Method and apparatus for decontaminating molten metal compositions

Abstract

A method and apparatus for decontaminating molten metal compositions. A molten metal composition (typically containing elemental lead or a lead alloy) is initially provided which also includes various inorganic contaminants. The composition is placed in contact with a specialized decontamination member which actively allows diffusion of the contaminants therein. As a result, the contaminants are removed from the molten metal composition. Contaminants of particular interest in lead-based molten metal compositions include arsenic, tin, antimony, tellurium, and combinations thereof. A reducing agent is optimally combined with the molten metal composition to prevent oxide formation on the decontamination member. The decontamination member preferably contains iron in the form of an iron alloy (for example, steel). Additional preferred components in the decontamination system include an iron trap for removing iron-containing contaminants from the molten metal composition. As a result, the composition is rapidly and effectively decontaminated.

Description

CONTRACTUAL ORIGIN OF THE INVENTION [0001] This invention was made with United States Government support under contract number DE-AC07-99ID13727, awarded by the United States Department of Energy. The United States has certain rights in this invention.FIELD OF THE INVENTION [0002] The present invention generally relates to the decontamination and purification of molten metal compositions and, more particularly, to the processing and treatment of lead-containing molten metal compositions in order to remove contaminants therefrom. The molten metal compositions being treated are particularly useful in cooling systems for nuclear power generating units and related applications, with the claimed decontamination apparatus and method facilitating the safe, continuous, and efficient operation of these systems in a rapid and effective manner. BACKGROUND OF THE INVENTION [0003] Most modern power generation systems produce significant quantities of heat as a by-product. Efficient heat removal ...

AI Technical Summary

Benefits of technology

[0013] Next, the molten metal composition having the inorganic contaminants therein is placed in contact with at least one decontamination member which is comprised of a composition that will allow the inorganic contaminants in the molten metal composition to diffuse into the decontamination member. In a preferred embodiment, the decontamination member will comprise iron [Fe] therein, with optimum results being achieved when an iron-containing alloy is employed (preferably steel). As extensively discussed below, the term “diffuse” shall be construed in the broadest possible sense to involve: (1) entry of the inorganic contaminants into and beneath the surface of the decontamination member to various depths without limitation; (2) interaction of the inorganic contaminants with the decontamination member at the surface thereof without necessarily passing beneath the surface; and / or (3) a combination of [1] and [2] above. Irrespective of the manner in which diffusion occurs, the decontamination member is of a type that will have a selective affinity for the inorganic contaminants of interest while avoiding affinity (and diffusion therein as defined above) of the various lead-containing materials (e.g. elemental lead, alloys, compounds, or complexes thereof) that are associated with the lead-containing molten metal composition. As a result (and as more fully described in the Detailed Description section), the inorganic contaminants can be efficiently removed from the lead-containing molten metal composition in order to effectively decontaminate it.

[0014] With continued reference to the decontamination process, specific operating parameters associated therewith (including preferred residence times and the like) will be presented in detail below. However, in order to obtain optimal results, it is preferred that the lead-containing molten metal composition be maintained at a temperature of about 400-600° C. during placement of the composition in contact with the decontamination member. This temperature level promotes favorable reaction kinetics and otherwise facilitates the decontamination process.

[0016] As stated above and in accordance with the claimed process, the inorganic contaminants of concern will diffuse into the decontamination member for removal thereof from the lead-containing molten metal composition. However and during this procedure, it is possible under some (but not necessarily all) circumstances that placement of the molten metal composition in contact with the decontamination member will cause at least one iron-containing contaminant (e.g. elemental iron [Fe], alloys, mixtures, compounds, and / or complexes containing iron) to be introduced into the molten metal composition. Removal of at least some (and preferably all) of the iron-containing contaminant is desired in order to preserve and maintain the overall purity, cooling efficiency, and non-corrosivity of the lead-containing molten metal composition and to likewise avoid undesired precipitation of the iron within the cooling system (which can cause flow restrictions and related problems).

[0018] It should likewise be recognized that in some (but not necessarily all) circumstances where a reducing agent is employed, the lead-containing molten metal composition will contain (after decontamination) at least some of the reducing agent therein which remains in an unreacted state. In particular, this reducing agent will be present (at least temporarily) within the molten metal composition after placement of the molten metal composition in contact with the decontamination member. This situation typically results in accordance with the use of excess quantities of reducing agent within the system as a “default” procedure in order to ensure that oxide formation on the decontamination member does not occur. In a preferred embodiment to be discussed in greater depth below, an additional feature of the claimed process can involve the step of removing at least some of the unreacted reducing agent from the molten metal composition (and the system as a whole). This step enables maximum operating efficiency to be maintained within the decontamination and cooling systems (namely, the minimization of corrosion and improved economic performance by the recycling of recovered quantities of reducing agent). As previously stated, additional information concerning the process discussed above and its various embodiments will be provided in the Detailed Description section below.

[0020] A containment vessel is also provided which is in fluid communication with the supply of the molten metal composition so that the composition can enter the vessel when decontamination is desired. In an exemplary and preferred embodiment, the containment vessel comprises therein the decontamination member outlined above. As previously stated, the decontamination member comprises iron therein (preferably an iron-containing alloy with optimum results being achieved when steel is used for this purpose). In accordance with the general information provided above, the decontamination member is of a type that will allow the inorganic contaminants of concern within the lead-containing molten metal composition to diffuse into the decontamination member when the molten metal composition comes in contact with the decontamination member. In this manner, the contaminants can be removed rapidly and effectively from the molten metal composition.

[0021] The containment vessel will further comprise at least one outlet port therein for passage of the molten metal composition out of the vessel after the composition comes in contact with the decontamination member. Likewise and in an exemplary embodiment, at least one additional outlet port is provided in the containment vessel for the passage of unreacted quantities of the reducing agent out of the vessel (with such quantities being previously combined with the molten metal composition as outlined above). In order to avoid corrosion and maintain structural stability, the containment vessel (and other components associated with the claimed decontamination apparatus) are optimally produced from a composition which is highly resistant to corrosion, chemical degradation, and the like. In a representative embodiment designed to provide optimum results, the containment vessel (along with the various conduits and other components of the decontamination system) are produced from a composition which comprises zirconium [Zr] therein (e.g. elemental zirconium or alloys, compounds, mixtures, and complexes which contain at least some zirconium).

Problems solved by technology

Notwithstanding the usefulness of lead-containing molten metal compositions as efficient coolants in nuclear reactor systems, various difficulties have likewise been encountered when these materials are employed which will now be discussed.

This corrosion (caused by a variety of complex chemical and physical interactions) can significantly degrade the operating components of the cooling systems, thereby leading to costly damage, leaks, and general system failures (including interruptions in reactor operation).

The presence of these materials (in even relatively small quantities, namely, 0.1% by weight or less) can cause significant corrosion of the cooling system under consideration and its operating components.

However, if the lead-containing molten metal compositions being used in the cooling systems are too oxidizing (which is characterized by the production of, for example, lead oxide [PbO] within the systems), reduced flow rates will occur as the oxide materials form.

Even under highly-oxidizing conditions, if one or more of the above-mentioned contaminants are present (namely, arsenic, antimony, tin, tellurium, and / or combinations thereof), corrosion will still occur.

Thus, while the active control of the overall environment in the cooling systems (from an oxidation-reduction perspective) remains a potentially viable approach for mitigating corrosion-based damage, the amount of oxygen in the systems must be precisely controlled in order to achieve this goal which can be difficult and complex.

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