etudiant said:
Interesting discussion that seems to skirt the obvious real life example from Fukushima.
The stacks are to vent the reactor space in case of problems and there was a lot of discussion at Fukushima about rupture discs failing to rupture appropriately, thereby causing more local contamination.
The major takeaway at least for me was that there really must be a very capable filter before the stack, otherwise it just spreads massive radioactivity widely, instead of leaving it local. Such filters are now mandatory in Finland, at least according to expert comments on this site. They are not similarly mandated in the US, partly because the retrofit would be a very costly exercise, but also because industry has convinced the NRC that the likely safety benefit is only achieved so rarely that it is wasteful to proceed.
One needs to remember that the Mark I/II containment was never designed with a containment vent system. These containments are designed for the transient plus decay heat only, with an expectation that a decay heat removal train be put in service anywhere from 10 to 30 minutes after the start of the event to maintain containment parameters within design limits. This is how it was licensed, and was considered completely acceptable at the time. However, between whistleblowing activities and further studies into decay heat and severe accidents, it was determined that the Mark I containment system did not have adequate capability to withstand severe accidents for an extended period of time, which is why a vent was required. In the US, Mark Is with vents don't utilize rupture discs, this was a Japanese idea. In all cases, the vent, even the rupture disc vents, were never intended to be opened and left opened, there are vent valves which also need to be opened and closed.
The stacks were never primarily intended for containment release. That was never their purpose. They are for standby gas release from the secondary containment only, which is a filtered release path.
Venting from the wetwell achieves a reasonable decontamination factor prior to release, especially if being vented to an elevated release point. Which is why even post Fukushima, wetwell vents in the US do not require filters. The cost of the filter does not justify the added decontamination factor. These studies are all available through EPRI by the way, with regards to severe accident venting in Mark I and II containment structures. The wetwell venting, especially when sprays are utilized, are as high as a dedicated external filter. Remember, BWRs have a wet pool to scrub fission products in. This is why BWRs are not required to vent their wetwell release paths. EPRI has demonstrated that decontamination factors of as high as 500 are easily capable using the wetwell gas space as a filter path.
As for drywell vents, not all BWRs need them, this is being done on a unit by unit basis. If a severe accident water addition strategy is employed to the drywell, the need for venting is minimized plus some level of scrubbing is now available to ensure releases are comparable to a vented setup. What the industry asked for, essentially, is credit for the fact that they would already be flooding containment and spraying containment per their severe accident strategies. Since these water additions to the drywell would be occurring to halt core damage, using FLEX equipment or other severe accident responses, it would mean that the drywell's heat capacity would be improved by the addition of water to the drywell, greatly reducing the time a release would need to be in progress compared to a case without water addition. This would result in a similar release rate to a dedicated drywell filter.
There are exceptions though, especially for some designs of Mark II drywell units, where there exists a higher potential for suppression pool bypass leakage, which would then result in unscrubbed releases. I believe some units will have to utilize a dedicated wetwell vent, along with a dedicated filter.
One of the big issues with regards to filtered release paths, at least a few years ago, was how decontamination factors greater than 1000 were kind of a "point of diminishing returns", and many/most studies pointed out that DFs > 1000 did not necessarily mean there was a 1 for 1 benefit for cost return, and later studies demonstrated that using lower DF strategies to minimize the overall venting required were just as effective in minimizing total radionucleide release as dedicated filters with > 1000 DFs.
(I'm not saying I necessarily support the US industry's approach, rather that I've read several reports and studies and followed this to the point of knowing the general logic/idea behind why filters were not required for BWR wetwell release paths)