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Modeling and Control of a Sodium Conversion Process Applied to Nuclear Decommissioning Activities

Humberto E. Garcia

Nuclear Technology

Volume 123 / Number 2 / August 1998 / Pages 166-183


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Production of sodium hydroxide has been an important process in the chemical industry. Sodium hydroxide can be derived in several ways. One way in particular is based on combining liquid sodium with water in a caustic medium. This reaction has appeared in the nuclear industry as an important process in current decommissioning activities for liquid-metal nuclear reactors. The significance is explained as follows. Liquid-metal reactors often use liquid sodium as a heat transfer medium. Being radioactive and chemically reactive, this sodium is a mixed waste that must be processed before disposal. An accepted solution is to convert the radioactive liquid sodium to sodium carbonate, a chemically inert low-level waste suitable for near-surface burial. The conversion can be carried out in two independent processes. A first process converts sodium to sodium hydroxide. A second process converts the resulting caustic product to sodium carbonate. The former process is addressed, i.e., the chemical process of combining sodium with water in a caustic medium to produce additional sodium hydroxide. Because of the particular dynamics, characterizing this chemical process is important to predict plant behavior to control actions, disturbances, and upsetting conditions. To this end, the describing formulations of this conversion are derived in a particular physical assembly. Based on the resulting description, a computer model was developed from mass and energy balance equations, swelling predictions, and hydraulic relationships present in the system. The model was then used to synthesize a simple control strategy and to analyze its performance. In particular, the control algorithms that regulate the sodium, water, and caustic flows are discussed. The controllers were then validated by computer simulation, and some plant responses are presented.

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