With nitric acid (HNO3) being at the core of nuclear technology through actinides separation and extraction processes, achieving a complete characterization of the complex processes involving concentrated HNO3 solutions under ionizing radiation equates bringing efficiency and safety into their operation. In this work, the three mechanisms contributing to the formation of nitrate radicals (NO3 center dot) in concentrated nitric acid were investigated by measuring the radiolytic yield of NO3 center dot in HNO3 solutions (0.5-23.5 M) at room (22.5 degrees C) and elevated (80 C) temperatures on time scales spanning from picosecond to microsecond by pulse radiolysis measurements. We conclude that the formation yield of NO3 center dot just after the 7 ps electron pulse, is due to the direct effect and to the ultrafast electron transfer reaction between NO3- and the water cation radical, H2O center dot+ The absolute formation yield of NO; radicals due to the direct effect, GNO(3 center dot)(dir), is found to be (3.4 +/- 0.1) X 10-7 molj-I, irrespective of the concentration and temperature. On longer time scales, >1 ns, an additional contribution to NO; formation from the reaction between *OH radicals and undissociated HNO3 is observed. The rate constant of this reaction, which is activation-controlled, was determined to be (5.3 +/- 0.2) x 10(7) M(-1)s(-1) for 22.5 degrees C, reaching a value of (1.1 +/- 0.2) X 10(8) M(-1)s(-1) at 80 degrees C.