Rive said:
There was a document linked somewhere back (years ago) about simulated results of handling a complete SBO on GE MK-I containment. As I recall that went exactly on the same way as you. The sooner the PCV depressurized is the better.
However, this contradicts the actual way of thinking about containing an accident with multiple barriers, even if with this the accident might end in a steam bomb slowly pumping up.
Cooldown is obviously the best way to protect the vessel long term, however an emergency blowdown or cooldown in excess of the ASME code limit has the potential for putting severe stress on the RPV and potentially causing a LOCA. Additionally, pressure changes will affect your water level instruments and make it very hard to control level. For this reason the EOPs direct stabizing pressure and level. Pressure should be stabilized within the 100 degF per hour cooldown limit and held as constant as possible, then level should be stabilized between the high and low water level trips. Once you have everything stabilized you commence a controlled cooldown. You're looking to minimize the challenges to level and pressure control, while also minimizing thermal stress or damage to the RPV. The stresses imposed on the RPV are huge during a blowdown, and the EOPs recognize this by not allowing you to exceed the cooldown limit unless the fuel or containment are challenged, where the risk to the public is larger by keeping the vessel hot than it is to blowdown and potentially have a LOCA.
As for level and pressure: when an srv opens up, you get a 25-35 inch spike in level, due to the swell effect, which continues to grow. The whole time you are losing inventory, with false high water level readings. This can cause your injection sources to trip off on high level. Then when the srv is closed, the shrink can cause another low level scram or ECCS injection signal. It's difficult to control. Additionally if you start rapidly cooling down, you need substantial inventory makeup to deal with inventory loss through steam relief and the water shrink during the cooldown. Something like IC provides no inventory. RCIC does, however it's nominal flow rate is 450-600 gpm, and it does not have sufficient makeup capability for the first 10-15 minutes, and until you let decay heat die a little RCIC doesn't have enough flow to support a rapid cooldown. You would have to rely on ECCS, which stresses your vessel nozzles and can damage fuel (either through foreign material in the suppression pool, or for plants with in-shroud ECCS water impingement on fuel bundles). So there's all these factors that have to be weighed. What we have done, is when we had to cooldown, we let decay heat die for an hour or two, use that time to take care of the secondary, then start cooling down. When you aren't fighting substantial decay heat, it's much easier to control. Also, at lower pressures, a single relief valve is going to pass less steam flow due to lower driving head, so you end up keeping relief valves open longer to achieve any meaningful depressurization which results in larger pool heat ups and larger makeup requirements. Above 500 psig, a single relief valve can almost always handle all decay heat. But below that, you'll need to cycle multiple relief valves which is outside of the containment and relief valve sparger loading analysis. It's assumed in the containment safety analysis that the only time you'll have multiple relief valves opening up for design basis events is during the initial load reject, after that only a single relief valve will be used which minimizes acoustic/water/structural loading on the suppression pool.
If condenser/Feedwater is available you can easily and rapidly cooldown. And in fact BWR procedures will demand a pretty quick cooldown to 500 psig to minimize thermal stress on the Feedwater nozzles, even if a hot restart is coming. But when you are isolated, the faster you move pressure, the harder it is to control the rest of the plant. Staying hot means you keep your steam driven injection sources, have more controllability, minimize stress on the vessel, and avoid spurious trips on your injection systems.
As for SBO, since it's only a 4 or 8 hour event per the design basis, you don't want to depressurize, as this adds heat to containment that can't be removed and also thermally challenges RCIC. Eventually, for long term coping, you either need to restore RHR HX, or wait until the last minute to blow down then reflood with fire pumps and seawater. Typically the suppression pool heat capacity is going to drive you to blowdown, not level, as RCIC/HPCI/HPCS operation is assumed for the coping duration.