Overview of solid breeder TBM concepts and programme for testing in ITER

The use of the solid breeder (SB) material technology offers attractive solutions for blankets of a first generation of Fusion Power Plant according to the possibility of a relatively simple design and operation of this component in reactor with high performances in term of thermal efficiency, tritium recovery and reduced dimensions. Almost all the ITER international parties have performed studies on such a concept and have presented an own blanket design for testing in ITER. Also if several different configurations of this kind of concept have been proposed during the past years, a set of common features can be identified that characterise this first generation concept, like the extern cooling of the breeder zone materials, the use of an independent low pressure helium flow for tritium recovery and Ceramic Breeder materials, mainly ternary Li-compounds, in form of a pebble bed. Furthermore, Ferritic/Ferritic-Martensitic steels at reduced activation grade have been selected for the structures; this choice dictates in strong way the performances of this kind of concept in term of minimum/maximum temperatures. All these concepts necessitate addition of large quantities (up to 4 times the amount of ceramic breeder) of beryllium or beryllium alloys as neutron multiplier in order to achieve a sufficient tritium breeder ratio with a reduced blanket thickness. Starting from this common base, different variants of this concept have been proposed. According to the coolant selection, two major classes of SB blanket can be identified, namely water cooled (SBWC) and helium cooled (SBHC) concepts. He cooling concepts have the advantage of a better chemical compatibility with the other materials and, in particular, with beryllium; water cooling concepts promise better thermo-hydraulics performances in term of cooling capacity of the first wall and a well proved cooling technology. Also the different arrangements of the breeder materials or the inclusion in the design of particular features typical of a reactor design (from DEMO or Fusion Power Plant Studies) lead to some variants for the reference blanket configuration. All these variants present, more or less, some different needs in part of the R and D programme. A recent study in the frame of the Test Blanket Working Group aimed to identify general issues connected to this kind of breeder blanket concept and to assess particular needs related to the development of its different variants. The present paper presents and discusses the results of this study, outlining its outcomes for the implementation of an international cooperation programme and giving guidelines for the identification of a limited number of basic configurations suitable for being tested in ITER. (orig.)