From real life to scale model (and back again): The life of CROCUS in the student’s mind

¹ École polytechnique fédérale de Lausanne (EPFL), Switzerland
² Paul Scherrer Institute (PSI), Switzerland

^* This email address is being protected from spambots. You need JavaScript enabled to view it.

Published online: 6 November 2025

Abstract

The CROCUS nuclear reactor is primarily motivated by education, from public visits to advanced teaching at the Bachelor’s, Master’s, and doctoral levels, as well as professional training. It is a uranium-fueled, light-water moderated, zero power reactor with a maximum power of 100 W. As simple as an operational reactor can be, CROCUS effectively supports conceptual understanding through its tangible materiality, offering virtually no distance from the system, no significant risk, and negligible dose rates in its vicinity.

Once the basic concepts and systems are grasped, however, a deeper effort is required to fully comprehend the machine and the underlying physics. Various tools are employed to accompany this learning process, including Monte Carlo modeling, operator licensing for PhD students, and tangible scale models – the focus of this contribution. Following the limited use of an earlier 3D-printed model, a sturdier and more manipulation-friendly version was built using LEGO^® bricks. This model reproduces all key features with minimal approximations, helping to visualize otherwise hidden elements, such as the startup source and the expansion vases.

We present the educational objectives, the learning methods applied, and the benefits of using CROCUS and its models. The impact of the LEGO^® model was evaluated in two teaching contexts: a large group of 2^nd-year Physics Bachelor students during a single 4-hour experiment, and a smaller group of 1^st-year Nuclear Engineering Master students over a semester-long practical course. Statistical analysis indicates improvement in the Master students’ understanding of CROCUS and reactor functioning.