[en] Using BWR (Benedict-Webb-Rubin) formula, authors have studied the relationships between the required inner pressure at 500 K, 400 K, 300 K, 77 K, 35 K, respectively, in the micro balloon's diameter of 100 μm, 200 μm, 400 μm, 520 μm, respectively, and the thickness of liquid layer in the target at 22 K. The authors found that the fuel pressure at low temperature is much more lower than that at room temperature. (authors)