Very promising results with the insertion of twisted tapes in tubular heat exchangers have suggested performance of experiments under more interesting conditions, i.e., at higher pressures (50 atm) with exchangers of larger dimensions (200 cm in length and 1-cm i.d.). The results show a great increase in the forced-convection heat transfer coefficient ranging from 100 to 150% at specific mass flow rates of 70 to 100 g/(cm2 sec). The law of dependence of the Nusselt number on the Reynolds number varies with an exponent unity instead of 0.8. The burnout thermal power increases by 30 to 50%; correspondingly, the burnout qualities increase from 60 to 90%, thus drastically reducing the post-burnout length. Furthermore, within this length there are no characteristic, dangerous temperature oscillations. The present results, together with others cited in the referenced literature, constitute a basis for improvement of the subcritical once-through heat exchangers for pressurized-water and liquid metal fast breeder reactor power plants.