MODEL 9975 LIFE EXTENSION PACKAGE 1 - FINAL REPORT

Daugherty, W.

Savannah River Site (S.C.)

关键词: Corrosion;    Mechanical Properties;    Storage;    Radioactive Materials;    Humidity;   

DOI  :  10.2172/1012547
RP-ID  :  SRNL-STI-2011-00113
RP-ID  :  DE-AC09-08SR22470
RP-ID  :  1012547

美国|英语

来源: UNT Digital Library

【 摘 要 】

Life extension package LE1 (9975-03382) was instrumented and subjected to a temperature/humidity environment that bounds KAMS package storage conditions for 92 weeks. During this time, the maximum fiberboard temperature was {approx}180 F, and was established by a combination of internal heat (12 watts) and external heat ({approx}142 F). The relative humidity external to the package was maintained at 80 %RH. This package was removed from test in November 2010 after several degraded conditions were observed during a periodic examination. These conditions included degraded fiberboard (easily broken, bottom layer stuck to the drum), corrosion of the drum, and separation of the air shield from the upper fiberboard assembly. Several tests and parameters were used to characterize the package components. Results from these tests generally indicate agreement between this full-scale shipping package and small-scale laboratory tests on fiberboard and O-ring samples. These areas of agreement include the rate of fiberboard weight loss, change in fiberboard thermal conductivity, fiberboard compression strength, and O-ring compression set. In addition, this package provides an example of the extent to which moisture within the fiberboard can redistribute in the presence of a temperature gradient such as might be created by a 12 watt internal heat load. Much of the moisture near the fiberboard ID surface migrated towards the OD surface, but there was not a significant axial moisture gradient during most of the test duration. Only during the last inspection period (i.e. after 92 weeks exposure during the second phase) did enough moisture migrate to the bottom fiberboard layers to cause saturation. A side effect of moisture migration is the leaching of soluble compounds from the fiberboard. In particular, the corrosion observed on the drum appears related primarily to the leaching and concentration of chlorides. In most locations, this attack appears to be general corrosion, with shallow attack of the drum surface. The primary areas susceptible to corrosion are weld/heat-affected zones. However, one corrosion location not immediately associated with a weld was tested for, and confirmed as having, throughwall penetration. An increase in the axial gap at the top of the package is also related to the migration of moisture within the fiberboard. As moisture redistributes within the package, a majority of the fiberboard loses moisture (on average) and shrinks axially. In addition, an increased moisture content of the bottom fiberboard layers locally reduces its compression strength, leading to compaction of those layers under the weight of the package internal components. In addition to these moisture-driven phenomena, the fiberboard will shrink due to slow pyrolysis in an elevated temperature/humidity environment. Under the collective influence of these effects, the axial gap in the LE1 package increased from an initial value of 0.58 inch and exceeded the 1 inch (maximum) criterion after approximately 18 weeks conditioning. The axial gap eventually reached 1.86 inches. Despite the degradation seen in several of the package components (drum, fiberboard, shield and O-rings), the package appears to have retained sufficient integrity to meet the functional requirements for storage in KAMS. This demonstrates a degree of robustness in the package design relative to the storage environment.