Answer to Question #880 Submitted to "Ask the Experts"

Q

What is a safe limit in bequerels or curies that plain carbon steel is allowed to emit to be considered safe for construction in buildings, oil rigs, and ships?

A

In accordance with federal radiation protection standards of 10 CFR Part 20, 100 mrem y^-1 above background is the limit for exposure to the general public from man-made radioactive sources (other than exposures received for medical purposes). The International Commission on Radiological Protection and the National Council on Radiation Protection and Measurements also prescribes "dose constraint" at the level of 30 mrem y^-1 to further limit the exposure from a single source. Compliance of the limits is also attached with a requirement to practice the protection according to the ALARA (As Low As Reasonably Achievable) principle. This latter requirement usually reduces the actual doses received at a level considerably lower than the prescribed limits. The federal regulators usually adopt more stringent requirements when dealing with releases containing residual radionuclides. For instance, the U.S. Environmental Protection Agency (EPA), under the Clean Air Act, has prescribed 10 mrem y^-1 for radioactive effluent release limit, and under the Safe Drinking Water Act it has prescribed 4 mrem y^-1 (beta and gamma emitters). For release of contaminated site, the U.S. Nuclear Regulatory Commission (NRC) has prescribed 25 mrem y^-1, and EPA's nonofficial guidance is 15 mrem y^-1. Release of solid materials to the public domain has not been officially standardized in the United States, although an ad hoc guidance (such as NRC Regulatory Guide 1.86 for release of surface-contaminated materials) has been used in the past. In the past, for instance, the case examples of metal release have shown doses at or below 1 mrem y^-1. The American National Standards Institute, Inc. (ANSI)/Health Physics Society (HPS) in 1999 published the N13.12 standards (Surface and Volume Radioactivity Standards for Clearance). The standards, however, are yet to be adopted by regulators for implementation. In fact, NRC is in the process of establishing a clearance rule for solid materials. Internationally, the International Atomic Energy Agency (IAEA) and EC (European Commission) have also published standards along the line. The ANSI N13.12 standards are based on 1 mrem y^-1 to the public. The standards are provided in units of Bq cm y^-2 for surface contamination and Bq g for volume contamination. These limits are provided in orders of 10, ranging from 0.1 to 100, representing various groups of radionuclides according to their radiotoxicity. For instance, these limits are 0.1 for ²³⁹Pu, 1 for ⁶⁰Co, 10 for ¹³¹I, and 100 for ³H. These limits are quite similar to those derived by the IAEA or other similar reports. It should be noted that currently steel mills are using sensitive radiation detectors to screen out the radioactive sources from entering the recycling practice for fear of contamination. As such, "detection limit" rather than "dose" has been used by the mills as a de facto standard to gauge the acceptability of metals. Such practice has complicated the distinction between "acceptability" and "protection." The HPS advocates adoption of ANSI/HPS N13.12 standards on the grounds that it provides adequate protection to the general public. S.Y. Chen, CHP, Ph.D.