Dismantling of Evaporators by Laser Cutting: Measurement of Secondary Emissions

In order to dismantle the evaporators of an obsolete reprocessing plant in Marcoule, studies were carried out by IRSN (Institut de Radioprotection et de Suˆrete´ Nucle´aire)/DSU/SERAC in cooperation with CEA (power laser group) on the laser cutting of steel structures, on the request of COGEMA (now AREVA NC)/Marcoule (UP1 dismantling project manager) and CEA/UMODD (UP1 dismantling owner). The aim of these studies was: • to quantify and to characterize the secondary emissions produced by Nd-YAG laser cutting of Uranus 65 steel pieces representative of UP1 evaporator elements and to examine the influence of different parameters, • to qualify a prefiltration technique and particularly an electrostatic precipitator, • to compare the Nd-YAG used with other cutting tools previously studied. The experiments, which took place in a 35 m³ ventilated cutting cell, allow to underline the following points: • for the Uranus 65 steel, the sedimented dross, the deposits on the walls of the cutting cell and the aerosols drawn in the ventilation exhaust duct (∼ 275 m³/h), represent respectively between 92% and 99%, between 0.01% and 0.25% and between 1% and 8% of the total collected mass, • the attached slag varies much from one configuration to the other and can sometimes amount to a relatively important fraction of the total mass, • the kerves vary from 2 mm up to 7 mm for the Uranus 65 steel plates (thickness: 13.8 mm for the single plate and 12.8 + 3.5 mm for the double plate), • the exhausted aerosol mass per cut length (g/m) decreases with the cutting speed, varies neither with the stand-off nor with the gas pressure, is dependent upon the gas nature (for the double plate), increases with the laser power, is strongly affected by the nature of the steel (stainless steel or mild steel) and is independent upon the plate position, • the size distribution of aerosols is multimodal with a main mode often around 0.45 μm, • the electrostatic precipitator has been a satisfactory prefilter in the experiments with a filtration efficiency greater than 85% and the acoustic declogging system was very useful, • compared to other cutting tools already tested in similar conditions (reciprocating saw, 50 A plasma torch, 200 A plasma torch, grinder, 1 kW laser without assistant gas, arc-air and arc saw), the 4 kW laser produced less secondary emissions than all other tools and less aerosols than the other thermal tools.