JPS6173803A - Production of powder mixture composed of uranium dioxide and gadolinium oxide for producing nuclear fuel pellet - Google Patents

Abstract

PURPOSE:To produce a stable powder mixture composed of UO2 and Gd2O3 as a raw material for nuclear fuel pellets by precipitating the compd. of U and Gd from an aq. nitric acid soln. of U and Gd having a specific concn. and filtering and drying the precipitate then subjecting the same to reduction roasting. CONSTITUTION:Thoroughly dried U3O8 and Gd2O3 are weighed to a suitable weight so that the content of Gd in UO2 pellets contg. Gd attains 6wt% in terms of the concn. of Gd2O3 and after the compd. is dissolved in a nitric acid, distilled water is added thereto to prepare the nitric acid soln. of 750 deg.C having 60-110g/l concn. of (U+Gd). Ammonia water is added to the soln. under stir ring to adjust the pH thereof to >8 thereby forming the precipitate consisting of (NH4)2U2O7 and Gd(OH)3. The precipitate is filtered and is dried for 16hr at 150 deg.C and is then roasted at >=550 deg.C in gaseous N2; in succession, the gaseous atm. is changed over to H2 and the precipitate is subjected to the hydrogen reduction at >=550 deg.C, by which the stable powder mixture composed of UO2+Gd2O3 is produced.

Description

[Detailed description of the invention] (industrial field) The present invention provides stable carbon dioxide for the production of nuclear fuel pellets.

Regarding the manufacturing method of uranium/gadolinium oxide mixed powder (prior technology and its problems) Nuclear fuel is used for about three years after being loaded into a nuclear reactor.

The longer the life of the reactor, the more efficient the reactor becomes. Fuel that can withstand long-term use must have high burnup performance. That is, fissile TJ-235
In this case, a flammable neutron absorber (flammable neutron poison substance, hereinafter referred to as combustible substance) is used to absorb excess reactivity at the beginning of the operating cycle. As an example of a burnable poison, borosilicate glass is currently used, but it accumulates as waste after use.

Therefore, instead of borosilicate glass, Gad IJ nium oxide, another burnable poison, is used in nuclear reactors as a dispersed mixture of nuclear fuel, and burnable poisons are removed during the subsequent reprocessing of spent fuel. A practical method is to process both at the same time.

One of the conventional nuclear fuels that have been developed with the addition of burnable poisons is uranium dioxide (UOv) nuclear fuel pellets containing gadolinium oxide (G(hos)). Conventionally, there are two methods for producing a mixed powder with gadolinium oxide.

(1) Powder mixer This method is a method in which uranium dioxide powder and gadolinium oxide powder are mechanically mixed and sintered at high temperature to create a UOtGdtOs solid solution and form Pev'yh.

(2) Precipitation reduction method In this method, gadolinium oxide and uranium oxide are dissolved and precipitated, mixed uniformly, and the precipitate is roasted and reduced to yield UOy
-GdtOs powder is formed, pressure molded, and then sintered to form pellets. The following methods utilize this precipitation reduction method.

(at Wada et al, BNES Con
f, (1973) 63.1-63.3 (bl Fu
kushima at al., J., Nucl., Ma.
T, 105 (1982) 201° In all of these methods, pellets are prepared and their physical properties are measured, but there are the following problems.

6 Questions Points) (a) In the above two papers, stable UO for pellet production was used.
t -GdtOs Since the purpose is not to produce mixed powder, there is no description of (U + Gd) # degrees and liquid temperature, and if those values change, UOz -G after sinter reduction
dtOs powder may oxidize in room temperature air.

(b) It is unclear whether pellets can be manufactured on an industrial scale.

On the other hand, another method of using the precipitation reduction method was
Although it is disclosed in Publication No. 634, UOt and (jcbo
Since the purpose is to uniformly mix s, there are the following problems.

Problem C Points) (a) It is unclear whether the UOz-GcbOs mixed powder after roasting reduction can be used directly as a berene)ff-forming powder without being oxidized in air at room temperature.

(b) It is unclear whether pellets can be produced on an industrial scale by scaling up the amounts shown in the examples.

(c) A mixed powder stable in air for use in pellet production cannot be obtained by simply setting the pH conditions and roasting temperature conditions.

C) The present invention solves the above problems by improving the precipitation reduction method, and produces stable UOm that does not oxidize in the air and can be used directly as a powder for pellet production after roasting and reduction. G
The purpose is to provide a method that enables FiJ production of dtOs powder (structure of the present dispute), that is, according to this Kakumei, Nitshin Suifu liquid containing uranium and gadolinium. In the process, (fJ + Gd) 4 degrees is in the range of 6017A to 111/bu, the temperature is raised to 50 degrees Celsius or higher, ammonia is added to make p)I 8.0 or higher to form a precipitate, and then the precipitate is Separate by filtration, dry pickle, culture and strain at 550℃ or higher in air or/and poisonous gas, and further add hydrogen or hydrogen-containing gas/atmosphere”1.
lrJ: Reduction at a temperature of 5s-o℃ or higher
Nucleoid pellets with ridges・→】Rainbow formation in the chamber 2/・r
＾ Il at the end of Gadolinicum Akira's costume! 31 construction methods are shouted.

As described above, the present invention is an improved invention of the above-mentioned precipitation/complementation method, and UOm-Gcb obtained by @decomposing and precipitating uranium and gadolinikumuihi metals, and then drilling and polishing the precipitate.
The Os mixed powder is not only uniformly mixed, but also does not naturalize during pelleting and pickling (stable and free-G).
dtOs(n) is a method for rPjJ4 of the U-QdlOs mixed powder that makes it possible to dispose of non-existent pellets.

The stability of the UOt/Gdt mixed powder after roasting and reduction is greatly influenced by the precipitation formation conditions. For that reason, +2 for stable UOm Gdt Os rff & combination! ! ! As for the precipitate for production, (1) the uranium precipitate must be somewhat large in order to prevent the UOt-GdwO□ mixed ω powder from being oxidized after roasting.

(21) In order to produce UOt pellets containing 0rbOs at a constant concentration of 9r, it is necessary to completely precipitate the uranium and gadolinium components and to have a small amount of sintering loss at the source of roasting. )fi9
For i'9, the sintered material is stable UOv Gdt Os powder that does not oxidize in the air (3) Furthermore, in order to put it to practical use in production. Therefore, it is necessary to increase the amount of chlorine/gadolinium used in experiments rather than a small amount.

By focusing on the above points and conducting various studies, we have achieved the present invention.

Next, the results of manufacturing a UOt Gdt Os mixed powder according to the present invention are shown below.

(1) Regarding the stability of 20 g of UOz Gd mixed powder after roast reduction under the precipitation conditions of uranium and gadolinium.

UO with gadolinium! Fuel pellets, i.e. (
U+Gd) Gadolinium-containing crystal in Ot is Gdt O
U3O8 and Ga so that s is 6.00% by weight.
After weighing a predetermined amount of TES, dissolve it in nitric acid, add ammonia, and add (NH4) to -0. A precipitate consisting of Gd(OH)s and Gd(OH)s was produced. The ammonia added may be ammonia water or ammonia gas. Table 1 shows the stability of the UOtGdtOs mixed powder obtained by drying, roasting and reducing the precipitates generated under different conditions of (U+Gd) concentration and liquid temperature of @dissolution, according to P. Table 1 sample magic 4 to 1
For each UO* Gch Os mixed powder of 67 and Ji9, 65 to 125 pellets were prepared;
All were stable without being oxidized during the pellet manufacturing process. In addition, the homogeneity of gadolinium in the obtained pellets was examined by observing the metal phase using a color-etched puffer using the ASTM-C968 (1983) method, but there was no free Gd*Os that was not dissolved in solid solution, and the pellets were found to be good as product pellets. there were.

From the above results, it was confirmed that (U + Gd) 11 degrees in the solution and the temperature of the solution are important factors in producing a stable mixed powder. That is, the solution temperature is 50℃
or more and (U + Gd) concentration is 60g/13~11o1
It was found that the precipitate produced in the range of 1/4 was a UOt GdtOs mixed powder that had a certain amount of Kura 7 powder and was stable even after the roasting layer, and could be used directly as a powder for pellet production. Furthermore, (U+Gd)# degree is 609
If it is less than 7A, the amount to be handled is too small to be used in mass production, so it cannot be used industrially.

(2) Regarding the stability of the composition ratio of uranium and gadolinium.

When GdtOs was dissolved in nitric acid, NHs water was added thereto to change the pH from 6.6 to 9, gadolinium hydroxide was precipitated, and P was separated using qualitative 1 paper of /I62.
Table 2 shows the relationship between the amount of loss of gadolinium transferred into the P solution and pH.

The amount of loss is expressed as the amount of gadolinium transferred to the f-liquid relative to the amount of gadolinium in the starting material Gd*Oa, expressed as 8% by weight.

Table 2 Relationship between gadolinium loss due to precipitation and pH Reference At pH 6.80, Gd(OH)s scuttling occurs, and p
Approximately 99 g of Gd (OH)s precipitates at H7.02 (loss amount of 1 g).

From Table 2, in order to reduce the amount of loss to 0.1% or less, pH8,
It was found that 0 or more is sufficient.

It is necessary that the composition ratio of uranium and gadolinium does not change consistently from the setting of the initial composition ratio by weighing the starting materials to the final pellet formation through melting, film formation, etc. To this end, it is necessary to completely separate the decomposed uranium and gadolinium into precipitates, and to prevent the loss of heat loss during roasting and reduction of the precipitated powder. In roasting and reduction, the uranium precipitate (N) t utoγ becomes UO, at 350 to 550°C during roasting, becomes U, O, at 550°C or higher, and then becomes UOt in a reducing atmosphere. On the other hand, gadolinium precipitate Gd(OH)s becomes Qd20g at 550°C or higher during roasting, and does not change to GdtOs even by reduction. If the roasting reduction temperature is high, the loss on burning due to dissipation and evaporation of the powdered precipitate during fractionation will be large, and the composition ratio of clan and gadolinium may change. Then,
On the other hand, if the degree of roasting reduction is low, the powder will have good sintering properties, but it will be an unstable powder that oxidizes in the air.

From the above viewpoint, we conducted a comprehensive experiment from melting to pellet production to investigate the relationship between changes in composition ratio and sinterability. Add sugar to the solution shown in Sample No. 5 in Table 1.

The weight composition ratio of uranium and gadolinium in the UOs Gd20s mixed powder obtained by adding water and precipitating at pH 8.3 and roasting and reducing at 760°C as shown in Table 1 is the initial composition ratio of 15°8.
7, there was no problem as the agreement was within 15.87±0.4%. In addition, the analysis result of the N content in this U (% GdtOs mixed powder) was 10 ppm or less, and there was no problem.
In the pellets produced by sintering in U2'f soot at 0°c5hr, the composition ratio of uranium and gadolinium matched within 0.4% of the initial composition ratio, and the sintering property was good, and the density was satisfactory as a product. pellets.

Furthermore, the analysis results showed that the amount of impurities in the pellets specified in the product specifications was below the specified amount, and there was no problem.

Also, UO of sample 7g64.6.7.9 in Table 1, = G'
d2σ.

Pellets were manufactured using the mixed powder, and the sinterability of the powder was good, and the composition ratio of uranium and gadolinium in each pellet was within 0.3%, which matched the initial composition ratio, so there were no problems.

From the above overall results, it was found that the roasting reduction temperature should be set to avoid fluctuations in the composition ratio of uranium and gadolinium and to produce UO
In order to obtain cbOs mixed powder, the temperature is not too high at 80°C.
In order to obtain a stable UO,-GdtOs mixed powder that does not oxidize in the air or during the pellet manufacturing process, the roasting reduction temperature should be 550°, at which the uranium precipitate changes from UOs to U,08 during roasting. A grade of C or higher is required. The desired UO2GdtOs mixed powder can be obtained by performing the roasting in an oxidizing atmosphere consisting of oxygen gas and/or nitrogen gas, and the reduction in an oxidizing atmosphere mainly consisting of hydrogen gas.

(3) Regarding the amount of uranium and gadolinium handled in the experiment, the amount of uranium, linium, and linium handled in the experiment was determined not by the beaker test amount, but by using the 7.6. ．． e. Install the sedimentation tank 25A, and
3O8974,4,@, GdtOs 60. Using Og, experiments were conducted consistently from dissolution and precipitation to pellet production. The produced pellets are acceptable as product pellets, and it seems that there will be no problem even if the device is scaled up for practical use. EXAMPLES Next, the present invention will be specifically explained with reference to Examples, but the following Examples do not limit the scope of the present invention.

Example 974.4 F, Gcb was added to sufficiently dried U3 so that the gadolinium content in the gadolinium-containing UOt pellet was 6.00 wt% as a Get, Q, concentration.
We weighed 60.0.9 Os, dissolved them in nitric acid, and added distilled water to make 83 g/e at 6 degrees (U+Gd).

The temperature of this solution was raised to 51.5°C, and while stirring, NHs water was added and the pH was adjusted to 8.3 (NH4)2 UtOy and Gd(O
H) A precipitate consisting of a is prepared, the precipitate is separated, and then dried at 150°C for 16 hours, and then heated in nitrogen gas for 76 hours.
Roast at 0℃ for 3 hours and switch the atmospheric gas to hydrogen gas.
The mixture was reduced for 1.5 hours to obtain a UOz Gdz Os mixed powder.

(Effects of the present invention) By adopting the above configuration, the present invention can obtain the following effects.

(1) Ura dioxide/pellets containing gadolinium oxide were produced using the obtained UOt Gch Os mixed powder in the same manner as the normal pellet manufacturing process, but the mixed powder was stable during the manufacturing process and did not easily destroy the pellets. It was a powder that could be used directly as is.

(2) The produced pellets had good Gd uniformity, which is important in terms of product specifications, with no free gadolinia (GdzOs), and the content of other impurities such as C and N did not pose any problems.

(3) The GdtOs concentration in the pellets was also within the product specification range, and only one of the uranium and gadolinium components was not lost during the entire manufacturing process.

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1973) 6Ll-633)+b+ Fukushima and others, Nuclear Materials Journal, 105 (1982) 21+1 (Fukushima at al, J, Nucl,
Mat., 105 (1982) 201) J.

Claims (1)

[Claims] (1) In a nitric acid aqueous solution containing uranium and gadolinium, the (U+Gd) concentration is 60 g/l to 110 g/l.
The temperature of the liquid is adjusted to 50°C or higher, and ammonia is added to raise the pH to 8.0 or higher to form a precipitate.Then, the precipitate is filtered out, dried, and heated to 550°C or higher in air or/and nitrogen gas. Uranium dioxide for producing nuclear fuel pellets, which is roasted at a temperature of 550°C or higher, and further reduced at a temperature of 550°C or higher in hydrogen or a reducing gas containing hydrogen.
A method for producing gadolinium oxide mixed powder.