Elaborating on a comment to that post, this essay will point out that exhaust stack elevation could have made a crucial difference in the Fukushima reactor accidents.
Row, 1971), pumping effluent gases to the main exhaust stack and maintaining a negative building pressure that disallows potentially contaminated air to escape unfiltered into the environment. Contaminated effluents are forced through filters and vented through the main exhaust stacks, except for hardened venting. During hardened venting, contaminated steam and gases may directly be routed to the exhaust stacks as a means of last resort in an emergency, relieving pressure from the reactor after all other options have been exhausted. Furthermore, hardened venting may prevent hydrogen produced by melting fuel rod cladding and radiolysis from reaching explosive concentrations.
Bird's eye view of Fukushima Daiichi Nuclear Power Station after hydrogen explosions destroyed the service floors of units 1 (left), 3 and 4. The tall exhaust stack towers can be seen on the left between units 1 and 2 and at center between units 3 and 4. The small sheds housing the fan gear are located at the base of the towers (source: cyptome.org).
But, the tall exhaust stacks at Fukushima need fan-support to generate sufficient draft. The fan gear is housed in small shed at their base. During a station blackout, the stacks must rely on draft alone.
According to the Fukushima Nuclear Accident Report (Interim Report) Dec. 2, 2011, of the stricken power station's operator Tokyo Electric Power Company (TEPCO), hardened venting released hydrogen in Unit 1's reactor building, flowing back through open valves and baffles. When mixed with air at concentrations above four percent, hydrogen may explosively deflagrate and detonate. At Unit 3, the hydrogen may have exploded inside the primary containment vessel.
|Explosion at units 1 and 4 (courtesy: Goddard's Journal).|
|Junction of SGTS effluent piping of Unit 3 and Unit 4 at the base of shared stack (arrow) (source: TEPCO).|
|Unit 4's SGTS filter contamination. The connection to the stack shared with Unit 3 is on the right (source: TEPCO).|
Had the shared exhaust stacks been tall enough to passively generate sufficient draft, the hydrogen explosions may have been prevented, particularly the one at Unit 4. Therefore, it is prudent to ensure that all nuclear power stations are outfitted with exhaust stacks tall enough to provide sufficient draft when all power fails.
Tomorrow, Nov. 21, 2013, the US Senate Committee on Environment and Public Works and the Subcommittee on Clean Air and Nuclear Safety will hold a joint hearing on the "NRC's Implementation of the Fukushima Near-Term Task Force Recommendations and other Actions to Enhance and Maintain Nuclear Safety."
Hopefully, the NRC has incorporated sufficient exhaust stack elevation into its recommendations for US reactor operators.
- Row TH (1973) Radioactive waste systems and radioactive effluents. Georgia Institute of Technology Short Course.
SimpyInfo.org provided the information for this post.