ML20195G605
| ML20195G605 | |
| Person / Time | |
|---|---|
| Issue date: | 09/08/1988 |
| From: | Surmeier J NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Lubenau J NRC OFFICE OF GOVERNMENTAL & PUBLIC AFFAIRS (GPA) |
| Shared Package | |
| ML20151C617 | List:
|
| References | |
| FOIA-88-470 NUDOCS 8811290004 | |
| Download: ML20195G605 (3) | |
Text
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- k UNITED STATES NUCLEAR REGULATORY COMMISSION
, g WASHINGTON, D C. 20666 r.
SEP - 819?8 MEMORANDUM FOR: Joel 0. Lubenau Acting Assistant Director for State Agreements Pre; ram State, Local, ar d Indian Tribe Program r FROM: John J. Surmeier, Chief Technical Branch Division of Low-level Waste Management and Decomissioning, NMSS
SUBJECT:
COMMENTS ON SOUTH CAROLINA RESPONSE TO SILLING REPORT We have reviewed the corrnents from the State of South Carolina (SC) on the report entitled, "Review of the Structural Design of Polyethylene High Integrity Containers." As you know we are continuing to study the technical issues regarding high-density polyethylene (HDPE) and are continuing our reviews of the three Topical Reports from vendors who market HICs made of HDPE. Many of the issues discussed in the letter from South Carolina are in the process of being evaluated by the staff in our efforts to make a decision regarding the Topical Report reviews by the end of the surtner. We do not believe it is appropriate to provide detailed technical coments on the South Carolina letter untti these issues are fully evaluated by the staff and our consultants. However, we have 4
enclosed brief responses to the SC coments. We will keep you apprised of progress in this area prior to making any formal decision.
If ycu have any questions regarding this memo or our responses, please contact me at x23439. '
s i s ,p- w_
John J. Surmeier, Chief Technical Branch Division of low-Level Waste Management and Decomissioning, NMSS
Enclosure:
Responses to SC cosinents BB11e 004 GMllD P
N1 ll-470
Comments on South Carolina Response to the Silling HDPE HIC Report Comment #1 - Secant Modulus In comment #1 it is stated that the "secant modulus is usually used for metals," and Implies that it may not b,e useful for plastics. The secant modulus is merely the total engineering stress divided by the total strain after a given time in a creep test. It is therefore a reasure of the effective stiffness of a material including creep strains, it is particularly as they would be in a HIC, appropriate neglecting when shape the loads changes. are nearly It is commonly constant,for plastics (see used Powell, Engineering with Polyrers, London: Chapman and Hall,1983). An alternate method of analysis should be suggested if the use of the secant modulus is considered inappropriate.
Comment #2 - Tensile Data In comment #2, it is stated that, "most of the loads experienced by the containers are compressive " and therefore the tensilt data for HDPE ray be inadequate. Tensileormultiaxialstresseswilloccurwhereverthereare bending forces within the HIC shell. Also, while there are some differences between the tensile and compressive responses of polyrers, the creep responses should be similar because creep mainly involves deviatoric strain.
Conment #3 - Linear HDPE Data Comment #3 says that the BNL data for unitradiated Marlex CL-100 are more appropriate than the linear HDPE data cited in the Silling report. The BNL data were short-term data. The linear HDPE data in the Silling report were intended only to show qualitatively that the long-term failure modes of polyethylene can differ significantly from thuse seen in the short term. In any case, regardless of the behavior of unitradiated HDPE, it is beyond question that radiation leads to embrittitment of polyethylene even in the short term.
t Comment #4 - Threshold stress for creep t
Comment #4 says that there may be a threshold stress for creep and suggests that if a container were designed so that all stresses were below this threshold, creep buckling would not be a problem. We find that there is no basis in the literature for the existence of a threshold stress for creep in polyethylene, j The Phillips data in figure 2 of the Silling report shows that even at 500 psi in a untaxial creep test, the total strain is 6 times the .tlastic strain af ter 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />. A threshold stress should be suggested and the soutee of the value stated if the use of such a threshold value is proposed.
I I
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6 Connent #5 - Credit for Backfill In connent #5, it is pointed out that the response of the containers will be affected by the waste and backfill. The Silling report states that these effects will be present. However there is no adequate method available for estimating these effects for highly flexible structures. The available analyses, such as those referenced in.the Silling report, assume linear elastic behavior of the backfill and therefore irplictly assume small deformation. The effect of the waste is also unknown, since the vendors did not characterize the mechanical properties of the waste and did not propose procedures for ensuring that all containers would contain waste with these properties. Also, since the empty-container buckling losds estimated in the Silling report are so far below the expected burial loads, the mere assumption that the waste and backfill will take care of the problem is unjustified.
Connent #6 - Absence of Data Connent #6 correctly states that the data for the mechanical properties of HDPE are incorrplete and suggests that further research is needed. However, this is not a convincing argument for licensing of the present designs. From the data available, the following patterns emerge: polyethylene creeps, cracks, and is sensitive to radiation.