ML20244B447
| ML20244B447 | |
| Person / Time | |
|---|---|
| Issue date: | 05/31/1989 |
| From: | Surmeier J NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Autry V SOUTH CAROLINA, STATE OF |
| References | |
| REF-WM-93 NUDOCS 8906130119 | |
| Download: ML20244B447 (8) | |
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Mr. Virgil R. Autry, Director Division of Radioactive Material
. Licensing and Compliance Bureau of Radiological Health South Carolina Department of Health and Environmental Control 2600 Bull Street Columbia, SC 29201
Dear Mr. Autry:
Thank you for your letter of May 2,1989, which provided comments on certain burial restrictions that are stated in Conditions 4 and 5 of NRC's Technical Evaluation Report (TER) for the LN Composite High Integrity Container (HIC).
This. response is not only to that letter but also to areas of concern discussed in the May 1,1989 telecon between you, Dick Bangart, John Greeves and ntyself. ' Those concerns were raised in a March 7,1989 memorandum to you from Jim Jerozal of your staff. The comments provided in the letter of May 2, 1989' appear.to be an expansion of the second of five areas of concern raised in the March 7,1989 memorandum. The five areas of concern are:
1 1.
The availability of long-term performance data for 316L stainless steel.
2.
The structural performance of the 316L shell for 300 years under burial conditions not considered in the submittal or NRC TER.
3.
The corrosive resistance of the 316L, including pitting and crevice corrosion, and the relevance of soil testing using samples _other than those found at the burial sites.
4.
The effect of galvanic corrosion, and possible resulting changes in the burial conditions linked to this.
1 5.
The effectiveness of medium density polyethylene to provide ultimate container integrity."
We will address those areas of concern in the order listed, as indicated j
below.
1.
Long Term Performance Data on 316L It is true that there is a lack of extensive data on the performance of stainless steels buried in soil. Such a statement also applies to any other material when considering design lifetimes of 300 years. The most comprehensive publication on underground corrosion (Reference 1) cites no quantitative data for periods greater than 20 years, even for the consonly available carbon i
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-2 steels. The data base on organic materials such as. polyethylene or any of the materials used as solidification media for LLW is similarly very restricted.
Thus, the selection of anJ material for use in a HIC must involve a significant degree of Engineering judgement. This is a necessary practice for NRC in decling with uranium mill tailings, high-level waste and reactor component safety reviews.
In these areas, it is not uncommon for us to have to use relatively short-term data to predict long-term performance.
(An example is the review of the design of high-level waste packages under 10 CFR 60,
" Disposal of High-Level Waste in Geologic Repositories." The rule requires that containment of the waste within the package be substantially complete for a containment period of 300 to 1000 years. The rule also requires that the release rate of any radionuclides from the engineered barrier system following the containment period shall not exceed one part in 100,000 per year of the 1000 year inventory).
The austenitic stainless steels came into being in the early part of the present century (Reference 2) and thus have been in use for several decades.
They have found broad application where hostile environments are involved and have generally demonstrated good performance. Review of the general corrosion characteristics of these steels can be found in many sources (for example, reference 3). The most commonly used austenitic stainless steel is Type 304 which is often specified when good general corrosion resistance is required.
(Type 304L,1/4 inch thick, is the reference material for the high-level waste package now under design by the Department of Energy). However, in certain circumstances, it can become susceptible to pitting attack, sometimes _
sufficiently severe as to cause perforation, as was demonstrated in the NBS tests.
Increased resistance to pitting attack can be acquired by adding two to three percent molybdenum, the modification thus obtained being Type 316. An additional contributory factor to pitting attack is sensitization, a phenomenon linked to the availability of carbon in the alloy and often encountered in the vicinity of welded joints and at grain boundaries.
Sensitization is brought about either by extended use of austenitic stainless steels in the temp 2rature range (approximately)of 800 -1500*F or by slow cooling of welded joints (such as would inevitably arise when joining heavy section plates). Sensitization, and the associated increased susceptibility tn pitting corrosion and intergranular attack, can be minimized or elidnated by reducing the carbon content in the alloy to less than 0.03%.
Such steels are termed "L" grades.
Hence, Type 316L is essentially Type 316 but with a reduced carbon level; its chemical composition, with the exception of the carbon content, is exactly the same as that of Type 316; it shares its generally good corrosion resistance and is even more resistant to pitting corrosion.
It should demonstrate increased resistance to pitting attack, particularly in the heat-affected zones of welds.
In summary, it is recognized that the sparsity of long term soil corrosion data on austenitic stainless steels leads to a degree of uncertainty in predicting the lifetime performance of the HIC.
However, the preponderance of information available on these steels indicates that the long term performance of Type 316L will be adequate and that the material will satisfy the stability requirements for the HIC's design lifetime of 300 years.
The issue was addressed in detail in the course of the review of the LN Technologies HIC
EW/LTR TO V AUTRY Topical Report and is discussed in Section 3.2 of the Technical Evaluation Report. The conclusion reached in that report, based upon the comprehensive review that was conducted, is that reasonable assurance exists that the LN Comnosite HIC will perform adequately in service.
2.
Burial Conditions Your letter states that burial conditions that exist (at the Barnwell Low-Level Waste Disposal Site) could not insure " sufficient spacing and intervening soil between HICs or HICs and othar objects" or " sufficient earth above and below adjacent levels." The letter goes on to state that the reverse may occur, i.e., stacking HICs and placing HICs or other liners in such a fashion as to cause non-uniform lateral loading.
The basis for the restrictions proposed in our TER may be described briefly as follows. The structural analysis used to demonstrate that the HICs will not buckle is based on the HICs being modeled as thin-walled cylinders.
To demonstrate analytically that a thin-walled cylinder will not buckle, it is necessary to show that it will be buried in a vertical orientation and with sufficient soil around it and with sufficient spacing between cylinders (or other objects harder than the earth) to produce the equivalent of uniform radial loading conditions. Similarly, the HICs must not be subjected to point or line loads froz a HIC or other hard objects above or below them.
This requirement is intended to prevent excessive concentration c' stress in local areas that may result in premature failure of the HIC due to buckling.
By including such a restriction in the TER, it is incumbent upon the vendor, and ultimately the user, to assure that these conditions can and will be satisfied by the disposal facility of choice.
If not, the user would not possess generic approval through the topical TER process to use such a HIC.
No explicit condition upon the disposal facility is created as a result of the TER process itself.
However, to provide reasonable assurance that a HIC will maintain its structural integrity over its 300-year lifetime, the burial restrictions stated in conditions 4 and 5 of the TER must be met. Otherwise, some of the HICs may fail prematurely during service.
For this reason, NRC will be addressing this issue regarding future burial of other HICs at all disposal sites. Please be advised that we now believe that any thin-walled cylinder HICs, regardless of material of construction, would be susceptible to premature failure due to buckling as a result of stress concentration-induced excessive local loading conditions if not properly placed.
This area of concern, i.e., the burial restrictions described in Conditions 4 and 5 of the TER for the LN Composite HIC, was brought to our attention by our consultant, SAIC, during the review of the Topical Report for the LN Composite HIC. This concern raises a generic issue covering disposal practices at all disposal sites, and we will be getting in touch with you and the other state regulatory agencies to discuss the matter further.
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, 3.
Corrosive Resistance of 316L L
It cannot be overemphasized that the pitting estimates are based on a worst case scenario, i.e., using data on corrosion of sensitized 316A in a very aggressive soil environment. The HIC shell will be made of the low carbon grade of Type 316 and is considered very unlikely to contain any sensitized We agree that the soil conditions at Barnwell are such that even within a reas.
the site, there will be variations, depending on where the samples are taken The backfill itself is an' unusual or spurious environment, in which the variations in the original soil due to depth have become obscured. Measurements performed on soil samples taken from different locations at Barnwell (reference 4), however, would be considered only mildly corrosive toward carbon steels.
In the case of the austenitic stainless steels, corrosion is considered to be most dependent on chloride concentration in the soil. At Barnwell, the concentration is orders of magnitude lower than that at the three most aggressive soils in the NBS tests (i.e., soils C, E and G).
Thus, the Barnwell soils would not be expected to be corrosive toward stainless steels because of the low chloride concentration alone. The superior resistance of Type 316L in many hostile environments has been established.
It is considered unlikely to be diminished during exposure to the Barnwell soils.
On the subject of crevice corrosion, the NBS tests did incorporate accelerated corrosion due to crevice corrosion. A series of tests was performed using welded Type 304 stainless steel. Corrosion was observed in the vicinity of the crevice when the material was exposed to soils C, E and G, but the degree was not significantly different from that observed over the remainder of the specimen. Thus, even if perforation of the 316L shell occurred and the conditions for crevice corrosion were createt on the inside surface, it does not appear likely that the corrcsfon rate would increase significantly.
In the case under consideration, it is likely that corrosion products would fill the pit. This would restrict access to the crevice of corrosive ions and moisture from the surrounding soil.
In summary, the potential for pitting and crevice corrosion was addressed in detail during the review. NRC considers that the conditions required for l
greatly accelerated corrosion are unlikely to occur in known disposal site environments. This subject is covered in paragraphs 3.2.2.1 and 3.2.3 of the TER.
4.
Effect of Galvanic Corrosion l
The March 7,1989 memorandum expressed concern that failure of the carbon steel skirt at the base of the HIC (due to galvanic corrosion) may impact the structural performance of the HIC.
j The review considered several potential soil conditions. One was the l
condition in which the skirt could cut into the so 1, allowing full earth l
contact on the bottom of the container. Another w-the condition in which the container would be supported by the skirt. A t.#rd was an intermediate condition in which the container would be supported partially from below by both the skirt and the soil under the base of the container. The principal l
l 1
J
EW/LTR TO V AUTRY effect of the skirt was considered to be that of providing the potential for less uniform loading on the base than might be achieved (depending on soil plasticity) without a skirt.
We consioer that loss of the skirt or uneven portions of the skirt would probably lead to a more uniform loading of the base of the HIC..In effect the LN Tech HIC was considered to be structurally adequate despite the skirt.
Loss or partial loss of the Akirt is not considered to be a significant problem under the required conditions of burial.
In summary, this concern was addressed in detail during the review.
It is covered under paragraphs 3.2.4, 3.3 and 4 (condition 5) of the TER.
5.
Medium Density Polyethylene The function of the polyethylene liner is to separate the LLW from the stainless steel shell. The liner serves no structural purpose but must be highly resistant to the chemical environments created by a variety of waste stream. All the polyethylene have this property.
From a structural engineering viewpoint, medium density polyethylene differs from the other two polyethylene mainly in the area of mechanical properties. These properties are intermediate between those of low density polyethylene and those of the high density form. The chemical resistance of medium density polyethylene is at least equal to (and often greater than) that of low density polyethylene and generally on a par with that of high density polyethylene (see, for example, reference 5).
In summary, NRC considered that the required chemical resistance by the liner to a variety of waste streams would be provided by all polyethylene.
- Thus, NRC considered it unnecessary for the TR to provide chemical properties data specific to medium density polyethylene.
NRC considers that this item was adequately addressed in paragraph 3.2.2.2 of the TER.
In conclusion, we remain convinced that all necessary technical safety issues were included in our review. They were all addressed in the TER for the LN Composite HIC, at least in summary fashion. The additional information provided above further clarifies our conclusion that there is reasonable assurance that the LN Composite HIC will meet the 300-year structural stability requirement of 10 CFR Part 61, if appropriate conditions are satisfied. We
. continue to encourage such discussions on technical safety issues related to areas of mutual concern.
We would be pleased to meet with you and your staff at your convenience to discuss these areas of concern more fully, if you would like further clarification.
Sincerely,
( E.,.
l John J. Surmeier, Chief Technical Branch l
Division of Low-Level Waste Management l
and Decommissioning, NMSS I
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Re'ferences r
F' 1.
M. Romanoff, " Underground Corrosion," NBS Circular 579 (April 1957).
L; 2.
J. G. Parr and;A'. Hanson, "An Introduction to Stainless Steel,"
- American Society for Metals, Metals Park, OH-(1985).
p.
~ 3.
A. J. Sedriks,." Corrosion of Stainless Steels," John Wiley & Sons, t
-Inc., New York, NY (1979).
4.
' P. L. Piciulo, C. E. Shea, and R. E._Barletta, " Analyses of Soils from L.
the Low-Level Radioactive-Waste Disposal Sites at.Barnwell, SC, and
- Richland,WA,"NUREG/CR-4083(March.1985).
5.
- " CRC Handbook of Chemistry and Physics,.61st. Edition," R. C. Weast, editor, CRC Press, Inc., Boca Raton, FL (1980).
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' Distribution:
%Centralifiles:.
JSurmeier, LLTB EWick, LLTB MTokar, LLTB NMSS r/f LLTB r/f MBell, LLRB Pl.ohaus, LLOB JGreeves, LLWM RBangart, LLWM KSchneider, SLITP VMiller, SLITP.
SSchwartz, SLITP PDR Yes:/~T 7 PDR No:/
/ Reason: Proprietary /
/ or CF Only. /
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ACNW Yes:C No:/~
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j SUBJECT ABSTRACT:-
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