Lifting Fuel Elements Out Of The Fukushima Daiichi Spent Fuel Pools
Dean Wilkie, 12-9-12
Over the course of the last 2 years, there has been speculation about the fuel elements melting in the spent fuel pools at Fukushima Daiichi in the event of the pools draining. Cooling of the spent fuel within the racks is performed by convective circulation. This sets up a flow path from the bottom of the rack, up through the fuel element coolant channels, and then back down the outside of the rack.
“As long as the water is maintained above the rack, this convective flow will continue. Water levels below that point result in a stagnated condition within the fuel rack and inhibit heat transfer from taking place which can overheat the fuel elements.”
One point that our research team has been looking into is the estimated power level of the newest elements in the pool, after having been in the spent fuel pools for over 2 years. Through the use of fuel element decay heat curves, an estimate can be made to determine if there is enough heat in one fuel element, which, when removed from the storage rack, vertical in the air, will cause it to overheat.
Our research indicates that at the present time since shutdown and defueling of the unit 4 reactor, sufficient time has passed for the fuel elements within the spent fuel pool to have reached a decayed power level. This would allow any element to be pulled out of the pool vertically into the air without reaching temperatures that would result in the ignition of cladding material or melting.
It should be pointed out that the fuel element would still be very highly radioactive and would be giving off an extremely high radiation level. In addition, this research does not imply that the entire pool would not have other catastrophic problems. These potential problems are being researched and reports will be made on our findings.
Fuel element removal history from units 1-4 are as follows:
|UNIT 1||9-27-2010||1/3 OF THE CORE WAS OFFLOADED AND REFUEL WAS COMPLETE|
|UNIT 2||11-18-2010||1/3 OF THE CORE WAS OFFLOADED AND REFUEL WAS COMPLETE|
|UNIT 3||9-23-2010||1/3 OF THE CORE WAS OFFLOADED AND REFUEL WAS COMPLETE|
|UNIT 4||11-29-2010||All of the fuel was removed from Unit 4 to prepare for a change out of the shroud. The fuel elements were stored in the same location in the spent fuel pool which is typically not done at BWR plants. The fuel elements are typically stored in a checkerboard fashion so all the recently removed elements are not all together. No reason has been given or identified as to why TEPCO did this at unit 4. Storage of the fuel in a concentration such as was done at unit 4 results in the highest gamma field possible and will increase radiation exposure to workers directly above the racks.|
From this data we can see that the times since fuel had been discharged in the spent fuel pools are as follows:
UNIT 1 804 days assuming 1/3 of core removed during refueling
UNIT 2 751 days assuming 1/3 of core removed during refueling
UNIT 3 808 days assuming 1/3 of core removed during refueling
UNIT 4 797 days assuming 1/3 of core removed during refueling
Nuclear power plants have two ways of rating power, one is the THERMAL power and one is ELECTRIC power. Both are expressed in megawatts and the electric power is based on the efficiency of the specific reactor type. Most reactors average around 30% operating efficiency and so the ELECTRIC power is derived by reducing the THERMAL power rated level for the unique reactor by 30% Since all of the fuel was removed from unit 4 most recently, which would result in the highest thermal loading, we will use the fuel in SFP 4 as the example for decay heat estimates:
- assume unit 4 is a 784 MW(e) and a 30% operating efficiency that equals about 2300 MW (th)
- divided by 548 fuel elements (unit 4 total fuel loading) is approximately 4300 kW/element
- then we take the 4300 kW/element and apply it to a decay heat curve which looks at WATTS (thermal) versus time in seconds
- Unit 4 797 days x 8640 sec/day = 6886080 seconds
After this decay time, the decay heat has reduced to <.2% of the total amount (4300 kW). According to a study done at Argonne National laboratory
Additional reference is made to Nureg -1726, Predictions of Spent Fuel Heat up After a Complete Loss of Spent Fuel Coolant,
In this study it was predicted, using the FLUENT CFD CODE (version 5), that fuel temperatures would remain below 800 C and 600 C after 26 and 35 months respectively.
“The view among the industry is that fuel assemblies that have aged for more than 120 days will rise to an elevated temperature and achieve equilibrium with the circulating air environment (a convective airflow establishes within the fuel element). This stable temperature will have a cladding below the 11-1200 C temperatures at which combustion of zirconium begins. This temperature is also below the melting temperature of commonly used steel alloys of around 1400 C”.
In addition, our team is concerned that:
- TEPCO is not accelerating the removal of all the new elements from unit 4 SFP since 2 were successfully removed.
- TEPCO is not informing the public about ongoing work
o TEPCO has released the long term base timeline schedule but has not provided more detailed schedules to the public
o In the removal of fuel from the common pool
o On the fabrication of new casks which will be needed next year (up to 190)
o Of plans to build another interim pool adjacent to the common pool (current plans is to split the common pool in ½ for older and then all the fuel from units 1-4 SFP)
o TEPCO is not providing translated updates into English or allowing them to be translated
o TEPCO has not provided information concerning the integrity of the spent fuel pool concrete located adjacent to the hottest fuel racks in SFP 4. The heat transfer through the stainless steel liner in the SFP can overheat the concrete walls and cause blistering or cracking which can weaken the SFP concrete walls
We will continue to research the situation at Fukushima Daiichi reactor units and provide further updates on fuel elements within the spent fuel pools.
We believe that this information has not been identified and is a crucial data point that should be incorporated into the planning of fuel element removal from the spent fuel pools. There is a possibility that fuel removal could start much earlier with specific engineered hardware, specifically at the unit 4 SFP.