Benefits and Challenges

Along with the physical and administrative monitoring, control and security measures currently in place, careful selection of the fuel composition and reprocessing techniques may further increase the proliferation resistance of the Gen IV 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.

Sodium is highly compatible with the reactor materials, which essentially rules out corrosion problems for the life of the plant. The first reactor to demonstrate inherent safety features that would eliminate the potential for catastrophic accidents like Fukushima was sodium cooled. Sodium is a highly efficient coolant compared to water, meaning that the system can operate at low pressure and high temperature.

The main engineering challenge for sodium as a reactor coolant is that it reacts energetically with water, so barriers between the coolant system and the steam system must be robust. Most demonstration and prototype sodium-cooled reactors have encountered water-sodium reactions as a result of design and manufacturing flaws. A second challenge is that sodium is opaque, which increases the difficulty of maintenance and inspection.

Molten lead is a very heavy coolant that provides advantages for radiation shielding, heat removal, and relative compatibility with the steam system. Lead has also been combined with bismuth to form a coolant with a lower melting temperature coolant, which simplifies design and improves operability. Both concepts would operate at low pressure.

Lead presents some unusual engineering challenges. In particular, high-temperature molten lead tends to corrode most metals. While lead-bismuth can reduce corrosion concerns, irradiated bismuth produces polonium, a highly undesirable radioactive byproduct. Lead is also opaque and has the odd characteristic that heavy objects such as nuclear fuel bundles and control rods will float if not secured.

Molten salt has some interesting benefits in reactor design, with unequaled flexibility. On the plus side, molten salt is an efficient high-temperature coolant whose transparency enables inspection and maintenance of components. The reactor fuel can be dissolved in the salt to allow continuous removal of impurities, or the salt can be used to cool more conventional solid fuel. Both concepts would operate at low pressure. Several salt mixtures have been proposed as reactor coolants, some of which have relatively high melting temperatures, which would complicate keeping the coolant in a liquid state throughout the system. High-temperature salt is corrosive, limiting the materials that can be selected for reactor design. Irradiation of the salt promotes compositional changes that modify the coolant’s properties.

Within GIF, helium is used as a coolant in two quite different system concepts. Helium has the advantage of being transparent, completely inert, and remains a gas at all temperatures and pressures of interest. Gas-cooled reactors operate at high pressure, but lower than current water-cooled reactors.

For reactor designers, the challenge with helium coolant is that its heat removal and retention properties are the weakest of the candidate coolants. For the Very High Temperature Reactor concept, this shortcoming is remedied by providing a large thermal buffer in the form of a graphite (a high temperature carbon material) structure. For the Gas Fast Reactor, structural graphite is not an option, so schemes for pressurized gas flow under all conditions are necessary to ensure safety.

Helium shortages can sometimes appear in the current market because it is thin. If a large market for gas-cooled reactors develops, sufficient helium could be captured from oil well production to satisfy the increased demand. Also, helium would be expected to behave like other commodities — short-term supply restrictions would drive up prices, stimulating more exploration and development.

Ordinary water subjected to very high pressure becomes supercritical water, which has a high boiling temperature, greater density, and enhanced chemical reactivity. Supercritical water has been successfully applied in modern coal plants around the world. Its advantages as a reactor coolant are much higher generating efficiency and a wealth of industrial experience that can be applied.

The disadvantages of supercritical water are chemical reactivity and a significant variation in density with change in temperature. These properties manifest as corrosion and safety issues for the reactor designer. Supercritical water reactors would operate at much higher pressure than current reactors.

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 learnt following incidents and accidents that have occurred (Three Mile Island 1979; Chernobyl, 1986; Fukushima, 2011) and of the resulting increasingly strict regulatory regime.

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 nuclear containment and there are no releases to the environment. Over the years, predicted core damage frequencies have been reduced from 0.001 to below 0.000001 (one in a million) per reactor-year; the probability of failure of the last barrier (containment) is another ten times lower.

The aim of Generation IV systems is to maintain the high level of safety achieved by today's reactors, while 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". Generation IV reactors would be equipped with both active safety systems and "passive" safety systems, which rely on natural laws of physics rather than people or machines. These safety systems would be at least as effective as those of Generation III reactors. 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.

Generation IV International Forum Response to the Fukushima Daiichi Nuclear Power Plant Accident

The GIF Experts Group has drafted a position paper on the use of Thorium in the nuclear fuel cycle.