Study on the aerosol generation of plutonium metal due to oxidation
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Study on the aerosol generation of plutonium metal due to oxidation Xianxun Peng1 · Dexin Ding1 Received: 15 May 2020 / Published online: 14 September 2020 © The Author(s) 2020
Abstract Transuranic metals such as plutonium usually need to be protected in inert gases to inhibit oxidation and produce aerosols. In the paper, the oxidation reaction process of plutonium was depicted based on the point defect model. The aerosol generation rate was approximated to the oxidation rate of plutonium when the oxidation layer keeps constant. The results show that the rate is strongly dependent on the vacancy diffusivity and reaction rate constant of the vacancy generation and consumption. The aerosol generation rate at the temperature of 298 K is in the order of 108 particles/cm2/s. Keywords Plutonium · Aerosol · Oxidation · Point defect model
Introduction
Pu + 3PuO2 = 2Pu2 O3
Plutonium in transuranic elements is a special nuclear material used in the nuclear energy and an important raw material for MOX fuel. Due to the active nature, highly radioactive and radiotoxic of plutonium, its manipulation is generally carried out in a glovebox with inert atmosphere, such as the argon [1]. However, the high oxophilicity property can still lead to the oxidation of plutonium. And its volume can be expanded by up to 70% during the process of oxidation [2], which may result in radioactive aerosol release to the atmosphere in a glovebox gas exchange. The U.S. Department of Energy estimates that the lifetime cancer risk from inhaling 5000 plutonium particles, each about 3 μm wide, to 1% over the background U.S. average [3]. Considering the severe consequence, the generation rate of the aerosol of plutonium in argon atmosphere needs to be estimated for the health of staff onsite. In the oxygen environment, there are mainly two compounds generated, P u2O3 and P uO2. The free energies of Pu2O3 and PuO2 are − 378 kcal/mol and − 238.5 kcal/mol [4], respectively. When the oxygen is enough, the reaction is
Pu + O2 = PuO2 When oxygen is scarce, the reaction below happens * Dexin Ding [email protected] 1
School of Resource Environment and Safety Engineering, University of South China, Hengyang 421001, Hunan, China
(1)
(2)
The above reaction can be backward again with more oxygen
2Pu2 O3 + 1∕2O2 → 2PuO2
(3)
Hydrogen would be generated with moisture appearance
Pu + 2H2 O = PuO2 + 2H2
(4)
PuO2 + xH2 O = PuO2+x + xH2
(5)
Studies have shown that the oxidation of plutonium is a two-stage process [1]. The first stage is a diffusion control process. With a thicker oxide layer accumulated on the surface, oxygen and metal ions have a higher barrier to diffuse to the interface to react, leading to a slower oxidation rate. The oxide layer thickness for the first stage can be depicted by a parabolic curve, as shown in Figs. 1 and 2. Then the oxidation enters the second stage, and the thickness of the oxide layer reaches a critical value, which is determined by the stress accumulation induced by the formation of lowdensity oxide (23.67 cm
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