Accelerator Driven Systems

13.1. INTRODUCTION/OBJECTIVES

Innovative accelerator driven systems (ADS) are under study nationally and inter­nationally to provide a possible alternative to critical reactor systems (considered so far in this book). The purpose of this chapter is to examine this technology. ADS are possible candidates for a range of applications; importantly, they provide a means of separating and eliminating actinides by a process referred to as partitioning and transmutation. They can transmute long-lived radioisotopes into short-lived or even non-radioactive isotopes, using an excess of neutrons available from a fission chain reaction. These neutrons are generated in a hybrid sub-critical reactor accelerator system, which forms the basis of the ADS. In such a system, high-energy protons produced by an accelerator bombard a ‘target’, producing an intense neutron source; this part of the process is termed ‘spallation’. These neutrons are multiplied up in a sub-critical reactor, referred to as the ‘blanket’, which surrounds the spallation target.

Thus, an important application for ADS technology could be to transmute high-level nuclear waste to non-radioactive materials or materials with much shorter half-lives. The issue of highly radioactive waste produced from reactor operation is a continuing problem with regard to the future of nuclear power. Such waste must be managed in a safe and efficient manner if nuclear power is to be sustained in the modern world. Other ADS applications include the ‘burning’ of weapons grade plutonium and energy production. These are developed further below.

In regard to international activity, the IAEA compiled a status report in 1997 (IAEA-TECDOC-985, 1997a), requested by participants in a special scientific programme initiated in 1994 on the ‘Use of High Energy Accelerators for the Transmutation of Actinides and Power Production’. The objectives were to review the various technical options available, including their advantages and disadvantages, including technical and economic viability and the future role of IAEA in developing international collaboration.

The process of nuclear transmutation has existed for some time. In 1919, Rutherford demonstrated the process for lighter elements, and Laurence in the US and Semenov in the USSR made attempts to promote accelerators to generate neutron sources in the 1940s. The evolution progressed through attempts to achieve transmutation using only spallation neutrons but these suffered from technical limitations and inefficiency. In recent years, hybrid systems have been produced involving the combination of a sub-critical reactor with a high-energy particle accelerator.

A number of different systems have been and are still being proposed. These include ADS using fast neutrons for higher actinide incineration; such systems have been proposed in the US and Japan. In the US, hybrid systems, using thermal neutrons with a linear accelerator has also been considered. Collectively the different approaches provide a means for the incineration of plutonium, for the transmutation of higher actinides and long-lived fission products (LLFP) to reduce radioactive waste activity, and for potential energy production using thorium fuel. In Europe, nuclear energy production from thorium-based fuel via the ‘Rubbia’ system has been put forward. The thorium fuel option reduces the concern about higher actinides in used fuel, and utilises relatively cheap and available thorium. These ideas have been tested using preliminary experiments at CERN.

ADS have the inherent safety feature that they are based on a non-self-sustained chain — reaction. This improves the safety characteristics of ADS and can also reduce or eliminate the need for control rods. A section on safety is included later in this chapter.

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