I’d like to add a little more information I am privy to. My chosen field is also Nuclear power and my specialization of the past 15 years is in Radiation Protection (with an additional 5-year stint in Operations)
The Earhquake/Tsunami:
My understanding is the intial plant response to the Earthquake was as expected. A complete loss of offsite power is anticipated since the distribution system is not siemically qualified (not earth quake proof). All or most of the Deisel generators fired up and provided ample power for shut down equipment.
The sea wall surrounding Daichii was ~7 meters. The wave that hit the facility is reported to have been 8-9 Meters high. So not only did the water spill over and take out the deisel generators (which were working post quake), but also some above ground fuel tanks are reported to have been swept away (I’d guess it was the DG Day tanks).
I suspect the wave, which continued to flow several miles inland, subsequently created a veritable swimming pool around the site where by the entire site sits in meteres of standing water. If the site is like many here in the US, (including mine) then most of the Emergency Core Cooling pumps, some buses, critical valves etc sit at levels near or below the site ground level. So when I envision what the scenario the plant operators had to contend with I’m thinking they would have to deal with water intrusion and outright submersion of anything at ground level (plus 15’) and below. Many plants (but not all) have water tight doors around critical equipment but i cannot imagine any systems that would remain operable under these conditions. When i think of all the contingency equipment that would be affected by a site submerged in this much water i can only gasp.
If you had submerge pumps you’d have essentially a compromised flow path (your not going to get much past an idle impeller) even if you were to restore power. An electrical pump is not going to fire up after being submerged. I’m wondering if the choice to go to seawater was as much a flow path issue as it was a water source issue. Sea water injection is the last chance option and in many cases involves the used of portable deisel pumps aligned to direct flowpaths into the core. We will know soon enough but I suspect this might be why.
My background is in PWR’s and we have steam driven turbines that can assist in a compltete loss of power. I suspect a BWR would have the same and I’m wondering what happened to this alternative.
The explosions:
The explosions in #1 and #3 were due to venting of the gas mixture from the core containment. The explosive mixture is largly due to Hydrogen produced from the steam/oxygen reaction withe the hot zirc rods. Zr + 2(H2O) (steam) + heat —-> ZrO2 + 2H2. A simpler explanation is Steam + High Temps + Zirconium = Hydrogen.
The fire at the Spent Fuel pool in #4 , as I have been told, was due to oil from a pump being used in that area. It was not a Zirconium reaction, or at least what I understand.
I am told the Pool at #4 lost inventory due to the debris that fell into it due to the exterior walls being damaged and dropped into the pool as a result of the explosion at #3. This lost inventory shortened time to boil and the debris i suspect blocked make-up flow paths so that the pool could not be filled and recirculated/cooled using normal means.
The Fallout
The fallout hazard is a big questionmark. As Dogs in a Pile mentione iodine is a hazard since it concentrates in the thyroid. However it has a short “half life” and will cease to be an appreciable hazard after 60 -100 days. If you have fuel damage and a breached core containment so that there is nothing to inhibit it’s release into the atmosphere then you could get a deadly combination of airborne an airborne release of fission products eminating from the fuel. So the feared combination that produces an un mitigated release is where you have degrading fuel and a breached core containment. If you have one without the other the situation is much less severe.
I think releases to date have been to alleviate pressure in the core containment structure. These releases probably would have went into a secondary vessel (the reactor building??) and then through filtration. Recall the explosions at #1 and #3 happened as a result of this vent into the secondary vessel where the Hydrogen formed an explosive atmosphere. In my mind that calls into question any filtration ability of the #1or #3 core containments. Again this could still be a relatively less severe issue if the core containment remains in tact and fuel degradation is not an issue. There is a third possibilty that there exists a filtration flowpath is not connected to the secondary pressure vessel flowpath which remains viable. Such design characteristics are likely but whether they are operable is another question.
The information I have received only leaves Fukushima #2’s core containment in question. I actually heard it is feared to be ruptured.
Reports I have heard put the airborne contamination from the site at very low levels as they relate to immediate health effects. I have yet to hear actual numbers but my impression is that the contamination is a combination of nuisance contamination and perhaps also short unfiltered concentrated “puff” releases that should dilute quickly (still not good, but far from worst case ). The best case scenario is where core material is largely contained and subsequent releases are filtered and pose little contamination hazards. My gut feeling is that the actual levels experienced will be closer to the best case scenario described above, versus the extreme other end of the spectrum which is what we saw from Chernobyl.
The severity
Time will tell on this one. One thing to keep in mind; radiation is VERY easy to detect with instrumentation. Therefore when you hear reports such as “iodine found in Tokyo drinking water” we should not automatically assume the worst is upon us. We are able to detect Cesium, Iodine and a host of other radio-nuclides as extremely trace levels. Their presence alone tells us little as to the severity of the accident. Their concentation, however, would tell us much more. We would then be able to postulate such things as unihabitable zones, delayed health effects, viable food and water supplies, etc. I will update the site with this information if and when I get it.
At this stage the governmental authorities WILL advise the public to take preventative measures. They have not advised taking Potassium Iodide (used to prevent iodine uptake in a severe accident release) which leads me to believe the worst case has been averted so far. As each day passes decay heat diminishes. Hopefully we are entering a recovery phase and are no longer contending with degrading conditions at Diachii.
Caveat : There is an information vacuum. Nothing I’ve said above is absolute and some of it may turn out to have been inaccurate. i’m just sharing what i know.
