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Structure of helium loaded stainless steel after short heat pulses
224
Journal
of Nuclear
Materials 148 (1987) 224-226 North-Holland, Amsterdam
LETTER TO THE EDITORS STRUCTURE
OF HELIUM
LOADED
STAINLESS
1. Introduction
In present days tokamaks plasma disruptions occur frequently. The next generation machines will, therefore, have to be designed to survive a considerable number of disruptions during their lifetime [l]. Since the damage to the first wall due to disruptions is expected to increase with the size of the machines, these phenomena are of major concern for the construction of machines of the INTOR/NET type. During plasma disruptions large amounts of energy (l-20 MJ m-*) are dumped in short times (l-20 ms) to parts of the first wall. As a consequence, the surface temperature of the wall may reach values in the range of 1500-3000” C. In the case of metallic first walls, surface melting and evaporation may damage the structure. In a number of theoretical studies [l-3], the temperatures, their time behaviour, melt depth and evaporation rate at the surface have been calculated. In order to verify the theoretical results and to investigate the metallurgical consequences, simulation experiments with an electron beam gun have been carried out at the JRC Ispra. The results obtained so far confirm qualitatively the results of the calculations. The experiments have also shown a considerable number of metallurgical consequences of the rapid melting and solidification [4,5]. Due to the interaction of the high energy neutrons which are generated in the burning plasma with the atoms of the wall material, substantial amounts of helium will be generated by (n, a) reactions. This helium will be randomly distributed in the wall material and will become mobile only at high temperature. In this
STEEL AFTER SHORT HEAT PULSES
Samples with concentrations of 200,400 and 600 appm helium in the implanted layer were prepared, the helium being homogeneously distributed in a surface layer of 135 pm thickness. Disruptions were simulated on these, and on blank specimens using an electron gun which allows high power densities to be achieved over a small area [4,5]. The beam power was 800 W, the diameter of the beam 2 mm and the pulse time about 100 ms. The helium profile in the specimen across the surface was checked by autoradiography after the injection. A variation of L-108 of the mean value was observed. The specimens were cut through at the center of the beam spot and analyzed in the hot laboratories of the JRC Ispra.
3. Results
In figs. 1, 2 and 3 metallographic sections through the melted zones after electron gun shots are shown. Fig. 1 shows a blank stainless steel specimen. Figs. 2 and 3 show two specimens containing in the surface layer 400 and 600 appm helium, respectively. The following conclusions can be drawn: (1) All specimen show a lense shaped molten zone.
paper some results will be reported on the behaviour of the helium contained in steel in the case of disruptions.
2. Experimental procedure Disc shaped specimens (diameter 15 mm, thickness 5 mm), were prepared out of stainless steel, AISI 316 (European reference heat). Helium was implanted by the Ispra Cyclotron into the surface by exposing the specimen to a 28 MeV a particle beam whose energy was varied by a degrader between 0 and 28 MeV. 0022-3115/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Fig. 1. Blank reference. Metallographic section through a stainless steel specimen to which an electron beam has been applied. The specimen did not contain helium.
B.V.
P. Schiller et al. / Structure of He loaded stainless steel
Fig. 2. Metallographic section through a specimen containing 400 appm helium in a 135 pm thick layer after the application of a short electron beam pulse. (a) Specimen mechanically polished, (b) specimen etched.
(2) The metallographic structure of these molten parts (3)
(4)
(5)
(6) (7) (8)
is dendritic, due to the rapid cooling after the end of the beam pulse. In the molten part of specimen 2 and 3 spherical bubbles are present, while in specimen 1 no bubbles can be found. No bubbles are visible with the optical microscope in those parts of specimens 2 and 3 which remained solid. The volume of the melted zones in specimens 2 and 3 was increased and protrudes above the surface of the specimens. The size distribution of the bubbles varies widely. There seems to be no correlation between size and position of the bubbles. The areas of the bubble sections on the micrographs through the 3 specimens with different helium content (200, 400, 600 appm) are related by the ratios: 1:1.7:3.9.
225
Fig. 3. As fig. 2, but with helium content of 600 appm helium.
4. Discussion The micrographs in figs. 2 and 3 show that helium, which is distributed randomly in the lattice, will coalesce in bubbles even if the metal is liquified only for a very short time. The micrographs present the situation at the moment of resolidification. The spherical shape of most of the bubbles is due to the interaction between gas pressure in the bubble and surface tension of the liquid steel. The coexistence of both large and small bubbles indicates that the nucleation and growth processes were still underway when the steel solidified. At the present time it is impossible to determine the amount of helium which remains in the lattice. Bubbles are also present in parts of the melt, where no helium was implanted (below 135 km). This is possible if within the melt considerable convection of the liquid takes place. In the case of equilibrium between the gas pressure in the bubbles and the surface tension of the liquid steel, the
226
P. Schiller et al. / Structure
gas pressure in the bubble is: p = 2y/r,
(1) where y is the surface tension of liquid steel and r the radius of the bubbles. The quantity of gas in the bubble can then be expressed with the help of the equation for ideal gases as:
of He loaded stainless steel (3) The quantity of helium is estimated from the area of the sections of the bubbles. In order to get reliable values, a high number of such measurements is necessary. Since in the present case only one measurement for each case has been done, the error in the results can be relevant.
5. Conelusions where N is the number of moles in the bubble. The quantity of gas in eq. (2) is proportional to the square of the bubble radius. The area covered by the sections through the bubbles on the micrographs is also proportional to the square of the radius of the bubbles. Therefore, the area of the sections through the bubbles is a relative measure for the gas volume present in the bubbles at the solidification temperature. The ratio of these areas in the 3 specimens should be 1:2: 3 (200-400-600 appm). The experimentally found values of 1:1.7:3.9 are in qualitative agreement with the expectations. The volume of the bubbles, which can be roughly estimated from figs. 2 and 3 shows that about 20-3041 of the originally implanted He has formed bubbles. For the quantitative results a number of uncertainties have to be considered. (1) What we observe is the result of transient heating and melting processes. Since neither the beam current nor the bum time of the electron beam are exactly reproducible, relative differences in bubble size and bubble number can be expected, due to differences in nucleation and growth of the bubbles. (2) The electron beam induces considerable convection in the liquid layer. Due to the convection, helium bubbles are transported to the surface and lost into the vacuum. Differences in the beam will create differences in convection and in the percentage of helium losses in the different experiments. Received 2 December 1986; accepted 9 December 1986
Disruption simulation experiments carried out with electronic beams on steel implanted with helium, have shown that the helium atoms will coalesce partly in bubbles of various size, even during the very short time for which the steel is liquified. The experiments have also shown that substantial convection takes place during the period in which the steel is liquified. Further experiments with reproducible beam energies will be necessary to understand the observed phenomena.
Acknowledgements
The authors thank Mr. Buscaglia, Looman, Qdriaud, Thornton and Weckermann for their technical help during the execution of the experiments.
References
Dl INTOR Phase One Report (IAEA, Vienna, 1982). 121H.Th. Klippel, ECN 137 (1983). [31 A.M. Hassanein, G.L. Kulcinski and W.G. Wolfer J. Nucl. Mater. 103 & 104 (1981) 321.
[41 D. Quataert, F. Brossa, P. Moretto and G. Rigon, Fusion Technology - Proc. 13th SOFT (Pergamon Press) p. 401.
[51 G. Rigon, P. Moretto and F. Brossa, to be published in Nucl. Eng. Des.
P. Schiller ‘, D. Quataert i, M. Cambini ’ and F. Brossa ’ 1Materials Science Division ’ Nuclear Support Division JRC Ispra (VA), Italy
Journal
of Nuclear
Materials 148 (1987) 224-226 North-Holland, Amsterdam
LETTER TO THE EDITORS STRUCTURE
OF HELIUM
LOADED
STAINLESS
1. Introduction
In present days tokamaks plasma disruptions occur frequently. The next generation machines will, therefore, have to be designed to survive a considerable number of disruptions during their lifetime [l]. Since the damage to the first wall due to disruptions is expected to increase with the size of the machines, these phenomena are of major concern for the construction of machines of the INTOR/NET type. During plasma disruptions large amounts of energy (l-20 MJ m-*) are dumped in short times (l-20 ms) to parts of the first wall. As a consequence, the surface temperature of the wall may reach values in the range of 1500-3000” C. In the case of metallic first walls, surface melting and evaporation may damage the structure. In a number of theoretical studies [l-3], the temperatures, their time behaviour, melt depth and evaporation rate at the surface have been calculated. In order to verify the theoretical results and to investigate the metallurgical consequences, simulation experiments with an electron beam gun have been carried out at the JRC Ispra. The results obtained so far confirm qualitatively the results of the calculations. The experiments have also shown a considerable number of metallurgical consequences of the rapid melting and solidification [4,5]. Due to the interaction of the high energy neutrons which are generated in the burning plasma with the atoms of the wall material, substantial amounts of helium will be generated by (n, a) reactions. This helium will be randomly distributed in the wall material and will become mobile only at high temperature. In this
STEEL AFTER SHORT HEAT PULSES
Samples with concentrations of 200,400 and 600 appm helium in the implanted layer were prepared, the helium being homogeneously distributed in a surface layer of 135 pm thickness. Disruptions were simulated on these, and on blank specimens using an electron gun which allows high power densities to be achieved over a small area [4,5]. The beam power was 800 W, the diameter of the beam 2 mm and the pulse time about 100 ms. The helium profile in the specimen across the surface was checked by autoradiography after the injection. A variation of L-108 of the mean value was observed. The specimens were cut through at the center of the beam spot and analyzed in the hot laboratories of the JRC Ispra.
3. Results
In figs. 1, 2 and 3 metallographic sections through the melted zones after electron gun shots are shown. Fig. 1 shows a blank stainless steel specimen. Figs. 2 and 3 show two specimens containing in the surface layer 400 and 600 appm helium, respectively. The following conclusions can be drawn: (1) All specimen show a lense shaped molten zone.
paper some results will be reported on the behaviour of the helium contained in steel in the case of disruptions.
2. Experimental procedure Disc shaped specimens (diameter 15 mm, thickness 5 mm), were prepared out of stainless steel, AISI 316 (European reference heat). Helium was implanted by the Ispra Cyclotron into the surface by exposing the specimen to a 28 MeV a particle beam whose energy was varied by a degrader between 0 and 28 MeV. 0022-3115/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Fig. 1. Blank reference. Metallographic section through a stainless steel specimen to which an electron beam has been applied. The specimen did not contain helium.
B.V.
P. Schiller et al. / Structure of He loaded stainless steel
Fig. 2. Metallographic section through a specimen containing 400 appm helium in a 135 pm thick layer after the application of a short electron beam pulse. (a) Specimen mechanically polished, (b) specimen etched.
(2) The metallographic structure of these molten parts (3)
(4)
(5)
(6) (7) (8)
is dendritic, due to the rapid cooling after the end of the beam pulse. In the molten part of specimen 2 and 3 spherical bubbles are present, while in specimen 1 no bubbles can be found. No bubbles are visible with the optical microscope in those parts of specimens 2 and 3 which remained solid. The volume of the melted zones in specimens 2 and 3 was increased and protrudes above the surface of the specimens. The size distribution of the bubbles varies widely. There seems to be no correlation between size and position of the bubbles. The areas of the bubble sections on the micrographs through the 3 specimens with different helium content (200, 400, 600 appm) are related by the ratios: 1:1.7:3.9.
225
Fig. 3. As fig. 2, but with helium content of 600 appm helium.
4. Discussion The micrographs in figs. 2 and 3 show that helium, which is distributed randomly in the lattice, will coalesce in bubbles even if the metal is liquified only for a very short time. The micrographs present the situation at the moment of resolidification. The spherical shape of most of the bubbles is due to the interaction between gas pressure in the bubble and surface tension of the liquid steel. The coexistence of both large and small bubbles indicates that the nucleation and growth processes were still underway when the steel solidified. At the present time it is impossible to determine the amount of helium which remains in the lattice. Bubbles are also present in parts of the melt, where no helium was implanted (below 135 km). This is possible if within the melt considerable convection of the liquid takes place. In the case of equilibrium between the gas pressure in the bubbles and the surface tension of the liquid steel, the
226
P. Schiller et al. / Structure
gas pressure in the bubble is: p = 2y/r,
(1) where y is the surface tension of liquid steel and r the radius of the bubbles. The quantity of gas in the bubble can then be expressed with the help of the equation for ideal gases as:
of He loaded stainless steel (3) The quantity of helium is estimated from the area of the sections of the bubbles. In order to get reliable values, a high number of such measurements is necessary. Since in the present case only one measurement for each case has been done, the error in the results can be relevant.
5. Conelusions where N is the number of moles in the bubble. The quantity of gas in eq. (2) is proportional to the square of the bubble radius. The area covered by the sections through the bubbles on the micrographs is also proportional to the square of the radius of the bubbles. Therefore, the area of the sections through the bubbles is a relative measure for the gas volume present in the bubbles at the solidification temperature. The ratio of these areas in the 3 specimens should be 1:2: 3 (200-400-600 appm). The experimentally found values of 1:1.7:3.9 are in qualitative agreement with the expectations. The volume of the bubbles, which can be roughly estimated from figs. 2 and 3 shows that about 20-3041 of the originally implanted He has formed bubbles. For the quantitative results a number of uncertainties have to be considered. (1) What we observe is the result of transient heating and melting processes. Since neither the beam current nor the bum time of the electron beam are exactly reproducible, relative differences in bubble size and bubble number can be expected, due to differences in nucleation and growth of the bubbles. (2) The electron beam induces considerable convection in the liquid layer. Due to the convection, helium bubbles are transported to the surface and lost into the vacuum. Differences in the beam will create differences in convection and in the percentage of helium losses in the different experiments. Received 2 December 1986; accepted 9 December 1986
Disruption simulation experiments carried out with electronic beams on steel implanted with helium, have shown that the helium atoms will coalesce partly in bubbles of various size, even during the very short time for which the steel is liquified. The experiments have also shown that substantial convection takes place during the period in which the steel is liquified. Further experiments with reproducible beam energies will be necessary to understand the observed phenomena.
Acknowledgements
The authors thank Mr. Buscaglia, Looman, Qdriaud, Thornton and Weckermann for their technical help during the execution of the experiments.
References
Dl INTOR Phase One Report (IAEA, Vienna, 1982). 121H.Th. Klippel, ECN 137 (1983). [31 A.M. Hassanein, G.L. Kulcinski and W.G. Wolfer J. Nucl. Mater. 103 & 104 (1981) 321.
[41 D. Quataert, F. Brossa, P. Moretto and G. Rigon, Fusion Technology - Proc. 13th SOFT (Pergamon Press) p. 401.
[51 G. Rigon, P. Moretto and F. Brossa, to be published in Nucl. Eng. Des.
P. Schiller ‘, D. Quataert i, M. Cambini ’ and F. Brossa ’ 1Materials Science Division ’ Nuclear Support Division JRC Ispra (VA), Italy