Introduction to Nuclear Fusion Prof. Dr. Yong-Su Na 1
Irradiation 2
1. 중성자와물질과의상호작용 중성자는물질을구성하는원자의궤도전자에 Coulomb force 를 미치지않기때문에원자, 분자를직접전리시키지못함. 주요상호작용 - 탄성산란 - 비탄성산란 - 중성자포획 - 하전입자의방출 ( 핵변환 ) - 핵분열반응 중성자와 plasma ion 간 scattering 효과는무시할만한가? 왜? 3
1) 탄성산란 중성자가원자핵과탄성충돌하여원자핵은운동에너지를가지고 recoil 되고, 중성자는그만큼운동에너지를잃고산란되는현상 (n,n) 중성자의에너지 < 원자핵의여기에너지 (~1 MeV) Maximum of energy transferred to the atom by head-on collision ( 반발원자핵의에너지 ) 1 2 4m1m2 4A ( E) max m1v 1 2 2 2 ( m m ) ( A 1) 1 2 E inc A: 반발원자핵의질량수 E inc : kinetic energy of the incident neutron - 수소원자핵일때최대 - 중원소의경우에는무시할수있을만큼작음. - 가벼운원소의물질은투과하기가어렵고, 무거운원소의물질은투과하기쉬움. 반발원자핵은궤도전자의일부또는전부를바꿀정도로고속으로움직임. 근처의원자, 분자를전리 여기시킴 ( 반발핵에의한간접적인전리 여기 ). 4
2) 비탄성산란 중성자가원자핵과충돌하면서원자핵에반발에너지를줌과동시에핵을여기시키는현상 (n,n ) 중성자에의해여기된원자핵은곧 γ선을방출하고기저상태로돌아감. 방출된 γ선이주위의원자, 분자를전리 여기시킴 ( 간접적인전리 여기 ). 중성자의에너지가높을수록일어나기쉬움 (> 핵의여기에너지 ). 수 MeV가되면탄성산란과같은비율로일어남. 핵의여기에너지보다작아지면일어나지않음. 5
3) 포획 중성자의속도가느려지면 (< 1 kev) 원자핵에충돌하여도산란이일어나지않고그대로원자핵에포획되어흡수됨. 포획직후원자핵은여기상태가되어에너지를 γ선 ( 포획 γ선 ) 의형태로방출하고안정화됨 (n,γ). 중성자를포획한원자핵은질량수가 1만큼많은동위원소가됨. 방사성인경우가많음. ex) 59 Co(n,γ) 60 Co 중성자의에너지가낮을수록일어나기쉬움. 1 ev 이하에서포획단면적은 1/v에비례하여증가열중성자에서쉽게발생 cf) 고속중성자 (500 kev ~ 10 MeV). 저속중성자 (1 ev ~ 500 kev) 공명포획 : 특정한원자핵이특정한에너지의중성자에대하여높은포획단면적을나타내는현상. 핵종에따라고유값을가짐. 결국중성자는열중성자로변하여원자핵에포획 흡수되거나 혹은베타붕괴를통해붕괴함 ( 반감기 ~ 14.8 분 ). n p e e 6
4) 하전입자의방출 ( 핵변환 ) 고에너지의중성자가원자핵에충돌하여복합핵을형성하고, 형성된여기상태의복합핵이양성자와알파입자등과같은하전입자를방출하고다른원소로변환되는반응. 방출된하전입자는주위의원자, 분자를전리 여기시킴 ( 간접적인전리 여기 ). 핵반응의단면적은산란단면적에비해압도적으로작음. 중성자의에너지가어느 threshold 이상일경우발생 ( > 수 MeV). 에너지가높을수록여기상태의복합핵의여기에너지가높아지기때문에하전입자의운동에너지의형태로방출함. Cf. 14 N(n,p) 14 C는저속중성자에의해발생. 10 B(n,α) 7 Li은열중성자에의해발생. 7
5) 핵분열반응 큰원자 ( 보통우라늄, 플루토늄 ) 의원자핵이두개이상의다른원자핵으로쪼개지는현상 핵분열의결과로보통 2, 3개의중성자가다시생겨남. 연쇄반응 1938년독일의오토한과프리츠스트라우스만이우라늄에중성자를조사시키면바륨의동위원소가생성된다는것을처음으로입증 8
2. 물질에대한조사손상 물질에대한조사손상의영향은다음과같이분류되어짐. - Impurity production - Atomic displacement - Ionization 9
1) Impurity Production 방사성핵으로변환되는것은중성자에의한중성자포획, 하전입자방출, 핵분열등을통해발생. 중성자가입사한경우, (n,α) 및 (n,p) 반응등을통해양성자및알파입자가생성되고, 재료내에서중성화되어양성자는수소로, 알파입자는헬륨이됨. 실내온도에서는이들이기체로존재하므로, 근접한원자에게압력을가하여고체에서는이내부압력에물질의 swelling을유발. 방사선조사에의해생겨난불순물은결정체에전기적, 기계적성질을바꿀수있는구조결함을야기함. 중성자포획을통해발생한방사성핵종 ( 동위원소 ) 은 decay scheme 에따라붕괴하면서화학원소를변화시킴. 붕괴진행과정에서방출되는방사선은물질에흡수되어방출에너지에따라물질과의상호작용을반복함. 이온이조사된경우, 핵의이온흡수는즉시화학원소를바꾸어핵변환이일어남. 10
2) Atomic Displacement 원자의정상적인위치에서원자의위치이동. 즉, 위치를이동한원자는빈격자공간을남기고, interstitial 위치에머물거나혹은격자구조에서다른원자와의내부교체가발생. 양성자, 중성자, 150 kev 이상의전자에의해주로발생 Atomic displacement는탄성충돌을통해또는방사선에의해유기된여기 (excitation) 를원자운동, 즉되튀김운동으로변환함에의해발생. ( 하전입자가물질을통과시, 하전입자에너지는궤도전자의여기및물질들의 nuclei와의탄성충돌에소비됨.) 탄성충돌에의해튕겨난원자는 primary knock-on. Interstitial, vacancy와함께 Frenkel pair 구성. 이차 displacement를야기하기도함. 결함 (vacancy, interstitial, Frenkel pairs, dislocation 등 ) 은이차입자의비적을따라발생. 비전리에너지손실 (no-ionizing energy loss: NIEL) 을사용하여정량화. NIEL, MeV/m, MeV m 2 /kg: 단위길이당비전리 events에의해손실된에너지 (displacement damage는충돌에의한에너지손실에비례 ) 11
2) Atomic Displacement Vacancy 결함 Interstitial 결함 Substitutional atom Frenkel 결함 ( 공공과침입형원자의쌍 ) Schottky 결함 ( 양이온공공과음이온공공의쌍 ) 12
2) Atomic Displacement Edge and Screw Dislocation ( 전위 ) Stress states around an edge dislocation http://en.wikipedia.org/wiki/dislocation 13
2) Atomic Displacement Temperature dependency of shape of dislocation loops Flower shape (420, Fe-8Cr-1) General shape (380, Pure Fe) Courtesy of Bumsu Park (Hokkaido University)
2) Atomic Displacement MD simulation of a displacement cascade produced by a 10 kev primary knock-on atom in an fcc lattice (Ghaly and Averback) 15
3) Ionization 원자로부터의전자제거와하전입자의이동경로에서이온쌍을형성하는것 전리는전자가중성원자에붙거나제거되는것. α, β, p은물질을직접적으로전리시킬수있지만, n, γ는간접적으로일으킴. 전리에의해아래순서로분자형성에대한손상이증가함. - Metallic bond (least damaged): 금속결합 - Ionic bond: 이온결합 - Covalent bond (most damaged): 공유결합 ex) 생체조직 - 작은체적에서의 large energy release. Thermal heating: 방사차폐체에중요 물과유기물에서흡수된전리에너지의대부분은화학적결합을깸. 금속의경우에는열로나타나고결과적으로물질의성질을변화시킬수있음. 중성자는 metallic bond, ionic bond에주로손상을가함. 16
Physical Phenomena in Radiation Effects 17
Simulation Hierarchy 18
Fusion materials 19
Radial Build in Fusion Reactors Blanket Shield Vacuum vessel Radiation Plasma Neutrons First Wall Tritium breeding zone Coolant for energy conversion Magnets 20
Candidate Materials in Fusion First Wall Blanket Shield Vacuum vessel - High Z (W, W-alloy) - Low Z (Be, Be-alloy) Radiation - C/C composite Blanket Structural Plasma material - FM steel (ODS steel) Neutrons - V-alloy - SiC f /SiC composite First Wall Tritium breeding zone Coolant for energy conversion Magnets Tritium Breeding material - Li based Oxides - Liquid Li (Li, Li-Pb) - Flibe / Flinabe Coolant - Li, LiPb, He - Molten salt, Water Magnets - Nb 3 Sn - Nb 2 Al 3 - HTS Neutron multiplier - Be, LiPb Shield - Ceramics Tritium Storage Bed - ZrCo - Uranium 21
Candidate Materials in Fusion First Wall Blanket Shield Vacuum vessel - High Z (W, W-alloy) - Low Z (Be, Be-alloy) - C/C composite Radiation Plasma Neutrons First Wall Tritium breeding zone Coolant for energy conversion Magnets 22
The First Wall and Other Materials The first wall - Withstand a tremendous amount of heat from the plasma - must not contaminate the plasma - be compatible with the fusion products that impinge on them Small scale PFCs for Tests (EU) Hot isostatic Pressing 23
The First Wall and Other Materials The Divertor - Bending outer magnetic fields away from plasma by means of auxiliary magnetic coils Removing outer layer of plasma to external chamber Cooling Neutralising Pumping away 24
The First Wall and Other Materials The Divertor - Bending outer magnetic fields away from plasma by means of auxiliary magnetic coils Removing outer layer of plasma to external chamber Cooling Neutralising Pumping away 25
The First Wall and Other Materials G. Lewis, Selection of Engineering Materials, Prentice Hall. Inc., K Englewood cliffs. NJ, 1990 26
The First Wall and Other Materials 27
Candidate Materials in Fusion First Wall Blanket Shield Vacuum vessel - High Z (W, W-alloy) - Low Z (Be, Be-alloy) - C/C composite Radiation Blanket Structural Plasma material - FM* steel (ODS** steel) - V-alloy - SiC f /SiC composite Neutrons First Wall Tritium breeding zone Coolant for energy conversion Magnets *FM: Ferritic Martensitic **ODS: Oxide Dispersion-Strengthened 28