EP0948794A1

EP0948794A1 - Nuclear reactor fuel element with high burn-up and method of producing the same

Info

Publication number: EP0948794A1
Application number: EP97942878A
Authority: EP
Inventors: Gerhard Gradel; Alfons Roppelt; Rudolf Meinl
Original assignee: Siemens AG
Current assignee: Areva GmbH
Priority date: 1996-09-09
Filing date: 1997-08-26
Publication date: 1999-10-13
Also published as: DE19636563C1; TW392178B; KR20000068512A; WO1998010428A1; JP2000502809A; US20010022827A1

Abstract

In order to increase the burn-up potential of fuel elements, pellets enriched to an unacceptably high degree are produced on the production lines (3 to 9) which are designed to produce large amounts of normally enriched fuel. The unacceptable enrichment is compensated by mixing with the fuel (T, P, N) in the powder mixer (M) at the production line inlet an amount of absorber material (U/B powder) such that the reactivity of the contaminated mixture does not exceed the reactivity of a normally enriched uncontaminated fuel mixture. Corresponding fuel elements thus contain larger amounts of these contaminated pellets (or only such contaminated pellets) which can be produced with conventional plants in large numbers (and hence economically).

Description

description

Nuclear reactor fuel assembly with high burn-up and process for its production

The invention relates to a method for producing a nuclear reactor fuel element with high burn-up, that is to say, for example, a nuclear reactor fuel element with a burning time of 5 or more cycles, and a corresponding enrichment of fissile material which corresponds to more than 5% U ₂₃ s. The invention is based on a manufacturing method that has the features of the preamble of claim 1.

In pressurized water reactors, some fuel elements, the usable energy content of which is used up in the form of enriched core material, are replaced at regular intervals (for example annually) with fresh, non-poisoned fuel elements. The manufacture of these non-poisoned fuel elements is shown in FIG. 1, starting from enriched fission material which is kept ready in transport containers T1, T2, ... Tn. These transport containers are delivered from a conversion system 1 in which a uranium oxide powder is produced from a uranium compound. The uranium of the uranium compound contains natural uranium (mainly the uranium isotope U ₃ β that cannot be used directly for the chain reaction of the reactor) and the uranium isotope U ₂₃₅ , which is important for the chain reaction, with the U235 content for safety reasons. this is the "enrichment", is generally limited and in any case must not exceed a maximum value (generally 5%).

In the conversion system 1, the oxide powder, which is generated, for example, from UF ₆ by reduction in an H ₂ / H ₂ 0 gas, is filled into the transport containers T1, T2,... Tn in a filling station 2, the volume the transport container Tl, T2, ... Tn is relatively small (for example, only for 100 kg of U0 ₂ powder), that is to say in each case only comprises a subcritical amount of fissile material and is also provided with rods S and / or a lining made of neutron-absorbing material.

The production plant comprises a first part 3 with a powder store and devices for powder processing, of which only one powder mixer M is shown in Fig. 1, e.g. a large mixing container with a stirring device, at the bottom of which a powder mixture is drawn off, which consists of the carefully homogenized contents of the transport containers Tl, T2, ... Tn emptied into the powder mixer M. This powder mixture can e.g. are conveyed (e.g. sucked in or blown with compressed air) via a powder feed line and other devices of the first part into the second part of the production system. Samples of the powder mixture are constantly examined at an analysis station 4 in order to control the homogeneity, fissile enrichment and quality of the mixture. In addition, it may be necessary to add lubricants and pressing aids to the gap material and / or to carry out suitable granulation measures on the powder.

The facilities of this first part of the production plant are designed with regard to their capacity so that they can hold so much powder that a filling of highly enriched material would come into dangerous proximity to the critical mass and would no longer be safe to handle. For security reasons, therefore, is a maximum value for €? Enrichment (eg 5%) is specified, and the capacity of the production facilities is designed so that the fissile material does not reach the critical mass, ie can be safely handled, even at the highest permitted enrichment value. For example, the volume and criticality dimensioning of the powder mixer M is usually designed for 1 to 4 tons, so that even a filling made from a non-poisoned powder Mixing with the permitted maximum value of, for example, 5% U ₂₃ 5 cannot come close to the critical mass.

This powder mixture is processed further in the second part of the production plant, a pellet press producing 5 pellet green compacts which are sintered in a sintering furnace 6. In a quality level 7 these pellets are ground to their final shape, measured, weighed and finally enclosed in a filling station 8 in corresponding metallic cladding tubes H, which usually consist of zirconium alloy (e.g. Zirkaloy). An assembling station 9 places these filled cladding tubes, which are gas-tightly welded by means of metallic end pieces, i.e. the fuel rods (FR), and other structural parts S of the fuel assembly, such as e.g. Head pieces, foot pieces and spacers as well as guide tubes or fuel boxes, together to form the finished fuel assembly ("fuel assembly", FA).

In addition to such non-poisoned fuel assemblies, "poisoned fuel assemblies" are also used to replace some of the spent fuel assemblies in a pressurized water reactor. In addition to the enriched fission material, these "poisoned fuel elements" contain a combustible neutron absorber, that is to say an absorber material whose absorption capacity for thermal neutrons decreases with increasing service life in the reactor. This "combustible neutron poison" neutralizes some of the neutrons emanating from the enriched material through nuclear fission, but the absorption effect has already decayed after one operating cycle to a residual, practically negligible absorption capacity. This makes it possible to maintain the value of the neutron flux, for which the reactor is designed and optimized, practically over the entire operating cycle and to compensate for the reactivity (excess reactivity) of the fresh fuel elements which goes beyond this. So far, non-poisoned and poisoned fuel elements have been used side by side in pressurized water reactors. In boiling water reactors, it is customary to use different enrichments for the individual fuel rods of each fuel element in order to achieve uniform combustion of the fissile material and optimal utilization. As a rule, all fuel elements in the core contain non-poisoned pellets and pellets with poisoned fuel. These pellets form the "active zone" of the fuel assemblies and are often surrounded by neutral pellets made of natural uranium, depleted uranium or other practically non-fissile oxide for reasons of thermal insulation and to limit the neutron flow.

The production of poisoned fuel elements is shown schematically in FIG. 2. The relatively expensive burnable neutron poison (usually gadolinium oxide Gd ₂ θ is only added to a few pellets of a fuel element, the powder mixture of which is produced in a special part of the production system, while the conversion system 1, the filling station 2, and the devices with the powder mixer M In the first part 3 of the production plant the powder mixture of the other pellets is used and the second part of the production plant can be used together with the pellet press 5, the sintering furnace 6, the quality level 7, the filling station 8 and the assembly stage 9 In a feed station 13, fuel powder of the poisoned pellets is removed from transport containers V, which come from a conversion system 10. There, the neutron poison was already added to the fissile material during the conversion of the uranium compound or added to the uranium oxide powder produced by the conversion For homogenization, lver is generally first filled into the transport container V in a filling station 11 and then Homogenization of the mixture fed to a tumble mixer 12.

In principle, other combustible neutron poisons can be used instead of gadolinium, for which the nuclear properties of boron appear particularly interesting. However, elemental boron or a boron-containing compound cannot simply be added to the uranium oxide powder, since then a readily volatile boron compound is formed, which cannot be kept in the pellets, but at the temperatures and in the reducing or inert gas atmosphere, which leads to Sintering can be used from which pellet is driven out. It has therefore already been proposed to first coat the finished pellets with boron. This coating layer can be sprayed on in a plasma process, by deposition from a corresponding vapor phase, by sputtering or other methods. An example is described in U.S. Patent No. 3,427,222. The coating layer can also consist of several layers in order to apply an adhesive intermediate layer and / or a protective layer and / or to improve the absorber properties by introducing a further absorber material with changed nuclear behavior. In DE-A-34 02 192 UO ₂ with

Niobium (3μ to 6μm thick) is charged, on which .ZrB _{2 is} then chemically separated from the vapor phase.

For the production of poisoned fuel elements, it has also already been proposed to introduce the boron into the fuel elements in the form of their own small absorber bodies. For example, steel tubes filled with boron glass can be inserted into the guide tubes of fuel assemblies via their own holders (so-called "boron glass spiders"), which are not required to control reactor operation and into which no control rods are therefore inserted. It has also already been proposed to produce boron-containing microparticles (eg from ZrB?) which are also protected by a coating (e.g. made of molybdenum). In principle, it is therefore possible, instead of the gadolinium oxide powder in FIG. 2, to mix a powder of such molybdenum-protected microparticles with the uranium oxide powder and to fill them into the transport container V.

Spent fuel assemblies still contain fissile plutonium, which can be separated from the spent fissile material in appropriate reprocessing plants in order to use it instead of fissile U235 to enrich fissile material for fresh fuel elements. For the production of fuel elements from such mixed oxide (MOX, i.e. mixture of uranium oxide and plutonium oxide), devices of the special part of the production plant shown in FIG. 2 are used. For this purpose, the transport container P (FIG. 3) and oxide of natural uranium (or depleted uranium from reprocessing) and the required absorber material delivered from the reprocessing plant and filled with plutonium oxide as well as the required absorber material in the filling station 11 can be filled into the transport container V and homogenized in the tumble mixer 12 become. The poisoned fuel powder is then fed into the second part of the production system, e.g. the elements 5 to 9 of FIGS. 1 and 2 fed.

Usually, about 1/4 of the fuel elements have practically burned down after each fuel cycle and must be replaced with new fuel elements. So far, the average life of the fuel element has been around four years, whereby this operating time is determined not only by the energy content (enrichment) of the fissile material, but also by the material properties of the cladding tubes. So far, fuel elements from areas in which weaker erosion occurs can only be used longer if, for example, sufficient corrosion-resistant cladding tube material is available. Meanwhile, cladding tubes are Structural materials and fuel element designs have been developed which also allow a longer period of use (e.g. 6 to 7 years). In principle, this enables considerable savings in the reloading with fresh fuel elements and the disposal of the spent fuel elements, since only about 1/6 to 1/7 of the fuel elements then had to be replaced. However, this presupposes a correspondingly high concentration, which, for example, had to be 6 to 8% U235 - a value at which, for example, the volume of the powder mixer M in FIG. 1, if it was filled with such enriched fission material, the maximum volume would exceed that is sufficiently distant from the critical mass and is still approved for safe handling. The quantities of pellets or filled fuel rods that were previously available for storage in production may no longer be used. For these reasons, the use of fissile material that exceeds a set maximum of 4 to 5% U23. or an equivalent level of plutonium is generally not permitted. The savings potential created by the advances in reactor technology cannot be used for these practical reasons, although this should be possible in theory.

Highly enriched fuel can namely. B. only m

Protective containers with a small volume and neutron-absorbing internals can be stored and transported. For fuel assemblies, it has already been proposed to use only plutonium without natural uranium, which is enclosed in hullnium tubes. But pellets that are added with

So far, boron coatings have only been considered in connection with the reactor physics of conventional poisoned fuel elements. However, it must also be possible to use fuel that would be enriched to increase the fire above the previous maximum value. However, producing such highly enriched pellets on an industrial scale seems to require particularly safe manufacturing processes and special equipment. As with the previously separately manufactured, poisoned pellets, it is considered to use only a few special pellets, which can be produced with such special devices, together with the largest possible number of the usual and easier to produce, normally enriched pellets, but a special production would be due to the small number of pieces not useful.

A further limitation of the enrichment arises from the requirement that the finished fuel elements must also be sufficiently removed from the critical act if they (unintentionally in the distribution center or during transport) are inadvertently straight into the vicinity of larger quantities of water (eg quenching water) in case of fire). Therefore, common fuel elements of the type 16 x 16 or 18 x 18 must not have an enrichment of more than 4.4% (for the type 17 x 17 the limit is somewhat higher). Safety would also be ensured if larger amounts of absorber material were installed in the structure of the fuel assembly. However, this either necessitates fundamental changes in the structure or structural material of the fuel elements, or special pellets containing absorber must be used. There are currently no concepts for either route that can be implemented quickly and economically. Rather, attempts are being made to extend the service life of the fuel elements without exceeding the enrichment limits by making better use of the previously available burn-up potential.

However, it should also be possible to create fuel elements with regard to the required safety, which enable safe use of highly enriched fissile material, and to modify the production processes and safety regulations accordingly. The invention is therefore based on the object of providing a method for producing fuel elements with such a highly enriched fissile material and of specifying corresponding fuel elements at all without lengthy and complex changes to the prior art having to be made.

The invention is based on the fact that basically the enrichment of the fissile core material itself, but only its reactivity and the reactivity of the finished fuel elements is the safety-relevant parameter. Instead of starting from the complete enrichment of the fissile material in order to maintain safety, it makes physical sense to subtract from this enrichment that part which may be compensated for by already added combustible neutron poison. It should therefore be based on the reactivity of the powder used, the resulting pellet and the fuel assembly. For the processing of a non-poisoned powder mixture, reactivity and enrichment are equivalent, and for the handling previously considered safe, the system according to FIG. 1 can still only be used with fissile material whose enrichment does not exceed the maximum value, for example 5% . 1, according to the invention, a powder mixture is processed with the same degree of security, in which the enrichment of the fissile material is above this maximum value, but the powder mixture also contains an amount such that the reactivity of the latter the poisoned powder mixture corresponds to the reactivity of a non-poisoned powder mixture, the accumulation of which does not exceed the maximum value mentioned. The corresponding pellets then have the required lower reactivity, although they have a higher concentration ("burn-off potential"). For the production of fuel elements with a higher burn-up - eg 60 up to 70 MWd / kg (U) - it is particularly advantageous to provide not only some of the fresh fuel elements, but all fuel elements of a pressurized water reactor with poisoned pellets, the accumulation of which is above a value of about 4 to 5% (e.g. at 6 up to 8%). It can even be advantageous - even in the case of a boiling water reactor - to enrich and poison all pellets of the fuel elements accordingly. As a result, the high production capacities that were previously only used for non-poisoned pellets are fully utilized. For the storage of such poisoned fuel elements, there are no changes compared to the previous fuel elements from a safety perspective.

This leads to a manufacturing method with the features of claim 1 and to a fuel assembly with the features of claim 12.

3 schematically shows the method steps and devices used for an embodiment of the method according to the invention.

The transport containers T, P and N, which are supplied by the conversion system or reprocessing system and are filled with enriched fissile material, plutonium-containing powder and powder with natural uranium, or in some other way contain the enriched fissile material. Cladding tubes H and the other structural parts required for the production of the fuel elements are also kept ready. Furthermore, a supply of absorber material is also assumed, e.g. can consist of gadolinium oxide according to the prior art.

The fuel powder to be processed in the pellet press is produced by a powder mixture in the powder mixer M. is produced, which on the one hand contains fissile material with an enrichment above the maximum enrichment value. On the other hand, this powder mixture contains such an amount of absorber material that the reactivity of the powder mixture has at most the reactivity that is equivalent to the reactivity of an unconfirmed, enriched fissile material.

3 can of course be a mixture of plutonium oxide, natural (or depleted) uranium oxide and enriched uranium oxide, but it is also possible to use only depleted uranium oxide and plutonium oxide, only enriched uranium oxide or another suitable cleavage material. This stock of highly enriched fission material can in particular be easily managed if the material is filled into many individual containers, the volume of which is only a fraction of the capacity of the powder mixer M. These containers can in particular consist of an absorber-containing material and / or contain additional absorbent components. The absorber material is mixed homogeneously with the contents of several such containers in the powder mixer. The absorber material can be present in the conventional way as gadolinium oxide, which can be mixed with the powder of the cleavage material, pressed into pellets and sintered in a known manner, directly or after additional measures for granulating and producing desired grain sizes. Tests on a laboratory scale have also shown favorable behavior during mixing, pressing and sintering for powders made of ZrB ₂ particles which are coated with molybdenum and mixed with uranium oxide powder. The burning behavior of boron particularly meets the requirements placed on the absorber by highly enriched fuel elements designed for a long period of use. Borides of rare earths such as gadolinium, erbium, euroblum, samarium etc. or hafnium are similarly also is suitable. Absorbent powders containing metal (e.g. hafnium, tantalum) also appear to be suitable. It is particularly advantageous to use not only one neutron absorbing chemical element, but several elements, in particular two elements. "Double absorbers" such as GdB ₂ , GdB or GdB ₆ allow MOX fuel elements with an increased content of fissile plutonium to be produced. Not only can the storage properties of the fresh fuel and the fresh fuel elements, but also the behavior of the fuel elements in the reactor be favorably influenced.

Standard dimensions and standard materials can be used for the cladding tubes and structural parts. However, while an extremely low hafnium content is usually prescribed for reactor materials, hafnium contents of up to 2% are quite possible here. This saves further costs because e.g. Zirconium sponge (the most common base metal for alloys used in nuclear technology) can only be cleaned of hafnium in a complex manner.

If it is planned to reprocess fuel elements with an enrichment of about 5% U ₂ 35 after a burn-up of 60 MWd / kg (or corresponding fuel elements designed for even higher burn-up) after their use in the reactor, poisoning with boron can Recycle problems that can be avoided by poisoning with gadolinium. However, due to fundamental considerations, the reprocessing of so heavily spent fuel elements no longer seems worthwhile; Boron poisoning is therefore particularly suitable for fuel elements that should be disposed of directly in the reactor after use.

In order to enable the long periods of use of the fuel assemblies, it is advantageous if the fuel assemblies are not only in the planes in which the fuel rod is attached for mechanical reasons Spacer grids must be supported, but also contain grids in intermediate levels. These intermediate grids are then provided with mixing devices in order to obtain better cooling of the highly enriched fuel rod by mixing the coolant. It is also advantageous if the enveloping tubes are particularly corrosion-resistant, for example, consist of a mechanically stable tube made of a zirconium alloy and contain a thin coating of a corrosion-resistant material on the outer surface exposed to the coolant, as described in the European patent

0 301 295. In this way, the fuel element is adapted to a long period of use not only with regard to its energy content and the enrichment of fissile material, but also with regard to the other chemical and physical conditions.

In order to increase the burn-up potential of fuel elements, pellets with impermissibly high enrichment are produced in the production lines (3 to 9), which are designed for processing large quantities of normally enriched fuel, the impermissible enrichment being compensated for by the fact that already in the powder mixer (M) at the entrance of the production line so much absorber material (U / B powder) is mixed to the fuel (T, P, N) that the reactivity of the poisoned mixture is the reactivity of an unpoisoned one

Fuel mixture does not exceed normal enrichment.

Corresponding fuel assemblies then contain larger quantities of these poisoned pellets (or - if necessary in addition to the neutral pellets mentioned - only those poisoned pellets) which are produced in large numbers (and therefore economically) with the conventional systems. Enriched fission material and an absorber material are then converted into poisoned pellets for the production of such fuel elements pressed and, if necessary, neutral pellets made from non-enriched material that is practically non-fissile (e.g. natural uranium or depleted uranium). These pellets are placed in columns, which consist exclusively of such poisoned pellets and possibly still neutral pellets and are enclosed in metallic cladding tubes. In this way, fuel rods are created, which are then assembled together with the structural parts (possibly also control rod guide tubes or water-filled rods, but without the use of unggi ted fuel rods) to form the fuel assembly.

Claims

claims

1. Method for manufacturing a fuel assembly (FA) for a

Light water reactor in which a) enriched fissile material (T, P), absorber material (U / B powder), metallic cladding tubes (H) for fuel rods and structural parts (S) of the fuel assembly are kept ready, b) in devices of at least one powder mixer Part (3) of a manufacturing plant containing (M) a powder mixture containing enriched fissile material is produced, the capacity of these devices (3), namely at least the volume of the powder mixer (M), being designed for a volume that is only available for one non-poisoned fissile material with an enrichment below a maximum value can still be handled safely, and c) in a second part of the production plant, a fuel powder made from enriched fissile material and absorber material is pressed and sintered into pellets and from the sintered pellets, the cladding tube and the structural parts the fuel elements are manufactured, the capacity of the device (4,5,6,7,8,9) of the second part does not exceed the safely manageable maximum volume of a non-poisoned fissile material with the enrichment below the maximum value, characterized in that a powder poisoned with the absorber material (U / B powder) is already produced in the powder mixer (M) as the powder mixture and the poisoned powder as Fuel powder is used for at least a portion of the pellets, the poisoned powder in the powder mixer (M) having an enrichment above the maximum value of the fuel enrichment and such an amount of absorber material that the reactivity of the powder mixture is at most equivalent to the reactivity of an unpoisoned the maximum enriched fissile material of the same volume.

2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that the enriched fission material is kept ready in individual containers, the volume of which is a fraction of the capacity of the powder mixer, and that in the powder mixer a powder of the absorber material is mixed with the content of several of these containers.

3. The method of claim 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the enriched fission material contains uranium oxide and / or plutonium oxide.

4. The method according to any one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t that the absorber material contains gadolinium.

5. The method according to any one of claims 1 to 4, that the absorber material contains boron or a boron compound.

6. The method of claim 5, d a d u r c h g e k e n n z e i c h n e t that the boron compound contains a rare earth.

7. The method according to any one of claims 1 to 6, that a powder with boron-containing particles, which are provided with a protective coating, are mixed with a powder from the enriched fissile material to produce the poisoned powder.

8. The method according to any one of claims 1 to 7, characterized in that pellets of all fuel rods (FR) of the fuel elements (FA), preferably all pellets of all fuel rods from the with the Absorbermate- rial poisoned powder with the enrichment above the maximum value.

9. The method according to any one of claims 1 to 8, so that cladding tubes (H) with a hafnium content are used which are above the permissible limit for the hafnium content in reactor-grade zirconium.

10. The method according to any one of claims 1 to 9, characterized in that the enrichment of the enriched fissile material is over 5 wt .-% U ₂₃₅ , preferably over 6%, or a corresponding value for fissile plutonium.

11. A method for producing a fuel assembly, in which enriched fission material and an absorber material are pressed and sintered into poisoned pellets, natural uranium or depleted uranium, if necessary, is also processed into sintered, non-poisoned neutral pellets, exclusively from the poisoned pellets and, if appropriate, the neutral pellets pellet columns placed and enclosed in metallic cladding tubes, and the fuel assembly is composed of structural parts and the metallic cladding tubes filled with the pellet columns.

12. Fuel element (FA) with fuel rods (FR), in which pellets with a fissile material are contained, whose enrichment is above the maximum value that is permitted for the safe processing of an unpoisoned enriched fissile material, the reactivity of the pellets being added by adding absorber material below the reactivity of a non-poisoned pellet from the non-poisoned fissile material enriched to the maximum value.

13. Fuel element according to claim 12, d a d u r c h g e k e n n z e i c h n e t that pellets of all fuel rods, preferably all enriched pellets of all fuel rods, contain this fission material.

14. The fuel assembly according to claim 12, so that all fuel rods of the fuel assembly contain only pellets with this fission material and possibly pellets of non-enriched material.