Axial growth in high Zircaloy-4 structural components due to the hydrogen incorporation

The zirconium based structural components under the severe operation conditions experiment changes in theirs original dimensions. These dimensional changes may be attributed to two causes: hydrogen incorporation and irradiation growth. Both causes play a role in the dimensional changes but only the reactor- in conditions can define if the two or one of them is significant. In structural components as the pressure tubes of the CANDU type reactors, where hydrogen content is less than forty ppm are acceptable, changes due to hydrogen incorporation are irrelevant. But in the case of the six meters long cooling channels of the Atucha 1 (CNA-1) nuclear power reactor, where hydrogen content up to 1000 ppm are possible, the situation could be different. In the present work the attention was focused on the volume changes due to the hydrogen incorporation. When the terminal solid solubility limit is exceeded in the zirconium based alloys, the hydride precipitation occurs. In Zircaloy-4 at the operating temperature, 300 deg C, about 90 ppm are in solid solution and the rest is precipitated as hydrides. Both situations produce a positive volume change which contributes to the growth. In this work the dimensional change produced by hydrogen incorporation in the CNA-1's channels was estimated. The expansion due to hydrogen absorption and the change of length caused by hydride dissolution and precipitation were measured by differential dilatometry in samples with hydrogen contents ranging from 20 to 650 ppm. The dilatometric data indicates that in the cooling channels of the CNA1, the axial growth is linear with the hydrogen concentration, and the calculated growing rate is 1,87 x 10-6 mm/mm material *ppm H. This rate for the upper hydrogen concentration limit of the channels implies a growth of 6,08 mm. As an accuracy testing method for the applied technique, the terminal solid solubility curve was determined by both, differential dilatometry and differential scanning calorimetry. The values obtained in the present work were compared with the values reported in literature and a very good agreement was found. (author)