Method of producing a nuclear fuel sintered body

Details Method of producing a nuclear fuel sintered body Method of producing a nuclear fuel sintered body

2002-03-27

2004-10-26

Fiorilla, Christopher A. (Department: 1731)

Plastic and nonmetallic article shaping or treating: processes

Shaping or treating radioactive material

C264S043000, C264S044000, C264S674000, C264S676000

Reexamination Certificate

active

06808656

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
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 Interaction—PCI). 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 UO
2
from UF
6
. 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 UO
2
powder is obtained indirectly from uranium hexafluoride (UF
6
) 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 (UF
6
) is generally reacted with water and hydrogen to form uranium dioxide directly, generally according to the following overall equation:
UF
6
+2H
2
O+H
2
→UO
2
+6HF.
The UO
2
-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 UO
2
-containing powder with further powders which contain fissile heavy metal oxides, for example U
2
O
3
, PuO
2
or ThO
2
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 Cr
2
O
3
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 Fe
2
O
3
, 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 p

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