Neutronics Modeling of NTUA’s Subcritical MAGNOX Graphite Pile in View of its Redeployment as a Reactor Mock-Up for Educational Purposes

Subcritical exponential graphite piles are commonly used in universities worldwide to train students in nuclear engineering. The NTUA’s Nuclear Engineering Laboratory continuously operated a MAGNOX graphite exponential pile (NTUA pile) from about 1974 until about 1984, training approximately 10 generations of nuclear engineering students as part of the undergraduate curriculum. The laboratory is planning to revive this training platform in the form of a Virtual Training Environment (VTE) for basic reactor operation using modern means. This paper presents the first steps in this endeavor focusing on the development of an adequate neutronics model capable of characterizing the pile considering the pile setup details as per its original configuration, the boundary conditions around its geometry and a selection of different modeling parameters. To achieve this, the pile neutronics models have been developed using OpenMC, in two main configurations with varied fuel enrichment. The calculated spectra and flux distributions showed increased fast neutron flux with enriched fuel and exponential axial flux attenuation. Sensitivity analyses showed that graphite thermal scattering S(α,β) and boundary conditions altered the neutron spectrum by only a few percentage points, with floor-reflection effects on the flux distribution localized near the pile-floor interface. For the natural uranium configuration, the calculated effective multiplication factor was 0.639, and decay-corrected thermal-flux predictions agreed with historical measurements within the reported uncertainties. Results concluded that this model meets the accuracy required to produce an educational simulator, while providing a replicable methodology suitable for other active or decommissioned piles worldwide.