# Uranium Fuel Cycle

The uranium fuel cycle is a process chain consisting of a series of differing stages. Uranium fuel cycle uses an enriched uranium fuel (~4% of U-235) as a fresh fuel. During the fuel burning, the content of the U-235 decreases and the content of the plutonium increases (up to ~1% of Pu ).

Most of PWRs use the uranium fuel, which is in the form of uranium dioxide. The uranium dioxide is pressed into pellets, these pellets are then sintered into the solid. These pellets are then loaded and encapsulated within a fuel rod (or fuel pin), which is made of zirkonium alloys due to its very low absorbtion cross-section (unlike the stainless steel). The surface of the tube, which covers the pellets, is called fuel cladding. Fuel rods are base element of a fuel assembly. The collection of fuel rods or elements is called the fuel assembly. The fuel assembly constitute the base element of the nuclear reactor core. The reactor core (PWR type) contains about 157 fuel assemblies

## Uranium – Fuel Breeding

Neutron capture may also be used to create fissile 239Pu from 238U, which is the dominant constituent of naturally occurring uranium (99.28%). Absorption of a neutron in the 238U nucleus yields 239U. The half-life of 239U is approximately 23.5 minutes239decays (negative beta decay) to 239Np (neptunium), whose half-life is 2.36 days239Np decays (negative beta decay)  to 239Pu.

Transmutation of transuranium elements (i.e. the chemical elements with atomic numbers greater than 92 ) such as the isotopes of plutonium has the potential to help solve some problems posed by the management of radioactive waste by reducing the proportion of long-lived isotopes it contains.

## Uranium Fuel Cycle

The uranium fuel cycle starts with the mining of uranium and ends with the disposal of nuclear waste. With the reprocessing of used fuel as an option for nuclear energy, the stages form a true cycle. In general, nuclear fuel cycle consists of steps in the front end (the preparation of the fuel), steps in the service period (fuel burnup), and steps in the back end (reprocessing or disposal of spent nuclear fuel).

• Front end of nuclear fuel cycle. The front end of the nuclear fuel cycle starts with the mining of uranium in the mines and ends with the delivery of the enriched uranium to the nuclear fuel assembly producer. Therefore, the front end of fuel cycle consist of:
• Uranium mining, milling and mill tailings,
• Conversion
• Fuel enrichment
• Fabrication of fuel assemblies
• Service Period. Service period includes transport of fuel assemblies within a power plant, in-core fuel management, fuel utilization and storage in the spent fuel pool.
• Back end of nuclear fuel cycle. The back end of the nuclear fuel cycle involves managing the spent fuel after irradiation. Therefore, the back end of fuel cycle consist of:
• spent fuel interim storage
• fuel reprocessing,
• final disposal of radioactive waste or spent fuel.

## Types of Nuclear Fuel Cycles

As was written, the back end of the nuclear fuel cycle involves managing the spent fuel after irradiation. Therefore, the back end of the fuel cycle consist of:

• spent fuel interim storage
• fuel reprocessing,
• final disposal of radioactive waste or spent fuel.

There are three main types of nuclear fuel cycle:

• Once-through fuel cycleIn fact, an open fuel cycle is not a real cycle. This strategy assumes that the fuel is used once and then sent to long-term storage without further reprocessing. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle or a once-through fuel cycle as the uranium components go through the reactor just one single time. The once-through cycle comprises two main back end stages:
• interim storage
• final disposal.
• Twice-through fuel cycle. The twice-through cycle strategy assumes, that the spent nuclear fuel will be reprocessed in order to extract the uranium and plutonium, which can either be recycled as fresh nuclear fuel for its use in a nuclear reactor that is adapted to this type of fuel.
• Closed fuel cycle. The closed fuel cycle is an advanced fuel cycle, which purpose is to achieve nuclear power sustainability by further reducing radiotoxicity of the final waste, as well as improving resource utilization, while maintaining its economic viability. There are currently different types of advanced fuel cycles under research, but most of them are based on the use of:
• Fuel Reprocessing

These types of fuel strategie are based on specific processes in entire fuel cycle (the back end, the front end, as well the service period). In fact, all these scenarios are theoretical and the practical solution will be in any case a combination of these options. In all cases, the fuel assemblies are first after irradiation stored in spent fuel pools at the reactor site for an initial cooling period. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away. After several years (> 5 years), decay heat decreases under specified limits so that spent fuel may be reprocessed or interim storaged. It must be added, any strategy for managing spent nuclear fuel will be built around combinations of many options, and all strategies must ultimately include permanent geological disposal.

The choices of nuclear fuel cycle (open, closed, or partially closed through limited spent fuel recycle) depend upon the technologies we develop and societal weighting of goals (safety, economics, waste management, and nonproliferation). Once choices are made, they will have major and very long term impacts on nuclear power development. Today we do not have sufficient knowledge to make informed choices for the best cycles and associated technologies.

References:
Nuclear and Reactor Physics:
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2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
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6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.