Home / We Must Consider In Japan a Potential Nuclear Explosion of the Critical Mass Type

We Must Consider In Japan a Potential Nuclear Explosion of the Critical Mass Type

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The world is well aware that on March 11, 2011, Japan was shaken to its foundations by an earthquake of 9.0 magnitude. Following the earthquake, a tsunami battered the island. The loss of life has been beyond comprehension. Japan is home to several nuclear power generating plants; these plants were so damaged by the quake as to be irreparable. Even today workers are laboring to prevent further damage to the island and beyond, that may be caused by deadly radiation pouring from the ruins of the nuclear plants. The efforts to contain the hazardous clouds of smoke and gasses are hampered by radiation already omnipresent. The workers may be forced to give up the effort, because of the threat to their lives. Now, another concern has arisen.  

Scientists tell us that in dealing with radioactive material, of the kind now bringing alarm to the population of Japan and to the world, at the nuclear power plant at Fukushima, Daiichi, there is a potential for a nuclear explosion which could engender far more impact than the radioactivity-escape dilemma already being encountered.

Nuclear rods, much discussed in recent days, are not solid rods at all. They are rather containers in the shape of long slender closed-end poles in the reactor, or in the spent rod holding area, for small pellets of fissionable material. Potentially these pellets could melt, if not contained by cooling water, and there is a possibility that the melted pellets could congeal into a single mass. Scientists tell us that such a mass, if it reaches or exceeds certain proportions may explode. Such an explosion would not be a mushroom cloud, nuclear bomb explosion of the type we have seen in tests, but rather would be similar to the detonation of a “dirty bomb.”

There are several million pounds of fissionable material at the Fukushima Daiichi nuclear plant. In some instances, thirty three pounds of uranium can produce a “critical mass”. Dirty bombs differ from more modern weapons in that they are often mixed with conventional explosives, which aid in spreading a radioactive cloud over a large area.

The nuclear weapons of the 1950s were inefficient. Of the destructive potential, only about 2% to 14% was in fact released. A dirty bomb is close in power to an ordinary explosive. The fear of contamination can cause widespread panic and terror. One difference between a weapon explosion and an accidental one rests in the fact that in bombs, reflectors are used to focus the fission toward the center of the fissionable material, achieving a much more disastrous blast.

There are three categories of mass involved in speaking of nuclear explosion; critical, sub-critical, and super-critical. In a critical, or super-critical mass explosion,  there will be a steady chain reaction of neutrons flying about, rupturing, or splitting atoms, thus releasing more free neutrons. Collisions are called fissions. Each fission usually produces one more fission. Critical mass means that there is enough fissionable material involved to produce and sustain a chain reaction, which grows exponentially within a miniscule passage of time. This chain reaction is precisely what happens in nuclear weapons.

While first readings from American data-collection flights over the stricken Fukushima Daiichi nuclear plant show that the worst contamination has not spread beyond the 19-mile range, in the perhaps unlikely event of a critical mass explosion, that range, and the resultant danger could be far greater.

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About John Lake

John Lake had a long and successful career in legitimate and musical theater. He moved up into work behind the camera at top motion pictures. He has done a smattering of radio, and television John joined the Blogcritics field of writers owing to a passion for the liberal press, himself speaking out about the political front, and liberal issues. Now the retired Mr. Lake has entered the field of motion picture, television, and video game (now a daily gamer!) critique. His writing is always innovative and immensely readable!
  • eeek!

    (runs away…)

  • John Lake


  • Glenn Contrarian

    Could there be a ‘dirty bomb’ type explosion? Perhaps – one of an order of the explosions we’ve already seen at the plants, yet spreading heavy radiation, most of which should – should – go out to sea due to the prevailing westerly air currents.

    But an atomic detonation due to a runaway chain reaction? No.

  • John Lake

    “a horrific mess”…
    There is concern, and the US maintains a 50 mile radius. “China Syndrome” is not a concern, we have to wonder where that idea ever came from.
    I am slightly more concerned today that I was yesterday about the potential for a ‘dirty bomb’ sort of explosion.
    It is remarkable in these global “social” media how areas are told what to believe.

  • Glenn Contrarian

    And you don’t have to take my word for it, John. Look at Iran – why is it they were spending so much on centrifuges? They have access to nuclear fuel, but they still don’t have a bomb. By your logic, they don’t need centrifuges to make a fission bomb…but they, on the other hand, KNOW that they need centrifuges to purify the fuel, to separate the normally non-fissile U-238 from the fissile U-235. Which, btw, is what we did at Oak Ridge during the Manhattan Project.

    Speaking of which, if you’re interested on the subject I strongly recommend Richard Rhodes’ The Making of the Atomic Bomb, which won the Pulitzer Prize. It’s a very enlightening book. It showed me how little I knew of what happened.

  • Glenn Contrarian

    John –

    MSNBC News points out that the total radioactive material in the Japanese plants amounts to several million pounds

    That matters not at all. What DOES matter is the purity and the geometry of the nuclear fuel.

    Furthermore, you can take all the uranium in the world, dump it all in one place, and while it might melt down and make a horrific mess, it will NOT explode unless it’s purified U-235 or purified plutonium. U-238, as it decays, does produce some plutonium, but not in purity or geometry needed for a runaway chain reaction.

  • John Lake

    Inas several readers have suggested I know nothing, and none even consider agreement, I should concede my ignorance. But sources say that temperature, shape, and mass all contribute to the likelihood of an explosion. If the amount of radioactive material is great enough, the remaining factors are out-weighed.
    Many believe, as was taught in the past, that the parts of the whole should smash together. In early bombs a round sphere was segmented into many parts; when it was time for detonation, explosive charges slammed them together. Not only was critical mass attained, but the smashing of the molecules undoubtedly added to the need for assurance.
    The smashing and slamming are not necessary; a quantity of radioactive material will explode (I am told) if the factors add up to such explosion. Again, substantial amounts over-ride other considerations.
    It is pointed out that if the fuel in the rods were to become molten, and drip or drop to a pool or puddle, that equation might be satisfied. MSNBC News points out that the total radioactive material in the Japanese plants amounts to several million pounds.
    Several million pounds multiplied by the speed of light, squared (that is, moving out in all directions, such as would occur inside a sphere) … even a little rocket scientist could predict considerable ill results.
    But since no one agrees, I surly must be wrong. Okay, I’m really wrong. I’m definitely wrong. Probably.

  • Glenn Contrarian

    John –

    I’m fairly liberal, but I know enough about nuclear power to know that there is absolutely no way that the nuclear fuel in the Japanese plants could ‘go critical and explode’. As SWoodberry explained, it requires a precise purity and geometry – otherwise you’d already be seeing lots of terrorists with lots of nuclear bombs.

    By the way, ‘critical’ is a bit of a misnomer. ‘Critical’ – when it comes to nuclear plants – merely describes a steady energy level. ‘Supercritical’ describes a rising energy state, and ‘subcritical’ describes a lowering energy state. When a plant is coming up in power from, say, 50% to 90%, it is supercritical during that stage – at least that’s what we were taught at the Naval Nuclear Power School back in ’82 (where I soon flunked after finding out the hard way that a rural MS education simply did not prepare me well enough). Anyway, such are technical terms that have been wildly blown out of proportion.

    One more thing – truly modern nuclear plants would not have been so affected and endangered by what happened to Japan. The plants that were affected there have been referred to as the “Model T’s” of the nuclear power industry.

    So yes, as liberal as you know that I am, I still support nuclear power, because I know better than to buy into the frantic fear-mongering that is so endemic among those who do not know so much about nuclear power.

  • John Lake

    Fear, uncertainty, and doubt. My understanding is that once salt water was utilized, the plants would never again be operable.

  • staying alive

    “these plants were so damaged by the quake as to be irreparable”

    once i got to this line, i stopped reading any further.

    I suppose the author isn’t a nuclear expert / physicist / seismologist.

    Sorry, FUD is not needed at times like this. thank you.

  • John Lake

    Friday afternoon:
    The are placing sand over the rods, which will prevent any explosion. If the stockpiles or rods in storage had exploded, experts say the worst would have been greater clouds of radioactive particulate matter.

  • John Lake

    Dr. Einstein’s early formula to determine resultant energy from a nuclear explosion, while interesting, has little practical application. As I stated in the article, early tests produced only a small percentage of the potential tumult. Energy produced equaling the weight of the nuclear material, multiplied by the squared speed of light, is not used by modern theorists, but has some value in determining the strength of the blast, which we can see will be exacerbated by the sheer quantity of the material involved.
    Having said that, we also consider that the rods are designed specifically to protect from the type of disaster we discuss. But we speculate the possibility that some of the hundreds of rods may be cracked or broken; the material inside may become molten, and pool. A resultant explosion could we concede compromise the remaining rods, which may as we see produce a calamitous situation.

  • John Lake

    The fission is more enabled if water is present, but the water is by no means required. The cement catch basins are below all the pellets, so they have no effect. the materials do not have to “come together”; simply being in critical mass is sufficient to produce an explosion that will spread the cloud farther and faster.

  • SWoodberry

    As near as I can tell, all your facts are incorrect. Fuel is ceramic and can melt but melting point is well above that of zirconium. Therefore pellets will fall out or be entrained in molten mat.

    Most U mat, (~96.5%) is not fissile and already depleted due to reactor operation. Each assembly contains about 392 lb of U and a complete core is 310,000 lbs of total U; @3.5% only ~10,900 is fissile.

    Typical assembly runs 3 cycles before depleted (1/3 of core is replaced every refuel). To keep math simple 1/3 of core depleted 1/3 is 1/2 depleted ~1/3 is “fresh;” total fissile inventory=10,900[(1/3*0)+(1/3*1/2)+1/3] = 5,450. That is dispersed in 200,000 lbs of metal from assembly structures & 300,000 lbs of non-fissile U. (ALL control blades (~200) are melted and interspersed in mat. and there is boron add. during shutdown to capture neutrons and reduce fission.)

    Chain reaction needs proper configuration, enrichment. How will 33lbs “walk” out of pellets and get together in a small area. For fission to occur, you need a moderator (e.g., water) to slow down neutrons so they can be captured by U-235 and the plant IS MISSING THE WATER!

    Each fission does not “usually produce another fission.” It produces 2 neutrons which must be moderated and captured by before lost or captured by a non-fissile mat. There is little, if no, moderator and only 5,450 lbs of randomized mat. and 700,000 lbs of inert mat. lots of neutron capture mat. what you hypothesize is impossible. (In order for 33 lbs of U235 to go critical it needs pure (>>90% enriched); a precise geometry; and brought together very quickly or it won’t do anything.

    All information I just provided can be found in a 1st year nuclear engineering book. I recommend that you take the time to understand the information; not just read it.