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Operating Microwave Ovens in the Laboratory

March 1999

•  Do not operate the oven if it is damaged. It is very important that the oven door close properly and that there is no damage to the door seals and sealing surfaces, hinges and latches.

•  Since there may be residual contamination, never use the laboratory microwave oven to heat food or drinks. Institute policy and federal regulations do not allow these items consumed or stored in the laboratory.

•  Do not use the microwave oven to heat up hazardous chemicals or radioactive materials.

•  Do not use ALUMINUM FOIL at anytime during the heating cycle. Metal utensils and utensils with metallic trim should not be used in the microwave oven.

•  Avoid heating materials in cylindrical-shaped containers. Liquids heated in certain shaped containers (especially cylindrical-shaped containers) may become overheated. When overheated, liquids may splatter during or after the heating cycle resulting in possible employee injury or damage to the microwave oven.

•  When heating liquids in screw-cap bottles, completely loosen the screw-caps to prevent pressure build-up within the container. This pressure build-up with a cap that is not sufficiently loose can cause the bottle to explode.

•  If steam accumulates inside or outside of the oven door, wipe with a soft cloth. This may occur when the microwave oven is operated under high humidity conditions and in no way indicates malfunction of the unit.

•  Be careful when removing containers from the microwave. Some containers absorb heat and may be very hot. Always use protective gloves and appropriate eye/face protection to minimize any possible injuries.

•  If materials inside the oven should ignite, KEEP OVEN DOOR CLOSED , turn oven off, and disconnect the power cord.

•  Do not attempt to tamper with or make any adjustments or repairs to door control panel, safety interlock switches or any other part of the oven. Repairs should be done by qualified service personnel only.

Near Miss from explosion in microwave oven
On October 8, 2001 , in a laboratory at the Oak Ridge National Laboratory (ORNL) West Complex, sealed glass tubes containing ferric chloride and hydrochloric acid exploded while being heated inside a microwave oven. The explosion broke the glass turntable in the microwave oven, opened the oven door, and released glassware fragments and chemicals outside the oven. Researchers for the experiment were standing near the oven while this occurred; however, no one was injured or contaminated. The contractor reported this as a near miss occurrence. (ORPS Report ORO--ORNL-X10WEST-2001-0012).

The experiment's principal researcher purchased a household microwave oven from a local department store for the experiment. The conventional oven he had used previously took several hours to heat the chemicals, and he hoped that a microwave oven would reduce the heating time to a few minutes. While the operating manual for the household microwave oven clearly cautioned against heating closed containers and chemicals, the researcher ignored these warnings. In preparing samples to heat in the microwave oven, he placed one-half milliliter of a ferric chloride and hydrochloric acid solution into each of four thin Pyrex ® tubes that were four to five inches long and heat-sealed the ends. The researcher believed that the small amount of liquid in the glass tubes would not cause pressures to exceed the 400 to 500 pounds per square inch pressure-retaining capability of the tubes. However, he did not perform a design analysis and failed to enlist the aid of a subject matter expert to verify his assumption. Subsequent calculations showed that he grossly underestimated the air space needed to accommodate the expansion of the solution from a liquid to a gas.

The researcher placed the tubes inside the microwave oven, set the oven at 60 percent power for two minutes, and waited beside the microwave oven while an assistant stood in front. The tubes exploded after about one minute, opening the oven door and splattering liquid chemical about 1 foot, which fortunately was not far enough to hit the researchers. Glass stained with ferric chloride landed near the assistant, but he was not injured. Both researchers were wearing safety glasses.

A subsequent investigation of this occurrence found that not only had the use of a microwave oven in this experiment not been subject to a design review by subject matter experts, but neither had the previous use of a conventional oven. The investigators determined that the root causes of the event were deficient work planning and inadequate hazards analysis.

During 1999, bioassay laboratories at the Savannah River Site experienced three separate explosions of sealed vessels inside microwave ovens. During the most recent explosion, on December 13, 1999 , the sealed ceramic container of a fecal sample burst, breaking the glass turntable in the microwave oven and opening the oven door. A subsequent investigation found that an electrical short in the pressure transducer prevented accurate monitoring of the vessel's pressure, and that the vessel's rupture disk had failed to function. The microwave oven was designed for laboratory use and had a safety device that restricted the opening of the door to a few inches. Although glass and other material fell onto the laboratory floor, the restricted door movement kept workers nearby from being injured. (ORPS Report SR--WSRC-LTA-1999-0039, OE Summary 2000-1).

These occurrences show that heating sealed containers or chemicals inside microwave ovens is hazardous. The ORNL occurrence illustrates the importance of hazard analyses and design reviews for laboratory experiments. Personnel selecting and using laboratory equipment should consider the hazards analyzed as well as the manufacturer's recommendations.

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Imperial Laboratory Oven