Shielding technology for upper structure of HTTR

Highlights: • Performance of shielding of HTTR during operation was demonstrated. • No unexpected neutron streaming path around the standpipes was observed. • The control rod withdrawal was reflected in the measured dose at the standpipe room. • A conservativeness of the shielding design around the standpipes was indicated. - Abstract: Shielding design for the high temperature gas-cooled reactor shall need special attention to avoid neutron streaming, since helium gas as coolant does not work for shielding, and the gap exists in the upper shield where the standpipe of the control rod pierces above the reactor pressure vessel. Performance of shielding of the High Temperature Engineering Test Reactor (HTTR) has been demonstrated to establish a design method for shielding of Very-High-Temperature Reactor (VHTR) as a Generation-IV nuclear energy system. With respect to the measurement of dose equivalent rate at on-operating acceptable areas, neutrons and gamma-rays were lower than the detection limits or were at the same level as the background, respectively. At the standpipe room of the HTTR, it was revealed that the dose consisted of over 90% of neutrons, and there was still a margin by a factor of 50 in addition to the design excluding the safety factor. The measured dose equivalent rates showed good agreement with the predictions considering the control rod withdrawal effect.