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Spent Fuel Research & Safety

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.fukuleaks.org/web/?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
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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).

 

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