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Impact of Storage Period on Safe Geological Disposal of Spent Fuel

B.B. Acar1, H.O. Zabunoğlu2

1Turkish Atomic Energy Authority, Ankara, Turkey2Department of Nuclear Engineering, Hacettepe University, Ankara,Turkey

e-mail contact of main author: banubulut.acar@taek.gov.tr

Abstract. Geological disposal is the widely accepted method for safe final disposal of spent fuel (SF) and high level waste (HLW). Currently, there are no active deep geological repositories. However, various geological disposal projects are under way in many countries. In geological disposal, canisters containing SF/HLW are simply placed into boreholes in a geological formation deep underground, specifically selected for final disposal of nuclear wastes. The main factor affecting the geological repository design is the amount of waste that can be safely emplaced per unit area of the repository (waste disposal density) and it strongly depends on the characteristics (amount, isotopic composition, heat generation rate etc.) of the waste. The isotopic composition and heat generation rate of SF discharged from reactor change during storage. This study aims to assess the effect of interim storage period on disposal density of SF in a geological repository. In the first part of the study, utilizing the code Monteburns, relevant compositions and decay heats of SFs discharged from a reference PWR (A 1000-MWe PWR loaded with 3.3 w/o enriched UO2 fuel, with a discharge burn up of 33000 MWd/tU and with an irradiation time of 1000 days) are obtained for selected cooling times. Then, using the code ANSYS, thermal analyses are performed for a reference repository concept and disposal areas needed for SFs with different ages are determined by ensuring that thermal criteria limiting the canister surface temperature is satisfied. Results of the analysis are used to assess the effect of storage period of SF on disposal layout and to derive the correlation between storage period and safe disposal capacity of geological repository.

Key Words: Spent fuel, geological disposal, storage, disposal density.

1. Introduction

Generally accepted method for the final disposal of Spent (nuclear) Fuels (SF) and high-level nuclear wastes is to bury them deep underground in a specially selected and designed repository, named “geological disposal”. This study focuses on estimating the disposal area required for SF as a function of the age of SF in granite formation (the KBS-3 concept developed by the Swedish Nuclear Fuel and Waste Management Company).

In the first part of the study, compositions and decay heat profiles of SF are determined for different cooling periods before the disposal. Then, using the decay heat profiles and taking into account the thermal constraints, disposal areas are obtained through thermal analysis.

2. Compositions of Spent Fuel

The code Monteburns, which links the Monte Carlo transport code MCNP with the radioactive decay and burnup code Origen2, produces a large number of criticality and burnup results based on various material feed/removal specifications, power(s), and time intervals. For a 1000-MWe PWR loaded with 3.3 w/o enriched UO2 fuel, with discharge burnup of 33000 MWd/tU and with an irradiation time of 1000 days, inputs are prepared according to the technical data of the reactor on the basis of the unit cell approximation. SF discharged consists of about 95.5 weight percent (w(o) U, 1 w/o Pu, and 3.5 w/o fission products and minor actinides. Note that the U in SF contains ~0.85 w/o U-235, and ~70 w/o of Pu in SF is composed of fissile isotopes (~59 w/o Pu-239 and ~11 w/o Pu-241).

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3. Decay Heat Profiles

The decay heat profile, obtained for the SF from Monteburns output, to be used as the source term in the thermal analysis is shown in Figure 1.

FIG. 1.Decay heat of SF with 33000 MWd/tHM burnup

In order to obtain heat generation rate equations for SFs with different storage periods (40, 50, 60, 80 and 100 years), a time-dependent decay heat curve is fitted to sum of four exponential terms «Put’ s formula»[1]:

tbi i

ieAtQ

where Q is decay heat in W/tHM; t is time in year elapsed since the discharge of SF. Values of the coefficients in Put’s formula are given in Table I.

TABLE I: VALUES OF THE COEFFICIENTS IN PUT'S FORMULA

A1 A2 A3 A4 b1 b2 b3 b4

990.18 120.73 14.27 11.60 0.02325 0.00166 0.00013 3.1375E-5

4. Disposal Density Calculations

Once SF is emplaced in the repository, temperatures of the repository components increase due to the heat generation. Increasing temperature affects many processes occurring in the repository, thus, during the repository design, it is necessary to investigate the resultant time-dependent temperature distributions and determine an appropriate density of emplacement for SF canisters.

4.1. Reference Repository

The KBS-3 concept developed by the Swedish Nuclear Fuel and Waste Management Company is taken as the reference repository. In this concept, SF is placed into copper canisters with a cast iron insert. The canisters are surrounded by bentonite buffer and placed vertically into holes in parallel tunnels at a depth of 500 m in granite rock. The depth of each hole is 7.55 and the diameter is 1.75 meters. Tunnel diameter is 5.5 meters. The distance between the tunnels is 40 meters [2]. Four SF assemblies would be packaged within a copper canister. Each SF assembly has a square cross-section 0.214 m by 0.214 m and 4.1 m long. Disposal canister is 4.5 m long and 0.9 m in diameter [2]. Figure 2 shows the reference repository concept.

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FIG. 2. The reference repository concept [2] and SF disposal canister

4.2. Thermal Analysis

Heat transfer in the repository is mainly by conduction. The finite element code ANSYS is used to develop a 3-D thermal model of the repository. It is assumed that the repository contains an infinite number of tunnels filled with an infinite number of canisters with the same thermal output. Due to the geometrical and loading symmetry of the repository, the thermal model is simplified to one quarter of a deposition hole with three symmetry surfaces. Vertical symmetry planes passing through the center of the holes, half distance between the adjacent holes and half distance between the adjacent tunnels constitute the lateral boundaries of the model. Figure 3 shows the ANSYS model of the repository.

FIG. 3. The ANSYS model of the repository

Constant temperature boundary conditions are applied at the top and bottom boundaries of the model. All symmetric boundaries are assumed to be adiabatic. The heat-source term is applied as volumetric heat generation in the waste region. The thermal constraint is that the temperature at the canister surface must not exceed 100 ºC. Bentonite will remain chemically intact for more than one million years as long as the temperature does not exceed 100 ºC [3]. In this study, the temperature limit is reduced to 80 ºC in order to include a margin of 10 ºC to cover for natural deviations in environmental parameters and another 10 ºC to cover the risk of occurrence of an air gap between the canister and the buffer [4]. Figure 4 shows how the temperatures on the canister surface and at the interface between the bentonite and rock change with time.

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FIG. 4. Temperature as a function of time at minimum canister spacing

4.3. Disposal Area

Disposal areas needed for one ton of SFs with different ages are calculated from the minimum distance between boreholes, distance between tunnels, and the amount of waste loaded into a canister. Results are given in Table II.

TABLE II: DISPOSAL AREAS FOR SFs WITH DIFFERENT AGES

Cooling time (years)

Canister spacing(m)

Disposal area per canister (m2/canister)

Disposal area(m2/ ton)

40 5.2 208 107.6350 3.9 156 80.7360 3.0 120 62.1080 2.3 92 47.61100 1.6 64 33.12

5. Conclusion

It is observed in Table II that, as the age of SF increases, a significant reduction in the disposal area can be achieved. Yet, the constraints of mechanical nature to maintain the physical integrity of the disposal area during the operational period become more important below a canister spacing of ~4 m. Then, it can be concluded that, for SF with 33000 MWd/t burnup from the reference PWR in this study, a cooling time of ~50 years prior to the disposal represents a point of compromise.

A follow-up study, based on the same models and using the same codes, is planned in order to obtain comparable results for SFs with higher burnups (with higher fresh fuel enrichments).

6. References

[1] PUT, M., HENRION, P., “Modeling of Radionuclide Migration and Heat Transport from an HLW-Repository in Boom Clay”, EC, Report EUR 14156 (1992).

[2] NIREX LTD., Outline Design for a Reference Repository Concept for UK High Level Waste/Spent Fuel, Number: 502644 (2005).

[3] CHOI, H.J., CHOI, J., “Double-layered buffer to enhance the thermal performance in a high-level radioactive waste disposal system”, Nuclear Engineering and Design 238 (2008), 2815–2820.

[4] SWEDISH NUCLEAR FUEL AND WASTE MANAGEMENT COMPANY, Heat Propagation in and around the Deep Repository, Technical Report TR-99-02 (1999).