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Fukushima Unit 4 Spent Fuel Won’t Melt if Spent Fuel Pool Drains

Fukushima Unit 4 Spent Fuel Won’t Melt if Spent Fuel Pool Drains

by Dean Wilkie has written several articles concerning the spent fuel pool (SFP) water chemistry, the racks and the current condition of the fuel to withstand raising one spent fuel assembly out of the pool water in air without experiencing cladding reactions or fire. We have completed research on the capability of the fuel assemblies located in the spent fuel pool (SFPs) to withstand draining of the pool with no excessive overheating leading to a fuel assembly zirconium cladding interaction or burning.

We have had several questions about the behavior of fuel in a reactor and are going to start this report with a recap of fuel behavior within the reactor.


General BWR (boiling water reactor) Information


  • The BWR uses demineralized water as a coolant and neutron moderator
  • Neutron moderator (hydrogen in the water has the same mass as a neutron and is used to slow the neutron down through successive collisions with neutrons until the neutron has reached the thermal energy level and makes it ideal for being attracted to the U235 atom which results in a fission).
  • Heat is produced by nuclear fission in the reactor core, and this causes the cooling water to boil, producing steam
  • Steam is directly used to drive a turbine
  • Steam is then cooled in a condenser and converted back to liquid water
  • This water is then returned to the reactor core, completing the loop
  • The cooling water is maintained at about 75 atm (7.6 MPa, 1000–1100 psi) so that it boils in the core at about 285 °C (550 °F)



BWR Fuel In Reator Operation

Cross section of a typical BWR fuel pellet showing the heat profile from the center of the fuel to the coolant water channel



Additional Barriers To Good Heat Transfer

  • Oxide film builds up on the surfaces of the fuel cladding which inhibits heat transfer. Just a few mils of film can significantly reduce the heat transfer

*Oxide film measurements are taken on fuel elements that are discharged from the reactor that would be reused in subsequent cycles to
ensure film measurements are within tolerances.

  • Crud from debris in the coolant system, fine particles etc. that deposits on the surface of the cladding and inhibits heat transfer




Diagram of a typical fuel pellet which consists of:

  • The uranium fuel, typically consisting of U235 combined with other isotopes like plutonium
  • The gap between the fuel and the cladding which is filled with Helium after the cladding is welded closed
  • The cladding material, a metal of zircaloy


The primary coolant water which flows through the fuel assembly and the heat generated by the fuel is transferred to the fuel assembly coolant channel Fuel pellets in BWR reactors have the most emphasis for safety in nuclear plants. The safety basis for the facility is directed mainly at keeping the fuel assemblies covered with water as well as preventing a condition where the pellets overheat. The accidents at Fukushima were an example of the magnitude of damage that can be caused by the fuel pellets. Protecting the fuel pellets requires a very complex system and the system must interact with the safety behavior of the pellets.

In the following section we will present how the coolant works in the reactor and how that interacts with the heat flux characteristics of the fuel. BWR reactors operate in the nucleate boiling region, ie: they boil to produce steam and this operating condition forces the reactor to run closer to undesirable boiling. The water cycle in a BWR operates to provide quality steam within the reactor vessel to drive the turbines. The flow cycle is very delicately controlled to prevent the reactor flow from entering a condition where the cladding pellets are unable to transfer sufficient heat and overheat.



BWR reactors operate in the nucleate boiling part of the classic heat transfer curve. At this temperature small beads of bubbles act to efficiently carry away the heat then the bubble is replaced by another and so on.  As the temperature of the water increases the reactor begins to enter the part of the curve called the transition boiling.

    • Transition boiling is the unstable transient region where nucleate boiling tends toward film boiling.
    • A water drop dancing on a hot frying pan is an example of film boiling
    • During film boiling a volume of insulating vapor separates the heated surface from the cooling fluid; this causes the temperature of the heated surface to increase drastically to once again reach equilibrium heat transfer with the cooling fluid. In other words, steam semi-insulates the heated surface and surface temperature rises to allow heat to get to the cooling fluid (through convection and radiative heat transfer).
    • During this transitional phase less and less bubbles form a heated surface and because of the already high centerline temperature in the fuel, the heated surface temperature climbs exponentially
    • The point at which the heat transfer curve reaches its peak is called the departure from nuclear boiling (DNB) temperature point. At this point less and less water is available to transport the heated surface and an instantaneous change to full film boiling can occur as follows:
      • Formation of hot spots are produced under a growing bubble. When a bubble grows at the heated wall a dry patch forms underneath the bubble as the micro-layer of liquid under the bubble evaporates. In this dry zone, the wall temperature rises due to the deterioration in heat transfer
      • Near-wall bubble crowding and inhibition of vapor release.
        • Here a “bubble boundary layer” builds up on the surface and vapor generated by boiling on the surface must escape through this boundary layer
        • When the boundary layer becomes too crowded with bubbles, vapor escape is impossible and liquid cannot penetrate to the heated wall and cool it, the surface becomes dry and overheat gives rise to burnout.
        • In plug or slug flow, the thin film surrounding the large bubble may dry out giving rise to localized overheating and hence burnout.
        • Alternatively, a stationary vapor slug may be formed on the wall with a thin film of liquid separating it from the wall, in this case, localized drying out of this film gives rise to overheating and burn out.


BWR nuclear fuel will be damaged by film boiling as this would cause the fuel cladding to overheat and fail. This condition is knows as fuel dryout and allows the fuel cladding to overheat and approach the point at which the cladding/fuel pellets will fail”



Draining The Unit 4 Spent Fuel Pool

“Can The Fuel Withstand This Without Melting?”

This section of the report focuses on the theory that the decay heat levels in unit 4 SFP have decayed to the point that the pool can withstand a complete draining without the fuel melting or catching fire.

After the reactor is shut down and decay heat cooling has been met, the top head is removed. Fuel assemblies are then transferred to the spent fuel pool

For our focus on unit 4 SFP, all of the fuel elements from the reactor were discharged and stored in a central storage pattern (close together). This ultimately has resulted in a negative situation where higher local heating and direct radiation has caused negative effects to the spent fuel pool liner and concrete wall and floor. released a report on the gamma heating effects in the SFP which has resulted in likely damage to the spent fuel pool liner as well as the concrete wall.

Fuel assemblies stored in the SFP have maximum allowable safety temperature and pressure limits:

  • 715 F (380°C) for cladding which is the temperature for indefinite storage
  • 265 psi (1.8 MPa) internal pressure at 25C


The classic heat transfer curve discussed earlier for reactor fuel applies to the spent fuel stored in the pool racks as well. Safety assumptions are made for the maximum fuel temperatures as well as water quality in the SFP.

As a refresher, the coolant method for the fuel assemblies in the spent fuel racks is for the water to travel down around the racks and then under the rack assemblies, then up through the spent fuel cooling channels. This is a convective flow and is the same for water or air in the case of a drained pool.

The unit 4 SFP currently has a temporary cover on the water with a protective steel cover over that. The pool’s water level is being maintained with temporary systems. Multiple leaks due to materials used or freezing has required constant system repairs. The pool water chemistry remains largely unknown at this time. The only news from TEPCO is that hydrazine may be added from time to time. No debris has been removed. Various large fragments are visible on the fuel racks and  Little detailed information has been made available to the public.

Thermal analyses used for draining accidents assume a robust and running building HVAC system which will be ignored for this review since the reactor building’s superstructure was demolished during the explosion on unit 4.

Decay heat levels will be those at the present time which were corrected from the time that the fuel assemblies were discharged to the SFP in November 2010

The report on lifting a fuel elements out of storage in the unit 4 SFP was shown to not cause melting. That is an initial basis which was developed to use in this report from the standpoint of present decay heat levels.



What We Know About Unit 4 SFP At The Present Time

    • The heat flux level of the fuel in the unit 4 SFP has reached a very low heat flux level which will result in the following:
      • After this decay (up to 12-10-12) time the decay heat has reduced to <.2% of the total amount (4300 kw). According to a study done at Argonne National laboratory
      • In that study it was predicted, using information from the Fluent CFD Code (version 5), that fuel temperatures would remain below below 800 C and 600 C after 26 and 35 months respectively.
      • Reported temperatures will be below the oxidation level for Zircaloy cladding with a lower temperature value of 1652 F (900C).
      • “The view prevailing in the industry is that the temperature of fuel assemblies aged for more than 120 days will rise, achieving equilibrium with the circulating air environment (a convective air flow establishes within the fuel element). The cladding will attain a stable temperature below 1100-1200 C at which zirconium begins to combust
      • This temperature is also below the melting temperature of commonly used steel alloys of around 1425 to 1540 C (2600 – 2800 F)”.


Based on US Department of Energy research these behaviors of spent fuel cooling are known:

  • The complete draining is essential to achieve sufficient convective air flow for cooling the spent fuel assemblies
  • If the water level drains more slowly and stopped at the top of the SFP rack the convective cooling of the water would be small enough that boiling and eventual fuel damage may occur.
  • A spray system placed around the SFP similar to the reactor vessel spray could be used to cool the fuel elements with the fine mist of spray acting as a heat sink by it’s latent heat of vaporization
  • The melting temperature for zircaloy is about 1800 deg C, but as it is in contact with water at only about 320 deg C in normal conditions, there is a good margin
  • UO2 is a poor thermal conductor. There is an appreciable temperature gradient from the outer radius of the fuel pin to its center. The melting temperature for UO2 is about 2800 deg C, creating a good margin of safety


Known factors in the Fukushima unit 4 spent fuel pool:

  • There has been debris scattered over and in the SFP
  • Structural debris is currently in the SFP resting on the top of some racks
  • There has been interface damage between the SFP and the fuel transfer area between the reactor and the SFP
  • The gate has sustained some damage although it is uncertain to what extent
  • There is a cooling system that has been jury rigged and attempts have been made to utilize a system which is fed from the Turbine Building.
  • These SFP cooling systems have been marginal at continuously feeding cooling water to the SFP and as a result the temperatures have been cycling.
  • The fuel at unit 4 was discharged in November 2010 which established the beginning time for calculating decay heat levels to the present time
  • The SFP has been reinforced with massive steel and grout in the hopes of providing sufficient support in the event of another large seismic event.
  • 2417 earthquakes were reported since 3-11-11 most likely hundreds of those could have impacted the Fukushima reactors



Negative Affects From Dealing With A Drained SFP

If the SFP were to drain the radiation/neutron levels at the top of the spent fuel pool would likely be enveloped by readings of

  • 2.4 X 10 6 rad/hr (10 e6 rem/hr)from gamma rays,compared with
  • 0.25 rem/hr from neutrons.0e5 Rem (10000 Sieverts)
  • The radiation field would prohibit work on or around unit 4 or 3 reactors
  • There would be airbourne contamination that would need to be dealt with and prevented from spreading
  • The radiation would be emitted around the spent fuel pool since it is elevated above ground thus adding and shielding as if it were below grade


The following figure shows the estimated radiation fields around the spent fuel pool. Everything would drastically change at the Fukushima disaster site while the water was drained from unit 4 SFP





  • The final conclusion theory we have reached at is that the unit 4 SFP water could be completely drained from the pool without causing any interaction from excessive temperatures. The fuel assemblies would heat but the convection air cooling would maintain the fuel elements at temperatures lower than damage thresholds. The following observations could affect this outcome

    • Debris in the pool bottom which inhibits cooling, we believe the fuel assemblies are currently being cooled despite this, air cooling should work as well.

    • Water level not draining completely ie: a partial draining to the level of near the top of the fuel rack. This condition would inhibit convective air flow and cause gradual heating of the fuel assemblies and subsequent boiling due to insufficient heat sink. Damage to the fuel elements including overheating and melting would be expected to occur in  this condition unless mitigative measures such as a spray mist system to help as a heat sink or the rapid addition of water were done. The fuel damage due to over heating would be expected to occur within days or short weeks.



  • TEPCO needs to expedite removal of debris from the SFP’s 3, 2, then 1
  • TEPCO needs to do additional cleaning to remove the small debris resting on top of the fuel assemblies and around areas near the bottom of the racks
  • We Strongly suggest that a standby emergency w a t e r spray system be placed in service to assist in cooling as well as reducing the effects of air activity which would come from the spent fuel assemblies in the event that the pool drains.
  • We Strongly suggest that TEPCO dedicates a separate storage tank with sufficient water supply to fill unit 4’s SFP rapidly to the level of at least 3/4s capacity (approximately 200,000-300,000 gallons) will continue to review this issue as time progresses and will present any further changing conditions at unit 4 SFP.




1. Maxium Allowable Temperature For Storage Of Spent Nuclear Reactor Fuel
Hanford  Engineering Development  Laboratory – 1078

2. Analysis of Spent Fuel Heatup Following Loss of Water in a Spent Fuel Pool
US NRC – 2002

3. Predictions of Spent Fuel Heat Up After a Complete Loss of Spent Fuel Pool Coolant
US NRC  - 2000

4. Operational Safety of Spent Nuclear Fuel
Joseph C. Braun - Lecture 6.1b - 2 December , 2010

5. Japan Quake Map – website tool

6. Spent Fuel Heatup Following Loss Of Water During Storage
Sandia National Lab – 1979

7. Technical Dictionary – Boiling Water Reactor 

8. Technical Dictionary – Nuclear Fuel


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