(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.
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.
(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.
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.