Calculation of nuclear level density relevant for thorium-based reactors

In this paper the problem of nuclear level density is investigated. The notion is intuitively introduced and then rigurously defined. Several procedures for its calculation, among the most performant, are described. We start with one of the first proposed approach, the equidistant model, continue with empirical methods like Back Shifted Fermi Gas and Gilbert-Cameron and arrive at some modern approaches, as Ignatyuk method and modified Ignatyuk method. Revealing for each method its advantages and limits, we concentrate on Ignatyuk-type methods. Practical recipes for the determination of the needed parameters are given. These are referring to the shape description and to the calculation of deformation energy, shell correction and collective enhancement factors (rotational and vibrational). The presented formalism is applied to study some protactinium isotopes, nuclei of practical interest which appear in the thorium fuelled nuclear reactors