TEPCO Confirms Fuel Microparticles At Daiichi Reactors

TEPCO has confirmed new details about the existence of fuel microparticles at the three reactors that suffered meltdowns. Deeper detail of the type and composition of these microparticles provides hints about the meltdowns and what may be encountered as they attempt to clean up the disaster.

We have documented this problem extensively here and here.

In TEPCO’s new report they provide analysis of collected substances. Some of these were collected as swipes taken from inspection equipment used inside containment. The sample from unit 1 was a collected sediment. One of the unit 2 samples was a swipe taken near the reactor well on the refueling floor.

The samples were tested by electron microscopy (SEM/TEM), conducted in FY 2018.
TEPCO’s report identifies the samples as follows (machine translated):

(A) Unit 1 PCV of sediment (float) at the bottom of the bottom sediments in April 2017

(B) Unit 2 PCV swipe of internal investigation equipment (Camera part Deposit)
The surface of the pan-tilt camera used in the internal investigation of containment The one that wiped off. Water droplets falling from the top of the pedestal. in January 2018

(C) Unit 2 Reactor Building Operating floor Curing sheet. It was near the shield plug of the top floor of the reactor building. A collection of curing sheets March, 2014.

(D) Unit 3 PCV swipe of internal investigation equipment. (Underwater ROV deposit)
The surface of the internal inspection apparatus (underwater robot) of the Unit 3 container
Wiped off July, 2017

Notes from the report on the samples in photos below:

U (uranium) is scattered in the micro-area of the μm order (left figure below).
Uranium dot the small area (B) PCV internal investigation of Unit No. 2 (also left figure below)
This is a common phenomenon in the whole unit, Uranium-containing solid particles are considered.
* whole unit likely refers to units 1-3. 

Zinc detected at high concentrations (A) Unit 1 PCV Bottom sediment (middle figure below)

Sample contains a large amount of mineral components, but is detected as a fiber-shaped Ca (calcium)
, It may be a component derived from the heat insulating material rather than a component derived from concrete. Fiber-shaped Ca (B) PCV internal investigation of Unit No. 2 (right figure below)

The citation by TEPCO that they found a fiber type calcium particle does not indicate that there was no concrete involvement in unit 2. Similar fuel particle samples found in the environment indicate concrete involvement. The fiber particle is a positive finding of insulation material, not indicative of an over all negative finding for concrete. The types of glass matrix found in the various types of microparticles can be caused by sufficient amounts of sea water, concrete or certain types of insulation due to the silica content. Further research into the properties of different types of microparticles may help determine their origin and behaviors during the meltdowns. No matter the origin of the silica, local temperatures over 2500c would be required to create these. These tiny particles would then solidify as they rapidly cooled.

This 2012 photo shows the concentrated steam leak out of the unit 2 reactor well. The cloud of steam can be seen blocking out the objects and light behind it. This is a likely point where some of the found microparticles left containment.

 

 

5. TEM analysis Result (a) Unit 1 PCV Bottom sediment
• The particles detected from the bottom sediments PCV Unit 1
Was about 2µm (left figure below).
• (U, Zr) O2 mother phase (bottom right figure analysis Point (1), (3), (4), (5))
The internal high ZR region (Analysis Point (2)) was confirmed.
• The region (2) is separated from the (U, Zr) o2-x during the cooling process It is considered α-Zr (O) phase (right figure).

TEPCO’s analysis shows the microparticle includes a section higher in zirconium (marker 2) compared to the rest of the microparticle that is higher in oxygen and uranium. Zirconium is used in nuclear fuel cladding. The presence of it may give some information about the creation of these specific microparticles.

 

6. TEM analysis Result (b) Unit 2 PCV internal investigation equipment Deposits (camera part deposit)
• Similar particle PCV internal investigation of camera No. 2 Detected from the smear sample (top figure).
• The composition of this particle is relatively uniform and Is considered to be the composition of (Zr 0.64, U 0.36) O2 (Right figure below).
• The presence of tetragonal crystals was shown as a result of electron diffraction (left figure below). 
• The possibility of being quenched from high temperature is suggested.

Points 1 to 5 tetragonal presence electron diffraction analysis (left above)

 

7. TEM analysis Result (c) Unit 2 operating Floor curing sheet
• The sample collected at the top floor of Unit 2 detects U-rich particles that contain almost no Zr (both the lower figure).
• The particles in the left figure look like secondary particles with a diameter of about 100nm particles aggregated.
• The particles in the right figure, the aggregated particles are visible to the particles became dense spherical crystal growth.
• These particles may be generated by the mechanism of evaporation condensation is suggested.
ZR is difficult to evaporate since it is, it is believed that the present amount is reduced to the particles produced by evaporation condensation.

These two microparticles from unit 2’s refueling floor include some key information provided by TEPCO. These are smaller than the other particles in TEPCO’s analysis. They lack significant amounts of zirconium. The left particle has notable amounts of chromium, iron and nickel. Iron, nickel, and chromium are primary components of stainless steel types SS 304 and SS 316. These types of stainless steel are frequently used in reactor metal parts. The breakdown of these components from the stainless steel indicates high levels of heat that caused this reaction.

TEPCO considers these to have been formed by “evaporation” due to the lack of zirconium. TEPCO’s characterization of evaporation may be the same mechanism we have referred to as volatilization or vaporization. Vaporization is evaporation at a faster rate with higher temperature. Our estimation is that the high heat of the meltdown in the reactor vessel caused the fuel to vaporize, creating microparticles. In the case of unit 2, these fuel microparticles leaked via the reactor well and out the blow out panel to the environment and likely leaked at other points of containment failure.

8. Results of TEM analysis (d) PCV bottom sediment sample unit 3
• Particles detected from the water ROV’s smear sample used in the internal investigation of the PCV Unit No. 3
•Is a composition in which the Zr-rich region and U-rich region are mixed, voids are in the boundary of the region (See below).
• The ZR-rich region is dense, whereas the U-rich region has a fine fissure.
• Produced in a process different from the tissues caused by phase separation during the cooling process of the molten core May be a particle.

TEPCO indicates that the zirconium exists in a different way than the uranium. Locations 1, 2, and 3 have higher uranium content than others. Locations 7, 6, 5, and 4 have higher zirconium. TEPCO notes that the uranium locations have fine fissures. They think different behaviors created these microparticles compared to the others in this report. This microparticle does appear to be more jagged than the unit 2 microparticles. Many of the black microparticles found in the environment have a jagged, conglomerate type appearance rather than looking like smooth spheres. A worker at Daiichi who was outside when unit 3 exploded described being covered in black soot. It isn’t clear if this indicates the worker was covered with microparticles. Unit 3’s explosion did seem to have many different behaviors compared to the explosion at unit 1 and to any of unit 2’s behaviors.

TEPCO provided the diagram below where they explain their theory of how some of the microparticles at unit 2 were created.

 

Machine translation of the diagram:

9. Phenomenological generation mechanism of uranium-containing particles
* label on the graphic: Molten fuel

Upper box translation:
Type II particles (evaporation and condensation process)
• Evaporation process involved from U-rich (low Zr) composition
(Zr is poorly volatile).
• 100nm order by condensation of evaporation components
Particles produced, through the secondary aggregation process It was estimated that it was the growth of micron particles.

Lower box translation:
Type I particles (droplet solidification process)
• Droplets generated during the fall of the core melt, etc.
Assumed to be solidified.
• (U, Zr) O2, (Zr, U) O2, α-Zr (O), etc.
The composition and the organization are close to the fuel debris body .

Bottom box translation:
On the second operating floor, both types of particles have been found and For α contamination, not only the type I particles, but also the type II particles may be involved.

This explains in more depth, what is detailed earlier in this report on the composition of these microparticles. TEPCO notes the alpha radiation contained in both types. Alpha radiation can be blocked by paper. The glass that incorporates these substances may provide shielding from the alpha radiation. Studies elsewhere have shown that the glass may break down over time later exposing the alpha radiation to the environment. In the case of inhaled or ingested particles this could mean internal alpha radiation exposure of the person or animal. This erosion potential combined with the long lived uranium isotopes create a long term risk factor from these microparticles. TEPCO notes that the microparticles appear to identify their creation source by the amount of zirconium in them. That low zirconium microparticles were created by fuel vaporization in the reactor vessel and higher zirconium microparticles were created in containment.

10. Overview of the results of TEM analysis (sample in PCV)
• The chemical composition is different for each sample, those of the oxide of the subject U, those containing Zr-rich oxide, those containing α-Zr (O) are observed.
• The sample of Unit 3 is accompanied by voids and fine fissures. These are observed in the other unit. Tissue is different from the sample seen as phase separation in the cooling process of the core melt.

More data for these unit 3 microparticles would be useful towards confirming if there are specific types of particles unique to unit 3. The jagged nature and fissures appears to set these microparticles apart from the others. Microparticles found in the environment outside the disaster site have been of distinct types including round glass spheres and black aggregates of materials with a very lumpy or jagged look.

11. Summary of TEM Analysis Results (Unit 2 Operating Floor Sample)
• In U-rich, almost Zr-free particles, a sphere indicating the possibility of generating during evaporation and condensation
characteristic particles of the form have been observed (type 1).
• Particles containing a considerable amount of Zr may be derived from the core melt, and those in amorphous (type 1).

Location: (C) Reactor Operating Floor
Type Particles

(lower right text)
Fe is the main
Note: The scale varies from diagram to figure

 

12. Summary
Sem-TEM analysis focusing on uranium-containing particles was carried out to obtain the following findings.
Uranium-containing particles include particles derived from core melt (type 1) and evaporation
There is an estimated particle (type II) generated in the condensation process.
Was confirmed.
 Fuel debris may have the same composition and structure as Type I particles
There is a character.
 Not only type I particles, but also alpha contamination (actinide behavior) inside the building
Type II particles may also be involved.
Currently, a study on small sampling from fuel debris in the pedestals.
As a result of the sample analysis of the inside of the containment vessel which we carried out this time,
And the sample handling experience gained through
We think that we will utilize for analysis of the resamples and to examine methods of handling them.

TEPCO’s conclusions, that the fuel debris may contain microparticles or microparticle similar inclusions and that these particles appear to have a distinct formation process based on location, tracks with some of the information discovered in environmental microparticle studies.

It appears that TEPCO plans to use these same analysis methods on the pedestal fuel debris samples that are planned for collection during future inspection work.

TEPCO has made incremental admissions of the existence of microparticles. The need to confirm the actual conditions inside containment and of the fuel debris should continue to force the admission of findings such as these.

One unexpected finding from this report was that the sand like deposits inside unit 1’s containment includes microparticles of fuel. These microparticles appear to be fuel debris type microparticles. These are the type TEPCO assumes were generated in containment by various corium (fuel debris) behaviors and not the in core vaporized fuel variety like what was found on unit 2’s refueling floor.

All of the samples tested by TEPCO found higher levels of uranium in them vs. cesium 137 or 134 as seen in the graphs for each sample. This appears to differ from some of the microparticles found in the environment far from the disaster site. Examples here and here. This study of microparticles in the environment DID find traces of Fukushima Daiichi derived plutonium and uranium in them. The latter study sampled materials deposited in the exclusion zone, near the highest levels of the fallout plume.
These various microparticle differences merit further exploration.

 

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