Nowadays, the operating nuclear reactors are able to utilise only 1 % of mined out uranium. An effective exploitation of uranium, even 60 %, is possible to achieve in so-called fast reactors. These reactors commercial operation is expected after the year 2035. Several design configurations of these reactors exist. Fast reactors rank among the so-called Generation IV reactors. Helium-cooled reactor, as a gas-cooled fast reactor, is one of them. Exchangers used to a heat transfer from a reactor active zone (i.e. heat exchangers) are an important part of fast reactors. This paper deals with the design calculation of U-tube heat exchanger (precisely 1-2 shell and tube heat exchanger with U-tubes): water – helium.