Tham khảo tài liệu 'electricity infrastructures in the global marketplace part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The Role of Nuclear in the Future Global Energy Scene 69 positively charged atomic nuclei ions . Normally fusion is not possible because the positively charged nuclei naturally repel each other. But as the temperature increases the ions move faster and they collide at speeds high enough to overcome the normal repulsion. The nuclei can then fuse causing a release of energy. In the sun massive gravitational forces create the right conditions for this but on Earth they are much harder to achieve. Fusion fuel - different isotopes of hydrogen - must be heated to extreme temperatures of over ten million degrees Celsius and must be kept dense enough and confined for long enough at least one second to trigger the energy release. The aim of the controlled fusion research program is to achieve ignition which occurs when enough fusion reactions take place for the process to become self-sustaining with fresh fuel then being added to continue it. Basic Fusion Technology With current technology the reaction most readily feasible is between the nuclei of the two heavy forms isotopes of hydrogen - deuterium D and tritium T . Each D-T fusion event releases MeV x 10-12 joule compared with 200 MeV for a U-235 fission . Deuterium occurs naturally in sea water 30 grams per cubic meter which makes it very abundant relative to other energy resources. Tritium does not occur naturally and is radioactive with a half-life of around 12 years. It can be made in a conventional nuclear reactor or in the present context bred in a fusion system from lithium. Lithium is found in large quantities 30 parts per million in the Earth s crust and in weaker concentrations in the sea. While the D-T reaction is the main focus of attention long-term hopes are for a D-D reaction but this requires much higher temperatures. In a fusion reactor the concept is that neutrons will be absorbed in a blanket containing lithium that surrounds the core. The lithium is then transformed into tritium and .
