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With regard to the pipes (Pa) that release vent gas into the outside air via the stack from the merging point of the Unit 3 SGTS pipes and Unit 4 SGTS pipes, and the Unit 4 SGTS pipes (Pb), while there is a certain error in the ratio of the effective pipe to the pipe inner diameter,
the ratio (Pa /Pb) of the effective pipe length divided by the inner diameter of Pa to Pb is approximately 1 to 3.8.
Concerning the velocity of fluid that flows from the Unit 3 SGTS
pipes into the stack side pipe through the merging point of the Unit 3 and Unit 4 SGTS pipes, and the velocity of another fluid that flows from the Unit 3 SGTS into Unit 4 SGTS pipes through the merging point above, the ratio of the velocity between the two fluids is inversely proportional to 1/2 power of the ratio (of 1 to 3.8) above and thus is approximately 2.0 to 1.0.
Moreover, the amount of inflow into pipes is determined by the product of the fluid velocity in the pipe and its cross-sectional area.
As such, the ratio of the amount of inflows of the stack side to the Unit 4 R/B side is approximately 2.6 to 1.0.
In addition, while a gravitational loss as a factor for suppression of inflow into the stack side is considered due to upward movement of the fluid following a flow into the exhaust stack , conversely, a factor for enhancement of inflow into the Unit 4 R/B is that the diameter of the pipes located in front of the SGTS filters on the second floor of the Unit 4 R/B is markedly larger and splits into A-train and B-train.
Meanwhile, a factor for suppression of inflow into the Unit 4 R/B is likely the existence of the SGTS filters.
However, even considering these factors it is still impossible to derive a factor that significantly influences the ratio of inflow into the pipes, and it would not be unnatural even if an estimated amount of at least around 25% of the fluid, that flowed into the merging points (of the Unit 3 SGTS pipes and Unit 4 SGTS pipes), flowed into the Unit 4 R/B.
Taking the above into account, and assuming that by midnight on March 15, 2011 at least approximately 800kg of hydrogen had been generated in Unit 3, even if 400kg of hydrogen had accumulated within the Unit 3 R/B, it is possible that 400kg of hydrogen flowed into the stack and Unit 4 SGTS pipes via the Unit 3 SGTS pipes once the containment vents were opened.
Assuming that 25% of this hydrogen flowed into the SGTS pipes of Unit 4, it is possible that a total of 100kg of hydrogen flowed from the Unit 4 SGTS pipes into the Unit 4 R/B.
(v) Therefore, if the air inside the Unit 4 R/B was assumed to be 30в„ѓ and the atmospheric pressure, and if approximately 74.2kg of hydrogen accumulated in a 5,000-m3 space, it would be possible for a detonation to occur within this space.
170 As such, there is ample room to state that a total of 80kg of hydrogen flowed into the Unit 4 R/B and then accumulated in a certain space, such as the west side of the fourth floor, for example, a portion of the hydrogen spread to the third and fifth floors through machinery service hatches and other passages, and then that this hydrogen caused an detonation inside the Unit 4 R/B
*According to interviews conducted with the plant manufacturer, there is one 90-degree bend in the pipes that
release vent gas into the outside air from Unit 3 R/B through the merging point between the Unit 3 SGTS pipes
and the Unit 4 SGTS pipes, and a stack. The total pipe length as converted to the straight pipe length is
143,530mm. If the length of the Unit 4 SGTS pipes, which run from the above mentioned merging point to the
merging point with the main air conditioning pipes through the second story Unit 4 R/B SGTS facilities, is the
length of the pipes, the total pipe length as converted to straight pipe length is approximately 481,256.5mm, as
there exist multiple butterfly valves, 45-degree bends, 90-degree bends, bifurcations and as the pipes near the
SGTS filter separate and run parallel between A-train and B-train
http://www.cas.go.jp/jp/seisaku/icanps/eng/03IIfinal.pdf