Categories
Nuclear Open Thread

Fukushima Technical Discussion Open Thread

It was suggested in a comment — and I agree — that the previous open threads on the Fukushima Daiichi Nuclear Accident were becoming difficult to read, because they are such a mixture of technical details and philosophical discourse. That is, it’s generally a bad idea to cater to two different audiences in one comment thread. So, I will split them up.

Please restrict all discussion here to technical information, analysis, criticisms and questions on FD — no philosophising or excursions into whether nuclear power is ‘good’ or ‘bad’ or the implications of FD for the future of nuclear power (except for new technical developments, e.g. safety standards), etc. You may impart your deep wisdom on how the world should work on the other open thread I’m about to open.

Besides the above guidelines, the other rules of the Open Threads on BNC apply. Read here for details.

To kick off discussion, below is the latest FEPC status report (I’ll update this as new reports come in). You will also be interested in:

— JAIF Updates #35 and #36

NISA Major Parameters 0600 March 29

NISA Summary Conditions 0600 March 29

—————-

  • Radiation Levels
    • At 11:45PM (JST) on March 28, TEPCO announced that plutonium 238, 239 and 240 were detected in the soil sampled on March 21st and 22nd at five spots in Fukushima Daiichi Nuclear Power Station. Concentration of detected plutonium 238, 239 and 240 are the same level of the fallout observed in Japan at the atmospheric nuclear tests in the past and poses no major impact on human health.
    • At 6:30PM on March 29, radiation level at main gate (approximately 3,281 feet from Unit 2 reactor building) of Fukushima Daiichi Nuclear Power Station: 177 micro Sv/hour.
    • At 6:30PM on March 29, radiation level at west gate (approximately 3,609 feet from Unit 2 reactor building) of Fukushima Daiichi Nuclear Power Station: 120.2 micro Sv/hour.
    • Measurement results of environmental radioactivity level around Fukushima Nuclear Power Station announced at 7:00PM on March 29 are shown in the attached PDF file. English version is available at: http://www.mext.go.jp/english/radioactivity_level/detail/1304082.htm
    • For comparison, a human receives 2,400 micro Sv per year from natural radiation in the form of sunlight, radon, and other sources. One chest CT scan generates 6,900 micro Sv per scan.
  • Fukushima Daiichi Unit 1 reactor
    • At 1:00PM on March 29, pressure inside the reactor core: 0.371MPa.
    • At 1:00PM on March 29, water level inside the reactor core: 1.65 meters below the top of the fuel rods.
    • At 1:00PM on March 29, pressure inside the primary containment vessel: 0.265MPaabs.
    • At 1:00PM on March 29, the temperature of the reactor vessel measured at the water supply nozzle: 570.9 degrees Fahrenheit
    • As of 3:00PM on March 29, transferring the water found at the turbine building to the condenser continues.
    • As of 4:00PM on March 29, the injection of freshwater into the reactor core continues.
  • Fukushima Daiichi Unit 2 reactor
    • At 1:00PM on March 29, the temperature of the spent fuel pool: 114.8 degrees Fahrenheit.
    • At 1:00PM on March 29, pressure inside the reactor core: -0.025MPa.
    • At 1:00PM on March 29, water level inside the reactor core: 1.5 meters below the top of the fuel rods.
    • At 1:00PM on March 29, pressure inside the primary containment vessel: 0.1MPaabs.
    • As of 4:00PM on March 29, the injection of freshwater into the reactor core continues.
    • As of 7:00PM on March 29, approximately 96 tons of water in total has been injected into the spent fuel storage pool.
  • Fukushima Daiichi Unit 3 reactor
    • At 12:00PM on March 29, pressure inside the reactor core: 0.029MPa.
    • At 12:00PM on March 29, water level inside the reactor core: 1.85 meters below the top of the fuel rods.
    • At 12:00PM on March 29, pressure inside the primary containment vessel: 0.1075MPaabs.
    • At 2:17PM on March 29, TEPCO began to shoot freshwater aimed at the spent fuel pool, with a specialized vehicle normally used for pumping concrete, until 6:18PM (approximately 100 tons in total).
    • As of 4:00PM on March 29, the injection of freshwater into the reactor core continues.
    • As of 7:00PM on March 29, approximately 4,697 tons of water in total has been shot to the spent fuel storage pool.
  • Fukushima Daiichi Unit 4 reactor
    • At 11:50AM on March 29, lighting was restored in the Central Control Room.
    • As of 7:00PM on March 29, approximately 960 tons of water in total has been shot to the spent fuel storage pool.
  • Fukushima Daiichi Unit 5 reactor
    • At 2:00PM on March 29, the temperature of the spent fuel pool: 101.5 degrees Fahrenheit.
  • Fukushima Daiichi Unit 6 reactor
    • At 2:00PM on March 29, the temperature of the spent fuel pool: 70.7 degrees Fahrenheit.
  • Fukushima Daiichi Common Spent Fuel Pool
    • At 3:10PM on March 28, the temperature of the spent fuel pool: 95 degrees Fahrenheit.
    • As of 7:00PM on March 29, approximately 130 tons of water in total has been injected to the spent fuel storage pool.

By Barry Brook

Barry Brook is an ARC Laureate Fellow and Chair of Environmental Sustainability at the University of Tasmania. He researches global change, ecology and energy.

368 replies on “Fukushima Technical Discussion Open Thread”

[Comment deleted for violation of the citation rule]
MODERATOR
Shelby – you have been warned before not to violate the citation rule.
Please read the rule below and abide by it. Any further such instances will be deleted.

CITING LITERATURE AND OTHER SOURCES
This is not a forum for cut-and-pasting slabs of text. Tell people why you think they should be interested in reading this, and what it means for this discussion. Otherwise, you’re not thinking and not contributing. Simple as that.
Appropriate and interesting citations and links within comments are welcomed, but please DO NOT cite material that you have not yourself read, digested and understood. As a general rule, please introduce any and every link or reference with a short description of the material, your judgement on its quality, and the specific reason you are including it (i.e. how it is relevant to the discussion).

Like

Re; MODERATOR
Shelby – you have been warned before not to violate the citation rule.
Please read the rule below and abide by it. Any further such instances will be deleted.

——-

Sorry I have to spell it out — I thought it would be interesting that the Union of Concerned Scientists stated that nitrogen injection has never been attempted before (to their knowledge) after a reactor has been running. They state it has only been done before the reactor is started. They imply there is no known case study to judge how this will be done or if it can be done effectively.

http://www.cnn.com/2011/WORLD/asiapcf/04/07/japan.nuclear.options/

Like

I have been thinking about : “flooding the Dry well/ area around the PRV” .

It is hard to understand this because this area vents thru huge pipes down into the Torus. ::It is for condensing any venting steam from the PRV..

Can’t imagine the CV to be filled very high with water.

It appears also that there are recirc pumps in the bottom of the dry well also. So another bad reason to flood the CV.

Like

I’ve also been thinking about the drywell flooding option. If there was no water left at the bottom of the pressure vessel then the corium could melt through, necessitating mitigation by flooding the drywell. Its basically a hot pan in a second pan of water then, which cools the reactor vessel bottom so that it doesn’t give way. In newer BWRs, such as GE’s ESBWR, there are automatic drywell deluge (flooders) fed by gravity from higher resevoir. These open automatically upon too high drywell temp and/or pressures.

Does anyone know if the drywell bulb bottom is designed to take a full corium release when not deluged, ie dry? The concrete at the bottom between the containment and the reactor vessel bottom appears very thick, so obviously this appears a design feature. But can it take full corium release when fully dry?

Like

I did find a model calculation which examines such a scenario for a BWR with Mark I containment, in a severe accident: short term station blackout with associated failure of all emergency core cooling systems.

The assumption of the model is that the RPV bottom head fails, and the corium begins pouring out onto the dry floor of the containment starting at t ~ 280 minutes after the loss of power and cooling.

It seems that the major concern turned up in the simulation was not melting the the drywell bulb bottom. The (model) temperatures found adjacent to the corium stayed less than the melting temperature of the containment shell.

The problem is that the atmospheric conditions in the hot environment above the molten debris are thought to be sufficient to fail the drywell head seals, and that the sustained temperature over time might lead to creep rupture of the primary containment pressure boundary.

I think you’ve stated correctly the reason why flooding the containment is considered an option, as you say, it’s to cool the bottom of the RPV and prevent it’s failure, with the release of corium onto the floor of the drywell.

I certainly would have thought that the time when that option would have been chosen (in the current case) was much earlier on than now, when it seems that core cooling (even if not by a closed loop) has been re-established, and there has been a lot of time for decay heat to begin to die away.

See:

Contain calculations of debris conditions adjacent to the BWR Mark I drywell shell during the later phases of a severe accident

Abstract

Best estimate CONTAIN calculations have recently been performed by the BWR Severe Accident Technology (BWRSAT) Program at Oak Ridge National Laboratory to predict the primary containment response during the later phases of an unmitigated low-pressure Short Term Station Blackout at the Peach Bottom Atomic Power Station. Debris pour conditions leaving the failed reactor vessel are taken from the results of best estimate BWRSAR analyses that are based upon an assumed metallic debris melting temperature of 2750°F (1783 K) and an oxide debris melting temperature of 4350°F (2672 K). COINTAIN analyses were performed for the debris/concrete interaction occurring without consideration of the possible existence of an overlying pool of water. Results indicate failure of the drywell head seals due to the extremely hot atmospheric conditions extant in the drywell. The maximum calculated temperature of the debris adjacent to the drywell shell is less than the melting temperature of the shell, yet the sustained temperatures may be sufficient to induce primary containment pressure boundary failure by the mechanism of creep-rupture. It is also predicted that a significant portion of the reactor pedestal wall is ablated during the period of the calculation. Nevertheless, the calculated results are recognized to be influenced by large modeling uncertainties. Several deficiencies in the application of the CORCON module within the CONTAIN code to BWR severe accident sequences are identified and discussed.

Like

In the media, here and there, and all over the place in scare narratives, there has been talk about recriticality incidents and it sounded like something along these lines was possible (but what exactly?) given the pumping of borated water into (what did they pump it into?)

Anyway, my question is, after the fission process has been stopped and the reactor has powered down to 7 %, and sheds decay heat from there, can a melting of the core lead to recriticality. what are the circumstances, probabilities, severity, etc? can recrit occur without the water?

Like

Thx Cyril, David for the responses.

If you go to the latest JAIF report (http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1302343542P.pdf), water injection to containment vessel is a reporting item. Status reads : to be confirmed (R1), to be decided (seawater) (R2), and, to be confirmed (R3).

I agree w/ the comments you made about WHEN they would have been considering this and I agree that perhaps it was a more critical discussion earlier in the accident.

It may be though that actually flooding the CV would be more difficult than than just pumping in the water as it would just drain out thru the turbine building and trenches (now plugged). see schematic at http://upload.wikimedia.org/wikipedia/commons/thumb/8/8f/Fukushima_I_nuclear_powerplant_nuclear_%28leak_case_of_high-level_radiation_water%2C_trenches_and_tunnels%29.PNG/800px-Fukushima_I_nuclear_powerplant_nuclear_%28leak_case_of_high-level_radiation_water%2C_trenches_and_tunnels%29.PNG

There would have to be some way the isolate the CV from the drain path shown in the schematic. Perhaps a closeable door is included in the plant design.

Like

From an opinion piece by Mark Lynas in the LA Times, comes this stark appraisal of what would be needed to replace Japan’s nuclear fleet in terms of land area and staggering costs. These figure seem to back-up what has been posted here on BNC in the TCASE series and stunningly re-but the figures provided by Beyond Zero Emissions as critiqued on BNC.

According to some recent number crunching by the Breakthrough Institute, a centrist environmental think tank, phasing out Japan’s current nuclear generation capacity and replacing it with wind would require a 1.3-billion-acre wind farm, covering more than half the country’s total land mass. Going for solar instead would require a similar land area, and would in economic terms cost the country more than a trillion dollars.

http://www.latimes.com/news/opinion/commentary/la-oe-lynas-nukes-20110410,0,3424093.story
It is well worth reading the whole article as he makes many more convincing arguments in favour of nuclear power.

Like

Are there any estimates of level, trend and nature of the aggregate emissions from the site?
Given the estimate of a decade long D&D program,
these emissions look to remain a factor for some time, even if the process runs smoothly.
It would be useful to have some assurance that the more extreme projections of a large swath of cesium contamination are ill founded.

Like

@Gregory Meyerson:

Recriticality

You can Google SARA+recriticality+BWR, or
NRC+recriticality+BWR for more details. SARA was a
European study of recriticality in severe
accidents in BWRs, and there are also NUREGs
dating to the mid-70’s which discussed such
possibilities.

My understanding is as follows, recriticality in a
BWR has been considered possibile under the
following circumstances:

(1) A complete loss of station power occurs,
accompanied by complete loss of emergency core
cooling systems. The control rods are
automatically inserted at the beginning of the
accident, terminating the fission reaction.

(2) No active efforts are made by the operators to
mitigate the accident.

(3) A short time after the loss of power and core
cooling the upper core becomes uncovered.

(3) The control rods melt in a part of the upper
core, probably near to the center and top. This
can happen without a simultaneous melting of the
fuel rods, if the control rods are made of boron
carbide, since it has a lower melting point than
Zircaloy fuel cladding. Control rods are relocated
in this way, out of a part of the upper core,
leaving that area with close to normal reactor
geometry, but no control rods. But there is no
water present, only steam; and therefore there is
still no fission reaction.

(4) Just at this point, and before fuel rods in
the upper core begin to melt and relocate
downwards together with the control rods, the
electrical power is suddenly restored to the
plant.

(5) The partially damaged core is then flooded by
cold fresh water, due to the restarting of the
emergency core cooling pumps. Fresh water is the
moderator in a BWR. So re-critcality can now occur
in that part of the core which has been cleared of
control rods, but still has close to normal
reactor geometry.

(6) The fission reaction restarts in part of the
damaged core (re-criticality).

Since the control rods are now gone, the factors
that will limit the re-initiated fission reaction
are the natural ones that always operate in a LWR:
namely void formation and Doppler broadening.

That is: as the fission reaction speeds up, it
generates more heat. The additional heat boils
water, creating steam bubbles in the liquid
phase. Steam is less dense than liquid water, so
it doesn’t slow neutrons nearly as
efficiently. This void formation in turn slows the
fission reaction down.

Doppler broadening of the neutron spectrum also
occurs due to heat generated by the fission
reaction. Neutrons which hit nuclei in the fuel,
or in the cladding, or that hit oxygen nuclei in
the water scatter elastically if they aren’t
absorbed and don’t cause fission reactions. But
due to heat, the nuclei are also moving
around. Such elastically scattered neutrons gain
or lose a little energy as they scatter, and this
will change the overall energy spectrum of the
neutrons. The neutrons will gain energy on
average, as the scattering material heats up. But
thermal neutron induced fission cross-sections
also drop significantly with increasing neutron
energies, so the rate of the fission reaction is
in turn reduced as temperature increases.

Depending on the rate at which fresh cold water is
pumped into the damaged core region, faster being
worse, and also depending on other details, there
can occur a big pulse of reactor power in the
small fraction of the core that is partially
damaged. Depending on how big a pulse and how that
energy is dissipated, the reactor power can either
oscillate and finally die away, continue
oscillating, or possibly die down to some constant
low fraction of the total reactor power.

This is potentially a big problem, if the
operators don’t do anything at all to try to
mitigate the situation.

But the whole sequence of events can be avoided,
though, if the operators inject borated instead of
fresh water into the damaged core once power comes
back, or as soon as they have the ability to do
so. Borated water acts as a neutron poison (like
the control rods) and will prevent re-criticality.

That was actually done in this accident, as I
understand it. The operators pumped borated
seawater into the RPVs, over the damaged cores.

I imagine that uncertaintly about the extent of
damage to the reactor cores and worry about the
above scenario may have been the main reason for
injecting the borated seawater into the cores.

More complete melting of the reactor cores makes
recriticality very much less likely, because the
fuel then relocates downward in the RPV together
with the melted fuel rods. Moreover, the geometry
of completely melted fuel is extremely
unfavourable for forming a critical configuration
with reespect to thermal neutrons: there is no
room to fit the moderator inside a blob of molten
fuel cladding, control rods, and fuel pellets, and
any water that gets trapped or sits on top would
simply be boiled away by the heat.

A critical configuration with respect to fast
neutrons isn’t a realistic possibility, due to the
low enriched fuel that is used in light water
reactors. Critical masses with fast neutrons are
simply too large (even for the optimal spherical
geometry).

Notice that the above scenario for re-criticality
has to occur pretty early on in the accident. It’s
very hard for me to imagine a scenario in which
re-criticality occurs at late times, or once the
fuel has become significantly damaged.

I suppose conceivably, such a thing might occur,
for a vey small fraction of the core, if enough
shattered or disintegrated fuel pellets somehow
survived melting, and found their way into a pool
of fresh water, say in the suppression pool. But
such a reaction would be limited by the same
mechanisms mentioned above, as well as probably
others, like the ability of such fuel to flow away
from the critical region.

Like

George Bower, on 10 April 2011 at 1:25 AM said:

If you go to the latest JAIF report (http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1302343542P.pdf), water injection to containment vessel is a reporting item. Status reads : to be confirmed (R1), to be decided (seawater) (R2), and, to be confirmed (R3).

George: I do seem to remember seeing that item about injecting water to containment in some of the earlier JAIF updates, but I wasn’t sure whether injection to containment meant just a slow spray of water into the containment, possibly for cooling purposes, or an actual attempt to flood the containment.

I certainly agree with you that if containment flooding were to be accomplished, then all of the major drainage paths out of the PCV, below the level of the bottom of the RPV would need to be closed off as tightly as possible.

Like

etudiant, on 11 April 2011 at 12:54 AM said:

Given the estimate of a decade long D&D program, these emissions look to remain a factor for some time

There are lot’s of unanswered questions at the moment, but for radiation to get far it needs a transport mechanism. The first job will be to deny a transport mechanism.

Like

[Comment deleted. Violation of citation policy]
MODERATOR
Do not post just a throwaway remark and a link.

Citation policy is this:

BARRY BROOK
The commenting rules are not meant to be confusing, they’re meant to be logical. This is not a forum for cut-and-pasting slabs of text, with no other comment other than a link. Tell people why you think they should be interesting in reading this, and what it means for this discussion. Otherwise, you’re not thinking and not contributing. Simple as that.

Citing literature and other sources: appropriate and interesting citations and links within comments are welcomed, but please DO NOT cite material that you have not yourself read, digested and understood. As a general rule, please introduce any and every link or reference with a short description of the material, your judgement on its quality, and the specific reason you are including it (i.e. how it is relevant to the discussion).

Like

Question: when does TEPCO run out of qualified workers? I mean workers who have both required skills and useful remaining radiation allowance.

I would like to know how fast the existing TEPCO manpower pool is being depleted (I mean their remaining radiological allowances). A week ago WNN reported that 21 out of 370 workers had reached 100 milliseverts [thanks to Hank Roberts 4/11/11 2:01]:

http://www.world-nuclear-news.org/RS_Deaths_confirmed_at_Fukushima_Daiichi_0304111.html

It would help to have an idea what part of the remaining work requires high-skill and high-site-experience vs. just cleanup labour. Is extensive site-experience even a big issue?

The labour supply is defined by the radiological allowance. Is there a consensus on what the worker allowance should be?

Like

Steve Darden, on 12 April 2011 at 8:07 PM said:

Question: when does TEPCO run out of qualified workers? I mean workers who have both required skills and useful remaining radiation allowance>

A fair portion of the maintenance and upgrades that are done at NPP’s is performed during refueling outages. Since refueling only occurs every 18 months or so there is an industry pool of ‘temporary help’.

2011 US Columbia Operating Station budget – includes $10 million for temporary help and overtime during refueling outage.

Click to access Final%202011%20Columbia%20Generating%20Station.pdf

Like

@ David Kahana

“(3) The control rods melt in a part of the upper
core, probably near to the center and top. This
can happen without a simultaneous melting of the
fuel rods, if the control rods are made of boron
carbide, since it has a lower melting point than
Zircaloy fuel cladding.”

false

The boron carbide has the melting point 2763°C – http://en.wikipedia.org/wiki/Boron_carbide, Zircaloy has melting point much lower – The pure Zirconium itself has 1855°C melting point – http://en.wikipedia.org/wiki/Zirconium, (and is alloyed by Sn – melting point 231°C – and or Niobium – melting point 2477°C – to form Zircaloys) and the Zirconium even at much lower temperatures already violently reacts with any water present. So the described mechanism of the recriticality is a patent nonsense and I don’t wonder that it is again not sourced by any link whatsoever – as was several times called for by the moderater here…

Personally I think the recriticality claims about the Fukushima are red herrings (I was looking into it after there were the weird Te-129 readings which subsequently showed being errors) because even if the whole cores would melt through the cladding and end on the bottom of the RPV’s there’s not the unpoisoned (borated etc.) moderator (crucial, without it the chain reaction is impossible in the low enriched fuel) for it to regain criticality) The same for the spent fuel pools, moreover there are the fuel rods packed in the boron carbide racks.

Like

@Jan:

false

You’re right: it’s false, at least the way I summarized the
scenario.

But, not being an expert, I’ve summarized it wrongly. The melting
of the B4C is said to occur at 1500K, due to a reaction of the
carbide with stainless steel. Possibly stainless steel may be in
proximity with control rods in these reactors.

So the described mechanism of the recriticality is a patent
nonsense and I don’t wonder that it is again not sourced by any
link whatsoever – as was several times called for by the
moderater here…

There are many sources describing the scenario for
recriticality. Here’s a link to the SARA study:

ftp://ftp.cordis.europa.eu/pub/fp5-euratom/docs/09-sara.pdf

Recriticality has been studied for a total loss of
electric power accident scenario. In a BWR, the B4C control rods
would melt at about 1500K (due to a eutectic reaction between B4C
and stainlesss steel) and relocate from the core before the fuel
would during core uncovery and heat-up. If electrical power
returns during this time-window, the duration of which is
predicted to be in the range of a few minutes to about 40
nminutes, it is likely that the core will be reflooded using
water from Emergency Core Cooling Systems (ECCS) or feed water
supplies. Since this water is normally unborated, recriticality
is possible …

Here’s a link to a report produced at ORNL:

IDENTIFICATION
OF AND ASSESSMENT OF BWR IN-VESSEL SEVERE ACCIDENT MITIGATION STRATEGIES

which describes the injection of borated water as a strategy to
prevent recriticality in a BWR accident with severe core damage
when

” … control blades have melted and relocated from
the core.”

(from section 5. of this report)

NUREG/CR5653 documents the conclusion that there is a
potentiality for recriticality in BWRs, if core reflood occurs.

See:

http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0933/sec3/155r2.html

Personally I think the recriticality claims about the
Fukushima are red herrings (I was looking into it after there
were the weird Te-129 readings which subsequently showed being
errors) because even if the whole cores would melt through the
cladding and end on the bottom of the RPV’s there’s not the
unpoisoned (borated etc.) moderator (crucial, without it the
chain reaction is impossible in the low enriched fuel) for it to
regain criticality) The same for the spent fuel pools, moreover
there are the fuel rods packed in the boron carbide
racks.

I agree with you about all of the late recriticality claims, and
I would add that the early recriticality possibility seems to
require a perfect storm. But it has, at least, been considered a
possibility.

Like

Here’s the relevant NRC snippet on recrit. Thanks to David.

ISSUE 155.4: IMPROVE CRITICALITY CALCULATIONS

DESCRIPTION

The Board believed that doubts still remained as to whether the TMI-2 core became critical, or was very close to critical, during the TMI-2 accident and recommended that the NRC establish guidelines that deal with criticality following a severe reactor accident.1362 These guidelines should take into account abnormal geometries and possible core conditions that could result from the accident. The Board believed that the accident scenario developed by the TMI-2 licensee was sufficiently detailed that a series of geometric configurations could be simulated for criticality calculations. Variables that could be estimated reasonably well included the presence of water, oxidation of cladding, melting and movement of fuel, melting of poison rods, and movement of poison.

CONCLUSION

The safety concern was addressed by DSR/RES in SARP Task 4.3: Investigate the Possibility and Consequences of Recriticality in Degraded BWR Cores.1382 The staff’s study was documented in NUREG/CR-56531379 in which it was concluded that there was the potential for recriticality in BWRs, if core reflood occurs after control blade melting has begun but prior to significant fuel rod melting. However, a recriticality event would most likely not generate a pressure pulse significant enough to fail the vessel. Two strategies were identified that would aid in regaining control of the reactor and terminate the recriticality event before containment failure pressures are reached: (1) initiation of boron injection at or before the time of core reflood, if the potential for control blade melting exists; and (2) initiation of RHR suppression pool cooling to remove the heat load generated by the recriticality event and extend the time available for boration.

The issue was not considered to be a major concern for PWRs because of their design that includes a safety injection system for supplying borated water to the core. Furthermore, it was concluded in NUREG/CR-58561417 that, during a severe accident, an unmoderated recriticality of the molten, consolidated portion of a degrading core cannot occur at U235 enrichments characteristic of a PWR. Based on the staff’s efforts in addressing the safety concerns in the SARP, this issue was DROPPED from further pursuit as a new and separate issue. In an RES evaluation,1564 it was concluded that consideration of a 20-year license renewal period did not change the priority of the issue.

Like

“Sorry I have to spell it out — I thought it would be interesting that the Union of Concerned Scientists stated that nitrogen injection has never been attempted before (to their knowledge) after a reactor has been running.”

Nitrogen in the containment is quite standard feature of the newer BWR designs by default to prevent hydrogen combustion in case of a LOCA accident and yes it must be sometimes injected in the CPV during operation to maintain the pressure.

se e.g. here:

Click to access nuclear-power-plant-units.pdf

Like

@ David Kahana

I’m not much convinced about the mechanism of the recriticality you describe anyway:
The B4C-steel eutectic melting would be impossible in cases the RPV would be completely dry, because the Zircaloy rapid oxidation would be impossible in such a case. If it would still not be completely dry as is most likely and Zircaloy starts to burn with steam then even if triggering the control rod eutectic it would have much higher temperature – partly because the exothermic violent reaction with the steam, partly because of the decay heat of the fuel inside, so it would most probably be the fuel rods not the control rods which will loose integrity first.
I think the theorizing about this scenatio is based on tests like this:
http://cat.inist.fr/?aModele=afficheN&cpsidt=15755028
But they have major flaw – no fuel with very high after scram decay heat is involved inside the Zircaloy tubes.

Btw. Only a crazy would flood the RPV with even partially melted core with a fresh unborated water. It would be better to leave the RPV dry and flood the CPV instead – as the document you link suggests.

Like

I would be interested in a technical discussion on environmental aspects of Fukushima. Has anyone compiled available data into coherent picture? Particularly interested in total Cs deposition as this will be the important one going forward. Sea water more difficult but should be less of an issue due to high salts which should inhibit significant uptake in marine organisms. Fukushima prefecture is starting to look ugly. Depends whether highs are coherent or spot samples. Anyone?

Like

An 18 March update on radiation levels around Fukushima from the DoE at their blog, http://blog.energy.gov/content/situation-japan/

Although it’s helpful in some ways, I find it a little irritating that they have only given a map of the estimated one-year dose, they have saturated their colour scale rather early at 20mSv, and they have included doses in the month just gone, likely more than half the dose. Is anyone aware of an accessible (less processed) version of current dose rates?

Like

The following is a quote from a WSJ article in this mornings WSJ

“Under its strategic plan unveiled Sunday, Tepco is planning to fill the containment vessels of the plant’s No. 1 and No. 3 reactor units with water and employ heat-exchanging systems that could include an air-cooling solution. Similar plans are in place for the heavily damaged No. 2 reactor unit but must await the repair of leaks that are allowing the escape of highly radioactive water contaminated by the damaged fuel rods.”

The entire article can be found here.
http://online.wsj.com/article/SB10001424052748703916004576270643486719236.html

As read from the quote above, Tepco is planning to fill the “containment” vessels.

This must be an error. I think they are just going to cool the water in the PRV by creating a new closed loop cooling system for the RPV and cooling this loop via an air cooled heat exchanger similer to french design. I would assume also that the air cooling would consist of an evap cooling tower.

Can anyone clear up what is going on here??

Thx,
GSB

Like

A better description can be found at IEEE:

TEPCO Announces a “Roadmap to Restoration” at Fukushima Dai-1

A link is given there to the TEPCO description of the roadmap.

I don’t detect a mention of filling the containment vessels with water. Mention is made of increasing the level of water to cover the “active fuel”.

1 Maintain Stable Cooling
– Nitrogen gas injection
– Flooding up to top of active fuel
– Examination and implementation of
heat exchange function.
2 (Unit 2) Cool the reactor while controlling
the increase of accumulated water until the PCV
is sealed.

Sounds more like they want to increase the water levels in the RPVs and for unit 2 to control the additional release of water until they can fix the hole in the suppression pool …

Like

Oops, sorry, spoke too soon. In the second pdf at the TEPCO site:

Current Status [1] (Units 1 to 3) Cooling Achieved by water injection while there is partial damage to fuel pellets.

-Continued injection of fresh water and further cooling measures are required.

Countermeasure [1]: Injecting fresh water into the
RPV by pumps.
Risk[1]: Possibility of hydrogen explosion due to condensation of steam in the PCV when cooled, leading to increased hydrogen concentration.

Countermeasure [2]: Injecting Nitrogen gas into the PCV (start from unit 1.)

Countermeasure [3]: Consideration of flooding the PCV up to the top of active fuel.

Like

Thx for the response David,

Perhaps they are anticipating a situation where they can not stop the leakage from the PRV in R2. I don’t know how they would repair leaking control rod seals. Pump seals could more easily be fixed. At any rate it is something “under consideration”. I would think that R1 and R2 would be cooled without flooding the CV’s. Set up a new closed loop on the PRV cooling water using an “air cooled” heat exchanger and that this air cooled heat exchanger would use air cooled water towers.

Can anyone agree or disagree??

Like

Oops.

Revise last statement to R1 and R3.

I would think that R1 and R3 would be cooled without flooding the CV’s. Set up a new closed loop on the PRV cooling water using an “air cooled” heat exchanger and that this air cooled heat exchanger would use air cooled water towers.

Like

Chhers, George.

It would seem crazy to flood the PCV of R2 if there is a leak in the torus which is what I though they suspected … that would only make the leak worse.

So maybe they’re thinking about that only for R1 and R3?

For R2, it would seem they’ld have to fix the hole first. And that fix will have to be able to take the hydrostatic pressure, before they could flood it.
I also don’t see how they could accomplish that fix with very high radiation levels that may exist near the leak.

It would seem to me to be better to cut the flow rate to the RPV in R2 to the minimum necessary to continue cooling so as to limit the leakage, and like you say, try to set up some kind of air cooling with a closed loop, putting the heat exchanger in a lower radiation area.

Like

I think the other problem is that the CV may not be able to stand another earthquake when said CV is filled w/ water.

So in summary I just don’t see flooding ANY of the CV’s w/ water as a likely route they would take.

Perhaps we will get some video from the Robot. Apparently it gets it’s camera all fogged up w/ steam when they try to go inside the CV of R2.

Like

David,
Cyril R posted this in the other thread. We are keeping the CV flooding issue here in this blog.

CyrilR quote:

Cyril R., on 20 April 2011 at 3:34 AM said:

George, the latest JAIF update says ‘to be confirmed’ in the category water injection to containment vessel, for units 1 and 3. For unit 2, it says ‘to be decided (seawater)’.

Click to access ENGNEWS01_1303211759P.pdf

There is also the AREVA presentation which says they flooded at least 1 unit (unit 1 probably).

Like

George, yes, that’s a very good point about
the additional stress on the PCV and the
chance of another earthquake.

I did see the Areva presentation, and it
certainly suggested that flooding of R1 PCV had
been done. There were also hints that R1 had
had a LOCA caused by the earthquake before the
tsunami hit — it was in a video presentation by a Hitachi engineer. I’ll try to find a link.

I understand why they would want to flood the containment in such a case, but I imagined it
would only be up to the level of the PRV bottom
to try to prevent melting through.

About the JAIF updates: I’ve never been quite clear
what they mean by “water injection to containment.”

Is there a separate water spray system inside the drywell, to be used for cooling to condense steam? Or do they really mean filling the whole thing up?

I’ld really love to see the video from the robot. I hope
it works.

Like

Martin Burkle, on 18 April 2011 at 9:59 PM said:

Does anyone know how much spent fuel is stored at Fukushima in casks?

Don’t know how much fuel is stored in casks but Tepco has a picture of a cask and cask mover on their website.

Like

George Bower, on 20 April 2011 at 3:15 AM said:

Pump seals could more easily be fixed. At any rate it is something “under consideration”. I would think that R1 and R2 would be cooled without flooding the CV’s.

In my simple mind, I think the problem is that the seals that are leaking are located inside the containment vessel which would make the inside of the containment vessel inaccessible.

As far as the cooling system, it appears they are talking about an air cooling heat exchanger…I.E. A large radiator rather then getting the sea water pumps and condensers functioning again.

Click to access f12np-gaiyou_e.pdf

Like

Thx Cyril for the link, an excellent presentation. I found the reactor core isolation pump/cooling slide especially interesting. The reactor was able to run it’s cooling loop pump off the turbine that used reactor steam as an energy source. No outside source required for the pump, just battery power….pretty ingenious.

Also R1 went the longest w/o cooling.

Perhaps 1 or more of the wet wells ARE flooded as shown in the slide.

Like

Thanks GRLC. Only Iitate is above 2microsieverts/hr of those monitoring stations, and a rough calculation shows that the dose from 15-19 Mar at that location is equivalent to a year at the current dose rate (which will drop further). So for evacuation-lifting purposes, almost all areas should be re-opened unless there is a realistic threat of further releases from Fukushima Daiichi on the same scale as mid March. Certainly people should be allowed back to check on their houses and possessions, where the roads etc. permit.

Like

@George Bower – your PCV/containment flooding question
I confirmed the PCV flooding strategy in my comments on the 18th:

TEPCO’s briefing slides (4) are available here. My speculation that they would flood the PCV appears to be correct:

Countermeasure [9]: Flood the PCV up to the top of active fuel.

I agree with David – if they achieve their objectives in 9 months I will be impressed and relieved. My understanding is that TEPCO knows little about the actual state of the machinery inside the buildings. And they have no experience of doing such construction in a hot environment – with or without robots.

The TEPCO plan describes the strategy for R1 and R3 fairly clearly: first fix the leaks, seal the containment, then flood to at least the top of the fuel and recirculate and cool as you outlined. For R2 the torus leaks must first be repaired (the “sticky cement” plan?). The annotated TEPCO diagram was excellent.

I’ve found two well-informed sources of analysis on the Daiichi stabilization plan. KBMAN worked 25 years ago on the same generation BWR/Type I containment. His latest on the TEPCO plan includes this

One of the primary steps mentioned in their plan is to flood the containment vessels of units 1 and 3 up to the level of the fuel to guarantee the fuel remains covered and to assist in cooling. They are also looking at building external heat exchangers to be used in conjunction with filtration systems to be able to recycle the water being pumped through the reactors and also cool the water in the containments. This step also helps to provide more effective cooling for the fuel sediment on the bottom of the reactors by conducting heat away from the bottom of the reactor vessel.

They also wish to do this for unit 2 but must first repair the damage to the containment vessel. There is mention of using “sticky cement” to make this repair, I’m not sure exactly what they mean by that. This is presumably the leak caused by the explosion in or near the torus during the first week of the emergency. Meanwhile, the leaks in the containment at unit 2 appear to be flowing directly to the service trench in the turbine building. They pumped thousands of gallons only to have the level rise back up again. They may end up needing to seal up the secondary leaks and treat the entire system – trench and all – as the “reactor vessel”. Stop adding new water and instead process the water from the trench and inject it back into the reactor.

Will Davis offers very timely analysis. His first on the TEPCO announced plan discussed the PCV flooding strategy.

The detailed releases by TEPCO do also indicate the desire to flood the PCV’s or Primary Containment Vessels (dry wells) up to the top of the active fuel region. The few illustrations released so far are not of what you might call a highly technical nature but they seem to indicate at least feed to the plant through the normal feed line and then removal of the water from the suppression pool. There is also one that shows a totally separate closed circuit cooling system that circulates only through the pressure vessel and a heat exchanger. Clearly the plans are in a state of flux and we’ll have to see how this shakes out. To help you understand why some of these considerations are of interest to us, look at the following illustration of an earlier style BWR reactor pressure vessel and internals. You’ve seen this before on this site, but it’s been marked up for this article…

Like

@Joffan on dose rates

Thanks for your rough calculations and observation that “almost all areas should be re-opened unless there is a realistic threat of further releases”.

The latest DOE dose-map slide I’ve seen was headed First-Year…commencing March 16. So that’s good, it does not sum up the earlier exposures. But as I read the map everything hotter than the blue will continue to be an exclusion zone.

Am I missing something?

Like

Steve, I haven’t seen the future plans of the definition of the evacuation zone anywhere. I doubt they exist in any formal sense. For reference, the actual current evacuation zone is the inner circle on that presentation.

But I don’t understand your “that’s good” remark – the significant impact on these areas was from 15 March onward, so practically all the initial higher-level dose rates are included in the calculation for that map. I’d like to see a map that starts its year from now, or at least from the start of April. It would look very different.

The continuation of an exclusion zone when access would not be harmful is a repeat of the mass psychology mistakes identified after Chernobyl. This land is not blighted or uninhabitable and should not be treated as such.

Like

@Joffan

But I don’t understand your “that’s good” remark – the significant impact on these areas was from 15 March onward, so practically all the initial higher-level dose rates are included in the calculation for that map. I’d like to see a map that starts its year from now, or at least from the start of April. It would look very different.

My bad – I wanted DOE to be doing what you said. That “must be” why I saw March 16 and thought April 16. There are alternative age-related hypotheses for synapse meltdowns like that…

The continuation of an exclusion zone when access would not be harmful is a repeat of the mass psychology mistakes identified after Chernobyl. This land is not blighted or uninhabitable and should not be treated as such.

I agree. I’ve not done my homework to verify what the projected dose would be. My guess is the authorities will continue the currently defined zone until they are confident there will not be any more “excitement” with new releases from NPP1. I.e., they are excluding in fear of new releases rather than based on existing contamination.

Like

@harrywr2

Thanks heaps Harry. Hope to read tonight (today is full-on boat projects)

Have you a view as to how much of the 20km exclusion zone could safely be reoccupied (baring future increased released)?

Like

The authorities have been proven right to undertake precautionary evacuations, so I certainly would not rule that out as a rationale.

But absent the precautionary aspect, I would personally say that (at the level of detail available to me) the entire area can be reoccupied now – the highest dose rate in Iitate is barely three times standard background, which is nothing, health-wise. I would expect to improve the resolution of the radiological information, check for agriculture effects and perhaps consider certain limitations on activities likely to stir up dust. Unfortunately this would mean that rebuilding the tsunami-shattered coastal communities will be quite hard. Actually though the coastal strip is one of the less contaminated regions, so perhaps my suggested injunction on dust should be restricted to the NW-running plume of higher fallout.

Like

Steve Darden, on 20 April 2011 at 12:14 PM said:

Have you a view as to how much of the 20km exclusion zone could safely be reoccupied (baring future increased released)?

Personally I wouldn’t have a problem visiting anywhere except portions of the Fukushima Plant Site. Radiation levels at the Fukushima plant site are currently running from 9 uSv/hr at Monitoring Post 1 to 500 uSv/hr South of the main building.

http://www.tepco.co.jp/en/nu/fukushima-np/f1/index-e.html

Having said that, if I look at a terrain map it’s fairly mountainous. The rainy season is in June in Japan, so some places may end up being more radioactive by the end of June then they are now do to runoff.

So if I put my ‘prudent’ hat on I probably will wait until after the rainy season then look at collapsing the evacuation zone. The iodine will be gone and the cesium will either have run off or attached to the soil.

Like

Steve,
Thx for the updates on flooding the CV. I would think you might need two closed loops one for the PRV and one for the CV water. However maybe there is enough natural heat transfer into the steel/concrete containment structure so a separate loop wont be needed.

As to this “air cooled” heat exchanger. I would think it would be an air cooled water tower. What do you think??

Like

If they just want to get the temperatures down to cold shutdown levels or lower, maybe to 50 degrees Celcius or so, air cooling is great. It could just be radiators like the ones in your car.

Like

Here are some ground measurements from MEXT:
http://www.mext.go.jp/english/radioactivity_level/detail/1304099.htm
I would think there are still some “hot” places even outside the evacuation zone with over 10 mikrosieverts/h of just external dose, so I would think it would first need the real on the ground measurements inside the evacuation zone to assess where it is already habitable -for -I would recommend gradual and on real measurements based – lifting of the exclusion zone.
I think the people should be let back as soon as possible to avoid the effects of protracted stress, which could have in summe more adverse effects than the radiation itself – after the plant is safely stabilized – which I think can fully be achieved after the short-halflives and mainly the radioactive water are gone, especially I-131, Te-132, Te-129m, Rh-103 as much as possible (>8 halflifes) to make the work there easier.

I think the rain season in mid June-July will help very much, because the most affected areas (acording to NNSA measurements) are relatively very close to the shore and as I checked in GE the terrain is quite a slopy in most of the parts and not much people in fact inhabite the mountain area and the Iitate willage is there one of the biggest places. So I think the rains will take bulk of the remaining medium halflifes like Cs-137 with, maybe creating secondary hotspots, but I would not think it will be too extensive in such a terrain. The hotspots created by secondary accumulation is also much easier to burry, because of much more localized.
For idea what did just a short rain with the values -here is fresh example from Oarai:
http://www.jaea.go.jp/04/o-arai/Oantai_e/html/graph168.html
To me seems quite clear that vast majority of the exclusion zone will be without much risk habitable after the rain season.
So quite clearly Fukushima consequences will be most probably nothing even close to the Chernobyl -as some “scientists”, even from UN organizations are trying to make the world buy into.

Like

CyrilR quote:
,
“If they just want to get the temperatures down to cold shutdown levels or lower, maybe to 50 degrees Celcius or so, air cooling is great. It could just be radiators like the ones in your car.”

The only problem is they are very expensive. Believe me, I worked for Garrett (now Honeywell). A cheap water cooling tower would be a better solution. (just speculating of course)

Like

The U&W approximation is, I suspect, no longer close enough.

Who has access to an accurate, maybe ORIGEN2-derived, tabulation of the decay heat in the Fukushima reactors? (Not a chart, please, and certainly not a chart with the heat shown logarithmically. People don’t know how to read that format. It hides the snuggling-up of the trace to zero, and nuclear savants who don’t get this do their cause harm.)

Like

Chris – but water’s heat of vaporization is so large that you dont really get much steam produced… for back of envelope calculations i use 1000 btu/lb to boil water, so even if there were say twenty degree temperature above normal boiling — for every cubic yard, 1/50th of that cubic yard flashing to steam will cool the other 49/50ths of that yard back down to 212…..

unit 4, being in refueling, probably had its pool filled to a higher level than normal operation like units 1-3.

Also it had more elements stored in it, 1331 if i recall vs like 500 for others.

Ground acceleration was reported 1/2G. That amounts to an extra 1 million pounds on the floor just to accelerate the fuel, plus half of whatever the water weighed. If a pool were going to be hurt by the earthquake i’d expect the heaviest loaded one to fail first.

I’m sticking with pool cracked by earthquake, and the violent zirconium-water fire hurt the fuel.
look up zirconium’s MSDS – the powder burns better underwater than it does in air and releases about 2500 BTU/pound.

old jim

Like

20-25 C above boiling, at the bottom of the pool. Works out to tens of cubic meters of steam per cubic meter of water.

Like

Leave a Reply to Martin Burkle Cancel reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s