SPENT FUEL POOLS AT FUKUSHIMA
FOLLOW ON REPORT
by Dean Wilkie
This paper with all graphics, companion documents, and tables can be downloaded as a PDF
Corrosion At Fukushima Daiichi
* A companion report that explains the basics and terminology of corrosion as it relates to Fukushima Daiichi can be found here. Corrosion At Fukushima Daiichi Explained
This is a follow-up to a report that was issued concerning the Fukushima spent fuel pools. The first report captured the general design, storage, and water quality necessary to protect the spent fuel for re-use or eventual storage in dry casks. At the time the report was written the details of water chemistry within the SFPs was not known in detail which would have been necessary to more accurately predict corrosion for the spent fuel elements, the structural racks, and the spent fuel liner/interface with the reactor wells. We have witnessed several conditions in or around the spent fuel pools since the first report. Foreign materials have been introduced into the pools as a result of cleanup on the main floor area, covering spent fuel pool #4 and dropping a large steel beam into the unit 3 SFP. Cooling systems to the SFPs have been shut off many times to make mechanical adjustments causing fluctuations in pool water temperature
Chloride levels in the SFPs were extremely high (in the 6900’s ppm as opposed to normal of 30 ppm or less) and were left unchecked for months. (10) Every worst-case corrosion analysis assumes Chloride levels at least a factor of 10 lower for their extreme conditions. Some corrosion rates have gone from 1mm to 10mm/yr which would suggest potential corrosion of the structural parts of the fuel elements (lattice holders, springs, handling assemblies, pellet holding tubing, and fuel pellet cladding. The spent fuel pools have had so many foreign materials introduced that it is very difficult to pinpoint the chemical makeup of the water. Season conditions around Fukushima have contributed to the addition of dust and other foreign materials which can change the chemistry in the pools. The following parameters were mentioned in the earlier report;
- Total Oxygen
- Dissolved Oxygen
- Algae/Slime Control
- Biological Fouling
- Filterable Solids
An IAEA document (1) examined the storage of fuel elements in spent fuel pools for short and long term storage. The following examples of water chemistry in storage pools is given below:
Cooling water Demineralized Water
Temperature <45 deg C
Water Activity 20MBq/m2
“Impurities in the water of the basins used for fuel decay or interim storage can have several consequences. Aggressive ionic species, un-dissolved particulates (if settled on the fuel cladding), and several microorganisms can accelerate the fuel corrosion rate. Excessive corrosion can cloud the water, and if through-clad penetration occurs, the concentration of radioisotopes (fission products) on the water can reach unacceptable values. Therefore, in order to maintain the integrity of the fuel for many years, it is necessary to use a purification system, to ensure and maintain good water quality, and minimize corrosion attack of the fuel cladding. As established for the primary cooling system, for storage basins, the utilization of purification systems is based on a filter and resin bed system to keep the water quality within specified limits. Flocculants may be added to remove turbidity in the water. However, the use of flocculants with the subsequent filter of the debris would be a special case of water quality management; considerations including impact to criticality and other potential impacts to the safety basis of the facility must be addressed before using any type of flocculants” (2)
Flocculants are chemicals used in water treatment processes to improve the sedimentation or filterability of small particles. Flocculants are used to remove microscopic particles which would otherwise cause water to be cloudy (turbid). Establishing chemistry control of the SFPs at Fukushima has been very fragmented and has involved minimal attempts at filtration and chemistry addition such as boric acid. Very limited information is available from TEPCO on the total water purity chemistry. It is our assumption that the water chemistry is completely out of the specifications mentioned above and has established conditions for optimal corrosion.
“In some cases, mostly in summer, microbiological or algae growth has occurred. Significant biological fouling (biofouling) was experienced in a spent fuel storage pool heat exchanger at one of the nuclear power stations in Canada. The fouling was attributed to a wide variety of bacteria which included pathogenic coliforms. Sulphate reducing bacteria (corrosion bacteria) were either absent or present in low numbers. The action taken to control the bacteria consisted of thorough cleaning of the heat exchanger with a brush, collection of the biological material, and incineration of the protective clothing worn by the workers. Appropriate biocides (hydrogen peroxide) at concentrations up to 1000 ppm were added (to the pool water) to control biofouling.” (3)
The normal cleanup systems at the SFPs at Fukushima have been replaced with at best temporary water systems with filtration and reverse osmosis. Approximately 8-9 months after the accident TEPCO did attempt to get the chloride concentration down in specifications after having been in the 6900’s of ppm/ml as compared to a normal level as seen above.
“In Japan at the BWR plants, boronated aluminum alloy racks have been used since 1978, and boronated stainless steel racks have been used since 1993. In the near future, there is a plan to install boronated stainless steel racks in further BWR and PWR plants.” (3)
There is no information that can be gathered to verify if the racks at Fukushima SFPs are of the original aluminum or the upgraded stainless steel. Knowing the type of rack material is central in determining estimates for corrosion.
“In the USA, one significant event that occurred during handling operations at a spent fuel storage pool involved the separation of the top end fitting (nozzle) from the remainder of a PWR fuel assembly as the assembly was being lifted out of a storage rack in the pool at the Prairie Island NPP. There were no radioactive releases from this Westinghouse assembly, and no fuel rod damage occurred. The fuel assembly was subsequently lifted and inserted into a storage position. Intergranular stress corrosion cracking of the Type 304 SS sleeves, which were welded to the top nozzle and mechanically joined to the Zircaloy control rod guide thimbles, was identified by Westinghouse Electric Corporation as the cause of the failure. The Zircaloy guide thimbles appear to have remained intact.” (3)
The only fuel that has been removed to date were 2 unused fuel elements that were removed from unit 4 SFP but since they were relatively new had no major signs of extensive corrosion but there was a lot of debris within the fuel element.
There are some seldom mentioned phenomena that occur in operating reactor fuel elements which depend on a term called “FUEL BURNUP”. All nuclear power plants (NPP’s) must determine to what degree they allow the reactor fuel element to burn up ie: what percentage of the actual fuel (like U235) is acceptable to burn up before the fuel element reaches its end of life. Since NPPs are in the business to make profits, many utilities have increased the burnup limits to decrease operating costs.
“To minimize operating costs and increase cost competitiveness, utilities are changing fuel designs and reactor operating conditions, including higher fuel burnups, longer cycles, increased enrichment, and new water chemistry. This increase in ‘fuel duty’ and these environmental changes directly impact fuel performance, and new performance issues have appeared. Fuel cladding is a key barrier in containing fission products and it is essential that this barrier is robust and remains intact. Fuel failure occurs when this barrier is degraded and breached. It contributes to increasing plant background radiation, which impacts planned outages and increases workers’ exposure. It can also contribute to the release of radioactive fission products into the environment. Thus, fuel performance should be sufficient to limit radiological releases into the environment and to be able to cope with ALARA issues. Finally, reactor fuel failure does not create confidence in the nuclear power industry and can influence public acceptance of nuclear power generation. For all of these reasons, it is a general goal of modern nuclear utilities to operate with a core free of defects” (4)
When the percentage of fuel burnup is increased many factors are introduced to the fuel elements such as:
• Increased internal pressures within the fuel pellets
• Cracking initiated at a massive hydride layer at the clad outer surface and propagation through the whole cladding thickness (delayed hydride cracking or DHC)
• Insufficient fuel rod support in the assembly due to improper design and/or fabrication, fuel rod vibration due to fluid elastic instability caused by crossflow in the assembly, and flow-induced
assembly and rod vibration. Debris fretting continues to be a common mechanism for fuel failure in all types of power reactors.
• Pellet–cladding interaction (PCI) fuel failure end
• Plug defects, and several types of end plug weld deficiencies.
These are a few of the major issues that can arise with NPP fuel elements and each can lead to what is called a “LEAKER FUEL ELEMENT” ie: a fuel element that has had some minute cracking or failure of the cladding around the fuel pellet which results in fission products to be dispersed in the primary coolant system. When the leaker fuel element is then placed in a spent fuel pool that same leak may seal up or it may open and release fission products directly into the spent fuel pool. The link above has a detailed section on fuel element leakage in operating reactors as well as spent fuel pools. The main result of a leaker element is the rise in gross activity levels, interaction with the leaker site, and the water in the reactor or spent fuel pool. These leaked fission products can result in levels of activity that would not allow work to be done in the area and in the case of Fukushima, would be directly released into the atmosphere. Increased levels of Xe133 is the main isotope that is watched for spotting a leaker. Iodines are also monitored for changes especially when the reactor is shut down. The most common ratios utilized as indicators of fuel failure are 133Xe to 135Xe, 133Xe to 138Xe, or 85mKr to 87Kr. A significant change in the value of such ratios is a clear indication of fuel failure. In addition, the presence of transuranic isotopes (actinides) in the primary coolant is an indication of the presence of fissile materials in the primary coolant due to erosion of fuel pellets through large defects.
The most significant actinides are:
—Plutonium 238Pu, 239Pu, 240Pu, 241Pu;
—Curium 242Cm,243Cm, 244Cm.
A technique called “sipping” has been successfully used to identify potential leakers. (4) Sipping is the most common technique used to locate fuel failures in both PWRs and BWRs. Identification
of fuel rod failure is based on the detection of fission product activity released through defects during sipping. The more common radioisotopes measured are xenon and krypton, and cesium or iodine in water samples. Various versions of sipping have been used to detect leaking fuel assemblies.
To date, TEPCO has not mentioned the use of sipping in any of the SFPs. No reported efforts have been reported by TEPCO on any efforts to directly determine if fuel element failures, including fuel leakers, have occurred in the pools. Failure to implement “sipping” or equivalent techniques” undermines the ability to be proactive and look for changes in the pools which would lead to fission product leakage/release. In addition, TEPCO has not released any information on the number of known defective fuel elements which may be stored in the SFPs including ones that are in canned storage.
Every analysis that has been researched concerning fuel degradation in spent fuel pools has included a qualifying statement, “assuming the chemistry of the spent fuel water is maintained” Clearly the SFPs at Fukushima have undergone unprecedented severe accidents, extreme thermal cycling, unknown chemistry, fires, damage from debris falling into the SFPs including a huge roof truss section which fell directly into the #3 SFP while removing debris. The key and most important parameters went to extreme levels out of specification and were unchecked for months. From these conditions as well as missed attempts to try and bring the water into some form of gross control, the assumptions for degradation rates will be made using the worst-case conditions.
The single most important event in the spent fuel pools at the Fukushima plants is creating conditions in which the fission product contents of a single or multiple fuel element pellets are released into the water and ultimately to the atmosphere. This report has tried to discuss some of the modes of failures that can present during storage in the spent fuel pools. Questions have been asked such as :
• What is the corrosion rate for the metals in the pools and fuel elements
• How will cooling of the fuel elements be maintained to ensure they don’t overheat
• What has all the debris/crud that has fallen and collected in the spent fuel pools do to challenge chemistry control
• What should be done to reduce the impacts of chemistry on the existing stored fuel elements
• What would the conditions be like around the spent fuel pools if the fission products were released into the atmosphere
• Would the activity levels be too high to work in the vicinity of the spent fuel pools if the fuel fission products were released
These are important questions that involve very complicated analysis to predict in a way that can relate to time, such that projections for failures can be made. The idea is to eventually remove the fuel from the spent fuel pools. However, the planning how to achieve this goal is in its infancy, since there has been no case like Fukushima from which to look back and see what was done.
OBSERVATIONS AND PROJECTIONS
SPENT FUEL POOLS
- BWR fuel elements are more prone to corrosion than PWR elements
- SPENT FUEL pools have undergone extreme conditions as noted earlier
- Specific water chemistry is unknown other than what few samples have been taken focusing on activity levels and isotopes (specifically Cs137)
- SPENT FUEL pools materials of concern are stainless steel, Inconel, some carbon steel, and aluminum
- LWR fuel structural material consists mainly of Zircaloy and stainless steel Zircaloy has proven to be insensitive to any kind of corrosion phenomena (uniform corrosion, stress corrosion cracking (SCC), electro corrosion) in the temperature range <=60°C (3)
- In such cases where Zircaloy and SS are in direct contact, in-service passivation prevents any electro-corrosive phenomena SS is also resistant to any kind of corrosion However, some attention has to be given to SCC in the neighborhood of welds Water chemistry quality control has proven to be a reliable remedy against SCC of SS components (3)
- During underwater storage of LWR fuel, sleeve corrosion has been reported for some LWR fuel with Zircaloy guide tubes and stainless steel upper sleeves. The corrosion phenomena, probably induced at the reactor, have occurred for high carbon content in the upper sleeve material and can jeopardize future spent fuel handling operations. (3)
- The corrosion rates of stainless steels stored for 18-month in the pool are very small ( 10~5 – 10~4 mm/year) regardless of stainless steel types and pre-treatment histories (3)
- Maximum corrosion occurs at the initial storage stage in small water volumes (cans) at the points of Zr/SS contact (under spacers). Corrosion of SS pool components amounts to 1 /µ/year. (3)
- A technique has been developed for assessing the fuel element integrity by calculating the ratio of dissolved ’37Cs and l34Cs in water. (3)
- Accelerated concentrations of Chlorides such as was experienced at Fukushima could elevate corrosion by a factor of 100. (3)
The reference link (5) is an excellent history of some of the erosion/corrosion problems experienced on reactors in Japan
- When predicting rates of erosion/corrosion, pipe sections of non-standard geometry tend to degrade faster than the rest of the pipe. Certain parts of the pipe will be subject to differing conditions (due to stream turbulence). These conditions can include temperature, chemical species, and fluid flow rate. As the rate of reduction in pipe thickness is determined by process conditions and materials, predictions cannot be made based on common corrosion data. The same is true of stress corrosion
- Pipe wall thickness reduction is mostly found in the cooling circuit of BWR’s. These pipes are made of carbon steel and hence management of water quality (temperature, dissolved oxygen, pH, etc.) as well as material properties (low alloy steel, Austenite stainless steel, etc.) is important
- In general, the acceleration of corrosion is caused by two factors: water impurities (seawater, etc.) and mechanical stress. Furthermore, the rate of thickness reduction caused by erosion/corrosion is much higher than that caused by corrosion only
- Erosion/corrosion causes are discussed below
o Position (T-junction, curve, joint, valve, and so on, downstream of an orifice, etc.)
o Downstream events (water injection, collision, hot water flushing)
o Design (inappropriate structure, unsuitable materials)
o Inappropriate structure composite, e.g T-junction and water injection
o Fluid-position composite, e.g. hot water flushing through an orifice
o Unsuitable structure and unsuitable fluid complex, e.g. water injection and joint
- The linked study arrived at the following conclusions regarding the rate of thickness reduction due to erosion/corrosion
o In general, corrosion proceeds at a rate of 0.15-0.3mm/year
o Accelerated corrosion occurs at a rate of 0.3-0.5mm/year
o In general, erosion/corrosion proceeds at a rate of 0.5-1.0mm/year
o Acceleration of erosion/corrosion occurs at a rate of 4mm/year
The table below lists estimates of corrosion rates for the feedwater pipes at Fukushima. The feedwater pipes are currently being used to supply water to the reactor vessels through a connection inside the reactor building, outside of containment. These pipes were already considerably old, records do not show TEPCO as having replaced these pipes. Browns Ferry NPP was used to estimate feedwater pipe diameter and schedule. (7) (8) According to the nominal pipe thickness of new pipes and the combined corrosion rates, the piping at Fukushima has an estimated life before failure of 3-6 years. TEPCO states they need to continue water cooling for 10 years. Details of the Browns Ferry feedwater system can be found in the companion report “Corrosion At Fukushima Daiichi Explained”
A larger version of the table is available as a PDF Fukushima_Daiichi_Corrosion_Table_Large
One final observation for Fukushima is that many systems have been stagnant for months, many systems have been modified temporarily which changes the hydrodynamics of the systems and could very well introduce accelerated corrosion conditions. The present estimates are based on the information available to the authors. Any unknown degradation in water quality could certainly increase the corrosion/erosion rates. (6)
1. Good Practices for Water Quality Management in Research Reactors and Spent Fuel Storage Facilities
IAEA No. NP-T-5.2
2. Good Practices for Water Quality Management in Research Reactors and Spent Fuel Storage Facilities
IAEA No. NP-T-5.2
3. Further analysis of extended storage of spent fuel
4. Review of Fuel Failures in Water Cooled Reactors
IAEA No. NF-T-2.1
5. Steam Eruption from Nuclear Power Plant Cooling System.
Failure Knowledge Database
6. Durability of spent nuclear fuels and facility components in wet storage.
7. BROWNS FERRY NUCLEAR PLANT (BFN) – UNITS 1, 2, AND 3 – LICENSE RENEWAL APPLICATION (LRA) – RESPONSE TO NRC REQUEST FOR ADDITIONAL INFORMATION CONCERNING THE UNIT 1 LAYUP PROGRAM (TAC NOS. MC1704, MC1705, AND MC1706)
NRC – ML051390237
8. BROWNS FERRY NUCLEAR PLANT (BFN) – UNITS 1, 2, AND 3 – SUBMITTAL OF UNIT 1 PERIODIC INSPECTION PROGRAM FOR LICENSE RENEWAL PROCEDURE IN FULFILLMENT OF LICENSE RENEWAL COMMITMENT
NRC – ML071150369
9. Wikipedia Nominal Pipe Thickness
10. Situation of Storage and Treatment of Accumulated Water including Highly Concentrated Radioactive Materials at Fukushima Daiichi Nuclear Power Station (65th Release)
September 19, 2012
Tokyo Electric Power Company