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A selection of presentations related to "Loss of Coolant Accidents with Release of Insulation Material" has already been published in the previous special issue of this journal.In the current special issue selected papers related to "Plant Behaviour Coupled with Reactor Physics" are presented, covering the areas:• code development and validation • uncertainty and sensitivity analyses • transient analyses Traditionally reactor physicists have probably used computer codes more intensively than other professional categories in the field of nuclear technology. Nearly from the beginning of the nuclear era computer codes have been used as tools for nuclear physics, mainly solving the Boltzmann equation for the neutron flux. The exact solution of this equation is impossible for realistic configurations. Regarding the limited computational resources, one has to simplify the methods gradually when going from simple to more complicated geometries.The nuclear core-layout calculation sequence starts with transport theory and a fine mesh for space, angle and energy. The next step, the fuel assembly design, is done by spacial homogenization and condensation into fewer energy groups. The last step, the calculation of the whole reactor core, requires an additional homogenization using two neutron energy groups and assembly homogenized cross sections. For this last step, neutron physics alone is not enough -thermo hydraulics must be taken into account, which supply feedback effects during neutron flux calculations.With increasing computer hardware power, the models became more and more sophisticated, so that the computer calculation time needed for a given problem was not reduced drastically but stayed more or less at the same level.As can be seen in the first four contributions, this traditional 3 step procedure from fuel pin via the fuel assembly to full core configurations seems to be merging together today -in particular for non standard applications. Full core transport calculations on a pin by pin basis are still expensive, but are possible today. Transport theory methods are used to solve complex time dependent transient problems and 3-dimensional core design codes are improved by implementing transport theory corrections into the coarse mesh solution method. And the codes can be used for different reactor designs: for LWR, hexagonal geometries and for HTGR's.A playground for the pure reactor physicist is the reactor core starting at the bottom of the fuelled region up to the top of the fuel assemblies. But this is not the whole story: the reactor core is only one part of the cooling circuit of a power plant. In the other areas, neutron physics is of minor importance, here the thermo hydraulics and the plant instrumentationand control-systems are essential. In these areas, system codes (like RELAP, RETRAN, ATHLET or TRAC) or more specialized thermo hydraulic methods (computational fluid dynamics, CFD) are used for design and accident analyses. For a long time, a simple nuclear core model was sufficient for system codes, b...