The simplest way to determine the mechanical strength of pebbles is by performing crush tests with individual pebbles. These tests are very useful for optimization of pebble properties and for quality control during pebble production.

In crush tests, pebbles are arranged between two plane plates. In a pebble bed, the pebbles in contact with the cavity wall experience similar conditions because there is only one contact at the corresponding pebble hemisphere. Pebbles that are in contact only with other pebbles have on an average six contacts86,87 and the contact forces are in general smaller than on the pebble-wall contact.

The crush loads of 0.5-mm diameter Li4SiO4 and 1-mm diameter Li2TiO3 pebbles used for the European HCPB project are in the range of 4-5 and 35-50 N, respectively, as shown by Knitter et at}9 and Roux et at}92 (see Figure 16).90 The crush loads of the Li2TiO3 pebbles were independent of 6Li enrichment.

Assuming coverage of the wall by pebbles of about 0.7, the lower values of 4 and 21 N for the 0.5 and 1.0 mm diameter pebbles correspond to pres­sures on the wall pebbles of about 12 and 15 MPa. These values are significantly above the assessed maximum pressures of about 6 MPa in the HCPB ceramic breeder pebble bed.91

However, there are several effects that decrease this margin:

• In pebble beds, more generally, in granular mate­rials, an inhomogeneous branching of forces occurs (see Jaeger et at.92), with the effect that some
pebbles experience much larger forces than the average value.

• Thermal annealing results in a significant grain size growth in Li2TiO3 pebbles; a slight decrease in crush load during aging was found for the Li4SiO4 and Li2TiO3 pebbles.77

• Neutron irradiation will have the strongest effect to reduce the pebble strength, as mentioned here.

Fortunately, there is one mechanism, as outlined below, that is expected to alleviate the problem con­siderably: thermal creep.

A general remark to be made here is that care is to be taken with respect to statistics: postirradiation tests from the EXOTIC-7 high burnup experiment showed fragmented pebbles as well as intact pebbles maintaining average strength levels.33, This statis­tical aspect is to be addressed more rigorously in optimization of pebble-bed technologies serving pre­dictable and reliable operation of breeding blankets.