Answer to Question #3486 Submitted to "Ask the Experts"

NUREG-1717 lists an exemption for steel mills using ⁶⁰CO as refractory wear indicators in blast furnaces. Can you explain how this works, how much ⁶⁰Co is used, and is this really common in today's mills or a thing of the past? If still used, how much ⁶⁰Co would be expected to enter the metal product produced in the furnace?

The answer to your question has been provided by Nuclear Regulatory Commission staff personnel responsible for the NUREG and a health physicist working in the steel industry.

The exemption is referenced in NUREG-1717, "Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials," on pages 2-18 and 2-19 (Section 2.2.3.3, Steel Contaminated with ⁶⁰Co).

A ⁶⁰Co source is embedded in the refractory material (typically bricks) for steel blast furnaces. A radiation detector located in the vicinity of the refractory indicates that the source is in place. When the refractory material wears, the source (typically 1-10 mCi) drops into the steel and the radiation detector no longer shows activity. This is how the ⁶⁰Co source is used as a refractory wear detector.

The NUREG only devotes two fairly short paragraphs to this and states that dose rates from a steel slab have been calculated and references a memorandum to the NRC dated 1 March 1996 titled "Radiation Levels from Steel Contaminated with ⁶⁰Co: A Limited Study of Potential Regulatory Issues."

The calculated doses were not high enough to exceed the threshold and warrant another calculation with less conservative, more realistic assumptions. Therefore, this process was listed as an exemption and below further regulatory concern. According to one individual in the industry, the practice is still in limited use in the United States and in more general use world wide. You may want to contact a health physicist in the steel industry for more precise information in this area.

Linda M. Sewell, CHP
ATE Editor, Nuclear Power/Industry