Twice-through Fuel Cycle

nuclear fuel cycle
Nuclear Fuel Cycle. Source: Nuclear Regulatory Commission from US. License: CC BY 2.0

The nuclear fuel cycle is a process chain consisting of a series of differing stages. The nuclear 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.

Twice-through Fuel Cycle

It must be added, there is also the twice-through cycle, which considers the spent nuclear fuel to be an energy source due to its composition. The twice-through fuel cycle involves more stages both in the back end and in the front end. It is based on the fact that the removed fuel (spent nuclear fuel) still contains about 96% of reusable material. It must be removed due to decreasing kinf of an assembly or in other words, it must be removed due to accumulation of fission products with significant absorption cross-section.

Fuel Depletion - Isotopic Changes
Isotopic changes of 4% uranium-235 fuel as a function of fuel burnup.

In summary, the following points are main isotopic changes for fuel at fuel burnup of 40 GWd/tU:

  • Approximately 3 – 4% of the heavy nuclei are fissioned.
  • These 3 – 4% are then fission fragments more or less
  • About two thirds of these fissions come directly from uranium 235, and the other third from plutonium, which is produced from uranium 238. The contribution significantly increases as the fuel burnup increases.
  • Approximately 96–97% of its components are recyclable materials, 94–96% of which is uranium.
  • Discharged fuel contains about 0.8% of plutonium and about 1% of uranium 235. It must be noted, there is a significant content (about 0.5%) of uranium 236, which is is neither a fissile isotope, nor a fertile isotope.

Thus, in order to exploit its energy potential, the spent nuclear fuel has to be put through a series of chemical processes known as reprocessing. Therefore, 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. As 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 transported to the reprocessing facility, where uranium and plutonium are separated from the minor actinides (MA) and fission products (FP), by means of the PUREX process, which is currently the only commercially available technology. Once they have been separated, the minor actinides and fission products are vitrified in a glass matrix and stored as high-level waste. Recovered uranium and plutonium can, if economic and institutional conditions permit, be recycled for use as nuclear fuel.

The uranium recovered from reprocessing, which typically contains a slightly higher concentration of uranium 235 than occurs in nature, can be reused as fuel after conversion and enrichment.

The plutonium can be directly made into mixed oxide fuel (MOX fuel), in which uranium and plutonium oxides are combined. In reactors that use MOX fuel, plutonium substitutes for the U-235 in normal uranium oxide fuel.

It must be added, the final high-level waste volume is reduced about 80%, its radiotoxicity decreases about 90% and its decay heat is also reduced compared to the once-through cycle. This is due to the fact, the spent fuel is composed of more than 96% of U and Pu.

See also: Conversion Factor

See also: Neutron Flux – Uranium vs. MOX 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 cycle. In 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:
    • Advanced Nuclear Reactors
    • 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. 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.

See more: The Future of the Nuclear Fuel Cycle. An interdisciplinary MIT study. MIT, 2011. ISBN 978-0-9828008-4-3.

References:
Nuclear and Reactor Physics:

Advanced Reactor Physics:

See above:

Nuclear Fuel Cycle