Patent Number: 
Section: claims

1. An indexing device installation unit provided at an access tunnel, in a nuclear reactor containment vessel, the access tunnel leading a plurality of guide tubes to a local power range monitor detector provided in a nuclear reactor, the indexing device installation unit comprising:an indexing device of a traversing in-core probe configured to selectively insert the guide tubes to a plurality of areas on the local power range monitor detector, the indexing device being provided in the access tunnel;a base mounted with the indexing device;a holding part configured to hold the base in a suspended state from a ceiling portion of the access tunnel; anda lift mechanism configured to uplift and down the base held by the holding part along with the indexing device in the access tunnel, the lift mechanism forming a predetermined clearance in the access tunnel when uplifting the base, so as to enable a worker to pass through the access tunnel,wherein the holding part has: a sleeve on whose inner surface a female thread is formed; a shaft screwed into the sleeve and having a male thread formed thereon; and a sprocket attached to one end of the sleeve, and one end of the shaft is fixed to the base; andwherein the lift mechanism has a driving source rotating the sprocket in a circumferential direction and, in moving the base up and down, changes a screwing depth of the shaft into the sleeve by rotating the sprocket. 2. The indexing device installation unit according to claim 1,wherein the base is formed in a rectangular shape;wherein the holding part is provided in four sets so as to hold corner portions of the rectangular base in a suspended state respectively; andwherein the lift mechanism includes at least a chain that rotates the sprockets in synchronization by transmitting a driving force of the driving source to the sprockets of the four holding parts. 3. The indexing device installation unit according to claim 1,wherein the driving source of the lift mechanism is an air motor. 4. The indexing device installation unit according to claim 2,wherein the driving source of the lift mechanism is an air motor.