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Conference Article: Seismic scrammability of HTTR control rods.

Nishiguchi, I.; Iyoku, T.; Ito, N. (Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan). Dept. of HTTR Project); Watanabe, Y. (Toshiba Corp., Kawasaki (Japan). Nuclear Engineering Lab.); Araki, T. (Toshiba Corp., Yokohama (Japan). Nuclear Energy Group); Katagiri, S. (Toshiba Corp., Yokohoma (Japan). Keihin Product Operations)

Abstract

Scrammability tests on HTTR (High-Temperature Engineering Test Reactor) control rods under seismic conditions have been carried out and seismic conditions influences on scram time as well as functional integrity were examined. A control rod drive located in a stand-pipe at the top of a reactor vessel, raises and lowers a pair of control rods by suspension cables. Each flexible control rod consists of 10 neutron absorber sections held together by a metal spine passing through the center. It falls into a hole in graphite blocks due to gravity at scram. In the tests, a full scale control rod drive and a pair of control rods were employed with a column of graphite blocks in which holes for rods were formed. Blocks misalignment and contact with the hole surface during earthquakes were considered as major causes of disturbance in scram time. Therefore, the following parameters were set up in the tests: excitation direction, combination or horizontal and vertical excitation, acceleration, frequency and block to block gaps. Main results obtained from tests are as follow. 1) Every scram time obtained under the design conditions was within 6 seconds. On the contrary, the scram times were 5.2 seconds when there were no vibration. Therefore, it was concluded that the seismic effects on scram time were not significant. 2) Scram time became longer with increase in both acceleration and horizontal excitation frequency, and control rods fell very smoothly without any jerkiness. This suggests that collision between control rods and hole surface is the main disturbing factor of falling motion. 3) Mechanical and functional integrity of control rod drive mechanism, control rods and graphite blocks was confirmed after 140 seismic scrammability tests.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Seismic behaviour of gas cooled reactor components. Proceedings of a specialists' meeting held in Gif-sur-Yvette, France, 14-16 November 1989
International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors
IWGGCR--22, pp:105-111