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The SOFIT program is described as a sub-task under the Finnish- Russian co-operation on VVER fuel research and consists of a series of irradiation tests in the MR reactor at the Kurchatov Institute, Moscow. The contracting parties are Imatran Voima Oy (IVO) and the Russian National Research Centre Kurchatov Institute (IRTM). The program is divided into three distinct phases each addressing specific objectives:
- SOFIT 1 Parametric fuel rod irradiations with basic steady state power histories up to moderate levels of burn-up as dictated by instrumentation endurance.
- SOFIT 2 Parametric studies based on irradiation of instrumented high burn-up rods.
- SOFIT 3 Irradiation testing under transient conditions.
The results of 2 assemblies of SOFIT 1 are at present available where the main objective was to obtain well qualified data on VVER-440 fuel for verifying and improving codes. In each assembly, rods of different design were irradiated with in-pile instrumentation to measure fuel centreline temperature, fuel stack and cladding elongation. PIE has been performed to obtain data on microstructural changes and measurement of fission gas release (FGR). The first series of irradiations were completed by May '92 and some destructive PIE has been performed.
Precharacterization and irradiation histories for SOFIT 1.1 rods 1-6, 7 and 12.
In-pile temperatures for rods 1-6 and PIE fgr from rods 7 and 12.
Precharacterization and irradiation histories for SOFIT 1.3 rods 1, 3, 4 and 5.
In-pile fuel and clad extension measurements for rods 1 and 3.
In-pile temperature data for rods 4 and 5.
Irradiation conditions in the MR reactor
The MR reactor is a pool type research reactor with a total power of 50 MW. The reactor contains several pressurized loops which can be connected to one or more in-pile test channels. These channels are located between blocks of beryllium which act as moderators. The reactor is operated in 30 to 40 day cycles of near constant power. At the time of the SOFIT tests, the axial form factor was 1.37 and the in-channel radial form factor was 1.2.
Rod power determination was based on the calorimetric bundle power measurements and calculations. The axial and radial power distributions were calculated using a model based on experimental data from the MR reactor. Axial power profiles were refined using signals from the 5 axially located neutron detectors by fitting a 5th order polynomial to the measurements to obtain a smooth power profile. The uncertainty in the axial power profiles is estimated at +/-5%. The in-bundle radial power profile was calculated using signals from 3 neutron detectors located on the same plane as the core central elevation. The detectors measured some azimuthal variation in power during irradiation and the uncertainty of the radial profile of +/-5% combined to provide an estimated total uncertainty of +/-10% in local linear power.
SOFIT 1.1 - 1.4 comprised 4 assemblies each of 18 rods arranged in hexagonal geometry around a central non fuelled tube used for instrumentation including neutron detectors and coolant temperature thermocouples. The fuel-to-clad gaps ranged between 140 and 290 microns, the fill gas, both helium and xenon were used, varied in pressure between .1 to 2 MPa. The UO2 density varied between 10.4 and 10.75 g/cc and the fabricated grain size was around 5 microns. The fuel active length was 1000 mm within a total rod length of 1200 mm. In most cases 6 of the 12 outer rods were instrumented with thermocouple hot junctions located between 300 and 500 mm above the lower end of the fuel stack and near the maximum power position.
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- V Yakovlev, A Moshajev, P Strizhov, J Johansson, P Liuhto, P Hyvarinen and R Terasvirta: SOFIT: A Joint Experimental Programme Between the USSR and Finland on VVER Fuel Performance, Paper presented at IAEA International Symposium on the Utilization of Multi-purpose Research Reactors and related International Co-operation, Grenoble, France, 19-23 October, 1987.
Keywords: VVER type reactor, experimental data, fission products, fuel elements, fuel pellets, fuel rods, fuel-cladding interaction, fuel-coolant interactions, xenon.