Analysis on the Coolant Radioactivity of HANARO

We analysed an overall history on the HANARO pool-top-radiation, improved a model to estimate pool radioactivity, estimated major sources produced by activation of aluminum and argon, and became to understand their production mechanism. The source of Ar-41 matched well with an estimation by core calculation. For aluminum activation products, equivalent thicknesses in which the products are released to the coolant, were obtained. These thicknesses are closely related to their solubility in the water and half lives. It seems that long half lived and easily dissolved Na-24 is released to the coolant from the whole oxide layer. Though Mg-27 and Al-28 would also be released from the whole oxide layer, their equivalent thicknesses are much smaller due to lower solubility and decay while they escape from the oxide layer. However, an increasing trend of pool activity is not found even though the oxide layer thickness increases about 50 % during an operation cycle. Transport of water below the hot water layer to the pool surface is composed of fast and slow components, and about 1.5 h transport time for the fast component to reach the upper region of hot water layer is confirmed. If a phenomenon observed in 2004 is taken into account, the fast component would be caused by fine air bubbles. If the reactor is operated at 24 ∼ 27 MW for about 12 h at the beginning of cycle, whether the air bubbles are actual cause could be verified. In order to understand the transport mechanism in the lower region of hot water layer, a measurement on vertical distribution for current status is needed. Should the transport mechanism be correctly understood, an effective way to keep the pool-top-radiation low would be found. Since the current pool-top-radiation is sufficiently low, tests decreasing the hot water temperature gradually would also be helpful. The analysis for HANARO enabled an estimation on the aluminum and argon activation sources for a new reactor, and a pool-top-radiation of conceptually designed PALLAS is estimated