The sources of non-stationary losses that occur in a XeCl^* excimer laser with nuclear pumping excited by a pulsed nuclear reactor are considered. Experimental studies of the spectral and temporal characteristics of the luminescence of Ar-Xe-CCl₄ gas mixtures of various compositions were conducted in the wavelength range of 200-1100 nm when excited by the products of the neutron nuclear reaction ²³⁵U(n,f). The main losses of the B-X band radiation of the XeCl^* molecule (λ_max = 308 nm) occurred due to the effect of the nuclear reactor's instantaneous gamma radiation on the laser's active medium and optical elements. The maximum value of the measured coefficient of non-stationary absorption of the active medium of the XeCl^* excimer laser at a wavelength of 308 nm at the moment of the gamma radiation pulse action was 2.5· 10^-2 cm^-1. Self-absorption at the B-level of the B-X radiation of the XeCl^* molecule (λ_max=308 nm) led to the appearance in the luminescence spectrum of a new B-X band of the XeCl^* molecule (λ_max= 236 nm). The non-stationary intra-cavity refractive index of the Ar-Xe-CCl₄ gas mixture for the B-X band of XeCl^* (308 nm) has been determined to be n_res = 1.1176. A new method has been proposed for determining the gain characteristics of the active medium of a nuclear-pumped excimer laser over the entire time range of the pump pulse. Keywords: nuclear pumping, excimer laser, gamma radiation, non-stationary absorption, pulsed nuclear reactor.