Can reactor fuel debris be safely removed from Fukushima Daiichi?

Date:
January 25, 2022
Source:
University of Helsinki
Summary:
Decommissioning and clean-up are ongoing at the Fukushima Daiichi
Nuclear Power Plant (FDNPP); however, many difficult problems
remain unaddressed.

Chief amongst these problems is the retrieval and management of
fuel debris.



FULL STORY ========================================================================== Decommissioning and clean-up are ongoing at the Fukushima Daiichi
Nuclear Power Plant (FDNPP); however, many difficult problems remain unaddressed. Chief amongst these problems is the retrieval and management
of fuel debris. Fuel debris is the name given to the solidified mixture
of melted nuclear fuel and other materials that now lie at the base of
each of the damaged reactors (reactor Units 1 -- 3). This material is
highly radioactive and it has potential to generate enough neutrons to
trigger successive nuclear fission reactions (uranium-235 breaks into two elements after capturing neutrons, emitting enormous amounts of energy, radiation, and more neutrons). Successive fission reactions would present
a serious safety and material management risk.


==========================================================================
One of the materials in nuclear reactors that can lower the number of
neutrons interacting with uranium-235 is boron carbide (B4C). This was
used as the control rod material in the FDNPP reactors, and it may now
remain within the fuel debris. If so, it may limit fission events within
the fuel debris.

Can the fuel debris be safely removed? On March 11th 2011, the control
rods were inserted into the FDNPP reactors to stop the fission reactions immediately after the earthquake, but the later tsunami destroyed the
reactor cooling systems. Fuel temperatures soon became high enough (>2000
DEGC) to cause reactor meltdowns. Currently, the fuel debris material
from each reactor is cooled and stable; however, careful assessment of
these materials, including not only their inventories of radioactive
elements but as well their boron content, a neutron absorber, is needed
to ascertain if successive fission reactions and associated neutron
flux could occur in the fuel debris during its removal. Many important questions remain: was boron from the control rods lost at high temperature during the meltdown? If so, does enough boron remain in the fuel debris to limit successive fission reactions within this material? These questions
must be answered to support safe decommissioning.

Study shows direct evidence of volatilization of control rods during
the accident.

Despite the importance of this topic, the state and stability of the
FDNPP control rod material has remained unknown until now. However,
work just published in the Journal of Hazardous Materials now provides
vital evidence that indicates that most of the control rod boron remains
in at least two of the damaged FDNPP reactors (Units 2 and/or 3).

The study was an international effort involving scientists from Japan,
Finland, France, and the USA. Dr. Satoshi Utsunomiya and graduate student Kazuki Fueda of Kyushu University led the study. Using electron microscopy
and secondary ion mass spectrometry (SIMS), the team has been able to
report the first-ever measurements of boron and lithium chemistry from radioactive Cs-rich microparticles (CsMPs). CsMPs formed inside FDNPP
reactor units 2 and/or 3 during the meltdowns. These microscopic particles
were then emitted into the environment, and the particles hold vital
clues about the extent and types of meltdown processes. The team's new
results on boron-11/boron-10 isotopic ratios (~4.2) clearly indicate that
most of the boron inside the CsMPs is derived from the FDNPP control rods
and not from other sources (e.g., boron from the seawater that was used
to cool the reactors). Dr Utsunomiya states that the presence of boron
in the CsMPs "provides direct evidence of volatilization of the control
rods, indicating that they were severely damaged during the meltdowns."
Ample boron likely remains in the reactors, but more research is needed
In the study the team also combined their new data with past knowledge
on CsMP emissions. From this, they have been able to estimate the total
amount of boron released from the FDNPP reactors was likely very small: 0.024-62 g.

Prof. Gareth Law, a co-author from the University of Helsinki emphasized
that this "is a tiny fraction of the reactor's overall boron inventory,
and this may mean that essentially all of the control rod boron remains
inside the reactors." The team hopes that this should prevent excessive
fission reactions in the fuel debris. Utsunomiya stresses that "FDNPP decommissioning, and specifically fuel debris removal must be planned so
that the extensive fission reactions do not occur. Our international team
has successfully provided the first direct evidence of volatilization of
B4C during the FDNPP meltdowns, but critically, our new data indicated
that large quantities of boron, which adsorbs neutrons, likely remains
within the fuel debris." Prof. Rod Ewing, a co-author from Stanford
University acknowledged the importance of these new findings but
highlighted that the team's measurements now need to be "extended
in follow-up studies, where the occurrence and distribution of boron
species should be characterized across a wide range of debris fragments."
Prof. emeritus Bernd Grambow, a study co-author from SUBATECH, Nantes, France,highlights that the work "paves the way for improving the safety assessment of debris retrieval during decommissioning at FDNPP," with the team's methods "providing a template for further studies." Utsunomiya
concludes that "it is nearly 11 years since the FDNPP disaster. In
addition to tireless efforts from engineers at the FDNPP, scientific contributions are becoming more and more important as tools to address
the major difficulties that will be faced during decommissioning." ========================================================================== Story Source: Materials provided by University_of_Helsinki. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Kazuki Fueda, Ryu Takami, Kenta Minomo, Kazuya Morooka, Kenji
Horie, Mami
Takehara, Shinya Yamasaki, Takumi Saito, Hiroyuki Shiotsu,
Toshihiko Ohnuki, Gareth T.W. Law, Bernd Grambow, Rodney C. Ewing,
Satoshi Utsunomiya. Volatilization of B4C control rods in Fukushima
Daiichi nuclear reactors during meltdown: B-Li isotopic signatures
in cesium-rich microparticles. Journal of Hazardous Materials,
2022; 428: 128214 DOI: 10.1016/j.jhazmat.2022.128214 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220125093041.htm

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