2.7 and circumferentially around the hulls, forming a distorted checkerboard pattern of 510 compartments. Typical compartment dimensions are 20m x 20m x Sm. Shielding from solar and galactic radiation is provided by lunar dirt and slag--by-product of the colony construction's refining process--packaged in bags and stacked inside the hull compartments. Thus the shielding material spins with the colony. Its area density is 5000 kg/m 2 ; its depth in the compartments is 2.5 meters. Figure 2.4 depicts a cylindrical section compartment. All compartments are assembled from prefabricated hull sections (roughly 20m x 20m) and bulkhead sections (roughly Sm x 20m). The prefabrication procedure uses electron beam welding to assemble lm x lm plates of AISI 4130 steel, separated by minor stiffeners of HY-80 or HY-100 steel, into the needed hull and bulkhead sections. Both hulls have plate thickness 4 cm; the bulkhead plates are 3 cm thick. The prefabricated sections are assembled by joining them to major stiffeners with threaded fasteners. Holes for the fasteners are drilled through the sections and the major stiffeners during prefabrication. Once assembled, compartments are sealed airtight. II.4.2: Damage Tolerance: The plate thickness of hulls and bulkheads satisfies the structural leak-before-break criterion, allowing detection of flaws before complete failure of a plate by a simple system of pressure measurements between the interior atmosphere and the compartments (the compartments are kept at different pressures). The Plate/Stiffener design is a Fail-Safe structure, avoiding large-scale structural failures by containing the spontaneous growth of cracks beyond the critical crack length. The tolerable damage scenario is the sudden loss of four lm x lm plates in a 2m x 2m square. The compartmentalization of the hull, leak-beforebreak criterion, and fail-safe structure permit inspection and repair schemes which require only simple technology and do not interfere with normal colony operations. In case of unexpected problems, some of the colony's buildings are designed as airtight emergency shelters.
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