The neutron window

2 Neutron Curtain in the HTR. A 6 ft 6 in. square sheet of -in.-thick boral mounted in an aluminum frame will serve as the neutron curtain. This curtain will be used during operation, if desired, to cover the face of the thermal column and reduce the number of thermal neutrons entering the column. It will have rollers at its four corners that will travel in vertical guides mounted in the 3-in. space between the two 4-in. thermal shield plates. 

A 2-in. by 7-ft vertical slot in the biological shield from the bottom of the thermal. shield to the sub-pile room ceiling will house the curtain when it is not in use. To shield against radiation streaming, the lower portion of this slot- will be filled with 2-ft-deep removable barytes concrete blocks supported from the sub-pile room ceiling by 6-in.-square steel bars which serve as additional shielding. 

A lifting mechanism mounted on the top of the reactor structure raises or lowers the curtain as desired. After removing the concrete and steel shielding at the bottom of the curtain slot, the curtain can. be lowered into the canal and a new curtain installed if necessary. 

Provisions have also been made for installation of a curtain on the west side of the thermal shield at the. face of the shielding facility however, no curtain or lifting mechanism will be furnished initially. 

3 The Neutron Window. The neutron window is a 3-in. sheet of lead placed between the permanent graphite and the thermal column to absorb gamma radiation but permit easy neutron passage. The lead sheet is mounted in a steel frame and pl aced in the approximately 6- by 6-ft hole in the outer thermal shield. 

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Two important experimental facilities, the thermal column and the shielding facility (see Sections 3.1 and, 3.2j, require that a section approximately 6 by 6 ft be removed from.the east and west thermal-shield side plates. Two components are added to the thermal-shield design by.these facilities (1) the neutron window, and (2) the neutron-absorbing curtain. The locations of these components may be visualized by referring to Figs. 2.B and C. When the 6- by 6-ft hole is cut in the thermal shield, it becomes necessary to replace... 

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