Tuesday, June 12, 2012

Estimating the Potential Impact of Failure of the Fukushima Daiichi Unit 4 Spent Fuel Pool

--- reposted from WashingtonsBlog

Estimating the Potential Impact of Failure of the Fukushima Daiichi Unit 4 Spent Fuel Pool
A Local Problem for Japan or a Global Mega Crisis?
Paul C. Gailey, Ph.D. , President& CEO, Holophi CHAG
Extreme opinions are being voiced about the risk of global catastrophe resulting from a possible collapse of the Fukushima Daiichi unit 4 spent fuel pool. These claims are appearing mostly among the public through internet media and other non-official channels. Officials sources remain largely mute on the subject or downplay the risks. This report provides an approximate bounding of the risks using available data. The results of this analysis suggest that a nominal release of 10% of the SFP 4 inventory of cesium and strontium would represent 3-10 times the March 2011 release amounts, substantially increasing risk levels in Japan and impacting marine life. Release of 100% of the SFP 4 inventory, or 30-100 times the March 2011 release amounts, could result in significant global impact.

More than a year after earthquake and tsunami that devastated areas of Japan in 2011, new alarms are being sounded concerning the state of the unit 4 spent fuel pool (see Note 1). Although the unit 4 reactor was not operating at the time of the tsunami, its fuel was being stored, along with a large quantity of spent fuel assemblies, in a pool within the unit 4 building. This building was seriously damaged by a hydrogen explosion. The spent fuel pool, which is located about 30 meters above ground, is considered to be in danger of collapsing.

The infrastructure for moving the fuel assemblies was rendered inoperable by the explosion, and high radiation levels make it extremely difficult to clear debris, perform repairs, or construct a new system for removing the assemblies. The cooling system for the spent fuel pool was also destroyed by the explosion, and Tepco has positioned temporary hoses to pump water into the pool for cooling. They have also installed steel pillars to help support the pool.

Reason for Concern
The unit 4 spent fuel pool (SFP) contains 1300-­‐1500 spent and active fuel assemblies. Because the structure of the unit 4 building was damaged by an explosion, the spent fuel pool is in danger of collapsing. If the pool collapses or develops serious cracks allowing the cooling water to drain, the fuel rods will be exposed to the environment. These concerns are elevated by a recent report that additional large earthquakes (magnitude 7) are expected in the area (Tong et al., 2012).

Spent fuel pools are not protected in the same way as reactor cores, and the unit 4 building is seriously damaged. Thus, there is no obvious second line of defense protecting the environment from the radioactive fuel and secondary isotopes if water cooling is lost. Together, the fuel rods currently produce about one megawatt (MVV) or more of waste energy in the form of heat (see Note 2) even though they are not operating in a reactor. This heat must be removed through the use of cooling water to avoid damage to the fuel rods including possible melting, fire, explosions and release of radioactive material.

If cooling water for the spent fuel pool is lost – either by collapse of the pool, formation of cracks in the pool, or other factors – a major release of radioactive material could result. Given the large amount of heat generated by the fuel rods, the temperature would rise quickly. These rods are surrounded by zirconium cladding, and at high temperatures, this cladding catalyzes hydrogen production, can generate additional heat, and even explode and burn (NRC, 2006).

The water surrounding the fuel rods in the spent fuel pools serves two purposes: First, it conducts heat away from the fuel assemblies to avoid overheating. Second, it provides shielding from the extremely high radiation levels near the rods. If a collapse or breakage of the unit 4 spent fuel pool occurs, the loss of shielding by the cooling water could critically increase radiation levels in the entire Daiichi complex. High radiation is already a serious problem limiting worker and even robot access to the plant to perform repairs and mitigation, and to maintain cooling of the other spent fuel pools and reactors. Thus, a catastrophic failure of the unit 4 spent fuel pool could potentially cascade into additional releases from the other spent fuel pools and reactors.

Estimated Amounts of Radioactive Material
Operation of a nuclear plant produces a number of radioactive isotopes. For this preliminary potential impact analysis, we will focus primarily on cesium, strontium, and plutonium, while noting that other radioisotopes are also of concern. Cesium and strontium are easily absorbed in plants and animals similarly to potassium and calcium, respectively. Their movement through the biosphere and long half lives (years to decades) mean that they represent substantial health hazards at relatively low concentrations. Plutonium is not as soluble and easily distributed in the environment, but is extremely carcinogenic when inhaled or ingested.

Because the various isotopes are produced by nuclear reactions during the operation of the reactors, the quantities present in the fuel rods must be estimated from calculations and measurements. For this analysis, we will use the estimated amounts below (Mertyurek et al., 2010, Pretzsch et al., 2011). If improved estimates are identified, the results of the analysis below can be easily scaled based on the new data.

[The remainder of this report can be found HERE]

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