Calculate the effective fission cross-section of 3 percent enriched UO ₂ if it is used in an ordinary water-moderated thermal reactor. The core lattice arrangement is such that the fuel/moderator ratio is 1:1.9 by volume. Assume the moderator is at 500'F and 2,000 psia, and that the other reactor-core materials contribute 2.0 barns of 1/V absorption per atom of hydrogen.