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7 Official estimates indicate that this exposure will cause 50 - 100 thousand extra cancer fatalities worldwide over the next 70 years . 19 28 . The core of the Harris reactor contains 157 PWR fuel assemblies . At shutdown, this core contains about 155 , 000 TBq (47 kg) of cesium" 137 . 20 When a spent fuel assembly is discharged from the reactor, it will contain more cesium- 137 than the average assembly at shutdown . CP &L plans an eventual , aggregate capacity in the Harris pools of 3 , 080 PWR assemblies and 5 , 304 BWR assemblies . Note that the cesium- 137 content in each BWR assembly will be about one quarter the cesium- 137 content in each PWR assembly, if both assemblies have been discharged for an equal period . 21 After discharge, the content of cesium437 in a fuel assembly will decay exponentially with ahalf--life of 30 years . 29 . As a simplified illustration, assume that all fuel assemblies in the Harris pools have been discharged for an equal period . Further assume that all four pools are full and contain 3 , 080 PWR assemblies and 5 , 304 BWR assemblies . The pools will then contain as much cesium437 as 4 ,406 PWR assemblies . (3 , 080 + 5 , 304 x 1 /4 = 4 , 406) Note that 4 , 406 PWR assemblies represent 28 cores of the Harris reactor. 30 . If an accident can be postulated that releases to the environment a significant fraction of the cesium437 in the Harris pools , then it is clear that the consequences of this accident would be large . The offsite radiation exposure could be an order of magnitude larger than the exposure from the Chernobyl accident . Activation of pools C and D could lead to an accident which creates offsite radiation exposure as much as two times higher than the exposure that would arise from a similar accident involving only pools A and B . H. Loss of Water from Spent Fuel Pools at Harris 31 . Loss of water from one or more of the Harris pools could initiate a release to the environment of a significant fraction of the cesium437 in the pools . This potential exists because the cladding of PWR or BWR fuel is a zirconium alloy which can react exothermically with air or steam . Thus , if the water in a fuel pool is removed and the fuel is partially or totally uncovered, one must be concerned about the possibility of a runaway air- zirconium or steam-zirconium reaction. Such a reaction could release cesium- 137 and other radioisotopes from affected fuel into the fuel building . That building was not designed to contain radioisotopes released during a vigorous exothermic reaction in the pools , and it can be assumed that most of the volatile radioisotopes entering the building from the affected fuel would be released from the building as an atmospheric plume . 32 . Several reports prepared by or for the NRC have examined the conditions under which a runaway zirconium reaction might occur. 22 However, these reports have 19 Allan S Krass, Consequences of the Chernobyl Accident (Cambridge , Massachusetts : Institute for Resource and Security Studies , December 1991 ) . 120 NRC, Final Environmental Statement, page 5 -50 . 21 The ratio of one quarter derives from the parameters shown in the license amendment application, Enclosure 7 , page 545 . 22 Relevant reports include : V L Sailor et al, Severe Accidents in Spent Fuel Pools in Support of Generic Safety Issue 82 , NUREG/CR-4982 , July 1987 ; E D Throm, Regulatory Analysis for the Resolution of Generic Issue 82 , "Beyond Design Basis Accidents in Spent Fuel Pools " , NUREG = 1353 , April 1989 ; and R