The Magnox design was an evolution and never truly finalised, and later units differ considerably from earlier ones. As neutron fluxes increased in order to improve power densities problems with neutron embrittlement were encountered, particularly at low temperatures. Later units at Oldbury and Wylfa replaced the steel pressure vessels with prestressed concrete versions which also contained the heat exchangers and steam plant. Working pressure varies from 6.9 to 19.35 bar for the steel vessels, and 24.8 and 27 bar for the two concrete designs.