Fig. 4-4 Accumulating behavior of noncondensable gas in steam generator U-tube Illustrated is the experimental result of injecting the same amount of nitrogen gas, heavier than steam, into the steam generator from downflow side. All of nytrogen gas injected was forced to flow into SG U-tube following after steam, pushed into locations #1,#2,-- #5 of U-tube, one after another, and then accumulated. As the number of injection increases, the volume of accumulated gas is decreases because of the primary system pressure increase.

Fig. 4-5 The primary as well as secondary system pressures, and the effective U-tube heat transfer area The figure shows relations between the primary as well as secondary system pressures and the effective U-tube heat transfer area against amount of noncondensable gas injected. Although the primary system pressure increases due to the reduction of heat transfer area, the rate of increase is rather small, so that reduction of heat transfer area down to 10% produces only a little increase in the pressure. The secondary pressure is hardly affected by accumulation of noncondensable gas.

During a nuclear power plant severe accident, the natural circulation heat transfer capability of the coolant fluid is expected to be the last dependable means of heat removal. However, during a severe accident, hydrogen gas may be generated by fuel oxidation, or nitrogen gas, which is used for pressurization, may be discharged through water injection from the accumulators. The natural circulation capability may be degraded if a noncondensable gas accumulates in the SG U-Tubes. An experiment utilizing LSTF demonstrated that noncondensable gases never prevent natural circulation heat removal capability and that this heat removal capability is very reliable.

Reference Y. Kukita et al., ROSA-V Program Integral Experiments on Preventive Accident Management Measures, 3rd Workshop on Severe Accident Research in Japan (SARJ-92), Tokyo, 1992.

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