MCNP model of L-54 M nuclear research reactor: development and preliminary verification

Abstract

Since shutdown , Politecnico di Milano managed L-54M nuclear research reactor according to deferred dismantling strategy. Recently, decommissioning activities have been started with preliminary radiological characterization, even though a more extended campaign would be required. In this work, a Monte Carlo N-Particle 3D model of the reactor has been proficiently developed to estimate graphite stack activation. To verify the model accuracy, several simulated criticality data have been satisfactorily compared with experimental ones, validating this computational approach as valid support to forthcoming radiological characterization campaign.

MCNP model of L-54M nuclear research reactor: validation by preliminary graphite radiological characterization

“…The development of L-54M reactor neutronic model has already been satisfactorily achieved, even if some knowledge gaps could not be overcome [15]. As well, the simulated criticality and neutron fluence data within the core have already been satisfactorily compared with the available experimental ones [14]. The evidenced differences were explained by unavoidable lack of information and accuracy of some input data [1].…”

“…experimental data could lay in some missing or misleading data about the operational history of the reactor and the exact composition, dimension and location of some components, such as shielding materials. As well, the activation precursors distribution has been considered uniform in the whole graphite stack, assuming the values of a previous elemental characterization on a virgin graphite sample [14]. Alteration of this parameter would more strongly affect the simulated activity concentration of the activation model, rather than the calculated neutron fluxes.…”

“…Several points had been identified as relevant on two accessible parts of the graphite stack: 7 samples were picked-up from the western surface of the stack (top) and other 10 along two (Top, Bottom) east side removable rods (center and bottom) of the at different axial and radial positions with respect to the core. and on the two east surface rods (center, bottom) [14] [15] The sampling activities took place in the Reactor Hall, inside an operative area continuously kept in a negative pressure status also during the L-54M CeSNEF Safe Enclosure Condition: the primary air pipes system continuously supplies ventilation in the Reactor Building with the exit pipe lines connected to absolute particulate HEPA filters (ε = 99.98%; d= 0.3 µm).…”

“…Moreover, experimental analyses were conducted to bridge some knowledge gaps, especially to accurately define macro constituents and impurities and better simulate neutrons diffusion and activation [14]. Given the unavoidable knowledge gaps and code implementation issues, the accuracy of the implemented model was satisfactorily verified by comparing several simulated criticality data (subcritical and supercritical reactivities, control rods total worth, inventory and calibration) and neutron fluxes in different core positions with the available experimental values [14] [15].…”

“…According to similar publications about irradiated graphite and coherently with time elapsed from reactor shut-down, these activation products were chosen: 3 H, 14 Accordingly, the main neutron activation reactions were chosen to identify the respective activation precursors (see Table 2). The precursors concentration in virgin materials was retrieved, either from available information or from new experimental campaigns.…”