I came across this article regarding a postulated BWR station blackout. http://www.ornl.gov/info/reports/1981/3445600211884.pdf. In my limited understanding of the finer points of nuclear power, is this a fairly accurate description of the early stages of the Fukushima incident? Granted they flooded the drywell before the core went through the floor, and it seems that Fukushima lasted longer on/after batteries than the projection indicated. The report shows the two most likely failure points of containment as a rupture of the torus, and the electrical penetration seal. Its been reported that the torus may have cracked at unit 2. Also, do you think the reports of contaminated water in an electrical trench would be consistant with a failure of the drywell electrical penetration seal?
An additional question. In the earlier pages of this post, many of you have indicated that HPCI and RCIC can be operated manually once the batteries are exhausted. The report indicates those functions being lost once the batteries go below 200 volts. Can you help us understand some of the technical details?
Thanks again to the industry professionals on this board. Nuclear power has been an interest of mine for a long time, and I appreciate the time and energy you have given to enlighten the rest of us!
There is no evidence thus far that the failed fuel breached the reactor vessels in any of the Fukushima reactors. You would be looking at reactor vessel-to-primary containment pressure equalization and very high bottom head temperatures (what is referred to as “core breach signature”).
There is indication that primary containment ruptured in Unit 2. The station blackout analysis you referenced assumes that it would have ruptured due to eventual overpressurization due to loss of containment cooling systems (all AC-powered); however in the case of Fukushima it was due to an internal hydrogen detonation . BWR/4s now have a primary containment vent capability that can be used to avoid the former. Unfortunately without AC power, hydrogen mitigation is difficult; it can be tough to vent all the hydrogen out without the nitrogen dilution system available to help purge it (again, AC-powered), and unfortunately for Fukushima, their vent system went directly to their reactor building secondary containment, where it detonated at units 1&3 anyway. Some plants vent to the outside, which in hindsight is a better idea, but this method of venting is an aftermarket mod aimed at reducing core damage frequency, and not tech spec required.
HPCI needs DC power to operate – it is a large system and I don’t know of any plants that have a contingency for local manual operation. Once batteries run out, RCIC can be operated locally (some operators call it “riding RCIC”, since you end up straddling 1000 psig steam supplies). However, as mentioned above, with AC-powered containment cooling lost, eventually containment pressures get so high that they impose too much backpressure on the RCIC turbine exhaust, and it will fail.
Water in cable trenches doesn’t necessarily mean an electrical penetration breach – those trenches are just lowpoints, and with the amount of external injection that was going on, the water just found its way there. There is a unit 2 break somewhere, though, since the primary containment pressure very quickly went to atmosphere, and water with fuel fragment traces had been identified.