RESUMEN
There is a lack of reliable experiments aiming at the prompt fission neutron spectrum of 235U for energies higher than 10 MeV. The presented experiment performed at the LVR-15 light water reactor aimed at the measurement of very high threshold reactions spectral averaged cross sections such as 55Mn (n,2n)54Mn, 197Au (n,2n)196Au, 197Au (n,3n)195Au, 209Bi(n,3n)207Bi, 209Bi(n,4n)206Bi. 209Bi(n,3n)207Bi and 209Bi(n,4n)206Bi reactions were measured for the first time. 58Ni(n,p)58Co reaction was used as a monitor reaction. The experimental spectral averaged cross sections are derived from reaction rates measured by means of high purity Germanium spectroscopy at the well defined detector. The experimental spectral averaged cross sections are compared with calculations using either ENDF/B-VIII.0 or JEFF-3.3 235U prompt fission neutron spectrum and IRDFF-II cross sections. The discrepancy is higher with higher mean response energy for JEFF-3.3 unlike ENDF/B-VIII.0, where the agreement is good within broad range of mean response energy. Moreover, due to the high thermal neutron flux in the reactor, the experimental reaction rate is compared with calculated 198Au (n,g)199Au reaction rate. The difference of -55.3% for double capture reaction was found in comparison with ROSFOND-2010 calculations.