Nuclear Power.net published an outstanding review of nuclear power plants, their use of fuel, and their operation. A typical nuclear power plant has an electric-generating capacity of 1000 MWe. The heat source in the nuclear power plant is a nuclear reactor. As is typical in all conventional thermal power stations the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity. The turbines are heat engines and are subject to the efficiency limitations imposed by the second law of thermodynamics. In modern nuclear power plants the overall thermodynamic efficiency is about one-third (33%), so 3000 MWth of thermal power from the fission reaction is needed to generate 1000 MWe of electrical power.
This thermal power is generated in a reactor core, which contains especially the nuclear fuel (fuel assemblies), the moderator and the control rods. The core of the reactor contains all the nuclear fuel assemblies and generates most of the heat (fraction of the heat is generated outside the reactor – e.g. gamma rays energy). The assemblies are exactly placed in the reactor according to a fuel loading pattern.
A typical 1000 MWe (3,000 MWthermal) nuclear core may contain 157 fuel assemblies composed of over 45,000 fuel rods and some 15 million fuel pellets, (or an energy equivalent of 255 billion cu. feet of natural gas). Generally, a common fuel assembly contain energy for approximately 4 years of operation at full power. Once loaded, fuel stays in the core for 4 years depending on the design of the operating cycle. During these 4 years the reactor core have to be refueled. During refueling, every 12 to 18 months, some of the fuel – usually one third or one quarter of the core – is removed to spent fuel pool, while the remainder is rearranged to a location in the core better suited to its remaining level of enrichment. The removed fuel (one third or one quarter of the core, i.e. 40 assemblies) has to be replaced by a fresh fuel assemblies. It follows, there are about 3-4 fuel batches which differ each other in the fuel burnup.
The total energy released in a reactor is about 210 MeV per 235U fission, distributed as shown in the table. In a reactor, the average recoverable energy per fission is about 200 MeV, being the total energy minus the energy of antineutrinos that are radiated away. This means that about 3.1⋅1010 fissions per second are required to produce a thermal power of 1 W. Since 1 gram of any fissile material contains about 2.5 x 1021 nuclei, the fissioning of 1 gram of fissile material yields about 1 megawatt-day (MW/day) of heat energy.
Natural uranium refers to uranium with the same isotopic ratio as found in nature. It consists primarily of isotope 238U (99.28%), therefore the atomic mass of uranium element is close to the atomic mass of 238U isotope (99.28%). Natural uranium also consists of two other isotopes: 235U (0.71%) and 234U (0.0054%). The abundance of isotopes in the nature is caused by difference in the half-life's. All three naturally-occurring isotopes of uranium (238U, 235U and 234U) are unstable. On the other hand these isotopes (except 234U) belong to primordial nuclides, because their half-life is comparable to the age of the Earth (~4.5×109 years for 238U).
Since natural uranium contains only 0.71% of fissile isotope 235U and most of current power reactors require enriched uranium, this natural uranium must be enriched. The level of enrichment required depends on specific reactor design (e.g. PWRs and BWRs require 3% – 5% of 235U) and specific requirements of the nuclear power plant operator. Without required enrichment these reactors are not able to initiate and sustain a nuclear chain reaction for such a long period as 12 months.
The enrichment process separates gaseous uranium hexafluoride (HF6) into two streams, one being enriched to the required level and known as low-enriched uranium, the other stream is progressively depleted in 235U and is called ‘tails’, or simply depleted uranium. Typically, to produce 1 kg of enriched uranium with 5% of 235U, about 10 kg of natural uranium is required with a byproduct of about 9 kg of depleted uranium. Therefore annual natural uranium consumption of 3,000 MWthermal reactor is about 250 tonnes of natural uranium (to produce of about 25 tonnes of enriched uranium).
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