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Metal carbonyl decomposition and carbon deposition in the advanced gas-cooled nuclear reactor
Corho~r. Vol. Printed
31, No. 3, pp. 467-472.
1993 Copyright
in Great Britain.
OCW6223193 96.00 + .M 0 1993 Pergamon Press Ltd.
METAL CARBONYL DECOMPOSITION AND CARBON DEPOSITION IN THE ADVANCED GAS-COOLED NUCLEAR REACTOR M. L. SYKES, I. A. S. EDWARDS, and K. M. THOMAS* Northern Carbon Research Laboratories, Department of Chemistry, The University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, U.K. (Received
26 May 1992; arcepted
in revised form
19 October
1992)
Abstract-Carbon deposition in advanced gas-cooled nuclear reactors has been found to occur preferentially in those regions of the reactor core which receive gas directly from the coolant reprocessing plant. This discovery has led to the hypothesis that such deposition may be catalysed by iron or nickel carbonyls being transported into the reactor core. This article describes experiments involving the decomposition of metal carbonyls in simulated AGR gas mixtures under conditions of temperature and pressure similar to those found within the reactor cores. Nickel carbonyl has been shown to promote significant carbon deposition from the simulated AGR, gas mixtures. The carbons deposited in the laboratory have been compared and contrasted with those found within the AGR. Those gases present which are most susceptible to decomposition under the influence of the decomposition products of nickel carbonyl have been identified, and the influence of AGR fuel cladding material examined. Taken together the results provide a mechanism accounting for the nature and distribution of the carbon deposits occurring in these areas of AGR cores. Key Words-Carbon deposition, advanced gas-cooled nuclear reactor, AGR, nickel carbonyl, carbonyl, metal carbonyl decomposition.
actions between circuit steels and carbon monoxide in the coolant. It is further postulated that, in the reactor core, these metal carbonyls decompose, producing metallic particulates capable of catalysing carbon deposition from the gas phase[5,6].
1.INTRODUCTION Advanced gas-cooled nuclear reactors (AGRs) used for electric power generation utilize high pressure (4.1 MPa) carbon dioxide as the heat transfer medium which takes energy, as heat, from the reactor core to the heat exchangers which provide steam to drive the turbines. Carbon dioxide is an efficient heat transfer medium for this purpose, but in conjunction with ionising radiation may cause oxidation of the graphite moderator used to control the reaction[ I]. In the AGR, carbon monoxide and methane are added to the coolant gas to protect the graphite moderator from radiolytic oxidation. Although providing excellent protection for the moderator this coolant strategy has been found, in certain cases, to result in carbon being deposited on the fuel elements[2,3]. Any carbon build-up causes a decrease in the conductive and convective heat transfer characteristics leading to the possibility of hot spots and of unacceptable degradation of the fuel elements. Recent investigation has shown that carbon deposition tends to occur preferentially on those fuel elements in the reactor zone which receive gas directly from the coolant reprocessing plant[4]. These observations have led to the hypothesis that nickel and/or iron carbonyls can be formed in the relatively cool environment of the reprocessing plant via inter-
*Author dressed.
to whom
correspondence
should
be ad-
iron
2. OBJECTIVES The overall objective of this study was to establish whether or not nickel carbonyl and/or iron carbonyl, present in coolant gases, was in part responsible for large and unacceptable amounts of carbon deposition found in specific quadrant areas of AGR cores, these areas being associated with a gas outlet from the coolant reprocessing plant. Carbon formation was studied from gas phase homogeneous reactions, and also from heterogeneous reactions occurring at the surface of AGR fuel cladding metals, each both with and without the influence of nickel and iron carbonyls. The specific objectives of the research were to use laboratory studies carried out under simulated AGR conditions of temperatures and pressure. The experiments used coolant gas doped with metal carbonyls, in the absence of ionising radiation, to provide possible explanations of: (a) why this quadrantdependent carbon deposition occurs in AGR cores; (b) where the initial carbon growth processes take place in the AGR; (c) which gases within the AGR coolant gas mixture lead to carbon deposition; and (d) how the structures found within the AGR cores are produced; and hence, to describe possible mech467
468
M. L.
SYKES
anisms defining the origin of these carbon deposits, also explaining why they do not occur throughout the reactor cores.
et al.
Table 1. Analytical data supplied by A. E. A. Harwell for the simulated A.G.R. coolant gas mixtures used (balance carbon dioxide) A.G.R.1.
3. EXPERIMENTAL
3.1 Carbon deposition
studies
Apparatus was designed and constructed thermally, to decompose nickel and iron carbonyls within simulated AGR coolant gas mixtures, under the conditions of temperature (350” to 700°C) and pressure (4.1 MPa) found in the AGR reactor cores, and to collect any particulate (>l pm) products between two sintered discs. A schematic diagram showing the layout of the apparatus is shown in Fig. 1. Two simulated AGR coolant gas mixtures were employed. These included both the methane and carbon monoxide which are added to the carbon dioxide coolant as well as the main hydrocarbons which are formed thereafter by radiolytic reactions within the reactor core. The compositions of the two simulated AGR gas mixtures are shown in Table 1. The mixtures were provided and analysed by the A.E.A. Harwell Laboratories. The compositions of the gas mixtures were based on those found within the AGR and were selected on the basis of previous operational experience[2]. Gas mixture 1 was selected as a mixture unlikely to cause carbon deposition in AGR reactor cores and gas mixture 2 as a mixture predicted to cause some carbon deposition in the AGR core under irradiation conditions. Experiments were conducted using the two simulated AGR coolant gas mixtures with injection of
Gas mixtureCarbon monoxide Methane Ethane Ethene Propane Propene Iso-butane n-butane 2,2 dimethyl propane Iso-pentane 2,2 dimethyl butane
(0.5%) 150 6.33 0.75 1.78 0.12 0.23 0.20 0.16 0.14 0.07
(1.5%) 300 13.76 1.61 3.86 0.25 0.50 0.44 0.36 0.31 0.15
5 and 55 ppm of either nickel or iron carbony1 in order to establish whether nickel carbonyl and/or iron carbonyl would promote homogeneous gas-phase carbon deposition from these mixtures, under the conditions of temperature (350” to 700°C) and pressure (4.1 MPa) found in the AGR reactor cores, and to collect any particulate products thereof. between
3.2 Quantitative
deposition measurements
A second technique was developed to measure carbon deposition from eight binary gas mixtures containing 5% of either carbon monoxide, methane, ethane, ethene, propane, propene, isobutane, or nbutane, with 95% carbon dioxide, under the conditions of temperature (350” to 700°C) and pressure (4.1 MPa) prevalent in the AGR reactor cores. This technique was a modified version of the apparatus described above, replacing carbon collection between sintered discs with carbon deposition on polished silica discs, and measuring the relative darkening of these discs, resulting from the deposited carbon, by the absorption of light. The experimental conditions were fixed such that only a thin film of carbon was deposited so that a wide variation in darkening of the silica discs could be measured. The relative amounts of carbon deposited from the eight binary gas mixtures, both singly and with nickel or iron carbonyls, were assessed using a Pye Unicam SP6-450 UV/Vis spectrophotometer with a modified sample holder.
3.3 Influence offuel
Fig. 1. Apparatus to decompose metal carbonyls within simulated AGR coolant gas mixtures.
A.G.R.2. @pm)
cladding
The two simulated AGR coolant gas mixtures were also used in experiments designed to assess the influence of AGR steel fuel cladding upon carbon deposition occurring at its surface, and to establish whether nickel deposited on the fuel cladding surface could promote heterogeneous carbon deposition from the coolant gas mixture. To this effect, nickel carbonyl was thermally decomposed onto small pieces of AGR steel fuel cladding under an atmosphere of carbon dioxide. These were then ex-
Metal carbonyl decomposition
posed to the simulated AGR gas mixtures at 500°C and 4.1 MPa for &hour periods with no further injection of nickel carbonyl, Similar uncoated pieces of fuel cladding were exposed to the simulated AGR gas mixtures under the same conditions but with injection of 2 ppm of nickel carbonyl. The pieces of fuel cladding were subsequently examined using scanning electron microscopy and polarised light microscopy. 4. RESULTS AND DISCUSSION
4.1 Carbon reposition Nickel carbonyl promotes homogeneous carbon deposition from both simulated AGR coolant gas mixtures. This occurs under the high pressure conditions prevalent in the AGRcoolant system (4.1 MPa) and over the range of temperatures found within the AGR reactor cores (3.50” to 700°C). This demonstrates that the gas mixture; predicted to be unlikely to cause carbon deposition within the AGR, deposits carbon in the presence of nickel carbonyl. Iron carbony1 is inactive at promoting carbon deposition under these conditions. Variation in the duration of the experiments of between 1 and 24 hours, of the amount of nickel carbonyl injected between 5 and 55 ppm, and of the amount of gas available by utilising a gas flow system has shown that, for experiments of duration 36 hours, the amount of carbon deposited is limited by the amount of nickel carbonyl injected. The nickel particulates responsible for carbon deposition are thought to become encapsulated and, hence, rendered inactive for promoting further carbon deposition once a certain amount of carbon has been deposited. 4.2 ~i~a~titative reposition rneu~~rernent~ Carbon deposition from the AGR gas mixtures was low and it was not possible to make quantitative deposition measurements. The binary gas mixture experiments were thus conducted at much higher concentrations in order to attempt to identify the carbon deposition propensities of the individual components, and to assess the influence of the metal carbonyls upon these. It is possible that, at the higher concentrations used in the binary gas mixture expe~ments, the deposition mechanisms may be altered. However, the experiments do provide useful data on the relative influence of the iron and nickel carbonyls upon the relative propensities for carbon deposition of each of the individual components and also provide information on which of the minor components are most likely to be responsible for carbon deposition. Carbon deposition from the eight binary gas mixtures has been assessed both with and without the influence of nickel and iron carbonyls. Without metal carbonyls, the mixtures deposit small amounts of carbon under the conditions of temperature and pressure found within the AGR reactor
469
in the AGR
Table 2. Comparative carbon deposition data derived from optical absorption measurements Gas (95% CO,) (+.5% of:) methane carbon monoxide ethane ethene propane propene isobutane n-butane
Without nickel carbonyl
1 1 it: 30 100 ::
With nickel carbonyl
1 1 20 530 8:: 20 30
The values of visible light absorbance for the silica discs were measured both before and after each experiment. The increases in absorbance were normalised by dividing by the lowest value, that for methane/carbon dioxide with no metal carbonyl injection, thus giving dimensionless comparative figures for the amount of carbon deposited. The values obtained for each binary gas mixture, both with and without nickel carbonyl, are shown in Table 2. The values have been rounded to the nearest ten and show some variation due to the nature of the technique, the trends however are clear. Nickel carbony1 promotes large increases in carbon deposition from the eight binary gas mixtures, the greatest effect being upon the mixtures containing ethene and propene. This suggests that unsaturated hydrocarbons within the AGR coolant mixture are the gases most susceptible to decomposition through interaction with nickel carbonyl, although some carbon deposition was observed for the alkanes, and this appeared unaffected by the presence of nickel carbony I. Iron carbonyl causes no increase in carbon deposition from the eight binary gas mixtures under these conditions. This indicates that iron carbonyl is not important with regard to carbon deposition in the AGR.
cores.
4.3 Carbon characterisation The carbon deposits produced in the laboratory experiments involving the simulated AGR gas mixtures and nickel carbonyl were characterised using scanning electron microscopy to examine the size and shape of the carbon structures and transmission electron microscopy to examine the electron density and underlying structure of the deposits, together with an electron diffraction study of the crystalline order and carbon lattice spacing. In addition, the distribution and degree of association of metals within the carbons were examined using energy dispersive analysis of x-rays. Carbon samples taken from the most heavily deposited quadrants of the AGR cores were characterised using the same techniques, and compared and contrasted with the laboratory obtained deposits,
470
M. L. SYKESet
Fig. 2. Scanning electron micrograph of laboratory obtained carbon deposit.
hence, showing how the thermally produced laboratory deposits differ from the deposits occurring under thermal and radiolytic depositing conditions within the AGR. Scanning electron microscopy of carbon deposits obtained in the laboratory experiments involving the simulated AGR gas mixtures and nickel carbonyl, at magnifications of up to 10,000x, shows these to consist predominantly of carbon platelets with diameters of ~20 Frn, and of agglomerations of spheres, of diameters ~0.5 pm. A typical micrograph showing the laboratory obtained deposit is shown in Fig. 2. Light field transmission electron microscopy of these carbon deposits, at magnifications of up to 220,000~ indicates elongated filaments of up to 100 nm in length with encapsulated electrondense particles of approximately 10 nm in diameter at their tips, which are thought to contain nickel. The deposits also contain spherical carbons of < 100 nm in diameter, which also contain electron-dense regions at their centres. Both areas are intermixed and both show distinctly different skin/core regions indicative of catalytically deposited carbons. Electron diffraction studies show that these carbons have doo2lattice spacings in the region of 0.39 nm. Energy dispersive analysis of x-rays has shown these carbons to contain finely dispersed nickel throughout. Scanning electron microscopy of carbon samples obtained from an AGR, at magnifications of up to 10,000~, shows these to consist predominantly of convoluted tree-like structures of approximately 100 pm in diameter, made-up of agglomerated spherical particles
al.
show these carbons to be more ordered than the laboratory deposits and to have doo2lattice spacings in the region of 0.36 nm. Energy dispersive analysis of x-rays has shown the AGR deposits to contain nickel very finely dispersed throughout. Iron is present as an associate of particles < 1 pm in diameter. Both types of deposit therefore consist of agglomerations of spherical carbons, these being
I
I
4
w
Fig. 3. Scanning electron micrograph of AGR carbon deposit.
411
Metal carbonyl decomposition in the AGR the initial stages, the most important with regard to carbon deposition in the AGR. These findings are in agreement with similar work conducted by Blanehard, et a/.[71 who found that fuel cladding steel, which had been electroplated with nickel and exposed to simulated AGR gas mixtures, remained cleaner from carbon than bordering unplated areas. These authors explained the findings in terms of a possible enhancement of the catalytic carbon oxidation reactions which can remove deposited carbon. 4.5 Mechanism In summary and with regard to the mechanism of production of the quadrant-dependent deposits found in the AGR, the following stages are proposed: 1. Ingress of nickel carbonyl into the reactor core occurs through the gas reprocessing plant outlet. The nickel carbonyl is formed by reaction between carbon monoxide in the coolant gas mixtures and metallic nickel in the coolant reprocessing plant. 2. Upon entering the higher temperature area below the reactor core, the nickel carbonyl thermally decomposes liberating nickel atoms, which aggregate to form fine nickel particulates of between 10 and 100 nm in diameter. Calculation of collision frequencies has shown that this aggregation can occur in the area beneath the reactor core and before entry into the fuel channels, with a IO-nm nickel particle being produced from individual nickel atoms in approximately tO0 ms. 3. The nickel particulates then promote carbon deposition from the carbon-containing gas species, especially the unsaturated molecules, producing fine spherical carbon particles of up to I pm in diameter. The unsaturated molecules are those most susceptible to decomposition under the influence of nickel carbonyl. This is most likely to be due to the possibility of coordination between the double bonds and the nickel, the resulting complexes having lower activation energy barriers for the decomposition reactions to occur. 4. The spherical carbon particles thus produced are blown into the fuel channels, in the area of the reactor core nearest to the gas reprocessing plant outlet, where they will collide with, and may stick to, either the surface of the fuel cladding or to any carbonaceous deposits already present on the fuel cladding surfaces. The carbon spheres deposited on the fuel cladding surfaces will continue to deposit carbon until the complete encapsulation of any nickel particles. The deposit will also grow by the build-up of further gas-borne structures, which will collide with and stick to the material already present, resulting in the convoluted structures, made up from fine
spherical particles, which have been identified in the examination of deposits taken from the AGR. 5. CONCLUSIONS This study has demonstrated that nickel carbonyl, present within simulated advanced gascooled reactor coolant gas mixtures, promotes carbon deposition, under the conditions of temperature (350” to 700°C) and pressure (4.1 MPa) prevalent in the AGR reactor cores. Iron carbonyl is inactive at promoting carbon deposition under these conditions. However, the influence of iron carbonyl may be altered under the influence of irradiation and iron may have a more important role with regard to carbon deposition in the AGR than is predicted by these results. Taken together the results suggest that the quadrant-dependent carbon deposits found within certain areas of the AGR cores result from small amounts of nickel carbonyl, produced by reaction between carbon monoxide in the coolant gas mixture and nickel in steel pipework, entering the reactor core through the coolant reprocessing plant gas outlet in the recirculation of the coolant gas. This nickel carbonyl then decomposes to nickel, and this promotes carbon deposition in this specific quadrant of the reactor core only. The initial carbon growth processes are proposed to occur within the coolant gas stream and before interaction with the fuel cladding surfaces. Unsaturated hydrocarbon molecules within the coolant mixture are those most susceptible to decomposition by interaction with nickel transported into the reactor cores as nickel carbonyl. The carbonaceous materials found on the surfaces of AGR fuel rods are produced by build-up of the carbon deposits originating in the gas stream. These may continue to promote further carbon deposition, after attachment to the fuel cladding surface, until all of the nickel has been encapsulated by deposited carbon. In the case of the AGR deposits the structures may also have been altered due to neutron damage effects which can modify structures after formation. It was not possible to study such processes in laboratory experiments.
Acknowledgements-The
authors would like to gratefully acknowledge the support of the United Kingdom Atomic Energy Authority and, in particular, Mr. Brian KelIy of Springfields Nuclear Power Development Laboratories. Dr. Alex Harper of A.E.A. Harwell Laboratories is thanked for the supply and analysis of simulated AGR gas mixtures and metal carbonyl-containing gas mixtures.
REFERENCES I. J. V. Best, W. J. Stephen, and A. J. Wickham, Prog. Nucl. Energy 16, 127 (1985).
472
M. L.
SYKES et al.
2. P. Campion, In Gas Chemistry in Nuclear Reactors and Large Industrial Plant (Edited by A. Dyer), pp.
53-66. Heyden Press, London (1979). 3. E. W. Carpenter and D. J. Norfolk, Nucl. Energy 23, 83 (1984). 4. J. Wilson, Atom 386, 2 (1988).
5. P. A. V. Johnson, Nucl. Energy 24, 381 (1985). 6. B. T. Kelly, Nucl. Energy 25, 225 (1986). 7. A. Blanchard, P. Campion, In Gas Chemistry in Nuc/ear Reactors and Large Industrial Plant (Edited by A. Dyer), pp. 90-97. Heyden Press, London (1979).
31, No. 3, pp. 467-472.
1993 Copyright
in Great Britain.
OCW6223193 96.00 + .M 0 1993 Pergamon Press Ltd.
METAL CARBONYL DECOMPOSITION AND CARBON DEPOSITION IN THE ADVANCED GAS-COOLED NUCLEAR REACTOR M. L. SYKES, I. A. S. EDWARDS, and K. M. THOMAS* Northern Carbon Research Laboratories, Department of Chemistry, The University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, U.K. (Received
26 May 1992; arcepted
in revised form
19 October
1992)
Abstract-Carbon deposition in advanced gas-cooled nuclear reactors has been found to occur preferentially in those regions of the reactor core which receive gas directly from the coolant reprocessing plant. This discovery has led to the hypothesis that such deposition may be catalysed by iron or nickel carbonyls being transported into the reactor core. This article describes experiments involving the decomposition of metal carbonyls in simulated AGR gas mixtures under conditions of temperature and pressure similar to those found within the reactor cores. Nickel carbonyl has been shown to promote significant carbon deposition from the simulated AGR, gas mixtures. The carbons deposited in the laboratory have been compared and contrasted with those found within the AGR. Those gases present which are most susceptible to decomposition under the influence of the decomposition products of nickel carbonyl have been identified, and the influence of AGR fuel cladding material examined. Taken together the results provide a mechanism accounting for the nature and distribution of the carbon deposits occurring in these areas of AGR cores. Key Words-Carbon deposition, advanced gas-cooled nuclear reactor, AGR, nickel carbonyl, carbonyl, metal carbonyl decomposition.
actions between circuit steels and carbon monoxide in the coolant. It is further postulated that, in the reactor core, these metal carbonyls decompose, producing metallic particulates capable of catalysing carbon deposition from the gas phase[5,6].
1.INTRODUCTION Advanced gas-cooled nuclear reactors (AGRs) used for electric power generation utilize high pressure (4.1 MPa) carbon dioxide as the heat transfer medium which takes energy, as heat, from the reactor core to the heat exchangers which provide steam to drive the turbines. Carbon dioxide is an efficient heat transfer medium for this purpose, but in conjunction with ionising radiation may cause oxidation of the graphite moderator used to control the reaction[ I]. In the AGR, carbon monoxide and methane are added to the coolant gas to protect the graphite moderator from radiolytic oxidation. Although providing excellent protection for the moderator this coolant strategy has been found, in certain cases, to result in carbon being deposited on the fuel elements[2,3]. Any carbon build-up causes a decrease in the conductive and convective heat transfer characteristics leading to the possibility of hot spots and of unacceptable degradation of the fuel elements. Recent investigation has shown that carbon deposition tends to occur preferentially on those fuel elements in the reactor zone which receive gas directly from the coolant reprocessing plant[4]. These observations have led to the hypothesis that nickel and/or iron carbonyls can be formed in the relatively cool environment of the reprocessing plant via inter-
*Author dressed.
to whom
correspondence
should
be ad-
iron
2. OBJECTIVES The overall objective of this study was to establish whether or not nickel carbonyl and/or iron carbonyl, present in coolant gases, was in part responsible for large and unacceptable amounts of carbon deposition found in specific quadrant areas of AGR cores, these areas being associated with a gas outlet from the coolant reprocessing plant. Carbon formation was studied from gas phase homogeneous reactions, and also from heterogeneous reactions occurring at the surface of AGR fuel cladding metals, each both with and without the influence of nickel and iron carbonyls. The specific objectives of the research were to use laboratory studies carried out under simulated AGR conditions of temperatures and pressure. The experiments used coolant gas doped with metal carbonyls, in the absence of ionising radiation, to provide possible explanations of: (a) why this quadrantdependent carbon deposition occurs in AGR cores; (b) where the initial carbon growth processes take place in the AGR; (c) which gases within the AGR coolant gas mixture lead to carbon deposition; and (d) how the structures found within the AGR cores are produced; and hence, to describe possible mech467
468
M. L.
SYKES
anisms defining the origin of these carbon deposits, also explaining why they do not occur throughout the reactor cores.
et al.
Table 1. Analytical data supplied by A. E. A. Harwell for the simulated A.G.R. coolant gas mixtures used (balance carbon dioxide) A.G.R.1.
3. EXPERIMENTAL
3.1 Carbon deposition
studies
Apparatus was designed and constructed thermally, to decompose nickel and iron carbonyls within simulated AGR coolant gas mixtures, under the conditions of temperature (350” to 700°C) and pressure (4.1 MPa) found in the AGR reactor cores, and to collect any particulate (>l pm) products between two sintered discs. A schematic diagram showing the layout of the apparatus is shown in Fig. 1. Two simulated AGR coolant gas mixtures were employed. These included both the methane and carbon monoxide which are added to the carbon dioxide coolant as well as the main hydrocarbons which are formed thereafter by radiolytic reactions within the reactor core. The compositions of the two simulated AGR gas mixtures are shown in Table 1. The mixtures were provided and analysed by the A.E.A. Harwell Laboratories. The compositions of the gas mixtures were based on those found within the AGR and were selected on the basis of previous operational experience[2]. Gas mixture 1 was selected as a mixture unlikely to cause carbon deposition in AGR reactor cores and gas mixture 2 as a mixture predicted to cause some carbon deposition in the AGR core under irradiation conditions. Experiments were conducted using the two simulated AGR coolant gas mixtures with injection of
Gas mixtureCarbon monoxide Methane Ethane Ethene Propane Propene Iso-butane n-butane 2,2 dimethyl propane Iso-pentane 2,2 dimethyl butane
(0.5%) 150 6.33 0.75 1.78 0.12 0.23 0.20 0.16 0.14 0.07
(1.5%) 300 13.76 1.61 3.86 0.25 0.50 0.44 0.36 0.31 0.15
5 and 55 ppm of either nickel or iron carbony1 in order to establish whether nickel carbonyl and/or iron carbonyl would promote homogeneous gas-phase carbon deposition from these mixtures, under the conditions of temperature (350” to 700°C) and pressure (4.1 MPa) found in the AGR reactor cores, and to collect any particulate products thereof. between
3.2 Quantitative
deposition measurements
A second technique was developed to measure carbon deposition from eight binary gas mixtures containing 5% of either carbon monoxide, methane, ethane, ethene, propane, propene, isobutane, or nbutane, with 95% carbon dioxide, under the conditions of temperature (350” to 700°C) and pressure (4.1 MPa) prevalent in the AGR reactor cores. This technique was a modified version of the apparatus described above, replacing carbon collection between sintered discs with carbon deposition on polished silica discs, and measuring the relative darkening of these discs, resulting from the deposited carbon, by the absorption of light. The experimental conditions were fixed such that only a thin film of carbon was deposited so that a wide variation in darkening of the silica discs could be measured. The relative amounts of carbon deposited from the eight binary gas mixtures, both singly and with nickel or iron carbonyls, were assessed using a Pye Unicam SP6-450 UV/Vis spectrophotometer with a modified sample holder.
3.3 Influence offuel
Fig. 1. Apparatus to decompose metal carbonyls within simulated AGR coolant gas mixtures.
A.G.R.2. @pm)
cladding
The two simulated AGR coolant gas mixtures were also used in experiments designed to assess the influence of AGR steel fuel cladding upon carbon deposition occurring at its surface, and to establish whether nickel deposited on the fuel cladding surface could promote heterogeneous carbon deposition from the coolant gas mixture. To this effect, nickel carbonyl was thermally decomposed onto small pieces of AGR steel fuel cladding under an atmosphere of carbon dioxide. These were then ex-
Metal carbonyl decomposition
posed to the simulated AGR gas mixtures at 500°C and 4.1 MPa for &hour periods with no further injection of nickel carbonyl, Similar uncoated pieces of fuel cladding were exposed to the simulated AGR gas mixtures under the same conditions but with injection of 2 ppm of nickel carbonyl. The pieces of fuel cladding were subsequently examined using scanning electron microscopy and polarised light microscopy. 4. RESULTS AND DISCUSSION
4.1 Carbon reposition Nickel carbonyl promotes homogeneous carbon deposition from both simulated AGR coolant gas mixtures. This occurs under the high pressure conditions prevalent in the AGRcoolant system (4.1 MPa) and over the range of temperatures found within the AGR reactor cores (3.50” to 700°C). This demonstrates that the gas mixture; predicted to be unlikely to cause carbon deposition within the AGR, deposits carbon in the presence of nickel carbonyl. Iron carbony1 is inactive at promoting carbon deposition under these conditions. Variation in the duration of the experiments of between 1 and 24 hours, of the amount of nickel carbonyl injected between 5 and 55 ppm, and of the amount of gas available by utilising a gas flow system has shown that, for experiments of duration 36 hours, the amount of carbon deposited is limited by the amount of nickel carbonyl injected. The nickel particulates responsible for carbon deposition are thought to become encapsulated and, hence, rendered inactive for promoting further carbon deposition once a certain amount of carbon has been deposited. 4.2 ~i~a~titative reposition rneu~~rernent~ Carbon deposition from the AGR gas mixtures was low and it was not possible to make quantitative deposition measurements. The binary gas mixture experiments were thus conducted at much higher concentrations in order to attempt to identify the carbon deposition propensities of the individual components, and to assess the influence of the metal carbonyls upon these. It is possible that, at the higher concentrations used in the binary gas mixture expe~ments, the deposition mechanisms may be altered. However, the experiments do provide useful data on the relative influence of the iron and nickel carbonyls upon the relative propensities for carbon deposition of each of the individual components and also provide information on which of the minor components are most likely to be responsible for carbon deposition. Carbon deposition from the eight binary gas mixtures has been assessed both with and without the influence of nickel and iron carbonyls. Without metal carbonyls, the mixtures deposit small amounts of carbon under the conditions of temperature and pressure found within the AGR reactor
469
in the AGR
Table 2. Comparative carbon deposition data derived from optical absorption measurements Gas (95% CO,) (+.5% of:) methane carbon monoxide ethane ethene propane propene isobutane n-butane
Without nickel carbonyl
1 1 it: 30 100 ::
With nickel carbonyl
1 1 20 530 8:: 20 30
The values of visible light absorbance for the silica discs were measured both before and after each experiment. The increases in absorbance were normalised by dividing by the lowest value, that for methane/carbon dioxide with no metal carbonyl injection, thus giving dimensionless comparative figures for the amount of carbon deposited. The values obtained for each binary gas mixture, both with and without nickel carbonyl, are shown in Table 2. The values have been rounded to the nearest ten and show some variation due to the nature of the technique, the trends however are clear. Nickel carbony1 promotes large increases in carbon deposition from the eight binary gas mixtures, the greatest effect being upon the mixtures containing ethene and propene. This suggests that unsaturated hydrocarbons within the AGR coolant mixture are the gases most susceptible to decomposition through interaction with nickel carbonyl, although some carbon deposition was observed for the alkanes, and this appeared unaffected by the presence of nickel carbony I. Iron carbonyl causes no increase in carbon deposition from the eight binary gas mixtures under these conditions. This indicates that iron carbonyl is not important with regard to carbon deposition in the AGR.
cores.
4.3 Carbon characterisation The carbon deposits produced in the laboratory experiments involving the simulated AGR gas mixtures and nickel carbonyl were characterised using scanning electron microscopy to examine the size and shape of the carbon structures and transmission electron microscopy to examine the electron density and underlying structure of the deposits, together with an electron diffraction study of the crystalline order and carbon lattice spacing. In addition, the distribution and degree of association of metals within the carbons were examined using energy dispersive analysis of x-rays. Carbon samples taken from the most heavily deposited quadrants of the AGR cores were characterised using the same techniques, and compared and contrasted with the laboratory obtained deposits,
470
M. L. SYKESet
Fig. 2. Scanning electron micrograph of laboratory obtained carbon deposit.
hence, showing how the thermally produced laboratory deposits differ from the deposits occurring under thermal and radiolytic depositing conditions within the AGR. Scanning electron microscopy of carbon deposits obtained in the laboratory experiments involving the simulated AGR gas mixtures and nickel carbonyl, at magnifications of up to 10,000x, shows these to consist predominantly of carbon platelets with diameters of ~20 Frn, and of agglomerations of spheres, of diameters ~0.5 pm. A typical micrograph showing the laboratory obtained deposit is shown in Fig. 2. Light field transmission electron microscopy of these carbon deposits, at magnifications of up to 220,000~ indicates elongated filaments of up to 100 nm in length with encapsulated electrondense particles of approximately 10 nm in diameter at their tips, which are thought to contain nickel. The deposits also contain spherical carbons of < 100 nm in diameter, which also contain electron-dense regions at their centres. Both areas are intermixed and both show distinctly different skin/core regions indicative of catalytically deposited carbons. Electron diffraction studies show that these carbons have doo2lattice spacings in the region of 0.39 nm. Energy dispersive analysis of x-rays has shown these carbons to contain finely dispersed nickel throughout. Scanning electron microscopy of carbon samples obtained from an AGR, at magnifications of up to 10,000~, shows these to consist predominantly of convoluted tree-like structures of approximately 100 pm in diameter, made-up of agglomerated spherical particles
al.
show these carbons to be more ordered than the laboratory deposits and to have doo2lattice spacings in the region of 0.36 nm. Energy dispersive analysis of x-rays has shown the AGR deposits to contain nickel very finely dispersed throughout. Iron is present as an associate of particles < 1 pm in diameter. Both types of deposit therefore consist of agglomerations of spherical carbons, these being
I
I
4
w
Fig. 3. Scanning electron micrograph of AGR carbon deposit.
411
Metal carbonyl decomposition in the AGR the initial stages, the most important with regard to carbon deposition in the AGR. These findings are in agreement with similar work conducted by Blanehard, et a/.[71 who found that fuel cladding steel, which had been electroplated with nickel and exposed to simulated AGR gas mixtures, remained cleaner from carbon than bordering unplated areas. These authors explained the findings in terms of a possible enhancement of the catalytic carbon oxidation reactions which can remove deposited carbon. 4.5 Mechanism In summary and with regard to the mechanism of production of the quadrant-dependent deposits found in the AGR, the following stages are proposed: 1. Ingress of nickel carbonyl into the reactor core occurs through the gas reprocessing plant outlet. The nickel carbonyl is formed by reaction between carbon monoxide in the coolant gas mixtures and metallic nickel in the coolant reprocessing plant. 2. Upon entering the higher temperature area below the reactor core, the nickel carbonyl thermally decomposes liberating nickel atoms, which aggregate to form fine nickel particulates of between 10 and 100 nm in diameter. Calculation of collision frequencies has shown that this aggregation can occur in the area beneath the reactor core and before entry into the fuel channels, with a IO-nm nickel particle being produced from individual nickel atoms in approximately tO0 ms. 3. The nickel particulates then promote carbon deposition from the carbon-containing gas species, especially the unsaturated molecules, producing fine spherical carbon particles of up to I pm in diameter. The unsaturated molecules are those most susceptible to decomposition under the influence of nickel carbonyl. This is most likely to be due to the possibility of coordination between the double bonds and the nickel, the resulting complexes having lower activation energy barriers for the decomposition reactions to occur. 4. The spherical carbon particles thus produced are blown into the fuel channels, in the area of the reactor core nearest to the gas reprocessing plant outlet, where they will collide with, and may stick to, either the surface of the fuel cladding or to any carbonaceous deposits already present on the fuel cladding surfaces. The carbon spheres deposited on the fuel cladding surfaces will continue to deposit carbon until the complete encapsulation of any nickel particles. The deposit will also grow by the build-up of further gas-borne structures, which will collide with and stick to the material already present, resulting in the convoluted structures, made up from fine
spherical particles, which have been identified in the examination of deposits taken from the AGR. 5. CONCLUSIONS This study has demonstrated that nickel carbonyl, present within simulated advanced gascooled reactor coolant gas mixtures, promotes carbon deposition, under the conditions of temperature (350” to 700°C) and pressure (4.1 MPa) prevalent in the AGR reactor cores. Iron carbonyl is inactive at promoting carbon deposition under these conditions. However, the influence of iron carbonyl may be altered under the influence of irradiation and iron may have a more important role with regard to carbon deposition in the AGR than is predicted by these results. Taken together the results suggest that the quadrant-dependent carbon deposits found within certain areas of the AGR cores result from small amounts of nickel carbonyl, produced by reaction between carbon monoxide in the coolant gas mixture and nickel in steel pipework, entering the reactor core through the coolant reprocessing plant gas outlet in the recirculation of the coolant gas. This nickel carbonyl then decomposes to nickel, and this promotes carbon deposition in this specific quadrant of the reactor core only. The initial carbon growth processes are proposed to occur within the coolant gas stream and before interaction with the fuel cladding surfaces. Unsaturated hydrocarbon molecules within the coolant mixture are those most susceptible to decomposition by interaction with nickel transported into the reactor cores as nickel carbonyl. The carbonaceous materials found on the surfaces of AGR fuel rods are produced by build-up of the carbon deposits originating in the gas stream. These may continue to promote further carbon deposition, after attachment to the fuel cladding surface, until all of the nickel has been encapsulated by deposited carbon. In the case of the AGR deposits the structures may also have been altered due to neutron damage effects which can modify structures after formation. It was not possible to study such processes in laboratory experiments.
Acknowledgements-The
authors would like to gratefully acknowledge the support of the United Kingdom Atomic Energy Authority and, in particular, Mr. Brian KelIy of Springfields Nuclear Power Development Laboratories. Dr. Alex Harper of A.E.A. Harwell Laboratories is thanked for the supply and analysis of simulated AGR gas mixtures and metal carbonyl-containing gas mixtures.
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