GB2359923A

GB2359923A - Method of processing the graphite used in nuclear reactors

Info

Publication number: GB2359923A
Application number: GB0005248A
Authority: GB
Inventors: Shigefumi Kuroyama
Original assignee: Nuclear Services Co
Current assignee: Nuclear Services Co
Priority date: 2000-03-01
Filing date: 2000-03-03
Publication date: 2001-09-05
Anticipated expiration: 2020-03-03
Also published as: GB2359923B; FR2805919A1; FR2805919B1; GB0005248D0

Description

2359923 1 MIETROD OF PROCESSING THE GRAPHITE USED IN NUCLEAR REACTORS The

present invention relates to a method of processing the graphite used in nuclear reactors, which is employed in, e,g., dccominissioning reactors of nuclear power plants, especially graphite-moderated nuclear reactors or graphite sleeves of nuclear fuel.

A core of a graphite-moderated nuclear reactor is composed of graphite moulded blocks stacked in multiple layers. The graphite blocks are radioactive because they are exposed for a long period of time to irradiation by neutrons radiated ftom nuclear fuel assemblies during, operation of the reactor.

Radioactive nuclidcs in those graphite blocks include radioactive carbons originating from nitrogen in air that is contained in bubbles present in the graphite, radioactive elements originating from impurity metals present in the graphite, other rare. gases, etc. Separating and enriching those radioactive elements from the graphite blocks is an important factor in dismantling a graphite-moderated nuclear reactor to be decommissioned and reducing resultant waste. It is however impossible to bring carbon in the form of solid graphite directly into the processes for separation, enrichment, and conversion, etc.

For that reason, processing the graphite has never been carried out in any countries when decommissioning nuclear reactors that employ graphite as a neutron moderator. The decommissioned reactors are left stored as they are.

As generally known, graphite is vaporised in very high temperature condition of the order of several thousands degrees Celsius, but processes at such a high temperature level is not actually practicable. It is therefore conceived to vaporise graphite in the form of a compound.

Also as is well known, burning graphite converts carbon into carbon dioxide ancL/or carbon monoxide in accordance with the following reactions:

C + 02 - C02 2C+02- 2C0 In attempting to burn graphite blocks for a nuclear reactor in air, however, the above oxidation reactions hardly proceed because the graphite blocks have high density and small surface area and hence the graphite temperature cannot be maintained within the range of reactions because of excessive heat losses. Experiments have proved that graphite can he oxidised by heating in an electrical furnace or direct heating by conducting electric power. As one inethod of increasing the volume of graphite and an oxidation reaction area per unit weight to develop satisfactory oxidation, it has been proposed to pulverise the graphite blocks beforehand and burn the pulverised graphite in a fluidising process.

Oxidation combustion of graphite using such a method apparently requires complicated equipment for preventing diffusion of radioactivity, in addition to work of heatina and holding the graphite blocks at a high temperature and intricacy in pulverising graphite. Another disadvantage is that the problem of secondary radioactive contamination of the equipment must be overcome. Furthermore, nitrogen not taking part in combustion occupies about 80% of air and dilutes carbon dioxide produced. thereby increasing the amount of combustion gas without any benefit.

The present invention has been made with the view of solving the problems set forth above, and its object is to provide a method of processing the graphite used in nuclear reactors, which can remarkably reduce secondary radioactive contamination of equipments and materials used for processing die graphite with radioactivity, and which can process the graphite effctively.

3 The present invention provides a method of processing the graphite used in nuclear reactors, the method comprising the steps of introducing the graphite used in nuclear reactors to a combustion furnace, wherein the graphite having a block shape as the same shape as that taken during use in the nuclear reactors; subjecting the graphite to oxidation combustion with oxygen of 50 to 100 volume % concentration supplied to the combustion flirnace, thereby generating gas and ash; and cleaning the generated gas through filtration and releasing the cleaned gas into the atmosphere or submitting to subsequent processes.

A graphite processing method comprising the step of subjecting the graphite used in nuclear reactors to oxidation combustion with oxygen of 50 to 100 volume % concentration.

In the above graphite processing methods, preferably, the oxidation combustion is performed in the range of 500 C to 1500 OC.

In the above graphite processing methods, preferably, the oxidation combustion is performed by using water/stearn as a catalyst.

In the above graphite processing tnethods, preferably the oxidation combustion with the high concentration oxygen is performed in two steps.

In the, above graphite processing methods, preferably, the gas generated by the oxidation combustion is heat exchanged with water through a metallic wall interposed between the generated gas and the water, followed by filtration.

In the above graphite processing methods, preferably, the combustion of the graphite is ignited by combustion of a liquid or gaseous fuel.

4 In the above graphite processing methods, preferably. the ash generated by the oxidation combustion is recovered by a ceramic container.

In the above graphite processing methods, preferably, an amount of the injected high concentration oxygen, a concentration of the oxygen, and an amount of the water/steam injected as a catalyst for the oxidation combustion are adjusted in a separate or combined manner during the oxidation combustion of the graphite.

As a result of conducting intensive studies on processing the graphite used in a nuclear reactor, the inventor discovered a technique of burning graphite with high concentration oxygen.

Looking first at experimental results, an experiment of introducing a high concentration oxygen gas consisted of 90% of oxygen and 10% of nitrogen to a graphite block of 2cm x 2cm x 2cm, which had been preheated to 1000 QC in an electrical furnace, proved that the graphite block continued combustion even after stopping external heating, and was totally oxidised and vanished after 70 minutes. It took about 3 hours to bum the same graphite block with air even by continued heating of the graphite block in an electrical furnace. Also, the inventor found that the time required for totally oxidising and vanishing the graphite block was cut down to 60 minutes by adding water vapour to the high concentration oxygen gas so as to produce a moist gas, and therefore water vapour served as a catalyst for oxidation combustion of graphite.

The inventor studied the above experimental results in more detail. The studies confirmed that when heated in air, graphite used in a nuclear reactor slowly started an oxidation reaction at the vicinity of 500 OC, showed a slightly intensified level of oxidation beyond 680 'C, and developed vigorous oxidation combustion in excess of 760 'C. Compared with an oxidation reaction of the graphite in a gas mixture of oxygen and nitrogen in place of air, a significant difference was not found in the oxidation start temperature Of 500 OC, but vigorous combustion begun from 650 'C at an oxygen concentration of 90%.

There was a great variation in data of the oxidation reaction of graphite. As a result of examining the cause, it was found that such a great variation was attributable to an oxide on a brass-plated piano wire which was a wire material for electrical discharge machining utilised in cutting a graphitc sample. Metal oxides such as an iron oxide, copper oxide and zinc oxide promoted oxidation of graphite, and lowered the temperature of 760 'C at which graphite developed the vigorous oxidation combustion by 15 "C.

Using, as an oxidation catalyst, a metal oxide or such a compound as becoming a metal oxide through pyrolysis is however not preferable because the metal oxide increases radioactive residual ash after burning. On the other hand, using water vapour as a catalyst to promote oxidation is advantageous in increasing the oxidising rate of graphite without increasing radioactive residual ash.

The higher the oxidising temperature of graphite, the greater is the oxidising rate thereof. A higher oxidising temperature is therefore preferable in order to increase the quantity of combustion per unit time. Taking into account a practically endurable temperature range of refractory materials in an oxidative atmosphere, however, there is a practical limitation at 1500 OC. If the oxidation temperature exceeds 1000 C, a greater proportion of carbon monoxide is produced in accordance with the following formula, and more secondary combustion to bum the generated carbon monoxide again is required:

2C + 02 - 2C0 In addition, since carbon monoxide is further increased by the endothermic reaction between carbon dioxide generated in a combustion area and graphite not yet burnt, expressed by the following equation, it is the fact that the combustion temperature is controlled, even with equipment including a secondary combustion apparatus:

C + C02 - 2C0 Further, if the temperature rises to a level beyond 1200 OC. there may occur such a risk due to local abrupt heating and a strong flow speed of the generated gas that broken graphite fragments and residual ash are whirled up and moved into an exhaust dust thereby impeding gas flow through an exhaust filter by clogging.

For the above reasons. the combustion temperature, at which graphite used in nuclear reactors is bumt using high concentration oxygen according to the present invention.. is desirably held in the range of 700 cIC to 1300 'C. The present invention however includes not only a process of burning the used gTaphite in the lower temperature range of 500 'C to 700 OC in which graphite can be subjected to a combustion reaction, but also a process of developing a combustion reaction of the used graphite in the higher temperature range of 1300 'C to 1500 OC by using practically usable apparatus materials and control.

Characteristics of graphite used in nuclear reactors are as follows: Graphitisation temperature Apparent density Thermal conductivity Specific heat Rate of thermal expansion Comprehensive strength Tensile strength Bending strength Young's modulus Ash 2750 "C 1.68- 1.75 94 - 162 Kcal/rn.s OC 0.129 KcaUKg 'C 2.66 - 3.63 x E (-6)/T 235 - 439 Kg/cm2 5 1 - 96.5 Kg/cin.2 192 - 24 8 Kg/cm2 9.3 13.5 x E(4)Kg/cm2 100 ppm The present invention is primarily intended to perform oxidation combustion of graphite blocks, which have been used in nuclear reactors, by employing high concentration oxygen.

7 A method of processing the graphite used in nuclear reactors will be summarised below.

The present invention provides a method of processing the graphite used in nuclear reactors, the method comprising the steps of introducing the graphite used in nuclear reactors to a combustion furnace, wherein the graphite having a block shape as the same shape as that taken during use in the nuclear reactors; subjecting the graphite to oxidation combustion with oxygen of 50 to 100 volume % concentration supplied to the combustion furnace, thereby generating gas and ash; and cleaning the generated gas through filtration and releasing the cleaned gas into the atmosphere or submitting it to subsequent processes. The graphite is in the form of acolumn substantially hexagonal in section, having a bore formed at the centre for insertion of fuel rods through the bore, and recesses and projection formed in and on outer surfaces thereof for combination with other blocks of graphite. The columns have a length ranging from about 85cm at maximum to about 40em at minimum, a thickness (diagonal length) of about 24cm, and a weight ranging from about 70kg at maximum to about 35kg at minimum, though not shown in detail.

Figure 1 schematically shows an apparatus applied for implementing a method of processing the graphite used in nuclear reactors according to an embodiment of the present invention; and Figure. 2 is a block diagram showing steps of the method of processing the graphite used in nuclear reactors according to the embodiment of the present invention.

Details of an apparatus used for implementing a graphite processing method of the present invention will be described below. Referring to Figure 1, numeral 1 denotes a gastight loading apparatus for graphite blocks. The loading apparatus 1 comprises a chamber having an inlet 3 through which a graphite block 2 is loaded, 8 and an outlet 5 which communicates with a combustion furnace 4. The inlet 3 and the outlet 5 can be closed respectively by doors 6, 7 in the best possible gastight condition. Conveying devices 8 such as a pusher (not shown) and rollers are provided in the chamber. The loading apparatus 1 is associated with an exhaust device 9 for returning gas leaked from the combustion ft=ace 4 to the combustion furnace 4 and for maintaining a sub-atmospheric pressure in the loading apparatus 1 to prevent radioactive combustion gas from being released to the outside.

The exhaust device 9 comprises an ejector provided between the loading apparatus 1 and the combustion furnace 4. The exhaust device 9 is operated with later-described oxygen (containing a small amount of nitrogen) supplied by a blower 10 so as to maintain a sub- atmospheric pressure in the loading apparatus 1.

Numeral 11 denotes an air intake and numeral 12 denotes a high concentration oxygen generator. The high concentration oxygen generator 12 may take the form of a cryogenic compressive air separating equipment or alternatively compressed oxygen or liquid oxygen may be used. From the viewpoint of convenience in practical use, however, it is advantageous to employ the high concentration oxygen generator 12 that operates based on the principle of a PSA (Pressure Swing Adsorption) process utilising adsorption of nitrogen by a molecular sieve 13. The high concentration oxygen generator 12 comprises molecular sieve 13 which is contained in a container 14 and adsorbs nitrogen while allowing oxygen to pass through the same. Thus, oxygen containing a small amount of nitrogen and argon originated from air (hereinafter referred to simply as oxygen) is supplied to the combustion furnace 4. In practice, two high concentration oxygen generators 12 are provided in parallel and used alternatively to provide a continuous supply of oxygen. Oxygen outgoing from the high concentration oxygen generator 12 has a concentration of 90 to 95%. Where liquid oxygen is used, oxygen has a concentration of almost 100%, Such a case is also included in the scope of the present invention.

9 Nunieral 15 denotes a communicating pipe, 16 denotes an exhaust port for nitrogen, and 17 denotes a tank. The oxygen produced as described above may be directly supplied to the combustion furnace 4, but the oxygen is preferably first supplied to an oxygen moistening device, such as a saturator, 18 for the purpose of increasing combustion efficiency. The oxygen moistening device 18 is intended to moisten the oxygen, and may be constructed by an enclosed container 19 to which water 21 is supplied from a supply source 20 and then heated by a heater 22. The oxygen is supplied to the enclosed container 19 from a supply port 23 for moistening. While equilibrium water vapour is generated under temperature control of the water in the oxygen moistening device 18, a sprayer or the like may also be used instead. As an alternative, the equivalent effect can be achieved by introducing a small amount of water/steam directly into the combustion furnace 4. The moistened oxygen is delivered through an outlet 24 such that a pW of the oxygen operates the exhaust device 9, while the remaining oxygen is directly introduced to the combustion furnace 4.

The combustion furnace 4 is a furnace in which the graphite blocks 2 are contained in number required for preheating and combustion, and they are directly subjected to oxidation combustion. The combustion furnace 4 is formed to have an outer wall endurable against the cornbustion temperature, to provide a mechanical strength, particularly in a bottom portion, endurable against loading of the graphite blocks, and to be gastight to prevent leakage of radioactive gas. In operation, the combustion furnace 4 is operated at sub-atmospheric pressure. The outer wall of the combustion furnace 4 is generally fonned of refractory bricks, but it is also effective to employ a directly water-cooled wall of corrosion-resistant steel in consideration of a difficulty in handling the refractory bricks later after being contaminated with radioactivity. The oxygen for combustion is injected the combustion furnace 4 through a plurality of nozzles provided at disp=ed positions for producing uniform combustion. In the case of a cylindrical furnace, it is preferred to inject the oxygen in the tangential direction of a section of the cylindrical furnace so that the oxygen is sufficiently diffused to the graphite surface. The oxygen temperature may also be raised beforehand through heat exchange.

The inventor burnt graphite used in a nuclear reactor by oxidation combustion. Mixed oxides of 0.025 to 0.040 % with respect to the graphite were obtained as residual ash. Analysing typical components of the residual ash resulted in 38 % of SiO2, 37% of Fe7,0.3, 15% of CaO, 4% of NiO, 3% of TiO2, and 3% of other oxides containing a small amount of chlorides.

Since graphite for a nuclear reactor has a high purity, the a-mount of residual ash is small., but 30 - 40 kg of ash is left when 100 tons of the graphite is burnt. While, as described above, the main components of the ash comprises oxides having melting points not lower than 1500 OC the ash has a high melting pQint., a small amount of chlorides and radioactive compounds are contained in the other components. It is therefore preferable to provide a tray 25 of a ceramic or heat-resistant metal on the ftirnace bottom beforehand so that the ash is prevented from permeating into the refractory of the furnace bottom and the ash can be easily taken out. or to lay a ceramic powdery material (not shown) such as silica powder or alumina powder in a layer of 0.5mrn to 5mm so that the ash is prevented from being stirred up as dust.

Numeral 26 denotes a carbon monoxide burning device for carrying out complete combustion of carbon monoxide generated during high temperature combustion of the graphite blocks 2. The carbon monoxide burning device 26 may comprise a ceramic or metal mesh burner (not shown) for achieving complete mixing of oxygen.

When combustion gas is at a high temperature, conversion efficiency into carbon dioxide is reduced. In such a case, the carbon monoxide burning device 26 may be installed in the stage after the gas has been cooled by a part of a gas cooler 27 described below. The gas cooler 27 is a boiler in a sense and is primarily intended to lower the gas temperature through heat exchange between a liquid coolant (water in many cases) and the gas. As a matter of course, constructing an outer wall of the gas cooler 27 as a water- cooled metallic wall similarly to the combustion furnace 4 is one of effective measures against contamination of radioactivity. Furthermore, effective utilisation of heat taken out with a heat medium in a secondary way also falls within the scope of the present invention.

Numeral 28 denotes a cooler body, and 29 denotes heat transfer tubes. Water is supplied through an inlet 30, being heated while flowing in the heat-transfer tubes 29, and exits through an outlet 30a in the form of steam. The steam is cooled by a apparatus (not-shown) for circulation. Numeral 31 denotes a dust collector that is intended to finally trap and collect scattered ash originating from the graphite blocks 2. The dust collector 31 is constructed by using, e.g., sintered filters of a ceramic or metal. Alternatively, the dust collector 31 may be an electrical for example an electrostatic dust collector. Also, any kind of filters to collect the radioactive dust effectively may be applicable. The combustion gas is cleaned while passing through the dust collector 3 1. Numeral 32 denotes a gas treating tower as equipment for absorption and conversion of the water vapour and NOx contained in the,combustion gas in small amount. The gas treating Lower 32 can be constructed by any suitable one of various adsorption and conversion equipment. Also, the gas treating tower 32 may comprise several units of equipment installed for removing the water vapour and NOx separately. Numeral 33 denotes a fan for operating the overall system under a negative pressure. Additional fans may be provided part way along the process or damper control may be employed for the purpose of compensating and controlling a pressure loss in units of apparatus. Numeral 34 denotes an exhaust stack.

The graphite processing method of the present invention implemented by using the above-described apparatus will be described with reference to Figures 1 and 2.

12 The graphite block 2 that has been used in a nuclear reactor is carried as a whole into the combustion furnace 4 through the airtight loading apparatus 1 for isolating the graphite block 2 from open air outside the combustion furnace 4, while the graphite block 2 has the same shape as that taken during the use in the reactor. Since another graphite block 2 is already preheated and subjected to oxidation combustion in the combustion ftimace 4 with auxiliary fuel or electric power supplied to the combustion furnace 4 beforehand, the graphite block 2 in the airtight loading apparatus 1 is preheated by the heat generated in the combustion furnace 4, For igniting the graphite block in the combustion furnace 4 for the first time, liquid petroleum gas (propane or butane) was supplied to the combustion furnace 4 from a gas cylinder 35 at the start of ignition in the embodiment. With this method, no electrodes are employed unlike the case of utilising electric power supply, and therefore processing the contaminated electrodes can be omitted.

Then. the blower 10 introduces air through the air intake 11 to the. ffigh concentration oxygen generator 12 in which high concentration oxygen is generated. In the embodiment. the generated oxygen has purity in the range of 85 to 95%. The oxygen is mixed with water vapour in the oxygen moistening device 18, and then supplied to the combustion furnace 4. The reason why combustion efficiency is improved by moistening oxygen presumably resides in that graphite has a molecular structure of multiple layers and moisture enters between the layers along with oxygen. The moistened oxygen is introduced to the combustion ftimace 4 in such a manner, thereby promoting natural combustion of the preheated graphite block 2. On this occasion, the auxiliary heating at the beginning is stopped and the combustion is primarily controlled by regulating the amount of oxygen while measuring the combustion temperature and the temperature of the generated gas. Also, the amount of water vapour as a catalyst is adjusted b the o en moiste i device 18. Residual ash 36 after burning is very small because the impurity content 1.7 1I.T c I'L A.'L5 13 of graphite used in a nuclear reactor is not more than several ppm from the own specific nature, it is not required to frequently take out the residual ash at regular intervals.

Then, because the combustion gas contains carbon monoxide, the generated gas must be burnt completely again using oxygen for converting the carbon monoxide into carbon dioxide. To this end, the carbon monoxide burning device 26 is employed to burn the generated gas using oxygen.

Since it is sometimes more effective to perform the combustion intended by the carbon monoxide burning device 26 after the generated gas has been partly cooled, the location of the carbon monoxide burning device 26 may depend upon characteristics of the gas cooler 27. In some cases, the carbon monoxide burning device 26 may be incorporated in the gas cooler 27. The gas cooler 27 is advantageously of the indirect heat exchange type. Preferably, dilution cooling with air or the like is not to be performed from the standpoint of minimising the amount of radioactive gas.

For the above reason, a liquid is used as a coolant and a typical effective coolant is water. In the case of using water, the water becomes steam or hot water. Then, the gas passes through the dust collector 31 in which a very small amount of dust, such as ash having radioactivity, is removed. Though depending on the composition of the oxygen gas, if the oxygen gas contains a small amount of nitrogen, the combustion gas possibly contains NOx. The gas is therefore passed through the gas treating tower 32 to remove the water vapour and separate other gaseous components. Finally, high concentration carbon dioxide is obtained and released into the atmosphere through the exhaust-stack 34.

Instead of releasing dust free gas into the atmosphere as described above, the gas may be supplied to a gas compressor 38 through a three-way valve 37 as shown in 14 Figure 1. In this ca-se, the dust free gas is compressed and liquefied in the gas compressor 38. and a resultant liquid carbon dioxide is recovered. Additionally, numeral 39 denotes a valve.

With the graphite processing method of the present invention, graphite used in nuclear reactors can be effectively converted into high concentration carbon dioxide with the least necessary aid materials and energy.

Claims

According to the feature of Claim 1, since graphite used in nuclear

reactors is burnt without being pulverised to pellets or powder, it is possible to avoid an apparatus for pulverising the graphite, and to avoid an increase in the amount of contaminated materials resulting frorn pulverisation of the graphite.

According to the feature of Claim 2, the amount of generated gas to be obtained after burning can be greatly reduced in comparison with the case of burning the graphite with air.

According to the feature of Claim 3. the combustion process of the graphite can be satisfactorily performed in the temperature range of 500 'C to 1500 '>C ftom the practical point of view.

According to the feature of Claim 4, because of using no metal compounds or the like, the graphite processing method can be implemented with a remarkably reduced cost. Further, since the post-processing process requires to be performed only on water vapour, the processing procedure is much easier than the case of processing the contaminated metal compound or the like.

According to the feature of Claim 5, complete combustion can be achieved.

According to the feature of Claim 6, since heat exchange is performed using water through a metallic wall interposed between the generated gas and the water, the amount of materials, such as a radioactive liquid or gas can be reduced in comparison with the case of cooling the generated gas by directly spraying water shower or introducing air or the like.

According to the feature of Claim 7, since the graphite is not ignited by conducting electric power through an electrode, the disposal process is facilitated because the problem of disposal of the contaminated electrode is avoided and gas used for igniting the graphite is disposed along with the combustion gas.

According to the feature of Claim 8, secondary radioactive contamination of a ceramic tray does not reach the inside of the tray, and the ceramic tray requires to be only cleaned at its surface. As a resul the cleaning process is simplified.

According to the feature of Claim 9, graphite blocks can be maintained in a satisfactory combustion state.

16 Claims 1. A method of processing the graphite used in nuclear reactors, the method comprising the step of. subjecting the graphite used in nuclear reactors to oxidation combustion with oxygen of 50 to 100 volume % concentration.
2. A method of processing the graphite used in nuclear reactors according to Claim 1 wherein the method comprising the steps of.. introducing the graphite used in nuclear reactors to a combustion furnace, wherein the graphite having a block shape as the same shape as that taken during use in the nuclear reactors; subjecting the graphite to oxidation combustion within the combustion furnace, thereby generating gas and ash; and cleaning the generated gas through filtration.
3. The method of processing the graphite used in nuclear reactors according to Claim 1 or 2, wherein the oxidation combustion is performed at a temperature in the range of 500 OC to 1500 OC.
4. The method of processing the graphite used in nuclear reactors according to any preceding claim, wherein the oxidation combustion is performed in the presence of added steam.
5. The method of processing the graphite used in nuclear reactors according to any preceding claim, wherein the oxidation combustion is performed in two steps.

17
6. The method of processing the graphite used in nuclear reactors according to any preceding claim wherein the gas generated ky the oxidation combustion is heat-exchanged with water through a metallic wall interposed between the generated gas and the water.
7. The method of processing the graphite used in nuclear reactors according to any preceding claim, wherein the combustion of the graphite is initiated by combustion of a liquid or gaseous fuel.
8. The method of processing the graphite used in nuclear reactors according to any preceding claim, wherein the ash generated by the oxidation combustion is recovered by a ceramic container.
9. The method of processing the graphite used in nuclear reactors according to Claim 4, 5, 6, 7 or 8, wherein an amount of the injected high concentration oxygen, a concentration of the oxygen, and an amount of the injected steam are adjusted in a separate or combined manner during the oxidation combustion of the graphite.
10. The method of processing the graphite used in nuclear reactors according to Claim 5 wherein oxygen is added to the gas from the combustion furnace to further convert carbon monoxide to carbon dioxide.
M- The method of processing the graphite used in nuclear reactors according to Claim 10 wherein the product gas from the combustion furnace is first cooled prior to the addition of further oxygen.
12. The method of processing the graphite used in nuclear reactors according to any preceding claim wherein the combustion step or steps take place at sub-atmospheric pressure.

is The method of processing the graphite used in nuclear reactors according to any preceding claim wherein the graphite is introduced to the combustion surface via a loading chamber.
13.
14. The method of processing the graphite used in nuclear rcactors according to Claim 13 wherein the loading chamber is partly evacuated of gas by an ejector driven by oxygen supplied to the combustion furnace.
15. The method of processing the graphite used in nuclear reactors according to Claim 9 wherein the amount of injected water is adjusted using a gas saturator vessel.