hazards, two main types of hazards we have to be concerned about, or they have to be concerned about, the heroic workers. one is exposed nuclear fuel, either the fuel in the reactor course or the spent fuel pools. when they become exposed you can start to have blistering and break down of the fuel and even a meltdown of that fuel, see that can release radiation. that's one effect. the second effect you're referring to, once the material starts to meltdown and then it resolidifies, the ooh rain yan and plutonium in that spent fuel, when it's recongealed it can get concentrated. when you put water back in the water can in effect reignite and cause a criticality accident. jenna: it's so incredibly complicatcomplicated. just as an expert what would you say we've learned so far.