Water is the most widely used moderator in neutron sources and nuclear reactors. Despite this fact, thermalization in water is still computed with models that are based on and validated with experimental data measured decades ago. A new model predicts a reduction of the total cross section around 25 meV when the temperature is increased from room temperature. This reduction is not predicted by the existing models, and affects the calculation of temperature reactivity coefficients in nuclear reactors. This reduction has been traced to a shift in the frequency spectrum in liquid water.To test the models, a very delicate measurement of the total cross section is needed, with high statistics in the thermal and epithermal range. VESUVIO has the thermal flux intensity and a neutron spectrum that peaks at the required energy to provide high statistics for the proposed measurements.