Frequently asked questions: Generation IV concepts

There must be many possible advanced reactor designs ... Why is the GIF looking at only six of these?

The fundamental design choices regarding a fission reactor are a) the fuel composition and average speed of the neutrons needed to propagate the fission chain reaction, b) the coolant to extract the heat produced in the core as a result of this chain reaction, and c) the materials able to withstand the operating conditions (temperature and irradiation) in the reactor. A number of appropriate safety concepts, devices and barriers must also be selected. The various combinations and permutations of these choices result in a vast number of possible designs. The GIF partners have looked closely at 130 proposed reactor systems, evaluated them and selected six promising concepts that comply most closely with the eight key Gen-IV technology goals, which are grouped into the four categories of sustainability, economics, safety and reliability, proliferation resistance and physical protection.

All isotopes of the family of actinides in the periodic table of the elements can undergo fission and liberate energy trapped in the nucleus, provided the right amount of energy is brought in by the collision of an incoming neutron. Because uranium-235 is the only isotope found in the Earth's crust and oceans that undergoes fission from collision with neutrons of any energy (so-called fissile isotope), it is the easiest one to be used for fission in fresh nuclear fuel in currently operating reactors. However, many other isotopes can be considered for power production: uranium-238, plutonium (especially the isotope plutonium-239), thorium, and some actinides present in today's nuclear waste, such as neptunium, americium and curium. Choosing the fuel (i.e. its composition) means not only choosing the isotopes to undergo fission but also the chemical and physical form of the fuel. There are also many ways of controlling - or moderating - the speed of the neutrons (through the choice of an appropriate "moderator"). There are even more possibilities when it comes to ways of extracting the heat produced by the nuclear fuel (i.e. choice of coolant). Equally, there are many technical options to consider regarding safe operation of the reactor (choice of safety components and barriers). Though some combinations of the above options will be unworkable and clearly inappropriate, the review by the GIF partners led to the further consideration of some 130 reactor concepts, from which the six most promising systems have been selected.

While the current intention is for all these concepts to use uranium-based nuclear fuel, there is no fundamental reason why thorium-based fuels could not be utilised in the future, provided a proper thorium fuel cycle is fully developed. However, such developments are not part of the present GIF Technology Roadmap, since natural uranium reserves and recyclable isotopes found in today's tailings from the enrichment process and nuclear waste are sufficient for the deployment of fast reactors for millennia to come.

On what basis were the six systems chosen for the R&D phase?

At the outset of the Generation IV Initiative, a top-down approach was used to define overriding guiding principles determining the technological goals that should be fulfilled by successful Generation IV systems. These goals were grouped in four main categories: sustainability, economics, safety & reliability, proliferation resistance & physical protection.

Goals for Generation IV Nuclear Energy Systems

Sustainability (1) Generation IV nuclear energy systems will provide sustainable energy generation that meets clean air objectives and promotes long-term availability of systems and effective fuel utilization for worldwide energy production.

Sustainability (2) Generation IV nuclear energy systems will minimize and manage their nuclear waste and notably reduce the long-term stewardship burden in the future, thereby improving protection for the public health and the environment.

Economics (1) Generation IV nuclear energy systems will have a clear life-cycle cost advantage over other energy sources.

Economics (2) Generation IV nuclear energy systems will have a level of financial risk comparable to other energy projects.

Safety and Reliability (1) Generation IV nuclear energy systems operations will excel in safety and reliability.

Safety and Reliability (2) Generation IV nuclear energy systems will have a very low likelihood and degree of reactor core damage.

Safety and Reliability (3) Generation IV nuclear energy systems will eliminate the need for offsite emergency response.

Proliferation Resistance and Physical Protection (1) Generation IV nuclear energy systems will increase the assurance that they are a very unattractive and the least desirable route for diversion or theft of weapons-usable materials, and provide increased physical protection against acts of terrorism.

An expert group defined along with these goals a comprehensive set of criteria with corresponding quantitative and semi-quantitative indicators. In parallel, a call for proposals was issued to the international nuclear community to submit concepts and designs that could be envisaged for deployment within the fourth generation of nuclear technology. This call resulted in more than 130 proposals from industry, research institutions and universities worldwide.

Additional qualitative evaluations regarding possible missions (i.e. uses, including electricity production, process heat generation, and actinide management), foreseeable R&D costs and deployment time horizon led to the narrowing down of these 130 proposals to 6 systems which showed a high ranking against the indicators, multiple missions, lower R&D costs and shorter deployment times. For each system, the same groups of international experts identified gaps in the technology and knowledge, which were then compiled in the Technology Roadmap for Generation IV Nuclear Energy Systems.

Will the six systems be safer than existing systems? Will the six systems be cheaper to run than existing systems?

In 50 years of nuclear energy development and deployment, the safety performance of nuclear power plants has been continuously improved. Some of these improvements are due to adaptation to state-of-the-art, as occurs with all technologies. Others are the result of lessons learned following incidents and accidents that have occurred (Three Mile Island, 1979, Chernobyl, 1986) and of the resulting increasingly strict regulatory regime.

The safety of earliest nuclear systems was based almost exclusively on active safety systems; it aimed at mastering design-basis accidents and included emergency plans for extended zones around the NPPs (nuclear power plants). These reactors were also partly unprotected against external events such as airplane crashes.

Generation II systems are considerably better equipped. They have a containment building protecting them against external impacts and providing one additional barrier to radioactivity release to the environment. The (active) safety systems have built-in redundancy and are diversified. They can master all design-basis accidents and certain accidents beyond design basis; consequently, the emergency zones around the NPPs could be considerably reduced in size. Following the TMI (Three Mile Island) core melt accident without radioactivity release, and the Chernobyl large accident with radioactivity release, great efforts were undertaken to reduce the probability of severe accidents involving core melt and subsequent radioactivity release to a minimum.

Generation III systems are equipped with reinforced or double containment buildings and a mixture of active and passive safety systems, which are not only redundant (up to fourfold) and diversified, but also spatially separated. Generation III systems can cope with the consequences of all accidents, including core melt, in a way that ensures the impacts are confined within the NPP site and do not affect the public; local emergency plans are therefore no longer needed from a technical viewpoint, although they may remain in place in order to reassure the public. Over the years, predicted core damage frequencies have been reduced from 10-3 to below 10-6 per reactor-year; the probability of failure of the last barrier (containment) is a factor of ten lower.

The aim of Generation IV systems is to maintain the high level of safety achieved by today's reactors, though at the same time shifting from the current principle of "mastering accidents" (i.e. accepting that accidents can occur, but taking care that the population is not affected) to the principle of "excluding accidents" in a quasi-deterministic manner. In both cases the impact on the population is small or non-existent, but in the second case the economic investment is equally protected and the "safety quality" is more evident to the laypeople. Generation IV reactors would be equipped with active and passive safety systems, at least as effective as those of Generation III. Certain Gen-IV concepts rely on physical principles which render the most severe accident (e.g. core melt) physically impossible; this is called inherent or intrinsic safety.

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 the ability to co-generate process heat and electricity, and therefore new models for their economic assessment are required. 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" (www.gen-4.org/Technology/horizontal/economics.htm) provides detailed information on this topic.

What inherent aspects of the systems under development will guard against nuclear proliferation?

Along with the physical and administrative monitoring, control and security measures currently in place, careful selection of the fuel composition and of the reprocessing techniques may further increase the proliferation resistance of the nuclear fuel cycle. Making nuclear material less suitable for use in a nuclear weapon, or less prone to diversion for such use, can be achieved in three different ways, which are not reactor but fuel cycle specific:

a) By increasing the radiological intensity of the material itself, so that it cannot be handled without severely exposing the people handling it or without heavy and specialized shielding equipment,

b) By assuring that at no point during the fuel cycle will the isotopic composition of the fuel be suitable for the production of an explosive nuclear device, without prior complex reprocessing,

c) By minimizing the opportunities for diversion, such as during intermediate storage, transport to and from reprocessing, etc.

Most of the Generation IV systems involve fast reactors relying on multiple reprocessing and recycling of fuel, which essentially address all three of the above strategies.

Whereas current reprocessing techniques such as the PUREX process use aqueous chemistry (dissolution of spent fuel in strong acids) to extract uranium and plutonium, independently of each other, from the remaining mixture of minor actinides and fission products, advanced reprocessing techniques aim at separating a mixture of all actinides (including U and Pu) from the fission products. This mixture of actinides can be recycled in a fast reactor but is not suitable for use in nuclear weapons.

Non-aqueous reprocessing techniques are also under development. These techniques are pyro-metallurgical processes, known as pyro-processing, and are based on the electrolysis of spent fuel using molten salts as an electrolytic bath. Heavy metals are separated on one electrode and fission products remain in the salt. The main advantage of pyro-processing, and its use for the separation of all actinides, is the relative simplicity of the process and its compactness. This allows the reprocessing facility to be installed on the same site as the fast reactor thereby maintaining maximum physical protection of sensitive material by avoiding transport to and from a central reprocessing plant.

An even more advanced version of on-site reprocessing is possible in the most futuristic of the six systems selected in Generation IV, the Molten Salt Reactor (MSR). In this reactor, the fuel and coolant are one, and co-exist as a molten salt circulating through the core. A small fraction of this liquid is continuously extracted from the primary circuit and processed, to extract fission products only, in an chemical plant integrated in the reactor building. The "cleaned" salt is then fed back into the primary circuit. Here again, all actinides stay together and are recycled thereby avoiding transports to a central reprocessing facility.

A cross-cutting activity in the Generation IV framework aims at developing a methodology for the evaluation of the proliferation resistance of the different Generation IV systems using number of criteria and quantitative or qualitative indicators (see http://www.gen-4.org/Technology/horizontal/PRPPEM.pdf ).

Will all six systems be ready for industrial deployment by 2030? When will the first prototype reactor be built? By whom? Is the intention to exploit all six of these Generation IV reactor types on a commercial basis?

The objective is for Generation IV nuclear energy systems to be available for wide-scale deployment by 2030 at the earliest. The anticipated deployment dates for the six Generation IV systems vary between 2030 for SFR and VHTR and 2045 for GFR, SCWR, LFR and MSR. These dates assume that considerable resources are made available for R&D. Deployment of intermediate systems (i.e. not fully compliant with Gen-IV objectives) may take place on a shorter time schedule. R&D on these near-term systems may be of direct benefit for the Generation IV program as a whole. Though each of the six Gen-IV systems has a comprehensive and complete R&D plan, the necessary work may be reduced by synergy with the developments made in the deployment of a relevant near-term system. In this regard, the Generation IV program continually monitors industry- and industry/government-sponsored R&D plans and progress in order to draw benefit and avoid duplication. Cases where industrial developments are halted or merged may signal needed changes in the Generation IV R&D plans. Likewise, current Generation IV R&D may result in significant advances for these near-term systems.

What is the difference between the fast neutron reactors developed in the past, and those to be developed within the Gen-IV framework?

Fast neutron reactors have been investigated since the early days of nuclear power, and a number of experimental, demonstrator or prototype plants have been constructed. A few are still in operation today. However, for the large-scale generation of electricity, they were not consisted to be an economically viable alternative to thermal reactors at a time when the cost of uranium was very low and waste minimization, certainly as regards the longer-lived isotopes such as the actinides, was not such an important attribute. Concerns regarding the long-term availability of uranium resources as well as the increased sensitivity over all aspects of nuclear waste management have re-awakened interest in fast neutron reactors. The key issue is the ability to provide a safe and sustainable energy supply for future global development, and in this regard the fast reactor option is being widely viewed with increasing interest. The fast reactors to be developed within the Gen-IV framework should be economically competitive, as safe as the best Light Water Reactors (LWRs) and able to burn all the actinides produced by LWRs or the fast reactors themselves. This requires significant developments and advancements compared with previous and current fast reactors, which were prototypes focussing mainly on breeding capability. Alternatives to the classical sodium-cooled concept should also be explored in view of the large number of reactors which would be necessary in the next decades: lead-cooled and gas-cooled systems are among those being considered.

Watch an Introduction to Generation IV Nuclear Energy Systems and the International Forum (requires Adobe Flash Player, or download in pdf format, 1.2 mb)

Download an Overview of the GIF (pdf, 161 kb)

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