Experimental study of prevention measures against sodium combustion residuum reignition

Ishikawa, Hiroyasu ; Ohno, Shuji  ; Miyahara, Shinya  

Nitrogen gas can be an extinguisher or a mitigating material in the case of sodium leak and fire accident in an air atmosphere, which may occur at a liquid metal cooled nuclear power plant. However sodium combustion residuum sometimes reignites in the air atmosphere even at room temperature when it was produced by nitrogen gas injection to the burning sodium. In this study we have been investigating the cause of reignition and prevention measures. Experiments were carried out with small type test equipment, which can handle 1g order sodium fire and extinguishment. Sodium combustion residua, which were made by our equipment and sampled, were analyzed by X-ray diffraction and chemical analysis. The chemical analysis of reignitable residua showed that the residuum contained metallic sodium of about 40wt-% (61 mol-%) to 60-wt % (76mol-%) and most of the rest was sodium-monoxide (NaO). Sodium-peroxide (NaO) was also included in less than 1wt-% of the residuum. Sodium or NaO cannot ignite by itself in the air atmosphere at room temperature in a few minutes. Therefore the reignition seems to be due to increase in the local temperature that is caused by oxidizing heat of Na and by adiabatic effect of NaO. It is important to deactivate this dispersed sodium on oxygen for prevention of the residuum reignition, hence it is considered as a rational measure to change the sodium to sodium-carbonate. Our experiments showed that the dispersed sodium on the exterior of residuum could be changed to carbonate by a mixture of carbon-dioxide (CO) gas (2 to 8vol-%), humidity (0.6 to 3vol-%) and nitrogen gas. The deactivated residuum did not reignite in the air atmosphere below 473K.