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Gen IV Reactor Design

· on 9/16/13 at 6:19 PM

1. How are Gen IV reactors different from reactors operating today?

The first generation of nuclear reactor prototypes were constructed in the 1950s and 60s and culminated in the construction of the first series of civil nuclear power reactors. The construction of the second generation of reactors started at the beginning of the 1970s and marked the widespread appearance of Light Water Reactors (LWR), either Pressurized Water Reactors (PWR) or Boiling Water Reactors (BWR), both using normal water as coolant and moderator. The LWRs constructed in the 80s and early 90s are essentially of the same "Gen-II" stock, and these now constitute the vast majority of reactors currently in operation worldwide.

More significant evolutionary developments have been integrated into the latest LWR designs available today, especially with regard to design lifetime (typically 60 years compared with 40 years in the past) and safety issues, in particular the behaviour under severe accident scenarios. These new designs are classified as Gen-III, and the first commercially available reactors of this generation are now under construction. In the year 2000, experts from around the world (in the context of the Generation IV International Forum - GIF) began formulating the requirements for a fourth generation of nuclear systems that could respond to the world's future energy needs, in particular increased demand for electricity and reduced CO2 emissions leading to more widespread use of nuclear energy. Making efficient use of uranium natural resources and minimising waste production become major concerns in such a scenario, in addition to satisfying economic competitiveness and maintaining stringent standards of safety and proliferation resistance. The possible designs that are currently under investigation, and around which the GIF organised an extensive R&D international collaboration, have collectively been labeled Gen-IV.

2. How much will it cost to operate a Gen IV reactor compared to current nuclear plants?

One of the fundamental goals for Generation IV nuclear energy systems is that they will have a clear life-cycle cost advantage over other energy sources. However, since Generation IV reactors are still at an early stage of development and will not be deployed commercially for at least two to three decades, it is difficult to quantify these cost benefits. In particular, Generation IV systems characteristics differ significantly from those of Generation II and III reactors. For example, some designs will have the ability to co-generate industrial process heat and electricity, which will require new models for their economic assessment. This work is currently ongoing within one of the GIF cross-cutting working groups — the Economics Modelling Working Group (EMWG). The "Cost Estimating Guidelines for Gen IV Nuclear Energy Systems" provides detailed information on this topic.

3. How energy efficient are Gen IV designs compared to previous reactor generations?

Since they have been designed to operate in a thermal (less energetic) neutron spectrum, current Gen-II and Gen-III Light-Water Reactors (LWRs) can extract fission energy from only a small fraction of the uranium in the fuel (effectively only the "fissile" U-235 component, which makes up less than 1% of natural uranium). Under such conditions, known and easily accessible uranium reserves are capable of sustaining only a few more decades of operation of the world's fleet of LWRs. Four of the six Gen-IV designs currently under investigation are so-called "fast-breeder" reactors, which have the capability of exploiting the full energetic potential of the uranium, thus extending resource sustainability by factors of 50 to 100.

4. Will Gen IV reactors be able to operate on spent nuclear fuel?

A fast reactor operates in a more energetic neutron spectrum, and is able, via nuclear transformations within the fuel, to "breed" fissile plutonium (Pu-239) from fertile uranium (U-238), which can then be recycled in fresh fuel. In this way, the energetic potential of U-238, representing more than 99% of the original natural uranium, can also be exploited.

Gen IV fast reactor designs will represent a radical rethink of current technology. In particular, the objective is to extract and recycle not only the bred Pu-239 but also the other (so-called minor) actinides that are produced in the fuel by nuclear transformation. This "full actinide" recycling will be a key attribute of Gen IV fast reactor systems and associated fuel cycles, as it will reduce the radiotoxicity and heat generation of the ultimate waste for disposal and increase overall proliferation resistance of the fuel cycle.

Is it true that the recycling of all minor actinides, along with the Pu-239, will result in much higher levels of radioactivity in many of the processes in the reprocessing and subsequent fuel fabrication plants, and the workforce will therefore need to be adequately protected with appropriate shielding and remote handling apparatus. This technology is already proven in current day facilities dealing with MOX (mixed-oxide fuel) production. In addition, all low-level operational radioactive waste produced in these facilities will need to be managed according to approved and controlled practices.

5. What are the waste products from Gen IV fuels? How will they be stored?

Recycling all the minor actinides back into fresh fuel enables them to be "burnt" in the reactor and transformed into so-called fission products. These fission products are separated out from the fuel in the reprocessing plant and constitute the "ultimate" waste from the process. This waste must be managed and ultimately disposed of in line with accepted and approved practice. The radioactivity of the waste will decay much more rapidly, and the radiotoxicity is much lower. But once the minor actinides have been removed, it will still be necessary to dispose of the waste in geological repositories to ensure optimal protection of people and the environment over the timescales during which the waste remains a potential hazard. Nonetheless, the quantities of this ultimate waste will be much less than from current fuel cycles for the same energy production. In addition, with the removal of the minor actinides, heat generation is also greatly reduced, enabling a much more efficient use of space in geological disposal facilities.

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