GB2116354A

GB2116354A - Nuclear fuel element

Info

Publication number: GB2116354A
Application number: GB08304111A
Authority: GB
Inventors: John Edward Garnier; Stefan Begej; Ralph Eric Williford; James Andrew Christensen
Original assignee: US Department of Energy
Current assignee: US Department of Energy
Priority date: 1982-03-03
Filing date: 1983-02-15
Publication date: 1983-09-21
Also published as: CA1195441A; FR2522865A1; GB2116354B; DE3307610A1; GB8304111D0; JPS58166286A

Abstract

The element consists of an outer cladding tube 10 a central core 14 of fuel material, slightly smaller in diameter than the inner surface of the cladding tube and a small lower accumulator section 16, the cladding tube being which is filled with a low molecular weight gas to transfer heat from fuel material to cladding during irradiation. A plurality of vertical extending grooves 28 in the fuel section extend downward and communicate with the accumulator section, and to allow thermal segregation to take place between the low molecular weight heat transfer gas and high molecular weight fission product gases produced by the fuel material during irradiation. The fission product gases migrate to the cooler cladding surface of the groove and descend by gravity into the accumulator section. The accumulation may also contain gettering material to sorb certain high molecular weight gases. <IMAGE>

Description

SPECIFICATION Nuclear reactor fuel element having improved heat transfer This invention relates to a nuclear reactor fuel element. More specifically, this invention relates to a nuclear reactor fuel element having improved heat transfer capabilities between the fuel material and the cladding.

Mechanisms which affect the radial heat transfer process between the fuel material and the cladding can be considered to be the actual fuel: clad contact area, the gap separation between the fuel and clad, the thermal and physical properties of the fuel and clad mating materials, the surface conditions of the fuel: clad interface, radiation, and the interstitial fluid (i.e., the gas).

Of these radial heat transfer mechanisms, the thermal conductivity of the gas plays an important role in determining the gap conductance contribution to the total heat transfer. Initially, the fuelcladding gap is filled with low molecular weight, high thermal conductivity gas such as helium. During burn-up of the fuel, fission products are generated and a portion of these fission products (xenon and krypton) escape to the plenum region of the fuel element. The resulting accumulation of these gases in the fuel:clad gap results in a lowering of the thermal conductance of the gap by virtue of the lower thermal conductivity of the gas mixture. The chemical state and concentration of the fission products (i.e., element, oxide or compiex compound) also influences the availability of oxygen within the fuel rod.

This in turn, curtails the oxygen potential of the fuel which is of first rank importance in determining whether fuel can react chemically with the cladding. These reactions, when they occur, result in corrosion of the metal and a consequent weakening of the cladding which is the primary barrier to the release of radioactivity. Thus any mechanisms which affect the local concentration and or distribution of gaseous species within the plenum is important.

Thermal diffusion is such a mechanism and it consists of a relative motion of the mass components of a gas mixture arising from temperature differences within the gas mixture as established by a temperature difference between two surfaces. Such a movement was experimentally confirmed by Chapman, S., and Dootson, F. W., Phial. Mag. [6] 33, 248 (1917). In 1938 Clusius, K., and Dickel, G., Naturvissenschaften, 26, 547 (1938) used this phenomenon to devise a continuous method of separating mixtures of gases and isotopes. A more recent theoretical study indicated that a potentially substantial thermal segregation of gases could occur within a fuel rod under certain conditions. S. K. Loyalka, V. K. Chandoia, L. B. Thomes, "Clusius-Dickel Effects in a Nuclear Fuel Rod", Nuclear Science and Engineering, July 1979.

A nuclear reactor fuel rod design has been developed which takes advantage of, not only thermal diffusion, but also convective segregation to separate the poor thermal conducting high molecular weight fission gases from the more thermal conducting low molecular weight gases in order to improve heat transfer across the fuel-cladding gap.

As hereinbefore stated, thermal diffusion consists in a relative motion of the components of a gas mixture arising from temperature differences within the gas mixture as established by the temperature gradient between two surfaces-in a fuel rod, between the outer surface of the fuel and the inner surface of the clad. Therefore, in a mixture initially of uniform composition, it leads to the development of a concentration gradient across the gap. Ordinary diffusion tends to eliminate the concentration gradient and steady-state condition is possible in which the separating effect of thermal diffusion is balanced by the remixing effect of ordinary diffusion. In a fuel rod, the axial, radial and circumferential temperature gradients seen by the gas in the gap all contribute to a partial separation of the components due to thermal diffusion.It has also been found that gravitationally induced convection effects will furthermore interact with the radial thermally induced concentration differences and ultimately cause a migration of the heavier and lighter gaseous components to the bottom and top of the rod, respectively. The resulting axial concentration differences thus become far greater when both convective and thermal diffusive effects operate together, than if thermal diffusion acts alone.

The nuclear fuel element of the invention therefore consists of a tubular-shaped outer metallic cladding having an inner surface and sealed ends, the interior of the cladding being divided into an upper, relatively long fuel section and a lower, relatively short gas accumulator section, the sections being in gaseous communication with each other. A central core of actinide fuel material is disposed within the upper fuel section, the fuel material being slightly smaller in diameter than the inner surface of the cladding to form an annular space therebetween. A low molecular weight, high thermal conductivity, heat transfer gas is sealed within the cladding for conducting heat between the fuel material and the inner surface of the cladding during irradiation of the fuel element.Extending the length of the fuel section down to and in communication with the accumulator section is at least one, generally longitudinal groove formed in either the outer surface of fuel, the inner surface of the cladding, or partially in both. The radial depth of the groove is sufficient to provide a zone of increased temperature difference between the hot outer surface of the fuel and the relatively cooler inner surface of the cladding, to result in radial thermal segregation between the low molecular weight heat transfer gas and the high molecular weight fission product gas, whereby the high molecular weight gases migrate to the relatively cooler cladding surface and flow by gravity down the groove to the accumulator.In one embodiment, the accumulator may contain material capable of reacting with, or gettering certain high molecular weight gas species as a means of retaining the gas to prevent diffusion into the fuel section to dilute the heat transfer gas. This can occur if the temperature gradient between fuel and clad were reduced to zero as when the reactor is shut down.

It is therefore the object of the invention to provide a nuclear reactor fuel element having improved heat transfer across the fuel-cladding gap. It is the other object of the invention to provide a nuclear reactor fuel element which contains means for separating the low molecular weight heat transfer gas from high molecular weight fission product gases, in order to improve heat transfer across the fuel-cladding gap.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

Fig. 1 is a longitudinal sectional view of the fuel element of the invention.

Fig. 2 is a cross-sectional view of the fuel element of Fig. 1 along line 2-2 showing one embodiment of the invention.

Fig. 3 is a cross-sectional view of the fuel element showing another embodiment of the invention.

Fig. 4 is a cross-sectional view of the fuel element showing still another embodiment of the invention.

Referring now to Fig. 1, there is shown fuel element 10, consisting of an outer cladding tube 12, divided into an upper, relatively long fuel chamber 14, and a lower, relatively short accumulator chamber 1 6, the chambers being separated by an inwardly extending support ledge 1 8. Within chamber 14, supported by ledge 1 8 is a central fuel core 20, having an outer surface 22 slightly smaller in diameter than the inner surface 24 of cladding tube 12, to provide a narrow annular space 26, therebetween.Evenly spaced about the diameter of surface 24, within the fuel section 14 are a plurality of longitudinal grooves 28, which extend through section 14 downward to, and in communication with accumulator section 1 6. Each groove 28 has a cladding surface 30 and a fuel surface 32 which must be a sufficient distance apart to provide a temperature differential between the surfaces sufficient to result in thermal segregation. Within section 1 5 is located a getter material 34 to sorb certain fission product gases and prevent them from re-entering fuel chamber 14 during periods when the fuel element is cool.

As shown in Fig. 2, grooves 28 are evenly spaced about the circumference of inner surface 24. In Fig. 3 the grooves are shown evenly spaced about outer surface 22 of fuel core 20, while in Fig. 4, grooves 26 are spaced about both inner surface 24 and outer surface 22. In this embodiment it will be necessary to provide an arrangement to prevent rotation of the fuel relative to the clad in order that the grooves remain opposite each other to insure that the temperature differential is provided. Other methods and configurations may be obvious to those skilled in the art, which will provide for local longitudinal zones-of increased temperature difference sufficient to cause radial thermal segregation.

The fuel element must contain at least one, preferably at least three evenly spaced grooves, to provide adequate separation of gases. Preferably the grooves will extend from the top of the fuel section down to the lower accumulator section and may be straight or they may have a vertical spiral configuration.

The depth of the grooves, that is the distance between the hot or fuel core surface, and the cooler or cladding surface, must be sufficient to provide a local radial temperature difference between fuel and cladding to result in a separation between the high molecular weight fission product gases and the low molecular weight heat transfer gas. It has been found that a temperature difference of at least 600C between the hot and cold walls is adequate to provide conditions for thermal diffusion connective segregation of component gases. The exact channel depth will depend upon the particular fuel material being used, the width of the annular space, thermal conductivity of the cladding material and other factors.In general, the depth of the groove, that is the distance between the hot and cooler surfaces, should be at least 0.005 cm (0.002 inches) and no more than about 0.254 cm (0.1 inches) to avoid the onset of convective gas effects from occurring in the groove. The width of the channel may vary from about 0.05 cm (0.02 inches) to about 1 cm (0.4 inches) depending on the interior circumference of the cladding and the number of grooves.

Gettering material suitable for sorbing certain of the fission product gases species such as Cesium32 from the decay of Xenon'33 include zirconium metal chips. This material may be present in the accumulator section as discrete particles or may be applied as a coating to the section wall.

Alternatively, the getter material may be present in the accumulator in the form of a vertical, open cylindrical-shaped porous sleeve which would support the fuel material in place of support ledge 1 8.

The following experiment, made to establish the feasibility of the invention, employs a single channel to minimize the accumulation of high molecular weight fission gases within the fuel-clad gap in a nuclear fuel rod.

A simulated fuel rod apparatus was constructed which consisted of a single bore 228 cm long aluminum oxide rod within an aluminum cladding tube. Spacers were fixed to the Al203 "fuel" to maintain a nominal radial fuel-clad gap of 0.063 cm. A metal heater wire was placed along the axis of the fuel bore. The fuel surface and centerline temperatures were monitored by thermocouples placed approximately 8 cm from the bottom and top of the fuel stack. Gas sampling ports were installed at the bottom and top plenum of the aluminum tube. The exterior of the aluminum tube was water-cooled and maintained at a temperature of 20 to 250C (inlet to output) to establish a horizontal temperature gradient between the "fuel" and "clad" surfaces. The results of two experiments are summarized in the Table below.

(At.% Xe) Bal . He Gas Gas Gas Top C.L. Bot. C.L.

press. comp. comp. temp. temp.

Comments (MPa) top bot. (K) (K) 11.7% we start gas* 80 W/m power input Zero hours* 0.52 11.7 11.7 320 317 96 hrs 0.31 10.1 26.8 318 325 52% Xe start gas 89 W/m power input Zero hours 0.35 52 52 371 366 18 hrs 0.35 25 96 344 563 48 hrs 0.24 29 99 343 553 *Equilibration time: Approximately 70 hrs.

**Equilibration time: Approximately 8 hrs.

C.L.-Centerline fuel temperature reading.

The results show essentially 99% Xe at the bottom of the rod for an initial starting gas mixture of 48% He:52% Xe, after 48 hours of operation.

Thus it can be seen that the channels in the fuel element in combination with the placement of an accumulator at the bottom of a fuel rod serve to collect the higher molecular weight fission gas products (i.e., Xe, Kr) created during fuel burnup, allowing for a high helium gas concentration to remain at the highest power region (i.e., near the center) of the fuel rod. This increases the gap conductance in this region, lowering the fuel centerline temperature. This also maintains the oxygen potential of the fuel by minimizing the buildup of fission products within the gap region, decreasing the extent of fuel/cladding chemical interaction.

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:

Claims

1. In a nuclear reactor fuel element having a tubular-shaped outer metallic cladding having an inner surface and sealed upper and lower ends, a central cylindrical-shaped core of actinide fuel material having an outer surface disposed within the cladding tube to form a fuel section, the diameter of the outer surface being slightly less than the diameter of the inner surface of the cladding to provide a narrow annular space therebetween, and low molecular weight gas sealed within the cladding for improving heat transfer during irradiation between the hot outer surface of the fuel material and the relatively cooler inner surface of the cladding, whereby during irradiation of the fuel element, fissioning of the actinide fuel material produces high molecular weight fission product gases which mix with the low molecular weight gas and reduce the heat transfer capability of that gas, the improvement comprising, a small gas accumulator section within the cladding tube below the fuel section in communication with the fuel section, and at least one longitudinal groove means formed in one of said inner surface of said cladding and said outer surface of said fuel material said groove means extending the length of said fuel section to communicate with said gas accumulator section, said groove means communicating with said annular space to provide a local zone of increased temperature difference between the hot fuel surface and the relatively cooler cladding surface throughout the length of the assembly, the temperature difference being sufficient to cause radial thermal segregation between the low molecular weight gas and the high molecular weight fission gas whereby the hole molecular weight fission gas migrates to the cooler cladding surface by thermal diffusion and decends by gravity down the surface to the lower accumulator section, while the low molecular weight gas remains in the upper fuel section, separating the high molecular weight fission gases from the low molecular weight heat transfer gas, thereby maintaining the heat transfer capabilitites of the heat transfer gas.

2. The fuel element of Claim 1 wherein the fuel section of the fuel contains at least three evenly spaced longitudinal groove means.

3. The fuel element of Claim 2 wherein the difference in temperature between the hot fuel surface and the relatively cooler cladding surface at the groove means is at least 600 C.

4. The fuel element of Claim 3 wherein the distance, between the hot fuel surface and the cooler cladding surface at the groove means is at least 0.005 cm.

5. The fuel element of Claim 4 wherein the distance between the hot fuel surface and cooler cladding surface at the groove means is from 0.005 cm to 0.25 cm.

6. The fuel element of Claim 5 wherein the accumulator section contains a getter to react with certain high molecular weight fission gases to prevent them from re-entering the fuel chamber when the fuel element has cooled.

7. The fuel element of Claim 6 where the getter is zirconium metal.

8. The fuel element of Claim 7 wherein the groove means are located in the inner surface of the cladding.

9. The fuel element of Claim 7 wherein the groove means are located in the outer surface of the fuel material.

10. The fuel element of Claim 7 wherein the groove means are located partially in the inner surface of the cladding and partially in the outer surface of the fuel material.