Highlight of development at Fukushima after the nuclear
accident
by
Ir Richard Fung
Recital
On 11 March 2011, the Great East Japan Earthquake occurred 130 km
offshore to the north east coast of Honshu in Japan. The earthquake was rated at Magnitude 9.0 on
the Richter Scale, that was accompanied with a 8-14 m high tsunami which
flooded the region [1], claiming over 15,800 lives with another 2,500 missing [2].
Of the 14 nuclear power generating units all located at 4 coastal sites
in the region, 11 were operating and were safely shut down after the earthquake
and the remaining 3 were already in planned shutdown. However, of the 6 Boiling Water Reactor type nuclear
units at the Fukushima Daiichi Nuclear Power Plant having 4696 MW of gross
total installed capacity and commissioned during 1971-79, Units 1 to 4
sustained serious damage, triggering a significant release of radioactivity and
forcing the mass evacuation of 154,000 people [3]. The nuclear accident was classified at Level
7, the highest on the International Nuclear Safety and Radiological Event Scale
[1, 4].
The situation has been much stabilized after seven years. However, recovery at the plant site and the
nearby community is still in progress, and is expected to last decades.
Status at the site
The reactor units
Units 1 to 4 of Fukushima Daiichi were seriously damaged in the accident. They initially sustained the loss of external
power connections in the earthquake, and then an hour later the loss of
internal power sources due to tsunami flooding because of low site elevation
above the sea [5]. Insufficient
robustness in plant safety systems together with the complete loss of
electrical power led to loss of reactor cooling at the first three units. Residual reactor heat led to over-temperature
producing steam which reacted chemically with fuel clad to produce hydrogen and
later fuel melt. In the next few days, fuel
melt and hydrogen explosions, that sometimes accompanied by deliberate if
inadequate venting, damaged primary containments and reactor buildings and resulted
in the release of radioactivity into the environment. Unit 4 reactor was not loaded with fuel during
the accident, and subsequent investigation showed that its hydrogen explosion
was caused by the hydrogen coming to the unit from the adjacent Unit 3 through
a common ventilation duct [5].
To remove residual reactor heat after shutdown, initially seawater
was injected using mobile equipment brought in from the outside. External power supply was restored after about
2 weeks so that plant cooling systems could function again to inject fresh
water into the reactors [6]. Nitrogen
was injected into the reactor pressure vessels to suppress hydrogen explosion
risks. Excess cooling water that had
been contaminated was transferred to storage tanks that were continuously added
at the site, and after several months the commissioning of a new treatment
plant for water used in reactor vessel cooling brought radioactive leakage
under control. With the cooling water
brought below boiling at the damaged reactors, “cold shutdown” was declared at
the plant in December 2011 following regulatory approval and Fukushima Daiichi completed
its accident phase [7].
Temporary covers were installed over the damaged external structures
of Units 1 to 4 to provide shelter against the elements, limit potential
radioactive release and facilitate remedial work. Inspections often employing robots have been
underway since 2015 to identify the extent of damage at the reactor units so as
to formulate a decommissioning plan [8, 9].
Meanwhile, decontamination and debris clearance were completed in 2016 at
the general site outside the plant [9].
Units 5 and 6 were under planned maintenance during the accident. They were kept cooled by an air-cooled diesel
generator that was unaffected during the tsunami, and were declared to be at
“cold shutdown” one week into the event.
Only minor damages were experienced at the other three sites. Indeed, the nuclear site in Onagawa to the
north was even safe enough to serve as an emergency shelter for the local
population in the first few weeks after the earthquake [10].
Spent nuclear fuel
Spent nuclear fuel at Fukushima was stored under water at the
reactor buildings of the 6 units and at the common site storage [5]. Its cooling at the reactor buildings was
interrupted during the accident and, despite a rise in temperature and a fall
in water level at the storage pools, restoration of spent fuel cooling was
accomplished after several days, initially by external water injection with
mobile equipment and then by plant systems, to be followed by newly commissioned
cooling systems. Despite having fallen
debris into some storage pools, no significant damage was sustained by the
spent nuclear fuel.
Because of possible structural damage caused by the earthquake, work
was conducted soon after the accident to reinforce Unit 4 spent nuclear fuel
storage pool which had also received the nuclear fuel unloaded from the reactor
during its refueling outage. Spent fuel
at Unit 4 was transferred to the common site storage in 2014 [8] and that at Units
1-3 will be removed in 2018-24 [11]. Older
spent fuel will be loaded in temporary dry storage modules.
Contaminated water discharge
With damage at the reactors of Units 1 to 3 and at the plant
buildings that contain the reactor cooling system, reactor cooling water containing
radioactivity has been escaping through damaged underground site conduits into
the site harbour and to the open sea. The
need for continuous nuclear cooling required a continuous supply of water that in
turn led to a continuous escape. The quantity
of the flow into the sea was increased by the high water table of adjacent
hills which brings groundwater into the damaged plant buildings hence adding to
the escaping reactor cooling water leaking to the sea. After the accident, wells were drilled on the
hillside of the site to extract inflowing groundwater to bypass the site or to
be treated for discharge into the sea. A
0.8km long impermeable wall was installed on the harbourside in 2015 and a 1.5km
long underground wall using frozen earth technique was installed around the
site in 2017 to isolate the environment from radioactive water at the
site. Groundwater flow into the site has
since much reduced and radioactivity in the sea water outside the site harbour
is within regulatory limits [7].
Current plan
It is planned to decommission the four damaged units in the next 30-40
years, to remove and dispose in particular the highly radioactive nuclear fuel
and fuel debris, and to remove both the radioactive and non-radioactive
structures of the plant buildings [11].
Before that, Units 5 and 6 will be used to practise decommissioning
techniques and demonstrate their feasibility.
Staff radiation dosage
Six workers have each received radiation dose above 250 mSv, the
limit set at the height of the accident, with the maximum recorded dose at
678.8 mSv, and the number of workers each received more than 100 mSv (the general
limit for an emergency) has been kept at 174 since 2012 [12]. In all cases, no radiation-induced casualties
were observed [13] though long term effects will need monitoring. However, two workers were drowned at
Fukushima Daiichi during the tsunami and another one was killed at Fukushima
Danni to the south [14].
Over 46,000 workers had been employed in radiation-related work at
Fukushima up to January 2016 [15] and the number of workers engaged has gradually
reduced from 11,500 in April 2015 [11] to 8,500 in November 2017 [16]. Their individual dosage is being kept at
within 100 mSv.
Environmental impact
It has been estimated that 408-770 PBq of Iodine-131 equivalent of
radioactivity was released into the air, equivalent to 10-18% of the release in
Chernobyl [4, 17]. Prevailing wind
initially took the released radioactivity east to the sea but with a change in
wind direction, radioactive material was blown inland 4 days into the
accident. An area up to 60km to the
north west of the nuclear power station was observed to have radioactivity at
about 30 times above local ambient, albeit the effect is within the regulatory
public health limit of 1 mSv/year [6, 18, 19].
Radioactivity in the affected land areas reduced by half for every 2
years after the initial release, possibly reflecting the decay of the released
radioactive constituents, uptake and runoff [20]. The trend will continue though the quantity
reduced will be at a slower rate.
Radioactivity in the sea nearby dropped in the first year after the
construction of a temporary sea wall in the site harbour but has been generally
maintained at a steady level, possibly due to leaching, soil drainage, and radioactive
leaks from the plant that has only recently been controlled. The levels found at 2km near shore one year
after [20] and at the harbour three years later [21], are already within the
limits recommended by the World Health Organisation for drinking water [22].
Public protection
Shortly after the release of radioactivity from the nuclear power plant,
the Japanese authority took emergency measures to protect the public, by
providing shelters and mass evacuation mainly to limit external exposure,
administering iodine tablets early in the accident to the public to mitigate
the effect of inhaled radioactive iodine, and identifying and isolating
contaminated foodstuff to limit internal exposure.
Sheltering and evacuation
In view of the increasing severity of the accident in the first few
days, the authority implemented an evacuation zone in steps up to 20 km from
the site, displacing some 76,000 people, and recommended voluntary evacuation
up to 30km in late March 2011. The
criterion was revised in late April to mark a zone to avoid an annual dose of
20 mSv (and a zone up to 30km from the site was identified for potential
evacuation but was later dropped), further displacing 69,000 people beyond 20km
and up to 50km to the northwest of the site [23]. In total, 154,000 people were evacuated [3]. The process was accompanied by conflicting
messages and inconsistent information disseminated by the authority during such
mass evacuation and has incurred considerable loss of public trust and
confidence [24, 25].
In the next two years, the evacuation zones were gradually classified
into one that evacuation orders could soon be lifted, one that people would not
be permitted to dwell, and one that it would be difficult to return for a long
time. Annual radiation dose limits of 20
mSv and 50 mSv were used to demarcate these zones. Decontamination and reconstruction
infrastructure restoration would meanwhile proceed [23].
In early 2014, after confirmation that annual radiation dose had
fallen below 20 mSv, sufficient restoration had been made and communication
channels had been found between the authorities and the local public,
evacuation was progressively if slowly being relaxed that began with certain
outlying municipalities [23]. Areas that
were still restricted in 2017 extend about 30km to the north west of the site
and affected 24,000 people [26]. However,
of the regions that people are permitted to return, only about 16% of the
people have returned by early 2017 [26] and a 2014 survey of the 80,000 people
considered as “evacuees” 22% were willing to return, 42% would not and 36% were
undecided [18, 19]. The low return rate
is considered partly as an outcome of ineffective public communication
compounded with an inadequate public understanding of stipulated criteria, that
the dose of 20 mSv per year is the criterion to return while 1 mSv per year is
the long term goal, with the latter being mistaken by the population as the
objective for complete contamination or else livelihood is not viable [18].
Food control
Radioactivity in foodstuff is monitored to ensure safety for
consumption. Restriction will be imposed
on foodstuff at a district found exceeded the limits and will be lifted if
continuous monitoring has showed that the limits are met [8]. In 2017, 234,740 samples were examined
nationwide with 0.06% exceeding their limits but with 0.2% from the Fukushima
Prefecture [27]. Game was found in 2017
to have the highest reject rate at 14% nationwide and 21% at Fukushima [27]. In contrast, the reject rate of marine fish
was at 57% immediately after the accident but has reduced to zero since 2015 [28].
The currently acceptable
levels of long life radionuclides in Japan, mainly cesium, are 10 Bq/kg for
drinking water, 50 Bq/kg for infant foods or milk products, and 100 Bq/kg for
general food [29]. The previous levels
before April 2012 were 200 Bq/kg for drinking water and milk, and 500 Bq/kg for
general food [25]. In comparison, the
corresponding EU limits are 1,000 Bq/kg for drinking water, 400 Bq/kg for
infant foods, 1,000 Bq/kg for milk products, and 1,250 Bq/kg for general food [30]. Similar limit to the EU for general food is
found in the US and in the CODEX STAN of the UN Food and Agricultural
Organisation [31]. Bearing in mind that
all people are unavoidably exposed to a small dose of natural radiation all the
time [32], the very strict food standards on radiation of Japan serves to
reduce the radiation dose of the people only by a very small margin but it
constitutes a significant economic impact to those people in food production [24].
Public health
Given that the radiation dose expected of the affected population
would be no higher than 12-25 mSv in the first year, the World Health
Organisation estimated that, even for the most vulnerable groups in the most
exposed locations, an additional lifetime risk leading to cancer is within 1% [33].
Since 2011, Japan has been committing to a 30 year surveillance
programme for the exposed public. It was
estimated that 15 people received a dose above 15 mSv, with the highest at 25
mSv, among the 460,000 people at Fukushima who were not employed by the
plant. Thyroid cancer screening of
children (age under 18) was also conducted.
The first screening in 2011-2014 gave an annual cancer incidence of 11
out of 100,000 for Fukushima compared to 23-130 out of 100,000 in unaffected
prefectures. The second screening in 2014-2016
gave an incidence of 4-14 out of 100,000 [34].
Both showed marginally lower incidences in Fukushima as compared with
other unaffected prefectures.
General health observation is being conducted on the evacuees. Overall improvement in their physical health
is found since the accident and is attributed to the gradual improvement in their
living conditions. On assessing
psychological consequences, of the 4,400 people given telephone support, the
problems for children were mainly excessive emotion, irritability, peer
relation problems and rebellious behaviour, while depression was the main
problem with adults [34].
Stress and the initial loss of health care of the evacuees have a notable
negative effect particularly on the elderly or bedridden [24, 25]. By early 2014, deaths from stress and related
illnesses among the evacuees from Fukushima much exceeded that of the adjacent
preferences and reached 1656, surpassing its 1,607 direct deaths due to
earthquake and tsunami [35].
Investigations and reform
Several national and international investigations were conducted to
study the sequence and impact of the accident as well as its underlying cause
to develop improvement measures [4, 5, 25, 36, 37].
In particular, the report to the Japan Diet in 2012 investigated the
underlying causes and considered that the disaster was caused by the
established but defective management culture and
ineffective decision process of the plant owner and the
government, as well as the submissive national culture [37]. It concluded that Fukushima was a “manmade
disaster” that was a coincident result of “collusion
between the government, the regulators and Tokyo Electric Power Company” and
the “root causes were the organizational and regulatory systems that supported
faulty rationales for decisions and actions.” The regulator was criticized for not sufficiently
maintaining independence from the industry in developing and enforcing safety
regulations or suffering “regulatory capture”, the government for inadequate
emergency preparedness and management, and the plant owner for its poor
governance and lack of safety culture. The
fundamental causes of the accident that was “Made in
Japan” were reportedly found in “the ingrained conventions of
Japanese culture: our reflexive obedience; our reluctance to question
authority; our devotion to 'sticking with the program'; our groupism; and our
insularity.” The national mindset prevented the country from “absorbing the
critical lessons learned from Three Mile Island and Chernobyl.” "The consequences of negligence at
Fukushima stand out as catastrophic, but the mindset that supported it can be
found across Japan. In recognizing that
fact, each of us should reflect on our responsibility as individuals in a
democratic society." [37]
The
report demanded fundamental changes across the Japanese nuclear industry, the
government and the regulators, to increase openness, trustworthiness and focus
on protecting public health and safety.
A new regulator was formed later in 2012, to be reportable to the
Ministry of Environment and no longer to the Ministry of Economic, Trade and
Industry [38]. All nuclear power
stations are required to be reviewed under new safety regulations before they are
allowed again to operate. In late March
2018, 7 nuclear power units have resumed (or scheduled to resume) operation and
another 17 are seeking approval for restart, out of a total of 42 that remain
operable today [39]. There were 54 operable
units before the accident.
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