JPS60131821A - Decomposing method of sodium nitrate - Google Patents

Abstract

PURPOSE:To decompose safely sodium nitrate to gaseous NOx and sodium borate without accompanying the volatilization of radioactive substance by adding boric acid to sodium nitrate contg. the radioactive substance and heating the mixture to melt. CONSTITUTION:In a process for recovering the remaining nuclear fuel substance or the like from the fuel after the usage, sodium nitrate contg. radioactive substance is obtained as a by-product. Boric acid (or boron oxide) are added to sodium nitrate and the mixture is decomposed to sodium borate and NOx at comparatively low temperature by heating and melting the mixture (it is preferable to use a microwave heating furnace). Also, in this method, it is possible to obtain a stable borosilicate glass instead of producing sodium nitrate by adding glass forming material such as silicates in addition to boric acid and heating the mixture to melt.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for decomposing sodium nitrate containing radioactive substances.

When a nuclear reactor is equipped with nuclear fuel and its combustion is controlled, fission products that absorb neutrons are produced in the fuel, reducing combustion efficiency. Therefore, in order to effectively utilize fuel, plutonium is separated and recovered from unburned nuclear fuel material (uranium) and newly generated nuclear fission products and reprocessed into fuel. During this separation and recovery, nitric acid solution is used as a chemical for dissolution, and some of this
L is neutralized with NaOH, and NaNO3 is produced here.

NaNo3 has been widely known as dust saltpeter for a long time, and is used in gunpowder, oxidizing agents, dyes, etc., and is designated as a Class 1 dangerous substance as a hazardous chemical under the Fire Service Act. Also, from the chemical properties, the specific gravity is 2.26 and the melting point is 308.
It is deliquescent at ℃ and easily dissolves in water. The problem is how to store NaNO3 with such unique properties stably over a long period of time, and the development of this method is desired.

This invention was made in order to solve such conventional problems, and makes it possible to store NaNO3 stably for a long period of time.

However, the first gist of the present invention is to add boron or boron oxide to 1-rium nitrate and to heat the heating bath 1.
11 to decompose boric acid into NOx gas such as mono-lithium.

The second gist is that sodium nitrate is decomposed into a glass substance and NOx gas by adding boric acid or boron oxide, further adding a glass-forming substance, and then heating bath WA11'. This is how it was done. In addition, the third gist is that boric acid or boron oxide is added to sodium nitrate, silicate is further added, and borosilicate glass is decomposed into NOx gas by heating and melting. It was designed so that In each of the above methods, it is preferable to perform heating and melting using electromagnetic waves because heating can be performed efficiently.

In general, when NaNO3 is heated above 380°C, it begins to decompose, releasing oxygen and becoming thorium nitrite, and when heated above 750°C, it produces sodium peroxide, which then oxidizes!・It becomes 5 lium. In this case, it is necessary to modify NaNO3 to the form of sodium oxide (Na20) for vitrification. That is, when NaNO3 is completely decomposed, approximately 40% by weight of the original amount of Na2O will be obtained, but when actually heated, this will not be as expected.

When NaNOx was decomposed using a thermobalance method, activities similar to those shown in the figure were obtained. Tsunawachi, Na, 'N
In the case of O3 alone, 5 decomposition as shown by line 1 begins at 690°C and shows a residual rate of 70% at around 960°C. On the other hand, in order to enhance the effect of the decomposition accelerator, SiC was replaced with NaNO.
The one added with 1:O,5 in molar ratio to 3 is 112
As shown in the figure, the decomposition temperature decreases slightly and decomposition starts at around 610°C, but the residual rate is also 70% at around 800°C, and it is not modified to Na2O. On the other hand, when B2O3 was added at a molar ratio of 1:1 to 71+1 L, the decomposition started at 450i' as shown by line 3, and it was modified to almost Na2O at around 800°C. Line 4 shows the case where NaNO3 is divided into Na2O.

Based on the results of the above 2, it is thought that the added boron oxide acted effectively with the decomposition promoting mound 1 or the decomposition bending material, and that it facilitated the decomposition by one of the following reactions. It will be done. -2Na NO3-1-2B2
Os −*N'a 2.0 ・21320'3+2
NO2 + ReiO2・・・・・・□・・・(1) 2'Na
NO3+t3203-+Na20- [32C)3'
+2NO2+ y20 2 ・・・ ・・・ ・・・
... - (2) That is, it is presumed that boron easily combines with sodium to form sodium borate, thereby promoting the decomposition of NaNO3. Other substances that have this role include pho 11 (1-1
'B 03 ) exists, and its addition amount is required to be 2Qwt% or more relative to sodium nitrate.

Next, vitrification was performed using the boron oxide used here. A stainless steel crucible was used as a crucible using 0 waves of heat as the heating source. - The force was set in the range of 3I≦W to 4°5KW.

The result is puzuyo in the first nose? In other words, when a decomposition accelerator is not added, for example, synthetic wollastonite (calcium citric acid: CaO and Sin/! as a component ratio) is used in a ratio of 1:1. ), NaN is fused into synthetic wollastonite powder.
It exhibits a phenomenon such as Os//b infiltration, and is caused by a melting reaction? 1
．． .

The situation was not observed. As a result, when immersed in water, it dissolves and becomes white after a few hours.On the other hand, when a decomposition accelerator is added, NaNO3 easily decomposes and becomes vitrified. Synthetic wollastonite (Ca 3i O 3 -calcium silicate), sewage car incineration ash and blast furnace slag were used, among which calcium silicate was converted into a sodium borate compound by adding boron oxide to nitrate-potassium. In particular, the boron component combines with the calcium component, and 3CaO-[3203, 2Ca 0-820
At the same time, the lithium component combines with the silicic acid component to form silicate glass, which is also insoluble in water, and has a great effect with a small amount of addition. - It is possible to escape.

The chemical composition of calcium silicate is shown in the ffT2 table.

Generally, two types of calcium silicate are known: a synthetic product and a natural product, and the synthetic product is a natural product and a bare alumina (AQ2
Synthetic products that contain a large amount of O3) components and contain alumina have a better degree of insolubilization in water, and the effect is recognized up to an fj content of alumina of 3 owt%. Table 3 shows the results of elubility in water by varying the ratio of calcium oxide and citric acid, which are the main components of calcium silicate. In this case, the weight ratio of calcium oxide and silicic acid was varied from 1 = 4 to 4 = 1.
wt% was added. As a result, it was possible to convert it into a water-insoluble assimilate.

Furthermore, in the above method, a heating melting method is used to convert NaNO3 into a water-insoluble substance, but in this case, if the heating temperature is too high (for example, 1000°C or higher), NaNO3
Among the various substances contained in it, for example, 1
37C8 and the like have problems such as evaporation at temperatures above 1000° C., and cannot be said to be a good method when radioactive substances are included. On the other hand, when boron oxide or boric acid is used, it is possible to decompose it into NOx gas at a very low temperature as described above, and it is possible to prevent the volatile substances from volatilizing. Follow-(
After the NOx gas decomposes, a sodium borate compound remains, but when modifying this sodium borate into a glass substance that is insoluble in water, what are the best glass modifying materials other than those mentioned here? Typical 11!2 salts such as aluminum silicate and magnesium tyate, as well as clay materials such as Honbushi clay and sandy gravel, can be used.

As explained above, this invention decomposes sodium nitrate containing a BX radioactive substance into a stable substance by heating and melting it, and it can be stored stably for a long period of time using a simple method. It is something.

[BRIEF DESCRIPTION OF THE DRAWINGS] The drawings are for explaining the present invention;
FIG. 3 is a diagram showing the relationship between the heating temperature and the residual rate of NaNO3. 1...Na Characteristic curve of NO3 alone, 3...Na
N. Characteristic curve when B'203 is added to 3. Special V [Applicant: Kobe Steel Corporation

Claims (1)

[Claims] 1. Decomposition of sodium nitrate, which is characterized by adding boric acid or boron oxide to sodium nitrate and heating and melting the mixture to decompose it into thorium borate and NOx gas. Method. 2. It is characterized by adding boric acid or boron oxide to sodium nitrate, further adding a glass-forming substance, and then heating and melting it to decompose it into a Ni-C glass substance and NOX gas. How to decompose sodium nitrate. 3. f1, which is characterized by adding boric acid or boron oxide to thorium nitrate, further adding silicate, and decomposing it into borosilicate glass and NOx gas by heating and melting. How to decompose sodium chloride. 4. The method for decomposing sodium nitrate according to claim 1, 2, or 3, wherein the heating and melting is performed using electromagnetic waves.