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Characterisation of mechanical and thermal properties of tungsten for high power spallation target applications

Habainy, Jemila LU (2018)
Abstract
The European Spallation Source (ESS), which is currently under construction in Lund, will use pure tungsten as the spallation neutron target. The tungsten will be irradiated by a 2 GeV proton beam, pulsed at a repetition rate of 14 Hz. Each pulse will deposit 357 kJ in the tungsten causing an instantaneous temperature rise of approximately 100 °C, during the 2.86 ms pulse duration. The target is wheel-shaped, 2.6 m in diameter, and will be rotating at 0.39 Hz to distribute the proton beam loads. The tungsten target consists of nearly 7000 bricks with dimensions 80×30×10 mm3. The bricks are separated by 2 mm wide cooling channels in which helium gas flows at a rate of 2.85 kg/s. The proton beam will be more powerful than the beam... (More)
The European Spallation Source (ESS), which is currently under construction in Lund, will use pure tungsten as the spallation neutron target. The tungsten will be irradiated by a 2 GeV proton beam, pulsed at a repetition rate of 14 Hz. Each pulse will deposit 357 kJ in the tungsten causing an instantaneous temperature rise of approximately 100 °C, during the 2.86 ms pulse duration. The target is wheel-shaped, 2.6 m in diameter, and will be rotating at 0.39 Hz to distribute the proton beam loads. The tungsten target consists of nearly 7000 bricks with dimensions 80×30×10 mm3. The bricks are separated by 2 mm wide cooling channels in which helium gas flows at a rate of 2.85 kg/s. The proton beam will be more powerful than the beam used at any other existing neutron spallation facility. Thanks to the high beam power and innovative moderator design, ESS will become the brightest neutron source in the world. 
However, designing the target is challenging, as its structure should withstand loads from the high beam power. The high intensity proton beam will not only cause cyclic thermo-mechanical loading of the tungsten, but also irradiation damage in the form of displacements of atoms in the microstructure, and production of a wide range of radioactive isotopes such as gaseous transmutation elements, and a significant fraction of solid transmutation elements, which will alter the thermal and mechanical properties of tungsten. Using such a powerful beam requires an optimal design of the target and a good understanding of the complex physical, mechanical, and thermal changes occurring in irradiated tungsten. It is also important to identify and mitigate potential issues and accidental scenarios during operation of the ESS target. 
The present work aims to characterise thermal and mechanical properties of high-energy proton and spallation neutron irradiated pure tungsten, for the use as a spallation target material. The work includes studies on fatigue properties of unirradiated tungsten from various processing routes, to determine which type of tungsten is the most durable under cyclic loads. The fatigue study served as a basis for setting the maximum acceptable stress of 100 MPa in the tungsten bricks during operation, as well as choosing rolling as the most suitable manufacturing method. The tungsten volume will be contained in a stainless-steel target vessel, which confines the helium gas target-coolant. Tungsten is known to be readily oxidised even at moderate temperatures, which makes even impurity levels of oxygen and water vapour in the helium a potential issue. Therefore, oxidation behaviour of tungsten in mildly oxidising atmospheres and temperatures relevant to ESS operation, was characterised. The results were used to set the upper temperature limit in the tungsten target during normal operation, as well as the maximum allowable temperature during off-normal incidents. However, both temperature and thermo-mechanical stress in the tungsten will alter as the properties of the material change due irradiation. 
The available data on proton irradiated tungsten is very limited. Therefore, in the present work, large efforts have been made for studying tungsten irradiated under similar conditions as the future ESS target material. Specifically, irradiation induced changes in thermal diffusivity, hardness, ductility, and ultimate tensile strength of tungsten irradiated by a high power proton beam at Paul Scherrer Institute (PSI), were studied. The results point towards a severe embrittlement of irradiated tungsten. Virtually zero plasticity was found in specimens tested up to 500 °C, and the hardness increased by more than 50%. Thermal diffusivity decreased by 28-51%, depending on the test temperature (25-500 °C). All tested irradiated specimens had lower irradiation damages than the ESS tungsten is expected to accumulate during the 5-year life time of the target.
Finally, a concluding study is presented in which new calculations of temperature and the maximum stress in the bricks were made, based on the obtained experimental data on the thermal and mechanical properties of irradiated tungsten. 
Keywords: ESS, Tungsten, Spallation target, Mechanical properties, Fatigue, Oxidation, Irradiation effects, Thermal diffusivity.
(Less)
Abstract (Swedish)
Inom ett par år kommer det nya materialforskningscentret ESS, The European Spallation Source, att stå färdigbyggt. Det kommer då vara det mest avancerade forskningsinstitutet i sitt slag och leverera upp till 30 gånger fler neutroner än någon annan neutronkälla i världen. En av de viktigaste komponenterna på ESS är det s.k. strålmålet som består av nästan 7000 bitar av det metalliska grundämnet volfram. Volframets enda, och för ESS helt avgörande uppgift, är att ge ifrån sig så många av sina neutroner som möjligt. Neutronerna produceras i den s.k. spallationsprocessen som går ut på att skjuta protoner med hög hastighet mot volfram för att på så sätt slå ut dess neutroner.

Protonstrålen levererar 14 pulser i sekunden och är mycket... (More)
Inom ett par år kommer det nya materialforskningscentret ESS, The European Spallation Source, att stå färdigbyggt. Det kommer då vara det mest avancerade forskningsinstitutet i sitt slag och leverera upp till 30 gånger fler neutroner än någon annan neutronkälla i världen. En av de viktigaste komponenterna på ESS är det s.k. strålmålet som består av nästan 7000 bitar av det metalliska grundämnet volfram. Volframets enda, och för ESS helt avgörande uppgift, är att ge ifrån sig så många av sina neutroner som möjligt. Neutronerna produceras i den s.k. spallationsprocessen som går ut på att skjuta protoner med hög hastighet mot volfram för att på så sätt slå ut dess neutroner.

Protonstrålen levererar 14 pulser i sekunden och är mycket kraftfull. Energin från protonerna som träffar volframet motsvarar energin som krävs för att stoppa en Mercedes-Benz S-klass som kör i 250 km/h – varje sekund. Varje puls kommer att öka temperaturen i volframet med ca 100 °C, vilket innebär att volframet får utstå höga cykliska termiska och mekaniska belastningar. Volframet kommer även att få strålskador av protonstålen och bli radioaktivt. Protonerna slår dels sönder volframatomerna, vilket omvandlar dem till en mängd andra, mer eller mindre stabila ämnen, dels skjuter de iväg hela atomen från sin ordinarie position vilket skapar vakanser i gittret. Mängden vakanser har en starkt negativ inverkan på materialets egenskaper.

Hanteringen av radioaktiva volframprover är mycket komplicerad och begränsad till särskilda laboratorier med s.k. hotcells, där proverna bearbetas med hjälp av robotarmar. På grund av svårigheten att dels producera, dels hantera radioaktivt volfram finns det i nuläget väldigt begränsad kunskap om hur materialet påverkas av strålning. Det är svårt att förutsäga hur väl det bestrålade volframet kommer att stå emot de kraftiga protonpulserna.

En stabil och tillförlitlig drift av ESS kräver en noggrann design av strålmålet. För att undvika att volframet oxiderar måste maxtemperaturen sättas på rätt nivå, detsamma gäller för den högsta tillåtna spänningen i materialet. Hänsyn måste även tas till att materialegenskaperna förändras av bestrålning, bl.a. försämras värmeledningsförmågan vilket leder till ännu högre temperaturer som i sin tur leder till ännu högre spänningar. Dessutom blir bestrålat volfram extremt sprött, och risken för utmattning som följd av den cykliska belastningen ökar kraftigt.

I den här avhandligen presenteras studier gjorda på både bestrålat och obestrålat volfram. Med hjälp av olika experiment kartläggs materialegenskaperna med särskilt fokus på volframets roll som neutronproducerande strålmål. Oxidationsstudier resulterade i ett förslag på 500 C som maxtemperatur. Utmattningsstudier visade att valsat volfram är överlägset bäst och att den högsta tillåtna spänningen borde sättas till 100 MPa. Mätningar av hårdhet, brottgräns och termisk diffusivitet av bestrålat volfram gav svar på hur strålning förändrar materialegenskaperna. Hårdheten ökade med mer än 50% och den termiska diffusiviteten minskade med 28-51%, beroende på testtemperaturen. Proverna blev extremt spröda och bröts utan några tecken på föregående deformation. Den experimentella datan användes för att simulera hur temperaturen och spänningen i volframet påverkas av de nya materialegenskaperna. Resultatet har använts för att förbättra designen av strålmålet.

Nyckelord: ESS, Volfram, Spallation, Mekaniska egenskaper, Utmattning, Oxidation, Strålningseffekter, Termisk diffusivitet. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Sevillano, Javier Gil, University of Navarra, Spain
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Utmattning, Termisk diffusivitet, Volfram, Spallation, Strålningseffekter, ESS, Oxidation, Mekaniska egenskaper, Mechanical Properties, Thermal diffusivity, Tungsten, ESS, Spallation target, Irradiation effects, Fatigue, Oxidation
pages
266 pages
publisher
Department of Mechanical Engineering, Lund University
defense location
M:B, Building M, Ole Römers väg 1, Lund University, Faculty of Engineering LTH.
defense date
2018-10-26 10:00:00
ISBN
978-91-7753-845-5
978-91-7753-844-8
language
English
LU publication?
yes
id
299cdf7c-17a3-49c2-b572-5a9f448beb6b
date added to LUP
2018-09-30 21:28:54
date last changed
2018-11-21 21:41:55
@phdthesis{299cdf7c-17a3-49c2-b572-5a9f448beb6b,
  abstract     = {The European Spallation Source (ESS), which is currently under construction in Lund, will use pure tungsten as the spallation neutron target. The tungsten will be irradiated by a 2 GeV proton beam, pulsed at a repetition rate of 14 Hz. Each pulse will deposit 357 kJ in the tungsten causing an instantaneous temperature rise of approximately 100 °C, during the 2.86 ms pulse duration. The target is wheel-shaped, 2.6 m in diameter, and will be rotating at 0.39 Hz to distribute the proton beam loads. The tungsten target consists of nearly 7000 bricks with dimensions 80×30×10 mm<sup>3</sup>. The bricks are separated by 2 mm wide cooling channels in which helium gas flows at a rate of 2.85 kg/s. The proton beam will be more powerful than the beam used at any other existing neutron spallation facility. Thanks to the high beam power and innovative moderator design, ESS will become the brightest neutron source in the world. <br/>However, designing the target is challenging, as its structure should withstand loads from the high beam power. The high intensity proton beam will not only cause cyclic thermo-mechanical loading of the tungsten, but also irradiation damage in the form of displacements of atoms in the microstructure, and production of a wide range of radioactive isotopes such as gaseous transmutation elements, and a significant fraction of solid transmutation elements, which will alter the thermal and mechanical properties of tungsten. Using such a powerful beam requires an optimal design of the target and a good understanding of the complex physical, mechanical, and thermal changes occurring in irradiated tungsten. It is also important to identify and mitigate potential issues and accidental scenarios during operation of the ESS target. <br/>The present work aims to characterise thermal and mechanical properties of high-energy proton and spallation neutron irradiated pure tungsten, for the use as a spallation target material. The work includes studies on fatigue properties of unirradiated tungsten from various processing routes, to determine which type of tungsten is the most durable under cyclic loads. The fatigue study served as a basis for setting the maximum acceptable stress of 100 MPa in the tungsten bricks during operation, as well as choosing rolling as the most suitable manufacturing method. The tungsten volume will be contained in a stainless-steel target vessel, which confines the helium gas target-coolant. Tungsten is known to be readily oxidised even at moderate temperatures, which makes even impurity levels of oxygen and water vapour in the helium a potential issue. Therefore, oxidation behaviour of tungsten in mildly oxidising atmospheres and temperatures relevant to ESS operation, was characterised. The results were used to set the upper temperature limit in the tungsten target during normal operation, as well as the maximum allowable temperature during off-normal incidents. However, both temperature and thermo-mechanical stress in the tungsten will alter as the properties of the material change due irradiation. <br/>The available data on proton irradiated tungsten is very limited. Therefore, in the present work, large efforts have been made for studying tungsten irradiated under similar conditions as the future ESS target material. Specifically, irradiation induced changes in thermal diffusivity, hardness, ductility, and ultimate tensile strength of tungsten irradiated by a high power proton beam at Paul Scherrer Institute (PSI), were studied. The results point towards a severe embrittlement of irradiated tungsten. Virtually zero plasticity was found in specimens tested up to 500 °C, and the hardness increased by more than 50%. Thermal diffusivity decreased by 28-51%, depending on the test temperature (25-500 °C). All tested irradiated specimens had lower irradiation damages than the ESS tungsten is expected to accumulate during the 5-year life time of the target. <br/>Finally, a concluding study is presented in which new calculations of temperature and the maximum stress in the bricks were made, based on the obtained experimental data on the thermal and mechanical properties of irradiated tungsten. <br/>Keywords: ESS, Tungsten, Spallation target, Mechanical properties, Fatigue, Oxidation, Irradiation effects, Thermal diffusivity. <br/>},
  author       = {Habainy, Jemila},
  isbn         = {978-91-7753-845-5 },
  language     = {eng},
  month        = {10},
  publisher    = {Department of Mechanical Engineering, Lund University},
  school       = {Lund University},
  title        = {Characterisation of mechanical and thermal properties of tungsten for high power spallation target applications},
  url          = {https://lup.lub.lu.se/search/ws/files/52092275/Jemila_Habainy_Digital.pdf},
  year         = {2018},
}