Channel opening/closing device in radiation utilizing facility

A closing plate is disposed to a maintenance port as a maintenance channel on the side close to a reactor, and a shielding plug is disposed to the outer side. A channel opening/closing device makes the shielding plug move and fit or remove the closing plate. A plurality of wheels are disposed to the lower surface of the shielding plug. A connector is disposed to the back end thereof for connecting with a tractor as a transfer-driving device. The closing plate has an outer circumferential lip portion which is gas tightly joined by welding to a lip portion on the side of the channel at the outer circumference of the closing edge of the region where the closing plate of the maintenance port is disposed. The welded and secured portion of the closing plate is cut and separated by a securement releasing device, and transferred by a transferring device. Then, the region where the closing plate is disposed is opened. (I.N.)