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1. Introduction<br />

In the framework of partitioning and transmutation (P&T), transmutation is the only technology<br />

which is capable of accelerating the natural decay sequence, of influencing the decay schemes of<br />

actinides and of reducing the radiotoxic inventory of some nuclides. For some radionuclides the<br />

natural decay reactions take hundred thousands of years to reach the initial uranium ore toxicity level.<br />

If partitioning of minor actinides (MAs) and long-lived fission products (LLFPs), such as e.g. 99 Tc<br />

and 129 I, is successful, the way to transmutation is open. Transmutation in thermal or fast neutron<br />

spectra has been thoroughly studied and both have their merits. Thermal neutrons are very effective<br />

for fissile trans-uranium (TRU) nuclides ( 239 Pu, 241 Pu, 242 Am, 245 Cm) whereas fast neutron spectra<br />

(in FRs and ADS) are indispensable for fissioning the fertile ( 237 Np, 241 Am, 243 Am) and even<br />

mass-number nuclides ( 238 Pu, 240 Pu, 242 Pu, 244 Cm...).<br />

The present paper makes an analysis of the possibilities of both approaches in relation with<br />

technological demonstration experiments which could be performed in BR2, and later on in the<br />

planned MYRRHA ADS-facility, to investigate the issues related to target optimisation, cladding<br />

selection and structural material behaviour under intense irradiation.<br />

2. BR2 and MYRRHA<br />

2.1 The high flux materials testing reactor BR2<br />

BR2 is a heterogeneous thermal high flux materials testing reactor [1-4]. Routine operation<br />

started in 1963, and, to this very day, BR2 continues to contribute to the development of nuclear<br />

projects within the European Community and for nuclear partners throughout the world. Figure 1 (left<br />

hand side) shows a horizontal cross-section of the reactor core at the reactor midplane with a typical<br />

loading.<br />

Figure 1. Horizontal cross-section (left) of the BR2 reactor at the reactor midplane<br />

with a typical loading and (right) of a type 6n-G fuel element with central aluminium plug<br />

BR2 is cooled and moderated with pressurised light water (12 bar) in a compact core of highly<br />

enriched uranium fuel elements, positioned in, and reflected by, a beryllium matrix. The ultimate<br />

cooling capacity, initially foreseen for 50 MW, has been increased in 1971 to 125 MW. The reactor<br />

nominal full power depends on the core configuration used; at the present time it ranges from 50 to<br />

796

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