The Fukushima reactor building that exploded March 12 is one of a series of identical General Electric reactors constructed in Japan and the US. In this reactor design, the used nuclear fuel rods are stored in pools of water at the top of the reactor building. These “spent” rods are still highly radioactive: the radioactivity is so great the rods must be stored in water so they do not combust. The explosion at Fukushima Daiichi reactor unit 1 apparently destroyed at least one wall and the roof of the building: some reports stated the roof had collapsed into the building.
Two days later, the nearby building containing the plutonium-uranium (MOX) fueled Fuksuhima Daichii reactor unit 3 exploded. So why bother about the rubble of reactor No 1? The WaPo quotes a nuclear engineer who knows the answer:
Although Tokyo Electric said it also continued to deal with cooling system failures and high pressures at half a dozen of its 10 reactors in the two Fukushima complexes, fears mounted about the threat posed by the pools of water where years of spent fuel rods are stored.
At the 40-year-old Fukushima Daiichi unit 1, where an explosion Saturday destroyed a building housing the reactor, the spent fuel pool, in accordance with General Electric’s design, is placed above the reactor. Tokyo Electric said it was trying to figure out how to maintain water levels in the pools, indicating that the normal safety systems there had failed, too. Failure to keep adequate water levels in a pool would lead to a catastrophic fire, said nuclear experts, some of whom think that unit 1’s pool may now be outside.
“That would be like Chernobyl on steroids,” said Arnie Gundersen, a nuclear engineer at Fairewinds Associates and a member of the public oversight panel for the Vermont Yankee nuclear plant, which is identical to the Fukushima Daiichi unit 1.
People familiar with the plant said there are seven spent fuel pools at Fukushima Daiichi, many of them densely packed.
Gundersen said the unit 1 pool could have as much as 20 years of spent fuel rods, which are still radioactive.
We’d be lucky if we only had to worry about the spent fuel rods from a single holding pool. We’re not that lucky. The Fukushima Daiichi plant has seven pools for spent fuel rods. Six of these are (or were) located at the top of six reactor buildings. One “common pool” is at ground level in a separate building. Each “reactor top” pool holds 3450 fuel rod assemblies. The common pool holds 6291 fuel rod assemblies. [The common pool has windows on one wall which were almost certainly destroyed by the tsunami.] Each assembly holds sixty-three fuel rods. This means the Fukushima Daiichi plant may contain over 600,000 spent fuel rods.
The fuel rods must be kept submerged in water. Why? Outside of the water bath, the radioactivity in the used rods can cause them to become so hot they begin to catch fire. These fires can burn so hot the radioactive rod contents are carried into the atmosphere as vaporized material or as very small particles. Reactor no 3 burns MOX fuel that contains a mix of plutonium and uranium. Plutonium generates more heat than uranium, which means these rods have the greatest risk of burning. That’s bad news, because plutonium scattered into the atmosphere is even more dangerous that the combustion products of rods without plutonium.
Chernobyl on steroids. When the nuclear engineer from an identical plant states there’s any possibility of such a catastrophe, Washington, we have a problem. Chernobyl’s contamination settled upon people and nations thousands of miles from that reactor’s location. How far would “Chernobyl on steroids” travel? And where are the up to 20 years of reactor no 1 spent fuel rods that could cause such a problem, and the spent fuel rods held – until the building exploded – in the spent fuel rod pool atop reactor no 3?
Update: Japan’s chief cabinet secretary, Yukio Edano, is holding a news conference, just started. (10:15pm ET).
Yesterday the spent fuel rod pool at Fukushima Daiichi reactor 4 caught fire. About that time instruments at the plant showed an exponential increase in radiation levels. After the fire was quenched, radiation levels fell. In the hour before I sat down to write this, there was an explosion at the same spent fuel rod pool. As I write, another fire is burning there. NHK reports the radiation level – 300 to 400 milliSieverts – is so high that firefighters cannot approach the area.
NHK reports that by Monday March 14 the temperature in the spent fuel rod pool was 84 degrees C: nearly double the usual temperature. NHK reports that there aren’t temperature readings for today: technical failure. We do know the pool temperature increased by roughly twenty degrees C per day after loss of power on Friday. And we know that water boils at 100 degrees C.
The spent fuel rod pool at reactor 4 is one of seven pools for spent fuel rods at Fukushima Daichii. These pools are designed to store the intensively radioactive fuel rods that were already used in nuclear reactors. These “used” fuel rods still contain uranium (or in the case of fuel rods from reactor 3, they contain both uranium and plutonium from the MOX fuel used in that reactor). In addition to the uranium and plutonium, the rods also contain other radioactive elements. These radioactive elements are created in the rods by the intense radiation around the rods when they are in the reactor core (before they are moved to the spent fuel pools).
Six of the spent fuel rod pools are (or were) located at the top of six reactor buildings. One “common pool” is at ground level in a separate building. Each “reactor top” pool holds up to 3450 fuel rod assemblies. The common pool holds up to 6291 fuel rod assemblies. [The common pool has windows on one wall which were almost certainly destroyed by the tsunami.] Each assembly holds sixty-three fuel rods. This means the Fukushima Daiichi plant may contain over 600,000 spent fuel rods. The fuel rods once stored atop reactor 3 may no longer be there: one of the several explosions at the Fukushima reactors may have damaged that pool.
Now that we have partial meltdown in the reactor vessels – the part of the reactor where nuclear reactions are supposed to happen – in at least three of the Daiichi palnt’s six reactors, why bother with swimming pools for fuel rods? Simple. Even after they are no longer usable to drive nuclear fission in the reactor vessels, the “spent” fuel rods are still highly radioactive. Part of that radioactive energy is emitted as heat. That’s no surprise: heat from radioactivity is the how the reactor core vessels generate the heat that drives the nuclear plant’s turbines to generate electricity. The fuel rods don’t know whether they are in the core or in the pools: they keep emitting heat and radioactivity until the radioactive material decays into non-radioactive elements. That process can take years, which is why spent fuel rods are still dangerous years after they leave the reactor core.
How can we prevent the spent fuel rods from bursting into flame once they’re out of the reactor core? The Fukushima plant – like many other reactors – keeps the rods in water, which absorbs the heat energy. But the pools – like the water in a teakettle – will boil away unless new water is added. After the Fukushima plant lost power in Friday’s 9.0 earthquake and got hit by the tsunami, the plant was no longer able to keep the pools topped up.
How long does it take the water in spent fuel rod pools to boil down to dangerously low levels? Yesterday FDL reader MtnWoman – who worked at TMI for twelve years – told us about the 2000 Nuclear Regulatory Commission study that looked at this very question. For boiling water reactors (BWR) such as the Fukushima reactors, the time required for spent fuel rod pool water levels to drop to dangerouslyy low levels is about 140 hours. The NRC study only looked at rods that had been out of reactors for six months or more: I don’t have data about how long the rods at the seven Fukushima pools have been out of reactors. Fortunately for the NRC, they weren’t studying fuel rod poos on the upper floors of reactor buildings housing reactor core vessels that had lost adequate cooling and were in partial meltdown. This may explain why the spent fuel rod pool at reactor 4 ignited on Monday, roughly 100 hours after the quake and power loss, but before the 140 hours the NRC calculated.
Why did the spent fuel rod pool at reactor 4 catch fire again today? Yesterday the Institute for Energy and Enviromental Research‘s Arjun Makhijani wrote a very detailed report that answers this question. In his report he quoted extensively from the 2006 study perfomed by the National Research Council of the National Academies. Their report tells us:
“The ability to remove decay heat from the spent fuel also would be reduced as the water level drops, especially when it drops below the tops of the fuel assemblies. This would cause temperatures in the fuel assemblies to rise, accelerating the oxidation of the zirconium alloy (zircaloy) cladding that encases the uranium oxide pellets. This oxidation reaction can occur in the presence of both air and steam and is strongly exothermic—that is, the reaction releases large quantities of heat, which can further raise cladding temperatures. The steam reaction also generates large quantities of hydrogen….
These oxidation reactions [with a loss of coolant] can become locally self-sustaining … at high temperatures (i.e., about a factor of 10 higher than the boiling point of water) if a supply of oxygen and/or steam is available to sustain the reactions…. The result could be a runaway oxidation reaction — referred to in this report as a zirconium cladding fire — that proceeds as a burn front (e.g., as seen in a forest fire or a fireworks sparkler) along the axis of the fuel rod toward the source of oxidant (i.e., air or steam)….
As fuel rod temperatures increase, the gas pressure inside the fuel rod increases and eventually can cause the cladding to balloon out and rupture. At higher temperatures (around 1800°C [approximately 3300°F]), zirconium cladding reacts with the uranium oxide fuel to form a complex molten phase containing zirconium-uranium oxide.
Beginning with the cladding rupture, these events would result in the release of radioactive fission gases and some of the fuel’s radioactive material in the form of aerosols into the building that houses the spent fuel pool and possibly into the environment. If the heat from one burning assembly is not dissipated, the fire could spread to other spent fuel assemblies in the pool, producing a propagating zirconium cladding fire.
The high-temperature reaction of zirconium and steam has been described quantitatively since at least the early 1960s….”
Translation for laypeople: Without enough water to cover the, the fuel rods will keep on igniting, just like trick birthday candles keep re-igniting after we blow them out. Just like trick birthday candles, the only way to put out the fuel rods is to put them under water. That’s why even after Monday’s reactor 4 spent fuel rod fire was quenched, the spent fuel rod pool caught fire again this afternoon.
Unlike trick birthday candles, the spent fuel rods burn hot (3300 degrees F) enough so that the radioactive material in the rods is aerosolized: carried into the atmosphere in clouds of hot smoke. And unlike our trick birthday candles, the spent fuel rods in reactor building 4 are four stories off the ground – just like the other five reactor spent fuel pools at Fukushima. And unlike our trick birthday candles, right now the radioactivity around the spent fuel rods is so high that no one can approach them to put out the fire.
I’m a slow typist: by the time I completed this the fire burning at reactor 4's spent fuel rod pool had gone out – apparently spontaneously. Fortunately, we’re not yet at the 140 hour mark by which the NRC calculated spent fuel rods in ideal conditions would be at risk of combustion. That’s a good thing, because there’s one other big difference between trick birthday candles and spent fuel rods. Trick birthday candles merely drip more wax on the cake. Uncontrolled spent fuel rod fires could pour enough radioactive waste into the atmosphere to cause what a nuclear engineer (at a Vermont plant identical to Fukushima reactors) calls “Chernobyl on steroids”.
Let’s hope the spent fuel rods at Fukushima are put back under water before we have the opportunity to test her hypothesis.
http://atomicinsights.com/2011/04/fukushima-nuclear-accident-exceptional-summary-by-murray-e-miles.html FUKUSHIMA NUCLEAR ACCIDENT (Notes, dispute as to nature of pool having 2 parts) THE KEESE SCHOOL OF CONTINUING EDUCATION MURRAY E. MILES MARCH 24, 2011
UNIT 4 FUEL POOL
Fukushima Daiichi has one fuel pool for each reactor and a seventh common pool that has not been in trouble. They also have some older fuel stored in dry casks perhaps a quarter of a mile away from the plants. The unit 4 fuel pool is nearly full with around 200 tons of fuel in a water tank with a capacity of around 400,000 gallons.
You need to hear one more complication in the design. The fuel pool is really two pools separated by a gate. Fuel removed from the reactor goes first into the small, upper pool which is only 20 or 25 feet deep. Later they move the fuel to the big, deep pool. There was apparently only a little fuel in this upper pool at the time of the quake.
This UPPER pool broke. The three-eighths inch steel liner is cracked and will not hold water. The concrete wall in front of this upper pool fell off. Fuel was severely damaged probably by explosion. Temperature profiles measured by helicopters show clumps of hot stuff that must be fuel scattered around the floor area. This scenario is consistent with the numerous reports of fire in unit 4.
This is a real nightmare. But the main fuel pool in unit 4 appears intact and full of water. The spread of radioactivity came fortunately from a small amount of fuel.
TWO SPENT FUEL POOLS CAUGHT FIRE AND SPREAD RADIATION
Reactor design at Japanese plant raises questions By Paul Handley (AFP) – Mar 17, 2011
http://www.google.com/hostednews/afp/article/ALeqM5hsPOSu-Nb_f1doHb7qculEvBAFNg?docId=CNG.4b1705bd8aeb3fb7c23a8cab0215312e.6b1 Questions are also being asked about another aspect of the Mark 1 design, namely the location of cooling tanks which hold highly radioactive spent fuel rods. They are placed outside the protection of the containment vessel.
These pools are now the source of intense anxiety in Fukushima, because pumps designed to circulate and top up the water that cools the immersed rods failed in the tsunami generated in the quake.
"At least two spent fuel pools at the Fukushima plant have caught fire and are releasing radiation into the atmosphere," said Edwin Lyman, a physicist and expert in nuclear plant design at the Union of Concerned Scientists (UCS), an NGO on nuclear safety.
"(...) The United States has 31 boiling-water reactors with similarly situated spent fuel pools that are far more densely packed than those at Fukushima and hence could pose far higher risks if damaged," Lyman said on Wednesday to the Senate Environment and Public Works Committee.
He called on US operators to withdraw some of the rods and place them in dry storage casks in order to reduce the heat load.
ibtimes: link wikipedia 4/8/2011: In the magnitude 9 earthquake which struck the Fukushima nuclear plants in March 2011, one of the spent fuel pools lost its roof and was reported to be emitting steam so was possibly boiling. According to The Nation, "Spent fuel pools at Fukushima are not equipped with backup water-circulation systems or backup generators for the water-circulation system they do have."  Later, there was some disagreement among sources as to whether the pool had boiled dry.  TEPCO, the plant owner, announced that if the rods were exposed, there was a chance they would reach criticality, setting off a nuclear chain reaction (not an explosion).  On April 1, 2011, United States Energy Secretary Steven Chu said that after efforts by workers to pour water on the Fukushima pools, these were “now under control.” 
Japan spent fuel pond on fire,radioactivity released: U.N. atomic watchdog
By IB Times Staff Reporter | March 15, 2011 11:19 AM GMT Japan has
informed the International Atomic Energy Agency (IAEA) that a spent
fuel storage pond at an earthquake-hit reactor is on fire and
radioactivity is being released directly into the atmosphere, the
Vienna-based U.N. atomic watchdog said.
Japan told the IAEA that dose rates of up to 400 millisievert per hour have been reported at the Fukushima Daiichi nuclear power plant site. "The Japanese authorities are saying that there is a possibility that the fire was caused by a hydrogen explosion," IAEA said.
... fire was extinguished on March 15 at 02:00 UTC
SPENT FUEL WILL BLOW UP IN 2 OR 3 HOURS AFTER LOSS OF WATER IN POOL!According to the Institute of Natural Resources and Security Studies, fuel discharged for one month can ignite in less than two hours after water is lost from a pool, while fuel discharged for three months from the reactor may ignite in three hours.
engineer live.com Fukushima disaster: filling spent fuel ponds The focus at the stricken Fukushima Daiichi nuclear power plant has turned to refilling with water spent fuel ponds in a bid to prevent dangerous radioactive releases, says IHS Energy Asia Pacific analyst Tom Grieder
The last reported temperature level in the spent fuel pond at Unit 4 was 84 degrees Celsius, although increases in temperature up to 1,000 degrees Celsius could cause the fuel rods' zirconium cladding to catch fire, leading to large, unchecked releases of radioactive isotopes. In view of these dangers, an expansion of the evacuation zone imposed by the Japanese government would be a prudent move.
....A fire was already reported in the spent fuel pond area at Unit 4 on 15 March, which is believed to have been the primary cause of increasing radiation levels recorded around the site—which reached 400 millisieverts per hour at one point. Unlike the reactors, which are better protected against radioactive releases by primary containment vessels, these spent fuel ponds are comparatively vulnerable.
In a status report from 9.00 am today, the Japan Atomic Industry Forum (JAIF) has conceded that
***in Unit 4 water levels are low and that damage to fuel rods is already suspected. ***
The extent of damage to the spent fuel may be affected by how long the fire burned for on 15 March. Based on reports from the Nuclear Industrial Safety Agency (NISA) the fire was reported at 9:38 am and 'extinguished spontaneously' at 11.00 am on Tuesday (15 March). However, a fire was reported earlier on the same day at 5.45 am, but could not be confirmed. This suggests that the fire may have been burning at a low level for a number of preceding hours before it was confirmed.
There has been a shift in the nuclear industry over preceding decades towards using high-density racks in a closed structure in spent fuel ponds, which is partially the result of a lack of spent fuel storage space. High-density racks mean that more radioactive material is stored in the pool, and so the potential dangers from releases of material are higher. If the fuel in the ponds has been discharged from the reactor relatively recently, then the risk of a fire is higher. According to the Institute of Natural Resources and Security Studies, fuel discharged for one month can ignite in less than two hours after water is lost from a pool, while fuel discharged for three months from the reactor may ignite in three hours.
Given that fire and fuel rod exposure have already occurred, and given the lack of containment vessels, radiation levels will probably be among the highest in Fukushima Daiichi in the spent fuel area of Unit 4. These radiation levels will impede cooling efforts, as workers will be unable to stay near the ponds without suffering high and health damaging radiation doses. The strategy of using a helicopter to spray the spent fuel pond at Unit 4 is perhaps an acknowledgement of this radiation risk. However, the idea of spraying water onto a spent fuel pond would not be feasible if the zirconium cladding around the spent fuel rods was to ignite. Instead of helping to fill the pool, the water would then feed a reaction which would generate flammable hydrogen
divvy (547 posts) Wed Mar-16-11 05:40 AM 37.
fuel pool notes 1) The spent fuel pools for Mark I containments are above the core, because the cores are top loaded and it makes fuel transport under water easier. Our control rods come in from the bottom. PWRs are the other way around, I know. Most of the heavy design work went into making sure that the pools couldn't be accdentally drained. There are no drains... the pool has leak detection equipment... all kinds of level, radiation, anti-syphon configurations, and temperature alarms... piping connections are all high in the pool. The easy way to get water up there was a fire hydrant right next to the pool (our fire pumps were redundant diesels) and if the fire system was down, a fire hose could be run up the stairwell. All you need is a tanker fire truck. Why, in 4 days, it never occurred to the utility to do it until dose was so high they can't do it, I don't know. 2) The last news I received is that they pulled everybody from the site.. those out of control fuel pools must have near lethal doses now. There is stuff they could have done in preparation of abandoning the reactor cores, but they don't seem to have done it (eg, flooding containment and draining he vessel). I can imagine a meltdown... I can't even imagine what those fuel pools will end up doing. We are way beyond TMI. I said from the very beginning that there were 6, not 3, plants in big trouble. The only thing that would have terminated this event was restoring AC power to site equipment. I never heard a single report on the status of doing that. The worst case has happened. There can be no good outcome now. I would be asking somebody if they can "shoot" a fire nozzle into the pool. There may not be that much they can do about the reactor cores, but they had better think about what they are going to do about those spent fuel pools unless they don't mind evacuating Japan. This is the worst possible outcome that I can imagine.
Tue Mar-15-11 05:24 AM Response to Reply #8 12. Burning fuel I am trying to think of what could cause a fuel pool to burn. 400 millisievert is 40 Rem/ hr. OMG! My guess is that the Unit was refueling... They probably had offloaded some hot fuel into the storage pool. There has been no refuel pool cooling since the event started. You need AC power. The clad must have gotten hot enough to start melting the cladding. That generates the hydrogen. I told you! These shutdown plants are not safe. They should start dumping fire water into the pool... Just let the water overflow. You can't stop the hydrogen generation unless you can cool off the water. Normally, there are temperature alarms to keep the fuel pool below about 140degreesF. They don't work. I bet the temp is way over that now with hydrogen burning on the surface. This is the highest reported dose rate yet... And from the "shutdown" plant. Glad to hear they got the fire out... Now get that pool cooled down! ... Or it will re-ignite.
%%SafetyIndian Point Energy Center
Highly Effective Barriers
Water serves as a natural—and one of the most effective—barriers to radiation. This is why spent fuel is stored in pools. The fuel is contained neatly in fuel rods in a 40 foot deep pool. The racks stand 13 feet high leaving the fuel completely contained and safely submerged under 27 feet of water.
Spent Fuel Pools at Indian Point
Spent Fuel Pool
Click for enlarged view.
The spent fuel pools for the two operating Indian Point plants have their own safety and construction features designed exclusively for their safeguard. They are constructed with concrete walls 4 to 6 feet wide and with a half-inch stainless steel inner liner. They have multiple, redundant back-up cooling systems.
- The spent fuel pool for Indian Point 1 is in a fully-enclosed concrete building.
- Both Indian Point pools are 99% to 100% underground making them virtually impossible to compromise from the sides.
- The roof of the spent-fuel pool building has no nuclear safety function. Damage to it would not have safety consequences.
- The fuel pools can easily be re-filled with water and have several backup mechanisms for doing so.
- It is very unlikely there would be significant off-site radiological consequences even if the pools were drained of their water.
Spent Fuel Pools
Engineers safely move spent
fuel from the fuel pool.
Click for enlarged view.
There are also several measures in place to protect each of the spent fuel pools at Indian Point. Once removed from the reactor core, the fuel rods which hold the radioactive isotopes are transferred under water to the spent fuel pool. The fuel assemblies, or fuel rods, rest in a pool of water approximately 40 feet deep to help cool the rods. The assemblies themselves stand 13 feet tall, so there is an ample 27 feet of water on top of them. This is important because water is a natural barrier to radioactive isotopes. Internally, there are other cooling systems in place and back up systems to replenish the water supply in the event of an emergency. As long as there is water covering the spent fuel, no radioactive materials will escape.
It is important to know that the fuel pool for Indian Point 2 is completely underground and Indian Point 3 is nearly 100% underground, so they are protected on all sides by rock and gravel and the 6 foot steel. External to the pools themselves, they are further protected by the containment structure on the north and west and hills to the south and east.
The fuel recently moved from IP3 remains safe in its new containment structure called dry cask. As the pools get closer to reaching capacity, we will continue to move spent fuel into these dry cask storage containers on the dry cask pad located on site.
%%United States71,862 tons of spent fuel in more than 30 states
Fukushima warning: US has 'utterly failed' to address risk of spent fuel csmonitor.com:
- Why are America's nuclear power plants allowed to store tons of used but still highly radioactive fuel in pools for as many as 100 years – despite the fact that those pools are far more vulnerable to terrorist attack than the reactors themselves?
- after just days without a cooling system, most water in at least one pool had apparently boiled away, a fire was reported, and radiation levels soared.
- nuclear utilities in the US have over decades accumulated some 71,862 tons of spent fuel in more than 30 states – the vast majority of it sitting today in pools that are mostly full
@@PhotoTEPCO releases 1st video of fuel rod storage pool. BY HIDENORI TSUBOYA STAFF WRITER 5/12/2011 shows the still submerged rods virtually buried under concrete and steel debris... ....collected a water sample from the pool. Radioactive cesium-134 was measured at 140,000 becquerels per cubic centimeter, cesium-137 at 150,000 becquerels and iodine-131 at 11,000 becquerels, all of which are not present under normal circumstances. The substances leaked into the pool from the nuclear reactor after fuel rods in the reactor were damaged in the explosion,
Destroyed Spent Fuel Pool SFP3 of Reactor Unit 3 at Fukushima Daiichi
8 May 2011
Labels are, clockwise from the top:
--Spent fuel assemblies, etc.
--Fresh fuel assemblies
Temperatures continue to rise in fuel pool #4 despite injecting an increasing amount of water.
Photo taken using camera connected to the end of a concrete pump arm.
Clockwise from the top:
1)Spent fuel assemblies, etc.
2) Control rods
4) Fresh fuel assemblies
5) Metal rack
Stock of SFP4
548 Spent/partially Fuel assemblies 4 month old about 2MW
783 Spent Fuel Assembles >> 4 months about 0.4kW
204 Brand new assemblies (heat negligable)
On 27/4/2011 Tepco stated that 70 tons/day of water is required to cool SFP4.
On the 28/4/2011 they stated 175 tons of water were boiling away.
write up how this refueling thing works. Fuel cask shows up in truck,
backs up into garage. Crane lifts up through the cask shaft up the
spider hole. Fuel goes into pool. "Upper pools" are flooded from
equipment over core and fuel pond. Reactor is opened up as per parts
in drawing . Equipment such as steam dryers which are above the fuel
need to be removed and stored in the dryer pool using the big overhead
crane while in the water since it may have radioactive water and be
radioactive. Then refuel platform moves fuel under water from pool
through the cattle chute gap into the reactor core. The gate is air
operated and probably failed when power is out, another not-so-safe
1:26 looks like split open fuel rod, bent and split opent at end - upside down "V"
Destroyed Spent Fuel Pool SFP3 of Reactor Unit 3 at Fukushima Daiichi
8 May 2011