Reexamination of Criticality Accident in JCO

Nuclear chain reactions are, by now, commonly used in the nuclear reactors, and thus it seems that there is no basic problem in fission processes from the scientific point of view. However, the criticality accident that occurred in JCO in 1999 suggests that one should carefully examine this accident from the nuclear physics point of view. Indeed the chain nuclear reactions should have taken place in the small area of space with 45 cm diameter disk times 30 cm height tank. In fact, when people carry the uranium nitrate solution into sedimentation tank, then this solution with uranium should get into the critical state at the 45? of uranium nitrate solution. The root cause of the accident should not be very simple from the nuclear physics point, and it should be quite important to examine why the uranium nitrate solution with 45? could have become critical.