Ron Chin and Bob Abboud of RGA Labs have compiled this analysis of the events at Japan's nuclear plant based on information found on the web and newspaper articles as of March 16.
Analysis of the conditions at Japan's Fukushima Daiichi Plant following the earthquake and subsequent tsunami.
Status 4/1/2011
It's been three weeks since Units 1, 2, and 3 shutdown from the earthquake. Once the Units shutdown, the production of iodine-131 should have stopped and the amount built up in the fuel should have begun decaying. Iodine-131 has a half life of 8 days, so after 16 days only 25% of the inventory should have remained, at 21 days there should be less than 20% of the original amount. But reports keep coming in about finding iodine-131 in samples taken further from the site and the amount in the ocean has increased. This indicates Daiichi is continuing to spew out the radioactive isotopes and there is a good possibility that the melted fuel is generating heat by going critical, thus producing more iodine.
Plans are underway to construct a storage pond, a storage tank and a water processing plant on the site to clean up the contaminated water. They hope to have the additional storage available in a couple of weeks and the processing plant running in a month. Meanwhile water continues to be added to the Unit 1, 2, and 3 reactors to keep the heat down and the spent fuel pools water inventories are being replenished on daily basis. Additional concrete pumping units are being brought in to provide a more controlled water injection into the spent fuel pools.
Status 3/31/2011
TEPCO has been pumping water from the condensers to other storage tanks to make space for pumping the flooded areas to the condensers. They are considering creating an artificial pond and bringing in an ocean tanker for additional storage. Until the pooled water can be removed work on restoring equipment is on hold. After the water is removed they will be dealing with highly contaminated areas which will be prone to become airborne when dirt and dust is kicked up.
They have determined a majority of the flooding in the trenches was deposited by the tsunami.
When the hydrogen exploded in the reactor buildings a large amount of radioactive contaminants were released into the atmosphere. The heavier dust and particles fell onto the plant grounds and are now being tracked and blown around thus spreading the radioactive materials. They feel the dust being blown around is the main source of the contaminants getting into the ocean. TEPCO will test spraying an adhesive resin onto the ground to contain the dust. If it works on the test area, they will spray it on all the contaminated areas.
Reports of Core Meltdown
Analysis of the radioactive isotopes being released and the data available from instrument within the reactor vessels and reactor containment are now indicating that fuel has melted. There are diverse opinions on how much has melted, where the molten mass resides, and how it is shaped. The worse case describes a large blob which has been going critical. Going critical means it is creating heat via nuclear fissions thus creating more contaminants. The best case is there are multiple smaller blobs which are hot from only the decay heat. TEPCO thinks it is in the form of smaller blobs which are at the bottom of reactor vessel. But the blobs are leaking out the penetrations which exist for the control rods. For a more detailed description of the meltdown see: http://spectrum.ieee.org/tech-talk/energy/nuclear/nuclear-engineer-says-theres-evidence-fuel-melted-through-reactor-pressure-vessel/?utm_source=techalert&utm_medium=email&utm_campaign=033111 .
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Writeup prior to March 31, 2011
The Fukushima Daiichi complex has six nuclear reactors. At the time of the earthquake three units (1, 2, & 3) were producing heat to generate electricity. The other three units (4, 5, & 6) were shutdown for refueling, a normal maintenance event.
When the earthquake occurred a blackout occurred at the plant causing Units 1, 2, and 3 to shutdown automatically. A shutdown stops the nuclear chain reaction which produces heat. A nuclear reactor requires electricity to operate its own safety equipment. Nuclear plants have diesel generators to provide backup power in the event of a power failure. The Daiichi has several diesel generators to provide backup power. When the tsunami hit the facility, it damaged the diesel fuel tanks thus resuming blackout conditions.
After a shutdown the fuel must cool. Despite stopping the nuclear chain reaction the radioactive isotopes continue to release heat, known as decay heat. Normally, there is cooling water available for circulating around the fuel to remove the decay heat. Reports of using sea water to remove the decay heat lead us to believe the water or associated piping was damaged by the tsunami beyond repair. Using seawater means they will not be able to produce power from that unit again, a sign that conditions required severe measures.
Fuel rods no longer usable for producing electricity are stored on site in spent fuel pools. The spent fuel pool is a large, steel lined, pool of water. There are seven spent fuel pools at Fukushima Daiichi. One common pool and six smaller pools associated with each unit. The amount of cooling each pool requires would depend upon the quantity of fuel assemblies in the pool and the amount of decay heat they are producing. Nuclear plants maintain 16 feet of water above the fuel rods.
What keeps the radioactive materials inside?
A nuclear reactor has multiple barriers to prevent the release of radioactive materials. The radioactive nuclear fuel, cylindrical pellets about 1 inch long and ½ inch in diameter, known as fuel pellets, are sealed into a zirconium metal tube about 12 feet long known as a fuel rod. This is the first barrier. A nuclear reactor contains tens of thousands fuel rods. The reactor vessel is a stainless steel lined canister with walls over 6 inches thick. It is about 14 feet wide and 25 feet high. This is the second barrier.
A concrete structure surrounds the reactor vessel. This is the third barrier which is known as the reactor containment. There is a large chamber below the reactor vessel for capturing steam called the suppression pool. The reactor building houses the reactor containment and the spent fuel pool. This is the fourth barrier.
When the plant lost backup power the decay heat from the fuel in Units 1, 2, and 3 boiled away the remaining water in the reactor vessel over a couple of hours. Without their normal pumps, the operators pumped seawater to cool the fuel with their fire system pumps. But by the time the seawater pumping began, the fuel pins had heated to temperatures exceeding 2000 C. When zirconium heats to 2000 C it disintegrates and rapidly oxidizes with the surrounding water releasing hydrogen. Hydrogen explosions clearly indicated that some of the fuel rods overheated. There are relief valves in the system which allowed the hydrogen to vent/escape into the reactor building which is why the initial explosion occurred in the reactor building. The radioactive fuel remained within the reactor vessel. Some small quantities of radioactive gases were released with the hydrogen, however they were. adequately mixed in the air to mitigate being a hazard.
Is this like Chernobyl?
This situation differs markedly from the last major nuclear emergency, Chernobyl. Then, a fire took place in the reactor during a nuclear chain reaction. The reactor building was the only barrier between the outside and the burning fuel. When fire destroyed the reactor building's roof fission products from the fuel were released into the atmosphere.
Wind carries the smallest particles for hundreds of miles, while the heavier particles drop to the ground closer to the plant, thus contaminating the immediate area around the plant. The area of contamination is dependent on the direction and speed of the wind for carrying the radioactive materials. The stronger the winds the further the contamination will spread. Only areas downwind from the site will be contaminated. .
Ordinarily a nuclear plant periodically releases pent up gas into the atmosphere through its vent stack. Before releasing the gas, the plant filters and removes particulates. While the gas may be radioactive, it has been mixed with enough air to make the radiation concentration less than what one would receive when taking a cross country airplane flight.
How do iodine tablets protect me?
Radioactive iodine can be one component in a gas release. In the event of a major release, such as Fukushima Daiichi, people who may be exposed to the fallout receive a potassium iodine tablet. This saturates the thyroid gland to prevent the body from accumulating radioactive iodine.
The first hydrogen explosion was in the Unit 1 reactor building on March 12 after venting the reactor vessel containment. The explosion created a small radioactive release which was measured to be 11 mrem/hr at the site boundary. The reactor building sustained some damage, but overall it continued to provide protection although the damage necessitated additional precautions and planning.
Lacking the normal ability to pump water to cool Unit 3, excessive heat damaged the fuel rods, which caused a hydrogen buildup in its containment vessel. Plant personnel had to pump seawater into Unit 3 to stop the damage. They also vented the containment vessel into the containment building. On March 13, however, a hydrogen explosion occurred in containment building which blew out a wall and ceiling. . The public was evacuated from the 12 mile radius around the plant.
The explosion in Unit 3 may have initiated a problem with the Unit 2 cooling system. The fuel boiled off all the water. Although they pumped seawater into the reactor to cool it down, the pressure buildup did not permit filling the reactor vessel more than halfway. The seawater boiled off. On March 14 the pressure buildup caused a hydrogen explosion in the reactor containment vessel. The explosion occurred because they did not detect a malfunctioning relief valve. The explosion, however, reduced the pressure, thus allowing the seawater to resume flowing into the reactor.
At this point onsite radiation readings were elevated throughout the site. Seven hundred-fifty non essential workers were evacuate leaving 50 to maintain the seawater cooling of Units 1, 2, and 3. Fukushima is using the industry guidelines for allowable exposure to radioactivity to guide the decisions on personnel evacuations.
On March 14, a fire, reportedly due to lubrication oil, broke out in the Unit 4 reactor building and took 140 minutes to extinguish. As this reactor was shutdown and fuel rods were still hot, we believe water in the spent fuel pool dropped to levels that could not keep the stored fuel rods cool. The water level dropped due to some of the pool water sloshing out during the earthquake Furthermore, on March 16, another fire began in the Unit 4 reactor building, which took two hours to extinguish. The announced plans to use helicopters to drop water confirmed to us the plant had problems maintaining the water inventory in the spent fuel pool.
Subsequent reports confirm there was a hydrogen explosion in the Unit 4 reactor building which caused the building damage. The hydrogen was from the fuel in the spent fuel pool heating up.
Is there fuel meltdown?
The only way to get water to fuel in Units 1, 2, and 3 is through existing piping. At this time there are five fire pumps injecting sea water into these three units. The company has provided estimated fuel damage to be 70% of the zirconium in Unit 1 has disintegrated and 33% in Unit 2. If one could look into the reactor vessel one would see the fuel pellets piled up around the base of the partial fuel assembly skeletons. One would not see is a molten mass of glowing uranium generating so much heat to melt through the vessel.
Fukushima Daiichi Status 3/18/2011
The Unit 1, 2, and 3 reactors are in a stable cooling mode with the sea water being pumped into the system.
The company has tried to check the rising temperature in the Unit 3 reactor building by dropping water from helicopters above and spraying water from fire trucks on the ground. The loss of water in the spent fuel pool has caused the emission of highly radioactive gamma rays into the atmosphere.
Radioactive release vs Exposure to a radioactive source.
The earlier radioactive releases consisted of gases which are diluted as they drift in the atmosphere. These gases do not give off high amounts of energy and over a short period of time lose a significant portion of their radioactivity. The radioactivity within the fuel is a high energy source which will maintain its energy for years. If you are in the line of sight with the fuel you will be getting radiation. The amount of radiation is reduced in three ways, distance, time, and shielding.
The current actions at Daiichi indicate the Unit 3 spent fuel has been uncovered so it is a major radiation problem. They are working on refilling the spent fuel pool to cover the fuel with water which is an excellent shield. The helicopter pilots were dropping water on the building without hovering to reduce the time they spent over the exposed fuel. The emergency crews spraying water into the building are probably positioned to use the shielding provided by the concrete structures to minimize the radiation level.
Below are some satellite photos of Daiichi taken since the earthquake. Figure 1 was taken shortly after the hydrogen explosion in Unit 3. Note the roof on Unit 1 is missing and the Unit 3 building is damaged and steam being emitted which indicates the water in the spent fuel pool is boiling off. Figure 2 was taken after the fires in Unit 4 were reported. The building is now damaged and open to the atmosphere indicating the fire was more than a lube oil fire.
Efforts are concentrated on getting water into the Unit 3 spent fuel pool. The fuel pools for Unit 4, 5, and 6 have risen above their normal temperatures but are still below boiling point. We hypothesize Unit 3 lost its water inventory so much faster than the other units because it has developed a leak.
Reports indicate diesel generators are supplying power to Units 5 and 6. Later today they plan to have power to Units 1 and 2 by tapping into the high voltage lines. They hope to have power to Units 3 and 4 in a couple of days. Having electric power will greatly alleviate using non conventional methods such as fire trucks and fire pumps to get water on the fuel.
Fukushima Daiichi Status 3/19/2011
The water injection into the Unit 3 reactor building successfully cooled the fuel pool as evidenced by a satellite photo which lacks the steam being emitted from the damaged building. The fire trucks will be used to spray water into the Unit 4 building.
Until power is restored and the normal cooling equipment can be used, it will be necessary to repeat water spraying to keep the spent fuel pool water from boiling off and exposing the fuel.
There are now reports of contaminated food stocks in the area around the plant. This would be caused by radioactive particulates which have been released from the explosions and buildings which are open to the atmosphere. In particular, some radioactive iodine has been measured in the affected zone.
Figure 1 Satellite photo from March 14 after explosions in Unit 1 & 3 reactor buildings
Source: http://isis-online.org/isis-reports/detail/satellite-image-shows-damage-to-reactor-buildings-at-fukushima-daiichi-comp/
Figure 2 Satellite photo from March 16, Unit 4 reactor building roof is also compromised
Source: http://isis-online.org/isis-reports/detail/new-satellite-image-of-fukushima-daiichi-nuclear-site-in-japan-from-march-1/
Fukushima Daiichi Status 3/20/2011
With electricity available to Units 5 and 6, the spent fuel pool cooling has been restored. So at this time Units 5 and 6 are in stable conditions.
Units 3 and 4 spent fuel pools continue to be sprayed by the fire trucks using seawater. Aerial reconnaissance indicates this is controlling temperatures in the spent fuel pools. They are working on getting power to Unit 4.
They are now unsure whether the Unit 3 reactor containment is damaged as earlier reported because it is holding pressure.
They have begun spraying the Unit 2 spent fuel pool with seawater from the fire trucks. The reactor continues to be cooled with seawater.
The Unit 1 reactor continues to be cooled with seawater. The spent fuel pool has not been sprayed yet.
It appears that they are close to having electricity connected to Units 1 and 2.
3/21/2011 Status
Getting electricity restored to each of the units is a first step. Electrical equipment which has been soaked from the seawater spraying is particularly susceptible to shorting out. Workers need to check out the pumps and motors prior to energizing them.
Unit 5 was on diesel generator power. It has now back on the normal AC power. Restoring Unit 6 to AC power is in progress. With AC power restored, these units are in a safe condition.
There were reports of grey smoke emanating from Unit 3 and later Unit 2. We suspect some smoke was from equipment which caught on fire when Unit 2 electricity was restored and the smoke followed a path out of the Unit 3 reactor building.
When electricity is available, they are working on getting the control room ventilation systems working. The control room contains all the switches and indicators needed to operate the unit. When the plant lost power and the hydrogen explosions occurred, one of the consequences was the control rooms were evacuated because the ventilation systems could not filter the radioactive contaminants. Plus without power they could not do much. Getting back into the control rooms will be helpful in assessing the condition of the unit.
Measuring radiation
Some media sources are quoting radiation doses in mrem and some are using microsieverts. The dose is the amount of energy which would be absorbed by human tissue. The higher the dose the more likely it will cause cellular damage. This damage could destroy a living cell or cause changes to the DNA. But there are acceptable amounts of radiation.
Mrem (millirem) and µSv (microsievert) are measures of radiation dose. There are 1000 mrem in a rem. rem is the customary unit in America. The international standard uses sievert.
1 mrem = 10 µSv
Some examples of radiation doses encountered by the general public:
| Airplane flight | 0.01 to 1 mrem depending on length and sunspot activity |
| Chest X-ray | 6 mrem |
| Mammogram | 70 mrem |
| Dental X-ray | 9 mrem |
| EPA Cleanup standard | 15 mrem/year (amount of radiation if at the site for a full year) |
| Avg Dose to human-made sources in a year | 66 mrem |
Status 3/22/2011
A crane with a boom has been setup to replace the spraying of seawater into the Unit 4 spent fuel pool. This equipment will produce less seawater runoff. Unit 3 continues to receive periodic spray downs to maintain temperature and levels.
The spray downs have been used to keep the spent fuel pool temperatures under control, but produce runoff. Several samples of seawater were taken from the ocean around the plant and found to contain elevated levels of radioactive Iodine and Cesium. Japanese officials indicate the elevated levels are still considered safe for someone consuming the average amount of water ever day for a full year. The ocean around the plant could have become contaminated by the runoff and the atmospheric releases.
The smoke seen coming from Unit 2 has subsided. The smoke from Unit 3 has changed from gray to white. No information has been provided as to the source.
AC power has been restored to Units 2, 4, 5, and 6.
Status 3/23/2011
AC Power is available to all units. But checking out the equipment before energizing it is taking time. They are restoring equipment based on a prioritized list. If a piece of equipment checks out it is energized, if it cannot be energized they determine whether to replace it or go to the next item on the list. They have restored lighting to the Unit 3 and 4 control room.
A plant status published by the Japanese Atomic Industrial Forum indicates "possibly damaged" for the Units 3 and 4 spent fuel pool. This is the first time we have seen a report recognizing the possibility of spent fuel pools are leaking, something we have suspected for awhile. Because some fuel rods in these pools are damaged, the continued replenishing of the water in these pools can be carrying off the radioactive contaminants from inside the fuel rods thus spreading the contamination.
Status 3/25/2011
On March 24 workers in Unit 3 crossed a flooded area. The water was deeper than the height of their boots resulting in their feet getting soaked. The water was contaminated with radioactive particles from the fuel causing the workers feet to be exposed to high levels of radiation. Their feet were washed but they had already been exposed to levels of radiation which caused burns.
Because the contaminants were from the fuel, there have been some concerns that this indicates the Unit 3 reactor vessel has been breached. We think it is more likely the contaminants are from the damaged fuel in the leaking spent fuel pool.
Status 3/28/2011
Experts were expressing concern about salt deposits building up on the fuel assemblies in the reactor which would prevent cooling from taking place. The US provided some barges of freshwater. The freshwater is now being injected into the reactors to cool the fuel. Now the concern is the contaminated water in the lower levels of the turbine building where pieces of equipment needed to establish normal cooling reside. The contaminated water prevents workers from safely entering the areas. The inventory of radioactive contaminants in the water is being used to determine the where the source of the water. Initial reports that it was a reactor was based on an incorrect measurement which identified a short lived iodine isotope in the water.
What happens to the radioactive contaminants?
When the reactors shutdown on March 11 from the earthquake, the nuclear chain reactions stopped, establishing a baseline inventory of radioactive isotopes in each fuel assembly. Each radioactive isotope has its own half life. The half life is the time it takes for only 50% of the baseline amount to remain. The shorter half lives are measured in seconds, the longer are measured in years. It takes 6 half lives to reduce the amount of a radioactive isotope to less than 1% of the baseline inventory. The isotope is changing because radioactivity is the emission of a subatomic particle or energy which results in a change in the inventory of neutrons, protons, and electrons in the atom. The radioactive emission can create a stable atom (i.e. the new atom does not emit radioactivity) or another radioactive isotope with a different half life. But all the chains eventually end up with as a stable atom.
Iodine-131 has been measured in Tokyo's water, crops around in the plant, and in the sea water. Since March 11, Daiichi has not produced any more. This isotope of Iodine has a half life of 8.1 days, so as of Sunday March 27 there were 2 half life periods which results in only 25% of the baseline amounts remains. If Iodine-131 is ingested, the body first replenishes thyroid gland with the iodine and allows any surplus to be excreted as waste.
Caesium-137 is another isotope which is being monitored. It has a half life of 30.1 years. So it will be around for decades. The concern about caesium is it will enter the food chain by accumulating in fruits and vegetables. When food with caesium is ingested the body will remove it through sweat or urine.
The amount of radioactive contaminated water which needs to be removed is a challenge. Normally the water could be processed through demineralizer tanks which would concentrate the contaminants, but result in clean water. But most likely these plant systems have been damaged. Daiichi is pumping the water into the condenser, a chamber which normally collects water and steam after it has been used to turn the turbine generators.
Status 3/29/2011
There has been a shift in actions being taken at Daiichi. Starting with the 3 workers getting contaminated by radioactive water on March 24, there appears to be a greater emphasis placed on the containment of the radionuclides which have been released from the damaged fuel.
In addition to the water collecting in the lower levels of the reactor buildings, contaminated water has now been found in trenches used for electric cable and piping which run under the site grounds. A trench which is 12 feet high, 9 feet wide and 228 feet long yielded a 100 rem/hr water sample. They are trying to determine the source of the contaminated water. Workers are putting sandbags around the trenches to contain any water which may overflow.
There are not enough tanks and places to which the contaminated water can be pumped.
The pumping of freshwater into the reactors to maintain cooling has been reduced by over 50% in an effort to reduce a source of the water. This will cause the reactors to heat up. But the control rooms are now inhabited again, so the temperatures are being monitored.
The reports of plutonium being found in samples taken from the ground on the site confirm the contents of the damaged fuel rods have been released to the atmosphere. The plutonium is a danger if it is ingested because it will be retained in the body emitting radiation.
Status 3/30/2011
Work at the plant centered on pumping the water from flooded areas. Pumping water from the Unit 1 reactor building into the condenser was suspended when the condenser became filled to capacity leaving about 8 inches of water in the building. Plans to pump water into the Unit 2 and 3 condensers were not executed because the condensers were already filled with water. Water will be transferred to other storage available storage tanks.
Freshwater is now being pumped to the Unit 1, 2 and 3 reactors using diesel generator driven electric pumps rather than the fire truck pumps.
How is the decay heat being removed?
Daiichi is working on keeping the fuel in Units 1, 2, and 3 reactors cool and the spent fuel in Units 1, 2, 3, and 4 pools cool by adding water to replenish the water being turned to steam by the hot fuel rods. Under normal conditions the steam is not released to the atmosphere. In the reactors and spent fuel pools decay heat normally heats up water which is run into a heat exchanger. The heat exchanger cools the water which has been in contact with the fuel without exposing it to the atmosphere. This is called a closed loop system. As an additional layer of protection the heated water from the heat exchanger is fed into another heat exchanger for cooling. The second heat exchanger system is cooled to the atmosphere.
From the information we have seen, Units 5 and 6 are being cooled in the normal mode.
Units 1, 2, and 3 do not have any of the heat exchangers working so water is either turning into steam or is collecting in the systems. In the reactor vessels the water level is about half way covering the fuel. As the steam pressure in the vessels builds up it is released into the reactor containment where it eventually condenses and collects in the suppression chamber. In the case of Unit 2, the reactor containment was damaged and the steam is able to enter the reactor building which is open to the atmosphere via a panel which has been removed on the side of the building. For Units 1 and 3 the reactor containments are able to maintain pressure. But without any heat removal systems for cooling the steam is building up and we hypothesize is venting to the reactor buildings which have been destroyed by explosions.
The spent fuel pools normally are cooled by the Residual Heat Removal (RHR) system and are not allowed to boil. The Unit 1, 2, 3, and 4 spent fuel pools are boiling off the water and water is being sprayed or injected to replace the lost inventory.
Until the cooling systems are available, TEPCO will need to find the balance between injecting water for cooling and causing more contaminated water to collect throughout the plant. Eventually they must stop the steam from being vented to the atmosphere without filtration because it is releasing radioactive contaminants.
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