March 17th, 2011 – Proposal to handle dried Spent Fuel Pool

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From: RST01 Hoc
Sent: Thursday, March 17, 2011 3:45 PM
To: Einziger, Robert
Cc: Ruland, William
Subject: N2 on dry spent fuel

Bill Ruland wanted me to request your input on putting liquid N2 into a dry spent fuel pool. You may have sent it previously. Please send again to this operations center address

Frank Collins
RST Coordinator


From:Einzicier. Robert
To:RST01 Hoc
Cc:Ruland. William; Dudes. Laura; McIntyre. David; Einziger. Robert; Haney. Catherine; Dorman, Dan Gordon, Matthew; Rahimi. Merai; Ordaz. Vonna Pstrak. David
Subject:RE: N2 on dry spent fuel
Date:Thursday, March 17, 2011 3:57:23 PM
Attachments,Proposal to handle dried Spent fuel oool.docx

In addition to the information in the attachment, my colleague Matt Gordon informed me that enough liquid N2 to fill a pool 40′ x 40′ x 100′ (probably on the order of the size of the SNF pool in Japan) could be contained in a container about 6′ x6′ x6′.

A tank this size should be easily attained from any Liquid gas supplier. The tank could be lowered by helicopter into the pool away from the fuel then opened. If you could get liquid Argon, you might not even have to get it directly into the pool but rather suspend the tank over the pool and open the valve remotely.


Proposal to handle dried spent fuel pool

RE Einziger, Ph.D.
SFST/MNSS/NRC

Credential: I have over 30 yrs experience at National Laboratories studying the behavior of Spent Fuel rods in an Oxidizing Atmosphere.

The spent fuel consists of UO2 fuel inside a Zircaloy-2 cladding covered with a Zirconium oxide layer and a layer of CRUD. If the pool goes dry, the rods will heat up until the rate of heat production is balanced by the rate of heat removal by conduction, convection, and radiation.

As the rod is heating up the gas inside the rod will be stressing the cladding, that has a lower yield strength and thus the cladding will expand. The expansion will cause the CRUD to flake off and break up to particulate in the 1-10 micron range as it hits the pool floor.

This CRUD, with a main radioactive component as Co-60 may become airborne Somewhere between 600 and 10O0OC the Zircaloy cladding that has a high concentration of zirconium hydrides on the outer surface may catch on fire. Zirconium hydride is pyrophoric but the ignition temperature is not an intrinsic properties but is dependent onn many features including the surface to volume ratio of the cladding..

Concurrently the pressure in the rod will continue to rise until -750C when the rod will burst, ejecting fission gases and volatiles in the gap including Cs and I compounds. Some fuel particulate will also be ejected.

As these temperatures the exposed fuel will rapidly oxidize to U308 with a -32% volume expansion. This expansion will split the cladding from end to end in a relatively short time. The fuel , now in grain size articulate (-10-15 micron) will stay in the cladding as a compact until it experiences a mild physical force at which time it will fall to the bottom of the pool.

Of primary important is to prevent the oxidation of the UO2 fuel that contains the preponderance of the radionuclides. Unless you can cover the fuel completely with water the fuel will oxide.

When water is sprayed on the fuel, steam is formed which further oxidizes the fuel. A better choice would be to put liquid Argon in the pool if it is available. In not then use liquid N2.

The benefits of these gases are that they have a large, (‘1000 times) volume expansion when they vaporize thus a small volume has to be put in the pool then the volume of water and more importantly, both will displace the oxygen and prevent the further oxidation of the fuel. (note that this is more effective for the Argon than the N2).

Once the oxygen is displaced, both the fire, if it occurs, and fuel oxidation will stop. At this point it would be best to fill the pool with sand and glass formers and let the fuel melt into a glass. Since the assemblies are – 50% dense, and the pool is about 1½ to 2/3 full (at least it was when I was there in November). The glass layer would be about 3-5 feet high.

The molten glass could then be quenched to solidify it. One has to be aware though, if the fuel is allowed to melt into a glass, most of the fission gases, and volatiles trapped in the fuel pellets would escape.

Any success of this plan would be dependent on the availability of liquid gases in Japan, and ability to deliver them. That is out of my range of expertise.

Just a suggestion.


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