DE2511054A1 - CYLINDRICAL NUCLEAR FUEL - Google Patents

Description

Applicant: Canadian General Electric Company, Ltd., Toronto, Ontario, Canada

Cylindrical nuclear fuel rod

The invention relates to a cylindrical nuclear fuel rod according to the preamble of claim 1.

In the operation of nuclear reactors, the types of cladding, the normal mode of operation and the special features have an impact of the fuel the stress on the fuel rods of the fuel elements. In boiling water reactors, their fuel rods have essentially rigid, self-supporting envelopes (US Pat. No. 3,119,747), lubricant layers are found in the envelope Use to avoid certain damage effects as far as possible. These fuel rods consist of one one-piece elongated fuel body, the length of which is about 35 cm, and which have a conical end piece of about 5 cm length on which a lubricant is provided is to avoid cumulative expansion effects in the cladding, which can lead to the cladding breaking. One of those Failure of a fuel rod can occur due to shifting

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gene in the longitudinal directions in the envelope at one end of the fuel body occur which shift occurs in each case when the nuclear reactor is switched on, with the fuel expands in the longitudinal direction more than the envelope. There is leeway to accommodate this axial fuel expansion provided at the end. Because of the interaction between the fuel and the cladding, however, local tensions occur, through which the envelope is sometimes expanded in the axial direction to the point that it is no longer under the external coolant pressure is longer self-supporting. This can lead to the failure of the cladding by cracking adjacent the end of the fuel.

In contrast, the invention relates to another problem with regard to damage to the envelope, namely one in the axial direction Directional and non-circumferential cracking that occurs in fuel elements for CANDU nuclear reactors can. In these nuclear reactors, thin-walled fuel rods with a locally flexible sheath are used, which in the is generally not self-supporting and the fuel usually consists of tablets 2.5 cm thick. At a thermal expansion, a peripheral expansion of the individual fuel pellets occurs localized relative to the envelope, so that the envelope does not expand evenly along the circumference of the tablets. It was found that the for inclusion the expansion of the tablet required peripheral expansion of the envelope along a significantly reduced stretching length the circumference of the envelope occurs. This can lead to so high stresses in the envelope that one moves in the axial direction extending cracking or the like occurs in the envelope.

Since fuel tablets can be deformed to a non-cylindrical constricted formation, tend the locally flexible thin-walled envelopes to tightly surround the constricted outer circumference of the tablets, whereby im

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essentially a differential axial expansion between tablet and wrapping is excluded. Also by notching the tablet ends to enable individual axial expansion Such an expansion between the tablet and the envelope can be avoided for each tablet. It is therefore a task the invention to improve nuclear fuel rods of the type mentioned in such a way that cracks that lead to failure be avoided as far as possible, which are caused by a concentrated differential expansion in the circumferential direction. These According to the invention, the object is achieved by the characterizing features of claim 1 solved. Advantageous further training are the subject of the subclaims.

A nuclear fuel rod according to the invention therefore has a longitudinal one at the ends of at least some fuel pellets Expansion compensation possibility in order to achieve a differential axial expansion between the tablet in question and exclude the enclosure during reactor operation, as well as an intermediate layer that distributes stresses between the outer surface of the tablet and the envelope in order to favor a peripheral distribution of stresses in the envelope and the likelihood to reduce cracking in the longitudinal direction of the envelope due to concentrations of peripheral expansion stresses.

The invention is to be explained in more detail, for example, with the aid of the drawing. Show it:

1 shows a known nuclear fuel rod;

FIG. 2 is an enlarged partial section along the line 2-2 in FIG. 1; FIG.

Fig. 3 is a sectional view along the line 3-3 in Fig. 1, to explain a first type of crack formation;

FIG. 4 shows a sectional view corresponding to FIG. 3 for explanation a second type of cracking; and

5 shows a cross section through a nuclear fuel rod according to the invention.

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The nuclear fuel rod 10 shown in Fig. 1 has a outer thin cylindrical envelope 12, for example made of a zirconium alloy and such. Thickness because a deformation in engagement with the tablets 14 can occur.

Fig. 2 shows in an exaggerated representation the deformation of the tablets 14, with a central one along the outer circumference Constriction takes place, but the envelope adapts to this shape under operating conditions of the reactor. However, it is It should be noted that a coolant pressure of about 100 atu acts on the casing 12.

Fig. 3 shows a type of cracking which can occur opposite a crack s in a tablet and to a crack F in of the casing 12, there being practically no change in the cross-sectional area of the casing 12. In the one shown in FIG In contrast, a reduction in the cross section of the casing 12 occurs in the area of the crack formation Fi.

The cross section through a nuclear fuel rod according to the invention shown in FIG. 5 is a stress-distributing one Intermediate layer 16 is provided between the casing 12 and the periphery of the tablets 14, which layer is a slight one Has shear strength.

The layer 16 can be made of a material such as siloxane, silicone or graphite, which materials are largely inert to the irradiation in the reactor, and good distribution ability of stresses at reactor temperatures and have a low shear strength, but without substantial Difficulty on the inner wall of the envelope 12 or the surface of the tablets 14 dry, as a liquid or to be dried Emulsion can be applied.

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Claims (3)

-5- March 10, 1975 C-3688 Claims Ij Cylindrical nuclear fuel rod for nuclear reactors, with a locally flexible metallic envelope which surrounds a number of closely spaced cylindrical fuel pellets, characterized in that the majority of the fuel pellets (14) have an axial expansion pressure relief option has at respective end faces to accommodate an individual axial expansion in operation and a differential to avoid axial expansion between the fuel pellets and the envelope (12), and that an intermediate layer (16) is provided with low shear strength between the outer surfaces of the fuel pellets and the envelope to a to favor peripheral distribution of stresses in the envelope and the likelihood of longitudinal cracking in the cladding due to concentrations of peripheral expansion stresses to reduce. 2. nuclear fuel rod according to claim 1, characterized marked e i c h ne t that the intermediate layer (16) consists of a material such as graphite, siloxane or silicone. 3. Nuclear fuel rod according to claim 1 or 2, characterized that the metallic shell has a thickness less than that which is at the operating pressure would prevent compression of the envelope in the reactor. 509838/0 7 37 Le e rs e ite