Reading and Answering 2
7 AUGUST 2020 • VOL 369 ISSUE 6504 621SCIENCE sciencemag.org
receptor with which they form a heterodi-
mer, but data are currently lacking.
Lotus and medicago have a narrow rhi-
zobial host range, which, at least in part,
can be explained by the occurrence of spe-
cific ligand recognition motifs in LjNFR1
and MtLYK3. However, several legumes are
more promiscuous and can establish root
nodules with a wide range of rhizobium
species that produce Nod factors with dif-
ferent structures. It should be feasible to
model the corresponding Nod factor recep-
tors and identify the structural character-
istics of such promiscuity.
An important issue is the evolutionary
origin of Nod factor perception in nodula-
tion. Nodulation is not specific to legumes,
but occurs in 10 plant lineages in four taxo-
nomic orders. It has been proposed that
nodulation has a single evolutionary origin
(~110 million years ago), driven by an acyl-
ated CO-producing, nitrogen-fixing Frankia
bacterium (14). Among nodulating nonle-
gumes, Parasponia (Cannabaceae) is the
only lineage that is nodulated by Nod factor–
producing rhizobia, and the corresponding
receptors have recently been identified (13).
Notably, Parasponia did not experience a
duplication of the CERK gene. Instead, a
single LysM-type receptor fulfills multiple
functions, including CO-induced innate im-
munity, AM symbiosis, and rhizobium Nod
factor–induced nodulation (13). These ob-
servations suggest that the ancestral gene
from which the legume Nod factor recep-
tors evolved already encoded a LysM-type
receptor that could perceive COs as well as
acylated COs. In legumes, the duplication of
this gene may have allowed the evolution of
highly specific Nod factor receptors. Subse-
quent coevolution of Nod factor structure
and the receptor ligand–binding site could
have resulted in host specificity through a
key-lock system, which is considered an im-
portant driver in the evolution of efficient
symbiotic systems (2). j
REFERENCES AND NOTES
1. C. Zipfel, G. E. D. Oldroyd, Nature 543, 328 (2017). 2. P. Remigi, J. Zhu, J. P. W. Young, C. Masson-Boivin, Trends
Microbiol. 24, 63 (2016). 3. Z. Bozsoki et al., Science 369, 663 (2020). 4. F. Maillet et al., Nature 469, 58 (2011). 5. A. Genre et al., New Phytol. 198, 190 (2013). 6. P. Lerouge et al., Nature 344, 781 (1990). 7. T. V. Nguyen et al., BMC Genomics 17, 796 (2016). 8. K. R. Cope et al., Plant Cell 31, 2386 (2019). 9. S. De Mita, A. Streng, T. Bisseling, R. Geurts, New Phytol.
201, 961 (2014). 10. Z. Bozsoki et al., Proc. Natl. Acad. Sci. U.S.A. 114, E8118
(2017). 11. C. Gibelin-Viala et al., New Phytol. 223, 1516 (2019). 12. F. Feng et al., Nat. Commun. 10, 5047 (2019). 13. L. Rutten et al., Plant Physiol. 10.1104/pp.19.01420
(2020), 14. R. van Velzen, J. J. Doyle, R. Geurts, Trends Plant Sci. 24,
49 (2019).
10.1126/science.abd3857
By Ken O. Buesseler
I n the time since Japan’s triple earthquake,
tsunami, and nuclear disaster in 2011,
much has improved in the ocean offshore
from the Fukushima Daiichi Nuclear
Power Plant (FDNPP). Concentrations of
cesium isotopes, some of the most abun-
dant and long-lived contaminants released,
are hundreds of thousands of times lower
than at their peak in April 2011. Since mid-
2015, none of the fish caught nearby exceed
Japan’s strict limit for cesium of 100 Bq/kg
(1, 2). Yet, enormous challenges remain in
decommissioning the reactors and clean-up
on land. Small, and sometimes unexpected,
sources of contaminants still continue to
enter the ocean to this day (3). Two of the
biggest unresolved issues are what to do
with the more than 1000 tanks at the site
that contain contaminated water and the
impact of releasing more than 1 million
tons of this water into the ocean.
The tank water is a combination of recov-
ered groundwater and deliberately injected
cooling waters, both of which became con-
taminated when interacting with the highly
radioactive nuclear reactor cores. From the
first months after the earthquake and tsu-
nami, these waters were contained in tanks
to prevent further radioisotope releases and
remediated by using several systems, most
notably the Advanced Liquid Processing
System (ALPS). ALPS was designed to effi-
ciently remove more than 62 different con-
taminants. The installation in an ice dam
and other groundwater barriers, as well as
the diversion of groundwater flow around
the site, also assisted in reducing the daily
accumulation of water from more than 400
to less than 200 metric tons per day.
Despite this effort, no decontamination
system can remove 100% of all radioactive
contaminants. Tritium, 3H, is notoriously
difficult to remove because it is a radioac-
tive form of hydrogen that is part of the
water molecule itself. Fortunately, tritium is
relatively harmless because it emits a low-
energy b particle that does little damage
to living cells. As a result, tritium has the
lowest dose coefficient for those radioac-
tive isotopes reported in the tanks (4) and
higher allowable release limits (see the ta-
ble). These properties do not detract from
the potential for large amounts of tritium
to have harmful effects, and debates remain
about the potential health effects.
The total amount of tritium contained in
the tanks also matters, which is reported to
be around 1 PBq (PBq = 1015 Bq) (5). That
total is far less than the 8000 PBq of tri-
tium still remaining from global atmo-
spheric nuclear testing in the 1960s or the
2000 PBq from natural interactions be-
tween cosmogenic particles and nitrogen
that form tritium in the atmosphere. In ad-
dition, all nuclear power facilities emit tri-
tium that, depending on plant design, can
be several PBq per year, or even higher, as in
the case of nuclear fuel reprocessing plans
such as at Cap de La Hague (6).
However, this story is not only about tri-
tium but what else is in the tanks. It was
not until mid-2018 when TEPCO, the op-
erator at FDNPP, released data detailing the Woods Hole Oceanographic Institution, Woods Hole, MA, USA. Email: kbuesseler@whoi.edu
NUCLEAR WASTE
Opening the floodgates at Fukushima Tritium is not the only radioisotope of concern for stored contaminated water
ISOTOPE MAX RELEASE (BQ/LITER)1
FOOD LIMIT (BQ/KG)2
HALF-LIFE (YEARS)3
3H 60,000 10,000 12.35
14C 2000 10,000 5730
99Tc 1000 10,000 211,000
125Sb 800 1000 2.77
60Co 200 1000 5.27
106Ru 100 100 1.01
137Cs 90 100 30.0
134Cs 60 100 2.06
90Sr 30 100 29.1
129I 9 100 16,000,000
Release limits and risk Different isotopes pose different environmental
and health challenges.
1Maximum levels allowed in Japan for waters released from nuclear reactor operations. 2Limits allowed for food safety (CODEX standard based upon adult consumer and annual consumption limit). 3Half-life is a physical property indicating the time it takes for 50% of an isotope to decay. A shorter value means a quicker loss.
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amounts of more dangerous isotopes, such
as ruthenium-106, cobalt-60, and stron-
tium-90 (7). The concentrations of these ra-
dioactive isotopes are orders of magnitude
lower than tritium but highly variable from
tank to tank (see the figure). By TEPCO’s
own assessments, more than 70% of the
tanks would need secondary treatment to
reduce concentrations below that required
by law for their release (7).
However, there are other important fac-
tors to consider. These radioactive isotopes
behave differently than tritium in the ocean
and are more readily incorporated into ma-
rine biota or seafloor sediments (see the fig-
ure). For example, the biological concentra-
tion factors in fish are up to 50,000 higher
for carbon-14 than tritium (8). Also, iso-
topes such as cobalt-60 are up to 300,000
times more likely to end up associated with
seafloor sediments (8). As a result, models
of the behavior of tritium in the ocean, with
tritium’s rapid dispersion and dilution, can-
not be used to assess the fate of these other
potential contaminants.
To assess the consequences of the tank re-
leases, a full accounting after any secondary
treatments of what isotopes are left in each
tank is needed. This includes the volume,
not just for the nine isotopes currently re-
ported but for a larger suite of possible con-
taminants, such as plutonium. Plutonium
may be present in FDNPP cooling waters
but was not released in large amounts to
the atmosphere in 2011.
The public has been told that there are
few options other than ocean discharge.
However, given the short half-lives of the
isotopes known in the tanks, time would
help. With a 12.3-year half-life, in 60 years,
97% of all of the tritium would decay, along
with several of the other shorter lived iso-
topes. In those intervening years of cleanup
on site, about four times the current volume
would be generated. The risk of tank leaks—
even if stored in earthquake-resistant tanks,
similar to what Japan already does for pe-
troleum or liquefied natural gas—needs
to be weighed against the greatly reduced
amount of radioactivity after decay. The
lack of space, the reason for the urgency in
ocean release, could be alleviated if tanks
were stored just outside the boundaries of
the current FDNPP.
Last, public fears should not be dismissed
because these decisions may have negative
impacts on local fisheries that are just now
rebuilding. Making data available is a good
start (9) but not enough. Seafood and ocean
monitoring should continue to involve local
fisherman, and studies that involve public
participation in sampling would be an ef-
fective tool to improve public education and
build confidence in the results (10).
The current focus on tritium in the waste-
water holding tanks ignores the other radio-
active isotopes but presents a solvable issue.
A solution includes reducing the concentra-
tions of non-tritium contaminants, reporting
after secondary treatment independently
verifies concentrations for all contaminants
in each tank, and reconsidering other stor-
age options. If there is a release, supporting
independent ocean study of multiple con-
taminants in seawater, marine biota, and
seafloor sediments should occur before, dur-
ing, and after. Although the operators have
promised some of this, actions will matter
more than words. What needs to be added
to the discussion is that the non-tritium iso-
topes in those tanks have vastly different tox-
icities and fates in the ocean. j
REFERENCES AND NOTES
1. K. Buesseler et al., Annu. Rev. Mar. Sci. 9, 173 (2017). 2. Radioactivity levels are measured in becquerels (Bq)
per unit volume or mass, with 1 Bq = one decay event per second.
3. V. Sanial, K. O. Buesseler, M. A. Charette, S. Nagao, Proc. Natl. Acad. Sci. U.S.A. 114, 11092 (2017).
4. International Commission on Radiological Protection (ICRP) publication 119, “Compendium of dose coef- ficients based upon ICRP publication 60” (ICRP, 2010).
5. TE PCO, Draft study responding to the subcommittee report on handling ALPS treated water, 24 March 2020.
6. P.-E. Oms et al., Sci. Total Environ. 656, 1289 (2019). 7. T. E. P. C. O. Treated Water Portal Site, www4.tepco.co.jp/
en/decommission/progress/watertreatment/index-e. html.
8. International Atomic Energy Agency (IAEA), Technical report series No. 422, “Sediment distribution coeffi- cients and concentration factors for biota in the marine environment” (IAEA, 2004).
9. TEPCO, “Radiation concentration estimates for each tank area (as of March 31, 2020)” (TEPCO 31 December 2019); https://www4.tepco.co.jp/en/sp/ decommission/progress/watertreatment/images/ tankarea_en.pdf.
10. Our Radioactive Ocean, www.ourradioactiveocean.org.
ACKNOWLEDGMENTS
This work was supported by the Deerbrook Charitable Trust and the Center for Marine and Environmental Radioactivity. Writing assistance by K. Kostel is also appreciated.
10.1126/science.abc1507
10,000,000
1,000,000
100,000
10,000
1000
100
10
1
0.1
1,000,000
100,000
10,000
1000
100
10
1
0.1
50% range 75% range 100% total range of all dataMean
3H 125Sb 60Co 106Ru 137Cs 134Cs 90Sr 129I14C 99Tc
3H 125Sb 60Co 106Ru 137Cs 134Cs 90Sr 129I14C 99Tc
Isotope
1.1 × 10–73.0 3 10–11Dose coefcient
Biological concentration factor Seafoor sediment-water distribution coefcient
Radioisotope concentration ranges for more than 200 tanks reported on 31 Dec 2019 by TEPCO (9) organized by their efective dose (dose coe1cient).
Radioisotopes concentrate to varying degrees in biological systems such as fsh (Bq/kg wet weight fsh per Bq/kg in seawater) and seaPoor sediment (Bq/kg dry weight sediment per Bq/kg in seawater).
Ta n
k le
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( B
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I N S I G H T S | PERSPECTIVES
Sorting out what is in the tanks One legacy of the Fukushima Daiichi nuclear disaster after the 2011 Tohoku-oki earthquake and tsunami is the
accumulation of water with a variety of radioisotopes in tanks. Assessing the risk of discharging water from
these tanks back into the ocean requires knowing radioisotope amounts and their ability to concentrate
in seafloor sediments and biological tissues.
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Opening the floodgates at Fukushima Ken O. Buesseler
DOI: 10.1126/science.abc1507 (6504), 621-622.369Science
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