Etching of UO2 in NF3 RF plasma glow discharge

A series of single effect, RF plasma, glow discharge experiments were conducted using NF3 gas to decontaminate depleted uranium dioxide from stainless-steel substrates. In the experiments, the plasma absorbed power was varied from 25 to 210 W and the pressure from similar to 10 to 40 Pa. The results demonstrated that UO2 can be completely removed from stainless-steel substrates after several minutes processing at under 100 W with initial etch rates ranging from 0.2 to 7.4 mu m/min. A primary etch mechanism is proposed in which F atoms created in the plasma diffuse to the UO2 surface and react to form successive intermediates of uranium fluorides and/or oxyfluorides with reactions continuing to form volatile UF6 which desorbs into the gas phase to be pumped away. Ions created in the plasma are too low in concentration to be the primary etch mechanism, yet they can deliver enough energy to enhance the reaction process. UO2 etching is a self-limiting process due to the formation of non-volatile uranium oxyfluorides and fluorides which form over the UO2 surface, slowing or completely blocking the reaction to UF6. (C) 2000 Elsevier Science B.V. All rights reserved.