Nuclear Wastes Technologies for Separations and Transmutation (1996) / Chapter Skim
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4 TRANSMUTATION SYSTEMS
4 TRANSMUTATION SYSTEMS
Pages 49-86
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From page 49... ...
and selected fission products from reprocessed LWR spent fuel. An LWR transmuter is a feasible approach in its own right, assuming the development of a fuel cycle to support such transmutation.
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From page 50... ...
For many fission products the neutron capture cross sections in a thermal (or epithermal) spectrum can give substantial transmutation rates.
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From page 51... ...
Second, the thermal neutron "capture-to-fission" ratios are typically higher than those for fast spectra. This effect is exaggerated for even-neutron isotopes; indeed, 240Pu and 241Am have large thermal neutron capture cross sections and are parasitic absorbers in thermal reactors.
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From page 52... ...
However, there are no universal figures of merit for the evaluation of the different transmutation approaches. The remaining sections of this chapter summarize the results of the evaluation using a variety of "measures" for comparison: · the rate and time for various percentage reductions in the TRU inventory: · the flexibility and rate for reducing key fission product inventories:
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From page 53... ...
The calculated TRU ratios and the corresponding times for various fractional changes in net inventory range widely among the proposals. For evaluation purposes, this report considers two nuclear power scenarios, under the assumption that the transmutation systems would produce electrical power for distribution to the electrical grid.
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From page 54... ...
French and U.K. efforts have looked at actinide burning in fast reactors, but more from a perspective of controlling reactivity swings.
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From page 55... ...
Overview of Critical Reactor Concepts This subsection gives an overview of a fast reactor system and two types of thermal reactor systems that are proposed or are being considered for transmutation of nuclear waste, namely, · transmutation of TRUs in an ALMR as part of an IFR proposed by GE and ANL; · transmutation of TRUs and fission products in LWRs, either existing designs adapted for the purpose or more advanced designs currently in certification review by the Nuclear Regulatory Commission (NRC) ; and · transmutation of minor actinides with a PER, as proposed by BNL.
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From page 56... ...
transmutation of TRUs and more fission products recovered by nonaqueous reprocessing of LWR spent fuel, using a nonaqueous neutron multiplying system, fueled partly with thorium and generating electric power (Case ATW-3~; and 4. transmutation of TRUs and some fission products in a nonaqueous thorium-breeder system for electrical power generation (Case ATW-4~.
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From page 57... ...
This suggests a waste management concept intermediate between the once-through LWR fuel cycle and the full transmutation of all TRUs and selected fission products. That is, the accumulated LWR spent fuel could be reprocessed and only the separated plutonium recycled to either an LWR or ALMR.
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From page 58... ...
REDUCTION OF TRANSURANIC INVENTORIES Introduction This section examines the extent to which the amounts of TRUs in wastes from the various proposed transmuters can be made significantly smaller than the amount of TRUs in spent fuel in the reference once-through LWR fuel cycle. Inventories of TRUs in wastes from fuel reprocessing and TRU recycle are considered, together with inventories of untransmuted TRUs in the transmuter and in the associated facilities for reprocessing and fabrication of recycled material.
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From page 59... ...
ratio" Butt, defined as total inventory of transuranics sent to waste disposal in time for the reference once-through LWR fuel cycle, if no fuel reprocessing, no recycle, and no transmutation r VJ total inventory of transuranics at time t in the bans muter, in its fuel cycle, and in process wastes TRUs supplied to the transmuter system for start-up and for make-up fuel can be obtained by reprocessing existing LWR spent fuel andlor by reprocessing spent fuel from future LWRs. In calculating quantities for the equivalent reference fuel cycle, i.e., for the numerator of city, that same amount of TRUs must be assumed to go directly to waste disposal.
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From page 60... ...
Each of these options could involve an initial period in which the transmuters are started and fueled with TRUs recovered from the stockpile of LWR spent fuel. The desired goal would be to reduce the total TRU inventory well below that of the reference oncethrough LWR fuel cycle.
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From page 61... ...
Our illustrations herein of the possible features of LWR plutonium and LWR TRU transmuters are based necessarily on data from Gorrell's calculations. Constant-Power ALMRs, Unlimited Supply of TRUs From Stockpile of LWR Spent Fuel The TRU inventories for ALMR burners of 0.65 breeding ratio for the first 100 years for the simple case of constant power and an unlimited stockpile of spent fuel containing TRUs are shown in Figure 4-1.
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From page 62... ...
12Recent calculations by ORNL, referred to in the ALMR chapter, suggest that if the cost of reprocessing LWR spent fuel for TRU recovery is as high as $1,000/kg, the LWR fuel cycle could be more economical than that of an ALMR even by using natural uranium from sea water, if uranium could be obtained from sea water by a new Japanese process for about $150/lb. If so, this would considerably extend the era for competitiveness of the LWR once-through fuel cycle compared with the ALMR fuel cycle that uses TRUs from spent LWR fuel.
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From page 63... ...
63 Assuming that nuclear power is to continue in the future at a steady power level, and assuming that transmutation by ALMRs is desired to benefit ultimate waste by reducing TRU inventories by even as little as an order of magnitude, Figures 4-2 and 4-3 show that a commitment would have to be made to continue ALMRs and their progeny for many centuries. Constant-Power ALMRs, Limited Stockpile of LWR Spent Fuel, Additional lLiWRs Figure 4-3 shows the time-dependent TRU inventories for the mixed ALMR-LWR fuel cycle.
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From page 64... ...
The total inventory of TRUs to waste disposal for this mixed fuel cycle is identical to that in Figure 4-1, and the TRU ratios for the two scenarios (Figures 4-1 and 4-3) are identical.
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From page 65... ...
- 6 _ 1 1 _2 65 FIGURE 4-3 TRU inventory and ratio versus time for limited stockpile of LWR spent fuel, constantpower ALMR.
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From page 66... ...
The total saleable electrical power from the aqueous ATW needed to utilize the 62,000-Mg stockpile of LWR spent fuel is lower than for ALMRs because of the relatively low net thermal efficiency of the aqueous ATW.~4 The aqueous ATW'S saleable electric power is also lower than that of the ALMR of |4The low net thermal efficiency of the aqueous ATW is a consequence of the limited system pressure and fuel-coolane temperature of the calandria-tvne pressure-tube reactor lattice and the need to supply electricity to the ATW's accelerator, as explained in the ATW section of this report.
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From page 67... ...
Time-dependent TRU ratios for constant-power ATWs, both aqueous and nonaqueous and effect of process losses, are shown as a function of time in Figure 4-7. Data in Table 4-2 show that the ratio B/I of the transmutation rate of LWR TRUs to the TRU inventory is much higher for the nonaqueous ATWs than for the aqueous ATW, a conse
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From page 68... ...
Even the nonaqueous ATW with 75% of the fissions from internal breeding from 232Th achieves more rapid net burn-up of LWR TRUs, per unit system inventory, than does the aqueous ATW. The TRU ratios for the nonaqueous ATWs are higher.
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From page 69... ...
Berkeley: University of California. Process Loss = 0.1% Process Loss = 0.01% Constant Transmuter Power 103 o ._ co .C' 1 o2 _ Cal cat , _ LWRIPu-burner ~ LWR/TRIJ-brIm~r PER LWR/Pu-bumer LWR/TRU-bumer 1 1 1 1o2 103 104 105 1o6 Time of Transmutation Operation, years FIGURE 4-9 TRU ratio versus time for constant-power LWRs and effect of process loss.
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From page 70... ...
Constant Power Transmuters Followed by Declining Power Shutdown The TRU ratios for constant-power transmuters are greater than those for the declining-power scenario. If constant-power operation of a transmutation system were to be suddenly terminated, it would not be necessary to send all of the TRUs in the reactor and fuel cycle to waste disposal, provided a commitment could be made to continue transmuter power in a stepwise declining mode, as illustrated in Figures 4-4, 4-5, and 4-8.
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From page 71... ...
4. For process losses of 0.1% per cycle, the maximum possible TRU ratios are a few hundred for ALMR, LWR, and nonaqueous ATW transmuters and less for the aqueous ATW.
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From page 72... ...
and about half as TcO4 (Baetsle, 1993~.2° The LWR-based transmutation concept has a high degree of flexibility for transmuting fission products. For a thermal neutron spectrum and flux level typical of a uranium-fueled PWR the transmutation rates of very dilute 99Tc and HI are about 1 1 %/yr and 3%/yr, respectively (Wachter 19It is routinely evolved quantitatively from the dissolver and recovered on silver iodide.
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From page 73... ...
For example, the PBR concept has almost two orders of magnitude higher power density than an LWR; an ATW operates with an order of magnitude higher thermal flux and a factor of 2.5 higher power density in the fissioning blanket than in an LWR; and the Phoenix concept has a higher fast neutron flux and power density than an ALMR. In addition to decay heat, the ATW and Phoenix concepts involve major target-heat dissipation and removal issues that affect reliable operation, as well as licensing.
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From page 74... ...
In contrast, the build-up of higher-mass TRUs to multiple-recycle fast reactor fuels is orders of magnitude lower. However, the experience with aqueous-based reprocessing of LWR spent fuel and with remotely operated fuel fabrication provides an initial basis for design and operation of the supporting fuel cycle.
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From page 75... ...
The ALMR/IFR alternative is expected to have an intermediate development risk, cost, and an intermediate time between LWR's and ATW's to complete demonstration about 15 to 20 years, assuming the use of existing facilities for in-reactor fuel testing and demonstration, exclusive of the cost of any full-scale demonstration facilities. Indeed, because a low threshold cost for LWR spent-fuel reprocessing is required for an economically viable ALMR/ IFR TRU burner, its development risk is higher than that of an ALMR for power production.
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From page 76... ...
Because only a scant amount of information to evaluate the two concepts is available, only a limited evaluation is presented here. PBR Transmutation System The PBR is a critical reactor concept with a very high thermal flux that could achieve rapid burn-up of TRUs and fission products, assuming a suitable fuel form could be developed.
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From page 77... ...
Reprocessing Costs. Appendix J analyzes cost information available on several foreign facilities that use a PUREXlike separations technology to reprocess LWR spent fuel, for a once-through fuel cycle.
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From page 78... ...
In addition to the reprocessing of the spent fuel, reprocessing of the fuel discharged from the transmuting reactor would be required. The amount of such fuel being reprocessed is about a factor of 10 less than the LWR spent fuel.
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From page 79... ...
Each LWR operates at 1.395 Owe. dRatio of the TRU inventory in the transmutors and fuel cycles, at the end of the generation, to the original 612 Mg of TRU.
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From page 80... ...
To process this in 5 years would require 2.6 reprocessing facilities, each of capacity 900 Mg/yr of LWR spent fuel. At the end of 30 years, 23 percent of the initial TRU inventory would remain.
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From page 81... ...
The asymptotic ratio of LWR power to nonaqueous ATW power is 3.42, corresponding to a reprocessing rate of 192 Mg HM/yr of LWR spent fuel. The aqueous ATW transmutor can transmute transuranics at almost twice the rate of that of the nonaqueous ATW, both at the same electrical power, because of the much lower thermal efficiency of the aqueous ATW.
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From page 82... ...
2. The proposed S&T systems require decades to centuries to achieve a significant net reduction in the total TRU inventory relative to that of a once-through LWR fuel cycle of the same electrical production capacity.
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From page 83... ...
It would be impossible for ATWs to effectively eliminate all long-lived radionuclides so that important residual radioactivity would persist "no longer than a human lifetime," as has been asserted by the ATW proponents. 24The "inventory fraction" is the ratio of the TRU inventory in the transmutor, fuel cycle, and waste to that in the spent fuel of a reference EWR once-through fuel cycle of the same electrical production.
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From page 84... ...
The times to reduce overall TRU inventories relative to the once-through LWR fuel cycle would be comparable to, but slightly shorter than, the times for the nonbreeding ALMR. During the first few hundred years of transmutor operation, the extent of TRU reduction would be little affected by the higher process losses of existing PUREX separations.
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From page 85... ...
Indeed, the high level of alpha and neutron radioactivity during onsite reprocessing and recycle-fuel circulation poses severe problems for the aqueous-based ATW-1 concept. For all the ATW concepts, the overall system economics are uncertain and are more sensitive than the other primary S&T concepts to the economics of feed material from reprocessing LWR spent fuel.
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From page 86... ...
/fuel cycle. Paper presented to STATS Symposium, Washington, D.C., January 13-14.
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