DE1234994B - Uranium alloys for nuclear reactor fuels - Google Patents

Description

GERMAN WTWW ^ PATENT OFFICE

EDITORIAL DEVELOPMENT German Cl .: 40 b -27/00

Number: 1234 994

File number: C 32214 VI a / 40 b

J 234 994 Filing date: February 21, 1964

Opened on: February 23, 1967

The invention relates to uranium alloys as nuclear reactor fuels as well as any other Uses. It relates in particular to such alloys of this type, which by adding small proportional amounts of appropriately selected elements one immediately after their Shedding existing delicacy of the structure and stability to radiation are given as they are usually only through a significant content of additional elements or through additional ones metallurgical treatments could be achieved.

It is known that the degree of utilization of uranium and thus the amount of energy that is produced Nuclear fuel can be supplied for such nuclear fuel based on metallic Uranium is strong due to the progressive decay of the fuel elements under the action of radiation is restricted. It is also known that the possible service life of these fuel elements Base of metallic uranium in the reactor before they become unusable due to decay a large part of their initial polycrystalline structure and the stability of this structure among the The working conditions of the reactor depends. Finally, it is known that the most favorable stability properties In the case of a fine-grain structure, the bodies develop without tension and pronounced directions within the structure, and that it is practically not possible in non-alloy uranium to combine these properties with one another and, in particular, to maintain them.

The remuneration of a uranium grain that is only slightly alloyed is currently based on one or the other of the two ways given below.

In the case of low-alloy alloys, the maximum a few thousandths of a weight percent Containing additional elements, this treatment is carried out by deterring them from the realm of existence the / MPhase to normal temperature or by isothermal conversion of the / ί phase within a narrow temperature interval in the area of existence of the «phase.

In the case of the higher alloyed intermediate alloys, especially those of the two-phase type (uranium- Molybdenum, uranium-chromium, etc.), the remuneration is done by reheating and slowly cooling.

Only the more heavily alloyed alloys that contain more than 1 percent by weight of additional elements can be subjected to normal cooling after potting.

A finely crystalline structure thus becomes the uranium alloys for nuclear reactor fuels

Applicant:

Compagnie pour l'stude et la Realization de
Combustibles Atomiques, Paris

Representative:

Dipl.-Ing. R. H. Bahr and Dipl.-Phys. Ε. Betzier, patent attorneys, Herne, Freiligrathstr. 19th

Named as inventor:

Henri Monti, Vitry-sur-Seine;

Michel Foure, Croissy-sur-Seine, Seine-et-Oise

(France)

Claimed priority:

France of February 26, 1963 (926 053) - -

Time applied method either by accepting the additional work process, which represents a further thermal treatment, or the addition of considerable proportions of alloying elements to the nuclear fuel, which is again associated with a considerable loss of reactivity of the fuel, which on the other hand, is again Enrichment with uranium 235 may be necessary. Achieving this structural fineness is therefore always an expensive additional measure. In addition, certain treatments, such as direct quenching from the area of the / J phase, generate stresses in the fuel element that cannot be neglected and, with probability, preferred directions, both as consequences of the transition from the ß- into the «phase.

It is also known that the resistance of uranium to thermal fatigue is reduced by adding it To improve quantities of two metallic elements. It can be one of these additional metals be aluminum, zirconium, chromium, titanium or vanadium; the second is about zirconium, Molybdenum or another metal from Group V of the Periodic Table. The one produced in this way However, uranium alloy requires a complicated thermal treatment; among other things too names include heating to around 1050 ° C, then cooling, followed by tempering at

709 510/458

about 600 ~ C. These procedures are complicated and time-consuming.

Since the shaping of the fuel elements takes place in the great majority of cases by casting, there is therefore a considerable effort to use alloys with low contents of additional elements create that through after potting without the need for subsequent heat treatment the desired fine crystalline structure can be obtained simply by cooling.

The invention therefore relates to a type of quaternary alloys with low contents of the Additional elements chromium, silicon and zirconium, which have a fine-grain structure immediately after the Potting with cooling within a very wide range of cooling speeds can be obtained. The main features of these can be described as "self-compensating" Alloys are described below on the basis of the micrographs, which are metallographic Represent cut surfaces by specimens of the following nature:

F i g. 1 illustrates, enlarged approximately 25 times, a metallographic sectional surface of a Specimen made of unalloyed uranium, which by cooling at a rate of 25 ° C / min. was obtained from the casting temperature of 1500 ° C;

F i g. 2 shows, likewise magnified approximately 25 times, the cut surface of a test specimen from a quaternary alloy consisting of 99.85% uranium, 0.07% chromium, 0.03% silicon and 0.05% zirconium exists, and after potting at a rate of also 25 ° C / min. cooled down became;

F i g. 3 shows, likewise magnified approximately 25 times, the cut surface of a test specimen from a quaternary alloy made from 99.77% uranium. Consists of 0.05% chromium, 0.08% silicon and 0.10% zirconium, and after potting at a rate of 8 ° C / min. has been cooled;

F i g. Figure 4 is a 100X enlargement of a small zone of the cut surface of Figure 4. 2, which reveals the fineness and regularity of the grains.

The uranium alloys according to the invention are characterized by the presence of three additional elements, namely of chromium, silicon and zirconium in a total proportional amount that is 0.4 percent by weight does not exceed, but is mostly only 0.1 to 0.2 percent by weight, marked. The this Maximum content of 0.4 percent by weight corresponding neutron absorption is less than that, which ζ. B would result in the presence of 0.15 percent by weight molybdenum (this proportional Amount of certain reactor fuels is based on natural uranium, which is produced by Graphite moderated and cooled with gas is exceeded considerably. That means that the neutron balance of the element is only loaded relatively little). The contents of chromium and silicon are in each case 0.02 to 0.1 percent by weight and the zirconium content is 0.01 to 0.2 percent by weight.

The total proportion of these three elements added to uranium is at least 0.1 percent by weight.

The optimal proportions of chromium, silicon and zirconium are determined as part of the

The limit values given depend on the cooling rate in the range of the conversion interval from the β to the phase and thus on the dimensions and shape of the cast moldings, the equipment used and the type of cooling that is used. The greater the cooling rate and the lower the lower the cooling rate, the higher the content of chromium compared to that of silicon. The optimal contents of chromium are to be understood with tolerances of ± 0.015 percent by weight for the chromium and the silicon and with tolerances of ± 0.05 percent by weight for the zirconium at a given cooling rate. The range of cooling rates extends from 3 to 80 ° C./min., Although it is also possible to increase the cooling rates to that of the immediate quenching by means of conventional water cooling from the / J range or the y range.

Whereas in the case of unalloyed uranium the grains of the structure obtained by normal cooling are coarse, very irregular and dendritic (Fig. 1), these are the grains which, in the case of the alloy according to The invention can be obtained (Figs. 2, 3 and 4) fine, omnidirectional, have polygonal approximately regular Contours on and are free from tension. Their mean "diameter" is always smaller than 0.5 mm, while that obtained with unalloyed uranium under the same cooling conditions Grains can reach and even exceed a size of several millimeters. If for maintaining the relationship between the proportionate contents of additional elements and the cooling rate Care is taken, so is the degree of tempering of the alloys according to the invention with the exception of this, in the case of very low cooling rates (below 5 ° C./min.), this is practical regardless of the cooling rate and the mean diameter of the grain is less than 0.1 mm.

The production of the alloys according to the invention presents no difficulty, provided that that in the case of the use of crucibles made of graphite, contamination of the melt by Carbon is prevented as much as possible. In order to achieve or secure that, one is required Thick, resistant, refractory lining of the crucibles is sufficient. The losses of chromium and Silicon can with regard to the permissible tolerance range within which the contents of these metals are changeable can be neglected. As a result of the intermetallic bond of the zirconium through the carbon occurring losses in this metal are very much lower than which would be expected from the stoichiometric point of view, and are also supported by the Compensated for range of possibilities for changing the content of the alloy in this metal. on the other hand For the same reason, the carbon content is calculated when measuring the amount of zirconium added of the uranium used.

The self-aging alloys according to the invention thus make it possible to use fuel elements of any shape and size to the simplest and most economical technological way to establish. As a result of the

Claims (4)

The loss of reactivity that occurs in the presence of the additional elements is very small and is particularly permissible in reactors that work with natural uranium. If the fuel element does not have the desired structure as a result of a manufacturing error or for any other reason, it is always possible to reheat it and then subject it to a continuous cooling treatment from the γ or / j phase. It could be observed that the dwell times in the areas of existence of these phases have no effect on the final structure, apart from a possible equalization of the cuts through the grains. Another advantage of the alloys according to the invention is the possibility of obtaining a small grain in welded joints, whereas in the case of the classic quenchable / Ϊ alloys, quenching is always required after welding to make the grain even. For the sake of clarity only, two examples of self-tempering alloys having the features of the invention are given below, which relate to the most common uses of the invention in the smelting of uranium. Example 1 This example relates to the production of small moldings or those with a small wall thickness by casting. In this case, the cooling rate is very high and corresponds in terms of magnitude to a temperature drop of 20 to 30 ° C / min. The optimal composition of the alloy is chromium 0.045 to 0.075 percent by weight silicon 0.025 to 0.055 percent by weight zirconium 0.02 to 0.12 percent by weight uranium remainder Die F i g. 2 and 4 show micrographs of cut surfaces of the cast alloys, which reveal the perfect quality of the crystalline structure obtained. The analytical examination of a sample of this alloy resulted in a composition of chromium 0.07 percent by weight silicon 0.03 percent by weight zirconium 0.05 percent by weight uranium remainder Example 2 This example relates to the production of massive moldings of large dimensions using a slow cooling rate of the order of magnitude a temperature drop of 4 to 10 ° C / min. corresponds, but possibly with different cooling rates in certain temperature ranges. The optimal composition of the alloy is chromium 0.035 to 0.065 percent by weight silicon 0.055 to 0.085 percent by weight zirconium .... 0.02 to 0.12 percent by weight uranium, the remainder F i g. 3 shows the micrograph of the cut surfaces of a by casting this alloy and cooling at a rate of 8 ° C / min. produced molded body. As can be seen, the structure of this alloy is very fine-grained. The analytical examination of a sample of the same resulted in a composition of chromium 0.05 percent by weight silicon 0.08 percent by weight zirconium 0.1 percent by weight uranium Remainder claims: 1. Quaternary uranium alloy with fine-grained, uniform structure, which is due to continuous Cooling has been established immediately after casting without subsequent heat treatment, for metallic nuclear reactor fuel elements of any shape and any dimensions or of bodies made of weakly alloyed uranium, which have a polycrystalline, fine-grain structure must, characterized in that the alloy consists of 0.02 to 0.1% chromium and silicon, 0.01 to 0.2% zirconium, remainder It consists of uranium and common impurities, the total of the three being added to the uranium Elements is at least 0.1%. 2. A method for the production of alloys according to claim 1, characterized in that the optimal contents of the additional elements chromium and silicon within the limit values according to claim 1 as a function of the cooling rate of the cast shaped bodies be chosen in such a way that the chromium content is higher than the silicon content, the greater, and the lower, the slower the cooling rate. 3. A method for the production of alloys according to claim 1 or 2, characterized in that the cast moldings from the β or 7 range at a rate of 3 to 80 ° C / min. be cooled down. 4. The method according to claim 3, characterized in that it is cooled from the ß or 7 range with a conventional water cooling. Considered publications:
French patent specification No. 1149 991. 1 sheet of drawings 709 510/458 2.67 © BundesdruckereiBerIin