[en] Graphite structures are used as the HTTR core internals. Graphite structures have high heat resistant property but strength degrades easily by oxidization. To prevent the oxidization of graphite structures, impurity concentrations in the helium coolant are controlled strictly. The helium sampling system is installed to measure the impurity concentrations in the helium coolant. Seal-oil leakage occurred at the rod seal mechanism of gas compressor in helium sampling system. The reasons are degradation of the seal material and contaminant abrasion powder of the grand-packing. Thus, the material of the seal material was changed and contaminant was decreased. A long term operation became possible by these countermeasures. Moreover, reliable data can be obtained and efficient impurity control is enabled due to renewal of data acquisition control computer of gas chromatograph mass spectrometer and improvement of liquid nitrogen trap. This report describes the improvement items of the helium sampling system. (author)