somaholiday said:
With chernobyl there is no question about how 30% of the inventory of a given radionuclide in the core would escape, but I don't see how even 1% to 3% would have gotten out in Japan. As far as I know, only water and steam that have touched fuel have gotten out, so is it possible that this high of a percentage of these isotopes has been carried out by these methods?
In contrast, three mile island, in which gas that was in contact with fuel was released resulted in a comparatively trivial 20 curies or less of I-131 escaping.
Can anyone share thoughts on this?
I think I've seen 50 million curies quoted for the I-131 at Chernobyl,
which would make the release 1/20 of Chernobyl. But never mind,
it's the right order of magnitude. The small discrepancy in Cs-137 and
I-131 seems odd, but then, what are the cumulative yields?
But most of all:
This has been a far more severe accident than TMI-2.
Remember that at TMI-2 core cooling was re-established within 4
hours, or so, of initiation of the LOCA. They never lost station
power. There was no damage to the RPV, and despite a hydrogen
burn inside - there was no damage to the containment
structure. They didn't even bother to vent the containment for
the first time until late April, well after the accident had
occurred in early March, if I remember rightly.
Mostly noble gases were released from the containment, and that
was done only some considerable time after the accident, at a
time of the operators choosing. A lot of the I-131 that may have
been in the atmosphere inside the containment had decayed to
Xe-131 by the time they vented it.
The other route for I-131 and other fission products to escape
from TMI-2 was via the primary coolant that leaked out through
the famous stuck pressure operated relief valve that led to the
loss of coolant. The leaked primary coolant ended up on the floor
of the containment building and was then transferred to an auxiliary
building (outside the containment) by pumps that were activated
when water was detected on the floor. Leakage rates of radioisotopes
from this coolant water were probably mitigated somewhat by the
intact structure of that auxiliary building.
The coolant water set off radiation detectors in the auxiliary building,
and at that point the operators realized definitely - I think for
the first time - that they had a LOCA underway; though I think at
least one knowledgeable person at the plant, as well as an expert
working on the situation from home, had earlier suggested closing
an auxiliary valve on the line that led to the stuck PORV.
Then the next shift arrived, figured out what was happening and
restarted the core cooling. The operators solved the other immediate
problems within a week or two. Since the containment building was
very large and undamaged, they had the luxury of simply keeping
it cool and waiting until I-131 decayed to Xe-131 to vent it.
At Fukushima, there is suspicion of primary containment damage at
one or more of reactors 1-3, and the releases have been far less
controlled than at TMI-2. This is no doubt due to the far smaller
volume of the containment structures for these early BWR designs,
and the much greater difficulty that they've had in re-establishing
core cooling in this accident.
It was always regarded as more likely that some degree of
containment failure could occur in early BWRs than in a PWR,
in the event of a severe accident. The two major causative
scenarios for a severe accident were thought to be station
blackout and anticipated transient without scram, and it seems
that the first is what they are now dealing with at Fukushima.
There was no cooling for a significant amount of time in all
three of the reactors that were online when the earthquake
hit. So all of them have significant core damage.
There's some evidence from temperature and pressure data for R1,
that's been officially released (though data for the very first
hours after the event haven't been made available as far as I
know) that what happened in that reactor was a loss of coolant
accident caused by the earthquake, followed immediately
afterwards by the total loss of station power, and so, all
emergency core cooling systems, soon after the tsunami hit.
The pressure in the primary containment of R1 is seen to be very
high, about 0.7-0.8 MPa, at 12 h after the earthquake, and the
pressure in the RPV is simultaneously seen to be very low, also
about 0.7MPa. The RPV normally operates at about 7MPa. So it lost
a lot of pressure somehow.
At 0.7-0.8 MPa, pressure tests have showed that the upper
head of the Mark I containment can be lifted by the internal
pressure, and that would then lead to a release of gases into
the area above the fuel processing floor. That may be how
the hydrogen, likely evolved from a zirconium-steam reaction,
found its way up there, and eventually blew the roof off of R1.
There could also, conceivably, be damage to the upper head
of the RPV in R1, depending on what happened in those early
hours after the loss of coolant (if indeed that is what happened).
There was also a hydrogen explosion at R3, which looks as if it
was even more damaging to the secondary containment than that at
R1. It's also suspected that there was a hydrogen burn inside
the torus at R2, which may have cracked the torus.
Without any cooling for the cores, the operators would soon have
been forced to vent steam, early on in the accident at least,
whenever that became necessary, to avoid overpressurizing the
containments. Now that decay heat has died away quite a bit, they
are probably trying to vent only when the winds are
favourable.
But all of this vented steam has been in direct contact with the
damaged cores, and it's by now a rather large volume of steam.
And there is also the apparent hydrogen explosion at R4, where
the hydrogen seemingly must have come from the spent fuel
pool. If that's true, it means that the spent fuel must have been
uncovered, in which case you could possibly have fission products
released directly into the secondary containment of R4, from the
core of R4 which had been off-loaded into the pool about 100 days
before the earthquake. Then there is a hydrogen explosion which
destroys the secondary containment ... this could have released
quite a lot of fission products directly into the environment.
As it is, they are saying the total release of I-131 is on
the order of 1/10 of Chernobyl, so far.
So possibly there has been a release of something like 1% of the
volatile fission products from four cores ... it doesn't seem impossible
since Cs and I are easily soluble in water and the containments are probably
somewhat damaged, and a lot of water has been passed over these
hot cores and vented to the outside.
But in any case, the bottom line in determining how serious the
health effects will be is not the total release, it's actually
the total exposure of the public. Indications are that, assuming
that things are controlled pretty soon, this may be much lower
than at Chernobyl, due to the early evacuation of the people, and
the careful scrutiny of the milk and other foods. For the I-131,
they will know to tell the people to take potassium iodide, thus
mitigating the thyroid cancers, which are probably the one
definitely directly attributable late health effect of Chernobyl.
