15.13.1 Critical Reactors

Many of the innovative systems under consideration have the capabilities to use advanced fuel cycles and there is a need for fuel performance research for many of these fuels (The US Generation IV Implementation Strategy, 2003). There are, however, a number of activities currently in progress (Table 15.8).

Innovative water reactor fuel cycle options are being considered whereby spent PWR fuel can be used for CANDUs, i. e. the DUPIC technology. This is attractive to avoid the separation of fissile material, particularly plutonium, during fuel cycle operations.

High-temperature reactor fuel design is also attracting research, e. g. on the stability of particulate fuel at very high temperatures and particularly under accident conditions. There is also relatively little experience on fuel fabrication. Regarding current research programmes, there is an OECD co-ordinated research programme on the physics of plutonium/innovative fuel cycles for pebble bed reactors. The IAEA is also co-ordinating analysis of experimental results for a number of high-temperature test reactors including, HTTR (Japan), HTR (China), GT-MHR (US and Russia) and ASTRA (Russia) (Newton, 2002).

Regarding innovative fuels, nitride fuels have been considered instead of oxide fuels, because they result in lower fuel temperatures, due to their improved thermal conductivity. A wide range of different fuels has been considered for minor actinide target fuels. There are several EC research programmes on advanced fuel-related issues. The CONFIRM programme is an experimental investigation of the high-temperature stability of actinide fuel in nitride form.

There are a number of initiatives in connection with fast reactor core physics, e. g. the CAPRA/CADRA programme, which includes studies of fast reactors, particularly in connection with plutonium burning and waste incineration fuel cycles. Uncertainties in fast reactor performance are the subject of a present IAEA research programme.

Thorium fuels have been considered as an alternative to uranium for some fuel cycles and some reactor types, but there is limited experience, certainly for commercial reactors. There is limited experience in general for the molten lead cooled and molten salt cooled systems.

15.13.1.1 Accelerator Driven Systems. In some respects, there are similarities between the important issues concerning subcritical ADSs and the critical reactors. However, the application of accelerators to different subcritical systems does require some new areas of research. There is also the issue of research on accelerator systems per se.

There has been research into the development of fuels for ADS. The EC FUTURE programme is investigating a number of innovative oxide compounds, in solid solution and inert matrix form. The ADS is also considered in the CAPRA/CADRA programme.

There is emphasis in designing new innovative reactors (critical and subcritical) in a way to reduce the radioactive waste burden compared with existing reactors. This is being achieved through design for high fuel burn-up; the utilisation of thorium could also achieve reduction in the higher actinides produced.

A feature of many of the new designs is that new coolants are being proposed for which little operational experience exists. These coolants are considered in Section 15.14.

15.9. ADVANCED COOLANTS

15.14.1 Present Experience

The future supercritical water concepts will operate with supercritical water on either the secondary side or also possibly on the primary side. It follows that the primary to secondary heat exchangers will require special attention. The performance of conventional PWR steam generators has not been without problems and the materials used, system chemistry control and the construction methods for the supercritical systems will need significant development research. For the supercritical heavy water concepts such as CANDU X, lower cost techniques will have to be developed for separating deuterium from hydrogen in light water.

There is some relevant experience of supercritical systems from coal-fired plants. However, there is no previous experience on the use of supercritical water in high radiation backgrounds. There is also the issue of system performance under fault conditions, e. g. LOCAs, given the very high system pressures.

For the liquid lead coolant systems, there is little experience in nuclear reactors outside of Russia. There will need to be significant effort to developing the chemistry specifications and control to ensure economic and reliable performance. Lead-induced stress corrosion cracking could also be an issue.

Molten salt systems will require developments on the control of their chemistry and the coolant composition during their extended periods of operation. The high-temperature performance of key components such as heat exchangers will need to be verified. There needs to be developed isotope separation technologies to separate out the lithium isotope 7Li from the naturally occurring 6Li.

15.14.1.1 Natural Circulation. Existing operating water reactors rely on natural circulation to remove decay heat when forced convection is lost. Many water and heavy water cooled designs include natural convection to remove decay heat after shutdown. Some of the simpler low-pressure water reactors rely on natural circulation to remove heat at all power levels. In general, there has not been a total reliance on natural circulation in the pressurised PWR and BWR systems. The innovative liquid lead and molten salt systems also allow for some level of natural circulation, namely the removal of decay heat after shutdown. Some of the ADS systems allow for natural circulation removal of heat at all power levels.

It follows that natural circulation will be an important phenomenon in innovative reactor technology. Regarding the current state of knowledge, there has already been much work on natural circulation in current and evolutionary plant. There is greater confidence in single-phase system performance, e. g. in the gas and liquid lead systems, than in the water systems, where two-phase flow can develop. Under accident conditions the presence of hydrogen can also be a problem. Additionally, there is a need for the development of correlations for transient heat transfer under all operating conditions.

There are several projects within the EC framework research programme for evolutionary systems; these were mentioned earlier, e. g. EUROFASTNET, ECORA and FLOMIX-R. Most of these are relevant to improving the understanding of natural circulation in the evolutionary and some of the innovative reactor systems. The above programmes also cover theoretical R&D, e. g. the development of appropriate numerical methods development for CFD modelling.