Results of the SPIZWURZ bundle test

The 21-rod SPIZWURZ test bundle with three types of unirradiated claddings (opt. ZIRLO, Zry-4, DX-D4) was used to conduct a long-term integral experiment, approximately simulating dry storage conditions. A variety of parameters typical of an integrated test allowed for the acquisition of a large amount of experimental data necessary for verifying the corresponding computer codes. The claddings were preliminarily hydrogenated to concentrations of 100 and 300 wppm in a specially designed HOKI tubular furnace, distributed as uniformly as possible over a 1.3 m length. Hydrogenation was carried out at 450 °C by sequentially feeding fixed masses of hydrogen through a specially treated inner surface of the claddings. It was noted that the rate of hydrogenation of the opt. ZIRLO claddings is 1.5 times lower than for the Zry-4 claddings. After the hydrogen loading of the cladding tubes, the axial distribution of hydrogen was determined by laser scanning profilometry calibrated by hot gas extraction.
During the experiment, two values of internal rod pressure were used: 106 and 146 bar, which were maintained constant throughout the experiment (250 days). ... mehrThe peak cladding temperature decreased in steps of ≈15 K from 400 to 165 °C (average cooling rate ≈0.9 K/day). The maximal cooling rate during each temperature step was 6 K/h, step duration was about 10 h.
The post-test laser scanner measurements of the outer cladding diameter showed significant creep: radial deformation values are between 0.2 and 3.3% (diameter increase and the corresponding wall thinning). The largest creep of 3.3% was measured for opt. ZIRLO claddings hydrogenated to 300 wppm. The corresponding maximum creep value was 0.93% for Zry-4 and 1% for DX-D4. A clearly visible dependence of the degree of creep on the hydrogen concentration is observed for the opt. ZIRLO claddings: the creep of claddings hydrogenated to 300 wppm is 1.2-1.5 times higher than that of claddings hydrogenated to 100 wppm. A number of claddings show radially asymmetric wall thinning, which can be associated with the radial shift of the pellets from the central axis of the rod and the corresponding asymmetric heat supply along the circumference of the cladding.
The metallographic investigations revealed a uniform distribution of hydrides throughout the entire cladding circumference for all three cladding types used. In the DX-D4 claddings, hydrogen primarily diffused toward the outer liner. The degree of hydride reorientation was significantly higher in the Zry-4 claddings compared to the opt. ZIRLO claddings.
The difference in the behavior of the Zry-4 and opt. ZIRLO may be due, in part, to their different grain microstructures. Opt. ZIRLO claddings have a finer grain size than Zry-4. Moreover, although the temperatures during hydrogenation (450 °C) and the experiment itself (max. 400 °C) were relatively low, EBSD measurements showed grain growth from approx. 6 μm for the initial state to post-test 16 μm for Zry-4 (12 μm after hydrogenation), and from initial 3 μm to post-test 4.3 μm for opt. ZIRLO.