Figure 1: A generalized conception of what is taking place beneath the Fukushima reactors, cores at very high temperatures burning their way into the ground.
Large problems are looming larger in Fukushima as reports of short-lived radioactive fission products detected by TEPCO in reactor number 2:
– There have been far too many self-serving assumptions made by operator about the conditions within the four reactors. The assumption is the cores are all sub-critical as designed and installed by the utility. The detection of short-lived fission daughters indicates the assumptions are wrong and that core(s) are critical.
– If the cores material is spread out over a wide area or inside the reactor buildings the cores would likely be sub-critical and unable to produce fission products.
– That fission products are being detected indicates the core beneath reactor 2 is in a concentrated mass. What matters is whether the cores can or will become super-critical causing an explosion.
– Cold shutdown is an abstract (advertising) concept unrelated to conditions in the destroyed reactors at Fukushima.
– This is typical of the ‘modern’ approach that insists that problems of physics are subject to public relations.
– Unlike the Soviets at Chernobyl shortly after the explosions and meltdown, the Japanese have not bothered to send nuclear scientists into the reactors to determine the condition and location of the three reactor cores. Consequently, nobody knows anything about the cores.
Figure 2: Schematic of a boiling-water reactor similar to the kinds used at Fukushima Dai-ichi. This is the Mark 1 containment as in reactor unit 1 (Click on image for big.)
When the fuel core melted through the pressure vessel it wound up in the in-pedestal area under the pressure vessel. Where to next?
Inside the reactor, a lot of energy is concentrated in a very small space. This density of components is essential to all reactors otherwise they will not work.
– Because the reactors are very small relative to the amounts of energy they release, there is no margin for error in dealing with malfunctions. All reactors operate at the bleeding edge of (1960’s) design and materials technology. It is possible that none of the four Fukushima reactors could have been saved after the earthquake due to damage and inherent fragility of the reactor equipment relative to the operating loads with greater shaking loads imposed upon them.
It is unknown whether the water being poured into the reactor buildings is effecting the cores, however the injections along with ordinary ground water means soils in the area of the plant that are saturated. Because the water has absorbed radio-nuclides, it is intensely radioactive. The consequence is that human workers are unable to tunnel into the ground beneath the reactors to determine the location or condition of the cores because of the dangerous radioactivity.
Determining the condition of the cores should have been a priority for the TEPCO operators and the Japanese government but so far no attempt has been made. This is during a period of eight months!
For the next seven months … five-hundred thousand men will wage hand-to-hand combat with an invisible enemy. For this battle, that has gone unsung which claimed thousands of unnamed and now almost forgotten heros.
Yet, it is thanks to these men that the worst was avoided … a second explosion, ten-times more powerful than Hiroshima, that would have wiped out half of Europe … @ 3.04
This ‘second explosion problem’ is what the Japanese are (not) facing up to now. In order to gain understanding some idea of how a reactor works is necessary:
Figure 3: (General Electric) This schematic identifies the core components in a BWR. The core occupies the space about the size of a bus. This confined space contains about 150 tons of low-enriched nuclear fuel. Notice the lattice of core elements arrayed so that water can flow between the fuel elements and carry away the heat. The water also acts as a moderator, slowing neutrons so that they are absorbed by Uranium-235 atoms in the fuel pellets, so that they might later split and release energy.
The ordinary power reactor operates at a barely-critical state with limited emission of prompt neutrons, this is controlled by the enrichment level of the fuel and the design of the fuel elements. What emerges into the core are the ‘delayed’ neutrons. The flux of delayed neutrons can be managed by neutron absorbing control rods or other ‘poisons’ such as boric acid added to the cooling water. By adjusting the flux of delayed neutrons, the operator can adjust the power output of the core.
In order for the moderated reactor design to work, all of the fissile components and the moderator(s) must have a strict physical- or grid relationship to each other. This is very important to keep in mind: when a reactor ceases to be a thermal reactor due to a malfunction and a ‘reconfiguration’ of the core, it becomes an accidental fast nuclear reactor.
“We were afraid, because it could have caused another explosion. it was terrifying. Scientists came and took readings. They were very worried. They were afraid the critical temperature would be reached and it would set off a second explosion that would have been a terrible tragedy,” Gen. Nikolai Antochkin USSR Air Force.
The cement slab below the reactor core is heating up and in danger of cracking. The magma is threatening to seep through. The water the firemen poured during the first hours of the disaster has pooled below the slab. If the radioactive magma makes contact with the water it could set off a second explosion even more devastating than the first.
The country’s top experts are called into action. Vassili Nesterenko was one of them, At the time, he was working on improving the Soviet Union’s intercontinental nuclear missiles.
“If the heat managed to crack the cement slab only fourteen hundred kilograms of uranium and graphite mixture would have needed to hit the water to set off a new explosion.”
The ensuing chain-reaction would set off an explosion comparable to a gigantic atomic bomb.
“Our experts studied the possibility and concluded that the explosion would have had the force from three- to five megatons …” said Nesterenko.
Since March 11, there have been no nuclear scientists on the Fukushima site, the efforts are ongoing to enforce media silence and cover up what has been taking place or not.
Figure 4: After melting through the bottoms of the reactor buildings the cores would consolidate into amorphous 150 ton blobs of metallic uranium, thorium and plutonium isotopes.
Taking place within the cores is heating that results from radioactive decay. Radioactive decay is not to be confused with fission which requires the splitting of atoms.
There are many different kinds of nuclide decay processes taking place within the three cores.
– This decay does not produce fission products such as xenon-135 or iodine-131.
– Because the isotopes in question have very short half-lives it is clear that fission is taking place right now within at least one of the cores.
– Heating of the core(s) would be the result of fast fission. Because cores emitting fission products cannot be sub-critical, the low detection levels of these gases is instead likely because the cores are underground.
– In order for fission to take place there must be a neutron flux. Because of the absence of any moderator, neutrons would be ‘fast’ or have a very high energy level. These high energy neutrons are not a part of ordinary nuclear reactor operation. Any moderation would be the result of impurities within the fuel mass or by neutron reflection. Both of these would add heat. Absorption of fast neutrons would depend on the neutron cross-section of target elements within the mass which is largely U-238.
– Unlike the commercial reactor which relies on the absorption of moderated neutrons by U-235, a fast reactor relies on the absorption of high-energy neutrons by U-238. This fission takes place at higher energy levels than exist within the commercial reactor.
– Fissions taking place now are ‘prompt’, that is the neutrons are produced by the fission of fuel nuclei rather than by decay nuclei.
– A chain reaction due to prompt neutrons can self-propagate with extreme rapidity under the right conditions.
– Additional small amounts of neutron emission from the core are the result of spontaneous splitting of fissile atoms such as Pu-240.
Because the cores are sub-critical (k< 1) by virtue of their level of enrichment, the core material under 'normal' conditions of pressure and temperature will not sustain a chain reaction. Amplifying the effective neutron flux would bring the nuclear fuel to increased criticality (k>1). Compression will do this. so does placing the fuel material adjacent to a neutron reflector. This is what Arnie Gunderson suggests took place on March 14 in the reactor 3 spent fuel pool: a compression of sub-critical nuclear fuel by a shock wave resulting from a hydrogen explosion above the fuel. This compression — according to Gunderson — amplified the flux of prompt neutrons that propagated and intensified a chain reaction in some of the spent fuel causing it to reconfigure explosively.
– Vassili Nestorenko was concerned about along with the others at Chernobyl about a second prompt criticality involving the fuel that had melted out of the reactor.
– This is the issue now, is it possible for events to bring the fuel cores to supercriticality.
As nuclear material fissions, a product is Xenon-135. This isotope is a powerful neutron poison. As the fuel material fissions, the resulting Xe-135 absorbs neutrons stifling the chain reaction until the Xenon ‘burns off’ by absorption of neutrons at which time the fission intensifies creating more Xenon. Because the fuel mass is borderline critical, Xenon-135 creation occurs at the same rate as the fission process to keep the reactions from becoming destructively super-critical.
Being near the ocean, the ground under the reactors is mostly silica sand. As the sand melts the core sinks through it. Above and surrounding the core material is a glassy substance that also includes the non-fissile material that once made up the core structures: the boron-carbide control blades, zirconium fuel cladding, stainless steel fuel racks, flow nozzles, steam dryers, rod drives along with concrete. This material forms a slag. The fuel doesn’t ‘burn away’ the soil in its path but simply sinks through it leaving behind a ‘plug’ made up of slag and the other debris.
This plug prevents water from reaching the core material to cool it. Any cooling at the heated core creates more layers of slag.
The plug, slag and other debris are intensely radioactive, being either the remains of the core or having come in contact with it.
Water poured into the reactors soaks into the ground. The rest fills the reactor buildings from where it eventually flows into the ocean.
While the sand absorbs neutrons and is a poor reflector, the bedrock some meters beneath the reactors is likely to be a good one. Any hard, dense material can be a good reflector (as is water). What the reflector does is bounce neutrons back into the fissile material to increase the neutron flux while moderating the neutrons at the same time. In this way, fissile U-235 atoms can also absorb neutrons and split.
More fission would amplify the flux increasing the energy release while compressing the fuel. The weight of the fuel along with the plug of slag would push fissile material onto the reflector causing a prompt criticality:
Figure 5: Super-criticality is an issue of time: as nuclei split energetically, the tendency is for the atoms to fly away from each other. The material separates and the reactions cease. The problem emerges when there is no place for the atoms to go. Chain reactions can then propagate for generation after generation with an accompanying energy buildup until the bonds of mass and inertia represented by the ground … are overcome.
– A low energy reaction would cause a fuel geyser that would blow core material and the plug through the roof of the reactor building, much like the explosion in reactor 3. This would require only a few generations of chain reactions in the super-critical core.
– A high energy reaction of many generations would cause a substantial nuclear explosion. Critical components would be: material of sufficient mass, this material confined by incompressible material (sandy soil), weight of the core and the plug above it pressing the core against the neutron reflector. More than fifty generations of chain reactions would cause a multi-kiloton explosion beneath the reactor.
– A powerful explosion would propagate a shock wave that would travel through the ground and compress other cores that might have burnt their way into the ground. This compression would cause even more powerful nuclear explosions. This was how a modest amount of fuel under Chernobyl would cause a shock wave capable of bringing the rest of the nuclear material into a super-critical state.
Remember, there were three other reactors at Chernobyl with each containing 195 tons of highly-energized nuclear fuel!
– The low enrichment ratio of fissile material within the cores is compensated by the cores’ mass. The fission of even a tiny percentage of a core would represent an immense amount of energy release.
– The relative lack of explosive energy is compensated by the amount of radioactive material at the site. Anything other than the most modest excursion would be exceptionally destructive due to radioactive fallout.
– The approximate largest fission nuclear test was @ 500 kilotons (Operation Ivy King, 1952). A Fukushima explosion would certainly be less powerful. The Ivy King ‘gadget’ was dangerously massive and inherently super-critical (k = 2) while the Fukushima fuel is inherently sub-critical.
– There are over a thousand tons of nuclear material in the reactors and spent fuel pools. A multi-kiloton detonation would destroy the reactors leaving an ocean-filled crater in place of the plant.
– nuclear reactors along with the cores and spent fuel would become part of the fallout cloud.
– The shut-down Reactors Five and Six at the Dai-ichi complex would be destroyed, their cores would melt into the ground setting a repeat of the super-criticality process a few month’s afterward.
– A reason for a modest explosion during a worst case scenarios would be the lack of x-ray emissions and ultra-high temperatures. Any fusion component is unlikely although tritium is no-doubt contained within reactor fuel.
– The radiation emitted and its extent is hard to estimate but certainly equal to the dirtiest above-ground weapons tests. The amount of fallout from Fukushima would be greater due to the fuel tonnage but the extent more limited because of the absence of explosive force. Weapons tests injected material high into the stratosphere spreading fallout over large areas. the Castle Bravo thermonuclear test took place in February, 1954:
The Bravo test created the worst radiological disaster in US history. Due to failures in forecasting and analyzing weather patterns, failure to postpone the test following unfavorable changes in the weather, and combined with the unexpectedly high yield and the failure to conduct pre-test evacuations as a precaution, the Marshallese Islanders on Rongelap, Ailinginae, and Utirik atolls were blanketed with the fallout plume, as were U.S. servicemen stationed on Rongerik.
Within 15 minutes after the test radiation levels began climbing on Eneu Island, site of the test control bunker, which was supposed to be upwind from the test and thus immune to fallout. An hour after the shot the level had reached 40 R/hr, and personnel had to retreat from the control room to the most heavily shielded room of the bunker until they could be rescued 11 hours later.
An hour after the shot Navy ships 30 miles south of Bikini found themselves being dusted with fallout with deck radiation levels rising to 5 R/hr. navy personnel were forced to retreat below decks and the ships retreated farther from the atoll.
As the fallout drifted east U.S. evacuation efforts lagged behind the plume. At Rongerik, 133 nm from ground zero, 28 U.S. personnel manning a weather station were evacuated on 2 March but not before receiving significant exposures. Evacuations of the 154 Marshallese Islanders only 100 nm from the shot did not begin until the morning of 3 March. Radiation safety personnel computed that the islanders received a whole-body radiation doses of 175 rad on Rongelap, 69 rad on Ailinginae, and 14 rad on Utirik.
The Japanese fishing vessel Daigo Fukuryu Maru (Fifth Lucky Dragon) was also heavily contaminated, with the 23 crewmen receiving exposures of 300 R, one of whom later died – apparently from complications. This incident created an international uproar, and a diplomatic crisis with Japan.
The entire Bikini Atoll was contaminated to varying degrees and plans for conducting test operations from the islands, including use of the firing bunker, had to be abandoned. All further Castle tests were controlled by radio link from the USS Estes.
After this test the exclusion zone around the Castle tests was increased to 570,000 square miles, a circle 850 miles across (for comparison this is equal to about 1% of the entire Earth’s land area).
An 850 mile circle covers most of Japan.
The need is for nuclear scientists to be engaged. Right now the emphasis is toward public relations. The detection of gaseous fission products indicates the time remaining to take action at Fukushima is running out.
What can be done:
– The Japanese government must seize the day and eliminate Tepco’s role in the reclamation process.
– Expand the exclusion zone to 50 miles from the plant until the cores are located then stabilized.
– Find the cores NOW by any means necessary: drilling, robots, pipeline cameras. If this exposes workers to radiation, so be it. If the cores are dispersed or within the reactor buildings there is less urgency and steps can be taken to treat the core material as spent fuel rather than incipient bombs.
– Horizontal drilling equipment MUST be used to drill under reactors. Bore holes can then be filled with boron. Liquid nitrogen can also be flooded under the cores, to freeze the ground beneath the cores and provide neutron absorption.
– Spent fuel in all the reactors, on the site and cores at Dai-ichi plants 5 and 6 must be removed off site by any means necessary and at whatever cost as rapidly as possible.
– If cores are located under the reactors and can be held in place by way of boron or ground freezing, the site can be surrounded by a cofferdam made of steel sheet piling. This cofferdam should have been built already. Wells can be drilled within the cofferdam and ground water removed and then treated to remove radioactive material. Water is a neutron reflector, the less water the better.
– If the cores are below the buildings it is likely adding water or boric acid into the buildings is counterproductive.
– Get a group of international nuclear experts onto the site and have them determine what is actually taking place so the appropriate steps can be taken.