Furthermore, the nuclear reaction w/in the MOX fuel has been poisoned by the affect of the moderating rods being fully inserted into the nuclear pile. That means that any more heat is not being generated within the nuclear pile.
That being said: the mass of U235, and zirconium fuel rods, and reaction moderating rods is immense. The residual heat contained therein is enormous and must be exhanged into heat-sink. That'll take time within the enclosed system as its been designed.
The problem is that if the temp of the core remains above some critical value for too long, the zirconium fuel rods holding the MOX pellets, catalyze water by dissociating it into its hydrogen and oxygen components. The catalyst then reacts chemically with the molecular oxygen (O2) to form zirconium oxide; the MOX pellet holders essentially are transformed to dust. This could allow raw MOX pellets to re-initiate criticality of some degree (nuclear exothermic reaction) as they deform configuration and get too close to their neighbors. That would be really really bad.
IF they have to refill the primary reactor loop again, there will be some release of radiation. Depending on how they're venting the primary loop pressure - most likely through radiation scrubbers - they don't have the enjoyment of the secondary scrubbers that were undoubtedly in place in the secondary containment building; its been Ba_BOOMED; subsequent primary coolant venting will be released immediately into the general environment.
My guess would be subsequent primary coolant loop pressure purge and / or coolant re-filling will result in some increased radiation ccntamination of the environment. However, it won't be doomsday either (primary coolant loop radiation scrubbers are still in place).
Now, if the primary pressure vessel containment is compromised: all bets are off. But if that's true: then the radiation Ah-OOOOOOH-Gah horns be sounding off to grimace proportion. But I have no confidence in that unless the engineers misplaced a two, or put the decimal place one spot to the left or right of where it should've been due to too much saki the day they designed the reactor.
BTW, if that be true: we'll be seeing news reports of several hundred (if not thousands) of hari-kari's happening all over the next few weeks by nuke plant engineers.
Even with so much being neuetralized? But yes, I understand that is the concern now..that it can be contained..unfortunately nobody will know until it happens.
The design is American, not Japanese. So the design engineers were American, not Japanese.
Fukushima Dai-ichi #1 was designed in the 1960s. The engineers who worked on that are in their 70's, 80's, or perhaps 90's, or already passed away.
Most of the scenarios that are being discussed in this and other forums were considered by those engineers, 50 or more years ago. The earthquake was 7 times more powerful than the maximum that the reactors were designed to withstand, so that is currently putting more stress on the reactors and the operators.
The reactors were mostly built, operated, and maintained by the Japanese.
The reactors must be built, operated, and maintained correctly, or else the best design will not prevent a catastrophe.
It seems to me (not an expert) that the Japanese could benefit at this time from consulting, in particular, expert out-of-the-box thinking. Hopefully they will accept Immelt's offer of help.
One potential problem is that the market for BWRs dried up in the late 1970s, and along with it, the engineering talent that was responsible for the design of these BWRs in the US was permitted to disperse and attenuate decades ago. Some of the expertise was shared or transferred to the Japanese.
Because the reactors are now in unorthodox modes of shutdown under pathologically unusual circumstances, out-of-the-box thinking (eg, of the Apollo 13 variety) may be of particular value. In any case now it seems like an ideal time for the enineers and operators of all countries to work together to regain control of the situation.