JPH0469592A - Nuclear fuel element - Google Patents

Abstract

PURPOSE:To enhance the protective capacity of a fuel cladding pipe by packing pure aluminum or low melting aluminum alloy into the spacing part between the inner peripheral surface of the fuel cladding pipe and the outer peripheral surfaces of fuel pellets. CONSTITUTION:Liquid sodium floats to the upper part of a nuclear fuel element and the contact of the sodium and the fuel pellets 3 is averted even if the sodium infilters the inside of the nuclear fuel element of a vent type or the fuel cladding pipe of the nuclear fuel element, the fuel cladding of which is ruptured in this fast breeder. Since the thermal conductivity of metal is good, the gap between the fuel pellets 3 and the fuel cladding pipe can be made larger than in the case of use of helium. The permissible width of a swelling quantity is thus increased.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to nuclear fuel elements used in the cores of nuclear reactors, particularly light water reactors and fast breeder reactors.

(Prior Art) A nuclear fuel element for a light water reactor and a fast breeder reactor consists of two parts as shown in FIG.

For example, in the case of oxide fuel, one is the combustion section 1 that houses fuel pellets 3 made of uranium or an oxide of uranium and plutonium (UOx, Punt/uO2), and the other is the combustion section 1 that contains the fuel pellets 3 that are generated from the fuel pellets 3 during combustion. This is a gas plenum section 2 that stores gaseous fission products (hereinafter referred to as Fp gas) and adjusts the pressure within the nuclear fuel element. In the combustion section 1, helium gas (lie) having high thermal conductivity is placed between the fuel pellets 3 and the fuel cladding tube 4 so that the heat generated in the fuel beret 3 is efficiently transferred to the outer surface of the fuel cladding tube 4.
is included. 6 is a neutron shielding material, 7 is a combustion t+1 blanket, and 8 and 9 are an upper end plug and an F section end plug, respectively.

In the case of a light water reactor, a moisture absorbing material 5 is installed in the gas plenum part 2.
Zirconium alloy powder is added as a zirconium alloy powder to prevent the hydrogen generated by the reaction between the trace amount of moisture remaining inside and the zirconium cladding material to be absorbed into the zirconium fuel cladding tube 4 and to prevent the tube 4 from turning into a silver oxide. There is. In addition, water as a coolant (in the case of light water reactors) or liquid sl.
In the case of a fast breeder reactor), the outside of this nuclear fuel element from below! - Passing towards the power section.

However, conventional nuclear fuel elements have the following problems.

■ As neutron irradiation progresses, nuclear fission products bromine (Br), iodine (1), and tellurium (To
), cesium (Cs), etc. are generated. These fission products corrode zirconium and stainless steel, which are fuel cladding materials, when the oxygen partial pressure within the nuclear fuel element is high, and there is a risk that the nuclear fuel element may be damaged.

■ Even though helium sealed between the fuel pellets and the fuel cladding has high thermal conductivity, its heat transferability is limited because it is a gas. Therefore, in the event of an abnormality, the center temperature of the fuel pellet may exceed the melting point of the fuel and cause the pellet to melt. In order to ensure a constant heat flow rate, it is necessary to reduce the gap between the fuel pellets and the fuel cladding tube.
If combustion progresses, the fuel pellets will expand in volume (hereinafter referred to as swelling) due to neutron irradiation, and there is a risk of damaging the fuel cladding, so the gap cannot be made too small.

Furthermore, even during steady operation, the center of the fuel pellet remains at a high temperature. t11kli change (equiaxed crystal → recrystallization, equiaxed crystal → columnar crystal), composition change (MO% Generation and diffusion of fission products such as Zr-, L Te, UO□, Pu01・U
O, changes in the amount of oxygen in the fuel pellet), morphological changes (formation of a central pore, increase in volume due to swelling, occurrence of cracks), etc. are likely to occur, resulting in instability, and due to mechanical interaction between the fuel pellet and the fuel cladding tube. There is a risk that the cladding tube may be damaged and FP gas may be generated abnormally, a small amount of fuel pellets may be released into the coolant, or the coolant may enter the inside of the tube and form compounds.

To address such problems, various countermeasures have been taken or proposed in the past.
The publication states that barium or a barium alloy (alloy consisting of aluminum, zirconium, nickel, titanium, or a combination thereof) is stored in the gas plenum and is chemically combined with moisture, FP gas, and other gases. The nuclear fuel element used as a getter and its manufacturing method are
JP-A No. 51-71498 discloses a fuel cladding tube (substrate)
A nuclear fuel element has been developed in Japanese Patent Laid-open No. 2003-110004, in which a metal barrier made of aluminum, copper, niobium, nickel, stainless steel or iron is metallurgically bonded to the inner layer of the substrate to protect the cladding from attack by fission products. No. 5169796 discloses that a metal layer of aluminum, chromium, molybdenum, niobium, or an alloy thereof is metallurgically bonded to a base material on the inside of a fuel cladding tube to form a composite cladding, which reacts rapidly with iodine, which is a fission product. JP-A-50-109397 discloses that refractory metals such as molybdenum, tungsten, rhenium, niobium, and their alloys are used between the nuclear fuel F4 material and the coating vessel. A nuclear fuel element is described that includes a liner to prevent contact between the cladding vessel and the fission products and to improve longitudinal thermal gradients.

(Problems to be Solved by the Invention) However, conventional nuclear fuel elements are not necessarily sufficiently effective in solving the various problems described above.

For example, even if a refractory metal liner is inserted between the fuel pellets and the fuel cladding, ``cracks'' will occur in the central high-temperature part of the reactor in a short period of time after the reactor is started, and the size of the cracks will depend on the amount of irradiation. As the burn-up increases, the adhesion between the refractory metal liner and fuel pellets is poor, and although the thermal conductivity between the fuel pellets and fuel cladding is improved compared to when using conventional helium, , its effect is not sufficient.

Furthermore, to the extent that coatings or thin films are applied to fuel cladding tubes, at least when aluminum is used, in the case of oxide fuels, it easily combines with oxygen in the nuclear fuel elements, and the resulting oxides has a high melting point,
In addition, in the case of metal fuels, the fuel and cladding interact at high temperatures, creating a eutectic alloy that penetrates the cladding and causes damage, making it difficult to maintain the expected performance for a long period of time. be.

Furthermore, it is extremely difficult to manufacture a fuel cladding tube having a small diameter and thin wall such as a fuel cladding tube, the inside of which is coated or metallurgically bonded from the beginning with a metal layer.

The present invention solves the above-mentioned various problems, increases the FP gas absorption capacity and the protection ability of the fuel cladding against other fission products, and improves the thermal conductivity between the fuel beam and the fuel cladding. ``Toward'', the nuclear fuel that was made ``'4 element 4 擢(j shi 4 action [1 target and -4.

(Solving the question - 4 people) As a result of various tests, the inventors of the present invention achieved the purpose 4 in 1. In order to achieve the purpose 4 in 1. Self-deprecating aluminum, ``-um (A IV), - is 1゛lumiJtsuJ,
It is preferable to use a base metal of 4, which is 1.2.

(8) Aluminum, 'llll Uno, or aluminum;, ram alloy has a strong affinity for oxygen, and moves with the oxygen getter.

(1)) Aluminum, ram, or low-melting point aluminum and nium alloys have low melting points (below 700°C), and during reactor operation, they melt and become flammable.
-1 The thermal conductivity between the cladding tube is much higher than that of 1'J Heliu J.

(C) Aluminum, 2゛J, 4: or aluminum, = 7um alloy does not dissolve at all in the liquid phase or solid phase, and the specific gravity is higher than that in the liquid phase or the solid phase. big.

(d) Words of War 1. Aluminum. -Ram or aluminum Ja alloy has activity) 7 cracks, 51 nuclear cracks! 4. Easily reacts with paraboloids such as bromine and tritium.
Tsum, tantalum 1. Active Φ group powders such as -ob1, -tsu J, and Ru have high reactivity with water and steam, and are effective in inhibiting corrosion of bamboo coated with t''1 by water and steam, and in CX corrosion 11.-1 Hydrogen concentration ζ, ″′, vs. 1.7 and effective V′
Working 6 (f) Aluminum, ram or aluminum 5: Uno 1. Nuclear Reaction Disruption of Alloys III] When loaded, it is close to water, and even when used in eaves water reactors, there are no negative effects.

The present invention utilizes the characteristics of aluminum (1) or aluminum alloy to the maximum extent possible, and its gist lies in the nuclear fuel elements (1) and (2) below.

■ The gap between the inner periphery of the fuel cladding tube and the outer periphery of the fuel pellets loaded in the fuel cladding tube is filled with pure aluminum or a low-melting point aluminum alloy. Nuclear fuel 149 elements.

(2) The nuclear fuel element according to (2) above, wherein the gas plenum portion is filled with an active substance.

The above-mentioned [low melting point aluminum base metal 1] is, for example, "A1".
3% Si, A1. -Like Ga, melting point is 700℃
[Hereinafter] refers to an alloy of PA.

In addition, the state ``filled with pure aluminum or low melting point aluminum alloy'' refers to l) pure aluminum or low melting point j'lumi J, lum alloy powder (particle size 0
．． 111) A thin-walled tube made of pure aluminum or a low melting point aluminum alloy is used as fuel. The inner surface of the cladding tube was strained (
No metallurgically bonded state, etc.

Pure aluminum or a low melting point aluminum alloy is
) or 11), when used, both melt and become the state 11), so 1) to 1j)) are substantially the same.

In addition, the above-mentioned "filled with 1" means that when melted,
The gap between the fuel belen 1~ and the fuel cladding tube, and the amount that is sufficient to fill the ``center'' that starts at the center of the knot and the ``cracks'' that occur throughout the knot during use. Pure aluminum or low melting point aluminum alloy is present.

The nuclear fuel element (2) is the same as the nuclear fuel element (2) in which the gas plenum portion is further filled with an active material. Active substances include aluminum, sodium, potassium J,
Powders of low melting point active alloys such as (particle size 0.IIIV),
Activated carbon or metal powders such as titanium, zirconium, tankle, niobium, nickel, etc. (particle size 0.1 mm or less)
Activated powders such as are suitable.

(Function) The function of the pure aluminum or low melting point aluminum alloy filled in the gap between the inner circumferential surface of the fuel pellet tube and the circumferential surface of the fuel pellet tube will be described below.

First of all, as mentioned above, the thermal conductivity of pure aluminum or low melting point aluminum alloy is significantly higher than that of helium, and when comparing the thermal conductivity and coefficient of aluminum and helium, as shown in Table 1, the thermal conductivity of aluminum is significantly higher than that of helium. is about 1,000 times thicker than helium, so the heat transfer from the fuel pellets to the fuel cladding tube is dramatically higher than when it is carried out through helium, and 6 FP gas release rate can be lowered.

Figure 2 is a graph showing the dependence of the FP gas release rate of fuel for fast breeder reactors on temperature (center temperature of fuel pellets), and the horizontal axis is the burnup (alo).
is ATOM%, which is the ratio of the number of uranium and plutonium that fission and contribute to heat generation in the fuel to the total number of uranium and plutonium.

In addition, M in the figure indicates oxide fuel, M-Zr indicates metal fuel, and MC indicates carbide fuel. 0M means U (uranium) or Pu (plutonium), and they are written together with each fuel. The value is the linear power, which is the calorific value per unit length of the fuel pellet (watts/cm).

As is clear from this figure, if X number of Ut3S fissions and contributes to heat generation, in MC (carbide salt $4), the center temperature is low and the FP gas release rate is low.

On the other hand, Mo2 (oxide fuel) and M-Zr (metallic fuel) have a high center temperature and a high FP gas release rate, but as mentioned above, pure aluminum filled in the gap between the fuel cladding tube and the fuel pellet The aluminum or aluminum alloy melts when the fuel is combusted, and heat is quickly transferred from the fuel pellet to the salt t-1 cladding tube, which lowers the temperature at the center of the fuel pellet and reduces the amount of FP gas released. becomes less. As a result, the amount of increase in pressure inside the fuel cladding tube is small, and the rate of increase is gradual, so the amount of increase in stress generated in the cladding tube is also small.
Since the rate of increase is also moderated, the burnup, which is limited by the allowable stress value of the fuel cladding tube, can be increased. That is,
The fuel life can be extended, and even if the burn-up is the same, the amount of FP gas released is small, so safety is improved if the volume of the gas plenum is the same. Furthermore, since the internal volume of the gas plenum can be made smaller, the fuel element can be designed shorter, and the reactor body can also be designed shorter.

Furthermore, the thermal conductivity between the fuel pellets and the fuel cladding is good, and the heat transfer in the vertical direction of the fuel cladding is rapid, so the uneven distribution of neutrons in the vertical direction inside the reactor The non-uniformity of the heat generation temperature in the vertical direction of the pellet is easily averaged out against the fuel cladding tube, and it is possible to prevent accidents where the fuel cladding tube becomes locally heated to a high temperature, leading to damage, resulting in excellent safety. There is.

In the case of oxide fuels, as mentioned above, the center of the fuel pellet forms within a short period of time after reactor startup.

However, the molten aluminum flows into that part regardless of the shape of the center hole, allowing the heat in the center to escape to the outside.

Pure aluminum or low melting point aluminum alloys have an extremely large free energy of oxide formation and a strong affinity for oxygen, so it is possible that a large amount of pure aluminum or low Filled with melting point aluminum alloy, it will not combine with oxygen and lose its function in a short period of time.
Aluminum is oxide fuel pellet L (UOx, Pu
nt・U(h) and combines with oxygen contained in the pellet, forming Al-U-0 system and All-Pu-U-0.
form oxides of the system. In addition, since it reacts with nuclear fission products such as bromine and iodine, it is effective in preventing corrosion of fuel cladding materials (zirconium and stainless steel).

The active powders such as aluminum, titanium, zirconium, tantalum, niobium, and nickel filled in the gas plenum are highly reactive with water and steam, and have an effective effect on hydrogen absorption. Activated carbon is also used for inactive gases such as xenon (Xe) and krypton (Kr) in FP gas.
), and sodium and potassium adsorb iodine and cesium in FP gas.

Aluminum and sodium do not melt together at all whether they are liquids or solids, and each has a density of 2.36 at 700°C.
g/cm', 0.78 g/cs+3, where liquid aluminum is heavier than liquid sodium. In a fast breeder reactor that uses natrilam as a coolant, Notriu J reacts with the oxide fuel F4 Belev 1 to form a compound. 7. Since this material causes swelling, it is possible to prevent F.lium from entering the fuel cladding tube as much as possible.However, in this fast breeder reactor, F. y! Even if sodium intrudes into the fuel cladding tube of a nuclear fuel element with a damaged tube, liquid I and lium will remain liquid "Fulminir".
) Since it is lighter, it floats on the -L part of the nuclear fuel element, avoiding contact between the sodium and the fuel pellets.

In addition, fuel used in fast breeder reactors is washed with water before reprocessing (-3 sodium is removed; Water could not be used because the plutonium monoxide (PuOz) would corrode and be leached.

However, if the fuel pellets are coated with aluminum, there is no risk of contact with water and cleaning with water is possible.

In fast breeder reactors, molten aluminum undergoes an alloying reaction with iron, the main constituent element of stainless steel used as fuel cladding tubes, and zirconium, the main constituent element of Zircaloy alloys used in light water reactors. Even if an alloy reaction were to occur, deterioration would only occur on the aluminum side at °C, and stainless steel and Zircaloy alloys are less likely to undergo deterioration.

Figure 3 shows aluminum and stainless steel III (SIJS)
304) was heated at 650°C for 20 minutes while in contact, the X-ray intensity of each element near the contact surface was measured using an X-ray microphone + 7 analyzer. While the change in X-ray intensity is large, no such large change is observed on the stainless steel side.

In the nuclear fuel element of the present invention, fuel pellets and
By simply inserting pure aluminum or low melting point aluminum alloy powder or thin-walled tubes between the cladding tubes, there is no need for any special processing of the fuel cladding tubes.Therefore, production can be carried out without changing the normal production line. It is possible to

In addition, the heat resistance of pure aluminum and low melting point aluminum alloys
Since the conductivity is low, it is possible to widen the gap between the fuel 24 pellets and the fuel cladding tube when using helium, and the allowable range of the amount of swelling can be increased by 4.

As mentioned above, the nuclear reaction cross section of aluminum is close to that of water, so there is almost no need for design changes when applying it to fast breeder reactors and light water reactors.

As explained above, the feature of the nuclear fuel element of the present invention is that the gap between the inner circumferential surface of the fuel cladding tube and the outer circumferential surface of the fuel I4 bellet 1 is filled with a large amount of pure aluminum or a low melting point aluminum alloy. Sometimes it becomes molten, resulting in a significant increase in the thermal conductivity between the salt 11 bellet 1 and the fuel cladding, an increase in the FP gas absorption capacity and the protection ability of the fuel tube against other fission products. such as a significant increase,
The reason is that it simultaneously provides functions for solving various conventional problems.

In the explanation so far, a nuclear fuel element using oxide fuel has been taken as an example, but the nuclear fuel element 1 of the present invention can also be applied to a nuclear fuel element using carbide, nitride, etc. as fuel. Figure 4 is a diagram showing the configuration of a core element (shielding body) for a fast breeder reactor using boron carbide (84C).
Carbonized boron belene NO is coated with a stainless steel cladding tube 4, and a gap 11 between the cladding tube 4 and belene HO is filled with pure aluminum or a low melting point aluminum alloy.

+2 is a spring for holding pellet I (l).

A control rod used to control a nuclear fission reaction is one in which a plurality of the above-mentioned elements are bundled in an outer tube material of a predetermined shape, and the present invention is of course applicable to this control rod as well.

Furthermore, in addition to the nuclear fuel element internal breeder reactor of the present invention, an advanced converter reactor (ATR; Advanced
It can be used as a nuclear fuel element in various types of nuclear reactors, such as the Theraal Reactor.

(Example) The thermal conductivity, pressure and oxygen partial pressure within the nuclear fuel element, moisture within the nuclear fuel element, and corrosion of the cladding tube were investigated for the nuclear fuel element of the present invention produced by the method shown in Table 2. This was compared with a case where the gap between the fuel cladding tube and the fuel pellet was not filled with aluminum.

In addition, in the same table, methods A and B of [aluminum filling method]
The method and C method refer to the following methods, respectively.

Method A: Vibratory filling of powder Pure aluminum powder (particle size of 5 μm or less) is filled while vibrating into a nuclear fuel element with one end plugged.

Method B: Filling of melt A pure aluminum melt is filled into a nuclear fuel element that has been evacuated in advance with one end plugged.

Method C: Inserting a thin-walled tube A thin-walled pure aluminum tube is attached to the inner surface of a fuel cladding tube by a drawing method to form a double tube, and fuel pellets are inserted into the tube.

Furthermore, "without vent" refers to a non-vent type nuclear fuel element as shown in FIG. 5, and "with vent" refers to a vent type nuclear fuel element having a vent hole 15 in the gas plenum as shown in FIG. It is.

In non-vented nuclear fuel elements, after filling the gap between the fuel cladding tube and the fuel pellet with pure aluminum using the method described above, the gas plenum is filled with pure aluminum powder (particle size of 5 μm or less) and the ends are welded. In plugged and vented nuclear fuel elements, pure aluminum powder (particle size 5 μm or less) is similarly filled into the gap between the fuel cladding tube and fuel pellets, and then titanium powder (particle size 5 μm or less) is filled in the gas plenum. Then, activated carbon powder (particle size of 5 μm or less) having a high physical adsorption capacity for inactive FP gases such as xenon (Xe) and krypton (Kr) was filled and the ends were plugged by welding.

The survey results are also shown in Table 2. The results of each survey item shown in Table 2 were judged based on the standard values shown in Table 3, and ◎ indicates very favorable.
indicates that it is preferable, and x indicates that it is not preferable.

As is clear from Table 2, the nuclear fuel element of the present invention (N
o, 1 to 7) are unfilled nuclear fuel elements (No. 8
．． It can be seen that the performance is significantly improved compared to 9).

Further, the nuclear fuel element of the present invention had the same effect whether it was a non-vented type or a vented type.

(Hereinafter, blank spaces) (Effects of the invention) By using the nuclear fuel element of the present invention in the core of a nuclear reactor such as a light water reactor or a fast breeder reactor, the thermal conductivity between the fuel vessel 1/1 and the fuel cladding tube can be improved. I can do a remarkable amount of improvement. In addition, the ability to capture FP gas and the protection ability of the fuel cladding against nuclear fission products will be increased, and the core temperature of the beret and the maximum temperature of the rupture tube will be increased, greatly increasing fuel safety. Fuel life can be extended.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view, not shown, of the structure of the nuclear fuel element of the present invention. FIG. 2 is a diagram showing the temperature dependence of FP gas generation from fast breeder reactor fuel. FIG. 3 is a diagram showing the state of alloying at the contact surface between aluminum and stainless steel. FIG. 4 is a joint cross-sectional view that does not show the structure of a core element for a fast breeder reactor. 5 and 6 are diagrams showing the configuration of the nuclear fuel element used in the example. FIG. 5 is a non-vent type nuclear fuel F4 element.

Claims (2)

[Claims] (1) A nuclear fuel element characterized in that the gap between the inner circumferential surface of a fuel cladding tube and the outer circumferential surface of fuel pellets loaded into the fuel cladding tube is filled with pure aluminum or a low-melting point aluminum alloy. (2) The nuclear fuel element according to claim (1), wherein the gas plenum portion is filled with an active substance.