Determination of effective thermal conductivity of beryllium pebble beds during progressive neutron irradiation up to 6000 appm He in the HIDOBE experiments

In present fusion reactors designs, beryllium pebble beds are the prime candidate for use as neutron multiplier for the helium cooled pebble bed (HCPB) breeder blankets. In the 2 HIDOBE experiments (HIgh DOse irradiation of BEryllium; [1]), performed in the High Flux Reactor (HFR) at NRG in Petten, constrained beryllium pebble bed samples are irradiated with four different target temperatures (425 C, 525 C, 650 C and 750 C) up to a doses of 3000 and 6000 appm He production in Beryllium. HIDOBE-02 reached a maximum neutron fluence (E > 1.0 MeV) of similar to 1.78 x 10 super(26) m super(-2) ( plus or minus 10%) in 1260 Full Power Days In this paper a relation is described to assess the state of the pebble bed during operation, using only temperature information, together with present models for effective thermal conductivity. Observed temperature gradients across the pebble bed are measured, reconstructed, and checked/corrected for 'external' causes, to obtain the corresponding thermal conductivities of the pebble beds. The resulting increase in effective thermal conductivity of beryllium pebble beds is caused by changing thermal-mechanical behaviour (creep) of the constrained pebble beds, which is determined using existing correlations from out-of-pile experiments. Creep values in pebble beds under irradiation correspond well with out-of-pile experiments. One pebble bed (T sub(irr) 525 C) shows signs of reaching a maximum creep value after a fast neutron fluence (E > 1.0 MeV) of similar to 0.8 x 10 super(26) m super(-2).