White Paper

DEPLETED URANIUM OXIDES AS SPENT-NUCLEAR- FUEL WASTE-PACKAGE INVERT ANDBACKFILLMATERIALS

C. W. Forsberg and M. J. Haire

Oak Ridge National Laboratory* P.O. Box 2008

Oak Ridge, Tennessee 37831-6180

Manuscript Date: July 7,1997

Prepared for

U.S. Department of Energy Depleted Uranium Workshop Las Vegas, Nevada

July 15,1997

‘The submitted manuscript has bean authored by a conbKtM d the U.S. Gornrnmh under mb’act No. DE- A-464. Accordingly, the US. GovMnrnrnt retains a noncmlusk, royaltyfree~ licsnw to publish or reproduce the published form of this contribution, or allow othen to do so, for U.S. Govsmment purporeo.’

y (y T.4’9 ~~~~~~~~ If * ffhjt s i ’ ” I P j f i ‘ x i ’ *Managed by Lockheed Martin Energy Research Corp. under contract DE-ACO5-96OR22464 for the

U. S . Department of Energy.

DEPLETED URANIUM OXIDES AS SPENT-NUCLEAR- FUEL WASTE-PACKAGE INVERT AND BACKFILL MATERIALS

C. W. Forsberg and M. J. Haire

ABSTRACT

A new technology has been proposed in which depleted uranium, in the form of oxides or silicates, is placed around the outside of the spent nuclear fuel waste packages in the geological repository. This concept may (1) reduce the potential for repository nuclear criticality events, and (2) reduce long-term release of radionuclides from the repository. As a new concept, there are sigmficant uncertainties.

1. INTRODUCTION

The use of depleted uranium (DU) compounds as a repository backfill material is proposed. A description of the concept is provided. The mechanisms for use of DU backfill to (1) slow long-term radionuclide release rates from waste packages (WPs) containing spent nuclear fuel (SNF) and (2) reduce the potential of long-term, external nuclear criticality in the repository are described. The major uncertainties are defined as are the required development activities.

2. CONCEPT: APPLICATION TO A REPOSITORY

The basic concept is to backfill the space around the outside of the WP with DU in the form of oxides or silicates. The concept can be implemented in several ways. The DU can be placed under the WP, over the WP, or totally around the WP. The DU can be a free flowing material or packaged to minimize concerns about DU dust. The DU (in appropriate chemical forms) can be incorporated into a Richard's barrier or other barrier concepts.

3. REPOSITORY BENEFITS

3.1 REDUCTION OF RADIONUCLIDE RELEASE RATE FROM THE REPOSITORY

The expected repository failure mode is radionuclide migration to the open environment by (1) WP failure, (2) leaching of SNF by water, (3) dissolution of radionuclides and generation of colloids, and (4) transport of those radionuclides in dissolved or colloidal forms to the open environment. The use of DU backfills may reduce radionuclide transport by reducing the groundwater dissolution of uranium compounds from the SNF that contain fission products and actinides.

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The DU backfill may be in several different chemical forms when placed into the repository. However, in

a repository environment similar to that of the proposed Yucca Mountain site, the DU will convert to U308

and various uranium silicates by the time of WP failure. Unlike the SNF, the DU will not be protected by a

high-integrity WP. As a consequence, it will chemically evolve in the geological environment. Studies of

natural analogs to the proposed Yucca Mountain repository show the evolution of uranium compounds in

such environments [l]. If the DU backfill is upstream of the WP and if groundwater flows through the backfill before it enters

the degraded WP, that groundwater will be partly saturated in uranium. Groundwater partly saturated in uranium can not dissolve as much uranium as groundwater without uranium. Therefore, the dissolution of the uranium fiom the SNF is decreased with slower transport of SNF uranium., fission products, and actinides

from the WP.

3.2 REDUCED POTENTIAL FOR REPOSITORY NUCLEAR CRITICALITY

Both short-term and long-term nuclear criticality is to be avoided in a geological repository. A nuclear

criticality event would generate added radioactivity and heat. The heat can accelerate degradation of WPs and

movement of water that may transport radionuclides to the environment. The added radioactivity and heat

also create uncertainties in the modeling of the long-term performance of the repository. These and other

considerations have led to the licensing requirement that nuclear criticality be avoided in a geological

repositov.

Use of DU as a backfill material reduces the potential for repository nuclear criticality events by lowering

the fissile assay of the repository below 1 wt % 235U. The average enrichment of SNF (all fissile isotopes) is

- 1.5 wt % [2 3. There is a wide distribution of fissile concentrations within the SNF inventory. It is generally

accepted that a nuclear criticality event will not occur over geologic time at enrichments <I wt % 2 3 5 ~

equivalent [3-51. Adding DU backfill lowers the overall enrichment of the repository below this value. Criticality is prevented in a repository by neutron absorbers and geometric spacing of fissile materials.

Neutron absorbers include 238U, boron, gadolinium, and other materials. Neutron absorbers (except 238U) leach from WPs and travel at rates different from the SNF uranium through the geologic media because of the different chemistries of the neutron absorbers in groundwater. Because of possible uranium groundwater transport and redeposition (a mechanism that creates uranium ore bodies), it has been suggested that the

potential for criticality events exists if the fissile concentration in the repository SNF is sufficiently high. In effect, the same phenomenon that created natural reactors in the distant past could cause nuclear criticality

events in the hture [ 61.

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The separation and concentration mechanisms for uranium in a repository are the same as those for

uranium in the natural environment. Uranium under oxidizing conditions is several orders of magnitude more

soluble than urauium under chemically reducing conditions. This allows uranium to be oxidized to the

+6 valence state by oxidizing groundwater, dissolved in groundwater, be transported by groundwater, and

precipitated from groundwater when the local geological conditions create chemically reducing conditions.

Potential chemical reducing agents are natural organics and many waste package materials, such as iron.

Figure 1 shows one such scenario.

In light-water reactor SNF, much of the fissile material is 239Pu. The foregoing analysis is based on the

assumption that plutonium remains with the uranium until the major plutonium isotope (239Pu) decays to

235U and can be isotopically diluted by the DU. This is ensured if the rate of plutonium decay to uranium is

faster than the rate of dissolution and transport of uranium within the repository. The primary plutonium

isotope, 239Py has a half-life of 24,000 years (i.e., the decay rate is 3 x 10'5/year). Performance assessments

indicate that plutonium migration is slow in most geological environments; thus, DU backill1 provides a basis

for long-term criticality control of 239Pu and its decay product 235U. DU backfill will address only long-term, external-WP criticality concerns. (Here, external refers to

criticality events external to the WP.) Potential internal WP criticality events are not addressed.

4. STATUS

No studies have been initiated on the concept. A preferred backfill chemical form for DU must be

chosen. Candidates include other uranium oxides, uranium silicates, and uranium glasses. In the natural

environment, UO, under oxidizing conditions evolves to higher-valence uranium oxides that eventually

become hydrated uranium silicates. It is desirable that the fill material have a composition similar to uranium

compounds found in this chronological sequence to minimize the uncertainties in the long-term behavior of

uranium in the WP.

Detailed mechanical and thermal analysis of various backfill options are required. These studies must be

integrated with the overall WP development. The scientific comrnuniw has indicated that the concept is worth consideration. The U.S. Nuclear Waste

Technical Review Board (NWTRB) was created by the U.S. Congress to provide technical review of the

Yucca Mountain Project. The NWTRB reviewed strategies to ensure that nuclear criticality is not a major

issue in the licensing of the repository and made the following rmmmendation [7]:

ORNL DWG 95A-783R2

FORMA TION OF URANIUM ORE DEPOSITS FROM URANIUM IN ROCK

ROLL FRONT URANIUM DEPOSIT

(US+ --L,U4+) DISSOLVE URANIUM IN \ \ OXIDIZING GROUNDWATER

REDUCING GROUNDWATER REDUCING GEOLOGY ____t

(ORGANIC, ETC.) OXIDIZING

GROUNDWATER (LITTLE URANIUM)

FORMA TION OF URANIUM ORE DEPOSITS FROM URANIUM WASTES

OXIDIZING GROUNDWATER

\ \

REDUCING GROUNDWATER __c

(LITTLE URANIUM)

1 '

\ /-\ DISSOLVED URANIUM ' ' - L t './ 1 IN GROUNDWATER 'r' DEGRADED

WASTE PACKAGE

~~~ ~ ~~

<l;N (WASTE PACKAGE, ROCK BOLTS, ETC.)

Fig. 1. Natural and man-made formation of uranium ore bodies.

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“In particular, the use of depleted uranium in mer, invert, or backfill material, or in all three, is a concept the program has not yet explored adequately. Conceivably, increasing the criticality control robustness of the EBS (Engineered Barrier System) could turn a potentially intractable analysis of external criticality into a comparatively easy one.”

5. TECHNICAL ISSUES

The primary technical issues are: (1) definition of the concept, (2) demonstration of a design, (3) licensing of the DU backfill design in a geological repository, and (4) quan-g DU backfill benefits as a function of the design parameters. The decision to use DU backfill depends upon its impact on repository performance and cost factors.

5.1 THERMAL AND MECHANICAL DESIGN

Evaluations of the thermal and mechanical impacts of DU backfill materials on the repository are required. These must include impacts on handling operations and retrieval operations, including constraints from potential dusting of DU compounds during operations.

5.2 REDUCIION OF LONGTERM RADIONUCLIDE RELEASE RATE FROM THE REPOSITORY

Modeling and experimental activities are required to determine effects of DU backtill materials. These must be integrated into current repository experimental and modeling activities. One clearly identified area of additional work is to develop models and to conduct experiments of the impact of a locally saturated DU environment on the degraded WP with SNF.

5.3 LONGTERM NUCLEAR CRITICALITY CONTROL

The repository program is evaluating strategies for long-tern criticality control. The use of DU backfill to control criticality is a different strategy than the current baseline strategy. Sigdicant investigations will be required in this area.

5.4 ECONOMICS

No economic analysis has been done. The key question is: What is the relative management costs of SNF and DU separately vs use of DU in W s ? The answer to this question is strongly dependent upon the requirements for disposal of SNF and the requirements for management of DU.

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6. CONCLUSIONS

DU as a backfill material has the potential for (1) improvements in repository performance; (2) reduction

of the potential for external to WP, long-term, nuclear-criticality issues; and (3) consuming large quantities of DU. As a new concept, sigTllficant uncertainties exist.

REFERENCES

1. W. M. Murphy, “Natural Analogs for Yucca Mountain,“ Radwaste Mag. 2(6), 44 (November 1995).

2. R C. Ashline and C. W. Forsberg, “U.S. Light-Water Reactor Spent Fuel Jnventory-Fissile Distribution,’’ in Proc. I996 International High-Level Radioactive Waste Management Conference, Las Vegas, Nevada, April 29-iMay 3,1996, American Society of Civil Engineers (April 1996).

3. S. R Naudet, “Etude Parametrique De La Criticite Des Reacteurs Naturels,” pp. 589-599 inNaturaZ Fission Reactors, Proc. of a Meeting of the Technical Committee on Natural Fission Reactors, Paris, France, December 19-21.1977, IAEA-TC- 119/22, International Atomic Energy Agency, Vienna, Austria (1978).

4. G. A. Cowan, “A Natural Fission Reactor,” Sei. Am., 235(36) (July 1976).

5. C. W. Forsberg, “Long-term Criticality Control in Radioactive Waste Disposal Facilities Using Depleted Uranium,” Proc. Criticality Safety Challenges in the Next Decade, Chelan, Washington, September 7-11,1997, American Nuclear Society, La Grange Park, Illinois (1997).

6. International Atomic Energy Agency, Natural Fission Reactors, Proc. of aMeeting of the Technicai Committee on Natural Fission Reactors, Pans, France, December 19-21,1977 (Vienna, Austria, 1978).

7. U.S. Nuclear Waste Technical Review Board, Report to the US. Congress and the Secretary of Energy: 1995 Findings and Recommendations, Washington D.C. (April 1996).

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any infomation, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recorn- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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