Hydride formation in Zircaloys reduces strength and ductility significantly. Hydrides form preferentially in areas of higher stress [36] such as near welds or at crack tips. In loss of coolant accidents (LOCAs), overheated Zircaloy cladding may react with cooling water in a complex manner with different phase-transformation temperatures depending on other species such as oxygen or hydrogen. The solubility of hydrogen in zirconium and Zircaloy differs by almost an order of magnitude between the alpha and beta phases [37], and excess hydrogen may form embrittling hydrides, particularly at

Fig. 4.10 Optical (a) and neutron-radiography images (b, c, d, e) of the welded Zircaloy-4 plates. Figure b was produced using neutrons with wavelengths between 1.4 and 2 A, whilst Figures c, d and e are images of the height of selected Bragg edges. In figure c the start of the HAZ clearly shows the disappearance of the (10-10) Bragg edges, whilst the (11-20) edge in figure d reveals the differences between the outer and inner layers of the plate. Reprinted with permission from (J. R. Santisteban, M. A. Vicente-Alvarez, P. Vizcaino, A. D. Banchik, S. C. Vogel, A. S. Tremsin, J. V. Vallerga, J. B. McPhate, W. Lehmann Kockelmann, J. Nucl. Mater. 425, 218 (2012)) [35]. Copyright (2012) Elsevier

crack tips etc. Despite decades of work, the mechanisms of corrosion of zirconium — based alloys, particularly in reactor conditions, are still an active research field.

As hydrogen is a strong neutron-scatterer, the hydrogen concentration (Fig. 4.11) can be measured by neutron radiography [38] with sensitivities of * 1,000 wt. ppm [39]. Neutron diffraction and radiography can be combined [40, 41] to identify and map hydride phases. The distribution [42] and kinetics [37] of hydrogen in mate­rials can be identified by neutron radiography.

The hydrogen concentration in metals (particularly in zirconium-based alloys) can be also be measured by cold neutron prompt-gamma activation analysis PGAA [44]. However, these techniques are not suitable for imaging and rather provide the
bulk average from the volume illuminated by the incident neutron beam. Cold neutron PGAA is based on measuring prompt gamma rays following the absorption of cold neutrons by hydrogen. This method has to be performed in a close prox­imity to a neutron source. The technique does not change the sample and so is well suited to assessing hydrogen uptake during interrupted corrosion testing.