OECD SCIP, Studsvik Cladding Integrity Project: the cladding tubes of the fuel assemblies surround the nuclear fuel, which is present in the form of tablets (pellets); these tubes form the primary barrier against the release of radioactive fission products. They must withstand a wide variety of stresses and loads, especially in case of increased burn-ups. Together with 24 other organisations, ENSI has been participating in the OECD’s SCIP project since mid-2009. This concerns the main damage mechanisms that can take place in cladding tubes made of zirconium alloys – including the pellet influence. For this purpose, experiments were carried out both at the Studsvik research reactor in Sweden and at the Halden reactor in Norway. The first phase of the SCIP project ran from 2004 until mid-2009. By the end of 2009, the programme had been redefined for project phase II, which is also scheduled for five years and will focus on the following aspects:
- Mechanical interaction between fuel and cladding tube: in case of power fluctuations, the fuel pellets change their form and may consequently exert strong pressure on the cladding tube in places.
- Interaction between fuel and cladding tube due to stress corrosion cracking: gaseous fission products generated during operation are partially released from the solid oxide pellet matrix into the cavities inside the cladding tubes. On contact with the cladding tube, they may encourage the formation of cracks, especially at points subject to mechanical stress (stress corrosion cracking).
- Hydrogen-induced weakening of the cladding tubes: hydrogen is released during the corrosion of the cladding tube, and it penetrates from outside into the zirconium alloy of the cladding tube. In this case, the hydrogen (in the form of hydrides) can lead to general embrittlement of the cladding tube, or may force the formation of cracks from outside.