The invention lies in the nuclear fuel production technology and relates, more specifically, to a method of producing a nuclear fuel sintered body. In the process, a powder that contains a fissile heavy metal oxide is produced, treated further, and sintered.
For nuclear reactors, nuclear fuels are generally provided in fuel assemblies. Depending on the type of reactor, these assemblies may have different structures and geometric forms (e.g. plates or rods). In conventional light water reactors, nuclear fuels are provided in the form of fuel rods which are combined in bundles to form a fuel assembly. The fuel rods are thereby generally arranged along a fuel assembly axis and are in each case guided through the meshes of a spacer in a plurality of planes perpendicular to the fuel assembly axis. As a result, they are laterally spaced apart and are mounted with partial resilience. Water generally flows from below onto the fuel assemblies arranged next to one another in a light water reactor core, and the water dissipates the heat generated in the nuclear fuel by the nuclear fission process and, at the same time, acts as a neutron moderator. The term light water reactors is to be understood as encompassing all reactors which operate with light water as a coolant, in particular boiling water reactors, pressurized water reactors, and also Soviet-design reactors (VVER reactors).
The cladding tube of a fuel rod which surrounds the nuclear fuel generally predominantly comprises a zirconium alloy which has only a low neutron-absorbing capacity. The nuclear fuel is usually arranged in the cladding tube in the form of a column which is stacked up from cylindrical sintered shaped bodies (pellets, nuclear fuel sintered bodies, sintered bodies). The cladding tube should in this case on the outer side have, inter alia, the best corrosion properties possible, i.e. a high corrosion resistance, with respect to the coolant. Moreover, it should be safely and reliably able to securely enclose the nuclear fuel and also nuclear fission products formed during the nuclear reaction, such as for example fission gases, at least throughout the entire dwell time of a fuel assembly, in order to reliably avoid contamination of the coolant. Suitable materials for this purpose are zirconium alloys, in particular Zircaloy alloys and zirconium alloys in combination with further materials (e.g. as a coating). However, different conditions, for example varying temperature and pressure conditions, and transient fluctuations in these parameters may occur in pressurized water reactors and boiling water reactors, in each case imposing different demands on cladding tubes and also on the fuel used in these reactors. Accordingly, various materials are customarily employed in boiling water reactors and pressurized water reactors.
Since the cladding tube generally tightly surrounds the nuclear fuel sintered body, it should be able to absorb on the inner side in particular changes in shape of the nuclear fuel sintered bodies during operation of the reactor. Since the outer side and the inner side of a cladding tube are therefore supposed to satisfy different demands, two-layer cladding tubes have by now become customary. In particular, cladding tubes are designed to be as ductile as possible on their inner side, in order, inter alia, to be able to absorb the changes in shape of the nuclear fuel sintered bodies and the resulting fuel/cladding tube interactions (Pellet Cladding Interactionxe2x80x94PCI). For this purpose, the cladding tube should be sufficiently capable of expansion and should be able to absorb high pressures which in some cases occur over a small area and in some cases for a very short time and variably and/or statically. This is the case in particular if, for example, fragments of a nuclear fuel sintered body become jammed in the cladding tube.
In principle, all substances which contain types of fissionable heavy metal, in particular heavy metal oxide, can be used as nuclear fuels. These substances comprise in particular nuclear fuels which are customary for light water reactors, containing uranium and/or plutonium and/or thorium and being in the form of powders and/or sintered bodies. To produce a nuclear fuel sintered body, first of all it is customary to obtain a nuclear fuel powder by way of a conversion process.
In principle, the state of the art conversion processes can be differentiated on the basis of the process used for producing UO2 from UF6. These are on the one hand dry-chemical conversion processes and on the other hand wet-chemical conversion processes. In the wet-chemical processes, a UO2 powder is obtained indirectly from uranium hexafluoride (UF6) after precipitation and separation of an intermediate stage from a solution. Known processes are named according to their intermediate stages, for example the AUC (ammonium uranyl carbonate) process and the ADU (ammonium diuranate) process. The wet-chemical conversion processes produce particularly high levels of radioactive waste, which entails considerable ecological and economic drawbacks compared to the dry-chemical processes.
In dry-chemical conversion processes (dry conversion, DC), uranium hexafluoride (UF6) is generally reacted with water and hydrogen to form uranium dioxide directly, generally according to the following overall equation:
UF6+2H2O+H2xe2x86x92UO2+6HF.
The UO2-containing powder (DC powder) which is formed can be used as the main raw material for production of a standard nuclear fuel powder and/or a mixed-oxide nuclear fuel powder (MOX nuclear fuel powder). To produce an MOX nuclear fuel powder it is possible, for example, to mix a UO2-containing powder with further powders which contain fissile heavy metal oxides, for example U2O3, PuO2 or ThO2 or compounds thereof, to form an MOX nuclear fuel powder.
After its production, the nuclear fuel powder is treated further. Additives are in some cases added to a nuclear fuel powder, inter alia to influence the properties of a nuclear fuel sintered body and/or for reasons which are of relevance to the production process.
This powder is usually pressed into shaped compacts, and the shaped compacts are sintered to form nuclear fuel sintered bodies. During sintering, the crystallites of the starting powder agglomerate to form grains in the sintered body. The size of the grains of a nuclear fuel sintered body can be influenced by a multiplicity of parameters during the production process and/or in the starting powder. The grain size has a decisive influence inter alia on the mechanical properties, in particular the plasticity of the sintered body and/or on its properties with regard to fission gas retention.
It has become known from U.S. Pat. No. 4,869,866 that a sintered body which has both good fission gas retention and improved PCI properties is of interest. However, for this purpose it is provided for virtually all the grains of a nuclear fuel sintered body to be completely surrounded by a glassy aluminosilicate phase. According to U.S. Pat. Nos. 4,869,867 and 4,869,868, it is also possible to provide a completely surrounding glassy magnesium silicate phase or a completely surrounding magnesium aluminosilicate phase.
According to Japanese JP 01029796, to improve the fission gas retention and the PCI properties, it is advantageous for Cr2O3 to be added to a nuclear fuel powder in an amount of 1000-3000 ppm, in order to accelerate grain growth during the sintering process.
Further publications are restricted either purely to measures for simply promoting grain growth or purely to measures for simply reducing the PCI.
Japanese JP 55151292 provides for an additive composition to be added solely for the purpose of increasing the grain size. In addition to a wide range of further additives, one example also mentions Fe2O3, in an amount of 50 ppm.
According to Japanese JP 55087993, a range of oxides are to be added, in high levels of 2000-50,000 ppm as additives to a nuclear fuel powder in order to produce a large number of oxygen defects, which serve as fission gas traps.
According to Japanese JP 55104791, a two-shell nuclear fuel sintered body is provided, having a multiplicity of additives in high levels of up to 50,000 ppm, in order, by means of the outer shell, to improve only the PCI properties of the nuclear fuel sintered body, while the inner shell is intended to improve the mechanical hardness of the nuclear fuel sintered body.
In normal operation, and particularly also in the event of ramp changes of power in the nuclear reactor, nuclear fuels for light water reactors release fission gases. The rate of release is dependent, inter alia, on the grain size of the nuclear fuel sintered body. Furthermore, the nuclear fuel sintered body, while it is being used in the nuclear reactor, is subject to dimensional fluctuations which, particularly in the event of an increase in dimensions, lead to interaction between the nuclear fuel sintered body and the cladding tube (PCI properties). This interaction is likewise dependent, inter alia, on the grain size of the nuclear fuel sintered body.
International PCT publication WO 00/00985 describes a nuclear fuel pellet wherein, through the addition of an Nb-containing dopant, a particularly favorable grain growth and a particularly favorable creep behavior are to be established.
It is accordingly an object of the invention to provide an improved method of producing a nuclear fuel sintered body from a powder which contains a fissile heavy metal oxide powder, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which results in a nuclear fuel that has properties which are improved both with regard to the release of fission gases and with regard to the nuclear fuel sintered body/cladding tube interaction (PCI properties). In particular, the properties of a nuclear fuel powder derived from a dry-chemical conversion process are to be taken into account both with a view to improved fission gas retention and with a view to improved PCI properties.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of producing a nuclear fuel sintered body, which comprises:
producing a powder containing a fissile heavy metal oxide, the powder comprising a sinter-active heavy metal oxide powder obtained predominantly from a dry-chemical conversion process;
adding a dopant to the powder for increasing a grain size and a plasticity of a resulting sintered body, the dopant containing an iron compound and being added such that a proportion by weight of the iron compound in relation to a proportion by weight of the heavy metal oxide in the nuclear fuel sintered body is at least 100 ppm; and
sintering the powder to form the nuclear fuel sintered body.
In other words, the objects of the invention are achieved by a method of producing a nuclear fuel sintered body, wherein a powder, which contains a fissile heavy metal oxide, is produced, treated further and sintered, the powder comprising a sinter-active heavy metal oxide powder which is obtained predominantly from a dry-chemical conversion process, and a dopant which increases both the grain size and the plasticity of the sintered body and contains at least 100 ppm of an iron compound, in particular of an iron oxide compound, is added to the powder.
The quantitative indication 100 ppm, as used herein, indicates the proportion by weight of the iron compound, in particular of the iron oxide compound, in relation to the total proportion by weight of the heavy metal oxide in the finished nuclear fuel sintered body (i.e. in xcexcg/g, which corresponds to ppm by weight). The iron compound may, if appropriate, also be iron.
The method according to the invention has in particular the advantage, firstly, that it can be carried out as part of existing production methods and sequences used in dry conversion. An iron compound, in particular an iron oxide compound, can easily (for example by direct addition) be added to the nuclear fuel powder, for example before or after a granulation operation or during milling. On account of the ferromagnetic characteristic of the iron particles, it is also easy to locate a nuclear fuel sintered body in a cladding tube, which has advantages for the production process and during process monitoring, in particular for the manufacture of fuel assemblies.
The invention is based on the surprising discovery that targeted addition of the iron-containing dopant, compared to a conventional fuel, both increases the grain size and increases the plasticity. Therefore, these two inherently contradictory parameters can be set particularly favorably, so that the sintered body has a particularly favorable operating performance both with regard to the release of fission gases and in terms of its PCI properties. Complex or additional measures, such as for example completely surrounding grains with a silicate phase of an additive, are rendered superfluous inter alia if the grain according to the invention already has a specifically favorable size.
Tests and experiments in connection with this invention carried out on a very wide range of nuclear fuel sintered bodies used in reactors have clearly demonstrated that the rate at which fission gases are released in sintered bodies with a coarse grain, i.e. a large grain size, is lower, i.e. the rate of release decreases as the grain size of the sintered body increases. This is because, on the one hand, when using a coarse grain the fission gases to a large extent remain in the grain and, on the other hand, when using a coarse grain the diffusion of fission gases at the grain boundaries is greatly reduced. By contrast, further tests and experiments carried out in connection with this invention have established that in sintered bodies with a fine grain, i.e. a small grain size, the interaction of the sintered body with the cladding tube is lower, i.e. in particular the cladding tube has a lower expansion despite a change in dimension of the sintered body. This is because fine-grained sintered bodies have a higher plasticity than coarse-grained sintered bodies, which manifests itself in an improved creep capacity in fine-grained sintered bodies. As a result, loads and, if appropriate, expansion of a cladding tube can be reduced considerably in particular on account of an increase in the dimensions of the sintered body.
There is in particular a discrepancy between an aim to reduce the release of fission gases in a nuclear fuel sintered body and also of reducing a nuclear fuel sintered body/cladding tube interaction. According to the inventive discovery, however, this discrepancy is overcome by targeted doping with an iron compound, which allows favorable setting of the grain size and, in addition, favorable setting of further characteristic parameters, in particular of the plasticity.
The doping of the nuclear fuel with a dopant which contains at least 100 ppm of an iron compound, in particular of an iron oxide compound, (and if appropriate, according to a refinement of the invention, with a dopant which additionally contains compounds of silicon, aluminum or chromium) moderately and deliberately increases the grain size in accordance with the inventive discovery. It is set in precisely such a way that, on the one hand, the release of fission gases is reduced and, on the other hand, the plasticity is increased compared to conventional fuels of the same or smaller grain size.
According to the invention, this measure has the combined effect of both a reduced release of fission gases and of a lower pellet/cladding interaction. The controlled doping of the fuel with at least 100 ppm of an iron compound, in particular with an iron oxide compound, and in particular, if appropriate, additionally with a silicon and/or aluminum and/or chromium compound, makes it possible to produce a nuclear fuel sintered body with properties which hitherto appeared impossible, namely both a coarse grain size and an improved plasticity.
According to the inventive discovery, it is advantageously provided for the addition of dopant to be carried out taking account of the properties of a powder which, specifically, is obtained predominantly in a dry-chemical or other direct conversion process.
This is because in the case of powders obtained from wet-chemical processes, the size of the crystallites which fuse together to form grains during sintering, at considerably less than 100 nm, is well below the crystallite size of powders obtained from dry-chemical processes (250 nmxc2x1100 nm). For this reason, among others, the wet-chemical powders sinter at an earlier time during the sintering process and starting from a low sintered density. By contrast, the sintering behavior of powders obtained from dry-chemical processes is slower and starts from a greater sintered density. By way of example, powder obtained from the ADU process has a BET surface area of at least 4 m2/g, whereas a powder obtained from a dry-chemical process has a surface area of, for example, 0.5-4 m2/g, generally around 2 m2/g (BET surface area: a specific surface area measured using the BET method).
If the starting point used were to be a powder obtained from a wet-chemical conversion process (e.g. ADU powder), a dopant would dissolve more successfully in the small crystallites of the ADU powder than in a crystallite of a powder obtained from a dry-chemical conversion process, and consequently the grain of the ADU powder would grow more quickly, and therefore the sintered body would become too hard on account of an excessively large grain. Therefore, identical quantities of dopant have completely different effects on a powder obtained from a wet-chemical process and a powder obtained from a dry-chemical process.
Specifically, in a DC powder, a dopant advantageously overall dissolves less well in the crystallite of the starting powder and the grain formed therefrom during the sintering process, since the crystallites are larger from the outset and sinter more slowly than in the case of a powder derived from a wet-chemical process, e.g. an ADU powder. This property of the DC powder, which initially appears to be a disadvantage, is, however, advantageously exploited according to the inventive discovery. This has considerable advantages both for a process and for a sintered body according to the invention. This is because, when a DC powder is being sintered in accordance with the invention, firstly the grain growth is not excessive but is sufficient to ensure adequate plasticity in combination with good fission gas retention, i.e. leads to a moderately sized grain. Secondly, some of the dopant remains outside the grain and increases the grain boundary slip with a moderately sized grain and therefore the plasticity with a moderately sized grain. The dopant is therefore added in particular in a quantity which results in a moderately large-grained microstructure and a high plasticity of the nuclear fuel sintered body.
Particularly with increasing reactor use, a nuclear fuel sintered body undergoes an increase in volume and accordingly an increase in diameter, which can lead to a considerable load and possibly expansion of the cladding tube. The importance of this phenomenon increases as the operating time of a fuel assembly having a nuclear fuel sintered body of this type increases, for example three or more use cycles. This is the case with fuels with a burn-up potential of 30 MWd/kgSM to 80 MWd/kgSM or more (high burn-up). Iron compounds, and in particular iron oxide compounds, as dopant have the advantage over other dopants of allowing plasticity of a nuclear fuel sintered body according to the invention even for long dwell times of a fuel assembly. This therefore has the advantage, with regard to the nuclear fuel sintered body produced using said process, that it substantially retains its plasticity even after three or more use cycles of a fuel assembly and also still has a sufficient fission gas retention.
Especially an iron compound, and in particular an iron oxide compound, compared to standard dopants with a moderate grain size, also does not accelerate diffusion to such an extent with respect to fission gas retention. Increasing the grain size by adding dopant usually also entails lower fission gas retention, on account of an interfacial diffusion for fission gases which is also improved by the dopant. However, this drawback of other dopants is not present when using an iron compound, and in particular an iron oxide compound, as dopant, as has surprisingly emerged.
In particular, an iron oxide compound can advantageously be successfully dissolved in the fluorite lattice of the heavy metal oxide and in the process releases oxygen. This is decisively responsible for the grain growth, since it greatly increases the mobility of the heavy metal atoms.
On the other hand, in particular iron oxide, compared to other dopants, on account of its high valency retains the oxygen for a sufficient time during sintering (Fe2O3, FeO, FeOH), and therefore promotes grain growth more successfully than other dopants for a nuclear fuel powder obtained from a dry-chemical conversion process (DC powder). Standard dopants are reduced to metals too quickly for the somewhat slower grain growth or sintering process described above for a DC powder (for example compared to an ADU powder). Nevertheless, iron oxide as dopant advantageously increases the grain size only up to a moderate size of 10 xcexcm to 25 xcexcm or even 30 xcexcm, so that both the build-up of large, i.e. also sharp-edged, grains is suppressed, and therefore sufficient plasticity of the nuclear fuel sintered body is also ensured. This is extremely advantageous for the PCI properties of the nuclear fuel sintered body. Grain sizes of 30 xcexcm to 80 xcexcm which can in some cases be reached with other dopants, such as Ti, Nb and Cr, are less advantageous for the PCI properties.
According to an advantageous refinement of the invention, the dopant therefore contains an iron oxide compound, in particular one such compound selected from the group consisting of: FeO, Fe2O3, Fe3O4, Fe(OH)2 and Fe(OH). Overall, iron oxide as dopant therefore has a good compatibility with the properties of a DC fuel powder, in particular even in production processes with regard to the mixing performance and the sintering performance.
In accordance with an added feature of the invention, the fissile heavy metal oxide advantageously contains an oxide of a heavy metal selected from the group consisting of: uranium, plutonium and thorium and/or a compound of the heavy metal, in particular UO2 and/or PuO2 and/or ThO2. This applies in particular also to the production of MOX fuels.
If appropriate, the powder may expediently also contain an element selected from the group consisting of beryllium and molybdenum, and/or a compound of these elements. This advantageously increases the thermal conductivity of the nuclear fuel sintered body.
The powder preferably contains a U3O8 powder which is obtained by oxidation of a UO2 powder obtained from a dry-chemical or other direct conversion process. However, if appropriate it is also possible for the powder to contain a U3O8 powder which is obtained by oxidation of an UO2 powder obtained from a wet-chemical process, in particular an AUC or ADU process.
The powder expediently includes finely dispersed particles of a size of substantially 1-1000 xcexcm, in particular of a size of on average 50 xcexcm to 150 xcexcm. In particular, a powder of this type also includes crystallites of a size of substantially 50-500 nm, in particular of a size of on average 100 nm to 350 nm. These are particularly advantageous properties for a powder produced using a dry-chemical or other direct conversion process, which according to the inventive discovery have advantageous effects in particular on the grain size which is established during the sintering process.
The powder is advantageously homogenized and/or milled for further treatment. In particular, the powder is pre-compacted and/or granulated for further treatment, expediently down to a particle size substantially of less than 2 mm. This improves the free-flowing properties of the powder, i.e. facilitates the process sequence, in particular the introduction of the powder into press molds for compression of the powder (prior to the sintering operation). In addition to an iron compound, the dopant expediently also contains a chromium compound selected from the group consisting of Cr2O3 and CrO3.
Furthermore, it is expedient for the dopant to contain a silicon and/or silicate compound, in particular a compound selected from the group consisting of silicon oxide, iron silicate and magnesium silicate. It is preferable for the dopant also to contain a compound of an element selected from the group consisting of magnesium, niobium, titanium, aluminum, vanadium and platinum, in particular an oxide of the element. This allows the abovementioned advantages of the invention in combination with an iron compound, in particular an iron oxide compound, to be improved further. This applies in particular to a silicon or aluminum compound, expediently not in excessively large quantities.
It is preferable for the dopant to be added in a quantity which leads to a microstructure of the nuclear fuel sintered body which has a grain dimension substantially of more than 8 xcexcm, in particular a grain dimension in the range from approximately 10-25 xcexcm. A grain size of 10-15 xcexcm is particularly preferred.
Furthermore, the dopant is expediently added in an amount which leads to a favorable plasticity of the nuclear fuel sintered body, which plasticity is determined by a moderate creep of the sintered body at temperatures of between 1100xc2x0 C. and 1700xc2x0 C. and pressures of approximately 70-160 N/mm2, in particular at temperatures of around approximately 1200xc2x0 C. and at pressures of approximately 70-90 N/mm2.
In particular, this quantity exceeds a quantity of the dopant which is soluble in the lattice of the heavy metal oxide. It has proven particularly expedient for the dopant to be added in an amount of between 100 ppm and 5000-6000 ppm, preferably in an amount of between 500 ppm and 3000 ppm. Amounts of between 750 ppm and 2500 ppm have proven particularly favorable. A quantity of this nature is not so high that a grain would be completely surrounded by the added additive (for example in the case of silicates).
It is advantageous for the dopant to be added to the powder in a step involved in the production and/or further treatment thereof, preferably prior to a granulation step. If appropriate, it is also expedient for the dopant to be added during and/or after a granulation step. For this purpose, the dopant is expediently added in powder form, the dopant powder having a particle size substantially of less than 5 xcexcm, preferably a particle size substantially of less than 2 xcexcm.
According to a refinement of the invention, a further dopant, which contains at least one compound of a substance selected from the group consisting of zirconium, cerium, yttrium, lanthanum, ytterbium, cesium, calcium, all further subgroup elements, lanthanide elements and actinide elements, is added to the powder. This dopant is in particular an oxide of the substance.
It is expedient for a neutron poison, in particular a gadolinium compound and/or a boron compound and/or an erbium compound, to be added to the powder. This has proven favorable in particular with a view to achieving a high burn-up in a fuel assembly. For this purpose, there is advantageously a higher level of the heavy metal oxide powder. This level may expediently exceed 3.5% or 4%, or may even be 4.5-5%. The level could even exceed 5%, for example could be 6.5%. To improve processing of the powder, it has proven favorable to add a lubricant, in particular a substance selected from the group consisting of stearate compounds, stearic acid, amide compounds, glycol compounds, and paraffin compounds, to the powder.
To set the sintered density, and for reasons of fission gas retention, it has also proven advantageous to add a pore-forming agent, in particular a substance selected from the group consisting of ammonium salts, ammonium carbonates, acetate compounds, oxalate compounds and sugar starch, to the powder. It has proven particularly advantageous to add a pore-forming agent in such a manner that a pore size of approximately between 2-200 xcexcm, in particular approximately between 5-80 xcexcm, is established in the sintered body.
The dopant or a plurality of further additives, for example a further dopant, a neutron poison, a pore-forming agent or a lubricant, are expediently added together to the powder and/or the further-treated powder. This additionally simplifies The method according to the invention.
The powder is advantageously pressed into shaped bodies prior to sintering, these shaped bodies expediently having a density of approximately 4.5-7 g/cm3.
The powder (a shaped body) is advantageously sintered substantially at temperatures of between 1500xc2x0 C. and 1900xc2x0 C. and in an atmosphere with a pressure in the region of standard pressure for a duration of 2-8 hours.
In particular, the powder (a shaped body) is sintered to approximately 93-98.5% of the theoretical density. Accordingly, the sintered shaped bodies expediently have a density of 10-20 g/cm3 in particular a density of 10.0-10.8 g/cm3.
For this purpose, the powder (a shaped body) is expediently sintered under a reducing inert gas atmosphere. The inert gas atmosphere preferably predominantly contains inert gases, in particular He and/or Ar, if appropriate also advantageously at least one further gas selected from the group consisting of H2, N2 and CO2. The ratio of H2 to N2 in the inert gas atmosphere is advantageously set to be substantially between 1:1 and 3:1. The ratio between the quantity of the further gases and the quantity of the inert gases is expediently set to be between 0.02:1 and 0.08:1.
During sintering, it is expedient to set an oxygen partial pressure of the inert gas atmosphere in such a manner that U3O8 fractions of the powder are reduced to form UO2 fractions of the powder. In particular, however, an oxygen partial pressure of the inert gas atmosphere is also set in such a manner that reduction of the iron oxide fractions of the powder to form iron fractions of the powder is avoided.
For this purpose, it is particularly expedient to set the partial pressure range approximately to a standard pressure lowered by 5% up to a standard pressure increased by 5%. The oxygen partial pressure of the inert gas atmosphere is preferably set at between 10xe2x88x9220 and 10xe2x88x926 atm.
In a very particularly advantageous configuration of the invention with regard to the process, the object is accordingly achieved by a method of producing a nuclear fuel sintered body, wherein a powder which contains at least one heavy metal oxide is produced, the powder containing a sinter-active UO2 heavy metal oxide powder which is obtained predominantly in a dry-chemical or other direct conversion process, and in particular has particles of a size of 1-1000 xcexcm with crystallites of a size of 50-500 nm, and contains a U3O8 heavy metal oxide powder, which is predominantly obtained by oxidation from some of the sinter-active UO2 heavy metal oxide powder, and if appropriate contains a returned U3O8 heavy metal oxide powder, the powder being treated further, and a dopant, which increases both the grain size and the plasticity of the sintered body and contains at least 100 ppm of an iron compound, in particular of an iron oxide compound, being added to the powder, the powder is pressed into shaped bodies, and the shaped bodies are sintered at temperatures of between 1500xc2x0 C. and 1900xc2x0 C. and in an atmosphere with a pressure in the region of standard pressure for a duration of 2-8 h.
In particular, the nuclear fuel sintered body preferably contains an iron oxide compound, in particular one such compound selected from the group consisting of: FeO, Fe2O3, Fe3O4, Fe(OH)2 and Fe(OH)3.
To achieve the advantages according to the invention, the nuclear fuel sintered body particularly advantageously has a microstructure which is wherein by a grain dimension substantially of more than 8 xcexcm, in particular up to 15 xcexcm, i.e. a grain dimension in the range from approximately 10-25 xcexcm. A grain size of 10-15 xcexcm is particularly preferred.
According to a refinement of the invention, the nuclear fuel sintered body preferably also has a pore size of approximately between 2-200 xcexcm, in particular of approximately between 5-80 xcexcm. It preferably has a density, which is influenced inter alia by the pore size, of 10-20 g/cm3, in particular a density of 10.0-10.8 g/cm3.
A nuclear fuel sintered body according to the invention or a refinement of the invention accordingly likewise has a favorable plasticity, which is determined by a moderate creep, in particular at temperatures between 1100xc2x0 C. and 1700xc2x0 C. and pressures of approximately 70-160 N/mm2, in particular at temperatures around approximately 1200xc2x0 C. and at pressures of approximately 70-90 N/mm2.
To prevent PCI as far as possible, the fuel should also be matched to the cladding tube according to the above design options.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of producing a nuclear fuel sintered body, and nuclear fuel sintered body, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.