Status Of The Spent Fuel In The Reactor Buildings Of Fukushima Daiichi 1-4

The ratios of the radio nuclides Cs-134g and Cs-137 deduced from measurements of liquid samples from the spent fuel pools in Fukushima Daiichi 1-4 are used to interpret the status of the spent fuel assemblies in the pools of the damaged reactor buildings. The different nature of the production of Cs-134g (neutron capture product of Cs-133) and Cs-137 (cumulative fission product from mass chain 137) and the different half-lives (2.06 years and 30.17 years respectively) require a complicated calculation of the mass and activity of the two nuclides. These masses are depending on the local burn up of the fuel, the burn up history and the radioactive decay. For the calculation of the radionuclide masses with ORIGEN-2 the user has to use different cross section tables depending on the maximum burn up of the fuel. These tables are based on data from activity measurements after irradiation of fissionable materials. Especially for the neutron capture product Cs-134g the calculation is more complicated, because the production of Cs-133 (stable cumulative fission product from mass chain 133) has to be taken into account. The neutron capture cross section for Cs-133 was fitted from a gamma scan of spent fuel rods in a hot cell. The method of the calculation of the nuclide activities and the interpretation of the gamma measurements of the spent fuel pool samples from Fukushima Daiichi 1-4 are described in detail.