JP3051289B2 - Coated nuclear fuel particles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coated nuclear fuel particle used for producing a fuel compact used as a fuel for a high temperature gas reactor or the like.

[0002]

2. Description of the Related Art Coated fuel particles currently used have a diameter of 20 which is composed of an oxide of a fissile material or a parent material (herein, these are collectively referred to as "nuclear fuel material").
The fuel compact is composed of fine particles in which a fuel core of 0 to 600 μm is multiply coated with pyrolytic carbon or silicon carbide. The fuel compact is obtained by mixing these coated fuel particles with graphite powder and a binder and press-molding the mixture. It is easier to load.

[0003] The most commonly used coated fuel particles of the TRISO type are a fuel core made of U or Th oxide having a diameter of 200 to 600 µm and a fuel core having a thickness of 60 µm.
Wrapped in a buffer layer made of low-density pyrolytic carbon, covered with a high-density pyrolytic carbon layer having a thickness of 30 μm, covered with a 25 μm-thick SiC layer, and further covered with a thickness of 25 μm. It has a triple coating structure coated with a high-density pyrolytic carbon layer of 45 μm.

[0004]

The greatest advantage of the coated fuel particles having such a multi-coated structure is that the fission gas can be retained in each particle, but chemical damage during irradiation is a major problem. ing. The cause is a chemical reaction between oxides (fuel nuclei) and carbides (coating layer) at high temperatures. In particular, oxygen ions in the fuel nuclei, called fuel nucleus migration, react with carbon in the buffer layer to form CO gas. It is believed that the particles move from the high temperature side (inside) to the low temperature side (outside) of the particles.

[0005] Further, since the coating layer is formed by mechanical vapor deposition by thermal decomposition of a deposition gas in a fluidized bed, little consideration is given to chemical bonding of the coating layer itself from a microscopic viewpoint. The coating layer itself is likely to be physically damaged.

Therefore, the present invention eliminates the chemical reaction between the oxide of the fuel core and the carbide of the coating layer, thereby reducing the chemical damage caused by the movement of the fuel nucleus and the like. It is an object of the present invention to provide a new and improved coated fuel particle that is less likely to be broken because it does not use a chemical vapor deposition.

[0007]

That is, the coated fuel particles according to the present invention are obtained by coating the outside of a fuel core made of a carbide of nuclear fuel material with a plurality of cage-like carbon clusters. The cage-like carbon cluster is a kind of polyhedral molecular capsule in which carbon atoms are connected in a network, and has a structure in which a fuel core made of carbide of a nuclear fuel substance is wrapped in the capsule.

In producing the coated fuel particles of the present invention, a method of performing arc heating in an inert atmosphere using a graphite rod filled with a nuclear fuel material as an electrode can be adopted.
Thus, coated particles having a structure in which the fuel nuclei of the nuclear fuel material carbide are wrapped in the cage-like carbon clusters in a multiple manner can be generated in the cathode portion.

A method for producing coated particles by coating fine particles with such multiple cage carbon clusters is already known. However, a fuel core made of carbide of nuclear fuel material is coated with multiple cage carbon clusters. No proposal has been made to produce coated fuel particles.

[0010]

According to the coated fuel particles of the present invention, since the outside of the fuel core, which is a carbide of the nuclear fuel material, is multiply coated with the cage-shaped carbon cluster, the oxidation of the nuclear fuel material such as the conventional coated fuel particles is performed. No chemical reaction occurs between the material and the carbide of the coating layer, and as a result, chemical damage due to fuel nucleus migration and the like is reduced.

Further, unlike the conventional coated fuel particles, the coating layer is not formed by mechanical vapor deposition, but has a structure in which the fuel core is wrapped in a multi-cage carbon cluster, which is a kind of molecular capsule, so that physical damage is also prevented. At least, the confinement of fission gas is also effectively and reliably achieved.

[0012]

EXAMPLES Examples of the production of the coated fuel particles of the present invention will be described below. FIG. 1 is a schematic view of an apparatus for producing coated fuel particles, in which a normal graphite rod is used as an anode and a graphite rod filled with nuclear fuel material is used as a cathode and installed in a chamber.

A graphite rod filled with a nuclear fuel material is manufactured as follows. Diameter 10mm, length 100mm
4mm diameter, 40m depth in the longitudinal direction of graphite rod
A hole of m is drilled, and the hole is filled with ThO ₂ . As a sample of ThO ₂ , a mixture of ThO ₂ (2%) + W (98%), which is easy to handle and easily available in an experiment, was used. The nuclear fuel material to be filled in the graphite rod may be a carbide, but an oxide is easier to handle and more readily available. After filling the graphite rod with the nuclear fuel material, it was calcined at 400 ° C. for 1 hour under a nitrogen atmosphere,
Heat treatment is performed at 00 ° C. for about 2 hours.

After setting the above-mentioned anode and cathode in the chamber, the inside of the chamber is evacuated to 10 ⁻⁴ T by a vacuum pump.
Exhaust to orr or less and reduce the amount of leak at this time to 10 ^-6 at
m.cc/sec (He conversion) or less. This is because it is necessary to remove water and oxygen in order to improve the yield in producing coated fuel particles. Next, helium gas is introduced into the chamber at about 400 to 500 Torr from the inlet. Next, the electrode interval is 2 mm
While moving the anode so that
00A, 25V voltage applied by DC power supply
Arc discharge for a minute. During this time, in order to protect the chuck portion of the electrode and the sealing surface of the chamber from heat, cooling water is flowed through these portions to cool them.

The carbon on the anode is converted into C ⁺ and gasified by the arc discharge, and is attracted to the cathode to be deposited on the front end surface as if it were deposited. Further, ThO ₂ filled in the graphite rod of the anode becomes ThC _{2 according} to the following formula, migrates to the cathode, and is deposited. ThO ₂ + 3C → ThC ₂ + CO ₂ Ultra fine particles that are not deposited on the tip of the cathode are collected by the ultra fine particle collecting system.

After the end of the discharge, the deposited portion at the tip of the cathode is recovered, the inner core portion is pulverized, and aggregated in a solvent such as ethanol by using ultrasonic waves to obtain a particle diameter of about 100 to 500.
Angstrom coated fuel particles are separated and recovered. Separation and collection of the coated fuel particles from the deposited portion at the tip of the cathode may be performed by a method other than the above-described method, such as a particle size sorting method or a weight sorting method.

FIG. 2 shows a high-resolution transmission electron micrograph of the obtained coated fuel particles. From this picture, the fuel core (X)
Is included in the multiple cage-like carbon cluster (Y). FIG. 3 shows the results of EDX analysis (energy dispersive X-ray microanalysis) of the coated fuel particles.
From this analysis result, it is understood that the fuel core (X) is a Th-W carbide.

The coated fuel particles of the present invention can
As in the case of the SO-type coated fuel particles, a fuel compact can be formed by, for example, mixing graphite powder and a binder and press-molding. In addition, the fuel compact in which the fuel compact is formed as a hollow pellet is filled in the sleeve by vibration or cold isostatic pressing (CIP).
Or hot isostatic pressurization (HIP) treatment to obtain an integral fuel rod.

[0019]

As can be seen from the above description, the coated fuel particles of the present invention have a fuel core made of carbide, and the outside of the coated fuel particles is covered with multiple cage carbon clusters. No chemical reaction occurs between the oxide of the fuel core and the carbide of the coating layer as seen in the particles, and as a result, chemical damage due to fuel core migration and the like is reduced.

Further, since the multi-cage carbon cluster is a kind of molecular capsule, it is possible to solve the problems such as physical damage as seen in the conventional coating layer formed by vapor deposition and to reduce the fission gas. It can be securely locked inside.

Further, since the physical properties of the coated fuel particles of the present invention are similar to those of graphite, the thermal conductivity can be expected to be improved as compared with the conventional nuclear fuel, and the heat resistance temperature is also about 2 times.
Since it is similar to graphite at 000 ° C., it is expected that the burnup of nuclear fuel and the improvement of safety can be expected.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view of an apparatus for producing coated fuel particles according to the present invention.

FIG. 2 is a high-resolution transmission electron micrograph of coated fuel particles.

FIG. 3 is a graph showing the results of EDX analysis of coated fuel particles.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G21C 3/62

Claims (1)

(57) [Claims] 1. Coated nuclear fuel particles characterized in that the outside of a fuel core made of carbide of a nuclear fuel material is coated with a plurality of cage-like carbon clusters.