(February 27, 1998) Rupture of a UF6 cylinder during a test may necessitate the addition of thermal protection caps at both ends. The 48Y cylinder failed tests under less strict circumstances as ordered by IAEA. Currently, 100,000 such cylinders are in use worldwide.
(487.4836) WISE Amsterdam - During a burst test conducted in France last December, a 48Y type cylinder with uranium hexafluoride (UF6) ruptured at a temperature of 650 degrees Celsius and a pressure of 52 bar.
The Nuclear Protection and Safety Institute (IPSN) said the container failed at one of the surrounding, stiffening bars.
New IAEA recommendations require that UF6 cylinders must provide the same resistance to fire as is required of type B containers (used to transport spent nuclear fuel), that is a surrounding flame of 800 degrees Centigrade burning for 30 minutes. This is more strict than the IPSN test, for which the 48Y cylinder already failed.
The IPSN test was the first in the Pecheur research program, conducted together with Japan's Central Institute of Electric Power Industry (CRIEPI) and Cogema, with a total budget of about US$1 million. At the end of the program, around mid-year, IPSN would judge whether thermal protection caps should be added to current 48Y cylinders to bring them into compliance with the new fire- resistance recommendations issued by the IAEA in 1996, as a late consequence of the 1984 Mont Louis accident. The Mont Louis sank with 30 UF6 cylinders aboard, which were all recovered. A valve of one of them ruptured.
The new requirements for UF6 cylinders are related to the high toxicity of gaseous 48Y which, in contact with atmospheric humidity, forms hydrofluoric acid and uranyl fluoride, a soluble aerosol. UF6 can be lethal at concentrations of 10 to 100 milligrams per cubic meter, according to the IPSN.
There are about 100,000 48Y UF6 cylinders in the world, nearly 80,000 in the US and about 10,000 in France.
Former tests with UF6 containers revealed the 'drastic' positive effect of thermal protection caps, such as that used in Japan. The caps create cold zones at both ends of the cylinder, causing the UF6 to crystallize and to be deposited at the extremities, partially counteracting the pressure increase due to the external fire. Without the caps the cylinders failed to meet the new IAEA criteria.
If the full Pecheur test results confirm the first test, IPSN thinks it is probable that companies wanting approval of UF6 transport cylinders under the new regulations would propose addition of the thermal caps.
In the US, many of the UF6 cylinders now show evidence of external corrosion. Four cylinders at Oak Ridge, two at Portsmouth and one at Paducah have developed breaches. UF6 storage activities in the US began in the early 1950s.
The current management plan of the Department of Energy (DOE) calls for continued storage of the cylinders through 2010. This plan, however, was based on reserving the depleted UF6 for future defense needs, re-enrichment or other uses. In the newly released Programmatic Environmental Impact Statement (PEIS), the DOE explored six alternative management strategies, including no action. At present, the DOE prefers to convert its estimated 557,000 tons of depleted UF6 to either uranium oxide (UO2) or uranium metal. The PEIS is available on the Internet at: http://www.ead.anl.gov/uranium.html.
Source : Nuclear Fuel, 25 March 1996 & 12 January 1998
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