March 25th, 2011 – Presentation from Japan Nuclear Technology invokes questions about official analysis of Reactor 4 explosion

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By March 25th, two weeks after the March 11th nuclear disaster in Japan, officials at the Nuclear Regulatory Commission were passing around an emergency estimation of what could potentially by happening inside of the reactor cores at Fukushima Daiichi, and presentation from the Nuclear Technology Institute in Japan to the Office of Nuclear Reactor Regulation (NRR).

The message notes a few critical details on the status of the reactors and the information being passed from the Japanese.

The presentation describes some of the differences between Three Mile Island (TMI) and Fukushima, one notable difference being the existence of a steam-water separator at the top part of the reactor core, because Fukushima uses the BWR system. This separator serves as resistance to releasing steam from the core to the top part of the pressure vessel. It therefore keeps steam in the core, undermining the injection of seawater.

This means compared to the example of a PWR, like at TMI, the BWR has a design that may make it difficult to cool the molten core.

One more major difference is the fact that TMI’s reactor core was stabilized by the use of the primary coolant pump (equivalent to the recirculation pump at Fukushima). With PWR, the primary cooling system is clearly separated and insulated from the turbine system.

In a BWR, simply activating a recirculation pump would do no more than agitating the reactor water unless a condenser is also used. The pump alone does not contribute to lowering the core temperature. However, using the condenser runs the risk of sending highly contaminated reactor cooling water to the turbine building, which has only limited shielding facilities.

Photo showing the debris formed on top of the Unit 1 reactor after the explosion
  • The presentation notes a large U-3 PV/RCS pressure spike on 3/13 and a primary containment pressure spike on 3/14.
  • They said that Reactor 1 (As well as none of the other units) had a concrete roof – which led to some confusion about the type of rubble which was observed at the top of Reactor 1 after the explosion (See photo above).
  • They said that TEPCO does not normally use a checkerboard fuel arrangement for the hot fuel in the pool.
    • This can affect the ability to effectively cool the spent fuel pools if a loss of coolant accident occurs
    • They believe Reactor 4 explosion was in fact from H2, and they have a heat-up “plot” for the SFP showing saturation temperature in less than 2 days from loss of power.
      • They also believe that some reflood would have occurred to refueling gate damage.
        • That said, with this heat-up curve, that water would not have lasted long either?
  • They believe that picture 2-17 shows a flooded pool after the explosion.
  • They were equivocal on the containment vent path used and the “hardened” nature of the vent.
    • The stack is where the venting should have occurred, but they understand that something does not make sense.
    • They had B5b-like connections that they used for the temporary fire pump tie-in to the primary systems.

Back to Reactor 4, the Fukushima Daiichi accident has revealed significant roles played by hydrogen produced through water radiolysis.

The presentation notes that the temperature of the spent fuel pool reached the critical boiling point less than 48 hours after the onset of the nuclear disaster, and the radiation dose recorded in the skies above Fukushima Daiichi Unit-4, was measured at 400 mSv. The conclusion given for these readings based them on the loss of water in the spent fuel storage pool, and hopes that they will decrease once the water level is restored (This comparison of radiation levels and loss of water in the spent fuel pool are in agreement with studies conducted at national labs in the United States).

The presentation notes that some transfer of water occurred between the refueling cavity and the spent fuel pool, but questions with the heat-up curve, how long that water would have actually provided cooling.

Once the spent fuel pool reaches the boiling point, it allows the generated hydrogen to be released from the pool at the production rate.  When the temperature gets high enough, any zirconium in the presence of water will oxidize.  This allows yet another mechanism for the production of hydrogen, as the zirconium “steals” the oxygen atom in the water molecule so it can make zirconium oxide, which in turn releases the hydrogen atoms.

Another interesting side note to keep in mind is the addition of boron to the spent fuel pool, which when it absorbs a neutron produces alpha particles, which in turn radiolyse more water.

Looking at this synopsis, it may imply that the build-up of hydrogen that led to the explosion in Reactor 4 was generated in that reactor building, and did not travel from Unit 3 as TEPCO has since stated.

The presentation is uncertain as to the root cause of the hydrogen build-up in the reactor, but there are two theories with how this may have occurred if the hydrogen did in fact source from the Spent Fuel Pool

A: The buildup was created from the steam bubbles being produced in the Spent Fuel Pool water, (as Dr. Genn Saji has stated), or

B: came from the zirconium at the top of the fuel being exposed to air (As Arnie Gundersen has proposed).

When the pool water temperature rose above the boiling point, practically all of the dissolved hydrogen (DH) gas generated, is likely to be released into the Spent Fuel Pool room.

If in fact the temperature of the water allowed it to reach a boiling point, the racks containing the spent fuel might have become distorted, which may make TEPCO’s removal plans more difficult.  Recently TEPCO announced that they will attempt to remove two spent fuel bundles that gave off no heat (new), which means that their test might not be meaningful.
Pages From ML12188A298 –12 Presentation From Japan Nuclear Technology Institute to NRR

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