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Reactor Heat Transfer
Published in G. Vaidyanathan, Dynamic Simulation of Sodium Cooled Fast Reactors, 2023
When a nuclear fission takes place, several fission products are formed. Also, neutrons, alpha, beta, and gamma rays are emitted. Most of the heat generated is instantaneously absorbed by the fuel and, to a lesser extent, in the coolant and structural material. The fission fragments are unstable (have excess neutrons) and undergo several transitions by beta emissions before reaching stability. Each of these processes results in heat generation, and this heat must be removed. Depending on the history of the core—i.e., duration of power production, power level, and isotopic content of fuel—the decay heat can be as much as 6–8% of the power at which the reactor operated before shutdown. Figure 3.7 shows the fission product decay heat vs. time curve for a typical fast reactor after shutdown. Decay heat data for all nuclides are available through data files such as evaluated nuclear data file (ENDF/BIV) available from Brookhaven National Laboratory (Rose and Burrows, 1976).
2 at High Temperatures under Reactor Accident Conditions
Published in J. T. Rogers, Fission Product Transport Processes in Reactor Accidents, 2020
C. E. L. Hunt, F. C. Iglesias, D. S. Cox
Oxidation of UO2 in both steam and air have been studied in the laboratory using unirradiated samples. Confirmatory tests were then done on irradiated UO2 in a “hot cell” facility. Fission product releases from the irradiated UO2 were measured using the arrangement shown schematically on Figure 1. The atmosphere at the sample was controlled by using flowing argon, steam, or air. A sensor, located downstream from the sample, gave a continuous record of the oxygen partial pressure (P(O2)). The fission products remaining in the sample were continuously monitored with a gamma-ray spectrometer. Gamma ray counting times were typically 150 to 300 seconds. Longer counting times decreased the statistical scatter of the fission product releases at the expense of resolution in time. This caused some difficulty in interpretation during periods of rapid changes in release rate. Deposition rods, located in the thermal gradient of the furnace downstream from the sample, were measured after the test to determine fission product deposition patterns as a function of temperature.
Vapor and Advanced Power Cycles
Published in Kavati Venkateswarlu, Engineering Thermodynamics, 2020
Nuclear fuel is a fuel used in a nuclear reactor to sustain a nuclear chain reaction. The general definition of the fuel that is a material used to produce heat is not applicable to nuclear fuel, since the heat in this case is generated by fission or disintegration of the fissile isotopes but not by the burning of fuel. Nuclear fuel is typically made up of one or more fissile isotopes such as 235U, 239Pu, and 233U, often in combination with a fertile isotope, 238U or 232Th. These fuels are in the solid form of metals, alloys, oxides, carbides, nitrides of uranium, plutonium, and thorium. The radioactive nature of nuclear fuels and their fission products are extremely hazardous to health. To avoid harmful effects of radiation, nuclear fuels are hermetically sealed inside a nonradioactive structural material known as cladding. Cladding is essentially an integral part of the nuclear fuel element, which serves different purposes. First it acts as a primary containment for radioactive fission products, and then it acts as a barrier avoiding direct contact of the fuel with coolant, and finally it transfers fission heat energy from fuel to coolant. Usually, nuclear fuel elements are manufactured in different shapes such as plates, pins, or rods and assembled in specific geometric configurations using spacers, end fittings, and other supporting hardware. The package of fuel elements is called a fuel assembly.
Safe, clean, proliferation resistant and cost-effective Thorium-based Molten Salt Reactors for sustainable development
Published in International Journal of Sustainable Energy, 2022
The online refuelling is also combined with a continuous removal of matter, which is beneficial for two reasons. The first is that most gaseous fission products (Xe, Kr, etc.) are continuously removed so there is no danger of release of these radioactive products, even under accident conditions (Furukawa et al. 2008; LeBlanc 2010; Moir and Teller 2005). If Xe-135 is allowed to burn off, the nuclear chain reaction will accelerate, which requires control rods to be reinserted in a carefully managed cycle until the reactor is stabilised. Mismanagement of this procedure contributed to the instability in the Chernobyl core that led to a runaway reactor and the explosion that followed (Hargraves and Moir 2010). Continuous removal of Xe-135 is therefore a great safety mechanism in itself. The second reason is that the fission products either quickly form stable fluorides that will stay within the salt during any leak or accident or are volatile or insoluble and are continuously removed (Furukawa et al. 2008; LeBlanc 2010).
Bifurcation Analysis of Xenon Oscillations in Large Pressurized Heavy Water Reactors with Spatial Control
Published in Nuclear Science and Engineering, 2022
Abhishek Chakraborty, Suneet Singh, M. P. S. Fernando
The safe operation of nuclear reactors is one of the most challenging aspects of the nuclear power industry. It involves a number of processes involving the generation of neutrons, their specific multiplications, their control, and the extraction of useful power out of the heat energy generated in the process. In most of the operating nuclear reactors, energy is produced by chain nuclear fission of 235U. In nuclear fission, a large nucleus like 235U is broken into multiple nuclei (fission products) with release of ~200 MeV of energy on absorption of a thermal/fast neutron(s). The heat which is produced in fission has to be removed by a coolant (generally H2O, D2O for thermal reactors and Na, Pb for fast reactors). This demands an efficient coupling between the neutronics and the thermal hydraulics to ensure a smooth, streamlined generation of power.
Reactor Physics Analysis Assessment of Feasibility of Using Advanced, Nonconventional Fuels in a Pressure Tube Heavy Water Reactor to Destroy Long-Lived Fission Products
Published in Nuclear Technology, 2021
The operation of nuclear reactors [including Generation 3+ (Gen-III+), Generation IV (Gen-IV), and Small Modular Reactor (SMR) technologies] produces high-level radioactive waste in the spent fuel, which contains numerous radioactive fission products, minor actinides (such as isotopes of Np, Pu, Am, Cm, and heavier elements). In addition, various radioactive isotopes are created in a nuclear reactor by neutron activation in structural components and in the coolant. Fortunately, many fission products have very short half-lives, ranging from seconds to less than a year, and will decay to relatively insignificant concentrations (similar to the radioactivity and radiotoxicity of natural uranium ore) in less than 10 years. Fission products such as 90Sr (Thalf-life ~ 29 years) and 137Cs (Thalf-life ~ 30 years) must be safely stored for at least 400 years [perhaps in a deep geological repository (DGR)] before they decay to less than 0.01% of their original level. In addition to minor actinides found in used nuclear fuel and the radioactive isotopes found in reactor structural components created by neutron activation, a key long-term problem for radioactive waste storage is the seven main long-lived fission products (LLFPs) listed in Table I, which have half-lives ranging from 100 000 years (for 126Sn) to 15 700 000 years (for 129I).