Research on Heavy-Water Reactors with CO2 Coolant

The first French heavy-water power plant EL4 is cooled by carbon dioxide under pressure. The fact that in France there is a widespread network of plants using this cooling method contributed to this choice. Moreover, power extraction by gas has certain advantages, such as ease of operation, cheap fluid, and the possibility of steam cycles fairly similar to those of conventional power plants. Lastly, gas can open up possibilities for yield increases going hand in hand with technological progress. Direct use in a gas turbine can give 35% yields, for a maximum temperature of 500°C, and even reach 40% with a more complex cycle. During construction of the EL4, study of a 600 MW(e) reactor of similar design was made. It became apparent that such an increase in capacity was possible without raising too difficult technological problems. Appreciable reductions in investment costs may be obtained, in particular, by increasing the number of rods per bundle from 19 to 31 or 37. Eventually, investment costs should be fully comparable to those of other types of heavy-water reactor. Under a co-operation agreement with the Siemens and Sulzer Companies, the CEA has been conducting a study for a year now on a power station with gas cooling and contained in a prestressed concrete vessel. A more compact design can be obtained by integration. Equilibration of pressures between the heavy-water and the carbon dioxide permits better neutron economy, the force tube maintaining the gas pressure being replaced by a single calandria tube. Finally, the safety of the facility is also improved provided that the various ducts running through the vessel are suitably designed. The main technical problems relate to: equilibration of pressures (several designs for reducing diffusion of gas into the water are investigated); the handling device which is incorporated into the upper part of the vessel and which must work in an atmosphere of dry CO2; and lastly the exchangers, which must be set up in such a way that a faulty pipe can be detected and isolated, and even the whole set replaced. Fuel elements. To derive the greatest advantage from the favourable neutron characteristics of heavy-water reactors, a low-absorption canning material with a high resistance to corrosion and good mechanical strength at a temperature of about 600°C must be developed. The work done on beryllium has enabled its ductility to be considerably improved, but the problem of post-irradiation embrittlement, due to accumulation of helium atoms at the grain boundaries, has still not been overcome. The alloy zirconium-copper (1.6% or 2. 5% Cu) should provide a suitable canning material. Irradiation results obtained in the reactor PEGASE, under more stringent conditions than those obtaining in a standard reactor, are encouraging enough for it now to be proposed that the stainless-steel cladding in EL4 should be gradually replaced by zirconium-copper cladding, which would enable the reactor to operate under the proposed conditions of temperature and specific capacity. (author)