rivers freeze over in winter, limiting navigation. Ukraine's rail network connects all major urban areas, port facilities and industrial centres with neighbouring countries. The heaviest concentration of railway track is the Donbas region. Although rail freight transport fell in the 1990s, Ukraine is still one of the world's highest rail users. Ukraine International Airlines, is the flag carrier and the largest airline, with its head office in Kyiv and its main hub at Kyiv's Boryspil International Airport. It operated domestic and international passenger flights and cargo services to Europe, the Middle East, the United States, Canada, and Asia. Energy Energy in Ukraine is mainly from gas and coal, followed by nuclear then oil. The coal industry has been disrupted by conflict. Most gas and oil is imported, but since 2015 energy policy has prioritised diversifying energy supply. About half of electricity generation is nuclear and a quarter coal. The largest nuclear power plant in Europe, the Zaporizhzhia Nuclear Power Plant, is in Ukraine. Fossil fuel subsidies were US$2.2 billion in 2019. Until the 2010s all of Ukraine's nuclear fuel came from Russia, but now most does not. Although gas transit is declining, over 40 billion cubic metres (bcm) of Russian gas flowed through Ukraine in 2021, which was about a third of Russian exports to other European countries. Some energy infrastructure was destroyed in the 2022 Russian invasion of Ukraine. In early 2022 Ukraine and Moldova decoupled their electricity grids from the Integrated Power System of Russia and Belarus; and the European Network of Transmission System Operators for Electricity synchronized them with continental Europe. Information technology The internet in the country is robust because it is diverse. Key officials may use Starlink as backup. The IT industry contributed almost 5 per cent to Ukraine's GDP in 2021 and in 2022 continued both inside and outside the country. Demographics Before the 2022 Russian invasion of Ukraine the country had an estimated population of over 41 million people, and was the eighth-most populous country in Europe. It is a heavily urbanized country, and its industrial regions in the east and southeast are the most densely populated—about 67% of its total population lives in urban areas. At that time Ukraine had a population density of 69.5 inhabitants per square kilometre (180 per square mile), and the overall life expectancy in the country at birth was 73 years (68 years for males and 77.8