GB2115212A - Nuclear fuel element - Google Patents

Abstract

The invention relates to a nuclear fuel element having a metal cladding tube (2) and nuclear fuel particles (20) packed within the cladding tube (2). Each of the fuel particles (20) comprises a substantially spherical fuel kernel (21) made of a nuclear fuel material of stable oxides and a composite multilayer coating on the spherical fuel kernel. The composite multilayer coating is composed of an innermost first layer (22) of low-density pyrolytic carbon coated on the spherical fuel kernel, a second layer (23) of high-density isotropic pyrolytic carbon on the innermost layer, a third layer (24) of carbides on the second layer, a fourth layer (25) of high-density isotropic pyrolytic carbon on the third layer, and an outermost fifth layer (26) of low-density pyrolytic carbon.

Description

SPECIFICATION Nuclear fuel element The present invention relates generally to a nuclear fuel element or rod, and more particularly to a nuclear fuel element comprising- a metal cladding tube sealed at its both ends and having particles of nuclear fuel. material contained within the cladding'tube.

A nuclear fuel element having a large.num- ber of cylindrical sintered pellets of nuclear fuel material which are stacked 3 to 4 m high.

in a metallic cladding tube sealed at both ends thereof by the upper and the lower end plugs is widely known and has been adopted as the conventional nuclear fuel element for many nuclear power reactors. The nuclear fuel element has, in general, a heat-insulating disc inserted between the bottom pellet and the lower end plug, and a coil spring between the top pellet and the upper end plug.

The cylindrical sintered pellets which are inserted into the cladding tube in this way, need to have the outer cylindrical surfaces ground, since the pellet as sintered has an hour-glass shape in which the upper and lower portions exhibit larger diameter than the middle portion. This grinding process is important and can not be omitted from the viewpoint of reducing the interaction between the pellet and the cladding tube, which will be described more in detail hereinafter.This grinding of the outer surface (by a centreless grinding machine), however, involves a troublesome problem from a standpoint of maintaining the contamination-free condition of the fuel production facilities, since it produces grinding scrap of fuel material, especially when cylindrical pellets of a mixture containing uranium dioxide and highly toxic plutonium dioxide and the like are prepared.

Moreover, the nuclear fuel elements prepared by using such cylindrical pellets also involves a problem in terms of the long life span of the fuel during the operation of the nuclear reactor. Since the temperature at the centre of the cylindrical pellets is high and the temperature gradient in the radial direction thereof is large, a number of cracks are generated at various places around the outer surface of the pellet with the result that a number of fragments of pellet are formed, and the fragments are forced to move outwards and to make a close abutment with the inner surface of the cladding tube. The fragments then produce a local stress on the inner surface of the cladding tube due to a thermal expansion and swelling etc., during the increase of the power output of the nuclear reactor.Furthermore, when the power output of the reactor reaches a higher level, the'fragments of pellet, which is initially in the shape of a right cylinder at the initial cold state, are deformed into a shape as shown in Fig. 1, in which fragments of pellets are warped outward from the central axis of the pellet due to the fact that the temperature at the centre of the pellet 1 is higher than that at the periphery thereof, -and due to the fact that the density of the oxide fuel material in the central part of the pellet is slightlysless than that in the end surfaces thereof. -This means that a cylindrical pellet wherein cracks are generated has an envelope shape of an hour-glass in the reactor use.Therefore, as also shown in Fig. 2 the fragments 11 .of the pellet 1 expand and contact with the inner surface of the cladding tube 2 to generate the interaction between the pellets and the cladding tube (pellet clad interaction, hereinafter referred to as PCI), at the inner surface of the cladding tube 2 facing the opening 10 of the crack in the fuel pellet 1, thus a large stress exists locally, and an incipient hair crack 1 2 is generated at a place where this stress is increased in the corrosive environment of iodine, cesium and the like from the fission products (hereinafter called FP) released from the nuclear fuel material.

There is a large possibility that a hair crack 1 2 grows in the course of operation of the reactor, particularly in the load-follow operation of the reactor, propagating through the thickness of the cladding tube and resulting in the breakage of the cladding tube 2 before the end of the designed lifetime of the fuel.

A principal object of embodiments of the present invention is to eliminate such imper fections of the conventional nuclear fuel ele ment as described.

Another object of embodiments of the pre sent invention is to provide a nuclear fuel element which has the advantage that PCI, which is considered as a troublesome pheno menon relating to the conventional cylindrical pellets, are not generated at all.

Another object pf embodiments of the pre sent invention is to provide a nuclear fuel element which can maintain the soundness of -the nuclear fuel during a long period of the load-follow operation of the reactor.

An additional object of embodiments of the present invention is to provide a nuclear fuel element which is suitable for the semi-auto matic remote preparation in the fuel fabrica tion facilities.

Another object of embodiments of the pre sent invention is to provide a nuclear fuel element which permits the elimination if the risk of harmful FP being released from a nuclear fuel material to-the outside of the cladding tube.

Another object of embodiments of the pre sent invention is to provide a nuclear fuel element which affords a high performance sufficient to meet the load-follow operation of a nuclear reactor and is capable of achieving a high burnup in a heavy-water reactor and light-water reactor.

A further object of embodiments the pre sent invention is to provide a nuclear fuel element which is suitable for a nuclear fuel element to attain a high burnup for a fast breeder reactor.

Yet another object of embodiments the present invention is to provide a nuclear fuel element which does not contaminate the interior of storage facilities even when the spent nuclear fuel is stored for a long period.

Accordingly, the present invention provides a nuclear fuel element having a metal cladding tube and nuclear fuel particles packed within said cladding tube wherein each of said fuel particles comprises a substantially spherical kernel made of a nuclear fuel material of stable oxides and a composite multilayer coating of said spherical fuel kernel, said composite multilayer coating being composed of an innermost first layer of low-density pyrolytic carbon coated on said spherical fuel kernel, a second layer of high-density isotropic pyrolytic carbon on said innermost layer, a third layer of carbides on said second layer, a fourth layer of high-density isotropic pyrolytic carbon on said third layer, and an outermost fifth layer of low-density pyrolytic carbon.

Preferably, the substantially spherical fuel kernel of oxide fuel material had a diameter of about 0.5-1.5 mm, and the third layer of carbides is silicon carbide or zirconium carbide.

The spherical coated fuel particles may be packed in the cladding tube together with fine particles of getter material which has small neutron absorption cross-section and also effectively captures gaseous FP released from the fuel material. Each of the getter particles is selected to have such a size that it allows location in the small spaces surrounded by several spherical coated fuel particles packed the cladding tube. The getter particles are preferably made of one or more kinds of materials selected from zirconium, zirconium alloy, titanium, titanium alloy or graphite.

The spherical coated fuel particles packed in the cladding tube may be divided by providing some partitions of graphite or metallic wool layer along the length of the cladding tube.

A coil spring and a heat-insulating disc may be disposed within the cladding tube so as to facilitate compressing and retaining the coated particles stacked within the cladding tube. The metal cladding tube may be formed of a single-walled tube made of one kind of material selected from zirconium, various zirconium alloys, stainless steel, titanium and vanadium alloys, or of a double-walled tube made of two kinds of the above-mentioned materials.

Nuclear fuels in a form of coated particles have been widely used so far in the hightemperature gas-cooled reactors, and as representative examples thereof, reference will be made hereinbelow to those used in a hightemperature gas-cooled reactor AVR in West Germany and in a planned multi-purpose high-temperature gas-cooled reactor project in Japan.

The nuclear fuel of the AVR (electrical output 15,000 KW), which is also called a pebble bed reactor is of a spherical shape and uranium (U) and thorium (Th) are used therein. The spherical graphite shell, which has an outer diameter of 6 cm and a thickness of 1 cm, includes therein a spherical fuel portion with a diameter of 4 cm. This fuel portion is prepared by coating a fuel kernal of about 400 ym formed of carbide or oxide of uranium/thorium with two layers of low-density carbon and high-density carbon (called BlSO-coating) to form a fuel- particle having an outer diameter of 740 ,um, mixing a number of thus formed fuel particles with graphite powder and phenolic resin as a binder, and sintering the mixture.The pebble fuel thus prepared is stacked up in a reactor vessel to form a reactor core bed and helium is circulated through the gap spaces formed among the spherical pebble fuels for cooling.

The planning design of the Japanese multipurpose high-temperature gas-cooled reactor has been made by the initiative of the Japan Atomic Energy Research Institute, and the fuel for the multi-purpose test reactor (thermal output: about 50,000 KW) which is designed as the first step of this project is described below. A so-called "annual fuel compact", which constitutes the fuel structural unit, is shaped as a cylinder with a central hollow therein and has an outer diameter of 3.6 cm, an inner diameter of 1.8 cm and a length of 3.6 cm.The annular fuel compact is prepared by a method wherein coated fuel particles are made by coating the surface of a spherical nuclear fuel kernal of uranium dioxide (UO2) having a diameter of 600 ,um with low-density carbon, high-density carbon, silicon carbide and high-density carbon layers in that order (called TRISO-coating), and a number of coated fuel particles thus formed having an outer diameter of about 920 pm are mixed with graphite powder and phenolic resin as a binder so that the mixture contains the 30% fuel particles, and then the mixture is sintered at an elevated temperature. The thus prepared fuel compacts are inserted into a graphite tube whose outer diameter inner diameter and length are 4.6 cm, 3.8 cm and 57 cm, respectively, to form a fuel rod or element. A number of fuel rods or elements thus formed are put into a hexagonal-pillar-shaped graphite tube, and helium is subject to circulation between the fuel rods and the graphite tube for cooling.

Since such annular fuel compacts are cooled by helium at a temperature of 750"C or more in the high-temperature gas-cooled reactor, a graphite tube is used in this system since it is a heat-resistant material having small neutron absorption cross section. However, a graphite tube is not a perfect barrier for gaseous FP.

A metal fuel cladding tube is suitable for heavy-water reactors and light-water reactors which have been commercialized already because the outer surface of the fuel is in contact with cooling water whose temperature is no more than 400"C. And a fuel rod or element having such metal cladding tube, which is sealed at both ends by welding, acts as a perfect barrier to gaseous FP. Assuming that the above-described annular fuel compacts are inserted into such metal cladding tube, the size of the gap between the fuel compacts and the inner surface of the metal cladding tube is an important factor in the same way as in the conventional cylindrical fuel pellets, and therefore the complex interaction between the fuel compacts and the cladding tube can not be avoided.Now, when the outer peripheries of the fuel compacts are ground, some of coated fuel particles near the outer periphery of each compact are exposed to grinding process and stripped of their TRISO-coating, and this results in the failures of the coating layers of many coated fuel particles previously provided.

The nuclear fuel element of the present invention wherein the specific coated fuel particles are employed eliminates the leakage of FP from the nuclear fuel element since several coating layers act effectively as a first-stage barrier to retain FP in coated fuel particles and since a metal cladding tube sealed at both ends is provided as a second-stage barrier to FP. Thus the present invention provides a nuclear fuel element which is far safer than the prior art nuclear fuel element, and also can demonstrate superior performance in the reactor operation as well as in the preparation of nuclear fuel, as mentioned above.

Other objects and features of the present invention will become apparent from the following detailed description of exemplary prefered embodiments thereof, which will be made with reference to the accompanying drawings, in which like reference characters denote like parts in the various views. In the drawings: Figure 1 is a perspective view of a deformed conventional cylindrical fuel pellet in the reactor use.

Figure 2 is an explanatory view showing the interaction produced between a metal cladding tube and the conventional pellet illustrated in Fig. 1.

Figure 3 is a sectional view of a coated fuel particle for use in the nuclear fuel element according to the present invention.

Figure 4 is a sectional elevation of a nuclear fuel element embodying the present invention, showing the coated fuel particles illustrated in Fig. 3 packed within a metal cladding tube.

Figure 5 is a diagram showing the measurement data of the thermal conductivity of a column of coated fuel particles packed within the cladding tube.

Figure 6 is a sectional view of a part of a nuclear fuel element according to another embodiment of the present invention.

Figure 7 is a sectional view of a part of a nuclear fuel element according to a futher embodiment of the present invention.

Reference is first made to Fig. 3 which shows the structure of the coated fuel particle employed in the present invention. As is clear from the drawing, the coated fuel particle 20 has at the centre thereof an almost-spherical nuclear fuel kernel 21 having a diameter of about 0.5 to 1.5 mm. This nuclear fuel kernel 21 is formed of nuclear fuel material selected from stable uranium dioxide (UO2), a mixture of uranium and plutoniium dioxides (UO2 + Pu02), a mixture of enriched uranium and thorium dioxides (UO2 + ThO2), etc. The almost-spherical nuclear fuel kernel of this kind is prepared as follows. If uranium oxide is employed as nuclear fuel material, an aqueous solution of uranyl nitrate is dripped into ammonia water through a plurality of parallel arranged capillary tubes to form spherical ammonium diuranate drops ((NH4)2U207) (ADU particles).The spherical ADU particles are roasted to produce uranium trioxide (UO3) particles which are then sintered in the atmosphere of hydrogen to thereby form almostspherical uranium dioxide (UO2) kernels. The nuclear fuel kernels thus prepared are selected by particle size and roundness.

The surface of each nuclear fuel kernel 21 is coated with a composite multilayer coating made up of a layer of low-density pyrolytic carbon 22, a layer of high-density isotropic pyrolytic carbon 23, a layer of silicon carbide or zirconium carbide 24, a layer of highdensity isotropic pyrolytic carbon 25 and a layer of low-density pyrolytic carbon 26 which are formed in sequence from the inside.

The layer of low-density pyrolytic carbon 22 forming the first layer (the innermost layer) has a thickness of about 30 to 60 jtm and bulk density of 1.0 to 1.2 g/cm3, and this layer fulfills the function of absorbing any volume change by expansion or contraction of the nuclear fuel kernel 21 located at the centre of the fuel particle which is caused by temperature change due to fluctuations of the reactor power output, and acts as a storage plenum for FP and as a getter material for capturing FP.

The second layer of high-density isotropic pyrolytic carbon 23 is a sealing layer having a thickness of about 5-20 ym and a bulk density of 1.7 to 1.9 g/cm3. Its main function is to seal in the gaseous components of FP, but it also improves the uniform deposition of the third layer on the outside thereof, the third layer being described presently, and prevents any adverse effect of halogen elements, which are produced during the formation of the third layer of silicon carbide or zirconium carbide and react chemically with the nuclear fuel kernel 21.

The third layer 24 of silicon carbide or zirconium carbide has a thickness of about 1 5 to 30 ,um and seals in and encapsulates solid elements of FP, and protects the kernel-coating shell against an external pressure (although it has little proof stress against internal pressure) to give the desired strength to the coating layer shell for buckling.

The layer of high-density isotropic pyrolytic carbon 25, the fourth layer, with a thickness of about 10 to 40 ,um, covers the layer of silicon carbode or zirconium carbide and thereby prevents the breakage of the spherical coating shell due to the internal pressure growth caused by the accumulation of FP. In other words, the fourth layer gives the necessary strength against an internal pressure.

The fifth layer of low-density pyrolytic carbon 26, the outermost layer, which has a thickness of about 5 to 20 'lem, damps any external forces imparted by any contact or collision between the coated fuel particles, and acts as a getter material if the kernelcoating shell is broken by any chance.

A fluidized-bed method or a physical vapour deposition method is employed as the method applying the above coatings depending upon the kind of coating materials. When carbon coatings of different densities are applied onto the fuel kernel, the characteristics of thermal decomposition of a raw material containing organic substances and the deposition conditions such as temperature and time duration should be selected as appropriate.

A coated fuel particle 20 with the above composite multilayer coating have an outer diameter of about 0.63 to 1.84 mm. The coated fuel particles 20 thus prepared are packed into a metal cladding tube in such a manner that they have a nearly uniform packing density in the longitudinal direction of the cladding tube, and then the ends of the cladding tube are sealed to complete the nuclear fuel element of the present invention.

The inner diameter of the cladding tube employed is in general 5 to 1 8 mm, but is selected as appropriate in accordance with the type of nuclear reactors, and coated fuel particles having an appropriate size and coating layers are selected from various combinations thereof corresponding to the kind of nuclear fuel materials and the designed burnup of the nuclear fuel.

When the nuclear fuel element thus constituted is employed in a nuclear reactor, most of the harmful radioactive FP which is produced through the operation of the reactor can be enclosed in the coated fuel particles, and since a number of the almost-spherical coated fuel particles are in contact with the inner surface of the metal cladding tube, such PCl (pellet clad interaction) as is seen at the opening of a crack in the conventional cylindrical pellet (as shown in Fig. 2) is absent even when the output of the reactor rises sharply and, therefore, the risk of the harmful radioactive FP being released from the damaged cladding tube is eliminated during the designed lifetime thereof.

The demand for further increasing the burnup of the fuel so as to enable its use in a reactor for a longer time, e.g. up to 8 years or more, has become more urgent very recently, since reprocessing of spent nuclear fuel is difficult owing to economic, international-political and local-environmental reasons. in order to safely achieve this high burnup of the fuel, it is advisable to incorporate fine particles of getter material capable of capturing FP in the cladding tube together with the spherical coated fuel particles, in case FP escaped from the fuel coating layer due to the damage thereof caused by an accident. The getter particles have a low neutron absorption cross section and a property enabling the effective capture of gaseous FP.As shown in Fig. 6, these getter particles 30 have particle sizes suitable for location in the small spaces surrounded by the spherical coated fuel particles 20. packed within the cladding tube, and are uniformly mixed with the coated fuel particles 20 and packed into the cladding tube 2. It is preferable that the diameter of these getter particles is not more than about one fifth (1/5) of the minimum outer diameter of the spherical coated fuel particles, and pure zirconium or various zirconium alloys, pure titanium or titanium alloys, or graphite is used as the material thereof.

As for the material of the metal cladding tube, stainless steel, titanium or vanadium alloys, pure zirconium, and alloys of zirconium such as Zry-2, Zry-4 and Zry-1% Nb can be used. At present, just one of these materials is generally used for a conventional singlewalled cladding tube, but a double-walled coextruded cladding tube made of two of these materials, as illustrated in Fig. 7, will be employed more frequently for high performance fuel in the future. In Fig. 7, a doublewalled composite cladding tube has concentri caljy disposed outer and inner walls 2a and 2b, each of these walls being made of different kind of metallic materials selected from those exemplified above.

In the course of burning of the fuel, the internal pressure of the coating layer shell will rise due to the accumulation of FP released from the fuel kernel and due to the vaporizing expansion thereof at high temperature, and the tensile stress exerted in the coating layer shell will be propagated with a gradual increase from the inner coating layer to the outer coating layer. So as to reduce the differential pressure between the outside and inside of the coating layer shell and to prevent the rupture of the spherical coating shell of the fuel particle due to the growth of the internal pressure thereof, it is effective to seal an inert gas such as argon or helium at high pressure in the cladding tube to thereby apply an external pressure to the coated fuel particles.

Helium, in particular, has excellent heat conductivity, and this heat conductivity is further improved by increasing its pressure. Thus, when helium is sealed at high pressure in the cladding tube, the temperature of the fuel decreases and the pressure of gaseous FP is largely reduced ultimately. In this case, it is preferable that helium is sealed in the cladding tube under a pressure of 3 to 50 kg/cm2, although this is also dependent on the conditions under which the fuel is used in a nuclear reactor.

With reference to Fig. 4 which shown the nuclear fuel element employing the aforementioned coated fuel particles, the coated fuel particles 20 are packed into a metal cladding tube 2 to form a packed column or stack of coated fuel particles which forms an effective fuel stack length. The cladding tube 2 is sealed by a lower end plug 5 welded around its periphery and an upper end plug 4. A graphite or metallic wool layer 1 7 is provided between the lower end plug 5 and the bottom of the packed-column of coated fuel particles.

The coated fuel particles packed-column are partitioned into several portions by inserting graphite or metallic wool layers 1 7a at several places along the length of the effective fuel stack length, i.e. the packed-column. Silverplated wires or strips of zirconium having a small thermal neutron absorption cross section is preferably employed as a metallic wool, as well as stainless steel and the like.These partitioning graphite or metallic wool layers 1 7a fulfull the function of reducing the relative displacement of the coated fuel particles column and the cladding tube due to thermal expansion differences between them along the length thereof caused during the start-up and shut-down of the reactor, so that excessive stresses do not exert along the length of the cladding tube, and also fulfill the function of facilitating the packing operation of the coated fuel particles into the cladding tube, as described later.

In addition, in the illustrated embodiment of the invention, a short coil spring 3 is provided above the packed-column of coated fuel particles, separated from them by a heat-insulat ing disc 7. This coil spring 3 is designed to allow for any increase in the gap between the heat-insulating disc 7 and the upper end plug 4 due to the elongation of the tube caused by radiation creep accompanying the burning of the fuel. Such a gap between the heat-insulating disc 7 and the upper end plug 2 is fundamentally different from a conventional upper plenum provided for a prior art fuel element.This is because a conventional upper plenum in a prior art fuel element needs a length equivalent to 20 to 70 cm of the length of the cladding tube so as to fulfill its function as a plenum space or reservoir for gaseous FP, whereas the gap required in the present embodiment is only about 5 cm in length. This provides an advantage in that the reactor core can be easily enlarged, since the coated fuel particles can be packed in the cladding tube to fill the upper plenum area of the prior art fuel element and thereby the length of the effective stack length of the fuel element is increased to meet the larger power capacity of a nuclear reactor. In view of the present facilities for the transport and repocessing of spent fuel, the total length of a nuclear fuel element cannot exceed 4.7 m.

As another embodiment of the present invention, it is also possible, as described above and shown in Fig. 6, to include fine particles 30 of getter material together with the coated fuel particles 20 for capturing FP in the cladding tube 2 in case of the series ruptures of the fuel coating shells. The fine getter particles are mixed with the fuel particles 20 and packed into the cladding tube 2 in such a manner that the fine getter particles 30 fill the small spaces surrounded by several coated fuel particles 20 and are distributed substantially uniformly in the packed-column of coated fuel particles in the fuel cladding tube.

At present, there exist no fuel fabrication facilities in Japan wherein nuclear fuel rods or elements having a total length of 4 to 5 cm can be prepared in a vertically standing position. It is our practice, of course, to hold the completed fuel assemblies in vertical position on a fuel rack for a while until they are shipped to a nuclear power plant, so as to prevent deformation of the fuel elements and reduce the total storage space. With using the existing fuel fabrication facilities, the following method will be applicable for packing coated fuel particles of this kind in a cladding tube of 4-5 cm.Firstly, a fuel cladding tube whose lower end plug is welded at the tube end is supported with the lower plug end inclined downward at an angle of about 20 degrees to the horizontal direction, and a predetermined quantity of coated fuel particles, or of a uniform mixture of coated fuel particles and fine getter particles, is inserted into the cladding tube from the open top thereof. Secondly, a graphite or metallic wool partition layer is put therein and the coated fuel particles which have already packed in the downwardly inclinded cladding tube is pressed down by using a rod through the partition layer, and then another predetermined quantity of coated fuel particles or of a uniform mixture of coated fuel particles and fine getter particles is again inserted. By repeating this operation the fuel element is to accomplish to have the packed column of coated fuel particles.

The packing of the coated fuel particles can also be conducted by a vibration packing method, wherein the fuel particles are inserted from the upper tube opening into a vertically standing cladding tube with its lower end plug sealed, while vibrations are being given thereto mechanically. This method however, is not always the best one, since such vertically standing cladding tube can not be positioned in fuel fabrication facilities unless the facilities are installed in a building with high ceilings, and since there is a danger of damaging the cladding tube when conducting a vibration packing.

Fig. 5 shows the experimental data obtained by measurements of the thermal conductivity of the coated fuel particles column packed in the cladding tube to test the fuel element of the present invention, which were carried out in a centre heating experimental apparatus to simulate the conditions of various heating stages of fuel elements.These experimental data were obtained from a fuel element or rod prepared by packing a graphite cladding tube having an inner diameter of 14.7 mm with coated fuel particles having a diameter of 1 269 lim (date indicated by circles in Fig. 5) and a diameter of 1 334 ym (data indicated by triangles in Fig. 5) with fuel kernels of about 900 and 1030 ym. The inner dimension of the cladding tube is the same with that used in an Advanced Thermal Reactor (ATR) which is a heavy-water reactor developed in Japan.According to the results of this experiment, it is seen that, in view of the temperature dependence, the degree to which thermal radiation through the gap spaces between the coated fuel particles contributes is very large at the elevated temperature region, in addition to the thermal conduction due to the contacts between the adjacent particles. With calculations on the basis of the experimental data, the highest temperatures of the coated fuel particles is sufficiently lower than the melting point (M.P.) of uranium dioxide fuel material (UO2), being at most about 2/3 of the M.P., when the linearheat- rate of ATR fuel is 1 3 kw/ft. Such lowered temperature of the fuel is desirable, because it means that the release rate of FP from nuclear fuel kernel can be also reduced.

The present invention, which offers a nuclear fuel element constructed as above, has the advantages that PCI which is a troublesome phenomenon for the conventional cylindrical pellets does not occur at all so that the load-follow operation of the nuclear power plant is possible, and that such grinding process as required in the preparation of the conventional cylindrical fuel pellets is unnecessary so that grinding scrap of nuclear fuel material is not produced and the necessity for the manpower and time required for the inspection of the appearance, dimensions, density, etc. of the fuel pellets is obviated. In brief, when using fuel particles, the coat of inspection is reduced by the appropriate application of a rapid automatic optical particle-size measuring method.Moreover, since the production of the coated fuel particles of this kind is inherently suitable for an automatic remote operation, it could well be said that it is suitable for nuclear fuels having a high toxicity such as, especially, plutonium dioxide or a mixture of plutonium-uranium dioxides. That is, once prepared into a coated fuel particle, the fuel kernal containing the toxic alpha-ray emitting plutonium can be sealed by a composite multilayer coating, and therefore the subsequent fuel production facilities can be free from the heavily complicated equipments used in the existing plutonium production facilities to provide radiation shielding and air-tightness, whereby the fuel production facilities becomes far more efficient than conventional fuel fabrication facilities for plutonium dioxide pellets, both from the economical and the environmental points of view.

Furthermore, the present invention is also suitable for nuclear fuels which are required to have a high burnup, such as a fuel for a fast breeder reactor. Liquid sodium is the primary coolant material for the fast breeder reactor core, and the pressure of the coolant is several kg/cm2 even with the pressure of an inert cover gas filled in the upper part of the reactor core. For a commercial fast breeder reactor power plant in the future, a high burnup of the nuclear fuel element such as 100,000 to 300,000 MW d/t maximum is expected.Therefore, the volumetric expansion or swelling, of the metal cladding tube due to the irradiation of fast neutrons is not avoidable, and further, when conventional cylindrical pellets are used, the internal pressure of the cladding tube will soon exceed the pressure of the coolant by the release of a great amount of FP from the pellets even though an upper plenum of several tens of centrimetre in length is provided in the nuclear fuel element, with the result that the amount of creep deformation of the cladding tube becomes extremely large. Moreover, oxidation of the inner surface of the metal cladding tube will increase progressively due to the accumulation of oxygen isolated by the fission reaction of oxide fuel materials, and the bonding of fuel pellets to the cladding tube will proceed to reduce the thickness of metal cladding tube to raise a serious problem concerning the soundness of the fuel. On the other hand, when the nuclear fuel element according to the present invention is employed, active oxygen isolated by the fission reaction is combined with the carbon contained in each coating layer, and even when a part of the FP leaks from the carbon coating layers, it is captured by the fine getter particles and therefore no oxygen corrodes the inner surface of the metal cladding tube.In addition, since the FP is substantially enclosed within the coated fuel particles, no creep deformation of the cladding tube occurs as is usual for the conventional fuel element using cylindrical pellets.

In this way, the present invention can provide a nuclear fuel element having a high performance sufficient to realize the load-follow operation of the reactor and capable of achieving a high burnup in a heavy-water reactor, light-water reactor or graphite-moderated advanced gas-cooled reactor (AGR) which have already been commercialized. It is also suitable as the fuel of the fast breeder reactor which is expected to be commercialized in the first half of the twenty-first century.

In view of the nuclear fuel cycle in the second half of the twenty-first century when it is considered that we will be forced to utilize the resources of thorium, the present invention also offers a promising technique for the safe achievement of the thorium cycle which will be economical in the types of reactors other than the high-temperature gas-cooled reactor. This is because a recycled thorium fuel will destroy the concept of existing nuclear fuel production facilities completely due to the pressence of strong gammarays, and because a plant for production of a recycled thorium fuel is imagined to be a chemical processing plant wherein an unmanned operation is performed by using robots, including critical mass control in each system wherein fuel particles are processed as semi-fluid substances.

An additional advantage of the present invention is that the interior of storage facilities is not contaminated by FP even when the spent nuclear fuel is stored for a long period (up to several tens of years) in the storage facilities without being reprocessed, since isolated FP hardly accumulate at all in the metal cladding tube of the nuclear fuel element.

Claims (11)

1. A nuclear fuel element having a metal cladding tube and nuclear fuel particles packed within said cladding tube, wherein each of said fuel particles comprises a substantially spherical kernel made of a nuclear fuel material of stable oxides and a composite multilayer coating on said spherical fuel kernel, said composite multilayer coating being composed of an innermost first layer of lowdensity pyrolytic carbon coated on said spherical fuel kernel, a second layer of high-density isotropic pyrolytic carbon on said innermost layer, a third layer of carbides on said second layer, a fourth layer of high-density isotropic pyrolytic carbon on said third layer, and an outermost fifth layer of low-density pyrolytic carbon.

2. The nuclear fuel element according to claim 1, in which said third layer of carbides is selected from materials of silicon carbide and zirconium carbide.

3. The nuclear fuel element according to claim 1 or 2, in which said substantially spherical fuel kernel of stable oxides has a diameter of about 0.5 to 1.5 mm.

4. The nuclear fuel element according to any preceding claim, in which said nuclear fuel material of stable oxides is selected from uranium dioxide, plutonium dixoide, thorium oxide and mixture thereof.

5. The nuclear fuel element according td any preceding claim, further comprising fine particles of getter material mixed with said coated fuel particles, said fine getter particles having a suitable size for location in the small spaces surrounded by several of said coated fuel particles packed in said cladding tube, said getter material having small neutron absorption cross section and effective capturing characteristics for gaseous fission products released from said nuclear fuel material.

6. The nuclear fuel element according to claim 5, in which said getter material is selected from zirconium, zirconium alloy, titanium, titanium alloy and graphite.

7. The nuclear fuel element according to any preceding claim, further comprising partitioning members disposed at places within said cladding tube along the longitudinal direction thereof, said partitioning members dividing a packed portion of said coated fuel particles into a plurality of shorter packed portions within said cladding tube.

8. The nuclear fuel element according to claim 7, in which said partitioning member comprises a layer of a material selected from graphite wool and metallic wool.

9. The nuclear fuel element according to any preceding claim, further comprising a coil spring disposed in the upper portion within said cladding tube and a heat-insulating disc disposed below said coil spring, said coil spring downwardly compressing said coated fuel particles packed in the cladding tube through said heat-insulating disc.

10. The nuclear fuel element according to any preceding claim, in which said metal cladding tube includes therein a pressurized inert gas selected from helium, argon and a mixture thereof.

11. The nuclear fuel element according to any preceding claim, in which said metal cladding tube is a single-walled tube made of one kind of material selected from zirconium, zirconium alloys, stainless steel, titanium and vanadium alloys.

1 2. The nuclear fuel element according to any of claims 1 to 11 in which said metal cladding tube is a composite double-walled tube having co-extruded inner and outer walls, each of said walls being made of a different kind of material selected from zirconium, zirconium alloys, stainless steel, titanium and vanadium alloys.

1 3. A nuclear fuel element substantially as herein defined with reference to any of Figs. 3 to 7 of the accompanying drawings.