In the 1970s, alternative fission fuel technologies had been developed based on packing of spheres to reduce the problems associated with excessive swelling and fragmentation of pellets.32 One of the early pebble-bed blanket designs was developed by Dalle Donne and coworkers6,7,9 at Forschungszentrum Karlsruhe (FZK), now called Karlsruhe Institute of Technology (KIT), Germany. In this concept, breeder ceramic and neutron multiplier were both shaped as small spheres or pebbles and arranged in a so-called mixed bed (see Figure 4).

The concept was based on small (0.1—0.2 mm diam­eter) pebbles of Li4SiO4 and a binary mixture of beryllium pebbles (0.1—0.2 mm and 1.5—2.3 mm diam­eter) (see Figure 5), taken from the extraction of tritium in ceramics 7 (EXOTIC-7) irradiation proj­ect.33 It was found that the compatibility of Li4SiO4 and beryllium was drastically reduced under neutron — irradiation conditions.34 This initiated the separation of breeder and neutron multiplier in different pebble beds in further blanket design evolution.

Purge gas

Be/Li4SiO4 pebble bed breeder zone

Inlet

Coolant systems

Outlet

Diffusion-welded first wall

Stiffening plate

Figure 4 Breeder-out-of-tube (BOT) with a mixed bed of Li4SiO4 and beryllium pebbles. Reproduced from Dalle Donne, M.; Anzidei, L. Fusion Eng. Des. 1995, 27, 319-336.

0 1 2 Figure 5 Mixed-bed test configuration of Li4SiO4 pebbles with small and large beryllium pebbles packed to high density used in EXOTIC-7 irradiation experiment. Reproduced from van der Laan, J. G.; Kwast, H.; Stijkel, M.; etal. J. Nucl. Mater. 1996, 233-237, 1446-1451.

Extensive R&D on breeder pebbles has also been performed by the Canadian Atomic Energy of Canada Limited (AECL), where in particular Li2ZrO3 and Li2TiO3 were developed.9,35

With growing insight into thermodynamics and the experimental results obtained from neutron — irradiation testing, the European breeder out of tube (BOT) concept evolved into the helium-cooled pebble-bed (HCPB) concept,7 in view of preparing a test module program for ITER. This concept evolved further in Europe within the scope of the Power Plant Conceptual Study.15,16 The key features of this early HCPB concept are given in Figure 6. It consists of

Figure 6 The Helium Cooled Pebble Bed (HCPB) blanket concept from the European Power Plant Conceptual Study (PPCS), model B. Reproduced from EFDA, A. Conceptual Study of Commercial Fusion Power Plants; Final Report of the European Fusion Power Plant Conceptual Study (PPCS); Report EFDA-RP-RE-5.0; 2005.

Ceramic

From caps and grid

To FW

To caps and grid

From FW

Bypass

He inlet

He coolant

Figure 7 Evolved helium-cooled pebble-bed concept. Reproduced from Poitevin. Y; et al. Fusion Eng. Des. 2010, 85, 2340-2347.

alternating beds of ceramic breeder and beryllium pebbles, perpendicular to the plasma-facing wall, between flat coolant plates of Eurofer-97, a so-called reduced activation steel based on conventional 9Cr steels.3637

In this study, the blanket box is considered a con­sumable component, with (1) the maximum irradia­tion damage of primary wall structures set at 150 dpa (about 5 FPY (full power year)) and probably the limiting factor of the box lifetime; and (2) the burnup of the ceramic breeder and swelling of the beryllium neutron multiplier depending on the design.38

Further evolution of the HCPB line in Europe concentrated on the strategies for the ITER TBM, as explained by Poitevin and coworkers.17,39-41

The internal structure of the blanket is given in Figure 7. All structures contain dense patterns of cooling channels, with beds of Be and ceramic breeder in the form of near-spherical particles (0 0.25-0.63 mm for Li4SiO4, 0 1 mm for alternative breeder Li2TiO3, and 0 1 mm for beryllium) separated by cooled steel plates and bed heights sufficiently low (about 10 mm) to conduct heat to the cooling plates without exceeding material temperature limits. Tritium is removed from the pebble beds by a slow purge flow of helium at near­atmospheric pressure, with hydrogen (typically

0. 1 vol%) and defined levels of other constituents such as H2O, and so on for optimized integral performance.

Some alternative concepts were explored such as those using a 9-Cr steel variant with higher

He in

pol.

He turn 2

tor.

He out

Beryllide
Cooling panel
beryllide	Ceramics
Tube with thermal insulation

Neutron multiplier
pebble
(Be, beryllide)

Tritium breeder pebble (Li2TiO3)

Figure 8 Schematic of water-cooled ceramic breeder concept developed in Japan. Reproduced from Akiba, M.; Enoeda, M.; Tsuru, D.; etal. Fusion Eng. Des. 2009, 84, 329-332.

temperature resistance, see Hermsmeyer et a/.,18 or SiC-based composite structure.2

The Japanese designed water-cooled solid breeder blanket consists of two submodules (see Figure 8). One submodule consists of a module box, tritium breeding pebbles, neutron multiplier pebbles, and cooling panels. The tritium breeding pebbles and neutron multiplier pebbles are separated by the cool­ing panels, and the beds are oriented parallel to the plasma-facing wall. The module box and the cooling panels are made of reduced activation ferritic — martensitic steel, F82H.36,42 For the tritium breeding pebbles, Li2TiO3 is selected as the primary candidate material. These pebbles are about 0.2-2 mm in diam­eter, with either a monodisperse or binary size distribution.

Among the pebble-bed concepts, there are also designs for a low-pressure water-cooled ITER driver blanket (see, e. g., Lorenzetto et a/.10) and the ITER 1998 design document by Ioki and coworkers.11, Nardi et a/.13 developed ideas for a driver blanket for the reduced size ITER-FEAT. Recent work reported by Ihli et a/.21 also included mixed bed options for DEMO blankets, as shown in Figure 9.

Figure 9 Breeder inside tube concept with ceramic pebbles. Reproduced from Ihli, T.; Basu, T. K.;

Giancarli, L. M.; etal. FusionEng. Des. 2008, 83, 912-919.