The Belesar arch dam, owned by Gas Natural Fenosa SLU, was built between 1957 and 1963 on the Miño River, north-west of Spain. The dam is 129 m high and impounds a reservoir of 655 hm³, used mainly for power generation. It is equipped with a bottom outlet (160 m³/s) at the central part of the dam body and two gated spillways, located at both gravity abutments (total capacity 4’000 m³/s). The total length of the dam crest is about 500 m, considering the arch (268 m) plus both abutments. During the 1970’s, shortly after construction, permanent displacement towards upstream as well as vertical deformation of the dam body were noticed. In addition, a peripheral crack near to the foundation appeared at the downstream face. This phenomenon has been caused by and ARR reaction, producing an expansion of the dam body and a concrete swelling. The irreversible deformations have been increasing up to now with values of 10 cm in the radial direction at the dam crest, without reaching an equilibrium state so far. After 50 years in service, Lombardi Ltd has been commissioned by Gas Natural Fenosa SLU to carry out a safety assessment of the dam, focussing on the expansive phenomenon due to the concrete alkali-silicate reactions. The expertise includes the evaluation for the medium-long term of the safety and operation conditions and the proposal of potential restauration measures, consisting probably of vertical slots using diamond wires to release the compressive stresses induced by the ARR reaction.