the reactor and the flux of quick neutrons emitted from the reactor. In both cases the loss of neutrons by the nucleus can follow in (gamma, n) and (n, 2n) reactions, however, these are low probability processes Therefore the only requirement is to moderate the quick neutrons, because the reactions with loss of neutron constitute a part of the reactions which hardly play important rule.

The reactor neutrons show a wide spectrum with average energy 0.72 MeV (the flux may contain also neutrons with energy 20 MeV), therefore it is advantageous to slow down (moderate) the reactor neutrons and the quick neutrons by the means of (n, 2n) reactions whereby the yield of neutrons can be increased. The beryllium moderator is in this case a further element after that applied for slowing down the reactor and quick neutrons.

The reactions of the reactor neutrons are characterized by small cross-section. Hence, they can be slowed down by means of the reaction 93

b (n, 2n) b. A very

effective reaction for slowing down the quick neutrons having energy in the range about 14 to 15 MeV is based on praesodymium: (n, 2n) . The processes mentioned

result in increased yield of neutrons. The advantageous character of these reaction can be seen from the following scheme of reactions:

10 days, decay e-, gamma, K) - (n, 2n)

(4 minutes, decay e⁺, gamma, K) - (140 days, decay gamma, K) -

139

Other reaction scheme are possible with low probability, because of the short half-period.

The target 2 includes advantageously a rear reflecting layer 5 for reflecting back the neutrons. This layer can be made of beryllium (₄Be).

The plate 4 of the target 2 is arranged preferably so that the neutron flux of the reactor falls under right angle (90°) on its surface.

Summarizing, the method of the invention should be realised with a target 2 including after the reactor a layer made of ^ and/or b, a moderator (of ₄Be), the metal plate 4 made of ₇₄W and/or ₇₀Yb and a mirror layer (rear reflecting layer 5, made of ₄Be) . The beryllium can be preferred because it is a neutron source under influence of the gamma radiation emitted by the reactor, with the following reactions:

wherein the neutrons at the output have energy 110 keV.

The process of the invention can be applied for preparing catalyzer substances - this improves the economy of operating a reactor. No specific security means or expenses are necessary. The metal mixtures can be separated into components according to the known thermal techniques or applied as alloys.

Claims

CLAIMS :

1. Method of utilizing the (n, gamma) reaction of thermal neutrons, comprising the step of arranging a target before a source of thermal neutrons, the target having a front surface directed to the source of the thermal neutrons and a rear surface behind the front surface,

characterized in the further steps of - preparing the target with a basic metal body made of at least one metal selected from the group including ₇₀Yb and ₇₄W,

- producing by the means of thermal neutrons a metallic mixture including the basic metal(s) and at least one pair of metals selected from the group including ₇₁Lu +

+ ₇₂Hf and ₇₅Re + ₇₆Os and

- storing the metallic mixture for reducing its activity.

2. The method as set forth in claim 1, characterized in preparing the basic metal body in the form of a plate and arranging it perpendicularly to the flux of the thermal neutrons.

3. The method as set forth in claim 1 or 2, characterized in the step of arranging on the front surface of the basic metal body a layer for slowing down fast and reactor neutrons by the means of (n, 2n) reactions, the layer consisting of at least one metal selected from the group including ₄₁Nb and ₅₉Pr.

4. The method as set forth in any of claims 1 to 3, characterized in the step of arranging at least one beryllium moderating layer on at least one of the front and rear surfaces of the target.

5. The method as set forth in any of claims 1 to 4, characterized in the further step of carrying out thermal decomposition of the metallic mixture after the storing period.