By Nina Chestney LONDON (Reuters) -EDF Energy aims to extend the life of its nuclear plants in Britain and invest a further 1.3 billion pounds ($1.7 billion) in its operational UK nuclear fleet to maintain output at current levels and energy security, the firm said on Tuesday. Several countries in Europe, including France, Belgium and Sweden, have announced plans to extend the operating lives of ageing nuclear reactors due to fears about a power supply crunch in the next few years. In the European Union and Britain, most reactors were built in the 1970s and 1980s and were commissioned to last …