CANDU REACTORS WITH THORIUM FUEL CYCLES

Transcription

1 CANDU REACTORS WITH THORIUM FUEL CYCLES Hopwood, J.M., Fehrenbach, P., Duffey, R., Kuran, S., Ivanco, M., Dyck, G.R., Chan, P.S.W., Tyagi, A.K. and Mancuso, C. Atomic Energy of Canada, Mississauga and Chalk River, Ontario, Canada 1. Introduction CANDU technology provides unequalled flexibility for the use of different fuel cycles. Its inherent high neutron economy, fuel channel design, on power refuelling capability and simple fuel bundle design allow for the optimisation of an assortment of different nuclear fuel-cycles. The suitability of CANDU technology for alternative fuel cycles, such as thorium and MOX, was first recognized during the conception of this unique technology [1]. To this end Atomic Energy of Canada Ltd. (AECL) has carried out theoretical and experimental investigations of thorium and other fuel cycles over the years in order to develop the expertise necessary to exploit them should market conditions and availability change [2]. However, because of the relative abundance and low cost of uranium, to date, there has not been a financial incentive to accelerate development of such fuel cycles. Today, approximately half a century after the first deployment of atomic energy for peaceful purposes, a number of factors have converged and added momentum to the possible exploitation of alternative fuel cycles involving thorium. For the first time in decades the price of uranium has increased substantially, having more than tripled over the last three years [3]. Although the impact of this increase on the cost of running nuclear plants is not substantial, neither is it insignificant. Uranium resources are not distributed throughout the world ubiquitously. Countries like India, China and Turkey, which currently have expanding economies, and a corresponding need for more electricity, do not have abundant uranium resources. However, they have a great abundance of Thorium and have a natural desire for self-reliance in energy supply. Only a few percent of the material in nuclear fuel is actually converted into energy, with the used fuel containing over 95% of the original energy content. This has long been recognized but, again, with the relative worldwide abundance of uranium there has not been financial incentive to exploit this potentially valuable resource. However, there is interest in reducing the volume of hazardous material in spent fuel, such as Plutonium, to make it easier to dispose of. These various economic and political drivers; the increasing cost of uranium, the abundance of thorium in countries with expanding economies and the desire to reduce the amount of long lived radioactive material in used fuel, have converged and motivated us to investigate a fuel cycle that combines Thorium with Plutonium. This paper will show that using fuel based on these fissile and fertile elements can be economical and can greatly extend the useful life of nuclear fission technology for production of electricity. We also examine the practical aspects of conversion of a CANDU reactor from operation with a 235 U fuel cycle to one operating with a Pu-Th fuel cycle. Necessary design changes can be implemented during a refurbishment outage or built into the design of a new plant, to make a mid-life conversion even easier. 2. Thorium Fuel Cycles There are a number of possible fuel cycles that employ thorium, in the form of ThO 2. Since Th is fertile, not fissile, a fissile material is needed to start the process. This fissile isotope is typically 235 U, 233 U (which is bred from an earlier Thorium cycle) or 239 Pu. The advantages of a thorium fuel cycle are quite well known. It is 3 to 4 times as abundant as uranium, and easy to mine, it being very abundant in sands found in India and China. ThO 2 is chemically more stable and has a higher radiation resistance than UO 2 [4]. ThO 2 is relatively inert and does not oxidize like UO 2. UO 2 easily oxidizes to U 3 O 8 and UO 3. This simplifies the long-term storage of thorium-based fuel or spent fuels since there is no problem of further oxidation. ThO 2 has a higher thermal conductivity and lower co-efficient of thermal expansion than UO 2. Thus ThO 2 based fuels have better in-pile performance than UO 2 based fuel. The lower atomic number for thorium, (Z=90) versus (Z=92) for uranium, reduces significantly the build up of heavy transuranic isotopes, which reduces the amount of long-lived radioactive isotopes in spent fuel.

2 For thermal neutrons, the absorption cross section of 232 Th is 7.4 barns and absorption cross-section of 238 U is 2.7 barns. Therefore, a higher conversion from fertile 232 Th to fissile 233 U is possible than for 238 U (fertile) to 239 Pu (fissile). Thus, 232 Th is a better fertile material than 238 U in a thermal reactor although the opposite is true in a fast reactor. However, a thermal reactor is much easier to build and control than a fast reactor and the fertile properties of 232 Th allow for the possibility of developing a breeder cycle in a thermal reactor. There is an obvious synergy between the generation of 239 Pu with a fast breeder reactor together with the generation of 233 U using a 232 Th/ 239 Pu thermal reactor fuel cycle. The 232 Th/ 239 Pu fuel cycle is actually relatively proliferation resistant. The 239 Pu is destroyed in this fuel cycle and, in the process of generation of 233 U from 232 Th, some 232 U is also formed. The decay chain of the latter includes strong gamma emitters and complicates the extraction of fissile material from used fuel. 2.1 Nuclear Physics Modeling A simple homogenous fuel was chosen for this study. The fuel is a slightly modified AECL CANFLEX fuel in which all of the elements, with the exception of the central one, are a homogenous mixture of PuO 2 and ThO 2. The central fuel element consists of a burnable poison; 60% by volume Dy 2 O 3 in ZrO 2. Twelve CANFLEX fuel bundles, which are approximately 50 cm long and 10 cm in diameter are placed end-to-end in a pressure tube and the arrangement of the fuel pencils in the 43- element bundle are shown in Figure 1. Modelling was carried out using WIMS-AECL version , together with nuclear cross-section data generated for thorium fuel cycle analysis. Figure 1: CANFLEX Fuel Bundle with PuO 2 /ThO 2 Mixture and Burnable Poison in Central Element Studies were carried out over a range of different Pu loadings but a loading of 3.4% PuO 2 by volume (3.19 wt% Pu) with the balance (96.6 Vol%) ThO 2, was chosen for this study. There is a trade off between fuel burn-up and fuel management in a CANDU reactor. More plutonium gives higher burn-up but can complicate on-power fuelling. 3.4% PuO 2 was chosen as a conservative loading of thorium for a CANDU reactor that offers improved burn-up and simple fuel management. Optimization of Pu loading is in progress. For 3.4% PuO 2 t he exit burnup for this fuel is approximately 21 MWd/kg, for a CANDU 6 reactor with no adjuster rods. Net plutonium destruction is shown in Figure 2. Each fresh bundle contains approximately 339 g of 239 Pu. Upon exit, each bundle contains only 53 g of 239 Pu. In addition, upon exit, each fuel bundle also contains 141 g of 233 U, which has been bred from 232 Th. Our models also show that the Pu-Th core allows great flexibility in tailoring reactivity characteristics.

3 Figure 2: Plutonium Isotope Evolution as a function of Burn-up From the point of view of nuclear physics, it is easily possible to achieve good fuel burn-up in a oncethrough fuel cycle with a simple fuel bundle geometry and composition. 2.2 Fuel Cycle Costs The levelized cost of Th-Pu fuel for a CANDU 6 is estimated and is listed in Table 1. For comparison purposes, the fuel cost for a CANDU 6 with natural uranium fuel is also given. As the table shows, the fuel cost for the 3.4 % PuO 2 case is significantly lower than for natural uranium even with spent fuel disposal included in the cost. However, it should be mentioned that it is assumed for the purposes of this estimate that Pu is available at no cost. This is a reasonable assumption since plutonium, which is currently classed by the nuclear industry as an asset with zero value [5]. There are significant costs incurred in storing Pu. Table 1 Levelized fuel cycle cost for CANDU 6 with various fuel type Fuel Burn up GWd/t Levelized fuel cycle cost (mills/kwe-hr) * ThO 2-2.5%PuO ThO 2-3.4%PuO ThO 2-5%PuO Nat-UO * 1.0 mills/kwe-hr is added to include waste disposal costs in accordance with US Federal Regulations. This study also uses 10% for carrying charges All weights are in volume % of PuO 2 in ThO 2

4 3 Impact of Pu-Th Fuel Cycle on CANDU Reactor Systems There is no question that the CANDU reactor core design will enable optimisation of various different types of fuel cycles, as already mentioned. However, there are unique aspects to all fuel cycles. One question that has not been explored in detail is to what extent, if any, an operating CANDU reactor needs to be modified to accommodate a different fuel cycle. 3.1 Impact of Using Pu-Th Fuel instead of Natural Uranium Fuel To address subject we have taken an existing reactor design, the CANDU 6, that has been successfully operating in several countries for more than 20 years and which AECL continues to build today. We have examined in considerable detail the key CANDU process systems to establish whether a system: a b c d Requires no modification, Likely requires no modification but should be investigated in more detail to determine with certainty that no modification is required, Likely requires some modification Definitely requires some modification It is necessary to make certain assumptions. The CANFLEX fuel bundle with homogeneously mixed Th and Pu fuel will be designed to meet the safety and operating envelopes established for CANDU 6 with natural uranium (NU) fuel. The thermal and electrical output of the CANDU 6 reactor with thorium-based fuel will be identical to a CANDU 6 operating with NU. We have found that in general minimal hardware changes will occur for most systems and balance of plant (BOP) will remain unaffected. However, the reactor physics of a Pu-Th-based fuel will be different than for uranium-based fuel. These changes in reactor physics will have the following general impacts: Reactor dynamics will be different because a Pu-Th fuel will have a smaller delayed neutron fraction. Hence, the reactor s response to transients will be faster Some fission products are different and the distribution of other fission products is changed. This may have an impact on storage and disposal, although the impact is likely a positive one. There is an additional source of gamma radiation due to the production of 232 U. There is a potential impact of this on some systems and operating processes. Due to the 233 Pa (27.5 days), transient, fuel handling needs to be examined. In addition, long term cooling (reactor shut down cooling system) needs to be re-examined because of the lag in decay heat. 3.2 Impact on Process Systems and Operations Fuel bundles will be designed to optimize fuel utilization and enhance inherent safety characteristics. The thorium reactor will have equal or better physics characteristics relative to as built conditions, especially in areas like core void reactivity and fuel temperature coefficient. All systems and operating activities have been examined and the conclusions identified. Those that require study or which may require modification are shown in Table 2.

5 Table 2: Summary of Reactor Systems and Processes that will likely Require Modification because of the Impact of Pu-Th Fuel Cycle System/Operation Fuel Management and Fuel Handling New Fuel Transfer and Storage Modification Required* d d Comments All fuel bundles are identical and refuelling is done every shift; essentially continuous refuelling similar to a CANDU 6. For 3.4% Pu-Th case, a 2-bundle shift will need to be used, instead of the traditional 8-bundle shift, during the equilibrium fuelling period. Fuel will be in the reactor for a longer period than for a CANDU 6, hence, this is within the capability of the current fuelling machine. The equilibrium period covers 95% of reactor life. For other periods, the refuelling scheme will need to be developed and be consistent with the different reactor physics for this fuel cycle. Fuelling Machine cooling water system: Design will need to be examined to determine if the margins will encompass the new fuel design. Spent fuel has an additional gamma-emitting component because of 232 U. Hence, there is a potential impact on lifetime of elastomeric components. If the impact is significant then additional shielding may be needed. Fuelling rates, as mentioned, will change. Design modifications are required for a new fuel transfer and storage system due to conversion from natural uranium fuel to thorium base fuel, since the driver fuel ( 239 Pu) has a stronger radiation field compared to natural uranium (background radiation). The container for the fuel bundles, fire protection system and storage arrangement will be different. Fuel Fabrication d Thorium is as almost as easy to work with as natural uranium but the presence of 239 Pu will require extra precautions and special handling during fabrication. Spent Fuel Bay c This system will require a thorough examination because of the presence of 233 Pa and 232 U in the spent fuel. The former may have an impact on the heat load that the Fuel Bay Heat Exchangers must dissipate and the latter on shielding requirements. Reactivity Control Units c Because there is a smaller delayed neutron fraction for Pu-Th fuel this system needs to be analysed in detail to determine if changes will be needed. We are in the process of assessing the reactivity worth of the control and shutdown systems to determine if any such modification is required. * See Section 3.1 for Definitions: c = Likely some modification; d = Definite modifications required. There are other systems that we believe likely do not require modification but that should be analyzed in further detail to make sure that this preliminary assessment is correct. Examples of such systems are: The reactor shutdown systems, SDS1 (shut off rods) and SDS2 (liquid Gd-nitrate injection) system dynamics need to be reviewed to make certain that the smaller delayed neutron fraction for the Pu- Th cycle is not an issue. See also my comment on the Reactivity Control Units. Containment Cooling systems need further analysis to ensure that 233 Pa decay is fully accounted for

6 Shutdown cooling system should be further analyzed to make sure that the margins are sufficient to account for 233 Pa decay. Emergency Coolant Injection system initiating parameters may need to be updated because of the different thermo-mechanical properties of Pu-Th fuel; greater thermal conductivity for example. All design basis accidents need to be re-analyzed to take into account the different properties of Pu- Th fuel. There are some characteristics of the fuel that may be beneficial from the point of view of analysis of accident scenarios, in particular the higher thermal conductivity and melting point of the fuel. 4 ACR-1000 The assessment of the impact of a Pu-Th fuel cycle on CANDU 6 systems applies equally well to AECL s ACR-1000 design. The reactor core of the latter is different from a CANDU 6, since it contains light water in the primary heat transport system and the pressure tubes are closer together. Hence, the reactor physics is slightly different from a CANDU 6 and there are subtle differences in terms of the impact on systems. The safety cases are in the process of being reviewed in detail as with the CANDU 6. 5 Generation IV Reactors The synergy between thorium fuel cycles and recent initiatives to develop Generation IV reactors is quite apparent. The primary goals of Generation IV initiatives are to improve nuclear safety, improve proliferation resistance, minimize waste and natural resource utilization, and to decrease the cost to build and run nuclear plants. A Pu-Th fuel cycle addresses several of these issues, in particular proliferation resistance and minimization of waste. Gen IV reactors are not expected to be deployed until at least 2030 and there is uncertainty as to the availability and cost of uranium a quarter of a century from now. Hence, development of advanced reactors using different fuel cycles is also prudent. 6 Summary Our preliminary studies suggest that it is feasible and economic to operate a CANDU reactor with a homogeneous Pu-Th fuel with a variety of different fuel loadings. Although further optimization is required, investigation thus far suggest that a loading of approximately 3.4% PuO 2 in a ThO 2 lattice in a once-through cycle will result in improved fuel burn-up (21 MWd/kg) without complicating fuel management of an existing reactor. Changes to the design that have been identified are not onerous and can be built into a new design or added on during refurbishment or during normal operation (e.g. New Fuel transfer and storage areas). No complex seed/blanket fuel arrangement is required to operate such a reactor, since all of the fuel bundles are identical. With the exception of the central dysprosium element, all of the fuel pencils are also identical. Our results suggest that a relatively simple Pu-Th fuel cycle can be implemented in existing or future CANDU reactors without substantial changes to reactor design or operation. 7 References [1] Lewis, B. W., AECL Report 968, August, 1952 [2] a.) Critoph, E., S. Banerjee, F.W. Barclay, D. Hamel, M.S. Milgram and J.I. Veeder, Prospects for Self-Sufficient Equilibrium Thorium Cycles in CANDU Reactors, Proceedings of ANS 1975 Winter Meeting, San Francisco, 1975 November b.) Boczar, P. G., Chan, P.S.W., Dyck, G. R. and Buss, D. B., Recent Advances in Thorium Fuel cycles for CANDU Reactors, AECL-CONF-091, November c.) Boczar, P.G., Chan, P.S.W., Dyck, G.R.,Ellis, R.J., Jones, R.T., Sullivan, J.D., Taylor, P., Thorium Fuel- Cycle Studies for CANDU Reactors, Presented at the IAEA Advisory Group Meeting on Thorium Fuel Utilization: Options and Trends, Vienna, Austria,28-30 September 1998 (and references therein) [3] Mohr, P.M., The Scotia Capital Commodities Report, May 30, 2006 [4] IAEA-TECDOC-1450, Thorium fuel Cycle Potential benefits and challenges, IAEA, Vienna, May 2005 [5] R.M. Consultants (prepared for UK Committee on Radioactive Waste Management), Position Paper on Plutonium, June 2005 (also CoRWM Document # 1281).