total fukushima I-131 release vs chernobyl, in curies

somaholiday

From what I can gather, the chernobyl release is most easily discussed in terms of the percentage of the core that was ejected into the environment. I have read the total activity in the core was approx. 9 billion curies, and that estimates of the total release range from 50 million curies all the way up to all 9 billion curies. Lets assume a 30% release, so roughly 3 billion curies. I also read the total core inventory of I-131 was around 80 million curies. Applying 30% to this gives me 24 million curies of I-131 released.

Recently it's been reported that Fukushima has released 2.4 million curies of Iodine, and this has been called roughly 10% of the cherbobyl iodine release (the basis for my 30% assumption above) The same article indicated the Fukushima Cs-137 release to be one seventh the chernobyl amount... This suggests that 3% of "a core inventory" of these radionuclides has escaped into the environment in Japan. Divide by 4 reactors and we get the better part of 1% of each core, or perhaps 3% of one core that may be the main contributer

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?

chironex

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 knowledgable 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.

clancy688

http://www.nisa.meti.go.jp/english/f...20110412-4.pdf

I'm not familiar with the dimension curie, but as far as I know, there were 1800 PBq of I131 and 85 PBq of C137 released during Chernobyl. Those are the same numbers as quoted in the NISA INES-7 paper. And calculated into Curie, it would be around 50 million curie I131 and 2 million curie C137.

http://www.oecd-nea.org/rp/chernobyl/c02.html (In TBq... but getting the numbers in Curie shouldn't be hard... so I don't know how you get billions of curie as reactor inventory)

Furthermore, NISA states, that between 130 PBq (NISA) and 150 PBq (NSC) I131; and between 6.1 PBq (NISA) and 12 PBq (NSC) C137 have been released.
In my opinion, that's consistent with being "10% of Chernobyl".

chironex

Thanks for the data on Fukushima ... I managed to find data on the Chernobyl
radiological release.

For conversions: 1 Becquerel is an activity of 1 decay per second, while 1 Curie is 37 GBq, or 37 x 10^9 decays per second.

The curie was defined as the activity of 1 gram of radium-226.

See for example:

http://www.translatorscafe.com/cafe/...urie-%5BCi%5D/

You can find the Chernobyl data in the pdf at this link:

http://lysander.sourceoecd.org/vl=78...92/v3n1/s1/p1l

It's in table 1. Note that at chernobyl there was a fire dispersing the radioactivity,
here there is none, it's mostly in steam and water that has been leaked into the ocean.

I'll type out a couple of lines from table 1 for comparison with your data.

Core Inventory April 26 Total release during the accident

133-Xe 5.3 d 6500 PBq 100% 6500 PBq
131-I 8.0 d 3200 PBq 50-60% 1760 PBq
137-Cs 30 y 280 PBq 20-40% 85 PBq

http://lysander.sourceoecd.org/vl=78...92/v3n1/s1/p1l

So using the NISA data we have I-131 release at Fukushima so far at about 7.9%
of Chernobyl, and Cs-137 at between 7.17% and 14% of Chernobyl ... consistent with
10% Chernobyl, I agree with you.

But at Chernobyl we have 6500 PBq of 133-Xe, 27000 PBq of Np-239 and many thousands Bq more,
in a long list of other fission products.

Meaning if we just look at the activity, then the Fukushima release is probably more like 3-6% of Chernobyl.

I say again, the main important thing is not the release, but the exposure that it leads to ... for that we'll have to wait and see.

NUCENG

1 Ci = 3.7e10 Bq

Based on results of the code ORIGEN2 for a BWR, Units 1 to 3 cores contained 7.5E7 Ci of I-131 at the moment of shutdown. The potential is there depending on how much has been released.

somaholiday

My hope in starting this thread was to get views on the difference between a release of SOLID fuel (i.e. chernobyl, where fuel was pulverized and cast out in an explosion of the reactor vessel) As opposed to a release of only water/steam that is contaminated from touching damaged fuel (Japan).

If my car's engine overheats and liquid coolant and steam burst out, you would likely find traces of iron and aluminum from the internal engine components in that coolant and steam, but you will not find 3%, or 10%, of the total amount of iron and aluminum that the engine is made out of.

If on the other hand my engine suffers a massive explosion and pistons are found several hundred yards away, we can weigh what remains under the hood and say something like "50% of the inventory of iron was released"

The only way I can think of to approach 10% of the amount released at chernobyl using only water/steam is if certain isotopes, like I-131, tend to fizzle out of hot fuel like carbonation out of soda, so that the water and steam can then carry significant fractions of the total available quantities of such isotopes away...

i.e the fuel is still in the core! how did such a high percentage of the fission fragments get out of the fuel mass? Anyone know specifics on this?

NUCENG

I'm going to look at this qualitatively. I can't yet speak to how much has actually been released either at Chernobyl or at Fukushima. I provided information earlier on the a conservative amount of I-131 that was available for release. This was based on a BWR analysis using ORIGEN2. I did a quick total of the radiation in the reactor including activation products, actinides, and fission products for just Unit 1 the smallest of the reactors an Fukushima. At shutdown the total is about 8.9E9 Ci or 3.3E20 Bq. This includes noble gasses, and other gaseous elements and a large number of soluble particulates and light elements that can be easily spread if released with steam or water. Chernobyl RBMKs rated at 1000 MW probably contained more than twice the initial source term as Fukushima Daiichi Unit 1. But at Fukushima we have full cores in the vessel in Units 2 and 3 and another full core in the spent fuel pool at Unit 4, plus hundreds of other spent fuel bundles.

At Chernobyl there was a large steam exoplosion and perhaps one or more smaller hydrogen explosions as well as a fire that created a significant airborn release.

At Fukushima we have seen a number of explosions most probably from hydrogen. Just guestimating, the total available radiactivity at Fukushima is probably twice that of Chernobyl. Even if the release from each unit were at half the rate of Chernobyl, the total release would be in the same ballpark.

What it really shows is that even with the very high radiation levels, the total release is actually a small fraction of the total available.

chironex

I'm very much looking forward to the quantitative results from NUCENG.

I'ld love to get hold of ORIGEN2 personally, and run it myself, but I've found
that it takes a couple of days to register and download this code and it would be
a while after that before I could get up to speed :)

In the meantime, here's a ballpark estimate of the I-131 that might have been released at
Fukushima, based on figures from an old study of a BWR with Mark I containment (the Peach
Bottom Plant), and using NUCENG's estimate for the I-131 content.

The study of the potential Peach Bottom accidents can be found in the following
paper:

Is Mark I shell failure really important? — Part two
Herschel Specter and Peter Bieniarz

Nuclear Engineering and Design Volume 121, Issue 3, August 1990, Pages 447-458

http://www.sciencedirect.com/science...4&searchtype=a

This paper examined severe accidents in which the reactor vessel fails, the core pours onto the drywell floor and begins to attack the concrete, and very shortly thereafter, the containment fails. The containment
failure is assumed to be in the drywell and to have an area of a few square feet, and the containment
pressure at failure is assumed to be 130 psia.

So these are prompt release scenarios, which may therefore not be applicable to Fukushima, but it has been previously thought that prompt releases would tend to maximize the source term.

Releases of the core inventories of I, Te, and Cs even in these severe cases are indeed estimated to be only a few percent of the total core inventories at the time of initiation of the accident. The calculations were based on Oak Ridge's BWRSAR code.

There were three accident scenarios examined, all following from a station blackout.

(1) TB1: Battery Failure after 6 hrs, no ADS available, ECCS off at 6 hrs.
(2) TB1A: Battery Failure after 6 hrs, ADS available, ECCS off at 6 hrs.
(3) TB1E: Battery Failure at T=0 hrs, ADS available, ECCS off at 0 hrs.

(ADS = Automatic Depressurization System, and it consists in part of
a mechanism to rapidly release pressure from the RPV to the suppression
pool, in accidents when the RPV is retaining pressure. It's a much better
case if this system functions. But, possibly, it may not in the case of a
complete loss of electric power.)

For scenario TB1, the fractional releases were:

Iodine: 0.03
Tellurium: 0.014
Cesium: 0.02

For scenario TB1A:

Iodine < 0.004
Tellurium < 0.014
Cesium < 0.004

For scenario TB1E: the releases were not calculated, but they were estimated to be between cases TB1A and TB1, and closer to TB1A. This was because, while the initial energy driving the releases (decay heat + metal water reactions) is different in the different cases, the dominant term is the metal water reactions, and that is the same in all three cases (at least for these short time release scenarios).

So the major difference is whether the ADS was available or not.

Only in case TB1E with no ADS available were the releases expected to be greater than for case TB1.

Taking NUCENG's value for I-131 for cores 1-3, we have 7.5E7 Ci of I-131
"at the moment of shutdown."

Assuming the TB1 scenario applies for all 3 cores there could have been a
release of 2.25 x 10^6 Ci, or 8.325x10^16 Bq, which is about 83 PBq.

So that scenario would seem to be reasonably consistent with the NISA estimated
releases that have taken place at Fukushima, I think, though of course, I do
not mean to suggest that this was the actual course of events!

There is also the spent fuel pool to consider, of course.

At least one conclusion is pretty clear: only a small percentage of the initial
core inventories needs to have been released in order to produce the
observed activities.

chironex

Indeed, you are right, but the situation is not like this at all.

What you should first try doing is to convert the total activity of iodine-131 that is reported into a number of iodine atoms. That's easy to do, you just divide the activity in Bq by the decay constant for I-131, which is log(2)/(8 days x 86400 s/ day). Then multiply by the atomic mass, and you'll see that the total amount of I-131 released is only a tiny fraction of the mass of all the fuel in the core (which is, say about 150 tonnes) for the larger reactors at Fukushima.

The fuel was initially in the form of small ceramic uranium oxide pellets, and the fission products and actinides produced by the reactor were initially contained in the fuel pellets, which were wrapped up in zircaloy cladding. But now it is very likely that a lot of that cladding is gone, and that fuel pellets are all damaged, in contact with cooling water.

Possibly even they pellets were all melted together and are now lying on the floor of the reactor vessel or even the floor of the containment building, in the worst possible case, and everything is in a molten state (though a crust has likely formed by now).

There may be a big hole in the containment, and cooling water passes over this very hot mass and dissolves everything that is easily soluble: which includes such fission products as Cs and I and some others as well as anything radioactive which has attached itself to small particles that may be around.

The water is turned into steam and floats around in the containment. Some, probably most, material dissolved in the steam will be plated out when the steam touches colder surfaces inside the containment - depending on what temperatures are like, of course. But some steam may pass out through holes in the containment, or be deliberately vented to the atmosphere. It's a complicated process and one needs computer codes and experiments to model it in detail.

In any case it will never be a large fraction of the mass of the fuel or the fission products that gets out:
that's the function of the containment.

At Chernobyl the situation was very different, because: there was an explosion (more than one), there was no containment, and there was a fire, which burned for ten days.

The explosions first of all, tossed large fragments of fuel all over the immediate area. These fragments were very radioactive, and posed a huge danger for those who eventually had to clear them away. But the fragments didn't release much of their inventory of fission products.

Some of the fuel was vaporized in the explosions and the fire. The fire made a huge difference: it provided material and surfaces for the heavier and less water soluble radioactive elements to chemically bind to, and the great heat of the fire carried some of the smoke way up to 30,000 feet. The smoke that didn't go so high was free to fall out as it cooled.

A lot of the smoke and larger particulates fell out pretty close by. But small amounts went for long distances.

Different radioisotopes transport differently, depending on their chemical properties.

What ended up on the ground near Chernobyl depended on the changing weather, the wind and the rain. There was a big hot spot near the plant, and another one located to the northeast.

Cesium and iodine were released from the vaporized fuel.

It seems that the release of Cs-137 and I-131 over ten days were about ten times the total releases at Fukushima, over a period of 30 days. And the initial inventory at Fukushima for the running reactors was larger than at Chernobyl. If there has been major damage to the fuel, such as I described above, then that shows the effect of the containment on the releases.

Also, there do not appear to be any large releases of actinides at Fukushima: the only one I've seen suggested were Pu-239 and Pu-240.

But these were at very low levels, corresponding to what could have been left from Chinese atmospheric testing, and Pu-238, which would have clinched the case that it came from Fukushima, was not found.

Again, that's an effect of the containment, and that the transport out of the cores has been via water and steam.

Dmytry

The fuel (uranium dioxide) heats up, even melts, the radioactive volatiles evaporate out of fuel. Think distillation. Put a little bit of wood in a test tube, and heat the test tube's tip up, to get the idea.

I think you simply misunderstood how they escaped at Chernobyl. If you just scatter freshly-spent fuel pellets on the ground, you'll get a huge local mess, and there will be some out-gassing of radioactive noble gasses (which are not very important), but the caesium and iodine will stay in. If you crush the pellets, you will get some dust into the air, but the iodine and caesium will still mostly stay in I believe.

If you put that fuel in a pile, though, it will heat itself to very high temperatures by decay heat. (The fuel lava is not very thermally conductive, so it won't rapidly melt through the ground or anything like that. Fully molten fuel can stay inside pressure vessel if it is cooled on the outside; the fuel in contact with the reactor wall would simply freeze).
Iodine and caesium will evaporate from the fuel, and will form aerosols. They will get out together with water and steam. Whatever settles on the walls of reactor would be washed out by water (caesium and iodine compounds are water-soluble). The water boiling (bubbling) makes aerosols.

It is the heat that releases radioactive volatiles. Once the fuel melted, and those separated from uranium dioxide, the only way to contain them would have been not to vent (but the reactor would have exploded) or to vent through filter (no filter of sufficient throughput was available).
Edit: Yes, some of the steam would get condensed in the piping, but don't hope for much - water has extremely high latent heat of vaporization, i.e. you need to take away a lot of heat from a tonne of steam to condense it. Furthermore, a huge amount of hydrogen was vented as well.

somaholiday

ok, it seems the conensus is that with sufficient heat, certain isotopes do escape from the fuel mass is large quantities, so you don't need any "fuel mass" to escape in order for those isotopes, or "volitiles", to escape. That pretty much answers my question, thanks for the replies.

The mention of a filter is interesting, it seems that if a filter capable of this task exists that it might be standard practice to have it in place, but that's separate question.

hank

> filter

Those were proposed for this design and some countries built them -- google
bwr sand filter and you'll find quite a few references. The idea seems to be that any overpressure would be vented out through a long underground sand bed, a really large area that would condense and capture material carried by gas or steam.

One example off the first page of Google hits:
" ... a large sand filter. This arrangement will be connected with ... core melt-down leading to a BWR-2 or BWR-3 release are reduced, ..."
130.226.56.153/rispubl/reports/ris-r-462.pdf

hank

a better search string:
http://www.google.com/search?q="boiling+water+reactor"+overpressure+sand+filter

Dmytry

I see your misunderstanding as example of impact of well crafted misinformation we all seen in the media. Very strong emphasis on 'containment' that is 'intact' (at the cost of venting it lol). And of the meltdown, which was 'averted' because 'down' part did not happen. I'd love to track what marketing genius invented the word 'meltdown' to utilize wordplay on the meaning such as 'situation going down' and shift emphasis away from the melt itself. All around, appeal to 'thinking' by verbal analogy, you've been actually thinking and from that you thought they were implying the fuel has to escape for volatiles to escape.

Ultimately, if the fuel would escape - e.g. if you would take the fuel out somehow and scatter it on ground - the situation outside the site would have been less severe, as the volatiles actually won't escape from escaped fuel ;)
Japan is very lax on nuclear safety. Filtering would be very expensive.

somaholiday

Dmytry, twice you have refered to fuel lying on the ground in undamaged chunks. This is completely irrelevent. If I wanted to be arrogant and condescending I would call that your misunderstanding and characterize you as a victem of media hype, but that would be wordplay...

When I say fuel "escaping" and refer to chernobyl, I mean in a powerful explosion that blows it into fine dust and also subjects it to enormous heat. My question was about comparing the potential for fission fragment release in that situation to the one in Japan, where the potential for fission fragment release is much less clear.

\thread

Dmytry

Only about 2% of fuel in Chernobyl was turned into dust. Secondarily, the releases stopped when the fire was extinguished (10 days after disaster).
Furthermore, the explosion has not subjected much of the fuel to 'enormous' heat. It was a steam explosion.
It is undeniably the case that the media had been really hyping the whole explosion / lack of containment building aspect of the Chernobyl. Not only the media but also many so called 'experts'.

chironex

The prmary containment can only contain fission products if:

(1) It is not breached.
(2) It is continuously cooled, so as to condense steam that is produced as a result of decay heat in the reactor, as well as heat produced due to chemical reactions between metals in the reactor core and steam, and possibly also air, in the event of severe overheating of the core.

Complete loss of station power for any significant period of time was known by all experts to be a beyond design basis accident, and certainly for the GE BWR's with the Mark I containment.

The containment was designed to survive a maximal LOCA caused by the abrupt rupture of the largest pipe feeding water to the reactor pressure vessel, and in that case, IF station power and at least minimal cooling systems were still functional, to also contain fission fragments released from damaged or melted fuel.

The system does this by venting fission product containing steam from the reactor vessel into the drywell and from there to the suppression pool, where it is condensed into water. This mechanism works, as long as the temperature of the suppression pool can be kept below 100 C.

If cooling systems function, then this temperature can be maintained. Venting of containment can be limited essentially to venting of noble gases in such a case, and these are very ineffective at providing exposure of the public to radiation.

If however, power is unavailable to run pumps, it has always been well understood by experts that radioactive steam would have to be vented to prevent failing the containment, and that that would result in releases of volatile fission products.

This does not mean that containment had no effect in such an accident, far from it. First, the design delays the time of release considerably. Second, there are many surfaces for volatile fission products to attach to inside the containment. The concentration that results in the steam is one which is in thermodynamic and chemical equilibrium with the condensed phase (molten or damaged hot fuel). To the extent that the re-establishment of cooling has been effective in this accident, even with a cobbled together open loop cooling system, and even with continuous releases of radioactive steam and water from the containments, they have still functioned better than a completely open system such as Chernobyl.

It's not clear to me, for example, what fraction of the releases have been dissolved in water that was released to the ocean and what fraction was in steam vented to the air.

At Chernobyl, where no mechanism hindered the release of volatiles fro heated and damaged fuel, it appears that up to 40-50% of the inventories of the most volatile Cs and I were released. And many were killed by acute radiation sickness. So far, here, the releases are about 1/10 of the initial inventories. And it's possible that some major fraction of the releases came from the spent fuel pools, which are outside the primary containment.

It seems also that the major releases here have coincided with the hydrogen explosions, and that ambient radiation levels in the evacuation zone have been on a decline since the first two weeks of the accident.

One hopes that this trend continues, and that no additional complications arise.