1. Lithuanian Energy Institute, Laboratory of Nuclear Installation Safety, Breslaujos g. 3, LT-44403 Kaunas, Lithuania; Vytautas Magnus University, Department of Mathematics and Statistics, Vileikos g. 8, LT-44404 Kaunas, Lithuania
2. Vytautas Magnus University, Department of Mathematics and Statistics, Vileikos g. 8, LT-44404 Kaunas, Lithuania
3. Lithuanian Energy Institute, Laboratory of Energy Systems Research, Breslaujos g. 3, LT-44403 Kaunas, Lithuania