Nowadays, the design of nuclear power plants (NPP) is characterized by enhanced safety level. In particular, GEN III+ and GEN IV reactors have to be highly secure and designed to withstand extremely severe external events such as floods, tsunamis, tornadoes, plane crashes, fires and, in particular, earthquakes. Seismic isolation is considered the most promising technology to protect NPPs from a seismic attack, because it allows to strongly reduce the horizontal components of the acceleration, giving to the isolated structure a — “rigid body” behaviour. Therefore, all the internal most critical components are subjected to the same horizontal acceleration, independently of their position in the building, allowing for a standardization of the plant, that can be designed independently of the construction site. Due to the typical huge masses of NPPs, the related isolators often have large sizes. This complicates the manufacturing process and the execution of the type tests, especially those to be performed in dynamic conditions. ENEA, in the framework of the second research programme of the agreement with the Italian Economic Development Ministry (MSE),designed elastomeric isolators addressed to the protection of Small – Medium Nuclear Reactors. Reference was made to the IRIS reactor, subjected to wide-ranging studies in cooperation with international partners. For application in NPPs, it is very important to know the behaviour of the isolators beyond the design conditions, thus tests up to failure are needed. As a first step of the testing campaign aimed at this scope, different tests were carried out on scaled prototypes of said isolators having 500 mm diameter, manufactured by FIP Industriale. The tests were performed at CESI and FIP Industriale laboratories. CESI performed non destructive tests on 6 isolators, to characterize the vertical and horizontal stiffness as well as damping both in static and dynamic conditions. Sinusoidal tests have been performed at different frequencies (0.1 ÷ 1.5 Hz)and different amplitudes (corresponding to shear strain 5% ÷ 150%). A combined tension-shear test was also carried out (0,1 Hz,100% shear strain, 100 kN of tensile load). FIP Industriale tested 4 isolators in extreme conditions. One isolator was tested in compression up to failure, and 3 isolators were tested in shear (under constant vertical load) at increasing amplitudes up to shear strain of at least 350%. The paper presents the very positive results of the experimental campaign, underlines the limits of the present European standard for antiseismic devices (EN15129,which is not specifically addressed to NPPs) and suggests some possible improvements in the type testing procedures of seismic isolators to be used for the protection of nuclear power plants.