摘要
The lineage of the Dragon Project can be traced back to 1955 when the United Kingdom launched a nuclear power programme which involved the construction of large graphite moderated reactors fuelled with natural uranium and cooled by carbon dioxide. Not long afterwards the European Nuclear Energy Agency (ENEA) of the then newly formed Organisation for European Economic Cooperation (OEEC), in the spirit of the time, sought to encourage the construction of nuclear power stations and the development of joint nuclear undertakings. The United Kingdom Atomic Energy Research Establishment (AERE) had, since 1949, been studying possible long term improvements in energy conversion efficiency resulting from higher coolant gas temperatures and the use of ceramic materials. A 1955 paper on gas-cooled reactors using the U-233/thorium cycle attracted interest and this progressed to the definition of an initial programme. The high temperature work led to a proposal for a 20 MW(Th) Reactor Experiment and one important consequence of the ENEA/OEEC initiative was the setting up in April 1959 of the international Dragon Project Agreement.Initial experiments at Harwell in 1957 had involved the coating of small spheroidal particles of uranium carbide or oxide with pyrolytic carbon which were then bonded with carbonaceous material. But experiments demonstrated that fission products such as caesium, strontium or barium could diffuse through such coatings. This led in 1961 to the modification of the coated particle design by the addition of an intermediate layer of silicon or zirconium carbide. The small size of the particles necessitated a statistical approach to quality during manufacture and effort was concentrated on the minimisation of the broken or defective particle fraction.The subsequent operation of the Dragon Reactor for over 10 years confirmed the benign nature of a HTR. It also proved that fuel bodies made with coated particles were capable of maintaining a high degree of fission product containment at high temperatures and for high burn-ups. What is remarkable is the speed with which the particle design evolved. The success of the choices that were made resulted in the High Temperature Gas Cooled Reactor system being studied in its various forms by many countries.Irrespective of the particular core design, the basic component of HTR fuel is the coated particle. In the intervening years since the early HTRs were launched it has been realised worldwide that there are now even more factors favouring the use of high temperature reactors. They enable more efficient use of fissile isotopes as well as providing inherent reactor safety. Unlike most other reactor designs, the HTR can take economic advantage from operation at high gas outlet temperatures, whilst maintaining the good retention of fuel and fission products within the fuel elements. The fuel cycle can use uranium, thorium or plutonium. In addition the burn-up can be high, permitting safe disposal of spent fuel without the need for reprocessing. Fast reactors based on coated particle fuel may be a future possibility.The Dragon Project was a successful political collaboration and a technical triumph in demonstrating a new type of reactor. However, overstretched resources coupled with a world-wide trend in that era to favour water reactors caused work on Dragon to be terminated in March 1976. By then the main objectives for the Dragon Project had been successfully achieved and extensively reported. The progression from the 20 MWE(Th) Dragon to a 1200 MW(e) power station as a single jump in technology was too large a step for the time. The experiences of the other major civil HTR projects of the same period in Germany and the United States of America resulted in the high temperature reactor system being studied in its various forms by many countries as is evidenced by the large number of papers presented at the biennial HTR conferences. In the intervening years since the early HTRs were launched it has been realised that there are now even more factors favouring the use of high temperature reactors.