LA-UR-97-3622

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ACCELERATOR TRANSMUTATION OF WASTE BLANKET CONSlDERATlONS

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MICHAEL G. HOUTS MIKAEL BJORNBERG DAVID 1. POSTON

1997 ANS TOPICAL MEETING November 16-20, 1997 Albuquerque, NM

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ACCELERATOR TRANSMUTATION OF WASTE BLANKET CONSIDERATIONS

INTRODUCTION

Michael G. Houts, Mikael Bjornberg, and David I. Poston Los Alamos National Laboratory

Los Alamos, NM 87544 MS-K55 1

(505)665-4336 / (505)665-3 167 (F)

Accelerator transmutation of waste (ATW) is one approach for reducing the amount of actinides and long- lived fission products that eventually will be sent to a repository. The ATW accelerator generates high-energy protons, which strike a target and produce spallation neutrons. The spallation neutrons transmute waste in a region that surrounds the spallation target. It is desirable for the waste transmutation region (WTR) to have significant neutron multiplication (a factor of 10 or higher) to keep the required accelerator size reasonable.

The WTR is subcritical and is thus not required to generate a self-sustaining fission reaction in the waste. The elimination of this requirement allows the ATW system to be optimized for reducing the hazard from nuclear waste without the concerns associated with safely maintaining criticality. Subcritical operation allows waste compositions with positive prompt reactivity feedback coefficients to be considered, allows waste forms optimized for processing to be considered, and allows additional design flexibility. The WTR will be designed so that criticality cannot be achieved during any credible accident scenario.

The primary advantage of the ATW approach is thus the design and operational flexibility gained from subcritical operation. The primary disadvantage of the ATW approach is the expense and complexity of integrating a large proton accelerator with a spallation target and the WTR.

DESCRIPTION OF WORK

It has been shown that operating the WTR in a fast neutron spectrum has several advantages over operating the WTR in a thermal neutron spectrum.’ Advantages include better neutron economy (higher eta), which allows the transmutation of fission products; reduced power peaking; flexibility in the choice of WTR materials; use of the spallation target as the coolant; and elimination of certain accident scenarios (such as fuel precipitation leading to criticality). The fast spectrum system also has a relatively flat power profile across the actinide-burning section of the WTR.

Because of the advantages noted above, the lead cooled, fast-spectrum WTR has been selected for further study. Parametric studies related to the design of two WTRs are now being performed. The first WTR is designed to be used in a demonstration device that will demonstrate the viability of ATW, while requiring orders of magnitude less accelerator power than an actual ATW device. The second WTR is designed to be used in an ATW capable of destroying hundreds of grams of actinides per day in addition to destroying some of the more hazardous fission products. The actinide mix used in both WTRs is representative of the end-of-life (EOL) actinide composition in a typical fast-spectrum ATW system assuming that the feed is pressurized water reactor (PWR) spent fuel, that select fission products are periodically removed, and that all actinides remain in the fuel mix. To obtain the EOL actinide composition, a code was written to link the MCNP’ and ORIGEN 2.13 codes.

Development of the linkage code (“MonteBwns”) continues, and the code now has the capability to burn multiple regions within the WTR. Detailed calculations using MonteBurns, MCNP, and ORIGEN will be performed to determine EOL actinide compositions for specific systems once the additional preliminary 3csign is completed. The actinide composition that was used in the calculations is given in Table 1.

The fuel used in both systems is assumed to be an actinide-bismuth eutectic, contained in molybdenum cylinders. If compatibility studies indicate that this is not an acceptable fuel form, alternative fuel forms could be used. Fuel form is being optimized based on chemical processing considerations. The primary advantage of the proposed fuel is that the chemistry for separating actinides and fission products from the fuel is straightforward. The primary disadvantage is that the fuel has never been used, and extensive development will be required.

The WTRs consist of a lead target, an actinide burning section, a fission product burning section (optional), and a lead coolantheflector. It may also be desirable to bum fission products in the actinide region to harden the neutron spectrum in the actinide region and to reduce the reactivity insertion resulting from the failure of pins containing fission products. From a chemistry standpoint, however, it may be desirable to keep the actinides and fission products separate. High-energy protons strike the spallation target, forming spallation neutrons that are multiplied in the actinide region and cause additional actinide fissions or fission product transmutations. Because the WTR is a sourcedriven subcritical assembly, the neutron flux is dependent on the distance from the neutron source. The fast-spectrum and high multiplication within the WTR help maintain an acceptable flux and power profile. For example, in the demonstration WTR, the minimum power density in the actinide burning section (uniform actinide density assumed) is 87.5% that of the maximum power density. A schematic of the WTR is shown in Figure 1.

Demonstration WTR

The demonstration WTR has a central lead annulus with a radius of 0.20 m that serves as the spallation target. The demonstration WTR has an outer radius of 0.41 m and a height of 0.82 m. The total actinide loading in the WTR is 330 kg. The demonstration WTR is lead cooled and lead reflected, although the low power level of the system allows the coolant fraction in the core to be small. The neutron spectrum in the demonstration WTR is shown in Figure 2.

Prototypic WTR

The prototypic WTR has a central lead annulus with a radius of 0.20 m that serves as the spallation target. The prototypic WTR has an outer radius of 0.695 m and a height of 1.19 m. The total actinide loading in the WTR is 587 kg. The prototypic WTR is lead cooled and lead reflected, with a large enough coolant fraction to allow several hundred grams of actinides to be transmuted each day. The neutron spectrum in the prototypic WTR is shown in Figure 3. As seen in Figures 2 and 3, the neutron spectrum in the prototypic WTR is quite similar to that of the demonstration WTR.

Sensit ivit ies

Several sensitivity studies are currently under way. One completed sensitivity study examined the effect of spallation target size on neutron spectrum. Figure 4 shows the neutron spectrum in a WTR with a 0.30-m radius spallation region. Figure 5 shows the neutron spectrum in a WTR with a 0.40-m radius spallation region. The size of the spallation region has only a small effect on the neutron spectrum within the WTR. However, reducing the size of the spallation region does reduce WTR actinide loading for a given multiplication.

Remaining Issues

The WTR is at the preconceptual design stage, and several issues remain. Compatibility of the bismuth- actinide eutectic, molybdenum, and lead must be confirmed. If those materials are not compatible, other options will need to be developed. Sensitivity studies have shown that actinide loading is reduced if the radius of the central spallation target region is reduced. Shock, peak flux, and neutron production (per proton) calculations must be performed to ensure that the radius of the spallation target is adequate. Pin configuration in the fueled region will affect natural circulation, performance of the liquid fuel, and other WTR characteristics related to safety. Acceptable lead coolant velocity, pumping power, and temperature change across the WTR will determine the quired lead volume fraction in the core and potentially affect system performance. System performance is also affected by actinide concentration in the actinide-bismuth eutectic. The maximum acceptable concentration needs to be determined for various actinide compositions. Safety issues include ensuring system subcriticality and ensuring adequate decay heat removal capacity.

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Future Work REFEREF CE

In the near future, work will focus on completing WTR sensitivity studies. After sensitivity studies are completed, two of the pmonceptual designs will be selected for further analysis and design work. This work will include calculation of the time-dependent actinide concentration and composition, as well as materials compatibility studies and evaluation of the spallation source. After this preliminary work is completed, a more detailed design will be developed.

1. M.G. Houts and D.I. Poston, “Accelerator Transmutation of Waste Blanket Considerations,” Transactkns of the American Nuclear Society, ISSN: 0003-018X (June 1997).

2. J. F. Briesmeister, “MCNP-A General Monte Carlo N-Particle Transport Code, LA-12625-M (March 1997).

3. RSIC Computer Code Collection Origen 2.1 Code Package, CCC-371(1991).

TABLE 1 MASS PERCENTAGES OF ACTINIDES IN TYPICAL EOL ATW ACTINIDE MIX.

Actinide Np-237 Pu-238 Pu-239 Pu-240 Pu-241 Pu-242 Am-24 1 Am-242 Am-243 Cm-242 Cm-243 Cm-244 Cm-245 Cm-246 Cm-247

Mass Percent in EOL Actinide Mix 2.86 4.45 24.75 37.03 7.80 12.28 3.40 0.28 2.59 0.50 0.04 3.03 0.71 0.25 0.03

Figure 1. Schematic of WTR showing beam, lead target/coolant, actinide waste region, and fission product waste region.

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Figure 2. Neutron spectrum within the demonstration WTR.

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Figure 4. Neutron spectrum within a WTR with a 0.30-m radius spallation region.

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Figure 5. Neutron spectrum within a WTR with a 0.40 m-radius spallation region.