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. Only uranium 235 is a fissile material (i.e. is capable of undergoing nuclear fission only after capturing a thermal neutron) but as it only accounts for 0.7% of the makeup of natural uranium, it is not present in sufficient proportions to be used in the LWR-type nuclear power plants. 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 (e.g. cycle length). Without required enrichment these reactors are not able to initiate and sustain a nuclear chain reaction for such a long period as 12 months (or more).
Enrichment is accomplished using any of several methods of isotope separation. Most commercial uranium enrichment processes (gaseous diffusion and the gas centrifuge method) require the uranium to be in a gaseous form, therefore the uranium oxide concentrate must be first converted to uranium hexafluoride, which is a gas at relatively low temperatures. Therefore the input material in this process is uranium hexafluoride, known also as “hex”. In the enrichment process gaseous uranium hexafluoride is separated into two streams, one being enriched to the required level and known as low-enriched uranium; the other stream is progressively depleted in uranium-235 and is called ‘tails’, or simply depleted uranium. The product of this stage of the nuclear fuel cycle is enriched uranium hexafluoride, which is reconverted to produce enriched uranium oxide. From a non-proliferation standpoint, uranium enrichment is a sensitive technology needing to be subject to tight international control.
Prior to enrichment, natural uranium consists primarily of isotope 238U (99.28%), therefore the atomic mass of uranium element is close to the atomic mass of 238U isotope (238.03u). 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-lifes. 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 (~4.5×109years for 238U).
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 3000MWth reactor is about 250 tonnes of natural uranium (to produce of about 25 tonnes of enriched uranium).
Consumption of a 3000MWth (~1000MWe) pressurized water reactor (12-months fuel cycle)
It is an illustrative example, following data do not correspond to any reactor design.
- Typical reactor may contain about 100 tonnes of enriched uranium (i.e. about 113 tonnes of uranium dioxide).
- This fuel is loaded within, for example, 157 fuel assemblies composed of over 45,000 fuel rods.
- A common fuel assembly contain energy for approximately 4 years of operation at full power.
- Therefore about one quarter of the core is yearly removed to spent fuel pool (i.e. about 40 fuel assemblies), while the remainder is rearranged to a location in the core better suited to its remaining level of enrichment (see Power Distribution).
- Annual natural uranium consumption of this reactor is about 250 tonnes of natural uranium (to produce of about 25 tonnes of enriched uranium).
- Annual enriched uranium consumption of this reactor is about 25 tonnes of enriched uranium.