THE CONCEPT OF PROLIFERATION-RESISTANT, ENVIRONMENT- FRIENDLY, ACCIDENT-TOLERANT, CONTINUAL AND ECONOMICAL REACTOR (PEACER) I. S. Hwang, S. H. Jeong, B. G. Park, W. S. Yang*, K.Y. Suh and C.H. Kim Department of Nuclear Engineering, Seoul National University, 56-1 Shinlim-dong Gwanak-ku Seoul 151-742, Republic of Korea *Department of Nuclear Engineering, Chosun University, 375 Sursuk-dong Dong-gu Kwang-ju 501-759, Republic of Korea Tel : +82-2-880-7215, Fax : +82-2-889-2688, E-mail : firstname.lastname@example.org Abstract As an effort to ameliorate generic concerns with current power reactors such as the risk of proliferation, radiological hazard of the spent fuel, and the vulnerability to core-melt accidents, the concept of a revolutionary reactor, named as PEACER, has been developed as a proliferation-resistant waste transmutation reactor based on the unique combination of technologies on the proven fast reactor and the heavy liquid metal coolant. In this paper, results of PEACER conceptual design are summarized by focusing on the estimated performance. The proliferation resistance of PEACER is built by installing both institutional and technical barriers. The latter includes denaturing of fissile materials, Pu in particular, as well as the intense radiation field associated with the pyrochemical partitioning method. When the fuel volume fraction and the core aspect ratio(L/D) are optimized, the transmutation capability of PEACER for long-lived wastes from LWR spent fuels is found to exceed their production rate of two LWR’s each at the same electric rating. In contrast with current power reactor design principles, the lower power density and the higher neutron leakage rate lead to the higher performance on the proliferation-resistance, transmutation capability and the accident-tolerance. The result of the present conceptual design showed promising characteristics in all the five targets defined by its name PEACER, which warrants more detailed studies. I. Introduction Due to the scarcity in domestic energy resources and strong energy dependence of her industry and society, Korea relies heavily on nuclear power for electricity generation. The nuclear power in the 21st century, however, is expected to confront with global controversy related with the risk of proliferation, the radiological hazard of the spent fuel, and the depletion of uranium resources if the disadvantage of water reactor technology is not overcome. The expectation calls for more fundamental steps to resolve the issues for the sake of the national energy security of resource-poor countries with high nuclear energy dependency. Recently, strong technical drive for nuclear wastes transmutation has been initiated and spreading world- wide to stimulate revolutionary approaches. A consensus is being achieved on the fact that most significant long-lived radioactive isotopes in spent fuels of the current power reactors can be transmuted into short-lived ones by using fast neutron spectrum with localized thermal traps.[Park, W.S., and et al., 1995] Geological disposal of the spent fuel, while being actively developed as one of the most visible options today, may be confronted with societal obstacles especially in countries with dense population and with political questions raised by the expected accessibility of plutonium by the decay of intense radiation barriers over hundreds of years. For sustained growth, plutonium and other actinides are better to be utilized as valuable fuel resources. Therefore the development of transmutation technology with built-in proliferation- resistance and inherent safety features can receive an international support. We believe that economical transmutation systems can be developed through creative combination of available technologies and an institutional approach among nuclear R&D communities in the world. The PEACER concept is driven towards this goal by utilizing a low power density core consisting of metallic fuels cooled by chemically stable Pb-Bi eutectic liquid metal. In the development of the conceptual design, the design targets and approach are established as follows. Proliferation-resistance: Both technical and institutional barriers are employed with the defense-in-depth approach in each step of nuclear materials processing. The adequacy of the barrier system should be judged from a view point of the international acceptance. The transmutation capability: One PEACER shall transmute long-lived wastes in the spent fuel from at least two LWR’s with the same power rating, leaving behind only short-lived wastes suitable for land-based disposal. Inherently safety: Using the inert Pb-Bi coolant with no fire hazard, the nuclear system shall be adequately cooled by natural convection even under the loss of ultimate heat sink accident scenarios. Fuel continuity: All actinides shall be made useable as fuel in order to extend fuel resources by order of magnitude from that for LWR’s. Improved economy: The power generation cost of the PEACER shall be competitive with that of LWR’s with a design life of 60 years. II. PEACER Design Approach Transmutation process requires partitioning that inherently raises concern with the possibility of diversion or theft of sensitive nuclear materials. As the PEACER is designed to have high transmutation rate, it is assumed that the facility is centralized as a waste burning energy park that can be operated under the international control. The utilization of pyroprocessing technique will enhance the proliferation resistance due to its inherent limit in separability and intense radiation field.[Laidler, J. J and et al., 1997] An additional approach of denaturing the sensitive fissile element is pursued especially for Pu. As a measure of the denaturing extent for proliferation-resistance, the odd ratio is defined as follows; total mass of odd mass number isotopes Odd Ratio (OR) ≡ (1) total mass of selected actinide element The OR indicates the possibility of explosion of nuclear fuel material. Volpi suggested that the explosive power of a nuclear weapon is drastically decreased when the OR value is lowered.[De Volpi, A., 1979] The OR of weapon grade Pu is over 0.93, the fuel grade is in the range of 0.8∼0.93, and the reactor grade is below 0.8.[J. Carson Mark, 1993] Uranium with OR value below 0.2 is internationally accepted to be proliferation resistant. Although no lower limit is established for Pu, the target of OR in PEACER is set to be 0.5 that presents significant additional technical barrier due to about ten times lower explosive power and higher decay heat compared with the case of typical spent fuel composition after the work of Volpi.[De Volpi, A, 1979] Environment-friendliness is measured by the transmutation rate that is defined by support ratio, as below; annual TRU destruction by PEACER at the same power Support Ratio (SR) ≡ (2) annual TRU production by an LWR For clarity in the performance comparison, the reactor power is measured in electric rating. The calculated TRU transmutation rate of PEACER is scaled linearly with the electrical power and cycle length of corresponding LWR. Considering its usefulness as a transmuter and associated economy, the target SR value for PEACER is set to exceed 2.0. For the conceptual design study, a reference power is selected to be 1,575 MWt in view of the Integral Fast Reactor (IFR).[Wade, D. C. and Hill, R. N., 1997] It is desired to maximize SR while minimizing OR through optimization of core design parameters including L/D and VF, the ratio of core active height to diameter and the fuel volume fraction, respectively. The fuel volume fraction is defined as the ratio of metallic fuel (U+TRU+Zr) volume to total core volume. It is expected that the neutron leakage is increased with decreasing L/D. The more neutron leakage represents less production of higher actinides due to neutron capture at a given transmutation rate that in turn will increase SR and decrease OR. Therefore the design approach is directed toward high leakage geometry. Decrease in volume fraction also increases the neutron leakage. In a typical Na-cooled fast reactor the fuel volume fraction is ranged from 35% to 40%[Waltar, A. E. and Reynolds, A. B.1981]. If core power density is lowered, for example to the level of Russian Pb-Bi fast reactors, the transmutation rate is significantly increased from that of IFR burner design corresponding with the increase in TRU loading. The design analysis is conducted using REBUS-3 code which is a multi-group diffusion-depletion analysis program developed at the Argonne National Laboratory[Toppel, B. J., 1990]. In REBUS-3 computation, the effective multiplication factors at the beginning and the end of an equilibrium core and the geometric data are specified to determine equilibrium core compositions. In order to assess the core performance, the equilibrium cycle behavior is determined using REBUS-3 with a on-power cycle length of 365 days and k- effective at the end of equilibrium cycle(EOEC) of 1.002. During calculation by REBUS-3, several assumptions are made as follows.[Yang, W.S. and et al., 1996] In fast neutron spectrum, fission products do not affect the core criticality as much as in thermal reactor case. Therefore, REBUS-3 analysis is simplified by grouping these fission products into four lumped fission products as an approximation. In this case the final fission products appear as groups. Although the procedure makes detailed follow-up of waste inventory difficult, its impact on neutronic analysis of prime interest here is insignificant. In tracking the actinide elements, isotopes heavier than Cm-245 and lighter than Th-232 are ignored from the chain analysis by the similar reasoning. For simplicity, all the actinide isotopes in the chain are assumed to be extracted 100% and recycled in fuel separation and fabrication process. The error by this assumption is not significant in the determination of SR and OR. More realistic data of 99.9 % recovery is used in economy analysis. External feed to the PEACER fuel cycle has the actinide isotopic composition corresponding to spent fuel with 150 days cooling after discharging from 1 GWe LWR[Benedict, M. and et al., 1981] except for U as the total amount of U is adjusted to meet the equilibrium cycle condition. All the control system including control rods are assumed to be fully removed from the core because we want determine a baseline behavior. III. Results of Design Analysis Fig. 1 shows the schema of PEACER reference design with pancake geometry to increase axial leakage and Fig. 2 shows the radial configuration of PEACER core. Radial leakage is enhanced by introducing a central reflector region.[Wade, D. C. and et al., 1997] The reference PEACER design also has significantly high ratio of fuel pin pitch to fuel pin diameter compared with that of Na-cooled fast reactors. The feature is utilized to allow sparse square lattice in a fuel assembly with adequate structural reinforcement against potential flow induced vibration with the heavy liquid metal coolant. Fig. 1. Schema of PEACER reference core. Fig. 2. Assembly arrangement of PEACER core. Proliferation-Resistance As the institutional barrier against proliferation, a waste transmutation complex consisting of multiple units of PEACER is proposed to be run by an international organization in compliance with the IAEA safeguard procedures. The fuel separation facility is made compact using the pyrochemical processing so that it can be housed within the physical boundary of the transmutation complex. The technical barrier represented by OR is examined as function of core design parameters. As shown in Fig. 3(a) and (b), OR of plutonium decreases as both L/D and VF decrease. This indicates that the low power density and high leakage core design also enhances the proliferation-resistance. This behavior is attributable to the slower breeding of Pu-239 from U-238 due to high leakage and the faster destruction of odd number isotopes due to the higher fission cross section and the greater abundance compared with those of even isotopes. In PEACER, OR of Pu decreases down to about 0.5 at EOEC, as shown in Fig. 3. This is much lower than that of LWR spent fuel, 0.7. The difference between two OR values at the beginning of equilibrium cycle(BOEC) and ending of equilibrium cycle(EOEC) is fairly constant at about 0.04 for the range of this study. Maximum fuel discharge burnup is found to be 11.5 % that is within the proven performance in EBR-II core.[Sackett, J. I., 1997] Environment-Friendliness Support Ratio(SR) shows us how much wastes can be transmuted. The higher SR means that the more wastes can be destroyed. Hence SR is regarded as the indicator of environmental influence. The performance parameter on environment-friendliness, SR, is calculated as function of the design parameter L/D, and VF. As shown in Fig. 4(a) and (b), the SR increases as both L/D and VR decrease. In the reference design, SR is found to be 2.07 meeting the design target of 2.0. The corresponding values of L/D and VF is 0.103 and 0.158, respectively. This indicates that one PEACER can transmute long-lived actinides in spent fuel from two LWR’s with the same electrical power rating as PEACER. The low power density and high leakage core design allows the loading of more fissile TRU elements including Pu. In the fuel with high TRU enrichment, the fertile isotopes are leaner and the TRU regeneration rate decreases. It should be noted, however, the inventory of individual isotope may not be accurate as the isotopic chain of TRU in REBUS-3 is simplified. For fission products no transmutation study is feasible with REBUS-3 due to its approximation scheme. It is speculated that transmutation of long-lived fission products such as Tc-99 and I-129 are possible by building localized thermal traps, as proposed by the Los Alamos group. This is particularly feasible in PEACER as the design is compatible with high neutron leakage rate. Accident-tolerance The Pb-Bi coolant has not only chemical inertness but high natural convection capability that leads to inherent safety, as presented by Chang.[Chang, J.E, et al., 1999] It is shown that core melting can be prevented by natural circulation of the Pb-Bi coolant during the loss of flow accident. A preliminary analysis for the loss of ultimate heat sink accident showed adequate cooling capability. As Pb-Bi coolant has much higher boiling point (1,670 ℃) than Na, the positive void coefficient is not an issue. The use of metallic fuel also contributes to the inherent safety. The safety nature of metallic fuel in fast reactor, as demonstrated at EBR-II in 1980’s can also be applied to PEACER. Continuity and Economy By REBUS-3 analysis, it is found that PEACER performance can be further improved by incorporating thorium as fuel in addition to TRU’s. Thorium resource world-wide is three times as much as U. It suggests that PEACER may extend fuel resources by orders of magnitude in comparison with the current LWR’s. It is predicted based on the oxidation kinetics of structural metals in Pb-Bi coolant that the PEACER can achieve the 60 years lifetime with the available technology. [Borisov, O. M. and et al., 1998] Preliminary cost analysis assuming 99.9 % recovery of long-lived isotopes showed that the nuclear waste transmutation by the PEACER with the near surface disposal plan can be made significantly more economical than its LWR counterpart with the once-through fuel cycle and geological waste disposal plan, despite of large uncertainty in the construction cost for PEACER and geological disposal.[Hwang, I.S., and et al., 1999] (a) (b) Fig. 3. Pu Odd Ratio of U-TRU-Zr metallic fuel at equilibrium cycle. (a) Pu odd ratio vs. the ratio of active core height to diameter (L/D): VF (fuel volume fraction)=0.158 (b) Pu odd ratio vs. the fuel volume fraction and the ratio of active core height to diameter (L/D) =0.103 (a) (b) Fig. 4. Support ratio of U-TRU-Zr metallic fuel at equilibrium. (a) Support ratio vs. the ratio of active core height to diameter (L/D): fuel volume fraction=0.158 (b) Support ratio vs. the fuel volume fraction and the ratio of active core height to diameter (L/D)=0.103. IV. Conclusions The principal conclusions of the conceptual design study of PEACER are as follows. 1) The conceptual design of Proliferation-resistant, Environment-friendly, Accident-tolerant, Continual and Economical Reactor (PEACER) has shown to meet all the design targets by lowering power density and increasing neutron leakage. 2) Proliferation-resistance, Environment-friendliness and Continuity: Fissile isotope fraction(odd ratio) of Pu are limited to 0.5 in order to enhance the proliferation-resistance. In addition, not only pure Pu production but its access are impeded by utilizing pyroprocessing with intense radiation barriers. The PEACER shall be operated under the international control for institutional barrier. With the conceptual design, one PEACER in an equilibrium cycle can transmute transuranic elements and possibly long-lived fission products from two LWRs each having the same electric power rating. Up to 99.9% of the long-lived waste elements are assumed to be recycled and residues including short-lived wastes are stabilized, with dilution when necessary, into a form suitable for the land-disposal. By utilizing fertile uranium, actinides and thorium, the PEACER may extend fuel resources by orders of magnitude in comparison with the current LWR. 3) Safety and Economy: The Pb-Bi coolant with its chemical inertness, high boiling point, and natural convection capability leads to inherent safety under the loss of flow or the loss of ultimate heat sink. It is predicted that the PEACER can achieve the 60 year lifetime with the available technology for the Pb-Bi chemistry control. Preliminary cost analysis showed that the nuclear waste transmutation by the PEACER can be made significantly more economical than its LWR counterpart with the once-through fuel cycle and geological waste disposal plan. 4) The success of meeting the design targets and promising features of the PEACER conceptual design warrants more detailed study. Acknowledgments This work was financially supported by the Nuclear Medium-Long Range Development Plan of the Korean Ministry of Science and Technology via the Korea Institute of Science and Technology Evaluation and Planning. Technical advice of Drs. S.T. Shin and M.H. Yang is deeply appreciated. References Benedict, M., Pigford, T. H. and Levi, H. W. (1981), Nuclear Chemical Engineering, 2nd edition, pp. 369, McGraw Hill. Borisov, O. M., Orlove, V. V., Naumov, V. V., Sila-Novitsky, A. G., Smirnov, V. S., Filin, A. I. and Tsikunov, V. S. 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