JPS6057039B2 - Method for recovering coated nuclear fuel particles from overcoated nuclear fuel particles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating overcoated nuclear fuel particles.

More specifically, the present invention relates to an improvement in a method for removing an overcoat from overcoat-coated nuclear fuel particles and recovering the coated nuclear fuel particles. Today, nuclear reactors called high-temperature gas reactors have double or quadruple coating layers of pyrolytic carbon or SiC applied to the particles of nuclear fuel materials such as UO2UC to confine the generated fission products. Furthermore, it is overcoated with a mixed powder of graphite powder and phenolic resin (usually referred to as matrix raw material), compacted using a warm press method (overcoat warm, press method), and fired. is used as nuclear fuel.

When producing these overcoated nuclear fuel particles (hereinafter simply referred to as overcoated particles), the permissible range of the weight ratio of the coated nuclear fuel particles to the matrix raw material at the time of overcoating is extremely narrow.

2. Overcoat particles containing a plurality of coated nuclear fuel particles and particles consisting only of matrix raw materials that do not contain coated nuclear fuel particles must not be mixed.

However, in the overcoating operation, the production of overcoat particles that do not meet the conditions in 1 and defective particles as described in 2 is unavoidable to some extent. Since a very uniform distribution of the nuclear fuel material is required in a nuclear fuel compact, the aforementioned defective overcoat particles must be excluded.

On the other hand, since nuclear fuel materials are radioactive and coated nuclear fuel particles are extremely expensive, it is desirable to remove the matrix material from defective overcoat particles and recover the coated nuclear fuel particles for reuse. When collecting coated nuclear fuel particles, extreme care must be taken in handling to avoid damaging their integrity, and they must be recovered quickly and with a high yield to avoid physical or chemical damage. .

Conventionally, the following method has been used to remove matrix raw material powder from defective overcoat particles and recover coated nuclear fuel particles.

In this method, the matrix raw material powder is desorbed from the coated nuclear fuel particles and the two are separated separately. Ru.
That is, first, the defective overcoat particles are immersed in alcohol for a long time (several hours or more) to remove them by utilizing the alcohol solubility of the phenol resin that serves as a binder. Then coated nuclear fuel particles and matrix using a sieve net! The raw material powder is separated, and furthermore, the matrix raw material powder adhering to and remaining on the surface of the coated nuclear fuel particles remaining on the sieve is washed and removed with alcohol. However, the above conventional method has the following drawbacks.

That is, several hours or more are required for immersion in solvent a.

b) The three operations of desorption, separation, and washing are performed separately, which increases the working time.

(c) The coated nuclear fuel particles tend to clog the sieve screen, reducing the yield or yield.

Alternatively, the recovery of the clogged coated nuclear fuel particles requires additional operating time. The present invention improves the conventional method and provides a new method that eliminates the above-mentioned drawbacks by performing the three operations of desorption, separation, and washing simultaneously and in parallel.

The present inventors have proposed the above-mentioned A. ．． b. ．． In order to solve problem c, the following measures were taken.

(1) Applying ultrasonic waves to the solvent solution in which the overcoat particles are immersed.

(2) A sieve plate with many small circular holes is used instead of a regular sieve screen.

(3) Continuously flow the solvent from the top to the bottom of the sieve plate carrying the overcoat particles.

(4) The solvent surface and the sieve surface on which the overcoat particles are placed are alternately rocked up and down relative to each other.

These effects are as shown below.

(1) promotes the desorption of the overcoat through the cavitation phenomenon, and (4) the dissolution of the binder from the coated nuclear fuel particles of the overcoat powder due to the mechanical action of collision between the solvent liquid level and the overcoat particles. accelerates desorption.

(1) also accelerates the dispersion of the detached overcoat powder into the solvent, and (3) and (4) accelerate the separation of the overcoat powder and coated nuclear fuel particles above and below the sieve surface.

(2) Since there are no irregularities caused by weaving metal wires like a normal sieve screen, there is no clogging when collecting coated nuclear fuel particles from the sieve plate.

(3) also has a cleaning effect on the surface of coated nuclear fuel particles.

Next, the present invention will be specifically explained with reference to Examples.

The experiment was carried out by assembling a device as shown in the attached drawings.

The apparatus includes a tank 1 equipped with a discharge pipe 2, a sieve plate 3, an ultrasonic oscillator 6 attached, and a liquid introduction pipe 4 facing.

The size is arbitrary, but the one used in this example is a stainless steel cylindrical one with a diameter of 11CFR and a depth of 1K1.
and a siphon tube with a bend for determining the highest liquid level H. The sieve 3 used was a 0.5m thick stainless steel plate with a diameter of 10C77! Small holes with a hole diameter of 0.5WR were bored in a 1wa equidistant array (i.e., an equilateral triangular three-scale array) over the entire surface. It is disc-shaped and can be fixed to the tank wall. The solvent is introduced from conduit 4 and discharged from discharge pipe 2, but since the discharge pipe is bent as shown, if the discharge capacity (diameter) of the discharge pipe is made larger than that of the introduction pipe, When the liquid level reaches H, the solvent is discharged from the tank by siphon action, and when it reaches L, the discharge stops and the liquid level rises again.

By doing so, the sieve surface and the liquid surface can be constantly alternately oscillated up and down relative to each other. In this device, the lowest liquid level is 5T1r1t1 below the sieve surface, and the highest liquid level is 15?
It was designed to be. The sample used in the experiment had a diameter of 920 mm.
Approximately 1.7 m in diameter, P7T1 coated nuclear fuel particles are overcoated with matrix raw material powder with a particle size of 105 μm or less, which is a mixture of 8% graphite powder and 21% phenolic resin.
overcoated particles were used.

The amount of overcoat was 1.073y of matrix raw material powder per 1f of coated nuclear fuel particles. In this experiment, ethyl alcohol was used as the solvent, and the tank was filled with it just up to the surface of the sieve plate. At the same time as the sample 50' described above was placed on the sieve plate, ultrasonic oscillation and solvent inflow were started and continued for 5 minutes. The inflow velocity of the solvent was 8 m1/sec,
During this time, the liquid level oscillated up and down 1 (2) times at intervals of about 30 seconds. After the operation was stopped, the sieve plate was removed and dried in an air bath at 110'C for 1 hour. By tilting the sieve plate, the coated nuclear fuel particles thus recovered could be easily transferred to a predetermined container without clogging a single particle. The weight of the recovered coated nuclear fuel particles was 24.13f, and according to calculations, the proportion of the matrix material remaining attached to the coated nuclear fuel particles was 0. It was less than % by weight. Since the present invention is configured as described above, the above-mentioned removal, separation,
Because the three types of cleaning actions are accelerated simultaneously and with a synergistic effect, it is possible to achieve extremely short processing times of less than 1 filtration compared to conventional methods, yet results comparable to those of conventional methods can be obtained. It will be done. Furthermore, as is clear from the above description, the amount of work is also greatly reduced. In this method, if the amount of overcoat particles charged is sufficiently small for the equipment, (4) of the four operations (1), (2), (3), and (4) described above can be performed.
) A similar effect can be obtained by using only a combination of three operations.

[Brief explanation of the drawing]

The accompanying drawing is a conceptual diagram of an example of equipment used to carry out the method of the invention. In the figure, 1: solvent tank, 2: siphon pipe (discharge pipe) 3
: Sieve, 4: Solvent introduction tube, 5:, (overcoat)
Coated nuclear fuel particles, 6: Ultrasonic oscillator.

Claims (1)

[Scope of Claims] 1. A method for recovering coated nuclear fuel particles by removing the overcoat using a solvent from coated nuclear fuel particles overcoated with a matrix raw material having a phenolic resin as a binder, comprising: (1) (2) Using a thin metal plate with many small holes as a sieving device instead of a screen; (3) Using a thin metal plate with many small holes held on the sieving surface A method characterized in that the solvent is continuously allowed to flow from top to bottom through a layer of overcoated nuclear fuel particles. 2 A method for recovering coated nuclear fuel particles by removing the overcoat using a solvent from coated nuclear fuel particles overcoated with a matrix raw material having a phenolic resin as a binder, the method comprising: (2) use of a thin metal plate with many small holes instead of a screen as a sieving device; (3) overcoat-coated nuclear fuel particles retained on the sieve surface; (4) The surface of the sieve carrying the overcoated nuclear fuel particles and the liquid level of the solvent are alternately oscillated up and down relative to each other. A method characterized by being carried out in combination. 3. The method according to claim 2, in which a siphon pipe is installed at the lowest liquid level in the solvent tank and has its highest point at the highest liquid level, and a siphon pipe is installed at a height midway between the two. A method characterized in that the sieve surface and the solvent liquid surface are alternately oscillated up and down relative to each other by introducing the solvent at an introduction capacity lower than the discharge capacity of the siphon tube.