Perfectly safe is a long stretch. As originally installed they had numerous design faults, the control rods operated too slowly, the carbon tips of the control

Perfectly safe is a long stretch. As originally installed they had numerous design faults, the control rods operated too slowly, the carbon tips of the control rods could locally increase reactivity when they were being inserted, the reactor was too large and consequently hard to control, they lacked the secondary containment structures which were common place in western reactors of the era.