CAN THERMAL REACTOR RECYCLE ELIMINATE THE NEED FOR MULTIPLE REPOSITORIES?
C. W. Forsberg, E. D. Collins, J. P. Renier, C. W. Alexander
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6170
Abstract
Recent partitioning-transmutation (P-T) systems studies for the Advanced Fuel Cycle Initiative Program have shown that partitioning of actinides from LWR spent fuel and subsequent transmutation in LWRs can contribute significantly to the extension of repository lifetime. If the correct conditions are chosen, an unlimited number of P-T cycles can be performed for all of the actinides, and only fission product wastes can be sent to the repository. In addition to extended lifetime for the repository, the scenario evaluated enables (1) lower costs for P-T of plutonium and the minor actinides and for storage of spent fuel, (2) maintenance of proliferation resistance in spent fuels, and (3) efficient use of fuel/target fabrication facilities.
Introduction
Because of the limited physical area of the geological strata, the planned YuccaMountain repository in the United States has limited capacity for disposal of spent nuclear fuel (SNF) and high-level waste (HLW). The amount of SNF or HLW that can be disposed of per unit of area is constrained by the decay heat generated by the radioactive wastes. Excessive decay heat raises the temperature and may degrade the performance of the repository. The decay heat is primarily from 241Pu-Am, 238Pu, 244Cm, 137Cs, 90Sr, and the associated decay products. If these radionuclides are removed, more waste can be put in the repository without exceeding temperature limits. Removal of the americium and plutonium can increase the repository capacity by a factor of 5, while the simultaneous removal of these actinides and 137Cs and 90Sr increases the capacity by a factor of 42.
Because of the ~30-year half-lives of cesium and strontium, these radionuclides can be separated from SNF and stored until they decay away, thus removing the repository heat load from these radionuclides. The long half-lives of americium and plutonium do not allow for this option. If they can be destroyed, the repository capacity can be dramatically increased. In the United States, the only existing machines that can destroy actinides are light-water reactors (LWRs). This raises the question of whether the actinides can be destroyed by recycle in these reactors. Far greater resources will be required if a new type of reactor is needed for actinide burning.
Recent Studies
Recent partitioning-transmutation (P-T) systems studies for the Advanced Fuel Cycle Initiative (AFCI) Program have shown that partitioning of actinides from LWR spent fuel and subsequent transmutation in LWRs can contribute significantly to the extension of repository lifetime. If the correct conditions are chosen, an unlimited number of P-T cycles can be performed for all of the actinides, and only fission product wastes can be sent to the repository.
Table 1 lists some of the key variables involved in P-T systems studies, the typical ranges of conditions that are considered, and those evaluated in the recent study. Two of the variables found to be most influential are “cooling time” (i.e., the time period since reactor discharge) and “blending scenario.” The latter is the sequence of blending of spent fuels, separations steps, re-irradiation, and cooling time. In other studies,[1],[2],[3] the variable conditions have been chosen that lead to the production of large and “difficult-to-handle” amounts of the radionuclide 244Cm because of its large decay heat and neutron emissions. Thus, added separations steps and storage requirements for curium are frequently considered, even though these are difficult and expensive process steps. The use of inert matrix fuels (IMF) is often chosen to avoid “re-creation” of actinides and to enable a greater extent of irradiation burnup. However, these conditions often lead to residual actinides that are more “difficult-to-handle” and difficult to transmute further. Thus, under these conditions, the number of P-T cycles is limited, and the irradiated actinides are sent to the repository. Since the irradiated actinides contain larger concentrations of radionuclides (244Cm, 238Pu, 241Am) that have greater decay heat emissions, storage of these spent fuels in the repository has a negative effect on repository loading and thus repository lifetime.
Table 1. Some Key Systems Studies Variables and ConstraintsTypical Range / Current Study
Irradiation Burnup / 20-50 GWd/T / 45 GWd/T
Cooling Time / 5 – 20 years / 30-y, 5-y
Blending Scenario / Series (Tiered) or Recycle / Recycle
Curium Separation / Separated/Stored or Recycled / Recycled
Fuel (Actinide Target) Matrix / Fertile (UO2) or IMF (Zr) / Fertile (UO2)
Actinide Transmutation / Homogenous or Heterogeneous / Heterogeneous
Number of P-T Cycles / Limited or Unlimited / Unlimited
Scenario Evaluated
The processing scenario evaluated (Fig. 1)is representative of the conditions that exist in the United States. The scenario assumed that (1) 2000 MT/year of spent fuel, irradiated to 45GWd/MT and decayed for 30years, is processed; (2) recovered plutonium and 90% of the neptunium are transmuted in LWR mixed-oxide fuel (MOX) fuel; and (3) minor actinides (MAs), consisting of americium, curium, and 10% of the neptunium, are transmuted in “burnable-poison”–type targets. Two key features of the scenario are long cooling time (30-year decay periods) and blending scenario (combination of LWR UO2 spent fuel, LWR Pu-Np MOX spent fuel, and LWR-irradiated Am-Cm targets in the feed stream to the separations plant.
Irradiation Configuration
In all cases (for both Am-Cm and Pu-Np), the transmutations were driven by enriched 235U drivers. The enrichment of the 235U in the driver fuel rods and in the Am-Cm target diluent was kept below the currently approved limit for commercial enrichment (5.0% 235U).By using enriched 235U drivers, the fuel/target rod loadings could be kept constant during the multiple P-T cycles. In most of the calculations made in this study, the MA “target” rods consisted of a loading of 10.0wt % MAs in a matrix of UO2 containing 5.0wt % 235U. Each fuel assembly consisted of 48MA target rods inserted into a standard 1717 pressurized water-reactor (PWR) fuel rod configuration, together with 216 standard “driver rods” containing UO2 fuel enriched to 5.0 wt % 235U. (Fig. 2)
Similarly, the MOX rods consisted of a loading of 9.28 wt % plutonium plus neptunium in a matrix of depleted UO2. Each MOX fuel assembly consisted of 104MOX rods, together with 160 standard “driver rods” containing UO2 fuel enriched to 3.5 wt. % 235U.
The fuel assemblies were irradiated for three reactor cycles of 18 months each in a 3400-MW(t) core, which contained 193 fuel assemblies. Detailed multi-dimensional neutronics calculations were performed with the HELIOS code using 45 neutron groups.
Calculations were also performed to determine the void reactivity coefficients for the MA target and the MOX fuel assemblies. These calculations were performed at full-power conditions for a relative void fraction of 90% and 50%. The void reactivity coefficients were negative for all cases. The peak power density for the MA target assemblies typically increases by 6% at the beginning-of-life.
Results from the Base Case (30-year Decay, 35-year P-T Cycles) Evaluation
The results of the base case evaluation are shown in Table 2 and Fig. 3. The calculations showed that both the Pu-Np and the Am-Cm can be brought to near equilibrium, and that the production of heavy isotopes (243Am, 244Cm, etc.) can be suppressed. The scenario enables the fissile content of the blended plutonium product from each separation to remain sufficiently high (≥ 40%) for multiple P-T cycles to be achieved. After 10 P-T cycles, the production rates of the radionuclides in the pathway to heavier elements (242Pu, 243Am, and curium isotopes) are still increasing, but at a relatively low rate. Overall, the production rates of all of the actinide elements are near equilibrium. The overall time span of ~350 years for 10 P-T cycles is only indicative of a sustainable strategy. Alternative strategies are likely to develop earlier in this time frame.
Comparative Results With 5-Year Decay (10-Year P-T Cycles)
A similar series of calculations was made to compare the results obtained when using 5-year decay periods (10-year P-T cycles) with the previously obtained results using 30-year decay periods (35-year PT cycles). Figure 4 illustrates the production rates of the key radionuclides, 242Pu, 243Am, and 244Cm. Although the production rates of 243Am are similar and those of 242Pu are not greatly different, the rate of production of 244Cm is significantly greater with shorter decay periods and cycle lengths.
When plotted against actual time, beginning with the start of the recycling scenario (Fig. 5), the differences are more prolific and indicate the difficulty that would be encountered in the near term (~50years after start of recycling). During this time, multiple tons per year of 244Cm would need to be handled if the 5-year decay fuel were processed; and expensive separations steps (to separate curium from americium) and storage requirements (for curium) might be needed.
Significant Features of the Scenario Evaluated
Two features of the scenario enable multiple cycles to be attainable. The first is the effect of the long cooling time. In the chart of the nuclides (Fig. 6), the primary path toward production of heavier nuclides (curium) is through
241Pu242Pu243Am 244mAm244Cm
by means of neutron capture and beta decay reactions.
With a 30-year decay period, more than 75% of the 241Pu decays to 241Am. Then, during subsequent irradiation, most of the 241Am is transmuted through the pathway
to produce predominantly 238Pu and 239Pu. Still, as indicated, some (~17%) of the 241Am is transmuted to 242Pu and thence to the heavier curium isotopes. However, during the 30-year decay period, ~2/3 of the previously produced 244Cm will decay to 240Pu. Thus, much of the transmutation pathway is altered to produce lighter plutonium nuclides rather than the heavy curium nuclides.
The second feature is the recycle blending strategy.
This strategy enables dilution of the heavier plutonium isotopes contained in the irradiated LWR-MOX with the lighter plutonium isotopes in the irradiated Am-Cm and LWR UO2 spent fuel. The effects are shown in Table 3for the preparation of 2nd-cycle feed.
Benefits of Scenario Evaluated
The scenario evaluated offers significant benefits, primarily extended lifetime for the repository but also lower costs for partitioning and transmutation of plutonium and the MAs and for storage of spent fuel,maintenance of proliferation resistance for the fissile plutonium in spent fuels, andefficient use of fuel/target fabrication facilities. The lifetime of the repository would be extended significantly because all of the plutonium and MAs would be “in process” or “in storage,” and only fission products would be put into the repository. The lower costs would be achieved primarily because no capital investment for a special transmuter reactor (fast reactor, accelerator-driven system, etc.) would be required. Instead, only existing and new LWRs would be utilized.
Moreover, no new storage capacity would be needed for spent fuels and irradiated targets because the number of spent fuel assemblies would remain the same after the scenario began. Even though the total inventory of plutonium would rise during the early cycles, ~98% of the plutonium would be contained in stored spent fuel and would be protected by high radiation (the “spent fuel standard”). This is because the spent fuel would be reprocessed and re-irradiated at intervals within which the fission products, 137Cs and 90Sr, both with half-lives of ~30years, exist in significantly high concentrations.
Further, the scenario evaluated would allow efficient use of the fabrication facilities because the larger fraction (uranium-plutonium-neptunium) can be fabricated into MOX fuel in conventional glove-box-contained equipment, whereas the smaller fraction (americium-curium-diluent) will require more expensive shielded containment. Also, the scenario would allow different irradiation and/or decay times for the MOX fuel and MA targets if necessary to optimize the transmutation process.
Finally, the decay heat generation and radioactivity of the fission product are exponentially decreased during the longer cooling time; therefore, the reduced heat and activity can enable significant simplification and cost reduction in the separations facilities.
[1] G. YOUINOU, F. VARIANE, and A. VASILE, “Plutonium and Americium Multirecycling in the European Pressurized Reactor (EPR) Using Slightly Over-Moderated U-235 Enriched MOX Fuel Assemblies,” Proceedings of the Global03 Conference, American Nuclear Society (Nov. 2003), pp. 135–140.
[2] R. A. WIGELAND, T. H. FANNING, and E. E. MORRIS, “Repository Impact of LWR MOX and Fast Reactor Recycling Options,” Proceedings of the Global 03 Conference, American Nuclear Society (Nov. 2003), pp. 251255.
[3] T. A. TAIWO, T. K. KIM, J. A. STILLMAN, R. N. HILL, M. SALVATORES, and P. J. FINCK, “Assessment of a Heterogeneous PWR Assembly for Plutonium and Minor Actinides Recycle,” Proceedings of the Global 03 Conference, American Nuclear Society (Nov. 2003), pp. 146163.