Study on the method of exchanging JRR-3 cold neutron guide tubes

Hazawa, Tomoya; Tamura, Itaru  ; Takazawa, Hiromitsu

The Department of Research Reactor and Tandem Accelerator has planned the project of increasing cold neutron intensities by ten times by upgrading the cold neutron guide tubes with super-mirrors also and the moderator cell of cold neutron source. An exchange of the guide tubes requires taking the beam shutter of about 6 ton out of the narrow guide tunnel. The cold neutron guide tubes in the front end are 3 times longer then thermal ones and set very close to the reactor core. Therefore, the work has to be done under a high radiation dose as well as paying close attention to setting the tubes precisely. This report describes the result of study on replacing the Ni-guide tubes with super-mirror guide tubes as the first step of the 10-times upgrading project for cold neutrons.