03/29/2011
Fukushima Daiichi Site
How Did the Accidents Happen?
I should preface this discussion with the fact that there still much speculation surrounding what really happened at the site. We do know that the earthquake and following Tsunami cause the major problems at the site. When I refer to the site, I am talking about the 6 reactors at Fukushima Daiichi site, which is operated by Tokyo Electric Power Co.. The site is at a elevation of about 10 meters, so a 15 meter tsunami wave would temporarily flood the site. As discussed before, the accidents progressed from a SCRAM of the 3 operating reactors, the failure of the emergency diesels generators (EDGs) and the loss of offsite power, and the failure of the residual heat removal system (RHRS). The RHRS needs power to operate successfully. When off site power is not available, the back-up power to the RHRS is provided by the EDGs. The RHRS provided cool water to the reactor core, which houses the fuel rods. The RHRS provides cool water to the reactor to keep the fuel cool. The loss of the EDGs and off site power caused what we call a station blackout where there is no electric power to the site except direct current (DC) from batteries. The reactor core cooling system (RCIC) is a steam drive pump that can also provide cooling to the core. The boiling water reactors at Fukushima are also protected by a RCIC system, which can operate without AC power because it is steam-driven and therefore does not require electric pumps. However, it does require DC power from batteries for its valves and controls to function. The RCIC should be able to operate for 8 hours, which is the expected life of the batteries.
When the earthquake occurred, the 3 site reactors, i.e., Units 1, 2 & 3, that were operating were shutdown, via an automatic or manual actuation called a SRAM. Three reactors. The other three reactors,i.e., Units 4,5 & 6, were already in cold shutdown for either maintenance activities or for fuel reload activities.
The SCRAM is the insertion into the fuel core boron carbide control rods that basically absorbs neutrons and ultimately causes the reactor to shut down. After a reactor shutdown, there is residual heat being generated from the core. This is called decay heat and it is caused by the decay of fission products that were created during operation of the reaction. This decay heat occurs for a long time, getting water to the reactor core is critical to ensure that the fuel does not get too hot. When the fuel gets too hot, the zirconium cladding that surrounds and the fuel pellets heats up to over 2000 degrees F, which causes the zirconium to chemically interact with the water to create hydrogen. This hydrogen is vented out of the reactor vessel out through the containment into the secondary containment. I believe the accumulation of hydrogen and an ignition from a DC power source caused the explosions in the secondary containment.
There are several components common to BWR reactors:
Fuel. Uranium is the basic fuel. Usually pellets of uranium oxide (UO2) are arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.
Moderator and Coolant. This is water (H2O) that flows in the reactor core, which slows down the neutrons that are absorbed into UO2 fuel that causes fissioning and the creation of fission products that create the heat after a reactor shutdown. In a BWR the water moderator functions also as primary coolant for heat removal from the fuel rods.
Control rods. These are made with neutron-absorbing material such as boron carbide and are inserted into the core to shutdown the reactor. The quick insertion of the control rods are performed during a SCRAM, which is an automatic shutdown of the reactor usually because of some safety parameter being exceeded.
Reactor pressure vessel. Is a robust steel vessel containing the reactor core and moderator/coolant.
Containment. The structure, which is about 3 foot steel reinforced concrete, around the reactor pressure, which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any malfunction inside. The containment along with the Mark 1 Torus are constructed to ensure that the pressure created from large pipe breaks do not exceed design structural failure limits of the Torus and containment.
Figure 1 shows the Mark 1 BWR reactor, which is similar to the containment and reactor at the Fukushima Daiichi site.
The Mark I containment, consisting of a rectangular steel-reinforced concrete building, along with an additional layer of steel-reinforced concrete surrounding the steel-lined cylindrical drywell and the steel-lined pressure suppression torus below. The Mark I was the earliest type of containment in wide use, and many reactors with Mark Is are still in service today. The reactor building of the Mark I generally is in the form of a large rectangular structure of reinforced concrete.
The Mark II containment, similar to the Mark I, but omitting a distinct pressure suppression torus in favor of a cylindrical wetwell below the non-reactor cavity section of the drywell. Both the wetwell and the drywell have a primary containment structure of steel as in the Mark I, as well as the Mark I's layers of steel-reinforced concrete composing the secondary containment between the outer primary containment structure and the outer wall of the reactor building proper. The reactor building of the Mark II generally is in the form of a flat-topped cylinder. The Mark III containment, generally similar in external shape to the stereotypical pressurized water reactor (PWR) containment, and with some similarities on the inside, at least on a superficial level. For example, rather than having a slab of concrete that staff could walk upon while the reactor was not being refueled covering the top of the primary containment and the RPV directly underneath. Additional changes include abstracting the wetwell into a pressure-suppression pool with a weir wall separating it from the drywell.
(Taken from http://en.wikipedia.org/wiki/File:Reaktor.svg )
1 Core with fuel rods
2 Concrete shield plug
3 Equipment pool
4 Drywell head
5 Fuel storage pool; spent fuel area
6 Refueling cavity
7 Drywell flange
8 Reactor pressure vessel
9 Biological shield
10 Secondary concrete shield wall
11 Free standing steel drywwell
12 Radial beam
13 Concrete embedment
14 Jet deflector
15 Expansion bellows
16 Vent header
17 Downcomer pipe
18 Water (wet well)
19 Embedded shell region
20 Basement
21 Reactor building
22 Refueling platform
23 Refueling Bulkhead
24 Pressure suppression
chamber (runs in a
torus around the reactor)
25 Vent (81 inch diameter)
26 Crane
27 Used Fuel
28 Coolant pipe
29 Cold water pipe (from generator)
30 Steam pipe (to generator)
31 Control rod drives
39 Control rods
40 Steam separators (water normally goes to this level)
41 Steam dryer
42 Vent and head spray
How the Heat is Created?The heat and fission products are produced in the uranium oxide (UO2) fuel pellet by the fission process as shown in Figure 2. The Urianiun-235 atoms, which are contained in the UO2 fuel pellet, are split by a neutron. This splitting (i.e., fission) of the U-235 atom causes other neutrons to be created that in turn split other U-235 atoms, the creation of fission products and a large amount of energy. When the reactor is shutdown, the control rods as shown in Figure 6 are inserted into the core thereby stopping the chain reaction that causes the creation of neutrons and the large amount of energy. What is left in the fission products, which produces decay heat. This relatively small amount heat has to be removed from the fuel or a heat up will occur that will eventually melt the fuel. The process of this heat-up is shown in Figure 3. As shown in Figure 3, the heat is generated in the fuel pellet and is dissipated to the Zr cladding that the surrounds the fuel pellets. The heat is transferred through the cladding and into the water coolant. If there is no water coolant removing the heat, the Zr becomes hot enough to cause a reaction with the water, which generates hydrogen that is ultimately released outside the containment. The Zr clad keeps getting hotter until it melts at 3000 degrees C and eventually the fuel melts at 5000 degrees C. If water is present, none of this happens. That's why it is so important to keep water in the reactor above the fuel.
![Fig2_fission[1]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_szlsHmsOeEPBBUqHRLIc_ysf0wbce54OjxVFsDD4ofUHqIsmndgHz3D34o203fYdnZWXgeA2EZsLjfXPGB7Uh6RboJfIC5544YglLZ6MdPvovRVPATSDTOTgh-QNgI17vs7_GH3oY0ZhegVYh3Tw=s0-d "Fig2_fission[1]")
Figure 2 Fission Process
Fuel Pin sometimes called Fuel Rod
As shown in Figure 3, the uranium oxide (UO2) fuel pellets that are about 3% enriched with U-235 are inserted into a Zr-2 cylindrical fuel rod. These fuel rods are arranged in a fuel bundle matrix (called an array) of varying sizes depending on the fuel manufacturer.
The fuel (UO2) height in a BWR is about 12.3-12.5 ft (146-150 inches). So the Zr-2 rod shown in Figure 3 is a cylinder that is about 12.5 feet long. The fuel pellets, as shown in Figure 3 being held by a tweezers in the individual's hand, are stacked in the Zr 12.5 feet cylinder sealed at both ends by Zr endplugs. Fuel in the 1970s and 1980s used an 8x8 array of fuel rods. In the mid 1980s, 9x9 arrays were introduced, and in the 1990s, 10x10 arrays became the norm. The rods got smaller in diameter and the different arrays all occupy the same envelope, which makes up the fuel bundle array. A 9x9 fuel bundle array is shown in Figure 4 and a 9x9 fuel bundle array being inspected in Figure 5. For example, the 9x9 fuel bundle array contains 72 12.5 feet Zr cylinders (72 fuel rods) that are held together by top and bottom tie plates and an spacers as shown in Figure 4. The other 9 positions in the 9x9 fuel bundle array are four large water holes that are used for fuel use optimization.
Figure 5 9x9 Fuel Bundles being inspected before they are shipped to the plant for use.
Control Rods
The control rod (Item 7) as shown in Figure 6 is surrounded by 4 fuel bundles. When the plant SCRAM occurs, these are inserted into the core from the bottom. The boron carbide control rods absorb enough neutrons to stop the nuclear reaction in the core and the plant shuts down. The decay heat caused by the fission products is present and must be removed by the coolant, which is water for a light water reactor (LWR) like the plants at the Fukushima Daiichi site.
What happens after the SCRAM?
When the earthquake occurred, the 3 plants that were operating at the time were immediately shut down (SCRAM by insertion of control rods). When the control rods as shown in Figure 6 are completely inserted (SRAM), the neutron stop the fission process and the plant is shut down. When the fission process stops, the radioactive fission products continue to decay and release heat. This decay heat is on the order of 6% of the power plant's 100% power at 1 second after shutdown and about 1 % of the power plant's 100% power (PO) at 1 hour after shutdown, so, for Fukushima unit 3 & 4 (760 MWe), this is on the order of 2400 MWt energy created at full power operation (PO), or about 144 MWt of decay heat right after shutdown and about 24 MWt of decay heat at 1 hour after shutdown. This is shown in Figure 2 where PO is the 100% operation power of the reactor. Now what does 144 MWt mean to you? It means that there are 144 x 106 watts being generated by decay heat of the fuel fission products in the reactor vessel immediately after shutdown. A watt is equal to 3.412 BTU/hour. A BTU is defined as the amount of energy that it takes to raise 1 pound of water by 1Fo at 14.7 psia. This is called the heat capacity factor of water at atmospheric conditions, Cp = 1.0 BTU/ Fo - lbm of water = 4.1855 [J/(g•K)] (15 °C, 101.325 kPa).
So if you have a 10 foot by 10 foot by 10 foot room filled with water 60 Fo. This room contains 1000 cubic feet of water, which is about 62,000 pounds of water. The decay heat from the Fukushima unit 3 is about 24 MWt at 1 hour after shutdown, so Fukushima unit 3 reactor decay heat could raise the water temperature in your 10 feet by 10 feet by 10 feet room from 60 Fo to 70 Fo in 27.2 seconds. Now if you used the decay heat right after shutdown, the decay heat is increased by a factor 6 (See Figure 2), therefore it would take about 4.6 seconds to raise your room water temperature by 10 Fo . This shows you how significant the decay heat in the first hour after shutdown and how much it changes. As you can see from Figure 8, there is a significant reduction in decay for the first 4 days after it is removed from the reactor. It reduces from 6 % of initial power at 1 second to about .10 % of initial power at 4 days. Based on initial information, the station lost all electrical power in the first hour after the Tsunami and about 8 hours the other system (i.e., RCIC) was lost. The RCIC system is used in a BWR to provide heat removal to the reactor fuel in the event of a station black out. The RCIC failed to operate due to lost battery power and also possibly due to the lost make-up water supply. By this time when the plant was in a complete station black out without batteries, the decay heat had reduced to approximately .6 % of the initial power or about 14.4 MWt and a heat up of the water in the reactor vessel began.
Eventually, since there is no heat removed from the Torus due to the failure of the RHRS, the water in the suppression pool reaches something we call “saturated steam” , i.e., it cannot absorb any additional heat and it, too begins to boil, increasing pressure in containment. To stay within design limits for the primary containment, operators reduce pressure by venting steam through filters (to scrub out any radioactive particles) to the atmosphere through the vent stack. This steam contains water vapor, Hydrogen, Oxygen and some radioactive material, mostly tritium and 16N. If the fuel pellet is melting you will also see Cesium and 131I. If the vented gases passed through the vent pipes and filters, much of the radioactive particles were scrub by these filters, although it is believed that some of this radioactive material leaked into the secondary containment and was not filtered. This caused the site radioactivity to go up. With these radioactive gases, significant quantities of hydrogen gas were being released that was created from the Zr water reaction in the core. The vents from the containment that are filtered prior to being released to the atmosphere are required in the US to be hardened vents. The Mark-I containment vents at the Fukushima Daiichi site may have been constructed of duct work, therefore due to the high pressure of the released gases, the duct work could have failed, which leaked hydrogen into the secondary containment that ultimately lead to the explosion in the secondary containment. The secondary containment on a Mark-I plant is a steel structure located directly above the primary containment (See Figures 1 & 7). It houses among other things, the crane that moves fuel from the reactor core to the spent fuel pool. The secondary containment is not as structurally strong as the primary containment. The hydrogen gases accumulated in the secondary containment from possible leaking during the venting and at the combustion level of hydrogen-air mixture (4-75% by volume), the secondary containment compartment holding this hydrogen-gas mixture burned/exploded caused a large pressure spike in the secondary containment that probably caused the secondary containment structure to fail and burn. The ignition source could be from batteries since the plant was in a "station blackout" and using battery power. The burn/explosion process, called combustion, involves the reaction of hydrogen with oxygen, and produces water and a large amount of heat. If allowed to expand rapidly from a high-pressure cylinder such as venting from a high pressure, hydrogen can also ignite spontaneously in air. These explosions that were seen in all three units probably created large holes in the secondary structure therefore hydrogen and noble gases escaped to the atmosphere that caused off site radiations levels to go up but the release of Hydrogen directly to the atmosphere also minimized the accumulation of explosive levels of Hydrogen in the secondary containment, thereby reducing the chance of explosions in the building. The first explosion, which occurred Unit 1 secondary containment at 15:36 local time on Saturday March 12, 2011 (about 10 hours prior to the explosion venting began in Unit 1) , was likely caused by a build-up of hydrogen gas inside Unit 1's secondary containment building. According to other sources, the fire involved oil and other lubricants in the building. The explosion blasted holes in the secondary containment building wall. Just over two hours later, fire broke out.
Following the loss of electric power to normal and emergency core cooling systems and the subsequent failure of back-up decay heat removal systems, water injection into the cores of all three reactors was compromised, and reactor water levels could not be maintained. In addition, the heat removal capability of the 6 spent fuel pools were also compromised. Tokyo Electric Power Company (TEPCO), the operator of the plant, resorted to injecting sea water and boric acid into the reactor vessels of these three units, in an effort to cool the fuel and ensure the reactors remained shutdown. It appears that primary containments for Units 1 and 3 remain functional, but the primary containment for Unit 2 may be damaged. TEPCO cut a hole in the side of the Unit 2 secondary containment to prevent hydrogen buildup following a sustained period when there was no water injection into the core.
In addition, Units 3 and 4 have low spent fuel pool (SFP) water levels. Efforts continue to supply seawater to the SFPs for Units 1 through 4 using various methods. At this time, the integrity of the SFPs for Units 3 and 4 is unknown.
Fukushima Daiichi Units 4 through 6 were shutdown for refueling outages at the time of the earthquake. The fuel assemblies for Unit 4 had been offloaded from the reactor core to the SFP. The SFPs for Units 5 and 6 appear to be intact, but the temperature of the pool water appears to be increasing. Emergency power is available to provide cooling water flow through the SFPs for Units 5 and 6.
What is a Spent Fuel Storage Pool?
Spent fuel pools (SFP) are storage pools for spent fuel from nuclear reactors. The SFPs at the Fukushima site, which are located in the secondary containment adjacent to the primary reactor containment (See Figure 9) , resemble large swimming pools. They’re about 40 feet (12.2 m) long by 30 feet (9.15 m) wide by 38 feet (11.59 m) deep, although they vary in size. In total they can hold about 1,300 to 1,400 metric tons of water, serving as both a shield radiation from spent fuel and as a coolant to lower the residual decay heat that spent fuel rods generate.
The pools contain anywhere from 400 to 700 fuel-rod assemblies. These fuel assemblies, which are the same fuel assemblies removed from the reactor core, sit on racks just above the pool floor, as shown in Figure 12. During normal operation, the water level in the pools is kept about 21 feet (6.40 m) above the top of the fuel assemblies.
Typically 38 or more feet (11.59 m) deep, with the bottom 17 feet (5.18 m) equipped with storage racks designed to hold fuel assemblies removed from the reactor. While only about 8 feet (2.43 m) of water above the spent fuel is needed to keep radiation levels below acceptable levels, the extra depth of approximately 13 feet (3.96 m) of water provides a safety margin and allows fuel assemblies to be manipulated without special shielding to protect the operators.
The SFP is shown in Figure 1 as item 5 and in Figure 7. A photo of the fuel rack in the Fukushima Daiichi Unit 3 plant is shown in Figure 10. This is similar to the SFP in Unit 4. These fuel pools are specially designed at the reactor in which the fuel was used and situated in the reactor building as shown in Figure 1. The storage of the spent fuel at the Fukushima Daiichi Site for each unit is shown in Table 1.
In Japan, the fuel assemblies, after being in the reactor for 3 to 6 years, are stored underwater for 10 to 20 years before being sent for reprocessing or dry cask storage. The Japanese are now building the first Japanese interim spent fuel storage facility at Mutsu and it should be ready for production in 2012. After the fuel has spent a significant time in the SFPs, they may be transferred to a dry storage cask faculty where the are store until the fuel is reprocessed. The dry storage facility uses air to cool the fuel since the fuel heat load is so low after spending 10 - 20 years in the SFP.
About one-fourth to one-third of the total fuel load of a reactor is removed from the core every 12 to 18 months and replaced with fresh fuel. Spent fuel rods generate heat and high levels of radiation that must be contained. Fuel is moved from the reactor and manipulated in the pool generally by automated handling systems or manual systems. Figure 11 shows the fuel assembly being lowered into the fuel pool.
The maximum temperature of the spent fuel bundles, which is proportional to the decay heat in the spent fuel, decreases as shown in Figure 13. The fuel pool water is continuously cooled by the use of pumps and heat exchangers to remove the heat that is produced by the spent fuel. The temperature is dependent on the decay heat created by the spent fuel and the heat removal by the cooling system. When there is no cooling system, the temperature in the pool will continue to rise until boiling in the pool occurs.
As you can see from Figure 13, the decay heat rate reduces at a much slower rate after 4 days. In the first four hours (See Figure 8), the decay heat rate drops by a factor of 12 and in the next 10 years the decay heat rate has drop by a factor of 100.
Pumps circulate water from the spent fuel pool to heat exchangers that cool the water, then back to the spent fuel pool. Radiolysis, the dissociation of molecules by radiation, is of particular concern in wet storage, as water may be split by residual radiation and hydrogen gas may accumulate increasing the risk of explosions. For this reason the air in the room of the pools, as well as the water, must permanently be monitored and treated.
What Happened to the Spent Fuel Storage Pools after the Earthquake?
When the earthquake occurred, the 3 reactors that were operating at the Fukushima Daiichi site were automatically shut down and the RHRS, which is the automatic cooling system, began to cool the 3 reactor cores. It does not appear that the earthquake itself damaged any of the emergency equipment. It was reported by TEPCO, that the RHRS operated for about 1 hour until the RHRS lost both off site power and the diesel generator emergency power, probably because of the flooding created by the Tsunami. The RCIC system, which is a steam driven system using steam from the reactor, cooled the reactors for about 8 hours until DC battery power was lost, then there was no cooling to the reactors and the stagnant water in the reactors began to heat up.
During this same sequence of events, all 6 spent fuel pools (SFPs), were heating up as soon as power was lost at about 1 hour. The SFPs are very forgiving since, due to the relative low decay heat in the pools, the pool water takes a significant amount of time to heat up and boil. Because the reactors require immediate attention to ensure proper cooling of the fuel is attained, all attention should be place on the reactors, although there were some early reports that suggested that attention to Unit 3 SFP caused the operators to neglect the reactors. This doesn't seem logical. The operators should be giving most of their attention to the reactors, especially early in the transient.
Early attention should have been given to Unit 4 SFP since a current full core off load had been performed about 110 days prior to the event. A summary of the Unit 4 events are provided below:
・ Because of the replacement work of the Shroud of RPV, no fuel was inside the RPV.
・ The temperature of water in the Spent Fuel Pool at Unit 4 had increased. (84 oC (183oF) at 04:08 March 14th, which is 16 oC from boiling)
・ It was confirmed that a part of wall in the operation area of Unit 4 was damaged. (06:14 March 15th)
・ The fire at Unit 4 occurred. (09:38 March 15th) TEPCO reported that the fire was extinguished spontaneously. (11:00 March 15th)
・ The fire occurred at Unit 4. (5:45 March 16th) TEPCO reported that no fire could be confirmed on the ground.(At around 06:15 March 16th)
・ The Self-Defence Force started water spray over the Spent Fuel Pool of Unit 4 (09:43 March 20th).
・ On-site survey for leading electric cable (From 11:00 till 16:00 March 20th)
・ Water spray over the Spent Fuel Pool of Unit 4 by Self-Defence Force was started. (From around 18:30 till 19:46 March 20th).
・ Water spray over the Spent Fuel Pool by Self-Defence Force using 13 fire engines was started (From 06:37 till 08:41 March 21st).
・ Works for laying electricity cable to the Power Center was completed. (At around 15:00 March 21st)
・ Power Center received electricity. (10:35 March 22nd)
・ Spray of around 150t of water using Concrete Pump Truck (50t/h) was carried out. (from 17:17 till 20:32 March 22nd)
・ Spray of around 130t of water using Concrete Pump Truck (50t/h) was carried out. (From 10:00 till 13:02 March 23rd)
・ Spray of around 150t of water using Concrete Pump Truck (50t/h) was carried out. (From 14:36 till 17:30 March 24th)
The complete summary will be provided later.
Since Unit 4 spent fuel pool (SFP) is the SFP that is currently having heat up issues, we will provide most of our discussion focused on Unit 4 SFP. During the Tsunami flood at Fukushima Daiichi site, Unit 4 was shut down for shroud repair. The shroud is located above the core as shown in Figure 14 was removed for repair and all fuel was removed from the Unit 4 reactor and placed in the SFP. In addition to the reactor fuel, other reactor internals, i.e., steam dryer, cycle separators and stand pipes, and the upper head, were all removed and placed in the SFP for shielding purposes since these materials are radioactive.
Based on what is know now, it is estimated that the fuel in the SFP of the secondary containment heated to a temperature of over 2000 oF. This caused a Zr-water reaction to occurred that released significant amounts of hydrogen gas in the secondary containment. The hydrogen accumulated in the secondary containment located at the top of the reactor building and exploded, which caused significant damage to the top of the reactor building as shown in Figure 15. The hydrogen could only come from the SFP since the Unit 4 reactor did not have any fuel in it. The fire/explosion took place at about 09:38 on March 15th. The building damage and the high radiation from the exposed spent and off loaded fuel in Unit 4 SFP created significant accessibility problems. In most cases, a small amount of water can keep the SFP covered. A fire hose delivering 200-300 gpm of water can typically keep the fuel cool. With the full core off load, it could be 30% more because of the additional decay heat. Without this cooling, the pool will heat up and eventually boil and loss water inventory. I believing the of loss of water because of the boiling in Unit 4 SFP caused the fuel to be exposed, which created high radiation levels. Because of the accessibility issues caused by the explosion and high radiation, no one could get to the pool area to put water in the pool and the pool became dry for some time. This caused some of the fuel to melt and release fission products. Some spray by fire water cannons began on March 20th at 19:46, which was about 10 days after the event began. This is a significant time period where the fuel in the Unit 4 SFP could have melted. As I write this paper it is not clear what is happening in Unit 4 SFP.
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