Some of the fuel in the damaged unit 3 of the Fukushima Daiichi plant has melted and dropped into the primary containment vessel, initial results from using a muon detection system indicate. Part of the fuel, however, is believed to remain in the reactor pressure vessel.
Structures within the reactor building of unit 3 can be seen in images obtained using muon data (Image: Tepco)
Muons are high-energy subatomic particles that are created when cosmic rays enter Earth’s upper atmosphere. These particles naturally and harmlessly strike the Earth’s surface at a rate of some 10,000 muons per square meter per minute. Muon tracking devices detect and track these particles as they pass through objects. Subtle changes in the trajectory of the muons as they penetrate materials and change in direction correlate with material density. Nuclear materials such as uranium and plutonium are very dense and are therefore relatively easy to identify. The muon detection system uses the so-called permeation method to measure the muon data.
Tokyo Electric Power Company (Tepco) installed a muon detection system on the first floor of unit 3’s turbine building. Measurements were taken between May and September this year.
Tepco said analysis of muon examinations of the fuel debris shows that most of the fuel has melted and dropped from its original position within the core.
Prior to the 2011 accident, some 160 tonnes of fuel rods and about 15 tonnes of control rods were located within the reactor core of unit 3. The upper and lower parts of the reactor vessel contains about 35 tonnes and 80 tonnes of structures, respectively.
The muon examination indicates that most of the debris – some 160 tonnes – had fallen to the bottom of the reactor pressure vessel and resolidified, with only about 30 tonnes remaining in the reactor core. Tepco said another 90 tonnes of debris remains in the upper part of the vessel.
The bulk of the fuel and structures in the core area dropped to the bottom of the reactor pressure vessel (RPV), Tepco believes. While part of the molten fuel is understood to have then fallen into the primary containment vessel (PCV), “there is a possibility that some fuel debris remains in the bottom of the RPV, though this is uncertain”, the company noted.
Similar muon measurements have already been conducted at units 1 and 2 at Fukushima Daiichi. Measurements taken at unit 1 between February and September 2015 indicated most of the fuel was no longer in the reactor’s core area. Measurements taken between March and July 2016 at unit 2 showed high-density materials, considered to be fuel debris, in the lower area of the RPV. Tepco said that more fuel debris may have fallen into the PCV in unit 3 than in unit 2.
The current understanding of fuel location in units 1-3 (Image: Tepco)
Tepco said the results obtained from the muon measurements together with knowledge obtained from internal investigations of the primary containment vessels using remote-controlled robots will help it plan the future removal of fuel debris from the damaged units.
In more than 6 years they have not been able to really find yet the melted fuel, despite their beautiful PR stunt last week, and now they talk about which techniques to use to remove it. Maybe they should better find it first before talking about how to remove it, that is if most of it has not been already vented and projected into our skies.
How to Clean Up Hundreds of Tons of Melted Nuclear Fuel
More than six years after three nuclear reactors melted down in Japan, the country is homing in on the lost fuel inside one of them. Japan’s biggest utility and owner of the wrecked Fukushima Dai-Ichi plant, Tokyo Electric Power Co. Holdings Inc., last week released images that for the first time showed what’s likely melted fuel inside the No. 3 reactor.
If confirmed, the nation will have to devise a way to remove the highly radioactive material, a mixture of melted nuclear fuel and reactor debris known as corium. The cleanup process that may last 40 years and cost 8 trillion yen ($72 billion) will require technology not yet invented.
Here are a few ways the removal could be done, including the government’s preferred approach by taking it out the side:
“Special tools and techniques will have to be developed to undertake such a task that has never been attempted before anywhere in the world,” said Dale Klein, an adviser to Tepco, as the utility is known, and a former chairman of the U.S. Nuclear Regulatory Commission. “Once Tepco has identified the characteristics of this material, then they can develop a plan to remove this material in a safe manner.”
The search for the fuel has left a trail of dead experimental robots specifically designed to find and photograph the estimated combined 600 metric tons of fuel and debris in the three melted reactors. While the No. 3 reactor was the last unit to be probed, its the first to produce a strong indication of where the fuel came to rest. The removal process is slated to begin in 2021.
Long-handled devices guided by a television monitor system were developed to remove fuel core debris at Three Mile Island in the U.S. after its 1979 meltdown. The so-called defueling process took from 1985 to 1990 and involved removing the partially melted fuel core from inside the pressure vessel of the No. 2 reactor, which remained intact. Fukushima offers a more complex challenge since three reactors suffered total meltdowns, with melted fuel rupturing pressure vessels and falling to the bottom of the units.
No such effort is being made at Chernobyl, where a concrete sarcophagus was used to entomb the wrecked plant that melted down in 1986.
Technology research and development should focus on removing fuel through the side, the Nuclear Damage Compensation & Decommissioning Facilitation Corp., Tepco’s top government shareholder, said Monday. That method is safer for workers than flooding the reactor, which would also require the management of radioactive water.
Japan will decide how it will remove the fuel by September, Industry Minister Hiroshige Seko said after the discovery last week, according to national broadcaster NHK.
New proposal suggests removing Fukushima plant’s melted nuclear fuel from side
A method to remove melted nuclear fuel debris on the bottom of the containment vessels of Fukushima No. 1 Nuclear Power Plant’s first, second and third reactors from the side was proposed by the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) on July 31.
Hajimu Yamana, head of the NDF, which is tasked with considering how to remove fuel debris from the reactors, for the first time explained the organization’s specific method proposal to the heads of local governments at a countermeasures for the decommissioning and handling of the contaminated water council meeting held in Iwaki, Fukushima Prefecture.
The method would focus on prioritizing the removal of debris from the bottom of the vessels from the side, using robotic arms and other remote devices while flushing water over the debris.
However, ways to block radiation and countermeasures against the scattering of airborne radioactive dust still remain unsolved. The central government and Tokyo Electric Power Co. (TEPCO) plan to finalize their policy to remove the debris and amend the decommission schedule in September.
In all three of the reactors, contaminated water has collected at the bottom of the containment vessels. The NDF had previously considered a “flooding method” that would fill the containment vessels completely with water to block radiation from leaking. However, measures to repair the containment vessels and prevent leakage of the radioactive water would be difficult, so the plan was put aside for having “too many issues.”
This underwater robot was used in the recent probe of reactor 3 at the Fukushima No. 1 power plant.
The first images of melted fuel from the crippled Fukushima No. 1 power plant indicate that it did not burn through the pressure vessel of reactor 3, but exited through the holes used to insert the control rods, officials say.
While the landmark robot footage from the primary containment vessel of unit 3 is helping Tokyo Electric grasp the reality of the damaged fuel assemblies, it may also force it to rewrite the road map for decommissioning the meltdown-hit plant.
Tokyo Electric Power Company Holdings Inc., better known as Tepco, sent an underwater robot into reactor 3 earlier this month to confirm its hypothesis that the core — the fuel assemblies in the pressure vessel — broke apart and fell to the bottom, letting molten fuel burn through and drip into the primary containment vessel.
According to Tepco spokesman Takahiro Kimoto, however, the images taken beneath the PCV indicate the pressure vessel probably withstood the heat of the molten fuel. He said the fuel apparently seeped through the holes for the control rods.
“We do not presume that the vessel, which is 14 cm thick, melted and collapsed together with the fuel, but that part of the fuel instead made its way down through holes,” Kimoto said. The control rods are used to moderate the chain reaction and are inserted vertically into the core.
Tepco said it estimates reactor 3 has about 364 tons of fuel debris, and that similar amounts will be found in reactors 1 and 2. Removing the fuel from the reactors is the largest challenge in defueling the aged plant — a process that could take up to 40 years to complete.
The camera on the underwater robot also captured images of rubble around the fuel debris, which could slow the removal process. The rubble includes devices for supporting the control rods at the bottom of the PCV and scaffolding for maintenance workers beneath the pressure vessel.
Minister of Economy, Trade and Industry Hiroshige Seko said the government and Tepco will try to draft a plan for removing the melted fuel in September, with an eye to hammering out the specifics in the first half of fiscal 2018 and starting the work in 2021.
But the findings from reactor 3 may force them to alter the state’s road map for decommissioning Fukushima No. 1, officials said.
An entity providing technical support for the project has urged that efforts be made to remove the melted fuel from the submerged lower part of the PCV by keeping air in the upper part, according to a source familiar with the plan.
Although filling the PCV completely with water would largely reduce the radiation risk to the robot probes, the Nuclear Damage Compensation and Decommissioning Facilitation Corp. is reluctant to do so because it is damaged and the toxic water will just leak out, the source said.
At the other two reactors, Tepco thinks most of the fuel in reactor 1 fell to the bottom of the primary containment vessel, and that some of the fuel in reactor 2 remained in the pressure vessel. The company made the estimates based on cosmic ray imaging analysis and by sending robots and endoscopes into the PCVs of the two reactors.