Motion control in radiation-intensive environments poses one of the most difficult design challenges possible to the engineer. Conventional step and microstepping motors are susceptible to high-energy gamma radiation particles that will attack non-metallic materials. As a result, lubricants, varnish, lamination bonding, and cable insulation will all deteriorate over time and finally crumble. Also, many radiation applications include additional environment problems, such as corrosive fluids.
Radiation-hardened step motors, however, have greatly expanded the design opportunities in highly radioactive environments. A recent motion control application at Oak Ridge National Laboratory (ORNL) in Oak Ridge, TN provides a good example of the possibilities created by these new designs, even in instances where there is more than one hostile condition in the environment.
Radiation Case History
An important step in the fuel recycling process is the separation of spent fuel oxides from their cladding by dissolving the oxides in nitric acid. ORNL, which is responsible for the development of nuclear fuel reprocessing technologies, recently developed a new concept for performing this separation process in collaboration with the Power Reactor and Nuclear Fuel Development Corporation of Japan (PRN).
The new concept devised by ORNL is a “multistage continuous rotary dissolver” that operates as a continuous, rather than batch-type, process. The new design provides a counter-current of nitric acid that is constantly forcing the highest acid concentrations against the hardest-to-dissolve areas of the fuel pins.
ORNL was able to find only one motor design that could withstand accumulated radiation dosages of 108 and which had all of the required performance specifications, a radiation-hardened hybrid permanent magnet step motor from Empire Magnetics. The Empire motor was a “zero backlash” model that features an in-line 87 to 1 cycloidal gear reducer, a design feature that increases motor output torque without lessening motor life. A radiation-hardened brushless resolver is connected to the gearbox output shaft. The resolver provided angular positioning capability with a resolution of 4,096 steps per revolution of the dissolver drum. The motor, gearbox and resolver were mounted inside a lead box to enhance the existing radiation resistance.
The key to a successful design for the ORNL project was in the proper selection of materials. Most critical was the specification of specific polymers, not only for magnet wire and cable insulation, but also for shaft and body seals. Finally, a heavy coat of radiation-resistant polymer paint provided the outer layer of defense against concentrated nitric acid vapor.
Recent Radiation-Hardened Step Motor Applications:
- Fusion research
- Material handling
- X-ray machine
- Fuel re-processing
- Fast breeder reactor
- Reactor inspection
- Beam research