JPS599585A - Fuel element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an improved nuclear fuel rod, and more particularly, to a nuclear fuel rod having a radial enrichment gradient that reduces operating temperature at a given power.

BACKGROUND OF THE INVENTION Conventional fuel rod designs feature a zirconium alloy sheath filled with a fissile fuel such as uranium oxide pellets. The uranium oxide is sintered and ground to close dimensional tolerances to provide good thermal contact with the pod. In recent years, it has been found that contact between the beret and the pod leads to a chemical interaction that causes the pod to crack and split.

The cracking of the pods inside the reactor results in the release of radioactive materials. The cooling water surrounding the fuel rods becomes contaminated, requiring a reactor shutdown.

To prevent this type of failure, newer fuel rod designs have been proposed in which the fissile fuel is made into spherical or granular shapes. A hybrid design using both pellet and spherical combustion combinations is described in U.S. Pat.
, 778,348 (issued December 11, 1973) and U.S. Patent No. 4,131,511 (issued December 1978).
(issued on the 26th of May).

The use of spherical or granular fuel, also known as "balls", reduces the peak contact stress between the fuel and the zirconium alloy sheath. This resulting reduction in surface interactions reduces the likelihood that cracks and tears will form in the pod.

However, the use of spherical fuel to reduce pod failure did not solve the problem of inefficient heat transfer between the fuel and the pod. To improve heat transfer, some patents have proposed mixing graphite with fissile grains (Canadian Patent No. 953, issued August 20, 1974).
, No. 437).

However, the use of graphite is limited to uranium oxide (U
02) at high temperature to produce carbon monoxide gas. −
Carbon oxides reduce fuel rod performance and therefore reduce the amount of carbon into the fuel! is not a viable solution.

This invention teaches how to use spherical fuel in a new way. The central portion of the fuel rod is filled with phase-depleted uranium oxide or other low enrichment fuel.

The depleted fuel is then surrounded by an outer ring of more enriched fuel, thus creating an enrichment gradient radially along the fuel cross-section. By concentrating the enriched fuel closer to the pod surface, more nuclear events occur closer to the pod. This improves the thermal efficiency of the fuel rod by shortening the heat transfer path through the low thermal conductivity of uranium oxide.

In this way, the centerline temperature of the fuel used in the fuel rod at a given power output is significantly reduced.

Consideration of related technology U.S. patents = “Q3,778.348 and No. 4,13
The above-mentioned invention described in No. 1.511 is a teaching contrary to that of the present invention. These patents teach placing concentrated 8 fuel in the core or center of the fuel rod.

This provides a larger volume of concentrated material in the center of the rod and a longer heat flow path to the sheath surface. The result of these fuel rod designs is that the thermal efficiency of the fuel rods is lower and the fuel rods operate at higher temperatures. Because higher temperatures increase fission gas release and increase stress in the pod, pod tears are more likely to form eyes and cracks. Therefore, these designs fail in preventing pod tearing and cracking.

The present invention synergistically prevents sheath failure by (1) lowering the operating temperature of the fuel rods and (2) reducing fuel sheath contact interactions using spherical combustion chambers.

SUMMARY OF THE INVENTION The present invention relates to fuel elements, or fuel rods, for use in nuclear reactors. The fuel material is contained within the elongated sheath. A radial enrichment gradient in the fuel material is created by employing at least two different fuel enrichments. The central portion of the fuel rod contains low enrichment fissile fuel and the outer annular portion contains substantially more highly enriched fuel. The radial enrichment gradient allows the fuel rods to work at lower temperatures for a given power output. Fuel rods contain a mixture of pellets and granular fuel.

For clarity, the term "granular fuel" or "granular fissile fuel" refers to powdered fuel, spherical fuel, mixtures of powdered and spherical fuel, and powdered fuel of different sizes and/or spherical particles of different sizes. Gives the fuel mixture a blue tinge.

The hybrid design has a grain center surrounded by a grain fuel term, a grain center surrounded by a benz)'if or fd, a lower enrichment grain core and a higher enrichment grain ring. Good too.

The enrichment range for the inner and outer elements is as follows:

Center part...depleted uranium oxide 0.2% U-2351
The desired enrichment outer annular portion by neutral design typically has a maximum of 2 modulus IJ-'235...approximately 2. OO%U-235 to 20
．． Other fissile isotopes such as rutonium and/or uranium-233 can be applied in place of or with uranium-235. Also, thorium oxide can be used in place of uranium oxide as an internal element.

In the present invention, the granular fuel has three different spherical diameter sizes: 1
500-600μm, 4'OO100μm and 50μm
can be a mixture in each range of up to m2
・, 1 These three size separation sections are smear density (s
mear density) mixed to reach 90 percent.

The fuel design of the present invention allows for equal or larger fuel volumes in the annular section.

Typically, the volume percent ratio between the annular portion and the central portion is 70,730. However, the fuel volume ratio in percentage between the outer annular portion and the central portion is 90/10.
It can vary from 501501 to 501501.

Uranium oxide is a relatively poor thermal conductor. The present invention overcomes the poor thermal conductivity of uranium oxide by providing greater enrichment near the sheath. By shortening the heat transfer path to the cooling sheath surface, the operating fuel centerline temperature at a given power output is significantly reduced. This results in less deformation and stress in the sheath material. Because the fissile fuel expands more than the sheath material at high temperatures, lowering the temperature reduces this expansion and therefore the deformation of the sheath. moreover,
At lower operating temperatures, the release of fission gases by the fuel is lower, the internal pressure on the potash pod is lowered, and the helium conduction path is not degraded by fission gases within the fuel. Iodine, cesium, cadmium-3, and any volatile fuel products are less likely to be deposited on the inner surface of the sheath. These elements promote cracking of zirconium alloy sheaths under stress and chemically assisted stress corrosion.

It is an object of the present invention to provide an improved fuel element for use in nuclear reactors.

Another object of the invention is to provide a fuel body that operates at lower temperatures for a given power output.

It is another object of the present invention to provide a fuel element with a design that reduces the likelihood of cracks and tears in the sheath.

These and other objects of the invention will be more clearly and better understood with reference to the following detailed description taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to fuel rods for use in nuclear reactors. One of the important considerations in the design and use of fuel rods is the possibility of cracking or tearing the cladding containing the fissile fuel. Cracks will form in the sheath due to chemical interaction between the zirconium alloy sheath and the uranium oxide fuel. Cracks may also occur due to stress created in the sheath by internal gas pressure or expansion of the uranium pellet.

The present invention 1. It features a unique fuel rod design that reduces the likelihood of cracking in the pod and improves thermal efficiency.

A cross-sectional view of a fuel rod 10 is shown in FIG. This fuel rod 10
is also shown in the enlarged sectional view of Fig. 2. An elongated zirconium alloy sheath 11 contains a hybrid fissile fuel. The fissile fuel comprises a central portion or core of a number of successively stacked pellets 12 forming a cylinder and surrounded by a nuclear fuel mixture 13. Granular fuel 13 occupies an annular portion around pellet core 12 inside the pod (FIG. 4).

The pod 11 has plugs 14 near each end to hold the fuel in place. At the bottom of the fuel rod there is a space 15 containing a coil spring 16. This space 15 allows the fuel column to expand or contract during its use. A coil spring 16 provides the necessary compressive force to maintain fuel column density.

The pellet core 12 is comprised of any depleted or lightly enriched fissile fuel such as uranium oxide (UO2). This core is 0
．． It can have a concentration of 20,000 to 2.0% U-235. Plutonium or TJ-2
Other fissile isotopes such as 33 can be used in place of or in conjunction with U-235. Thorium oxide can also be used in place of or in combination with uranium oxide.

Nuclear fuel 13 is substantially more enriched than core 12 .

Uranium oxide grains are 2.0 to 20.0 weight ft% U-23
5. Plutonium and U-233 isotopes may be used in place of or in combination with uranium oxide. A typical mixture of grains is 58 parts 1500
-60011m size sphere%20 40n 100
/1m size sphere and 22 508m size spheres.

The central operating temperature l of the fuel is lowered by several hundred degrees Celsius relative to the nominal stable output, as shown in Table 1 below.

Table 1 Median fuel temperature 13y, nominal nwR @ steady state operating output, 25KW /
Calculated at M (,7,5K9/Ft) 8X8
0/100 984 hybrid annular p1/t&
Sphere 8x8 10/90 869 Hybrid Ring Beret & Sphere 8x8 30/70 780 sphere
Body 8X80/100 973 8X8 30/70 666 * In the above calculation, average concentration 1i 3.78 is used for maximum outer concentration 5.00.

FIG. 3 shows a cross-section of an alternative embodiment to the fuel rod of FIG. FIG. 4 is an enlarged view of the fuel rod in FIG. 3 [(Ji side view).

In the embodiment of FIG. 3, nuclear fuel 23 is fuel rod 20
, and a pellet 22 occupies an annular portion around the nuclear fuel 23 within the sheath 21 .

All other actual information for this example embodiment is substantially the same as that of the fuel rod 10 pool of FIG. The elements 14, 15 and 16 are the same as shown in FIG.

The use of nuclear fuel in contact with the pod will reduce the likelihood of pod failure due to the occurrence of cracks and tears. The radial enrichment gradient across the cross-section of the fuel material lowers the plasticization temperature of the fuel rod, which also reduces the likelihood of cracking in the sheath.

The reduction in the probability of sheath breakage is synergistic and is greater than the combined reduction in probability even if applied separately by each methodology.

This unique interaction is the result of several factors: 1. The lower operating temperature in the fuel center due to the radial enrichment gradient causes smaller fuel expansion, thus reducing stress in the pod. Make smaller.

2. Lowering the fuel temperature to 11"V and thermal expansion allows the use of a smaller fuel-sheath gap, which will result in lower fuel operating temperatures.

3. The reduced fuel operating temperature also reduces the release of fission gas into the rod helium atmosphere, and this increases the fuel operating temperature as burn-up progresses due to either helium bulk or gap conductance effects. will lower the

4. The relief fuel in contact with the sheath distributes circumferential stresses evenly and does not provide radial cracks that provide tunnels for transport of fission products from inside the hot fuel.

5. 'tJ-shaped fuel reduces stress due to fuel deformation in the center of the fuel^8*-1a, reduces pellet ridging at the ends of the pellet, and reduces stress in the pod when the burn-up intensifies. Reduce stress.

[Brief explanation of the drawing]

FIG. 1 is an axial cross-sectional view of a fuel rod of the present invention, FIG. 2 is an enlarged cross-sectional view taken along line 2--2 of FIG. 1, and FIG. 3 is an alternative embodiment of the present invention. and FIG. 4 is an enlarged cross-sectional view taken along line 4--4 of FIG. 3. DESCRIPTION OF SYMBOLS 10... Fuel rod, 11... Sheath, 12... Pellet core, 13... Granular fuel, 14... Plug, 20...
Fuel rod, 21... Sheath, 22... Annular pellet, 2
3... Granular fuel. Remaining days below

Claims (1)

Claims: 1. A fuel element for use in a nuclear reactor comprising fuel material contained within an elongated sheath, the fuel material comprising both a pellet fuel portion and a nuclear fuel portion. , each of these portions occupying inner and outer substantially concentric regions, respectively, within the sheath, the outer fuel portion having a substantially higher enrichment than the inner fuel portion, thereby providing a radial An enrichment gradient within the fuel material is enforced, which results in improved operating temperatures for the fuel element. 2. The fuel element of claim 1, wherein the outer portion does not contain the nuclear fuel and the inner portion contains the nuclear fuel. 3. The fuel element of claim 1, wherein said inner portion comprises fissile fuel having a uranium enrichment of about 0.2 to 2.0% by weight. 4. The inner portion comprises a fissile fuel mixture or a depleted fissile fuel containing at least one of the group of fissile fuels such as uranium (235) oxide, uranium (233) oxide, platonium and thorium oxide. A fuel element according to claim 1. 5. The fuel element of claim 1, wherein said outer portion comprises fissile fuel having a uranium enrichment of about 2.0 to 20.0% by weight. 6. The outer part comprises a fissile fuel mixture or fissile fuel containing at least one of the group of fissile fuels such as uranium oxide (235), uranium oxide (233) and plutonium. A fuel element according to range 1. 7. The fuel according to claim 1, wherein the nuclear fuel comprises a mixture of three types of spherical particles having a diameter size range of approximately 1500 to 600 μm, 400'i'i to 100 μm, and 50 μm or less. element. 8. The fuel element of claim 1, wherein said nuclear fuel has a smear density of about 85 to 90 percent. 9. The combustion element according to claim 1, wherein the outer portion comprises the fuel to be let and the inner portion comprises the nuclear fuel bar. 10. The fuel material comprises the pellet or grain portion occupying an annular area cross-sectionally around a central area within the sheath, the annular area being /4' with respect to the central area.
2. A fuel element according to claim 1 having a volume ratio in the range from 90/10 to 50,150 cents.