The neutronics scheme adopted for the HELIOS irradiation experiment in the High Flux Reactor Petten

Highlights: • The schema separates effects on the reactor cycle timescale from effects of the day-to-day timescale. • The calculated reaction rates in flux detectors were compared with experimental measurements. • A novel method of real-time measurement of sample power was proposed. - Abstract: A dedicated neutronics computational scheme is a crucial tool in the preparation and the post-analysis of an irradiation experiment, as it defines the key parameters of the experiment. The complex and very dynamic environment of a Materials Testing Reactor (MTR) core, on the other hand, represents a challenging object for neutronics calculations. Within the framework of the HELIOS experiment conducted at the High Flux Reactor (HFR) in Petten (The Netherlands) in 2009 under the European Project EUROTRANS, a new neutronics scheme has been developed with the objective of finding the optimum balance between accuracy and calculation performance. This scheme is based on the Monte-Carlo technique and employs a two-level approach by separating the problem into overlapping spatial and temporal domains. The underlying physical phenomena are presented, together with elements of validation which gives a further insight into the effects governing the neutron fluxes in the core.