Radiological characterization of the activation of the structure materials of the BR1 research reactor

This paper presents a radiological characterization of structure materials of the BR1 research reactor. BR1 is a natural uranium fuelled, graphite moderated and gas (air) cooled reactor. The objectives of this study are (1) to predict the type and the quantity of activation products (isotope-by-isotope) formed over the operating life of the reactor BR1 and remaining after a given cooling time and (2) to evaluate the structure materials activation (, or activations) at different time scales after the definitive predicted shutdown of the reactor. For this purpose, a calculation methodology was defined: The SCALE4.3 code system, in conjunction with the 238 neutron group library (based on ENDF/B-V data file) was used to generate cross section for all relevant materials. For the 2-dimensional spatial flux determination, the neutron transport code DORT was used. From the 2-D flux map, activations are obtained by using the ORIGEN2 code and taking into account the detailed operational history. This work concerns both the graphite (reflector) and the concrete (biological shielding) of the BR1 reactor. Main results are as follows: we notice that in the concrete and the graphite, the flux is almost thermal (more than 90% of total flux has energy below 0.5 eV), so this enhances the thermal activation reactions such as⁴⁰Ca(n,)⁴¹Ca,⁵⁴Fe(n,)⁵⁵Fe, etc. For the graphite zone,¹⁴C remains the dominating participant to the activity after 5 years of cooling time. There is also production of other activation products such as⁵⁵Fe and⁶³Ni. In graphite, we underline the generation of⁶⁰Co formed by capture reaction on Fe and Ni (Fe and Ni are initially present in graphite as impurities),⁶⁰Co is a high energy gamma emitter. For concrete,¹³³Ba is by far the principal producer of activity but we underline also⁵⁵Fe and⁴¹Ca contributions:⁵⁵Fe is the most active during the first years of cooling time and the⁴¹Ca contribution becomes more significant after a long term scale. In concrete, the specific activity is strongly spatially dependent.