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Nuclear reactors and their fuel cycles
Published in R.J. Pentreath, Nuclear Power, Man and the Environment, 2019
A simplified plan of a magnox reactor is shown in figure 4.1. In the earliest types, the reactor pressure vessel was contained within a steel sphere connected by ducts to the steam-generator units. Later designs consist of pre-stressed concrete pressure vessels with an integral arrangement of steam generators; this allowed more than a two-fold increase in reactor size. The reactor core of a magnox reactor is large relative to those of other designs and requires a more or less continuous refuelling process. For this to be done while the reactor is on-load requires a complex fuel-handling system which, operated by remote control, removes spent fuel rods and inserts new ones. A variation of the graphite-moderated reactor, which does not use natural uranium, has been used in the USSR. These reactors have ordinary light water instead of carbon dioxide as coolant; they are therefore referred to as light water graphite reactors (LWGR).
Moderator and Reflector Materials
Published in C. K. Gupta, Materials in Nuclear Energy Applications, 1989
A summary is now in order. The purpose or chief function of a moderator is to moderate or slow down fast fission neutrons from high energy to thermal energy in a thermal reactor. On the other hand, the purpose of the reflector is to reflect leaking neutrons back into the core in either thermal or fast reactors. From a nuclear consideration, the primary requirements of moderator and reflector materials are the same: (1) low neutron absorption, (2) high neutron scattering, and (3) low mass number (large energy loss by neutron per collision). For solid moderators and reflectors, desirable characteristics include (1) fabricability, (2) structural strength, (3) corrosion resistance, (4) high density of moderators or reflectors, (5) high thermal conductivity, and (6) good thermal and radiation stability. For fluid moderators or reflectors, desirable characteristics include (1) melting point below room temperature, (2) low vapor pressure at high temperature, (3) being noncorrosive to structural materials, (4) high density of moderating or reflector atoms, (5) good heat transfer properties, and (6) good thermal and radiation stability. Table 1 shows the major moderator-reflector materials along with their requirements. Their leading characteristics are presented in Table 2. Among the major moderator and reflector materials, in a thermal reactor, ordinary water (H2O) and heavy water (D2O) not only perform the functions of moderator and reflector simultaneously, but also function as coolant material. Because of its abundance and economics, ordinary water has been used extensively as the coolant and working substance in LWRs. Of all the elements, beryllium and carbon are the only attractive solid moderators. They can simultaneously perform the function of moderator, reflector, and structural materials. Graphite has long been used as moderator, reflector, and structural material in the gas-cooled, graphite-moderated reactor or high-temperature gas-cooled reactor. The first nuclear fission reactor, CP-1, in 1942 was a natural uranium-graphite reactor. Table 3 presents moderators employed in commercial thermal reactor types along with their required fuels.
Three tons of uranium from the International Atomic Energy Agency: diplomacy over nuclear fuel for the Japan Research Reactor-3 at the Board of Governors’ meetings, 1958–1959
Published in History and Technology, 2021
The JRR-3 was a heavy water reactor (HWR) that used natural uranium. According to Yoshioka Hitoshi, the Ministry of International Trade and Industry’s committee on the budget for nuclear power research mentioned above decided to adopt an HWR design as early as November 1954. An HWR can use natural uranium and produce plutonium. By using natural uranium, Japan could avoid relying on enriched uranium from the US, and an HWR was considered easier to construct than other natural uranium reactors, such as a graphite-moderated reactor.40 In the final design, the JRR-3 had a relatively large thermal output of 10 megawatts. It was a multipurpose reactor, whose purposes included not only academic research but also nuclear power-related data collection and the accumulation of experience in the construction and operation of a nuclear reactor. Additionally, the reactor was expected to produce radioisotopes and plutonium.41 To secure natural uranium, Japan started massive uranium prospecting within the country and discovered deposits in Ningyotōge Pass in 1955, while also initiating negotiations with outside suppliers.42 Requesting the IAEA’s assistance was an extension of Japan’s effort to secure natural uranium.
Thermal Design of the TREAT Facility
Published in Nuclear Technology, 2020
The TREAT Facility is an air-cooled graphite-moderated reactor located at Idaho National Laboratory that is used to performed transient irradiations of nuclear fuel and material. The reactor recently resumed transient operations in 2017. An overview of the reactor thermal properties and characteristics is provided. The large thermal capacity of the reactor core and relatively small cooling rate allow for near adiabatic heating during transient operations. Although not required in any way to protect the reactor core, the reactor has two blowers capable of providing up to 8000 cfm of air flow through the reactor cavity to accelerate reactor cooling time and conduct heat balances across the core that are essential for the calibration of nuclear instruments. The temperature profiles of the core vary widely depending on core configuration, experiment, transient shape, and air flow. Various profiles are provided based on both measured (in core and experimental) and calculated values. Additionally, maximum transient energy and steady-state power are calculated based on measured fuel and coolant characteristics.
Safe, clean, proliferation resistant and cost-effective Thorium-based Molten Salt Reactors for sustainable development
Published in International Journal of Sustainable Energy, 2022
It is interesting to note that some of the first commercial LWRs developed in the late 1950s and early 1960s in the United States were initially operated with thorium-based fuels (OECD/NEA 2015). In fact, 3 thorium reactors have been operating for longer periods already (Kazimi 2004) and more are planned (Schaffer 2013). The first was a gas-cooled, graphite-moderated reactor called Peach Bottom Unit One, located in Pennsylvania, which used a combination of thorium and highly enriched uranium in the mid-1960s. Then, another gas-cooled reactor at Fort St. Vrain in Colorado was run on a similar thorium-based fuel between 1976 and 1989. Lastly, was the German THTR-300 – a high-temperature gas-cooled, 300-megawatt reactor outside Hamburg which operated in the 1980s.