Subtask 2.1 – Microstructure related to fuel fragmentation:
The existence of a burnup threshold for fuel fragmentation in LOCA scenarios has been a key question in several studies and research efforts. As the experimental evidence grows, it seems that high burnup is only one of several factors determining the susceptibility of the fuel to fragment. Several hypotheses have been brought forward to explain this behaviour, such as effects of the power history inducing residual stresses in the pellet, or repartitioning of the fission gas inventory to closed grain boundary networks or bubble populations that weakens the integrity of the fuel under a LOCA event. Recent results from SCIP III have identified some potentially very important effects related to the development of the fuel microstructure in the course of fuel operation. In order to study the impact of these phenomena further, it is proposed to continue on the advanced microscopy examinations performed in SCIP III on fuels with high burnup that fragment to a large extent in LOCA like conditions, as well as to study high burnup fuel that appears resistant to fine fragmentation.
Subtask 2.2 – Fuel fragmentation, relocation and dispersal in non-standard fuel:
In SCIP III, investigations focused on the performance of “standard fuel”, i.e. UO2 fuel with relatively small grains, whereas use of large grain fuel with dopants or additives has become more and more common. Moreover, the microstructure of MOX and gadolinia fuel might also develop differently during reactor operation, compared to standard fuel. Work to be performed under this Subtask aims at extending data base and understanding of fuel fragmentation, relocation and dispersal to fuel types that have not yet been investigated within SCIP III or elsewhere. The data will support estimates of fuel dispersal in LOCA safety assessments carried out by utilities and regulators, as well as refinement and extension of fuel fragmentation models to be incorporated in fuel performance and transient codes.
Subtask 2.3 – Separate effects tests:
Tests in SCIP III have indicated that for fuels susceptible to fine fragmentation critical parameters may be both the temperature ramp rate and the magnitude of the depressurisation transient upon burst. The possibility to control temperature ramp rates was rather limited in SCIP III heating tests. Therefore, it is proposed that a new furnace is constructed to better control the temperature ramp rate in tests of similar size as the existing heating test apparatus (testing a few pellets worth of material). The equipment will be made compatible with a new depressurisation rig being able to simulate the burst event with high degree of control, including an expansion chamber to contain and collect the ejected fuel fragments for further study.
Subtask 2.4 – Transient fission gas release and axial gas communication:
During a loss-of-coolant accident, rapid and large changes of temperature may cause transient fission gas release from the fuel, by mechanisms such as fuel grain boundary fracture or diffusion and interconnection of fission gas bubbles. Understanding of the transient fission gas behaviour is important to determine factors such as increase in rod inner pressure and margins to cladding burst and loss of rod integrity. Knowledge of the transient fission gas release also allows for a more accurate determination of the source term in an accident scenario. In order to properly assess the effects of transient fission gas release on local pressure and ballooning and burst, it is important to know the axial gas communication inside the fuel rod. As a continuation of a limited number of tests performed in SCIP III, it is proposed to perform a parametric study of axial gas communication against burnup and temperature. The results will support improving fuel performance code models of gas communication under transient conditions.
Subtask 2.5 – Spent fuel pool LOCA:
Loss of coolant in a spent fuel pool, with high temperature oxidation of cladding in an air-steam mixture as well as transients leading to ballooning and burst of fuel rods, can have severe consequences. Within SCIP III, only two LOCA tests under simulated spent fuel pool conditions have been performed. Moreover, the scope of post-test examinations was rather limited. Therefore, additional spent fuel pool LOCA tests, covering a broader band of potential conditions, will be performed in this Subtask. The scope of post-test examinations will be extended, providing additional data to define the fission product source term for this type of events.