In short, the events that culminated to result in the Fukushima Daachi disaster in 2001 were largely unavoidable, but as with most situations of this nature sometimes the event itself is not the issue, but how it is handled. A high-caliber tsunami and earthquake created the perfect storm for a nuclear accident, with widespread damage that continues to be noticeable today. In short, the plant lost AC and DC power, and the plant operators used improper readings to falsely assume the situation was managable, when in actuality it was a very critical situation that should have been met with more force in hindsight. In addition, when it comes to Fukushima, TEPCO employees, as well as the employees of the auditing company, were immoral and dishonest in terms of divulging the errors made by the company. These factors came together to result in contamination that is found in the air and water nearby, making the impact very intense locally, and on a level of contamination that can be found globally. This report will touch on the background information that lead up to the events that took place at the plants, as well as the key decisions that were made by TEPCO before and after the accident took place. It will also look into how these decisions impacted the local Japanese culture as well. Lastly, it will be discussed how if the plant had been outfitted with a different selection of core materials as well as a passive core cooling system, this accident may have resulted in a much less severe impact.
I. BACKGROUND INFORMATION ON THE FUKUSHIMA ACCIDENT
In March of 2011, a 9.0 earthquake caused a significant amount of damage to the Japanese coast of Daaichi. Down the coast, several sets of Reactor plants were shut down due to the loss of power. Fukushima Daaichi was the most affected by this loss of power. When the reactor plant was originally designed, the engineers only accounted for a 7m high wave, The height of the tsunami was approximately 13m*. The whole area surrounding the major buildings of Units 1 to 4 was flooded to a depth of approximately 1.5m to 5.5m. (Fukushima Nuclear Accident Analysis Report (Interim Report)). Due to poor designing of the layouts of the reactor plants, the emergency generators were installed below the water level that the tsunami reached instead of above. Although the earthquake caused a loss of power, causing the reactor plants to shutdown, the tsunami had effects that were much more severe. When a reactor plant that has been operated for a long time at high powers is shutdown, decay heat is created. This decay heat caused a rise in temperature and subsequently the explosion of the 1, 2, 3, and 4 reactor plants.
II. INITIAL CONDITIONS AND EVENTS AFTER THE EARTHQUAKE
Before the earthquake occurred, Fukushima Daachi Units 1-3 were operating at power producing electric power for the grid while Units 4-6 were shutdown. Unit 4 was depressurized and undergoing a refueling overhaul with its contents removed from the core. Units 5 and 6 were also undergoing specific inspection, but still had their contents within the core.
Due to the seismic acceleration caused from the earthquake, Units 1-3 were shut down after operating at full power. Within minutes, the Emergency Diesel Generators were started up automatically due to the loss of the off-site power supply. All was occurring as expected with a loss of power: the EDGs had started up and were supplying emergency power to the decay heat removal systems within each plant. Roughly 45 minutes after the onset of the earthquake, the tsunami arrived with an estimated 15m, which was much larger than the sea wall of 5m. (FUKUSHIMA DAIICHI: ANS Committee Report.)
FIG 1 – The Epicenter of the Great East Japan Earthquake, IAEA (2015)
This caused the loss of all but one EDG operating at plant number 6. This caused the loss of all AC power generation for cooling components at units 1-5 due to the electrical connections being submerged in water. Unit 6 emergency AC power generation continued due to Unit 6 having air cooled components within it instead of water like the others.
The shutdown of the operating reactor plants was necessary to minimize the amount of heat that was built up in the reactor plant. This shutdown of each of the reactor plants was caused by the response acceleration in the ground caused by the earthquake. Concurrently, the transmission lines for off-site emergency power distribution were damaged by the earthquake. The effects of this earthquake, and the resulting response accelerations can be seen below.
FIG 2 – Observed and Design Basis Seismic Data, INPO (2011)
Before 2011, the probability for exceeding the design basis acceleration was in the range of 10-4 to 10-6 per reactor-year (INPO, 2011). As can be seen from the above table, the largest horizontal acceleration seen was 550 gal in Unit 2, and 302 gal in the vertical direction. Not only were several design basis accelerations exceeded with the earthquake, each of the plants scram set points for the plants were also exceeded during the earthquake. Once the scram had occurred at each of the operating plants, operators began attempting to operate within protocol.
After roughly 45 minutes on emergency power, the tsunami that struck the coast caused a loss of all the operating EDGs in reactors plants 1-5, and all but one EDG in reactor plant 6. Not only did the tsunami cause a loss of power, it caused a significant amount of damage to operating sea water pumps in the plants as well.
The emergency diesel generators produce 3 phase alternating current to be used by the emergency cooling systems. In order for 3 phase Alternating Current to be produced, there are three things that are required for inducing a voltage: current carrying conductor, magnetic field, and relative motion between the two. The electricity for the plant is produced by the interactions between the three and can be seen below.
In order for 3 phase AC voltage to be generated, 3 assumptions need to be made:
• What the north pole is under any unprimed winding, the output is max (+)
• The RHR for coils must be used for rotor flux
• The rotation of the rotor is in the clockwise direction
As the rotor spins around, voltage is induced in the armature 3 phase generation is produced. Frequency of the 3 phase sine wave can be calculated by multiplying the number of poles and speed of the rotor and dividing that by 120. For example, if a generator spins at 3600 revolutions per minute, and contains 2 poles, the frequency can be calculated as follows:
=> Where F= frequency, N= speed of the rotor
Emergency generation of power is achieved by attaching a diesel engine to the other end of the rotor field. In the case of the reactor plants in Fukushima, the time emergency generation of power was available was cut short due to the tsunami that struck shortly after the earthquake.
ATTEMPTING A RECOVERY
With Unit 6 being the only unit with emergency power, operators began to attempt to recover power by cross connecting power in units 5 and 6. When all emergency power was lost to the plants, TEPCO personnel began informing the government of the emergency conditions that existed. The problem was that the roads on the path to the plants were damaged by the flooding from the tsunami. After the first plant was scrammed, operators began frantically trying to remove decay heat in accordance with written procedures. The problem with the first 3 reactors was that the off-site power was lost due to the damage of transmission lines as well. This loss of off-site power resulted in the loss of normal systems used in the event of a reactor scram to control pressure and temperature.
In addition, the steam stops for plant 1 were shut as well. This resulted in the ability to remove decay heat, and subsequently pressure began to rise in plant 1. Because unit 1 was an older model, operators were forced to utilize the plant isolation condensers. When operators utilized the plant isolation condensers, they felt that it lowered plant pressure and temperature too rapidly. Instead, the operators began to cycle the system as necessary to lower pressure to prevent core damage and minimize the chance of exceeding plant design pressure changes.
Units 2-6 were designed after 1 and as a result, did not utilize the isolation condensers to remove heat and pressure from the RPV. Units 2 and 3 had their steam stops shut as well, however, the steam reliefs that were designed to prevent RPV from overpressure lifted. This sent steam from the RPV into the suppression pools, and allowed operators to lower pressure within the plants. Due to the loss of AC and DC power in all plants except plant 6, the operators were at a loss for deciding the next actions to take to control plant temperature and pressure. It took roughly nine days to return power to units 1 and 2 after the tsunami and took over 2 weeks to restore power to units 3 and 4. Unit 5 was able to receive power from the operating EDG at unit 6.
DECAY HEAT REMOVAL (Program Outcome 1)
Although it proved futile in the end, the amount of decay heat that was removed from the plants was crucial in allowing more time to save the plants. The decay heat that was produced in the reactor plants was brought on by two major factors, power history and the time since the shutdown. This power history can be calculated by using the number of days the reactor has been operating and the levels it had been operating at. The following equation can be used to calculate beta and gamma decay heat generation:
Where P is the decay power, P0 is the nominal reactor power; τ is the time since reactor startup and τs is the time of reactor shutdown measured from the time of startup.
Formula 1 (Decay Heat Estimate for MNR)
Using the following information containing the plant history and time since shutdown, decay
heat can be calculated:
Power At Shutdown (%)
Duration at Power (days)
Duration Shutdown (days)
Thermal Power (MWth)
Core Decay Heat Generation at Time of Earthquake (MWth)
No Fuel In RPV
(Program Outcome 6)
Units 1 and 3 had the highest amount of decay heat generation produced in their plants. Using the information above, the curves can be generated showing their decay heat generation by entering the information into Microsoft Excel and producing the following graph. It’s important to note that the left side of the graph is the decay heat, and the bottom is the time since shutdown:
The most important time for decay heat removal in a plant is the period immediately following a SCRAM. When there is a reactor SCRAM, a condition where all the control rods are inserted causing the reactor to shutdown, the amount of fission reactions in the core almost reach a standstill lowering power to about 7% of full power in 1 second. The power doesn’t fully die off to zero simply because of the radioactive isotopes that remain from the prior fission events in the fuel. These fission products go on to create the different types of radiation as they decay, such as alphas, betas, and gamma rays particles. The formation of decay heat results when the radiation energy is deposited in the fuel.
After the shutdown, Unit 1 used a cooldown system where the heat and steam from the reactor was transferred to a condensing pool that was vented to atmosphere. While the steam from the reactor was allowed to enter the condenser pool, the heat was removed by a heat sink created from the ocean. Eventually, this condenser pool boiled away, resulting in the loss of the only heat sink cooling the reactor plant. This system when used properly had enough water to cool the RPV for eight hours. This method of core cooling is advantageous because it requires less power supplies due to being driven by gravity and thermal driving head.
FIG 3: Diagram of Reactor Core Isolation Cooling active system (IAEA, 2015).
UNITS 2-6 COOLING METHOD
Due to units 2-6 being newer models than unit 1, the decay heat was attempted to be removed by a reactor core isolation cooling system (RCIC). This system takes cool, borated water and injects it into the core. This system would seemingly continue for days if the RCIC pump began pulling from the suppression pool once the condensate storage tank ran out.
FIG 4: Diagram of Reactor Core Isolation Cooling active system (IAEA, 2015).
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