A brief review is given of the problems connected with the methods of radiation damage prediction by means of reaction rate measurement. The least-squares adjustment with singular covariance matrices of the pre-information group fluence values is discussed. An alternative method of unfolding is proposed. This method is based on the principle of maximum entropy and includes the possibility of assigning confidence intervals at a certain confidence level to integral parameters. The results obtained for the²³⁵ U fission spectrum have been compared with least-squares results of the STAY'SL program.