The Studsvik Cladding Integrity Project SCIP (Studsvik Cladding Integrity Program), is an OECD/NEA supported international program operated by Studsvik, launched in 2004 and now prolonged to 2019, with participants from Europe, Japan, the US, China and Korea. The participants represent four categories; those who supply and manufacture the fuel, the power companies themselves, regulators and laboratories with similar assignments to Studsvik’s.
Thus, SCIP studied basic phenomena of fuel rod failures driven by pellet cladding mechanical interaction, thus contributing to a better understanding of fundamental failure mechanisms. Pellet cladding mechanical interaction, primarily as a function of burnup, was studied in a number of ramp tests. Key parameters important for hydrogen-induced failures, in particular delayed hydride cracking and failures due to embrittlement of the cladding as a consequence of hydriding, are now much better understood thanks to SCIP and can in many cases be quantified. In the case of failures caused by stress corrosion cracking from the inside of the fuel rod (“classical” pellet cladding interaction, PCI), equipment simulating in-core conditions was significantly improved. Such understanding enables safer operation, more effective product development as well as optimal operating restrictions, supporting economical operation.
From the very beginning, SCIP has focused on studies of the behavior of nuclear cladding materials. Studies on pellet-related parameters were usually not considered, primarily due to limited resources and funding. Already at an early stage of SCIP, it became obvious that dramatically changing pellet properties as a function of burnup cannot be excluded in a holistic description of PCI/PCMI. Consequently, the continuation of SCIP, SCIP II, aimed at improving the knowledge of the behavior of fuel pellet materials and investigating the mechanisms generating fuel failures from both nuclear fuel and cladding points of view. An important part of SCIP II consisted also of extended parametric studies using the improved PCI failure simulation equipment that has been developed within the SCIP program.
In SCIP II advanced pellets with additives and larger grains were included. The studies of new materials aimed on the one hand at confirming similarities with well established materials and on the other hand at identifying potential differences and consequences of these differences on failure-related performance.
During SCIP II the Program Review Group followed up a LOCA test program, performed by Studsvik on behalf of the U.S.NRC. Significant fuel fragmentation, relocation and dispersal occurred during the tests with higher burnup fuel. Similar fragmentation was observed in LOCA tests with very high burnup rods performed in the Halden.
The interest of the SCIP community moved to a new area and therefore the focus of SCIP III will be on LOCA issues, in particular on fuel fragmentation, relocation and release. The consequences of cladding overheating and its impact on mechanical cladding properties, due to off normal transients at lower than LOCA typical temperatures and the axial constraint on fuel rod performance during a LOCA transient, will be investigated as well.
Nevertheless, as a spin off from SCIP II, some PCMI and PCI issues will be further studied, as the beneficial effect on PCI of slow and staircase ramps compared with fast ramps and oxygen impact on PCI resistance.
The technical work in SCIP III is performed in different laboratories. Studies of the irradiated rods and semi integral LOCA test are made at the Studsvik Hot Cell Laboratory, leading to a series of LOCA integral-test and mechanical tests in other laboratories at Studsvik. Irradiation takes place at the Halden HBWR (Heavy Boiling Water Reactor), where rods can be power transient tested.