FIELD OF THE INVENTION

The present invention refers to a method of utiliz¬ ing 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 mak¬ ing use of the thermal neutron flux of a nuclear reactor, with disregard to the kind of the reactor, whereby the e- conomy of operating of the different reactors can be high- ly improved. The proposed method can be realised with re¬ actors 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 (π, gamma) reaction can be applied for producing some isotopes. For example, the reaction

27^° ^ ^π ' 9 ^{a ma} ) 27^° is the basis of generating the very important isotope of cobalt having mass πumer 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-ac- tivity). This process may not be ralised without special security measures.

The theory of atomic nuclei recites lots of theore¬ tical and practical reactions for transforming chemical elements, i.e. atomic nuclei. In the handbooks e.g. the process

^X ^P (n, gamma) ¹ ^Pt — ¹ ^Au

can be found for producing gold, wherein the half-period of decay of the intermediate platinum isotope is relative¬ ly short, about 20 hours. This way of producing gold is very expensive and inconvenient: the substance at the be- ginning 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 pro¬ cess 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 meas¬ ures 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 radioacti- vity 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 di- rected with its front surface to a source of thermal neut¬ rons, especially a reactor, wherein according to the inven¬ tion the target is consisted of ynYb and/or _7Λ ^W - ■***" ^t i- ³ especially advantageous to apply before the target a plate shaped body for slowing down the quick and/or the reactor neutrons, consisted of ,,Nb for slowing down the reactor neutrons and/or gP 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 cov¬ ering the rear side of the target - this ensures reflec- tion of the neutrons back to the target.

By the means of the method proposed by the iπven-* tion about 30 % of the amount of ytterbium can be trans¬ formed into lutetioπ 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 res¬ pective melting points.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further disclosed in more de¬ tail 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 .,Nb and/or 59P1-'. If necessary, .Be can be applied to. The mentioned metals slow down the flux of the neutrons leaving the interior of the reactor 2. The ref¬ lecting 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 neut¬ rons before entering the metal plate 4.

The neutron flux 6 can be directed to the target 2 through the wall 7 of the reactor 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 pro¬ duced by the front layer in a (n, 2n) reaction should re¬ sult in an alloy like mixture consisting of the following metals (the composition is given with approximate data): a) on the basis of ytterbium: 37 112 101.4 127

₇₀ Yb + ₇₁ Lu + _?2 Hf (+ ₆₉ T )

60 30 % 10 % 0.1 % b) on the basis of tungsten: 19.2 86 15.3 21 74 ⁺ ₇₅ Re + ₇₆ 0s (+ ₇₃ Ta)

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 fol¬ lowing 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:

^°Yb (n, gamma) ^X ₇ jγb

² ^Yb (n, gamma) ¹ ^Yb

^X ^Yb (n, gamma) ^Yb

^Yb (n, gamma) ^Yb

The following reactions result in transformation of elements:

¹ _7Q Yb (n, gamma) ^Yb - ¹ ^Tm 100 ^* %

¹ ₇ gYb (n, gamma)* ¹ ₇ gYb - ¹ ₇ JLU (e ^" , gamma) ^Hf 18 %

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

The process of stabilization of _7ι --u is characteris¬ ed by the half-period 6.75 days.

When taking tungsten, the metal includes the follow¬ ing isotopes:

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:

¹ γ w (n , gamma ) ¹⁸ ^W

¹ f2 ^{w (n} ' aamma) ¹⁸ *W The following reactions result in transformation of elements:

₇ ?W (n, gamma)

1 ₇ 84. (n, gamma)

₇₄ (n, gamma)

In normal circumstances, characterised by half-period about S.IO" ^1" " years by weak e ^" radiation. In a (n, gamma) reaction, however another pro¬ cess dominates:

¹⁸⁷ Re (π, gamma) ¹⁸⁸ Re - ¹⁸ ₇₆ 0s - ¹⁸⁸ 0s 13.3 %

The half-period of rhenium is 18 hours, the isomeric os¬ mium nucleus shows half-period 26 days. In these condi- tions the rhenium 18 ₇ 5c e can be also activated and in de¬ cay processes (e~, gamma, K) it can be transformed partly into tungsten, partly into osmium: a dominate part, how¬ ever, 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 process¬ ing. 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:

3410 ^β C 3180 ^β C 2700 °C 2996 °C

In realising the method of the invention it is ad¬ vantageous 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 neut¬ rons 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 con¬ stitute a part of the reactions which hardly play impor¬ tant rule. The reactor neutrons show a wide spectrum with aver¬ age 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 neut¬ rons 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 characte¬ rized by small cross-section. Hence, they can be slowed down by means of the reaction 9 _Λ 3iNb (n, 2n) 9*2iNb. A very effective reaction for slowing down the quick neutrons having energy in the range about 14 to 15 MeV is based on praesodymium: gPr (n, 2n) „Pr. The processes mentioned result in increased yield of neutrons. The advantageous character of these reaction can be seen from the following

scheme of reactions: _o ^Nb (n, 2n) _A ?Nb (10 days, decay e~, gamma, K) -

- Jjjzr (n, 2n) (n, 2n) °Zr

QPΓ (n, 2n) cg r -4 minutes, decay e ⁺ , gamma,* K) -

- c _o Ce (n, 2h) ^Ce (140 days, decay gamma, K) -

- ⁽ⁿ ' ^l ll ^Ce

Other reaction scheme are ^' possible with low proba¬ bility, because of the short half-period.

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

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 5 ₀ ?^ and/or 93Mb, a moderator (of ₄ Be), the metal plate 4 made of ₇ .W and/or _7Q Yb and a mirror layer (rear reflecting layer 5, made of _^ Be) . The beryl¬ lium can be preferred because it is a neutron source under influence of the gamma radiation emitted by the reactor, with the following reactions:

9 .Be (gamma, n) 8.Be - 2et

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 ex¬ penses are necessary. The metal mixtures can be separated into components according to the known thermal techniques or applied as alloys.