A visual inspection by remote controlled camera has shown no significant damage to the used fuel pond of Fukushima Daiichi unit 4.
There had been fears of serious damage to nuclear fuel stored in the pond after a series of fires and explosions in the vicinity. Highly radioactive and heat-emitting used nuclear fuel is stored for a few years in the ponds before transfer to a larger storage pond shared by all six reactors at the site. However, the reactor was in a period of maintenance with the full core temporarily stored, requiring very much more cooling than the years-old fuel. This contributed to problems at the pond as water heated up and evaporated after the tsunami of 11 March disabled cooling and water top-up systems.
At least two fires as well as an explosion occurred in the area of the pond around 15 March, although at that time radiation levels prevented workers from making a direct check on the pond's status. Engineers became worried that the pool had dried out, the fuel overheated and zirconium cladding reacted with water to produce hydrogen, but this visual inspection initially discounts that scenario by showing no serious damage of the kinds that would be expected. Some debris was scattered in the pond as a result of the damage to the building but it is thought that fuel integrity has been maintained.
Tokyo Electric Power Company continues to regularly top up the water level in unit 4 as well the others on site. At unit 4 this is done with a concrete pumping truck dubbed an 'elephant' due to its long flexible delivery tube.
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1. Reports indicate that some fuel melted and fell to the lower containment sections of units 1-3, where it dispersed in a fairly uniform residue — but this does not seem to have breached containment in any of the reactor pressure vessels. Re-criticality of this ‘corium’ seems very unlikely, but no details can of course be confirmed until the reactor cores are finally dismantled — which may be years away.
2. Two automated PackBot robots entered units 1 and 3, took photos, and measured temperature, pressure and radioactivity within the buildings. Peak levels were 40-60 mSv/hr.
3. An anti-scattering agent is being sprayed on the ground around the damaged units (about 1,200 square metres in area) to prevent further spread of radionuclides (see photo above).
4. Excess radioactive cooling water continues to be transferred from unit 2′s basement and tunnels to a waste processing facility.
5. Further surveys are being made of the area surrounding the Fukushima evacuation zone and the exclusion area is being policed more strictly. Highest levels were measured at Itate, at about 4 microsieverts per hour (by comparison, the background level is 0.2 — 0.4 uSv/hr).
6. TEPCO have now released a roadmap plan for the restoration of stable conditions at the site, over a 3 — 6 month timetable, leading to a cold shutdown at units 1-3 and various other stability targets. They also released a 27-slide presentation on the timeline of the accident and current situation, that is definitely worth a look through.
NEI update as of April 29
* Priorities this week at Fukushima continued to be cooling the reactors and fuel pools, draining water from the turbine buildings and concrete structures that house piping to reduce radiation levels, and containing the spread of radioactive materials. Tokyo Electric Power Co. (TEPCO) is increasing the amount of cooling water injected into reactor 1 at the Fukushima Daiichi plant as part of a plan to cover the fuel.
* TEPCO plans to build a storage and processing facility that can hold 70,000 tons of highly radioactive water at the plant.
* Overall, site radiation dose rates are stabilizing or decreasing. The most recent radiation readings reported at the plant site gates ranged from 4.8 millirem per hour to 2.2 millirem per hour. TEPCO has released a map showing radiation levels around the site, based on readings taken on different days since the incident began.
* TEPCO said this week that it will build a wall of sandbags along the shoreline at the Fukushima Daiichi site as a temporary measure against another possible tsunami. The company also moved emergency power generators to higher ground to prevent the reactors' cooling systems from failing in case a major tsunami hits the plant again. The utility will sandbag the shoreline at the plant to a height of several meters. Priority will be put on the area near the waste processing facility, where highly radioactive water is being moved from around the reactor buildings. TEPCO is also planning to build a breakwater on the shoreline, as the sandbags cannot remain the long-term solution for a possible tsunami.
* Japan's Nuclear Safety Commission (NSC) asked the government April 28 to review the ability of the country's nuclear power plants to withstand earthquakes. The commission has requested that the Nuclear and Industrial Safety Agency "re-examine the fault lines and geographical changes where plant operators have so far said the risk of earthquake damage was low." The utilities’ reassessment of earthquake resistance "will likely take several years," the NSC said, and will likely affect the start of operations at new nuclear power plants and the construction of new reactors.
* TEPCO said April 28 that it does not believe the spent fuel pool at reactor 4 of Fukushima Daiichi is leaking, according to a report by Japan television station NHK. The utility said it initially believed that declining water levels in the pool indicated that it might have been damaged in an explosion soon after March 11, but it "now believes that the water has been evaporating at a rate in line with calculations by experts.” The fuel storage pool "will be reinforced by July," TEPCO said.
IAEA update April 28
IAEA update from April 28
On 25 April the power supply for the temporary electrical pumps that supply water to the reactor pressure vessel of Units 1, 2 and 3 was switched from the off-site power supply to temporary diesel generators to allow work to enhance the off-site power supply. The power supply has now been returned to the off-site supply.
White smoke continues to be emitted from Units 2 and 3. No more white smoke was seen coming from Unit 4 as of 21:30 UTC on 25 April.
In Unit 1 fresh water was being continuously injected into the reactor pressure vessel through the feedwater line at an indicated flow rate of 6 m3/h using a temporary electric pump with off-site power. On 27 April at 01:02 UTC an operation was initiated to increase the flow rate for injected water gradually from 6 m3/h to 14 m3/h to determine the amount of water required to flood the reactor core.
In Unit 2 and Unit 3 fresh water is being continuously injected into the reactor pressure vessel through the fire extinguisher line at an indicated rate of 7 m3/h using temporary electric pumps with off-site power.
In Unit 4 water continues to be sprayed on to the spent fuel pool using a concrete pump truck. An amount of 85 tonnes of water was sprayed on 27 April.
Nitrogen gas is still being injected into the containment vessel in Unit 1 to reduce the possibility of hydrogen combustion in the containment vessel. The indicated pressure in the reactor pressure vessel is still increasing.
In Unit 1, the indicated temperature at the feedwater nozzle of the reactor pressure vessel is 132.0 °C and at the bottom of reactor pressure vessel is 110.5 °C.
In Unit 2 the indicated temperature at the feedwater nozzle of the reactor pressure vessel is 120.4 °C. The reactor pressure vessel and the dry well remain at atmospheric pressure. On 26 April an amount of 47.5 tonnes of fresh water was injected into the spent fuel pool using the spent fuel pool clean-up system.
In Unit 3 the indicated temperature at the feed water nozzle of the reactor pressure vessel is 72.0 °C and at the bottom of the reactor pressure vessel is 110.7 °C. The reactor pressure vessel and the dry well remain at atmospheric pressure.
There has been no change in the status in Unit 5 or Unit 6 or in the common spent fuel storage facility.
Spraying of anti-scattering agent at the site is continuing. An area of 7500 m2 to the east of the Unit 3 turbine building was sprayed on 27 April.
Deposition of Cs-137 was detected in four prefectures on 26 and 27 April, the values reported ranging from 4 Bq/m2 to 29 Bq/m2. I-131 deposition was reported for one prefecture on 26 April, with a value of 3.3 Bq/m2.
Gamma dose rates are measured daily in all 47 prefectures. A general decreasing trend has been observed in all locations since around 20 March. For the Fukushima prefecture gamma dose rates remain at 1.8 μSv/h. In Ibaraki prefecture gamma dose rates were slightly below 0.12 µSv/h. The other 45 prefectures had gamma dose rates of below 0.1 µSv/h, falling within the range of local natural background radiation levels. Gamma dose rates reported specifically for the eastern part of Fukushima prefecture, for distances beyond 30 km from the Fukushima Daiichi plant, showed a similar general decreasing trend, ranging from 0.1 to 13.6 µSv/h, as reported on 26 April.
On-site measurements at the west gate of the Fukushima Daiichi plant indicate the presence of I-131 and Cs-137 in the air in the close vicinity of the plant (within approx. 1 km). The concentrations in air reported since 31 March show a maximum on 14 April of 11.8 x 10−4 Bq/cm3 for total I-131 and 2.7 x 10-4 Bq/cm3 for total Cs-137. The values reported for 26 April are 9.0 x 10−5 Bq/cm3 for total I-131 and 2.4 x 10−5 Bq/cm3 for total Cs-137.
Since 1 April there has been one remaining restriction on the consumption of drinking water relating to I-131 (with a limit of 100 Bq/L), which applies to one village in the Fukushima prefecture and only for infants.
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