Fracture mechanics approach for failure mode analysis in CABRI and NSRR RIA tests

Titre du congrès :WRFPM 2008 - Water Reactor Fuel Performance Meeting
Ville du congrès :Séoul
Date du congrès :19/10/2008

Reactivity Initiated Accident (RIA) tests performed in Nuclear Safety Research Reactor (NSRR) and CABRI reactor showed that zircaloy cladding brittleness increases with burnup, especially over an average fuel burnup of about 50 GWd/tM. It is now fully accepted that the main reason of this embrittlement is the hydrogen absorbed with oxidation of the cladding during the base irradiation. The hydrogen solubility being rather low in zircaloy, especially at room temperature, the hydrogen precipitated in the cladding before the test can act as a preferential site for incipient crack nucleation if hydrides are concentrated in a blister or an hydride rim. An analysis based on elastic plastic fracture mechanics approach has been derived to better understand the failure conditions of the cladding during pellet cladding mechanical interaction phase of RIA transient. Examination of the clad metallograhies performed on CABRI an NSRR fuel rod tests has been done to evaluate the depth of the brittle zone, that is to say the size of incipient crack in each rod. The deleterious impact of incipient cracks versus fuel averaged enthalpy, has been both evaluated through the calculation of the stress intensity factor and the Rice integral (or J-integral). The clad stress and stress intensity factor during transient is calculated with RIA code SCANAIR. The J-integral is evaluated through a module (CLARIS) coupled with SCANAIR code and based on a large set of finite element calculations. The evolutions of these two parameters during the transient has been compared with fracture toughness (KIc) and critical value of J-integral (Jc) of zircaloy deduced from a literature review. This analysis shows that during the PCMI phase of the transient in elastic domain, the stress intensity factor can be a good parameter for the failure analysis. On the other hand, the use of J-integral is more complex but can be used both in elastic and elasto-plastic domain. The study, shows also that due to the strong dependence of hydride solubility with temperature, the brittle zone is deeper in room temperature conditions than in PWR conditions. The incipient crack to be considered in NSRR test is then not only the depth of the hydride rim in the cladding but also an underlying zone containing a significant hydrogen concentration. NSRR condition is thus a testing condition much more severe than the CABRI one and that expected at typical PWR conditions.