The adjacent table lists the critical masses for curium isotopes for a sphere, without a moderator and reflector. With a metal reflector (30 cm of steel), the critical masses of the odd isotopes are about 3–4 kg. When using water (thickness ~20–30 cm) as the reflector, the critical mass can be as small as 59 gram for 245Cm, 155 gram for 243Cm and 1550 gram for 247Cm. There is a significant uncertainty in these critical mass values. Whereas it is usually on the order of 20%, the values for 242Cm and 246Cm were listed as large as 371 kg and 70.1 kg, respectively, by some research groups.