Measurement of the fission yield of ¹³⁶Cs in the ²³⁹Pu(n_th,f) reaction

A recent experimental campaign performed at the LOHENGRIN spectrometer at ILL aimed at measuring the independent fission yield of 136 Cs in the 239 Pu(n th ,f) reaction. This nuclide can have an important contribution to the total dose rate coming from spent light water reactor UOX and MOX fuels. Moreover, its impact is of first order on the uncertainty of the total dose rate calculated in specific areas of Nuclear Power Plants within accidental conditions. One of the most important sources of uncertainty is its independent fission yield. Therefore, a new measurement of its independent yield along with a rigorous uncertainty analysis was performed. Due to its low independent yield, a new measurement technique has been applied. Ions recoiling from neutron-irradiated 239 Pu target were collected by implantation into an Al foil placed inside a vacuum chamber. This foil was then transferred to a low γ-ray background setup located at nearby laboratory (LPSC). The procedure was then repeated for different LOHENGRIN settings. Compared to the JEFF-3.1.1 library, a decrease of the fission yield of 136 Cs with a reduced uncertainty is obtained.