Measurement of the delayed-neutron multiplicity and time constants in the thermal neutron induced fission of 235 U at ILL

Delayed neutron (DN) data is essential in inherent reactor safety and reactor control since it is neededfor the estimation of the reactivity. Nowadays, discrepancies among the data in dierent international databases(JEFF, ENDF, JENDL) are large and bring excessive conservatism in the safety margins. The ALDEN experiment,built in a collaboration of CEA and CNRS, aimed at re-measuring the delayed-neutron data associatedwith several fissioning systems (average delayed-neutron yield and kinetic parameters). The measurement hasbeen performed with a long counter made of a polyethylene matrix and 16 3He proportional counters. Thefirst experimental campaign, with an 235U target, took place at ILL (Institut Laue-Langevin) at the beginningof September 2018. The data regression analysis gave an average delayed-neutron yield of 1.631E-02 1.4%DN/fiss, and a mean precursors half-life of 8.93 0.98% s. The results are consistent with the literature(WPEC-6), but they are aected by a much smaller uncertainty