GB2297189A

GB2297189A - Improvements Relating to Nuclear Reactors

Info

Publication number: GB2297189A
Application number: GB9600666A
Authority: GB
Inventors: George Hall; William Andrew Watson Mair; Katherine Alison Simpson
Original assignee: SCOTTISH NUCLEAR Ltd
Current assignee: SCOTTISH NUCLEAR Ltd
Priority date: 1995-01-17
Filing date: 1996-01-12
Publication date: 1996-07-24
Anticipated expiration: 2016-01-12
Also published as: GB9500803D0; GB2297189B; GB9600666D0

Abstract

A problem exists with gas cooled nuclear reactors, such as MAGNOX and AGR reactors, in that reactions occurring in the coolant under irradiation lead to the formation of deposits, eg, of carbon within the primary coolant circuit. Carbon deposits on the boiler and reheater tubes has a detrimental effect on the efficiency of the plant. Accordingly the invention provides a method for at least partially removing deposits from a reactor-side surface(s) of at least one heat exchange apparatus of a gas cooled thermal nuclear reactor comprising injecting a gas in a localised manner into a pressure vessel of the reactor such that a portion of the gas is directed to and reacts with deposits on said surface(s) of said heat exchange apparatus, any remainder of said gas being substantially consumed within said heat exchange apparatus by coolant gas. Advantageously the method may be performed during normal reactor operation. The gas may be predominantly O 2 .

Description

IMPROVEMENTS RELATING TO NUCLEAR REACTORS This invention relates to nuclear reactors, and in particular to a method for the removal of deposits found on heat exchanger surfaces within the primary cooling circuit of a gas cooled thermal nuclear reactor.

Gas cooled thermal nuclear reactors are operated in the UK, and in a small number of other countries to produce power for electricity generation. The thermal energy produced in such a nuclear reactor must be converted to mechanical energy for the generation of electricity and this is ordinarily achieved by means of a steam cycle. The gas cooled reactor is analogous to a conventional fossil fueled plant in that gas at high temperature transmits heat to the steam generation equipment. In Magnox reactors and Advanced Gas Cooled Reactors (AGR's), the two types utilised for power generation in the UK, heat generated in the core is transmitted by means of a coolant including pressurised carbon dioxide (C02).

In addition to C02 the coolant contains other minor components in specified concentrations (known as the coolant advice). For AGR and Magnox reactors these include carbon monoxide (CO), water vapour (H20) and methane (CH4). These additions to the coolant are needed to maintain the function of major reactor components such as the graphite moderator, the boilers and other such items which are not replaced during the operational lifetime of the plant.

A problem exists with gas cooled nuclear reactors in that reactions occurring in the coolant under irradiation lead to the formation of organic compounds which in turn decompose and result in the deposition of carbon within the primary coolant circuit. The deposit is most prevalent on the high temperature heat exchange surfaces, ie boiler and reheater tubes, but is also present on other surfaces such as fuel pins and the steel linings of the circuit. The carbon deposition on the boiler and reheater tubes has a detrimental effect on the efficiency of the plant by reducing the heat transfer between the coolant and the steam cycle working fluid.

The problems of reduced plant efficiency caused by carbon deposition will now be illustrated with reference to the Torness nuclear installation which operates two AGR's. The primary effect of carbon deposition is to reduce the heat transfer coefficient between the coolant and the reheaters. As a result of this, reheater steam outlet temperature falls which results directly in a reduction in reheater thermal power output and steam cycle efficiency. Consequently, as less heat is removed from the coolant in the reheaters, the coolant temperature at the inlet to the main boilers increases.

This tends to increase the boiler steam outlet temperature unless the secondary superheater is coated with carbon deposit. Therefore, in order to maintain certain boiler component temperatures within allowable limits, boiler feed flow must be increased. The effect of this is to increase boiler output but this only partially offsets the loss in output due to the fall in reheater and superheater steam outlet temperatures.

Thus, though the thermal power output of each reactor does not change, the poor quality of the reheat steam results in an unavoidable reduction in steam cycle efficiency and hence generated output.

Inspect ion of deposits removed from the reheater and superheater surfaces of Torness reactor 1 indicate that the deposit consists of a dense sub-layer of carbon approximately 100 microns thick, surmounted by a fluffy surface layer of low density and more variable thickness.

Heat transfer calculations have indicated that the fluffy deposit component would probably have sufficient insulation properties to account for the major part of the reduction in reheater/boiler performance.

In addition to reducing plant efficiency, carbon deposition on internal surfaces also hinders the inreactor inspection of components whose satisfactory condition needs to be established periodically. The physical removal of the deposit is not possible due to the limited access available to the heat exchanger surfaces.

It is known that carbon deposits have in the past been removed from boilers and fuel elements in reactors of the Magnox type by oxygen (02) injection into the coolant. This technique involves the introduction into the coolant of sufficient 02 to convert all the CO present into CO2, CO being the most plentiful minor coolant constituent. Once this has been achieved further 02 is added to provide a coolant composed predominantly of a mixture of CO2 and 02. The mixture is then circulated throughout the entire coolant circuit where the 02 reacts with the carbon deposits to form oxides of carbon.

A drawback with this technique is that it is non selective and cannot be directed to a specific part of the coolant circuit. The C02/02 mixture is circulated circuit-wide and the 02 can act on both core surfaces (e.g. fuel elements and graphite moderator) and out of core surfaces (e.g. boiler/reheater tubes). The oxidation of core materials in an operational reactor (the fuel sleeves and moderators of AGRs being manufactured predominantly from graphite) is especially undesirable. Weight loss through oxidation can seriously affect the mechanical properties of graphite, most notably its strength. This in turn can have a detrimental effect on the fuel sleeve performance and on moderator integrity and hence lead to a reduction in the operational lifetime of the plant.

As can be seen from the above there are strong technical and commercial incentives to control and, if possible, reduce the carbon deposited on the heat exchanger elements of the coolant circuit. These are, however, tempered by the need to control the oxidation of the graphite core and, in the case of AGR reactors, the oxidation of the graphite sleeves which surround the fuel pins in a fuel element.

It is an object therefore of the present invention to obviate or mitigate at least some of the aforementioned disadvantages.

According to a first aspect of the present invention there is provided a method for at least partially removing deposits from a reactor-side surface(s) of at least one heat exchange apparatus of a gas cooled thermal nuclear reactor comprising injecting a gas in a localised manner into a pressure vessel of the reactor such that a portion of the gas is directed to and reacts with deposits on said surface(s) of said heat exchange apparatus, any remainder of said gas being substantially consumed within said heat exchange apparatus by coolant gas.

It is believed that a minor portion of the gas injected in such a localised manner is consumed by the deposits on the surface(s) of the heat exchange apparatus, the remaining, major portion of the gas reacting with component(s) of the coolant within the heat exchange apparatus. In this way any oxygen in the gas does not detrimentally react with core materials since normal coolant composition is maintained within the core.

The at least one heat exchange apparatus may comprise at least one boiler stack which boiler stack(s) comprise(s) part of a plurality of boiler stacks provided circumferentially around a core of the reactor within the pressure vessel.

Advantageously the method may be performed during normal reactor operation.

The gas may be predominantly 02, the deposit being predominantly carbon.

The gas may be controllably injected in a localised manner into the pressure vessel through one or more existing channels (penetrations) of the pressure vessel.

Use of existing penetrations is beneficial in that injection apparatus may be installed and commissioned whilst the reactor is on-load. Suitable penetrations may terminate within the pressure vessel in locations where the injected gas is entrained in the circulating coolant after the coolant has passed over the fuel and before the coolant enters the heat exchange apparatus. The selection of penetration(s) controls the distribution of injected oxygen and hence directs oxygen to areas from which deposit is to be removed.

Suitable penetrations may include Burst Can Detection gas sampling lines, boiler inlet sampling lines, above-gas baffle pressure monitoring lines (also known as low pressure dome D.P. lines), ISI penetrations and/or superheater moisture sampling lines.

Additionally, one can add (possibly off-load) additional pipework to existing penetrations inside the pressure vessel so as to improve the efficiency of distribution of injected gas.

The gas may be blended for injection. In the case where the gas is predominantly oxygen, the oxygen may be blended with a portion of coolant gas drawn from a suitable point in the coolant gas circuit. The oxygen may alternatively be blended with carbon dioxide drawn from the carbon dioxide make-up supply. Alternatively, the oxygen may be blended with a dedicated supply of carbon dioxide.

Thus, the method may further comprise the steps of: bleeding of a portion of the coolant gas from the coolant circuit; and blending said portion of said coolant gas with said gas prior to injecting said blend into said pressure vessel.

The portion of the coolant gas may be bled from the pressure vessel via a sampling line.

The portion of the coolant gas may be compressed before or after being blended with said gas.

The blend may comprise an oxygen concentration by volume in the range 5-35%.

For delivery into the circulating coolant, the gas to be injected should have a pressure in excess of the local coolant pressure at the point of delivery.

The pressure may also be increased to overcome restrictions peculiar to the penetration(s) selected for the injection route.

The required pressure may be obtained for blending of coolant with oxygen by taking coolant gas from a high pressure part of the coolant circuit.

Alternatively, the required pressure may be obtained for blending of coolant by use of a dedicated compressor to compress the coolant preferably before blending with oxygen.

Alternatively, the required pressure may be obtained for blending of make-up carbon dioxide from the apparatus already existing at the nuclear installation for delivery to the reactors of make-up carbon dioxide.

Alternatively, the required pressure may be obtained for blending of make-up carbon dioxide by using a dedicated compressor.

Alternatively, the required pressure may be obtained from a pre-pressurised mixture of oxygen and carbon dioxide.

The extent of pressurisation of gas, immediately after blending, above the coolant pressure at the point of delivery in the coolant circuit may be between 2 and 15 bar.

The blended gas may be injected through the selected penetration(s) at flow rates such that the integrated rate of oxygen delivery to the circulating coolant may be between 0.lg/s and 9g/s per reheater.

The method may further comprise the initial step of purging the necessary channel (s) The gas cooled thermal nuclear reactor may be of the AGR type.

According to a second aspect of the present invention there is provided apparatus for injecting a gas in a localised manner into a pressure vessel of a gas cooled thermal nuclear reactor so as to at least partially remove deposits from a reactor-side surface(s) of at least one heat exchange apparatus comprising: a first pipe for accepting a portion of a coolant gas bled from said vessel; a blending unit for mixing said portion of said coolant gas with a gas; and a second pipe for feeding said blend to a location where said gas can be injected into said vessel.

The invention will now be described with reference to the following drawings in which: Fig 1 shows the general arrangement of the major components of an Advanced Gas Cooled Reactor (AGR) , employing low pressure dome D.P. injection line(s) as the point(s) of gas injection according to a first embodiment of the present invention; Fig 2 shows a plan view of the core and boiler stacks of the AGR of Fig 1; Fig 3 shows a view of a dome D.P. line in a quadrant of the AGR of Fig 1 along with a boiler inlet sample line; Fig 4 shows the general arrangement of the major components of an AGR employing Burst Can Detector (BCD) sample pipe(s) as the point(s) of gas injection according to further embodiment of the present invention; and Fig 5 shows a more detailed view of a BCD sample line of the AGR of Fig 4.

Referring firstly to Figure 1 there is shown a thermal nuclear reactor, generally designated 1, conforming to AGR classification, comprising a core 5 surrounded by a thermal shield/gas baffle 30. Positioned around the periphery of the shield/baffle 30 are a plurality of boiler stacks 10, each one incorporating in an upper portion a reheater unit 15. Figure 2 shows the circumferential arrangement of the boiler stacks 10 relative to the thermal shield/baffle 30 and pressure vessel 25. The lower portion of the boiler stack 10 comprises (from bottom to top) economisers, evaporators, primary and secondary superheaters.

The core 5 consists essentially of fuel elements (not shown) supported in an ordered grid like arrangement and surrounded by a suitable neutron moderator. In the case of an AGR the neutron moderator is manufactured from graphite and the fuel is in the form of an enriched uranium oxide clad in stainless steel. In addition to fuel channels 80 in the moderator for receiving the fuel elements, the core 5 also includes conduits for control rods (not shown) which regulate the operation of the reactor 1.

The core 5, thermal shield/gas baffle 30 and the boiler stacks 10 are housed within a biological shield (pressure vessel) 25. This acts both to contain the harmful radioactive elements of the reactor 1 and as a pressure vessel, and is constructed from pre-stressed concrete.

In use, a controlled self sustaining nuclear fission chain reaction within the core 5 produces energy in the form of heat. This heat is removed from the core 5 by means of a pressurised gas coolant contained within the biological shield/pressure vessel 25. The coolant is circulated within the vessel 25 by a combination of convection and the action of a plurality of circulation pumps 20. The main coolant circulation paths are depicted by arrows 35. The coolant transfers heat from the core 5 to the reheaters 15 and to the boiler stacks 10 where it is utilised to raise steam. The steam is then expanded through turbines and the mechanical energy produced thus is used to generate electricity, as is known in the art.

The AGR coolant is typically composed chiefly of CO2 at a pressure of about 40 bar. Mixed with the CO2 are a number of minor constituents such as CO, H20 and CH4, which are required to maintain the function of major reactor components. Reactions in the coolant under irradiation lead to the breakdown of some of the minor constituents. To maintain a specified coolant composition the coolant must be continually treated to remove breakdown products and replenish methane. The reactor 1 is, therefore, provided with a coolant treatment line 40 (gas bypass plant) which allows a fraction of the coolant flow to be removed from the reactor 1, treated and returned.

Referring to Fig 1, the reactor 1 is also provided with a carbon dioxide (CO2) make-up line 36 which line 36 may be connected to a CO2 source (not shown) for replenishing CO2 lost by leakage.

Irradiation of the coolant leads to the formation of organic compounds which in turn decompose and result in the deposition of carbon within the reactor 1. As stated previously these deposits, when situated on the boilers 10/reheaters 15, have an adverse effect on the efficiency and output of the plant as a whole. The present invention seeks to alleviate these problems by the introduction of 2 in measured amounts into the vicinity of the fouled boilers 10/reheaters 15. A detailed description will now be given of one way of performing the method according to the present invention.

According to a first embodiment of the invention, coolant is bled from the coolant treatment line 40 through a junction 45 and passed to a compressor 50 before being conveyed to a blending unit 55. Once in the blending unit 55 pure 2 can be mixed with the coolant to produce an 02/C02 mixture of the desired variable proportions. These proportions will be apparent to a skilled person. In this embodiment, the mixture can then be passed via pipework 65 to one or more low pressure dome D.P. lines before being reintroduced into the reactor 1. The mixture is communicated through the pressure vessel 25 by means of the low pressure dome DP line(s) 70 which terminate(s) within the pressure vessel 25 above the gas baffle 30.A low pressure dome DP line 70 is provided in each quadrant of the reactors at Torness in conjunction with a twin line which terminates under the gas baffle 30 to measure the pressure drop across the baffle 30 as shown in Fig 3.

During 02/C02 injection the low pressure dome DP line(s) 70 cannot be used for their/its normal function, but can easily be reinstated when required. The dome DP pipework is shown in more detail in Fig 3.

The in-reactor termination of the line(s) 70 may be modified to improve oxygen distribution without impairing the normal function of pressure differential measurements. The injected gas mixes with the main flow of reactor coolant which has passed up through the core 5, and is carried to the reheater unit 15 in one or more quadrants as depicted by arrows 35.

However, prior to the injection of an 2/C 2 mixture of sufficient proportions to remove carbon deposits from a reheater 15 or boiler 10, an operation to clean the chosen low pressure dome DP line(s) 70 is preferably carried out. For this the blending unit 55 produces an 02/C02 mixture with an 2 concentration of less than 5% by volume, typically 1-2%. This mixture is passed through the chosen low pressure dome DP line 70 to purge it of any carbon fouling.

Once this operation has been completed the blending unit 55 is set to produce an 2/C 2 mixture with an 2 concentration of between 5% and 35% by volume, typically between 10% and 25%. This second mixture is injected, via the same low pressure dome DP line 70 as mentioned in the previous paragraph, at a flow rate which results in a 2 concentration of, for example, 0.5 to 3g/s in the vicinity of the chosen reheater 15. It is envisaged that these concentrations will have to be maintained for periods of a few days to a few months depending on the amount and location of deposit to be removed. Progress can be followed via increases in reheater and superheater steam outlet temperatures.

The introduction of 2 into the reactor 1 results in a number of chemical reactions. The introduction of oxygen local to a reheater 15 allows the carbon deposit to react with oxygen at a concentration which gives a practically useful removal rate. It is believed that the deposit consumes only a very small proportion of the injected oxygen. The remainder may be consumed by rapid reaction with carbon monoxide in a reaction considered to be catalysed by undeposited steel surfaces which occurs lower in the boiler stack 10 than the reheater 15.

The presence of 2 in the core 5, as mentioned hereinbefore, is highly undesirable. Therefore the injection flowrate of the 02/C02 mixture through the chosen low pressure dome DP line 70 should be optimised such that effectively all the injected 2 is combined with carbon and/or CO before it leaves boiler stack 10, and any residual 2 is available to be conveyed to the core 5 is inconsequential. The chosen oxygen injection rate and duration will vary depending on a number of factors. These factors will be apparent to a skilled person when presented with this disclosure, and will, therefore, not be discussed further.

According to a second embodiment of the present invention, oxidation of reheater/boiler deposits may alternatively be carried out by injecting carbon dioxide/02 mixtures directly into the coolant through boiler inlet sample pipes. Figure 3 also shows the position of a boiler inlet sampling pipe 90. On the reactors at Torness, a pipe 90 is present in each of two of the four quadrants depicted in Figure 2. Using a pipe 90 as an injection point, it is not as readily possible to direct the injected 2 to specific sites within the reactor 1 to obtain uniform deposit oxidation without internal modifications to allow access to adjacent quadrants.

As an example, after pipework cleaning with a 1% oxygen mixture, oxygen can be injected as an oxygen-rich, c35k by volume, mixture in carbon dioxide at 2g/s of oxygen down a boiler inlet sample line, resulting in a reheater outlet steam temperature recovery of 1OC/day.

According to a further embodiment of the present invention, oxidation of reheater/boiler deposits may alternatively be carried out by injecting Cm2 /02 mixtures through reactor Burst Can Detection (BCD) apparatus.

Referring to Figs 4 and 5 there is illustrated apparatus for use in the further embodiment. In Figs 4 and 5 like parts are identified by the same numerals as in Figs 1 to 3 but suffixed with a ""'.

According to the further embodiment of the invention, coolant is bled from the coolant treatment line 40' through a junction 45' and passed to a compressor 50' before being conveyed to a blending unit 55'. Once in the blending unit 55' pure 02 can be mixed with the coolant to produce an 02/CO2 mixture of the desired variable proportions. These proportions will be apparent to a skilled person. In this embodiment, the mixture can then be passed via pipework 65' to a roof of the pressure vessel 25' before being reintroduced into the reactor 1'. The mixture is communicated through the pressure vessel 25' by means of the a network of channels 75' which form part of the reactor Burst Can Detection (BCD) apparatus. A BCD channel is provided for, and positioned above, each fuel chanel 80' in the core 5'.

The BCD apparatus is normally used to monitor the integrity of the fuel cladding. During O2/CO2 injection the BCD channels 75' are taken out of normal service and modified to allow fluid to pass through into the reactor 1'. A BCD sample pipe is shown in details in Fig 5.

The use of the BCD channel network 75' has the advantage that the O2/CO2 mixture can be directed to specific parts of the reactor 1'. The injected gas mixes with the main flow of reactor coolant which has passed up through the core 5', and is carried to the reheater unit 15' in one or more quadrants as depicted by arrows 35'.

Thus, with a knowledge of the coolant flow patterns present within the reactor 1' it is possible, by injecting the gas mixture at the correct place, to "target" a specific reheater 15' and boiler stack 10'.

However, prior to the injection of an O2/CO2 mixture of sufficient proportions to remove carbon deposits from a reheater 15' or boiler 10', an operation to clean the chosen BCD channel 75' is preferably carried out. For this the blending unit 55' produces an O2/CO2 mixture with an 2 concentration of less than 5% by volume, typically 1%. This mixture is passed through the chosen BCD channel 75' to purge it of any carbon fouling.

Once this operation has been completed the blending unit 55' is set to produce an O2/CO2 mixture with an 2 concentration of between 5% and 35% by volume, typically between 10% and 25%. This second mixture is injected, via the same BCD channel 75' as mentioned in the previous paragraph, at a flow rate which results in a 2 concentration of, for example, 0.5 to 3g/s in the vicinity of the chosen reheater 15'. It is envisaged that these concentrations will have to be maintained for periods of a few days to a few months depending on the amount and location of deposit to be removed. Progress can be followed via increases in reheater steam outlet temperatures.

It will be understood that the embodiments of present invention hereinbefore described are given by way of example only, and are not meant to limit the scope of the invention in any way.

Particularly, it should be appreciated that according to the present invention it is possible to remove deposits from out of core surfaces, whilst deposits in the core region are not oxidised and whilst the oxidation of sleeve and moderator graphite continues to be protected by use of the normal reactor coolant, as prescribed in the coolant advice. This is beneficial for reactor lifetime and fuel cycle optimisation. The ability to avoid oxidation of in-core carbon also assists in maintaining normal circuit activity levels during oxygen injection.

The invention offers the option to select, via choice of injection point(s), the region from which deposits will be removed. The invention can be employed whilst the reactor is on-load, thus avoiding interruption of power generation, and allows normal reactor operation to continue during the oxygen injection.

Claims

1. A method for at least partially removing deposits from a reactor-side surface(s) of at least one heat exchange apparatus of a gas cooled thermal nuclear reactor comprising injecting a gas in a localised manner into a pressure vessel of the reactor such that a portion of the gas is directed to and reacts with deposits on said surface(s) of said heat exchange apparatus, any remainder of said gas being substantially consumed within said heat exchange apparatus by coolant gas.

2. A method for removing deposits as claimed in claim 1, wherein a minor portion of the gas injected in such a localised manner is consumed by the deposits on the surface(s) of the heat exchange apparatus, the remaining, major portion of the gas reacting with component(s) of the coolant gas within the heat exchange apparatus.

3. A method for removing deposits as claimed in any preceding claim, wherein the at least one heat exchange apparatus comprises at least one boiler stack which boiler stack(s) comprise(s) part of a plurality of boiler stacks provided circumferentially around a core of the reactor within the pressure vessel.

4. A method for removing deposits as claimed in any preceding claim, wherein the method is performed during normal reactor operation.

5. A method for removing deposits as claimed in any preceding claim, wherein the gas is predominantly 02, the deposit being predominantly carbon.

6. A method for removing deposits as claimed in any preceding claim, wherein the gas is controllably injected in a localised manner into the pressure vessel through one or more existing channels (penetrations) of the pressure vessel.

7. A method for removing deposits as claimed in claim 6, wherein the one or more channels terminate within the pressure vessel in locations where the injected gas is entrained in the circulating coolant after the coolant has passed over the fuel and before the coolant enters the heat exchange apparatus.

8. A method for removing deposits as claimed in either of claims 6 or 7, wherein selection of channel(s) controls the distribution of injected oxygen and hence directs oxygen to areas from which deposit is to be removed.

9. A method for removing deposits as claimed in claims 6 to 8, wherein the channel(s) is/are selected from Burst Can Detection gas sampling lines, boiler inlet sampling lines, above-gas baffle pressure monitoring lines (low pressure dome D.P. lines), ISI penetrations and/or superheater moisture sampling lines.

10. A method for removing deposits as claimed in claims 6 to 9, wherein additional pipework is added to existing penetrations inside the pressure vessel so as to improve the efficiency of distribution of injected gas.

11. A method for removing deposits as claimed in any preceding claim, wherein the gas is blended for injection.

12. A method for removing deposits as claimed in any preceding claim, wherein the gas is predominantly oxygen, and the oxygen is blended with a portion of coolant gas drawn from a suitable point in the coolant gas circuit.

13. A method for removing deposits as claimed in claims 1 to 11, wherein the oxygen is blended with carbon dioxide drawn from the carbon dioxide make-up supply.

14. A method for removing deposits as claimed in any of claims 1 to 11, wherein the oxygen is blended with a dedicated supply of carbon dioxide.

15. A method for removing deposits as claimed in claims 1 to 12, wherein the method further comprises the steps of: bleeding of a portion of the coolant gas from the coolant circuit; and blending said portion of said coolant gas with said gas prior to injecting said blend into said pressure vessel.

16. A method for removing deposits as claimed in claim 15, wherein the portion of the coolant gas is bled from the pressure vessel via a sampling line.

17. A method for removing deposits as claimed in claims 12, 15 or 16, wherein the portion of the coolant gas is compressed before or after being blended with said gas.

18. A method for removing deposits as claimed in claims 11 to 17, wherein the blend comprises an oxygen concentration by volume in the range 5-35%.

19. A method for removing deposits as claimed in any preceding claim, wherein for delivery into the circulating coolant, the gas to be injected has a pressure in excess of the local coolant pressure at the point of delivery.

20. A method for removing deposits as claimed in any preceding claim, wherein the pressure is increased to overcome restrictions peculiar to the penetration(s) selected for the injection route.

21. A method for removing deposits as claimed in claims 19 or 20 when dependent on any of claims 12, 15, 15 or 17, wherein the required pressure is obtained for blending of coolant with oxygen by taking coolant gas from a high pressure part of the coolant circuit.

22. A method for removing deposits as claimed in claims 19 or 20 when dependent on any of claims 12, 15, 15 or 17, wherein the required pressure is obtained for blending of coolant by use of a dedicated compressor to compress the coolant preferably before blending with oxygen.

23. A method for removing deposits as claimed in claims 19 or 20 when dependent on claim 13, wherein the required pressure is obtained for blending of make-up carbon dioxide from the apparatus already existing at the nuclear installation for delivery to the reactors of make-up carbon dioxide.

24. A method for removing deposits as claimed in claims 19 or 20 when dependent on claim 13, wherein the required pressure is obtained for blending of make-up carbon dioxide by using a dedicated compressor.

25. A method for removing deposits as claimed in claims 19 or 20 when dependent on claim 14, wherein the required pressure is obtained from a pre-pressurised mixture of oxygen and carbon dioxide.

26. A method for removing deposits as claimed in claims 19 or 20 when dependent on claim 11, wherein the extent of pressurisation of gas, immediately after blending, above the coolant pressure at the point of delivery in the coolant circuit is substantially between 2 and 15 bar.

27. A method for removing deposits as claimed in claim 11 when dependent upon claims 6 to 10, wherein the blended gas is injected through the selected penetration(s) at flow rates such that the integrated rate of oxygen delivery to the circulating coolant is between 0.lg/s and 9g/s per reheater.

28. A method for removing deposits as claimed in claim 6, wherein the method further comprises the initial step of purging the channel (s).

29. A method for removing deposits as claimed in any preceding claim, wherein the gas cooled thermal nuclear reactor may be of the Advanced Gas Cooled Reactor (AGR) type.

30. Apparatus for injecting a gas in a localised manner into a pressure vessel of a gas cooled thermal nuclear reactor so as to at least partially remove deposits from a reactor-side surface(s) of at least one heat exchange apparatus comprising: a first pipe for accepting a portion of a coolant gas bled from said vessel; a blending unit for mixing said portion of said coolant gas with a gas; and a second pipe for feeding said blend to a location where said gas can be injected into said vessel.

31. A method for at least partially removing deposits from a reactor-side surface(s) of at least one heat exchange apparatus of a gas cooled thermal nuclear reactor as hereinbefore described with reference to Figs 1 to 3, or Figs 4 and 5.

32. Apparatus for injecting a gas in a localised manner into a pressure vessel of a gas cooled thermal nuclear reactor so as to at least partially remove deposits from a reactor side surface(s) of at least one heat exchange apparatus as hereinbefore described with reference to Figs 1 to 3, or Figs 4 and 5.