Thursday, August 7, 2014

(UPDATED) #Fukushima I NPP: Source of Neutron Detected on March 13-15, 2011 May Be Plutonium and Uranium Released by Core Melt

(UPDATED with clarification on the neutron dose rates in the translation, and information on the monitoring car at the bottom.)


that then somehow escaped from the Reactor Pressure Vessel, Containment Vessel, and finally Reactor Building, according to TEPCO.

TEPCO just released the latest reports of its on-going data analysis and simulation of the nuclear accident at Fukushima I Nuclear Power Plant that they've been doing since 2011. I'm reading the report, but for now I quickly share a short snippet that piqued my interest.

In sum, TEPCO now thinks the source of neutrons detected right after the start of the accident in March 2011 was the actinide species including plutonium and uranium that somehow escaped the reactors as the reactor core melted (which more or less coincided with the rise in pressure inside the reactors), and neutrons were emitted by spontaneous fission of plutonium and curium.

Neutron detection on March 13 is attributed to the core melt of Reactor 3, and detection on March 14/15 to the core melt of Reactor 2.

Just how did those actinide species escape the pressure vessels and containment vessels? TEPCO only says continued investigation is needed to understand the mechanism and to secure the safety of workers at the plant.

From one of TEPCO's reports titled "Relationship between the neutrons detected at the time of the accident and the core melt" (original in Japanese, quick translation is by me, subject to change later; part):


モニタリングカーは、3 月 13 日早朝、及び、3 月 14 日夜から 3 月 15 日未明にかけての二つの期間に中性子を検出した。検出された中性子の線量率は、中性子検出器の検出限度の 0.01μSv/h、および、その 2 倍の 0.02μSv/h と非常に小さい値である。測定場所は正門近辺であり原子炉建屋からかなり距離が離れた場所であることから、原子炉から直接飛来した中性子を検出したものではないと考えられるが、放射性物質が放出され発電所内のガンマ線の線量率が上昇したタイミングで中性子が検知されたという状況でもないため、これまで中性子の検出の原因については不明としてきた。また、発電所内の土壌から、ウランやプルトニウム等が検出されているが、その漏えいタイミング、経路についても不明としてきている。 ここでは、これまでに明らかになった福島第一原子力発電所 1~3 号機の事故進展挙動から、中性子検出についての説明を試みる。

Our monitoring car detected neutrons in two periods - in the early morning of March 13, and from the evening of March 14 to the early hours of March 15. The dose rates measured were extremely low: 0.01μSv/h (detection limit of the neutron detector) and 0.02μSv/h. The location where neutrons were detected was near the main gate of the plant, far away from the reactor building; thus it is considered that the neutrons detected did not come directly from the reactors. At the same time, the neutron detection didn't coincide with the rise in gamma ray dose rates as radioactive materials were released. So far, the reason why neutrons were detected has been deemed unknown. Uranium and plutonium have been detected in the soil inside the plant, but the timing of the leak and the leak process have also been undetermined. In this paper, we will attempt to explain the detection of neutrons based on the accident development behavior of Reactors 1 through 3 at Fukushima I Nuclear Power Plant which has been revealed so far.

(Detection of neutrons on March 13, 2011)

表1に 3 月 13 日に中性子が検出された時刻を示す。図 5 は正門付近での線量率(ガンマ線)の時間変化を示したものに、中性子の線量率が 0.01μSv/h となった時刻のプロットを青、中性子の線量率が 0.02μSv/h となった時刻のプロットを赤としたものである。中性子が検出された 3 月 13 日の午前は、9 時頃の原子炉減圧とそれに伴う S/C からのベントの影響で線量が上昇しているが、中性子検出とガンマ線線量率の変化に相関関係は見られない。すなわち、中性子検出は、ガンマ線線量率の上昇の原因となった放射性物質放出とは、関係しない現象によって引き起こされたものと考えられる。

Table 1 shows the time when neutrons were detected on March 13. Chart 5 shows the change of the dose rate (gamma ray) at different times near the main gate, with times when the neutron dose rate was 0.01μSv/h marked in blue and with times when the neutron dose rate was 0.02μSv/h marked in red. In the morning of March 13, the dose rate rose due to reduction of pressure in the reactor and vent from the suppression chamber. However, no correlation can be observed between the detection of neutrons and the change in gamma-ray dose rate. In other words, the phenomenon that caused the neutron detection was not related to the release of radioactive materials that caused the rise in the gamma-ray dose rate.

(Table 1)

(Chart 5)

一方で、厳密には一致しないが、原子炉水位の変化から予想される、3 号機で燃料溶融が発生したと推定される時間帯を考慮すると、中性子検出と燃料溶融の関連が示唆される。すなわち、燃料溶融により一部気中に放出されたウランやプルトニウムなどのアクチニド核種が、ガンマ線線量率の上昇を引き起こした放射性物質放出とは異なる経路で原子炉建屋外に漏えいし、これらに含まれるプルトニウムやキュリウム等の自発核分裂により放出された中性子が検出された可能性がある。実際、過去の核実験時のフォールアウトによって蓄積したプルトニウムと同程度ではあるものの、プルトニウムの同位体組成から、明らかに福島第一原子力発電所での事故起因と考えられるプルトニウムが、発電所構内の土壌中から 検出されている。

On the other hand, although it is not an exact match, when we consider the time period when the core melt started in Reactor 3, as estimated from the change in water levels in the reactor, there may be a relationship between the detection of neutrons and the core melt. In other words, some actinide species such as uranium and plutonium were generated because of the core melt, and leaked outside the reactor building through a different route than the one in which the release of radioactive materials that resulted in the rise in the gamma-ray dose rate took place. In fact, plutonium has been detected in the soil inside the plant; even though it is about the same level as plutonium accumulated in the soil from the past atmospheric nuclear testing, it is clearly originated in the Fukushima I Nuclear Power Plant accident as determined by the isotopic composition.

なお、中性子が検出された正門付近は、図 6 に示すとおり約 1km 距離が離れていることから、原子炉からの中性子を検出したものである可能性は小さい。

The main gate where neutrons were detected is about 1 kilometer away from the reactors as shown in the Chart 6, and the possibility is small that neutrons from the reactor were detected.

(Chart 6)

(Detection of neutrons on March 14/15, 2011)

一方で、図 7 に中性子の検出時間と 2 号機の原子炉・格納容器圧力の関係を示すが、中性子は原子炉圧力の上昇が始まった後に検出されている。前述の通り、この原子炉圧力上昇は、消防車による注水が炉心部に到達し蒸気が発生したことによると考えられ、また、この際には水-ジルコニウム反応が発生して大量にエネルギーが放出され燃料が溶融したと考えられる。すなわち、原子炉圧力の上昇のあった時間帯に発生した燃料溶融によりウランやプルトニウムなどのアクチニド原子が一部気中に放出され、ガンマ線線量率の上昇を引き起こした放射性物質放出とは異なる経路で原子炉建屋外に漏えいし、これらに含まれるプルトニウムやキュリウム等の自発核分裂により放出された中性子が検出された可能性がある。

Chart 7 shows the time when neutrons were detected and the pressure inside Reactor 2's reactor/containment vessel. Neutrons were detected after the pressure inside the reactor started to rise. As we said before, this rise in pressure in the reactor is considered to have occurred when the water injected from the fire engine reached the reactor core and generated steam. This also caused the water-zirconium reaction which released a large amount of energy, causing the fuel to melt. In other words, our hypothesis is that core melt took place during the time period when the reactor pressure rose, releasing part of the actinide atoms such as uranium and plutonium. These actinide atoms then leaked outside the reactor building through a different route than the one in which the release of radioactive materials that resulted in the rise in the gamma-ray dose rate took place. Then the spontaneous fission of plutonium and curium released neutrons, which were detected.

(Chart 7)

I'll try a separate post on the mechanism of Reactor 2's core melt, which was accelerated, or so TEPCO thinks, by water injection by the fire engines.


As far as I remember, there was one monitoring car at the time of the accident at Fukushima I NPP. I'll try to verify, but I do recall wondering aloud why there wasn't any more monitoring car at the plant. There was no electricity at the plant during the time period this TEPCO's paper covers, and the plant's regular monitoring stations were not working (as they operate on electricity). Power to the plant wasn't restored until early April, 2011.


Anonymous said...

'that somehow escaped the reactors as the reactor core melted ' indicates the thing was already broken before it exploded. Meaning, it was the earth quake (too?) that caused all this mess, something Tepco and their nuclear buddies want to prevent to come out, especially because the power of the earth quake at the fukushima plant was something around 6,5 or so, and NPP should be able to withstand that.... Think about it, what that will mean for all excisting NPP...

Anonymous said...

Is this the ONLY monitoring 'car' which was available? Are there other sources with dosimeters able to measure neurons? The dose measured was relevant or dangerous it that..since its the highest measureable, think that means bad.
What did the neuron release do to the humans in the bombardment area? Does anyone know the sideeffects. Guess looking at the sailors on the USS Ronald Reagon might be a starting point as over 1500 of the 5 of 6 K crew are now reporting issues. Sadly, in Japan, its not reported, we really need to know what devils have been released.

Anonymous said...

When will Japan/TEPCO admit there was a nuclear detonation? A nuclear explosion..not just hydrogen...

Anonymous said...

The release of actinides and caesium / iodine is expected to be somewhat different: Cs + I are volatile at high temp and can be directly released as gases, whereas actinides will need to be entrained, i.e. as water droplets, from water that was in contact with the melt. So nothing very surprising IMHO.

Anonymous said...

Just a bit of math, but neutrons move at about 2200 meters per second. Anyone in a 60 mile radius of Fukushima would have been exposed in 43 seconds. US reported .6 of gamma 'sky shine' on the 15th of March. How long did TEPCO measure the neutron radiation? Once, days? Ongoing?

Anonymous said...

"The main gate where neutrons were detected is about 1 kilometer away from the reactors .."

As 4:27 noted above was the measurer the only one, and did he retreat after measuring it?

Obvious it was airborne, see "pluming" ..

Anonymous said...

- "'that somehow escaped the reactors as the reactor core melted ' indicates the thing was already broken before it exploded. "
Nope. That time the accident was already beyond DB. Damage from the EQ cannot be ruled out but it's not a required to get the given results.

- "The dose measured was relevant or dangerous it that..since its the highest measureable, think that means bad. "
Actually, I think it's the lower limit of that equipment, not the higher. If you think back: in some cases when the 'beyond limit' was confirmed it was over 1-10 Sv, not uS.

- "What did the neuron release do to the humans in the bombardment area? "
Practically nothing. A full fledged N-bomb is needed to cause for lethal effects within a kilometer or so. The measured strength is negligible, has only scientific/engineering relevance.

- "When will Japan/TEPCO admit there was a nuclear detonation? "
There was no nuclear detonation.

- "Just a bit of math, but neutrons move at about 2200 meters per second. Anyone in a 60 mile radius of Fukushima would have been exposed in 43 seconds. "
At the distance of the gate the strength of N-radiation was barely measurable. Any further away it was much lower than the natural background.

Yosaku said...


Great article. I'm always impressed with your translations, but I think you missed the following phrase entirely in the second sentence of the first translated paragraph: "...と非常に小さい値である。" (i.e., that the measured values were extremely low). I think that might have saved Anonymous at 4:27am some heartache!

Anonymous at 4:40am and 12:23pm, thank you for your sanity.

arevamirpal::laprimavera said...

Yosaku, thank you. I somehow managed to skip the entire phrase, didn't I? It's now put in.

Additional info at the bottom of the post about monitoring car/posts.

Anonymous said...

From Wikipedia:
"Exposure to free neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to molecules and atoms, and can also cause reactions that give rise to other forms of radiation (such as protons)."

Interesting how the graphs and data are tailored not to show all the inventory being released. Not that TEPCO could have monitored it all in such high dose rates. All explanations of events are guesses by TEPCO with caveats with a bent to avoid mentioning criticalities which doesn't have to include sustained chain reactions. They do mention spontaneous fission which is fissile materials overloaded with neutrons fracturing on their own into two or three pieces which leaves a signature.

TEPCO does the kabuki around U(s) and Pu(s) distilling out first like they are missing somewhere from the equation. Fissile U and PU doesn't require water to react, hydrogen is also a moderator. Plus the unknown of a couple thousand, hour or less, half lives of daughter products created besides some new parent elements or tritium assists a nuke pit but the apologist don't believe a melt could configure itself to fissile a little bit even with Unit 3's core being 1/4 MOX about 5% Pu rich or about a ton of Pu just a tiny bit to deal with, huh? Not counting any new creation. Designed supposedly not to have a runaway reaction but that is in a controlled environment, melting down adds new parameters. What if the Pu settled together, wouldn't that suck? Plus Unit 3 was without any water, heat generation must have been tremendous causing everything to vaporize ending in a fireworks show. So you have nuke solids turning atomized and 3 meltdowns going on at once in various stages. Makes the facts as clear as a bell. The neutron beams were clearly observed from choppers in the aftermath of the explosions.

They knew Unit 3 was about to blow and evacuated. Unit 2 caught them off guard or when do you add water or when do you don't add water during meltdown stages? Autopsy the couple of firemen who died when Unit 2 went lit off, might give some clues. On the job training in real time.

Flame away.

Anonymous said...

ex-SKF: " .. I do recall wondering aloud why there wasn't any more monitoring car[s] at the plant. "

One thing that could be counted on from the workers remaining at the plant was their paying quite close attention to these types of measurings, watching events going "progressively north", as it were. Watching supervisors growing increasingly alarmed as their options of response were occluded.

Somehow, thinkings like 4:40's "actinides will need to be entrained, i.e. as water droplets" were quite likely not in effect.
Seriously, cores are melting down at super-high temperatures and actinides "need to be entrained" to be detected?!

12:23's "- "What did the neu[t]ron release do to the humans in the bombardment area? "
Practically nothing." fails to metion the reason nothing is because they evacuated from the neutron sources' areas. Did the helicopter stay within the beam's path? Would you expect those pilots to do so?

The reason Tepco's being sued is they knew the reactors were melting down and suppressed the information.
Yosaku, "heartache" from 4:27 can be avoided if the foreigners accept that they must accept what their hosts feel is appropriate for they stay?
'Sadly, in Japan, its not reported .. ", other than by ... Yosaku?

Anonymous said...

I might add, as TEPCO drips information out slowly, that JNES (old name?) has diagrammed the most likely areas of failure due to where venting/leaking would occur. The most notable area beside the commonly known seal(s) O ring for the caps that are first to fail with overpressure then heat is a 'top head manhole' for primary containment access positioned somewhere near the edge or between the sectional plug and SFP. Notice all the pictures of blown Unit 3 showing deformed and discolored roof tresses that sag into a certain location, neither directly above the SFP or any other corresponding area except above this access manhole location. If the manhole lid went missing, any venting from primary containment would go straight up through a narrow hole. Later during during de-construction and cleanup, pictures of the area of the working floor show some type of equipment sitting on and over where this manhole would be located. Fascinating.

Page 67 of 82 Titled: Places of Possible Leakage (Example of Type-I Mark Reactor)

Elsewhere, there are pics and videos of Unit 3's operation floor during and after cleanup showing the area in question.

Anonymous said...


On page 35 of the pdf any idea what they mean by "Counterflow"?, especially at Fuku Daiini?

Anonymous said...

August 9, 2014 at 5:58 PM : Not sure what they are referring to but counterflow usually means water or liquid flowing through piping or plumbing is reversed and if one-way valves or flappers are present, did not function properly or as designed ex. Surface storm drains and their connecting pipes fill at high tide causing street flooding.

Anonymous said...

For some reason people still wish to believe there was a nuclear explosion at Fukushima. People still cling to buzzwords like skyshine and hot particles, neutron beams, etc. Rather than raise the level of knowledge about nuclear physics, I fear this accident has made everyone a bit more stupid. I just read a news article from February 2014 that said the USS Ronald Reagan "is now so radioactive it cannot be scrapped or used again", which is nonsense that can be easily verified as such.

Anonymous said...

August 9, 2014 at 11:08 PM:
Is there a reason they can't remove the melted cores at Fukushima, Daiichi and clean the mess right away or does it have something to do with nuclear physics?
Why can't they stop the radioactive runoff into the Pacific.
How much money does nuclear physics require to find a solution to spent fuel storage?
Does nuclear physics include cancer rates studies or is that not germane to the nuclear industry?
You read as a little numb to the situation, is that by design?

Anonymous said...


Let's define-down your "a bit more stupid". Refresh us as to when neutrons are detected in nuclear materials.

Anonymous said...

Neutron emission events tend to be associated with criticalities. So 11:08, let's not present the lack of measures as more of your "a bit more stupid". Speak in terms of what's understood about nuclear events like neutron detection.

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