Challenges for Future Reactor Systems (part 1)

by Eddie Lopez Honorato and Joseph Somers

The ever increasing worldwide demand for energy along with concerns over green house gas emission and future security of energy source of supply are at the forefront of energy policy across the globe. The role of nuclear energy with its low carbon imprint has rapidly become recognised, and has been promoted by well known figures that once opposed nuclear energy as Patrick Moore (co-founder of GREENPEACE). Recent events in Japan, with three nuclear reactors stricken as a result of an unprecedented natural disaster, have renewed the public debate about nuclear energy. It is not, however, the purpose of this article to contribute to this debate, rather to provide an overview of how future systems could evolve, if and when required.

Towards the end of the last century, a number of nations gathered to deliberate on the future of nuclear energy for the coming century. Thus the Generation IV International Forum, also known as GIF, was founded. Various panels were established to select the most promising reactor systems for the future, based on the following criteria:

• Safety of the reactor system

• Resource sustainability i.e. maximising the amount of energy that can be extracted from natural resources, while not compromising future generations

• Waste minimisation e.g. capability to curtail long term waste legacy

• Economy (capital, operation and decommissioning)

• Proliferation resistance, i.e. capability to guard against misuse of materials

After much debate, six reactor systems were recognised, as capable of fulfilling these criteria and are discussed below.

Fast Neutron Reactors

Nuclear reactors can be classified based on the energy of the neutrons that produces the fission reaction responsible for the heat generated in the reactor. Today’s commercial reactors use “thermal” neutrons that are slowed down by a moderator, i.e. water. In contrast, “fast” reactors use neutrons without the use of a moderator. The fast neutron reactors with sodium lead, and gas coolants (denominated SFR, LFR and GFR) optimise uranium resource use by a factor of 50 above today’s light water reactors (LWR). They pave the way for improved radioactive waste management through fuel cycle closure with recovery of valuable fissile material for irradiation in the next reactor cycles, thus improving the uranium resource utilisation. In addition, the minor actinides (i.e. neptunium, americium, curium), the major contributors to long term radiotoxicity, can be recovered and transform to elements with lower radiotoxicity (transmutation) through further irradiation. Thus, in theory at least, the long term radiotoxicity of the spent fuel can be reduced from some 100,000 years down to 300 years. Even if the transmutation efficiency is not 100%, a reduction to 1000 years seems reasonable (see figure 1), and is within the bounds one can foresee for engineering barriers against release to the biosphere. Transmutation of minor actinides has a further benefit. They contribute significantly to the heat load of the spent fuel. Thus, their transmutation increases the effective capacity of a repository by at least a factor of 10, meaning the need for fewer repositories.

Radiotoxicity of spent nuclear fuel as a function of time

The separation of the elements of interest from nuclear waste (partitioning) and transmutation can be achieved in all of the foreseen fast reactors and in a dedicated system, such as the accelerator driven system (ADS). The latter device yields improved safety. It is a nuclear reactor operating in a sub critical condition, with additional neutrons required to maintain the reactor core criticality being provided by a particle accelerator coupled to the reactor. When the proton accelerator is switched off, the reactor essentially switches off.

The effort required to enable the deployment of such minor actinide management systems is significant. Advanced fuels need to be fabricated and their safety qualified. Fuels could be traditional oxide, metal, carbide or nitride chemical form. Table 1 compares some of their characteristics. Both oxide and metallic fuels operate at about 80% of their melting point. In contrast, carbide and nitride fuels operate at about 40% of their melting point. Despite their favourable margin to melt, neither carbide nor nitride have been deployed on a large scale, mainly due to difficulty in fabrication and a less well understood irradiation performance.

Table 1: Properties of metallic, oxide, nitride and carbide fuels

Fuel Metal Oxide Nitride Carbide
Melting Point (K) 1350 3000 3035 2575
Centreline temperature (K) 1050 2350 1000 1000

 

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