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NRX (National Research Experimental) was a heavy water moderated, light water cooled, nuclear research reactor at the Canadian Chalk River Laboratories, which came into operation in 1947 at a design power rating of 10 MW (thermal), increasing to 42 MW by 1954. At the time of its construction it was Canada's most expensive science facility and the world's most powerful nuclear research reactor.〔(Scientific Research Helps World War II War Effort - National Research Council Canada )〕 NRX was remarkable both in terms of its heat output and the number of ''neutrons'' it generated. When a nuclear reactor is operating its nuclear chain reaction generates many billions of free neutrons, and in the late 1940s NRX was the most intense neutron source in the world. NRX experienced one of the world's first major reactor accidents on 12 December 1952. The reactor began operation on 22 July 1947 under the National Research Council of Canada, and was taken over by Atomic Energy of Canada Limited (AECL) shortly before the 1952 accident. The accident was cleaned up and the reactor restarted within two years. NRX operated for 45 years, being shut down permanently on 8 April 1993.〔(【引用サイトリンク】url = http://www.candu.org/candu_reactors.html )〕 It is currently undergoing decommissioning at the Chalk River Laboratories site. NRX was the successor to Canada's first reactor, ZEEP. Because the operating life of a research reactor was not expected to be very long, in 1948 planning started for construction of a successor facility, NRU, which went critical in 1957. == Design == A heavy water moderated reactor is governed by two main processes. First, the water slows down (moderates) the neutrons which are produced by nuclear fission, increasing the chances of the high energy neutrons causing further fission reactions. Second, control rods absorb neutrons and adjust the power level or shut down the reactor in the course of normal operation. Either inserting the control rods or removing the heavy water moderator can stop the reaction. The NRX reactor incorporated a calandria, a sealed vertical aluminium cylindrical vessel with a diameter of 8 m and height of 3 m. The core vessel held about 175 six-centimetre-diameter vertical tubes in a hexagonal lattice, 14,000 litres of heavy water and helium gas to displace air and prevent corrosion. The level of water in the reactor could be adjusted to help set the power level. Sitting in the vertical tubes and surrounded by air were fuel elements or experimental items. This design was a forerunner of the CANDU reactors. The fuel elements contained fuel rods 3.1 m long, 31 mm in diameter and weighing 55 kg, containing uranium fuel and sheathed in aluminium. Surrounding the fuel element was an aluminium coolant tube with up to 250 litres per second of cooling water from the Ottawa River flowing through it. Between the coolant sheath and the calandria an air flow of 8 kg/second was maintained. Twelve of the vertical tubes contained control rods made of boron carbide powder inside steel tubes. These could be raised and lowered to control the reaction, with any seven inserted being enough to absorb sufficient neutrons that no chain reaction could happen. The rods were held up by electromagnets, so that a power failure would cause them to fall into the tubes and terminate the reaction. A pneumatic system could use air pressure from above to quickly force them into the reactor core or from below to slowly raise them from it. Four of these were called the ''safeguard bank'' while the other eight were controlled in an automatic sequence. Two pushbuttons on the main panel in the control room activated magnets to seal the rods to the pneumatic system, and the pushbutton to cause the pneumatic of the rods into the core was located a few feet away. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「NRX」の詳細全文を読む スポンサード リンク
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