355. 高速増殖原型炉「もんじゅ」における窒素ガス注入によるナトリウム火災対策

抄録

A nitrogen gas extinguisher system will be installed at MONJU as one of countermeasures against sodium fire. The basic design specifications of this system were determined by some experiments. Following experiments were conducted with the object of confirming (1) an oxygen concentration to suppress sodium fire, (2) a nitrogen gas mixing efficiency to decrease the oxygen concentration, and (3) a nitrogen gas feed rate to prevent air in-leak from the outside to keep the low oxygen atmosphere.
(1) The sodium combustion rate, which was evaluated by a calorimetric heat, depended on oxygen concentration, but it could be rearranged simply by a mole flow rate of oxygen gas. The sodium combustion heat was sharply changed at 1 mole/min, and the oxygen concentration of less than 5% was sufficient to suppress sodium fire under the natural convection condition. Since the sodium combustion heat was approx. 15kW/m² at 5% of oxygen concentration, while approx. 200kW/m² at 21%.
(2) The mixing efficiency at the initial nitrogen gas feed rate is important to determine the capacity of the nitrogen gas tank. It was verified that over 0.9 of the mixing efficiency could be obtained by a 1/10 scale model and a large scale model test.
(3) It was confirmed that 4, 000m3/h of nitrogen gas should be fed for the largest partitioned area of the secondary cooling system in order to keep the low oxygen atmosphere.