Metal Disintegration Machining System for Three Mile Island Nuclear Reactor Vessel

Metal Disintegration Machining System for Three Mile Island Nuclear Reactor Vessel

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Usage conditions apply
The metal disintegration machining (MDM) system consists of a box enclosing a cutting head assembly and an attached articulating arm assembly. The lower end of the arm is attached to the top of the cutting head assembly. The MDM system is the principal object of accession no. 2012.0171 in the NMAH Modern Physics Collection.
Background on Metal Disintegration Machining (MDM) System
Edited excerpts from "Phase 4 Status Report, Removal of Test Specimens from the TMI-2 Reactor Vessel Bottom Head, Project Summary, MPR-1195", October 1, 1990, prepared for U.S. Nuclear Regulatory Commission (NRC) Office of Nuclear Regulatory Research (RES). [Copy of Report in NMAH Curator's file for this accession.]
Following the accident in March of 1979, the Three Mile Island Unit 2 (TMI-2) [nuclear] reactor vessel sustained significant internal damage. The resulting damage to the lower head and the margin to failure have been of interest to the nuclear industry. In early 1988, GPU Nuclear, the owner/operator of the TMI facility, had completed a large portion of the disassembly and defueling work inside the reactor and was preparing to remove the lower structural internals and defuel the lower head. At that point, NRC-RES initiated a project with MPR Associates to remove metallurgical specimens of the lower head and adjacent areas to determine the extent of damage to the vessel and to gain insight into the events that took place inside the vessel during the accident. The project was set up such that sampling work in the vessel would be performed by MPR after GPU Nuclear had completed defueling. Defueling ended on Jan. 30, 1990, at which time GPU Nuclear turned control of the reactor vessel over to MPR.
Techniques and Special Tooling
[In order to satisfy NRC-RES, GPUN and MPR project objectives], electrical discharge machining was selected for vessel sampling. The cutting technique used is referred to as metal disintegration machining (MDM). Both the MDM process and the more commonly used electric discharge machining (EDM) remove conductive material by melting away small bits of material using an electric arc between an electrode and the work piece. EDM makes and breaks electrical arcs by switching the electrical current on and off using transistors. MDM makes and breaks electrical arcs by moving the electrode into and away from the work material. MDM was used in the project due to the high conductivity of the reactor vessel water. The MDM cutting head has two U-shaped graphite electrodes to cut the triangular [boat-shaped] samples. The electrodes are attached to hydraulic cylinders and slide on tracks mounted at angles to provide a sample with an included angle of about 60 degrees. The electrodes are consumable and were designed to be replaced after each sample cut. PCI Energy Services, under contract to MPR, developed and tested the MDM cutting equipment. The MDM head was positioned at the vessel sample locations with a delivery system consisting of several long pipe sections and a hinged arm which articulated the MDM cutting head to the desired angle for sampling the vessel head. The MDM cutting system was used to cut samples both in open areas of the vessel and at incore penetrations.
For details on the MDM cutting system and associated equipment, and for a full description of the development and qualification program, refer to the above Report document in the Curator's file for this acquisition.
Object Name
metal disintegration machining (MDM) system
date made
PCI Energy Services, Inc.
Physical Description
metal (overall material)
graphite (electrodes in cutting head material)
plastic (overall material)
upper shaft of arm, diam.: 4 7/16 in; 11.27125 cm
top of middle tube clamp to axis of pivot arm: 13 3/4 in; 34.925 cm
top of cutting head box to axis of pivot arm: 55 7/8 in; 141.9225 cm
total radius of swinging arm: 79 5/8 in; 202.2475 cm
power cable conductor diam.: 9/16 in; 1.42875 cm
overall height: 108 in; 274.32 cm
cutting head box height: 23 3/4 in; 60.325 cm
top of cutting head box to top of middle tube clamp: 42 1/2 in; 107.95 cm
top of middle tube clamp to top of tubes: 42 in; 106.68 cm
cutting head box: 23 3/4 in x 13 1/8 in x 11 5/8 in; 60.325 cm x 33.3375 cm x 29.5275 cm
cutting head jaws (electrodes) (outside length): 7 in; 17.78 cm
cutting head jaws opening (retracted): 3 3/8 in; 8.5725 cm
cutting head jaw plate thickness: 1/4 in;.635 cm
ID Number
accession number
catalog number
Credit Line
U.S. Nuclear Regulatory Commission
Science & Scientific Instruments
See more items in
Medicine and Science: Modern Physics
Science & Mathematics
Measuring & Mapping
Modern Physics
Energy & Power
Data Source
National Museum of American History
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