Suppression pool testing at the SIET laboratory (1) Experimental Investigation of Critical Phenomena Expected in the Fukushima Daiichi Suppression Chamber

In the unlikely event of a Nuclear Power Plant (NPP) long duration station black-out, as in the Fukushima Daiichi (1F) severe accident (SA), it was recognized that the Suppression Chamber (S/C) functions of heat sink and Fission Product (FP) scrubbing will degrade, resulting in the S/C pressure increase, reduction of the scrubbing efficiency and subsequent necessity of venting operations. Consequently, a relatively large amount of fission products is likely to be dispersed into the environment. As a method to evaluate the degradation of the pool characteristics under typical conditions expected at the 1F NPP, an experimental campaign was recently started at the SIET research laboratory in Italy. Three different sparger geometries and several combinations of steam and air mass flow rates were tested to scale down the most critical phenomena that occurred during the 1F SA, such as steam condensation and water thermal stratification. In addition, iron oxide powder is employed to simulate the water capability to scrub the FPs depending on the pool temperature evolution. Measurements of pool water temperature in different points, visualization with high-speed camera and concentrations of the oxides carryover, represent the main outcome of the experimental activity. The preliminary experimental results have demonstrated that spargers inducing large chugging and steam jetting at the bottom of the pool are effective to avoid and break the temperature stratification, thanks to the large water recirculation and vertical mixing within the pool. Differently, spargers with holes disposed in the vertical direction, as in the RCIC exhaust pipe of the 1F unit 3, are not able to produce water vertical mixing, thus resulting in intense stratification that drastically reduces the condensation efficiency of the S/C pool. From the experimental results of the scrubbing simulation with the iron oxide powder it was extrapolated that the FP carry-over rates in subcooled condition was about 30% while it increased up to around 50% during boiling. The ongoing experimental activity aims at constructing a database based on the high-speed filming, measurements of major quantities such as water temperature, steam pressure and fission products concentration to foster the development of physical models for both lumped parameter SA codes and detailed computational fluid dynamics softwares, in an effort to enhance the understanding of the complex phenomena following the Fukushima Daiichi accident.