ML20210A858
| ML20210A858 | |
| Person / Time | |
|---|---|
| Issue date: | 06/13/1986 |
| From: | Greeves J NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Nussbaumer D NRC OFFICE OF STATE PROGRAMS (OSP) |
| References | |
| REF-WM-91 NUDOCS 8609170367 | |
| Download: ML20210A858 (7) | |
Text
wm-9/
o
- s JE 18 EIS 202.2/TLJ/6/9/86
-1 d
b
' MEMORANDUM FOR:
Donald A. Nussbaumer Assistant Director for i
State Agreements Programs Office of State Programs FROM:
John T. Greeves, Chief i
Engineering Branch i
Division of Waste Management
]}
Office of Nuclear Material Safety and Safeguards
SUBJECT:
NRC STAFF RECOMENDATIONS ON BACKFILLING 0F BITUMEN WASTE 2
FORMS Enclosed is a draft copy of the NRC Staff Recommendations on Soil Backfill for LLW Land Disposal Excavations. The draft recommendations have been developed l
as a result of concerns expressed by the State authorities in South Carolina and i
Washington about the disposal of bitumen waste forms. Please transmit a copy of the enclosed to the licensing authorities in South Carolina and Washington.
i We also request the Office of State Programs to arrange a meeting with V. Autry (SC) and N. Kirner (WA) during the afternoon of June 25, 1986. We understand that they will be in Washington, DC that week. The meeting will provide the States the opportunity to ask questions concerning the subject draft document, so that all three parties have the same understanding.
If you have any questions, please contact Tom Jungling at x74540 or Tim l
Johnson at x74088.
Mf Q.% $ 4 :- 7]_
.%g,
w Reconr File (DR. -
~
gPOR.
John T. Greeves, Chief Engineering Branch
~ _ _ _.
Distpb.utite. _
p
~~ ~- -
Division of Waste Management Office of Nuclear Material Safety
, _., l. _._
~
and Safeguards g~Ti%h 67%%. -
~
~~
Enclosure:
As stated 060917o367 060613 I
(*Seepreviousconcurrences)
OFC
- WMEG*
- WMEG*
- W k j$
.f.....:............:............:............::...........
..... :............ :............ :.L NAME :TJungling:gh:TCJohnson
- J r eyes d
DATE :06/ /86
- 06/ /86
- 06/tf/86
202.2/TLJ/6/9/86 MEMORANDUM FOR:
Donald A. Nussbaumer Assistant Director for State Agreements Programs Office of State Programs FROM:
John T. Greeves, Chief Engineering Branch Division of Waste Management
SUBJECT:
NRC STAFF RECOMMENDATIONS ON BACKFILLING 0F BITUMEN WASTE FORMS y
/
Enclosed is a copy of the NRC Staff Recommendat W on S0'11 Backfill for LLW Land Disposal Excavations. The recommendations
been developed as a result of concerns expressed by the State authorities 1 rauth Carolina and Washington about the disposal of bitumen waste forms.
Pleoce transmit a copy of the enclosed to the licensing authorities in South Car,olina and Washington.
7 We also request the Office of State Programs to arrange a meeting with V.
Autry (SC) and N. Kirner (WA) during the afternoon of June 25, 1986. We understand that they will be in Washington.gDC that week. The meeting will provide the States the opportunity to ask questions concerning the subject document, so that all three parties have the same understanding of the position.
If you have any questions, please contact Tom Jungling at x74540 or Tim Johnson at x74088.
/
John T. Greeves, Chief Engineering Branch Division of Waste Management
Enclosure:
As stated
/
%.FC :WMEG q :WMEGf 0
- WMEG
...:........... :...... p _:....____....:...._____...:..__________: __.......__:__..._____.
'NAME :TJungling:gh:TCJoh on
- JTGreeves BATE!bbfk/bb bbff[fbb
!bb/ hbb s
i JK i
l ;.
4 l
NRC STAFF RECOMMENDATIONS ON SOIL BACKFILL FOR LLW LAND DISPOSAL EXCAVATIONS
1.0 INTRODUCTION
At current shallow land burial sites for low-level radioactive waste (LLW), the i
solidified waste is usually contained within' carbon steel 55-gallon drums and 50-200 cubic feet liners. However, within the burial trench which is normally excavated into natural soil materials, the steel containers will eventually corrode over some period of time. The rate of corrosion is affected by the soil characteristics and the moisture content of the soil.
If bitumen is utilized as the stabilizing medium, deformation of the waste fom may occur following the loss of confinement previously provided by the steel containers due to corrosion and deterioration. A potential problem may result if the bituminized waste (BW) were to signficantly deform into unfilled voids which may have existed between the original steel containers. Large defonnations of j
the BW could then result in significant and undesirable differential settlements and cracking of the trench cover. This condition of large deformations may also permit localized water pockets in contact with the waste form to develop over the impermeable bitumen that had deformed into the previously existing voids j
. hich may now be fed by water infiltration through the cracked trench cover.
w 10 CFR 61 sets forth technical requirements for emplacement of waste packages in land disposal facilities as well as the requirement that void spaces between the packages be filled to reduce future subsidence within the trench (Section l
61.52). This brief staff report is intended to provide guidance to states, site operators, and waste generators on proper procedures that should be used for filling the voids between waste containers in LLW disposal trenches that contain bituminized waste. Specific criteria on soil material to be used for filling the voids are provided in order to ensure long-term trench stability.
This report does not cover the placement and compaction requirements for 4
materials placed above the top of the waste in the disposal trench cover.
l 2.0. POTENTIAL DEFORMATION AND BACKFILL MATERIAL CONSIDERATIONS Several mechanisms exist by which deformation of the BW could potentially occur i
after the waste drums and liners corrode and deteriorate from around the waste form. First, if large voids are permitted to exist between the containers when the waste packages are initially placed, then deformations of the BW into the unfilled void spaces could result, followed by sub:idence of the LLW disposal unit. Additional deformations which would likely be small, might also occur because of large lateral pressure differences between the waste form and the i
i
?
i n
WAFT JK soils placed around containers due to lateral compression and because of a possible flow of low viscous bitumen into large pores of a coarse fill that may exist around the containers.
It is the recognition of these mechanisms for deformation and the resulting problems with disposal unit subsidence that encourages the selection of a stable backfill material that would have the following characteristics:
1.
Conformability so that when placed by the usual construction placement method (dumping over the waste drums and liners without any formal spreading effort in order to avoid requiring workers to enter the trench being filled with the LLW) the backfill material would freely move into and fill the voids between waste containers without permitting bridging and the formation of soil clumps that would result in large void openings between containers.
2.
Low compressibility in the backfill material in spite of the usual method of backfill placement with no formal densification effort.
3.
Gradation which would ensure a sufficiently permeable backfill material that would allow any percolating water to drain to the trench bottom, thereby avoiding prolonged contact of water with the waste, but yet have an upper size limit that would prevent migration of the bitumen into the intergranular pores of the backfill material.
The staff has estimated the volume of voids which could reasonably be expected to exist between containers within a typical LLW burial trench under two types of backfills:
(a) allowing for the placement of a cohesionless backfill soil, and (b) allowing for a cohesive soil placement.
The conditions assumed in the estimate include:
1.
A burial trench that measures 150 feet in width,1,000 feet in length and 37 feet in depth.
2.
A systematic placement of the 55-gallon drums which are stacked vertically and in a 6-pack arrangement. On the basis of this assumption, the volume of open space between four adjacent drums was computed to be approximately 2 cubic. feet.
3.
Backfilling with a cohesionless soil which has a maximum dry density of 115 pounds per cubic foot and a minimum dry density of 95 pounds per cubic foot. A 30 percent relative density is conservatively assumed for the cohesionless backfill at the time of placement.
(Relative density i
.n.n--
e--.
~
e -
JK expresses the degree of denseness of a cohesionless soil with respect to its loosest and densest condition. A soil in the loosest condition would have a relative density of 0 percent and in its densest state would have a 1
relative density of 100 percent).
4.
Backfilling with cohesive soils that allows for bridging and clumping of the soil and results in only one-half of the open space between the drum containers being filled. This estimate of filling is not a calculated value but is assumed based on experience in the excavation and fill placement of cohesive soils under a wide range of naturally occurring moisture contents with no tamping or compaction.
If the cohesionless backfill soil is assumed to eventually reach its maximum density, an increase in the trench void volume of 0.20 cubic feet is computed to occur in the space between the four containers, because of the cohesionless soil densification. This is in comparison to the 1.0 cubic feet void volume change that could be expected to occur in the cohesive backfill.
If the above changes in void volume were assumed to occur over the entire assumed trench area, where drums are adjacent to each other, it can be seen that the potential for settlement with resulting cracking of the trench cover would be on the order of 5 times greater for the cohesive backfill in comparison to the cohesionless backfill.
In comparison to the total disposal volume of the assumed LLW trench, this void volume change resulting from the compression of the backfill materials is 1.8 percent for the trench backfilled with cohesionless soil and 8.8 percent for the trench backfilled with a cohesive soil. The results of this comparison are consistent with the statements in NUREG/CR-3144, Trench Design and Construction Techniques for Low-Level Radioactive Waste Disposal, where the use of sands and gravel (cohesionless soils) are indicated to make better backfill materials because of their being less compressible than silts and clays (cohesive soils).
On the basis of the comparison of the void volume change, the staff recommends the use of a cohesionless soil, with material controls that are subsequently provided, for backfilling LLW disposal facilities in order to meet the j
technical requirements of 10 CFR 61.
It is of interest to note that the estimated volume of voids in a single 55-gallon drum that is 90 percent filled with bitumen is 0.80 cubic feet, which if considered over the entire assumed trench area would be approximately 7.0 percent of the total disposal trench volume. This recognition should encourage I
increased efforts to be made for obtaining more complete filling of the drums with bitumen because of the resulting subsidence that could be anticipated if the containers were to fully corrode and deteriorate.
i f
y-_-
m.
3.0, RECOMMENDATIONS Backfill material for LLW disposal trenches containing BW should consist of cohesionless soils having less than 12% fine particles by weight passing the No. 200 mesh sieve and with not more than 40% by. weight coarser than the 3/4 inch size and with a maximum particle size not greater than 3 inches. Limiting the percentage of fines to 12% will help provide a relatively free draining soil that is not subject to bridging and the formation of soil clumps.
Establishing a limit on the 3/4 inch size is intended to ensure that the backfill soils will not have too great a percentage of large stone sizes, but will be reasonably graded with smaller sizes in order to fill the irregular void spaces.
The 3 inch maximum particle size is recommended based on the anticipated size of the unfilled intercontainer void space when 55-gallon drums are used. This maximum particle size may be changed, and in some cases should be chanced, if different size containers are used or if specific site placement conditions (e.g. random arrangement of containers in trench) differ significantly from those assumed by the staff in this study. The recommendation is made to assure that bridging of large stones and rocks between containers will not occur and the smaller sizes of the cohesionless backfill materials will move freely into the void spaces around containers. Wooden pallets, if used in handling and placing the waste containers, should be removed from the disposal trench prior to backfilling because of potential problems in filling around the pallets and with subsidence following future decomposition.
The cohesionless backfill material should be in a loose, dry condition during placement and should be placed after each successive waste container layer is placed. Allowing several layers of waste containers to be placed on top of each other before backfilling the intercontainer voids should not be permitted because of the reduced effectiveness in completely filling the voids and the resulting adve~rse and larger trench settlements which could then be expected.
If a soil other than what has been recommended by the staff is considered for backfill material at a proposed land disposal facility, there should be a requirement for an early demonstration including confirmatory field testing that the intercontainer voids are being filled and that bridging and clumping of the backfill materials around containers are not occurring. A test project that duplicates anticipated waste container placement and backfill conditions (e.g. configuration of excavation, similar container arrangements and construction methods and equipment, similar material type and range in placement moisture contents) should be required for proposed cohesive soils using the backfilling procedures planned for the disposal excavation. The volume change resulting from soil compression around the containers and the corresponding percentage of the total disposal trench volume that will not be 1
JK filled because of voids in the backfill should be determined and the results of the test project should be submitted to the proper regulatory authority for evaluation and approval of the proposed backfill operation.
Other proposed options for backfilling around containers (grouting, densification measures, etc) would be required to demonstrate their effectiveness in meeting the technical requirements of 10 CFR 61 by successfully completing a field test demonstration and by submitting a technical report to the proper regulatory agency for evaluation and approval.
4.0. CONCLUSIONS 10 CFR 61 sets forth technical requirements for emplacement of waste packages in disposal trenches as well as the requirement that void spaces between the packages be minimized to reduce future subsidence within the trench. The staff has estimated that the potential for settlement would be on the order of 5 times greater for a disposal trench that uses a cohesive soil as backfill between waste containers, than for a trench backfilled with a cohesionless soil.
The factors which would influence the compressibility of a cohesive backfill are more numerous and their impact less predictable than those that would influence the compressibility of a cohesionless backfill. For a cohesive backfill the factors that would influence compressibility would include the natural moisture content at time of placement, the extent of soil clumping and bridging and the higher natural compressibility characteristics of the cohesive soils. The large uncertainties associated with these widely varying factors would suggest that the use of cohesive soils as backfill in LLW disposal facilities be determined on a site specific basis.
The cohesionless soils have desirable backfill material characteristics which (1) allow it to better conform to the irregular openings between containers, (2) exhibit lower compressibility even when initially placed without a compactive effort and (3) minimize the time that the LLW would be in contact with percolating water, if any, because of the backfill soil's permeability.
Because of these desirable characteristics the staff recommends that cohesionless soils be required in backfilling LLW disposal facilities and has l
provided guidance on backfill material specifications and placement procedures.
Alternatives (e.g. grouting, densification) to using cohesionless backfill would be acceptable to the staff provided a field test is completed prior to the actual placement of LLW and provided the results of the test project successfully demonstrate that the technical requirement of 10 CFR 61 covering reduction of voids spaces between waste packages will be met.
w--.-
w _
.._,..m,
,