Generation and Characteristics of Radioactive Wastes

Abstract

Mining refers to the act of collecting ore containing a target metal from mines. This process generates gangue—rock that is commercially valueless and therefore subject to disposal. The process of extracting metal from its ore is called melting. Uraninite, pitchblend and brannerite are uranium containing ores, and they contain uranium in the form of oxide. Their grade generally ranges from 0.1 to 0.3 % triuranium octoxide (U₃O₈) equivalent. Although extracted ore of ordinary metals undergoes “dressing,” the process of separating valueless rocks based on physical or chemical characteristics by such methods as fire refining and aqueous refining, the dressing process is not effective for low-grade uranium ore. This kind of ore is crushed into pieces and dissolved in acid or alkali solution. Then uranium is refined and concentrated, followed by precipitation using strong alkali. An intermediate from this milling process is uranium concentrate, which is U₃O₈ powder, called yellow cake for its color. Its uranium content (U₃O₈ content) is about 70–80 %. This material is further refined (or purified) to increase purity and is converted to forms such as UF₆, UO₂ or metallic uranium, suitable for use as reactor fuel in the next process at fuel fabrication facilities. In this connection, refining means increasing the purity of metal resulting from melting by electrolysis or other processes; in the uranium melting process, purification corresponds to refining.

The nuclear regulatory system in Japan has been changed significantly after the Fukushima Daiichi Nuclear Power Station accident in March 2011. Descriptions in this chapter have been translated from the book originally published in Japanese before the accident, with minimal update where appropriate.