Conceptual aspects of melting unit vessel cooling by heavy liquid metal coolant
The goal of the article is to present conceptual analysis of feasibility of lead-bismuth coolant application for cooling steel vessel of melting unit designed for implementation of the new effective technology of radwaste reprocessing. In support of lead-bismuth coolant feasibility, the main advantages and specific features of its application are presented, taking into account significant experience gained in Russia in handling this coolant (nuclear submarines reactor units and test facilities), availability of job-proved methods and equipment for the coolant quality control, and coolant properties assuring fire and explosion safety and heat removal capability under high temperature and low pressure conditions. The preliminary evaluations were made on temperature distribution in the mode of cooling of the melting unit steel vessel by the liquid lead-bismuth using simple one-dimensional heat transfer model. Forced coolant flow provided by the pump was considered in the analytical model. It was assumed that the coolant was flowing within the gap between the external and internal steel walls to cool the melting unit, while the excess heat was removed by the external heat exchanger. Radiation heat flux from gas in the burning chamber, heat flux from the slag to the inner wall surface of the vessel, and heat removed by the liquid metal coolant were taken into account in the analysis. Emissivity factor of gas in the burning chamber was calculated taking into account correction for mutual interference of CO<sub>2</sub> and evaporated water absorption bands. Calculations were made for the normal operation mode of the melting unit, assuming presence of refractory coating and slag lining of certain thickness formed on the inner surface of the vessel. As follows from the results of evaluations, lead-bismuth coolant is capable of maintaining melting unit vessel surface temperature within permissible limits with the reasonable coolant flow rate under normal operating conditions. Data presented in this article have been obtained for the first time and may be useful in designing melting units for radwaste reprocessing.