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By monitoring the irradiation-induced embrittlement of materials, the Pressure Vessel Surveillance Program (PVSP) contributes to the RPV integrity and lifetime assessments. This program is implemented on each PWR Unit in France; it is mainly based on Charpy tests, which are widely used in the nuclear industry to characterize the mechanical properties of the materials. Moreover, toughness tests are also carried out to check the conservatism of the PVSP methodology. This paper first describes the procedure followed for the Pressure Vessel Surveillance Program. It presents the irradiation capsules: the samples materials (low alloy Mn, Ni, Mo vessel steel including base metals, heat affected zones, welds and a reference material) and the mechanical tests performed. Then it draws up a synthesis of the analysis of about 180 capsules removed from the reactors at fluence levels up to 7.1019 n/cm2 (E > 1 MeV). This database gathers the results of more than 10,000 Charpy tests and 250 toughness tests. The experimental results confirm the conservatism of the Code-based methodology applied to the toughness assessment.

It has been observed that hydrogen either in solid solution or precipitated under the form of hydrides has an impact on the viscoplasticity of CWSR Zircaloy-4 cladding tubes, increasing significantly the creep resistance. The use of TEM on the structurally complex CWSR material being unlikely to identify the deformation mechanisms, it has been decided to complete this R&D program on recrystallized material. A study has been carried out on fully annealed unirradiated Zircaloy-4 sheet used for the manufacturing of the fuel subassembly grids. Mechanical tests were performed for large ranges of temperatures (300 to 400°C), stresses (120 to 250 MPa), and strain rates (2 ∙ 10-7 to 2 ∙ 10-3 s-1) on as-received and hydrided specimen. The results emphasize: • Hydrogen in solid solution induces a softening of the material. The TEM observations have revealed identical structure of dislocations for both as-received and hydrided specimens. The softening has been particularly observed when dynamic strain aging is activated. It is assumed that atomic hydrogen decreases the dislocation pinning caused by interstitial and/or enhances the intrinsic mobility of the dislocations. With respect to abinitio calculation, atomic hydrogen might be trapped easily by the core of the dislocation, this phenomenon contributing to decrease the lattice friction and to enhance planar glide. • Precipitated hydrides induce a hardening of the material as observed for CWSR Zircaloy 4. The magnitude of the phenomenon depends upon temperature and stress. An analysis of the unload sequences for tension tests and of the secondary strain rates for creep tests leads to the conclusion that hydrides change the kinematics hardening by increasing the internal stress with respect to the as-received material. TEM observation combined with this viscoplasticity approach has revealed that: first, as long as the internal stress is increasing versus plastic strain, hydride are obstacles to the dislocation glide; second, once the internal stress reaches saturation, the hydrides can be jumped over by the dislocations.

This paper deals with the impact of hydrogen on plasticity and creep of unirradiated CWSR Zircaloy-4 cladding tubes. Specimens with low tin content (≈1.3%) were charged with hydrogen between 100 and 1100 ppm using gaseous technique. Uniaxial and biaxial mechanical tests (tension and creep) were conducted on as-received and hydrided material at room and elevated temperatures (350 and 400°C). Analysis of the results showed that both precipitated hydrides and hydrogen in solid solution play a role in the viscoplastic behavior of the Zircaloy-4 cladding. With respect to plasticity, hydrogen affects the hardening kinetics by decreasing the yield stress, increasing the linear hardening coefficient, and enhancing the dynamic recovery kinetics, especially in the hoop direction. It has no effect on the strain anisotropy. Hydrogen increases the creep resistance. The creep rate of the hydrided material decreases by a factor of 2 to 4 depending on the temperature, the hydrogen concentration, and the stress biaxiality. Potential mechanisms such as localized plasticity in the vicinity of hydrides are discussed, as well as the composite effect induced by hydrides acting like inclusions within the ductile metal matrix, and the respective role of hydrogen in solid solution and of hydrides on the dislocation mobility and recovery. Even if no firm conclusions regarding those mechanisms have been reached, the results of this study are already useful in predicting the mechanical behavior of hydrided Zircaloy-4 cladding under handling and long-term interim dry storage conditions.

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