Optimization of the cold moderator for the new pulsed reactor NEPTUN

•The possibility of using mesitylene as a cold moderator in the new research reactor NEPTUN was examined.•To achieve a high cold neutron flux and longest working time, mesitylene was examined in two forms (bulk form, small pellets).•Two designs for the cold moderator chambers were tested.•A comparison between five different materials at varying temperatures to be used as a cold moderator was done.•The technology currently used in the IBR-2 reactor is not suitable for use in the new NEPTUN reactor.