What is a graphite water-cooled stack?
Graphite water-cooled reactor is a thermal neutron reactor with graphite as moderator and water as coolant. In the early stage of industrial development, graphite water-cooled reactors were mainly used to produce weapon charge plutonium and neon. Such reactors typically use natural uranium metal components as fuel. The 235 of uranium in the natural uranium in the reactor absorbs neutrons to produce a nuclear fission reaction that releases neutrons and energy. Some of these neutrons are used to sustain the chain of nuclear fission reactions, and some are absorbed by uranium-238 in natural uranium and converted into plutonium-239 and other plutonium isotopes. Nuclear pure graphite masonry is used as moderator and reflector for structural graphite water-cooled stacks.
There are two or three horizontal channels (horizontal stacking) or vertical channels (vertical stacking) in graphite masonry. Replaceable graphite sleeves were inserted into these channels, and aluminum alloy process piping was inserted into the sleeves to separate the cooling water from the graphite moderator. There are ribs on the inner wall of the process pipe to maintain the gap between the process pipe and the fuel element. The temperature of each part of the graphite masonry is not uniform. By changing the gap between the graphite sleeve and the process pipe and the water flow in the process pipe, the temperature of the masonry can be partially adjusted to make the temperature distribution more flat. Typically, the fuel elements are made of rods with a diameter of about 35-38mm and a length of about 100-200mm. In order to increase the specific power and to even out the radial burnup of the elements, tubular fuel elements are also used. In the early development of production reactors, an open cooling mode was used. Even though the river flows through the reactor core, the hot water is discharged into the river. This method has been discontinued due to reasons such as high water consumption, high levels of radioactivity in the drainage, and outstanding environmental protection issues, and a closed cooling method is widely used, i.e. cooling water flows out of the core, heat is output to the reactor, and heat is transferred through heat exchangers The water to the other loop side is then returned to the reactor core through the main pump, forming a closed loop main cooling loop or main loop.
There are two ways to deal with the heat of the primary circuit water: one is to transfer the heat from the primary circuit to the secondary circuit water through a heat exchanger, which is then cooled by cooling towers or river water, and the heat is discharged to the environment. Another method is to transfer heat to a waste heat utilization system through a heat exchanger to provide heat to the outside world or generate electricity as a heat source. An important feature of natural uranium cold reactors is that the reserve reactivity is small. The reactivity of early graphite water-cooled reactors increased with temperature, and so did the reactor power (the so-called positive temperature effect), which led to an increase in reactivity until the reactor was placed in an external neutron absorber (control rod). etc.), or lead to vicious accidents such as core melting. In 1986, after the Chernobyl nuclear accident, the problem of positive temperature effect attracted attention from all quarters. In terms of reactor physical design, negative temperature effects are obtained to ensure the critical self-stability of the reactor.
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