Improving Conversion Ratio of PWR with Th-U 233 Fuel Using Boiling Channels

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1 67 Reactor Physics and Technology I (Wednesday, February 12, :30) Improving Conversion Ratio of PWR with Th-U 233 Fuel Using Boiling Channels M. Margulis, E. Shwageraus Ben-Gurion University of the Negev, P.O.B. 653 Beer-Sheva 84105, Israel INTRODUCTION The goal of this study is to develop a Pressurized Water Reactor with improved fuel utilization. This can be achieved through the use of Th-U 233 fuel cycle. In recent studies, a number of options for employing Th- U233 fuel cycle in a typical PWR core were investigated (Kotlyar and Shwageraus (1), Volaski et. al. (2) and Boldova and Fridman (3) ). These studies focused on achieving breeding through optimization of fuel geometry, pins arrangement within the assembly and fuel composition. The results indicated that creating a self-sustainable Th-U 233 PWR fuel cycle requires transition from homogeneous to heterogeneous configuration, a fissile rich (seed) zone and the fertile (blanket) zone to improve the neutron absorption by Th 232. However, unavoidable consequence of this heterogeneous arrangement would be much higher than typical power peaking, leading to higher fuel temperature and heat fluxes in the high power locations. As a result, the safety constraints will inevitably limit the achievable core power density. Although the mentioned studies (1, 2, 3) presented designs that potentially could achieve conversion ratio advantage of the concept. This study presents the first attempt to optimize a high conversion ratio PWR without reducing the core power density below nominal 104 W/cc. The main idea is to create an intermediate spectrum by allowing partial two phase flow regime inside the seed zones. This in turn reduces moderation and improves the breeding performance. In addition, large latent heat of coolant phase transition allows removing much higher power from the same volume. Boiling heat transfer is also more efficient as long as bubbly flow regime is maintained. Although, in conventional PWRs, limited sub-cooled boiling is allowed, it is an undesired phenomenon because it leads to preferential deposition of crud, precipitation of soluble boron and resulting axial power shape distortion. In high conversion PWRs (1, 2, 3) however, the excess core reactivity is very small and may allow operation entirely without soluble boron using only mechanical (control rods) shim. In such case, boiling may no longer be an issue as long as the operating and safety envelope of the core is preserved. Some thermal hydraulic design issues however may still need to be addressed requiring further studies. The fuel center line temperature (T CL ) could remain above the limiting value of no melt incipient under anticipated transient conditions. A possible solution to reduce T CL, is to increase the number of seed fuel pins by reducing their lattice pitch without changing the original seed region volume fraction. Larger number of seed pins would reduce the linear heat generation rate and the heat flux. This modification however, implies that seed and blanket pin lattices would no longer have the same pitch. The high conversion partially boiling PWR core design would have to meet the following design objectives and constraints. The core design should have conversion ratio greater than unity to assure sustainable fuel cycle. Reasonably long fuel cycle length of at least one year of continuous operation should be maintained. Core safety margin at steady state should not be compromised and maintained similar to the conventional LWR designs. In addition, despite the boiling in the seed channels, the outlet core average coolant enthalpy should be the same as in a typical PWR. In other words, the coolant must still be sub-cooled at the core outlet after two-phase flow from the high power seed channels is mixed with a single phase coolant from the lower power blanket channels. Finally, the coolant flow pressure losses should not require replacement of the existing main recirculation pumps. The 2D analysis performed here, confirms that such PWR (with boiling channels) design is feasible in principle.

2 HIGH-CONVERSION Th-U233 FUEL ASSEMBLY OPTIMIZATION A number of modifications were made to the original reference design (2) to allow boiling regime in the seed region, while keeping fissile inventory ratio (FIR) above unity: 1. Blanket size optimization (section 1). 2. FIR as a function of seed size (section 2). 3. Blanket fuel material - ThO 2 vs. ThH 2 (section 3). The end of life (EOL) was chosen to be 900 days and requires adjustment of the initial fuel assembly enrichment. For each studied case, we examined the reactivity as a function of time behavior in order to determine the discharge burnup. This study relied on two computational tools in order to perform the optimization. The first is BOXER code, which is a part of the ELCOS package for steady state simulation of light water reactor cores, Paratte et al. (4). The second is SERPENT, which is a continuous energy Monte Carlo (MC) neutron transport code with burnup (BU) capabilities, Leppänen (5). 1. Blanket size optimization The effect of reduced moderation on breeding performance in PWRs was studied in detail by Utoinen et al. (6). In their study, reduced moderation resulted in better breeding performance, which was also observed here. Introducing boiling in the seed region improves the breeding performance (i.e. higher FIR at EOL) compared to zero void value. This is because higher void fraction increases the leakage rate into the blanket and somewhat hardens the spectrum also increasing the resonance absorption in Thorium. The blanket region size in the reference model (2) was selected to maximize the capture of neutrons leaking from the seed region. Due to the fact that the operating conditions in the seed region were changed, the blanket region had to undergo a modification in order to improve neutron captures. Moreover, we investigate the use of two possible fuel forms for the blanket region - ThH 2 and ThO 2. Each fuel form presents potential advantages. On one hand, admixing moderator material to the fuel in the ThH 2 case also improves the resonance captures in Th because of the lower self-shielding, ultimately resulting in better breeding performance. In addition, thermal conductivity of hydrides is much higher than in the oxide fuel. On the other hand, ThO 2 has a much higher melting point and its performance as a nuclear fuel is much better understood. A model of seed assembly surrounded by a large blanket zone was created in SERPENT. We then examined the cumulative neutron captures in the blanket as a function of distance from the seed, while changing the seed coolant void fraction. The blanket region size was selected at the point where the cumulative captures reached 95% of total, as shown in Figure 1 for ThO 2 and ThH 2 blanket materials with the latter being much smaller. The results in Figure 1 are not surprising due to the less efficient neutron moderation in ThO 2 in comparison to ThH 2. The less expected result is that the void fraction in the seed has remarkably small effect of the capture rate distribution in the blanket. 2. FIR vs. seed assembly size and pin dimensions In the next step, different seed assembly arrays of N N pins were examined. The seed pins were placed in the assembly center and surrounded by the blanket with dimensions determined in the previous section. The core power density was assumed to be fixed at 104 W/cc and have different void fractions in the seed, varied from 10% to 30%. The fissile material content was adjusted to achieve the same cycle length of 300 days (or 900 days total in core residence time). Furthermore, the effect of the seed pin geometry modifications on FIR was also examined. The reference seed pin radius selected to be that of a typical PWR, cm. Then, the pin radius was increased by 10% and 20%, while the cell pitch remained fixed at 1.26 [cm] (although it may need to be reduced to reduce the linear heat generation rate as discussed earlier). A setup example is shown in Figure 2. The results for the 2D simulations obtained from BOXER are shown in Figures 3 and 4.

3 Figure 1. Cumulative fraction of neutron capture in Th for ThH 2 and ThO 2 Seed Pin Blanket Pin Figure 2. Quarter of a high-conversion Th-U233 fuel assembly Figure 3. FIR at EOL as a function seed size, ThO 2 fuel, R s =0.4095, [cm] The results presented in Figure 3 suggest high sensitivity of FIR to the seed pin dimensions and coolant void fraction. Reduced moderation (higher void fraction and larger pin diameter) improves the breeding significantly. The seed region size (total number of seed pins) also has significant effect on the breeding. Reducing the seed region size improves FIR but it also requires higher initial fissile content, as can be seen in Figure 4 for the void fraction of 20% and different fuel types. Smaller seed region also implies smaller relative volume fraction of the seed (Figure 5) and thus higher power peaking, which will most definitely challenge the core thermal design. Seed fuel pin radius of , average void fraction of about 20% and

4 15x15 to 17x17 seed array size seem to provide reasonable combination of parameters to achieve FIR~1 at the reference core power density. Figure 4. Initial fissile material content as a function of seed size with void fraction of 20% (Left ThH 2, Right ThO 2 ) Figure 5. Relative seed region volume fraction vs. seed size 3. Blanket fuel material - ThO 2 vs. ThH 2 The calculation in the previous section suggested that ThO 2 fuel in the blanket would be preferred over ThH 2 fuel in order to achieve FIR above unity at EOL. However, the use of hydride fuel form improves the moderation and creates a much more thermalized neutron spectrum in the blanket region. This, in turn, should increase the neutron absorption in thorium and reduce the relative blanket volume fraction, consequently allowing tolerating higher seed region power density. Figure 1 clearly demonstrates that the blanket thickness should be only cm as opposed to 25.2 cm in ThH 2 and ThO 2 cases respectively. Although, high thermal flux increases the probability of neutron capture in the thorium, it is also increases the burnup rate of U 233 once it is accumulated in substantial quantities resulting in less efficient breeding performance on the overall balance. Furthermore, absorption in hydrogen contained in the fuel is also nonnegligible which also negatively affects breeding. Therefore, it can be concluded that ThO 2 is a preferable choice of blanket fuel form the breeding performance point of view. This assertion however will have to be confirmed by thermal hydraulic analysis in which power density distribution considerations may prove otherwise. CONCLUSIONS This work explored the basic neutronic design considerations of a PWR seed-blanket fuel assembly with boiling channels in order to achieve FIR above unity at EOL. More precisely, the effect of various parameters, such as void fraction in the seed region, seed size (number of seed pins) and seed pin diameter, on breeding was investigated. The optimization process also included selection of the blanket size

5 and selection of the blanket fuel material. The results of the optimization suggest that certain combination of studied design parameters (void fraction, seed region size and pin radius) would indeed result in net breeding. It was found that, in general, reduced moderation improves the breeding performance. Such reduced moderation can be achieved through combination of void fraction on the order of 20% and larger than standard pin diameter. The calculations performed in this work were based on the assumption of a uniform axial void distribution, which would clearly not be the case. Therefore, 3D fuel assembly analysis coupled with thermal hydraulic feedback would be required to confirm feasibility of this design concept. AKNOWLEGMENTS This work was sponsored by research grant from the US-Israel Binational Science Foundation Grant # REFERENCES 1. D. Kotlyar, E. Shwageraus, Neutronic Optimization in High Conversion Th-233U Fuel Assembly With Simulated Annealing, PHYSOR, Knoxville, TN USA (2012) 2. D. Volaski, E. Fridman, E. Shwageraus, Thermal Design Feasibility of Th-233U PWR Breeder, Proc. Global, Parish, Frnace, p (2009) 3. D. Boldova, E. Fridman, High Conversion Th-U233 Fuel Assembly for Current Generation of PWRs, Before submission 4. J. Parette, K. Foskolos, P. Grimm, C. Maeder, Das PSI Codesystem ELCOS zur Stationaren Berechnung von Leichtwasserraktoren, Proc. Jahrestagung Kernetchink, Travemünde, Germany p. 59, (1998) 5. J. Leppänen, Development of a New Monte Carlo Reactor Physics Code, D.Sc. Thesis, Helsinki University of Technology, VTT Publications 640 (2007) 6. V.O. Uotinene et al, Thecnical Feasibility of Pressurized Water Reactor Design With a Low Water Volume Fraction Lattice, EPRI NP-1833 Final Report (1981)