GB2381649A - Treatment of radioactively-contaminated graphite blocks - Google Patents

Abstract

A method is provided for controlling the starting temperature of oxidation and the oxidation velocity of graphite blocks, when the oxidative combustion is conducted to process the graphite blocks without actually changing the shapes of the graphite blocks which have already been used in a nuclear reactor and have been radioactively contaminated. The method comprises the steps of adding catalytic compounds to the surface of the graphite blocks used in a nuclear reactor, in accordance with a desired oxidation starting temperature and desired oxidation velocity; and performing an oxidative combustion of the graphite blocks in an oxygen-rich air supplied to a combustion furnace, thereby producing gases as well as ashes. A high concentration oxygen generator is provided as a later stage of an apparatus for properly adding catalytic compounds to the surface of the graphite blocks used in a nuclear reactor.

Description

METHOD OF AND APPARATUS FOR CONTROLLING OXIDATIVE

COMBUSTION OF THE GRAPHITE WHICH HAS BEEN

RADIOACTIVATED AND/OR RADIOACTIVELY CONTAMINATED

WHEN BEING USED IN NUCLEAR REACTORS OR HAS THE

POSSIBILITY OF HAVING BEEN RADIOACTIVELY

CONTAMINATED

TECHNICAL FIELD

The present invention relates to a method of and an apparatus for controlling an o cidative combustion of a sort of graphite which has been used as a moderator or as a fuel assembly in the core of a nuclear reactor and has become unwanted when the nuclear reactor iB to be decommiasioned; another sort of graphite which has become unusefb1 when used fuel iB replaced by fresh fuel and which has become radioactive due to neutron irradiation or at least having such a possibility; a further sort of graphite which has been contaminated or at least having such a doubt since it has been used in nuclear energy technique, in an experunent and a measurement of a radioactive substance, or used as a container storing a radioactive substance; a Bti further sort of graphite which has not been used but still has such a doubt to have been contaminated. (Here, a graphite doubted to have been contaminated means a graphite which is handled in the same way as a graphite which is clearly proved to have been contaminated, because the graphite doubted to have been contaminated is the one which has been existing in a radiation treatment equipment or a nuclear energy equipment, and therefore it is impossible to prove that the graphite is not contaminated, or it is difficult to ensure that the graphite does not contain a radioactive energy or to specify a radioactive substance by, for

- 2 examp}e, separating and dividing the graphite and measuring the radioactivity of it.) In more detail, this invention relates to a method of and an apparatus for controlling an oxidative combustion starting temperature as well as an oxidation velocity, by adding a catalyst to a graphite during a process in which the graphite is subjected to an oxidative combustion.

TECHNICAL BACKGROUND

Conventionally, a core of a graphite-moderated nuclear reactor using a graphite as a moderator, iB composed of molded or machined graphite blocks stacked in multiple layers, each of the blocks iB formed by molding or by machining graphite material into a block shape.

These graphite blocks are usually radioactive, because they are exposed for a long period of time to a great amount of irradiation by neutrons radiated from nuclear fuel assemblies during operation of the nuclear reactor. For this reason, when such a graphite-moderated nuclear reactor iB to be dismantled or reduced in its volume, it is important to separate the radioactive eubetancee contained in the graphite blocks.

As a method of treating the above graphite blocks, it is conceived to use a process in which the need graphite blocks are burned so as to convert carbon into a fluid such as carbon monoxide and/or carbon dioxide, thereby separating radioactive metal oxides as residue.

However, since the graphite blocks molded or machined ibr forming the core of a graphite-moderated nuclear reactor have all been subjected to a high purification treatment, they contain only an extremely small amount of impurities, presenting a fine surface and a high density. AB a result, these graphite blocks are noncombustible in _._ 1- Il- alelllel llaarlllele 111 till 1111118111111111 11181 111 1 515111

!, - 3 air. In more detail, since the graphite blocks processed in a high purification treatment has a very small amount of impurities, a fine surface, and a small surface area, it is rlifficult to ensure a sufficient combustion surface area per unit volume of the graphite block.

Consequently, using only a reaction heat malcee it difficult to maintain a combustion temperature against a heat radiation amount. As a result, it is imposaible to maintain the combustion.

As described above, since the problem of noncombustibility of the graphite blocks has not been solved, it is often been that the graphite blocks of a decommissioned nuclear reactor are left as they are without receiving any treatment all the world over.

Experiments have proved that graphite can be oxidized by external heating in an electrical furnace or direct electrical heating by conducting electric power, however, there is problem that it requires a long time and much labor to maintain an obtained heat, as well as a problem of secondary radioactive contamination of the equipment used in treating the graphite blocks.

Further, it is also conceived to use a method and an apparatus in which the graphite blocks are crushed into granular powder in advance so as to obtain an increased surface area for an oxidative reaction, followed by a combustion treatment in a fluidized bed.

However, in this case, as coarse particles are deposited in the combustion area, they fail to be burned, while some fine particles fly about without being burned, it results in a low combustion rate on the whole. At present, there is not a astiefactory technique capable of crushing the graphite blacks without producing very fine particles and it requires a long time and much labor to remove very fine particles by classification. And also there are iota of problems to be solved related to

-4 a secondary pollution happened in a prOCeBB of retreating classified fine particles as well as in an equipment used in the retreating process.

In view of the above, for the purpose of processing the graphite blocks as a block shape without crushing them, and for the purpose of solving the above-mentioned problem of noncombustibility of the graphite b10cl B HO as to realize a desired combustion treatment on these graphite blocks, the inventor of the present invention has disclosed (in Japanese Patent No. 3051869) the idea in which an amount of air having a high oxygen concentration is supplied to a combustion area in which the graphite blacks are to be burned, thereby overcoming the above-

mentioned problem of noncombustibility of graphite blocks and thus ensuring a continuous combustion.

Usually, when graphite is burned, it becomes carbon dimide and/or carbon monoxide in accordance with the following reactions: C+O2 CO2

2C+O2 2CO

Further, in order for the graphite blocks to react with oxygen so as to effect a desired combustion of the graphite bloclm, it is necessary to preheat the graphite blacks to a predetermined Reaction atarting temperature" and to maintain a combustion temperature during the combustion process.

Generally, when an oxidation of graphite is conducted at a higher temperature, it produces a higher carbon monoxide generating rate. On the contrary, when the oxidation is carried out at a relatively low temperature, the carbon monoxide generating rate is decreased at the time of oxidation while carbon dioxide generating rate is increased.

Accordingly, if it is possible to obtain a freedom for controlling a reaction starting temperature, it will also become possible to select a gas A, -, err Art am 11111 11 ape 1111118II I F I iI!I-1 All 1111 111 11 1 11 1!11

- s - obtainable in the oxidation.

However, in the above-discuseed graphite treatment method suggested in Japanese Patent No. 3051859, although a reaction starting temperature in an atmosphere having a high oxygen concentration is lower than that in an atmospheric air, this method has not arrived at an idea of controlling such a reaction starting temperature.

In view of the above circumAtancee, it is an object of the present invention to control an oxidizing reaction temperature in a process for oxidizing graphite used in a nuclear reactor.

SUMMARY OF THE INVENTION

A method for oxidizing a graphite used in a nuclear reactor according to the present invention, comprises the steps of adding one or more catalytic compounds to the surface of graphite blocks used in a nuclear reactor, wherein the graphite blocks having a block shape as the same shape as that taken during use in nuclear reactors inclusive of those which are somewhat broken or cracked; and performing an oxidation combustion of the graphite blocks in an oxygen-rich air supplied to a combustion furnace, thereby producing gases as well as ashes. Further, in the method for oxidizing a graphite used in a nuclear reactor, the catalytic compounds to be added to the surface of the I graphite blocks are selected from compounds containing alkali metal elements and/or heavy metal elements which are effective for lowering an oxidation starting temperature and/or an oxidation reaction continuing temperature.

Moreover, a surfactant is added to the catalytic compound for use in the method for o cid ng the above graphite, or the surface of the

graphite blocks is coated with the surfactant in advance.

Furthennore? a combustion furnace connected with a high concentration oxygen generator is disposed as a later stage of an apparatus for adding catalytic compounds such as a catalytic solution coating apparatus to the surface of the graphite blocks used in a nuclear reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 is a TG-DTA chart for measuring a combustion starting temperature and a combustion ending temperature during a graphite combustion experiment according to the present invention.

Fig. 2 is also a TG-DTA chart showing the graphite oxidising reaction using lead as a catalyst, indicating an embodiment of the present invention.

Fig. 3 is a flow chart showing how to carry out the embodiment of the present Invention.

Fig. 4 is a longitudinal sectional view showing the embodiment of the present Invention.

13 T FOR CARRYING OUT THE INVENTION

The inventor of the present invention, at the time of accomplishing the invention disclosed in the above-mentioned Japanese Patent No. 3051859, found that several sorts of metal oxides such as iron oxide, copper oxide and zinc oxide can enhance the oxidation of graphite, thereby lowering the remarkable combustion temperature in air by 1 5 C at which graphite developing the vigorous oxidative combustion. (See the paragraph [0007] in the specification of the patent). Namely, it was found that some impurity

elements coming from a brass plated on a _,,,,,. -11_1 1_l ll la illl Illlll I 111 11 1illlllli 11-ilB I 11 liBlil 1511 L

piano wire, which has been used in cutting a graphite sample material has an effect of lowering a graphite combustion starting temperature.

The inventor of the present invention selected, from among a plurality of available compounds ranging over 39 sorts of elements, some specific compounds which can maintain some oxides containing desired elements even after they are subjected to pyrolysis at a temperature lower than an ordinary combustion starting temperature of a graphite for use in a nuclear reactor, and then added the selected compounds one after another to graphite so as to carry out an oxidative combustion experiments. As a result, it was understood that when a small amount of specific compounds is added in advance to the surface of graphite blacks to be oxidized, the added compounds act as a catalyst capable of promoting the oxidation. Further, it was found that the above selected compounds can be added either singly or in combination, and it was also found. that a desired catalytic effect is reduced when they are added in an amount exceeding the proper quantity.

Now, description will be given to explain several experiments

serving as the bases of the embodiments of the present invention.

Q) EXPERIMENT FOR SPECIFYING ELEMENTS HAVING A

CATALYTIC EFFECT.

The elements which have been used in an experiment for investigating their catalytic effect in oxidation are listed in the following, arranged in an order according to their atomic numbers.

Li,B,Na,Mg,ALP,K, Ca,TiV, Cr,Mn, Fe, Co, Ni Cu. Zn, Ga, Rb, Sr, Y. Zr, Nb, Mo,

Rh,Pd,Ag,Cd,In,Sn,Sb,Ce,Ba,La,Ce,W, Hg, Pb and BE The above elements are 39 sorts in ale Aqueous solutions used in an actual experiment are all first class reagents of these elements, including a nitrate, a sulfate, an ammonium salt, an o cyealt, a halogenide, a boric acid, a phosphoric acid and the line.

Moreover, some elements (S. halogen, and noble gases) which fly about in a process of heating the graphite and are thus unable to get attached to the surface of the graphite blocks, other elements (C, N. and O) which are considered meaningless with respect to an object of the invention, elements (be, AB, Se, Te, Po) having a strong toxicity, radioactive elements, elements (Sit Ge, So, Au, and platinum group) each of which is not easy to form an aqueous solution of a single element, and elements (including most of the rare-earth elements) of which high purity reagents are difficult to obtain, are all intentionally excluded from the experiment, since the catalytic effects of some of these elements are predictable from other elements tested in the above experiment.

Further, the adding of one or more salts was effected in the form of aqueous solution, since it was convement in carrying out the invention.

It is also possible to use other solutions than aqueous solution, provided that one or more desired elements can be still exist, even if their corresponding compounds are pyrolyzed before arriving at a combustion temperature of the graphite blocks.

In practice, when the present invention is applied to a process in which graphite blocks having a large size are treated, this is different from an experiment in which only a small amount of graphite is used as a sample, it is preferable to add an extremely small amount of surfactant in the water. This is because the surface of the graphite blocks repels _. _ _ ',, en,, a r l la l. ia.11 11;1 11 E 11

! _ 9_ water and not easily get wet.

Here, since moat of surfactants are comprised of Na salt or K salt, and since Na salt or K salt can pronde the following oxidation promoting effects, it is necessary to find out the respective contents of these substances beforehand and reflect the results in carrying out blending.

Although the graphite to be used in a nuclear reactor has a high purity as discussed above, it still contains some impurities shown in the following Table 1.

[Table 1

EXAMPLE OF ANALYSIS OF IMPURITIES OF

GRAPHITE FOR USE IN A NUCLEAR REACTOR

(when converted into oxide) SiO2 0.01 05'h . Fe2O3 0.01 01% . _ CaO 0. 0045% _ NiO 0.0013% TiO2 0.0008% Total 0.0272% At the time the invention disclosed in Japanese Patent No. 3051859 was accomplished, since it was still not clear whether the aforementioned heavy metal impurities have a catalytic effect, the graphite used in a nuclear reactor was used as an experiment material, and it was found that the remarkable combustion temperature in air was 680 C.

In contrast, the present invention needs to find out an action of heavy metal impurities, the graphite sample material used in the present invention was a plurality of pure carbon pieces (hereinafter, simply referred to as "graphite pieces) which do not contain the

- lo -

aforesaid impurities.

When the remarkable combustion temperature in air of the graphite pieces was measured, it was found that such a temperature was 850 C. (This measurement was performed on lo mg sample, using TG -

DT Such TG - DTA will be described in detail later.) Table 2 shows a number of compounds added in graphite pieces.

An aqueous solution containing one or more of these compounds is dropped onto graphite pieces which have been weighed in advance.

After the compounds contained in the aqueous solution get adhered to the graphite pieces, a drying process iB conducted. The "compound adding amount" in Table 2 can be calculated from an increased amount of the weight of dried sample of the graphite pieces weighed in advance as noted above.

Further, "element adding rate of added elements to graphites in Table 2 iB a percentage in element value represented by chemical calculation with respect to graphite amount.

The graphite sample prepared in the above manner is put into a thermal analyzing apparatus TG-DTA (manufactured by MAC SCIENCE CORPORATION), and its temperature was elated from a room temperature at a speed of 20MC/min.

In the present espenment (I), it iB an object to specify elements that exhibit a remarkable catalytic property, and at first the experiment Q) wee conducted with a combustion area in air.

[Table 21 (on next page.) ,.,, _,_...,. - - B I11 11 1 111 111|1B, 1 111 1111 15511

- 11 [Table 2] EFFECT OBTAINABLE BY ADDING ELEMENItS (IN AIR) (1) (2) (3) - (4) (5)(6) (7)

[md [md [x][ c] [ c] Fe Fe(NO3)3 9H20 0.76 10.72 0.98762 -88 Co Co(NO3)2 6H20 0.53 10.69 1.00726 -124 .. _ _ _ _

Ni NiSO4 - 6H20 0.40 10.45 0.86 894 +44 Na NaNO3 0 30 10.76 0.75 694 -156 Li LiNO3 1 55 j 10.67 1.46 714 -136 Mg Mg(NO3)2 - 6H20 0.52 10.54 0.47 974 +124 Mn MnCH3COO 4H20 0.45 10.93 0 92 906 +56 _ _. _.

Al Al(NO3)3 9H20 1.79 10.43 1.23 1006 +156 Cu Cu(CH3CO0)2 H20 0.40 10.97 0.58 742 =108 _. _ _ _

K KNO3 0.21 10.70 0.75 674 -176

Ca Ca(CH3CO0)2 0.43 10.42 0.94 782 -68 Cr Cr(CH3CO0)3 9H20 0.65 10.33 0. 82 694 - 156 Zn Zn(NO3)2 6H20 0.47 10.94 0.94 822 -28 Ag AgCH3COO 0.11 10. 64 0.67 690 -160 -- _ _._ _

B H3BO3 0.51 10.85 0.82 930 +80

P H3PO4 _ 0.33 10.75 0.97 1010 +160

Rb RbNO3 0.16 10.77 0.86 618 -232 . _ V NH4VOa 0.25 10.86 1.29 678 -172 Pd Pd(NO3)2 0.21 10.25 0.95 662 -188 Y Y(NO3)3 0.40 10.65 0.94 840 -

_Zr ZrO(NO3)2 0.33 10.84 0.92 870 +20 Ba Ba(NO3)2 0.09 10.65 0.94 850 0 Hg Hg(NO3)2 0.15 10.54 0.95 840 -10 in In(NO3)3 0.26 10.93 _0.91 860 Pb Pb(NO3)2 0.20 10.35 0.97 530 -320 Bi Bi(NO3)3 0.16 10.36 0.97 570 -280 Mo (NH4)2MoO4 0.19 10 26 0.97 710 -140 Cs CsNO3 0.09 10.90 0.92 650 -200 Rh Rh(NO3)3 0.18 10.47 0.96 750 -100 Sr Sr(NO3)2 0.29 10.33 1.16 746 - 104 Cd Cd(NO3)2 0 29 10.22 1.35 754 -96 Nb NbFs 0.18 10.57 0.85 910 +60 Sb SbCI3 1 0.14 1 10.89 0.69 790 -60 Ce Ce(NO3)3 j 0.25 10.73 1.00 710 - 140 Sn j Sn[HCI Soil] 1 0.30 10.35 1.00 838 -12 La LaCI3 7H20 0.34 1 10.19 1. 30 810 -40 Ga Ga[ HNO3 Sol.] 0.52 1 10.24 1.00 866 +16 W (NH4)2WO4 - 5H20 0.23 10.66 1.10 764 -86

_ _ _ _ _. _. _

Ti Ti[tartaric acid Sol.] 0.63 1 10.87 1.00 878 l +28 ote) AiX sol.] represents a solution for ned by d ssolv g A rn X.

- 12 Explanation of columns in Table 2: (1) element (2) added compound (3) added compound amount [mid (4) sample graphite amount tmg] (5) element adding rate of added elements to graphite [%] (6) combustion starting temperature ['C] (7) combustion starting temperature deviation pC] And, a TG-DTA chart iB shown in Fig. 1. The TG-DTA chart is formed by three curves plotted with respect to an operation time, with a temperature curve acting as a nearly straight line rising towards the right. Here, DTA is a differential thermal curve, representing a heat absorption or a heat radiation of the sample. Further, TG is a thermobalance which can continuously indicate a change in the weight of the sample.

As can be seen from Fig. 1, an o cidative combustion of graphite is such that when it approaches a combustion starting temperature, DTA rises sharply, showing a heat radiation peak Tom, when combustion ends, DTA returns to its flat state.

On the other hand, TG shows a sudden drop towards the right as soon as the combustion is started. Such a drop continues until the combustion ends. From that point onward, TG returns to its flat state.

It shows the process of decrease of the weight of the sample due to combustion. Then, the TG-DTA chart obtained in the above-discussed manner is processed in accordance with a thermal analyzing method prescribed by Japanese Industrial Standards (for example, JIS H7101), such that 11 1__ 151 511 1 _1 81ll B111811 11111- 111541,l115 1111111 1 1 11 111 1111111111 11111 it,l1 1 11 11ilEEEI

- 13 some tangent lines may be drawn on the curves of the chart, thereby calculating and thus obtaining a combustion starting temperature, a combustion ending temperature as well as a combustion time, in accordance with two intersection points PI and P2 of the two tangent lines. Here, in order to calculate and thus obtain the above temperatures, two vertical broken lines are drawn from the above points PI and P2, in parallel to the temperature axis. The two broken lines are intersected with the temperature curve, forming two intersection points T' and T2, thereby the respective temperatures can be read and obtained from these intersection points To and T2.

Results of the experiment are shown in Table 2. Here, the order of the tested 39 elements is in line with an experiment order, while the compounds actually added in the graphite pieces are represented in their molecular formulas.

Without adding any salts, and in place of salts, distilled water is dropped to the graphite pieces followed by a drying process, then blank samples can be obtained. The blank samples show an average combustion starting temperature of 850 C. At this time, the temperature of 8509C is used as a reference temperature, and then a difference between the reference temperature and the sample data of each of the 39 elements is referred to as "combustion starting temperature deviations.

The elements which are listed in Table 2 and which affect the combustion starting temperature of the graphite can be classified as follows: (i) Elements which lower the combustion staring temperature.

From the most effective element to the least effective one.

- 14 Pb, Bi Rb, C8, Pd, it, V, Ag, Na, Cr, Ce, Mo, Li, Co, Cu. Sr, Rh, Cd, Fe, W. Ca, Sb (ii) Elements which raise the combustion starting temperature.

From the moat effective element to the least effective one.

P. Al, Mg, B. Nb, Mn (hi) Elements which do not show remarkable effect on the combustion starting temperature. Namely, the elements with which the combustion starting temperature wee not changed more than850 C + 60 C.

La, Zn Sn, Y. Hg, Ba, In, Ga, Zr, Ti Hi In view of the above results, it can be presumed that although not used in the present experilnent, acme platinum group elements such as Ru, Oe, Ir, Pt and Au, and some rareearth elements such as Pr, Nd, Pm, Sm. En, Gd, Tb, Dy, Ho, Er, Tm and Lu, also have an effect on lowering the combnation starting temperature of graphite.

(TI) EXPERIMENT ON COMPOUND ADDING AMOUNT AND

EFFECT OBTAINABLE BY AI IT N.

Next, the inventor of the present invention conducted an experiment on compound adding amount and the effect obtainable by combined addition.

In preparing compounds for use in the present expel iment, potassium (an alkali metal) and lead (a heavy metal) were selected from the elements showing a remarkable effect on the combnation starting temperature among the elements listed on Table 2.

And, an added compound was nitrate respectively. Then, in the same procedure as used in the above experiment 0), the compounds were 111111111 11 1 In 111_1111111115! 11 111 1 1 11111111 11111 111! 1 1 1 1111111 15 1

- 15 added as an aqueous solution to graphite samples, followed by a drying process, and an investigation was conducted to investigate a catalytic effect obtainable when the graphite is burned in air.

Results of the present experiment are shown in Table 3.

[Table 3]

E I'd ACT OBTAINABLE BY ADDING CERTAIN AMOUNT OF ELEMENTS FOR COMBUSI ION OF GRAPHITE (IN AIR)

-ADDING AMOUNT AND EFFECT OBTAINABLE BY COMBINED ADDITION-

(1) (2) - (3) 1 (4) (5) - (6) 1

[ ma] [X] i No. KNO3 Pb(NO3)2 [mg] | K Pb [ C] [ C] 30 -I - 10.29 _- - 850 +0

31 D 39 _ 10 70 076 6 74 176

33 _ 0.16 10.35 _ 0.97 530 -320

1 34 0.34 11.00 1.93 562 -288 1

35 0.22 0.17 10.65 0.80 1 0 97 622 -228

36 0.48 0.34 1 10.87 1 1.70 1.95 634 -216

Explanation of columns in Table 3: (1) experiment No. (2) added compound amount [mg] (3) sample graphite amount [mg] (4) element adding rate of added elements to graphite [%] (5) combustion starting temperature [ C] (6) combustion starting temperature deviation t C] In Table 3, the top row data shows the results of the experiment in which distilled water was added to the sample and then dried. It can be seen that a combustion starting temperature of it was 850 C which is the same as that of the above experiment (1).

On the other hand, with the other samples to which one or more compounds have been added, it wee found that their combustion starting temperatures have been lowered irrespective of whether K and Pb were used singly or in combination.

Further, with respect to an adding amount of the compounds, it was found that when the adding amount exceeds a specific value, the combustion starting temperature is increased by contranee. It wee also found that a high catalytic effect can be obtained when the compound adding amount is controlled at 1% or lees.

Accordingly, it iB understood that for effecting an o cidative combustion of the graphite for use in nuclear reactors, it is not necessary to add a large amount of elements, such as K and Pb. Further, it is understood that a high catalytic effect can be obtained when the compound adding amount is controlled at 1% or less with respect to the weight of the graphite, irrespective of whether these compounds are added singly or in combination (TII) EXPERIMENT OF THE EFFECT OF ADDED COMPOUNDS

WHEN GRAPHITE IS BURNED AN A tOf;P E HAVING A HIGH CONCENTRATION OF OXYGEN.

It is clearly understood from the aforesaid Japanese Patent No. 3051859 that the combustion starting temperature of graphite can be made lower and the combustion speed can be made quicker in an atmosphere having a high oxygen concentration than in air. It has also been made clear that graphite bloclcs can be subjected to an oxidative combustion with their block shape unchanged.

In the present invention, pure graphite pieces were used to prepare samples to which one or more compounds have been added. The .4.. l l,'.i.. '..; al lel';'llIlll le llllllil Illl I I Ill lll lll'li 11115 ACME

prepared samples were burned in an atmosphere having a high oxygen concentration, thereby marring a comparison between a combustion at this time and a combustion in air.

Procedure used in this experiment is just the same as that used in the above experiments O) and (I0, however, combustion wee performed in an atmosphere having a high oxygen concentration, that is, with oxygen of 90 volume % concentration (and nitrogen of 10 volume % concentration), and the results are shown in Table 4.

[Table 41 (on next page.)

- 18 _- | o E I _; 1 if' o o | Ad I it r r t O.C X o O _ _ _ _ _ _ k = 0o O O O O O Z Z 9. _ _ _ _ c Z= + 1 8 8 8

O Mu r r r C 9 -

O̳z = of To =- 8 8 C _ rl _ _ rl -iC 'Sc o c = i' L E o 1 9 S S : 8

a z = _ 0 ' _ _ 0-| E a; | S E | o o o | o I N I e l. E l E E E 0 iZ; LLL L I 1- 1 L o E l S - 3m 5 =- O -i, 0 -. -

1 o 1 = = 1 1 = t. 8 E V I _ o | | et | | | I [ Q -

I.11. 1,1111 1 11 1 5111 1 111111511181 11 11 111;1 niBlli

- 19 It has been confirmed by a comparison between Table 3 and Table 4 that the combustion starting temperature of graphite can be made lower in an atmosphere having a high oxygen concentration (Table 4) than that of in air Cable 3) an d cuesed in Japanese Patent No. 3051859, even when one or more compounds have been added.

As to the concentration of oxygen, it has been found that there was no significant difference in effectiveness between the o cidative combustion performed with oxygen of 50 volume % or more concentration and the oxidative combustion performed with oxygen of 100 volume % concentration. As the concentration of oxygen began to fall and approach to 20 volume % concentration, the effectivenese of oxygen-rich atmosphere was gradually decreased. Even though, same effectiveness wee observed m its own way.

OV) RELATIONSHIP BETWEEN COMBUSTION TEMPERATURE

AND COMBUSTION VELOCITY.

Referring to Table 4, if an attention is paid to Combustion velocity" which can be calculated from a time period necessary for completely burning entire amount of graphite, it can be understood that the graphitecontaining one or more added compounds has a lower combustion velocity than that of not containing the compounds. In other words, graphite containing one or more added compounds needs a longer time for completely burning the entire amount of the graphite, than another graphite which does not contain such compounds.

In order to make clear the above diacueeion, the inventor of the present invention has obtained another TG-DTA chart where Pb has been added as shown in lTig. 2, using the same method as used in the above experiment U)

- 20 It is understood from Fig. 2 that an oxidation of a graphite containing an added lead chows several endothermic reaction peaks along DT The reasons as to why the above phenomenon occurs may be explained in such a manner that during an oxidation of a graphite, lead is reduced and carbonated and then loses its catalytic effect, however, such lead is oxidized again and becomes a combustion catalyst again.

Here, the inventor of the present invention directed his attention to the results obtained when Pb and K were added in combination, and found that during the combustion of the graphite, the combined addition acts to prevent Pb from losing its catalytic effect.

In Table 4, as shown in Experiment No. 46, when Pb is added in an amount of 0.97% and K is added in an amount of 0.80%, the combustion starting temperature was lowered by 228 C and the combustion ending temperature was lowered by 1641C, however, its combustion velocity was 4.20 mg/min that is nearly as high as that (4.31 mg/min) of the example which does not contain any added compounds.

This should be considered as an aetonwhing result in light of common knowledge ate-out temperature dependency nature, that "the velocity of a chemical reaction is sharply increased as the reaction temperature is increased.

When using a catalyst containing both Pb and K in combination, at first Pb provides an effect of lowering the reaction starting temperature, so that the graphite begins to be oxidized at a temperature of 5529C that 228 C lower than that of the example which does not contain any catalyst. Then, half way through the reaction process, E acts to prevent the catalytic effect Mom losing, SO that the oxidation goes on smoothly and ends at a temperature of 652 C that is 164 C lower than the _...,_e,_' i, i,'' ',e, Ha. e a Ace' a 11 111 1 151 111 1 11 -! 111 1! 1111

- 21 combustion ending temperature (816 C) of the graphite in Experiment No. 40 which does not contain any catalyst. At this time, the total amount of the graphite has been burned completely. Further, its combustion velocity was 4.20 mg/min that is almost the same as that (4.31 mg/min) of the graphite in Experiment No. 40 which does not contain any catalyst.

Referring further to Table 4, when Experiment No. 45 is compared with Experiment No. 46, it can be understood that there is a proper catalyst adding amount, similarly to the combustion conducted in air.

Experiment No. 46 having a catalyst adding rate of higher than 1% was proved to have a lower catalytic effect than Experiment No. 45, in all aspects including combustion starting temperature and the combustion ending temperature as well as the combustion velocity.

In view of the facts discussed above, it is conceived that when a heavy metal having an effect of lowering the above temperature is combined with an alkali metal also having an effect of lowering the temperature at the same time, it is possible to effect a control on the combustion starting temperature and the combustion ending temperature as well as the combustion velocity.

It iB also possible to replace K with an alkali metal such as Rb, CB, Na and Li and to replace Pb with Bi Pd. V, Ag, Cr. Ce, Mo, Cu. Sr, Rh, Cd, Fe, W. Ca, Sb, platinum group elements, rare-earth elements, and the line.

Further, it is presumed that the above elements can be used not only singly, but also in combination including two or more sorts of these elements in accordance with characteristics of each element.

[EMBODIMENT

An embodiment of the present invention will be described with

- 22 reference to Figs. 3 and 4.

Graphite bloclre 1 used in a nuclear reactor are materials to be subjected to an oxidative combustion in the present invention.

Although it iB not shown in the drawings in detail each graphite block iB in the form of a column substantially hexagonal in section, having a bore formed at the center for insertion of fuel rods through the bore. When in a nuclear reactor, since the hexagonal columns with different length are combined together to form a large block moderator, each hexagonal column has linear recesses and linear projections formed along its longitudinal direction. Each graphite bloclc has a diagonal length of about 24 cm, a longitudinal length of about 40 to 85 cm, and a weight of above 35 to 70 kg.

In a nuclear reactor, besides the above graphite materials, other kinds of graphite is used, for example, graphite for forming fuel sleeves and graphite assembly for specific functions for use in the nuclear reactor. Unless these graphite materials are pulverized specifically for the purpose of burning treatment, they are still referred to as graphite blocks generically in the method of the present invention, even if they are more or less crushed or severed, more specifica kr, the Mung a of 1 mm or more in screen size are included.

Further, graphite materials in the form of broken pieces which are difficult to be handled as a block, can be formed into one body by using plastic, cloth, paper, or wooden packing materials which do not bring about any unfavorable influence on combustion. These graphite still fall within the concept of the above-mentioned graphite blocks.

The graphite blocks 1 are moved to a catalytic solution coating apparatus 2 in which the graphite blocks 1 are coated with a catalytic solution 2a supplied from a catalytic solution dissolving and stonog .,_,_,,,,,_ t',,, r, _,,_,,.,, _, I,_ _ B.1 1 1! 111 1 111 1 11

- 23 apparatus 3 provided for dissolving the catalyst and adjusting its concentration in advance.

It is possible to select various coating methods for coating the graphite blacks with the catalytic solution. For example, the graphite blocks 1 can be dipped into the catalytic solution 2a, or the graphite blocks can be coated with the catalytic solution using a brush (these two examples are not shown), or spraying can be applicable as shown in Fig. 4. Among them, spraying is the simplest and the moat convenient on the point that it allows easy determination of the quantity of catalytic solution 2a.

Further, as to a spray, it is of course possible to use not only a pump pressure type, but also an air pressure type.

Moreover, when a plurality of different elements are to be added to the graphite blowfly, the addition can be effected either using an aqueous solution in which these different elements have been dissolved in advance, or these different elements may be added to the graphite blocks individually and successively. Both methods make no significant difference in the constitution of the present invention.

Then, as to a surfactant, it may be minced into the catalytic solution 2a in the storing apparatus 3 by an agitator 3a, or it may be applied to the graphite blocks 1 in advance of the addition of the catalyst.

Subsequently, the graphite blacks 1 coated with the catalytic solution 2a are transported to a catalyst drying apparatus 4 to be dried therein, thereby a solvent of the catalytic solution is removed.

It is also possible to select other kinds of drying methods, including natural drying and another separate drying.

Further, once a combustion furnace is operated, an excessive heat is produced from an entire system, 80 it can be conceived to recover and

- 24 malre use of the excessive heat.

If a water content in the solvent iB not so large, it may be no problem to make use of a heat generated within the furnace to effect the drying and evaporation without using any specific drying means. But it should be noted that there may be a possibility by accident that the catalytic solution 2a begins to drop and gather downwardly during the drying process, with a result that the presence of the catalyst becomes uneven. Incidentally, it is not an absolute requirement to include the catalyst drying apparatus 4 in the constitution of the present invention.

The catalyst drying apparatus 4 shown in Figs. 3 and 4 receives a high temperature gas from a heat source 5 for drying.

If the drying apparatus is an electric heating type, an electric power is supplied from the heat source 5.

This heat source 5 can utilize a chemical reaction heat by a direct or indirect heat exchange after the combustion furnace started to work.

A wet gas 6a generated after separating the water component from the solvent by the drying apparatus 4 is discharged by a drymg gee discharging apparatus G. Such a wet gas 6a can be diseh ged outside environment because usually it does not contain any radioactive substances unless the heat source contains any radioactive ingredients.

In this way, the graphite blocks 1 containing the added catalyst are subsequently transported from an air-tight graphite loading apparatus 7 to a combustion furnace 8.

An oxygen lea is fed from a high concentration oxygen generator 10 to the combustion furnace 8, thereby causing the graphite blocks 1 to burn. As the high concentration oxygen generator 10, for example, it iB ,,,_,,,,,,, ,,,_.,, -r 1 1 1111 111_ 11111 111 1 1 1111 Biilil 111111111111 1 1 1 1 111 1 111 111111

- 25 possible to employ an apparatus baaed on a PSA (I?resaure Swing Absorption) process utilizing a molecular sieve, or an apparatus using a selective transmission diaphragm, or an apparatus based on a separation of oxygen from a liquified air.

At the time of starting an oxidative combustion, similar to what discussed earlier in the present specification, since part of the graphite

blocks 1 within the furnace have to be heated to a combustion starting temperature, fuel 9 for preheating to combustion is burned within the furnace 80 as to raise the temperature of the graphite 1.

Reference numeral 11 represents a residual ash discharge lid.

A gas obtained by combnation in the combustion furnace 8 is moved to a gas cooler 12 so as to lower its temperature.

The gee cooler 12 is a boiler in fact. A cooling water 13 is caused to flow through the combustion gee 17 and a heat transfer tube, so that the temperature of the combustion gee 17 is lowered and the cooling water 13 is changed to steam or warm water 14.

The combustion gee 17 with its temperature lowered is treated easily by a dust collection treatment and gas treatment apparatus 15.

This gas treatment apparatus 15 iB meant to generically represent a series of equipment arranged together for cleaning flying ashes in the aforesaid gas and for removing gaseous impurities in the aforesaid gas.

A fan 16 is prodded for operating the whole system including the combustion furnace at cub-atmospheric pressure and for making up for a pressure 10BB in each equipment and at each joint portion. Such a fan can be also disposed before the dust collecting gas treating apparatus for the same purpose.

combustion gas 17 coming out of the fan 16 is processed aB needed in view of different conditions.

- 26 Since the combustion gas 17 contains more or less nitrogen and argon besides carbon monoxide and carbon dioxide, this combustion gas should be subjected to separation treatment if necessary. Further, if environmental conditions allow, it is possible for the combustion gas to be directly discharged to the surrounding atmosphere or to be poured to sea. INDUSTRIAL APPLICABILITY

The present invention was constructed as described above, it is possible to control the oxidation starting temperature and oxidation velocity of a radioactively contaminated graphite, such as a graphite for use in a nuclear reactor (hereinafter referred to as nuclear reactor graphite), without being changed its block shape. The advantages obtainable by performing such control will be described as follows.

(i) ADVANTAGES OBTAINABLE BY LOWERING THE OXIDATION STARTING TEMPERATURE.

When nuclear reactor graphite iB burned without being changed its block shape, a preheating treatment is Usually conducted as described above. At this stage, when an oxidation at a relatively low temperature, it becomes easy to start the operation of an oxidizing equipment. Once an oxidative combustion is started even partially, such a partial oxidative combustion can spread to an entire combustion furnace, thereby causing an o cidative combustion over a wide range.

Further, if an oxidation temperature low, a carbon monoxide generating rate during the oxidation also becomes low, and then the concentration of carbon dioxide iB increased. Accordingly, if it iB desired to increase the concentration of carbon dioxide in the generated gas, it iB , ___,,, __,' 1 1 t1. 1_11_11-1 '11 11 11 11111 11 1111 - 1111 1 111 1 1 11_11 111 1 51

- 27 effective to lower the combustion temperature. Moreover, if several different catalysts in combination are added to the graphite blocks, it becomes possible to prevent an undesired partial drop of an oxidizing reaction, such a partial drop may occur when the combustion temperature is lowered too much, as a result ensuring a stabilized and continued oxidizing reaction.

In addition, if the combustion temperature is allowed to be relatively low, a heat proof designing for an oxidation reactor can be made easy, making it possible to use some refractory materials which are cheap in price and easy to handle. It is also possible to design water-

cooling metal wall to form the oxidation reactor.

If the temperature within the reactor is low, it can be made sure that the combustion ashes do not get melted, so that it is possible to prevent a quick wear of the reactor body and refractory material Further, since there is no melted corrosive ashes adhering to the pipes within the heat exchanger, it is possible to prevent the metal material portions such as the pipes of the heat exchanger from being affected by high temperature corrosion. As a result, the operation and the maintenance of the combustion furnace and the gas cooler can be made easy. Besides, since the combustion residual ashes are free from melting and adhesion, the combustion residual ash treatment can also be made easy.

In the case where the gas obtained after combustion is to be compressed and liquified in a subsequent step, it is required that a harmful carbon monoxide be burned by using an after-burner 80 that it can be converted into carbon dioxide. However, in the present invention this step can be omitted.

(ii) ADVANTAGES OBTAINABLE BY RAISING THE

- 28 OXIDATION STATING TEMPERATURE.

An oxidation reaction of graphite can produce a higher generation rate of carbon monoxide at a higher temperature. If it is necessary in a subsequent step to conduct separation and concentration of carbon, which is radioactive isotope contained in the combustion gas 17, carbon monoxide is better as a gas to be supplied to a separation step in terms of efficiency of separation of isotopic carbon irrespective of what method is used in an isotope separation, because carbon monoxide contains larger carbon ratio than carbon dioxide in terms of molecular weight, as carbon monoxide contains lees oxygen ratio than carbon dioxide. Further, even if an oxidation is conducted at a high temperature, at the start of the operation of the graphite combustion furnace, in order to partially start combustion and then increase the combustion temperature afterwards, it is effective to perform a combined operation in which a low temperature combustion catalyst is attached to part of the graphite in the combustion furnace at which an initial ignition of the graphite is conducted, while a high temperature combustion catalyst is attached to other portions of the graphite in the t n- ii AB described above, the present invention provides a method for oxidizing a graphite used in a nuclear reactor and having a radioactivity.

In particular, using the invented catalyst technology of the present invention, it is possible to select a combustion starting temperature as well as a combustion velocity from a wide range in consideration of total evaluation of a designed structure of a combustion furnace, conditions for operating the combustion furnace, the maintenance of the combustion furnace and its auxiliary equipment, the composition of generated gas, and subsequent steps therefor.

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Claims (1)

1. Claims

   I. A method of controlling an oxidative combustion of a graphite which was used in a nuclear reactor and has been radioactivated and/or radioactively contaminated or at least has such a possibility of having been radioactively contaminated, characterized in that the method comprises the steps of adding one or more catalytic compounds to the surface of graphite blocks used in the nuclear reactor, wherein the graphite blocks having a block shape as the same shape as that taken during use in the nuclear reactor; and performing an oxidative combustion of the graphite blocks in an oxygen-rich air supplied to a combustion furnace. thereby producing gases as well as ashes.

   9. The method according to claim 1, wherein an oxygen concentration of the oxygen-rich air is equal to or higher than the oxygen concentration of air, but lower than 1 00% oxygen.

   3. The method according to claim I or 2. wherein the catalytic compounds to be added to the surface of the graphite blocks are selected from compounds containing alkali metal elements and/or heavy metal elements which are effective for lowering an oxidation starting temperature and/or an oxidation reaction continuing temperature.

   4. The method according to claim 3, wherein the alkali metal elements include lithium, sodium, potassium, rubidium, and cesium.

   5. The method according to claim 3 or 4, wherein the heavy metal elements include lead bismuth, palladium, vanadium, silver? chromium, cerimn, molybdenum, cobalt, copper. strontium, rhodium, cadmium, iron, tungsten, calcium, antimony, platinum group elements and rare-earth elements.

   (. The method according to any one of claims 3 to 5, wherein the catalytic compounds containing alkali metal elements and heavy metal elements are formed by a combination of two or more kinds of compounds containing alkali metal elements or heavy metal elements.

   7. The method according to any one of claims 3 to 5. wherein the catalytic compounds are formed by a combination of heavy metal compounds and alkali metal compounds.

   X. The method according to any one of claims 3 4, 5, 6 or 7, wherein the amount of catalytic compounds is such that catalytic elements are about 0. 01% to about 1% of the total amount by weight of the graphite used in a nuclear reactor or supposed to have been used in a nuclear reactor or that catalytic elements are about 0.05 g/m2 to about g/m2 on the basis of the surface area of the surface of the graphite.

   9. The method according to any preceding claim wherein a surfactant is added to the catalytic compound or the surface of the graphite blocks is coated with the surfactant in advance. 1(). The method according to claim 9, wherein when the surfactant contains an alkali metal. the content of the metal is made to fall within the range of the amount of catalytic compounds recited in claim 8.

   11. The method according to any preceding claim, wherein the catalytic compounds to be added to the surface of the graphite blocks are selected from compounds containing phosphorus aluminium, magnesium, boron, niobium, and manganese, all of which are effective for raising an oxidation starting temperature and/or an oxidation reaction continuing temperature.

   1. An apparatus for controlling an oxidation combustion of a graphite used in a nuclear reactor and has been radioactivated and/or radioactively contaminated or at least has such a possibility of having been radioactively contaminated, characterized in that a combustion furnace connected with a high concentration oxygen generator is provided as a later stage of a catalytic solution coating apparatus for adding catalytic compounds to the surface of the graphite blocks used in a nuclear reactor.

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