Contribution of research reactors to the programmes for research and technological development on the safety

The most significant current use of nuclear reactors is for the generation of electrical power, for instance with pressurized water reactors (PWR) which constitute the most commercial type of reactor. Nevertheless, since the beginning of nuclear activities, important applications of nuclear reactors are research applications including: the providing of neutrons source for the basic research (beamline experiments), the development of nuclear technology, and of new types of reactors, for example to bring solutions of the radioactive waste management. Moreover, nuclear reactors are used for training, nuclear propulsion, production of radioisotopes for medicine and industry. Therefore, the research reactors constitute major equipments to the scientific and technological research. Research is of benefit for both reactors currently in operation and for future reactors as the phenomena occurring during a severe accident are the same. Nevertheless, if severe accidents were not taken into account in the initial design of nuclear power plants currently in operation, improvements have been made in France in terms of prevention and limitation of consequences. The objective of the present article is to illustrate the contribution of the research reactors to the programmes for research and technological development led to increase the safety of the nuclear power plants, more particularly in the field of the accidents of insertion of reactivity (RIA), loss of primary coolant (LOCA) and partial or total melting of the core; indeed, the physical phenomena occurring during a severe accident are extremely complex. Research aims to better understand these phenomena and to reduce the associated uncertainties in order to assess the extent to which the state of acquired knowledge can be used to make reliable predictions. The examples will concern accidents that may occur in pressurized water reactors and in sodium fast reactors (SFR). Some given examples will show the fundamental importance of experiments in research reactors to establish and support the safety demonstrations; indeed, such in-pile tests: - allow to characterize badly known physical phenomena, - can put in evidence some not foreseen phenomena, - contribute in fine to the development and in the global validation of the codes used in the safety demonstrations, such as ASTEC (Accident Source Term Evaluation Code) used to simulate an entire accident from the initiating event to the possible radionuclide release outside the containment. (author)