We need to clear up this misconception about what happened at unit 1.This might have been the reason before Fukushima.
But what are the reasons why filtered vents are not mandated in US even _after_ Fukushima experimentally demonstrated that meltdowns are a realistic possibility?
Well, they definitely could have properly engaged IC on Unit 1, if accident manuals has clear directives to do so in station blackout. Thus, Unit 1 could have been saved.
And if manuals would have sections directing them to do so, they could have vented RPVs of Units 2 and 3 down to atmospheric pressure _before_ fuel started to melt. This would have released small amounts of radioactivity, yes, but then they could start injecting water with much less powerful pumps, since there would be no pressure difference to fight against. Units 2 and 3 could have been saved, too.
In reality, and as the video above clearly explains, with no manuals, they tried to reach two opposing goals at once: keep RPVs pressurized, and pump water into them. It was not possible to achieve both at once.
First off, the operators performed all required actions in accordance with the BWR Owners Group Emergency Procedure Guidlines (EPGs).
You have to simultaneously stabilize level and pressure, then commence a controlled cooldown. You are not allowed under the conditions they were in to exceed the cooldown rate of 100 degF per hour. That means manually cycling the IC motor operated 003 valve to turn the IC on and off to stabilize pressure. This is as written in the EPGs, and is how the operators are trained. There are very limited cases where EPGs allow you to exceed the 100 degF per hour cooldown rate. (Post Fukushima we have a lot more, but I’ll write about that later).
Anyways, when the IC is running, there’s really no need to inject water to the reactor, because it was isolated at the time. That’s the purpose of the IC, to allow an isolated reactor to cool down without steaming or feedwater supply. The only leakage would be if a reactor coolant pump seal started to leak, and as they cooled down, the leakage would decrease greatly. The leakage is based on time at high pressure without seal cooling, so by following the EPGs you would avoid gross seal failure or leakage. All signs and data point to the operators doing that during the time between the earthquake and tsunami. They followed their procedures and training correctly.
Anyways, it didn’t matter if the IC was on service or not. The best data we have tells us the containment inboard IC isolation valves went partially or fully closed during the flooding and electrical failures that occurred. The operators even tried to manually open the 003 valve and had no success.
So we need to get away from incorrect statements deriding the operators for following the EPGs as written. (I’m on the emergency procedure committee).
Post Fukushima, we have a lot of changes, and one of them is if you are in a situation with no high capacity reflood capability (similar to Fukushima), you do have permission to exceed 100 degF per hour in order to prevent the core from being uncovered while trying to maintain level with low capacity reflood pumps. I should note that during the first hour at Fukushima daiichi unit 1, this would not have changed the operator response. During the first hour, the plant had all safety systems available and there was no need or requirement to violate the cooldown limit. You can’t just violate the EPGs without a need to do so.
Also hardened vents ARE mandated in the US for Mark I and II plants and are being installed in upcoming refuel outages. They are mandated by adequate protection requirements, however when 10CFR50.155 is issued within the next 6 months they will be required by regulations as well. This regulation is about mitigating severe /beyond design based accidents. The BWROG emergency procedure committee also just issued revision 4 of the EPG/SAGs which complies with hardened vent and all remaining post Fukushima requirements. Rev 3 was the initial/immediate changes required for safety after Fukushima, and rev 4 is based on all the data and learnings we’ve had in the last 5 years since then. Rev 4 also includes more comprehensive scram failure response actions, better actions and strategies for when core flooding is required, procedures for emergencies with a shutdown or refueling reactor, and a lot of cleanup of legacy items in the EPGs including calculation and model updates.
Talking about units 2/3, if they had depressurized those units earlier they would have lost their injection systems (RCIC/HPCI). Even today that’s not the right action initially. When RCIC and HPCI failed, the operators did attempt to perform an emergency blowdown. Unit 2 they were unsuccessful. Unit 3 they couldn’t get relief valves to open manually, but later on the ADS system automatically did blowdown the reactor, but water wasnt being pumped in adequately for many reasons (bad lineups mostly). But there’s a lot of learnings here and there are technical reasons why you have to be very careful in these station blackout situations with depressurizing the core, because it consumes a massive amount of inventory during the depressurization and causes you to lose your steam driven injection systems.
If you have any questions please let me know.