Welding with non-consumable thoriated tungsten electrodes

Abstract

This document was prepared by the International Institute of Welding, Commission VIII Health and Safety under the chairmanship of the Author. It constitutes the considered judgement of this expert group on the hazards and risks added to arc welding by the use of thoriated tungsten electrodes and the specific protective measures which are required to control the risks and thus protect the health and safety of welders and those working with them directly or indirectly as in cleaning and maintenance. The experts have drawn on views expressed in previous Commission VIII documents on this topic and information on thorium and its compounds published in papers in the scientific literature. The experts recommend that use of thoriated electrodes ceases as soon as is practicable and that, until that change is completed, special care is taken to inform workers of the hazards and to implement all the other protective measures which are detailed in the report.

Appendix 1—additional material

The following abstracts are not for publication; they are provided here for the assistance to Commission VIII

• 1. Jankovic JT, Underwood WS, Goodwin GM. Exposures from thorium contained in thoriated tungsten welding electrodes. Am Ind Hyg Assoc J. 1999 May-Jun;60(3):384-9.

Information provided in this article can be used for estimating the radiation dose associated with the use of thoriated tungsten electrodes in tungsten inert gas welding. Area and breathing zone concentrations of 232Th generated by welding and electrode sharpening along with particle size information; isotopic composition of electrodes from two domestic manufacturers and one European manufacturer; and process variables and estimates on the number of thoriated tungsten electrodes manufactured are presented. Past literature is reviewed and compared with the results of this study. Isotopic analysis of a nominal 2 % thoriated electrode found 0.6 ppm +/- 0.4 ppm 230Th and less than 0.1 ppm 228Th. Analysis of a ceriated tungsten electrode and a lanthanated tungsten electrode for 232Th found 124 ppm and 177 ppm, respectively. Electrode consumption during welding was primarily the result of tip sharpening. Less than 3 % of the weight loss was attributable to the welding process. The in-mask concentration of respirable thorium particulate in the welder’s breathing zone was 0.002 × 10(-12) microCi 232Th/mL. The concentration of respirable thorium particulate from electrode sharpening was 1.3 × 10(-12) microCi 232Th/mL. The measured sharpening time was 20 sec per electrode. Estimates of the activity median aerodynamic diameters for the respirable fraction of the welding and electrode sharpening aerosols were 3.5 and 5 microns, respectively, when measured in the breathing zone at 0.3 m (12 inches) from the point of operation. The respirable fraction of the total welding and sharpening aerosols was 45 and 60 %.

Oak Ridge National Laboratory, TN 37831-6292, USA.

• 2. Crim EM, Bradley TD. Measurements of air concentrations of thorium during grinding and welding operations using thoriated tungsten electrodes. Health Phys. 1995 May;68(5):719-22.

An evaluation was performed to determine whether thorium was present in concentrations above the derived air concentration during grinding and welding operations using thoriated tungsten electrodes. A few of the advantages of using thoriated tungsten electrodes in industry include easier are starting, greater stability, and reduced weld metal contamination. The electrodes used in this evaluation contained 2 % thoria (thorium oxide) and were either 2.4 mm or 3.9 mm in diameter. Personal breathing zone and area air samples were collected for the experienced welders participating in this evaluation during grinding operations. The results during the grinding operations for personal and area air samples were generally below the derived air concentration (DAC) for 232Th for solubility class Y of 0.04 Bq m-3 (1 × 10(-12) microCi mL-1) as per 10 CFR 20. The area samples collected during welding operations were below the DAC.

Dow Elanco, Industrial Hygiene, Indianapolis, IN 46268, USA.

• 3. Gafvert T, Pagels J, Holm E. Thorium exposure during tungsten inert gas welding with thoriated tungsten electrodes. Radiat Prot Dosimetry. 2003;103(4):349-57

The exposure to 232Th from TIG welding with thoriated electrodes has been determined at five different workshops. Welding with both alternating and direct current was investigated. The exposure levels of 232Th were generally below 10 mBq m(-3) in the breathing zone of the welders. Two samples from AC welding showed significant higher exposure levels, probably due to maladjustment of the TIG welding power source. Samples of the respirable fraction of 232Th from grinding thoriated electrodes were also collected showing exposure levels of 5 mBq m(-3) or lower. A dose estimate has been made for two scenarios, one realistic and one with conservative assumptions, showing that the annual committed effective dose from inhalation of 232Th, 230Th, 228Th and 228Ra, for a full-time TIG welder, in the realistic case is below 0.3 mSv and with conservative assumptions around 1 mSv or lower. The contribution from grinding electrodes was lower, 10 microSv or lower in the realistic case and 63 microSv or lower based on conservative assumptions. The study does not exclude occurrence of higher exposure levels under welding conditions different from those prevailing in this study.

Dept of Radiation Physics The Jubileum Institute, Lund University, SE-221 85 Lund, Sweden. torbjorn.gafvert@radfys.lu.se

• 4. Saito H, Hisanaga N, Okada Y, Hirai S. Thorium-232 exposure during tungsten inert gas arc welding and electrode sharpening. Ind Health. 2003 Jul;41(3):273-8.

To assess the exposure of welders to thorium-232 (232Th) during tungsten inert gas arc (TIG) welding, airborne concentrations of 232Th in the breathing zone of the welder and background levels were measured. The radioactive concentrations were 1.11 × 10(-2) Bq/m3 during TIG welding of aluminium (TIG/Al), 1.78 × 10(-4) Bq/m3 during TIG welding of stainless steel (TIG/SS), and 1.93 × 10(-1) Bq/m3 during electrode sharpening, with 5.82 × 10(-5) Bq/m3 background concentration. Although the annual intake of 232Th estimated using these values did not exceed the annual limit intake (ALI, 1.6 × 10(2) Bq), we recommend reducing 232Th exposure by substituting thoriated electrodes with a thorium-free electrodes, setting up local ventilation systems, and by using respiratory protective equipment. It is also necessary to inform workers that thoriated tungsten electrodes contain radioactive material.

National Institute of Industrial Health, 6-21-1, Nagao, Tama-ku, Kawasaki, 214-8585 Japan.

• 5. Ludwig T, Schwass D, Seitz G, Siekmann H. Intakes of thorium while using thoriated tungsten electrodes for TIG welding. Health Phys. 1999 Oct;77(4):462-9.

Thoriated electrodes are used in TIG welding. TIG welders, along with persons who grind thoriated electrodes and persons located near relevant welding and grinding sites, might be at risk of thorium intake. The isotopes of radiological relevance are 232Th, 230Th, and 228Th. The studies described in the literature do not provide a consistent picture of the actual hazards, and changes in European and German radiological protection laws have now made it necessary to determine the risks. To accomplish this, a field test was conducted under real working conditions in 26 different welding shops. The airborne activity generated through welding, and through grinding of electrodes, was measured using personal air samplers. Stationary samplers were also used. The filters' samples were evaluated by means of direct alpha spectrometry with proportional counting and by means of gamma spectrometry following neutron activation. The results clearly showed that considerable intake can occur during both alternating-current welding and electrode grinding, if no suction systems are used. The range of 232Th intakes to welders were estimated from 0.1 Bq y(-1) to 144 Bq y(-1) during welding and from 0.02 Bq y(-1) to 30.2 Bq y(-1) during grinding. In 6 of the 26 cases the recent annual limit on intake derived from the most recent ICRP publications was exceeded--in the worst case it was exceeded by a factor of 10--if it is assumed that the persons studied were not exposed workers (not routinely monitored for radiation exposure). When the significantly more restrictive German limits are applied, the amounts by which the limits were exceeded were even greater. Because many qualified welders have very long careers, the risks can thus be considerable. The paper also discusses parameters that influence exposure, and it presents a catalogue of recommended measures for dosage reduction.

Institut fur Strahlenschutz der Berufsgenossenschaften der Feinmechanik und Elektrotechnik und der Chemischen Industrie, Koln, Germany. Ludwig@bgfue.de