Experimental study of the field-of-view of neutron detectors towards thermohydraulic perturbances

The aim of the present study was to identify different stages of boiling during the HOR experiments at the 2 MW reactor of IRI (Interfaculty Reactor Inst.), Delft, Netherlands. The electrical heating power was increased while the flow rate was constant, thus inducing a single-phase flow, and afterwards subcooled boiling and finally developed volume boiling of the coolant in the experimental assembly. Subcooled boiling and the onset of boiling could not be detected using neutron detectors. This behaviour shows that the field of view of the NDs is very limited. The neutron noise signals remained unchanged, though at a distance of 3-10 cm developed volume boiling arose. Only the steam-packets in the slug-type flow during measurement III caused changes in the neutro noise. (author) 12 refs.; 20 figs