Fluctuation analysis of various physical parameters, mainly neutron flux or other radiation, has long been used for the diagnostics of both fission and fusion reactors. However, it appears that there has been relatively little exchange of information regarding research in the two fields. Some noise diagnostic methods used in fission reactors that may have some relevance for or resemblance to fusion plasma diagnostics are described, and this may contribute to the exchange between the two areas. An example is given to illustrate the possibility of such a transfer of experience. Namely, a method is described, taken from experience with fission reactor technology, that has been suggested for fusion applications. The method is used in general for nonintrusive determination of the correlation length of density fluctuations by spectral and correlation analysis. It can be applied to the analysis of plasma soft X rays for investigation of turbulence and magnetohydrodynamic effects. The proposed method has been partially tested on data from the Joint European Torus (JET) tokamak.