The spent fuel pools at Fukushima Daiichi have shown a considerable risk and design flaw in some reactors. The fuel pools have proven to be a considerable problem and safety risk.
This document is a work in progress, check back for updates and new information.
SimplyInfo member Dean Wilkie has written a detailed report on the spent fuel pools at Fukushima and the technical challenges involved. The pool chemistry and many other factors impact the stability and corrosion of the fuel assemblies. Read Dean’s report here http://www.simplyinfo.org/?p=5542
TEPCO recently released some information on the desalination work for unit 2’s spent fuel pool. They document work done to the spent fuel pool water in an attempt to control the problems with the water. handouts_120702_02-e
The Idaho National Lab published this research document on corrosion and microbial impact on stored spent fuel. Below are some excerpts from the paper. It can be read in full here: 766409
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.
RESEARCH RESULTS - The Cs-137 adsorption rate of the stainless steels stored for 18-month in pool appears to be about 78 pCi/cm^-month for 144 pCi/ml of Cs-137 concentration in pool water, while that of Co-60 seems to be about 7 pCi/cm2-month for 27 pCi/ml of Co-60 concentration in pool water - The preliminary results indicate that gamma irradiation enhances the corrosion of stainless steel - The corrosion rates of stainless steels stored for 18-month in pooJ_are very small ( 10~5 - 10~4 mm/year) regardless of stainless steel tvpes and pre-treatment histories
Microbially Influenced Corrosion’s Role in Wet Storage Several factors have led to extensive deterioration of the fuel, such as physical damage due to handling, but the prinmy corrosion mechanism is pitting. Pitting is a form of localized corrosion of a metal surface that results in cavities. Pitting is most common in metals that form an adherent passive surface film such as Al and 304 SS. The pits tend to develop at defects or flaws in the surface film and at sites of mechanical 1 ,,r-- ,..,
The rates of pitting can be quite rapid (e.g., 5,000 mils/yr) under specific conditions and can be one of the most destructive forms of corrosion. In the case of stored Al-clad- or SS-clad SNF in basin water, pitting can penetrate the clad material and allow the release of uranium, plutonium, cesium-137, and other radionuclides due to reaction with fuel meat. As an example, tests performed at the SRS P, K, L basins showed that pitting can completely penetrate coupons 750 pm (30 roil) thick in 45 to 100 days in water with the following characteristics: conductivity >180 pS/cm, pH 6.3 to 7.1, and chloride up to 18 ppm (Howell 1993; Howell 1995a). MIC is a form of localized corrosion. Pit initiation can be a result of microbial activity on the surface of the metal. Once the pit in initiated, propagation can continue despite environmental changes.
"The rates of pitting can be quite rapid (e.g., 5,000 mils/yr) under specific conditions and can be one of the most destructive forms of corrosion. In the case of stored Al-clad- or SS-clad SNF in basin water, pitting can penetrate the clad material and allow the release of uranium, plutonium, cesium-137, and other radionuclides due to reaction with fuel meat. As an example, tests performed at the SRS P, K, L basins showed that pitting can completely penetrate coupons 750 pm (30 roil) thick in 45 to 100 days in water with the following characteristics: conductivity >180 pS/cm, pH 6.3 to 7.1, and chloride up to 18 ppm"
The ecosystems and the chemistry contained in the bacterial exopolymers can also encourage stress corrosion cracking and hydrogen embrittlement.
Physically, biofilms act as a diffusion barrier, tending to concentrate chemical species produced at the metal film interface and to retard diffusion of species from the bulk water towards the metal surface. ~ The presence within the biofilm of aerobic species capable of hydrocarbon degradation and subsequent fermentative metabolism will provide an oxygendepleted area witlin the deeper layers of the biofilm.
The stages of biofilm development are shown in Figure 5: (1) conditioning film accumulates on submerged surface; (2) planktonic bacteria from the bulk water colonize the surface and begin a sessile existence by excreting exopolymer that anchors the cells to the surface; (3) different species of sessile bacteria replicate on the metal surface; (4) microcolonies of different species continue to grow and eventually establish close relationships with each other on the surface, the biofilm increases in thickness, and conditions at the base of the biofilm change; (5) portions of the biofilm slough away from the surface; and (6) the exposed areas of the surface are recolonized by planktonic bacteria or sessile bacteria adjacent to the exposed area.
Several factors have led to extensive deterioration of the fuel, such as physical damage due to handling, but the prinmy corrosion mechanism is pitting. Pitting is a form of localized corrosion of a metal surface that results in cavities. Pitting is most common in metals that form an adherent passive surface film such as Al and 304 SS. The pits tend to develop at defects or flaws in the surface film and at sites of mechanical damage. The rates of pitting can be quite rapid (e.g., 5,000 mils/yr) under specific conditions and can be one of the most destructive forms of corrosion. In the case of stored Al-clad- or SS-clad SNF in basin water, pitting can penetrate the clad material and allow the release of uranium, plutonium, cesium-137, and other radionuclides due to reaction with fuel meat. As an example, tests performed at the SRS P, K, L basins showed that pitting can completely penetrate coupons 750 pm (30 roil) thick in 45 to 100 days in water with the following characteristics: conductivity >180 pS/cm, pH 6.3 to 7.1, and chloride up to 18 ppm (Howell 1993; Howell 1995a).