Annals of Nuclear Energy

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1 Annals of Nuclear Energy 35 (2008) Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: Core physics analysis of 100% MOX core in IRIS Fausto Franceschini a, *, Bojan Petrović b a Westinghouse Electric Company LLC, Science and Technology Department, Pittsburgh, PA 15235, USA b Georgia Institute of Technology, Nuclear and Radiological Engineering, G.W. Woodruff School, Atlanta, GA , USA article info abstract Article history: Received 20 November 2007 Accepted 11 March 2008 Available online 28 April 2008 International Reactor Innovative and Secure (IRIS) is an advanced small-to-medium-size (1000 MWt) Pressurized Water Reactor (PWR), targeting deployment around Its reference core design is based on the current Westinghouse UO 2 fuel with less than 5% 235 U, and the analysis has been previously completed confirming good performance for that case. The full MOX fuel core is currently under evaluation as one of the alternatives for the second wave of IRIS reactors. A full 3-D neutronic analysis has been performed to examine main core performance and safety parameters, such as critical boron concentration, peaking factors, discharge burnup, reactivity coefficients, shut-down margin, etc. In addition, the basis to perform load follow maneuvers via the Westinghouse innovative strategy MSHIM has been established. The enhanced moderation of the IRIS fuel lattice facilitates MOX core design, and all the obtained results are within the operational and safety limits considered thus confirming viability of this option from the reactor physics standpoint. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction International Reactor Innovative and Secure (IRIS) is an advanced small-to-medium-size (1000 MWt) PWR with integral primary system (Carelli, 2003; Carelli et al., 2004; Carelli and Petrović, 2006). It relies on combining proven and worldwide accepted LWR technology with innovative engineering. While the use of proven technology enables IRIS to target deployment already around 2015, its innovative solutions address all the key requirements for the next generation of reactors. Consistent with its aggressive development and deployment schedule, the first IRIS reference core design assumes current, licensed fuel technology, i.e., UO 2 fuel with less than 5% 235 U enrichment. However, more advanced core configurations are being examined in parallel for the second wave of IRIS reactors (Franceschini and Petrović, 2004). These studies which include evaluation of the full MOX core capability are presented in this paper. 2. IRIS reference core design 2.1. UO 2 core design The IRIS core includes 89 fuel assemblies employing square lattice, standard layout (with 264 fuel rods, 24 locations for control rods and the central location for instrumentation), and a standard fuel rod size. The IRIS fuel design considered in this study incorporates standard Westinghouse fuel features, except for the: (a) lattice pitch, which has been increased from to 0.523, thus increasing the moderator-to-fuel volume ratio V m /V f from 1.7 for Standard Westinghouse Fuel to 2.0 for IRIS, and (b) plenum volume, which has been roughly doubled, compared to present PWRs. The increased moderation leads to better fuel utilization thus enabling cycle length longer than with a Standard PWR lattice, while the larger plenum volume eliminates internal rod pressure issues and enables higher fuel burnup. A reference 2-batch core design and reload strategy have been previously devised for UO 2 fuel, as depicted in Fig. 1a. Erbia (Er 2 O 3 ) is employed as the integral burnable absorber (IBA), at four concentrations (erbia 1 through erbia 4 in Fig. 1a). Erbia has a slower depletion rate than other burnable absorbers employed in PWRs (Gd in form of gadolinia, Gd 2 O 3,or 10 B in ZrB 2, IFBA), which makes it suitable for reactivity and power distribution control over the long cycle. As the baseline for comparison, the two-batch UO 2 core configuration satisfies all operational and safety constraints, and provides a cycle length of about 3.5 years MOX core design * Corresponding author. Tel.: ; fax: addresses: FranceF@westinghouse.com, faustopitt@gmail.com (F. Franceschini). A MOX core design has been developed analogous to the UO 2 core design (Fig. 1b). The same 2-batch reloading strategy has been used, but the fuel rod size has been reduced from standard to thinner rods. The thinner fuel rods further enhance the /$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.anucene

2 1588 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) Twice Burned Feed Once Burned Feed Feed Once Burned Erbia Erbia Erbia 3 Feed Once Burned Feed Once Burned Feed Once Burned Erbia Erbia Erbia 3 Once Burned Feed Once Burned Feed Feed Erbia Erbia Erbia 4 Feed Once Burned Feed Once Burned Once Burned Erbia Erbia 2 Feed Feed Feed Once Burned Erbia Erbia Erbia 4 Burned Burned fuel Once Burned Once Burned Feed Fresh fuel Erbia 1 Initial U-235 w/o and Erbia content Twice Burned Feed Once Burned Feed Feed Once Burned IFBA IFBA IFBA 3 Feed Once Burned Feed Once Burned Feed Once Burned IFBA IFBA IFBA 3 Once Burned Feed Once Burned Feed Feed IFBA IFBA IFBA 4 Feed Once Burned Feed Once Burned Once Burned IFBA IFBA 2 Feed Feed Feed Once Burned IFBA IFBA IFBA 4 Burned Burned fuel Once Burned Once Burned Feed Fresh fuel IFBA 1 Initial fissile Pu w/o and IFBA content Fig. 1. (a) IRIS reference 2-batch UO 2 core (quarter core map) and (b) IRIS reference 2-batch MOX core (quarter core map). moderator-to-fuel volume ratio to 2.3 with respect to 1.7 of typical PWR lattice or 2.0 of UO 2 IRIS lattice. This softens the spectrum making it closer to UO 2 spectrum, and facilitates satisfying safety requirements, which are otherwise challenged in current PWRs with MOX by safety constraints related to reactivity feedback, control rod worth, etc. Additionally, the fissile content has been increased from 4.95% 235 Uto5.5% fissile Pu ( 239 Pu Pu) to obtain similar cycle length as in the UO 2 core. Radial distribution of burnable absorbers has been kept qualitatively similar, but ZrB 2 (IFBA) instead of erbia is employed, as discussed later.

3 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) Spectral differences between UO 2 and MOX fuel and the effect of enhanced moderation in IRIS lattice 3.1. Spectral difference The spectral differences between UO 2 and MOX fuel have been investigated by 2-D transport theory, single fuel assembly calculations. The fissile content chosen for the assembly study is the same as in the respective equilibrium core design, namely 235 U at Table 1 Moderator-to-fuel volume ratio (V m /V f ) for UO 2 and MOX in IRIS and standard PWR lattice UO 2 fuel V m /V f IRIS enhanced moderation lattice Standard PWR lattice MOX fuel V m /V f 4.95 w/o for the UO 2 fuel and fissile Pu at 5.5 w/o for the MOX fuel. Fuel dimensions and lattice type for IRIS are as indicated above, yielding V m /V f of roughly 2.0 for UO 2 and 2.3 for MOX fuel. In order to assess the impact of the enhanced moderation, calculations are repeated using Standard Westinghouse Fuel lattice parameters with V m /V f of 1.7, and the results are compared to those obtained for IRIS. Since the 1.7 V m /V f value is representative of typical PWR V m /V f values, the Standard Westinghouse Fuel lattice is referred to as standard PWR fuel in the course of this study. The cases analyzed and the associated moderator-to-fuel volume ratios are summarized in Table 1. The spectrum for the different cases is shown in Fig. 2 at BOC and Fig. 3 at 40 GWd/tHM. The MOX spectrum is still substantially harder than the UO 2 spectrum for all the analyzed cases; however, the IRIS increased moderation approach yields noticeably softer spectrum than that in the standard fuel. As the fuel depletes, UO 2 and MOX spectra become closer (Fig. 3); this is mainly due to actinide buildup in UO 2 fuel and Pu depletion in the MOX fuel. On the Flux per unit lethargy IRIS w UO 2 (Vm/Vf~2.0) Std. PWR w UO 2 (Vm/Vf~1.7) IRIS w MOX (Vm/Vf~2.3) 0.02 Std. PWR w MOX (Vm/Vf~1.7) E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Log Energy (ev) Fig. 2. Spectrum in UO 2 and MOX for IRIS vs. standard PWR lattice (BOC) Flux per unit lethargy IRIS w UO 2 (Vm/Vf~2.0) Std. PWR w UO 2 (Vm/Vf~1.7) IRIS w MOX (Vm/Vf~2.3) 0.02 Std. PWR w MOX (Vm/Vf~1.7) E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Log Energy (ev) Fig. 3. Spectrum in UO 2 and MOX for IRIS vs. standard PWR lattice (40 GWd/tHM).

4 1590 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) other hand, the difference in spectrum between IRIS and standard PWR fuel is larger at higher burnup, which reflects the cumulative effect of the spectral softening produced by the enhanced moderation. The implications of the MOX harder spectrum on neutron thermal absorbers behavior are discussed next. The benefits of the enhanced moderation are examined in Section Neutron absorber behavior in MOX fuel As a result of the harder spectrum, the worth of thermal neutron absorbers (soluble, integral and inside the control rodlets of the control banks) is reduced in the MOX core with respect to the UO 2 core. Therefore, adequate countermeasures are required to maintain the core control capability in the full-mox core. Consistent with their lower worth fuel burnable absorbers (BAs) in MOX deplete at a slower rate than in UO 2. The lower worth is partially compensated by the lower excess reactivity in the MOX fuel but the slower depletion rate may lead to a substantial endof-cycle reactivity penalty if burnable absorbers are not adjusted. The approach taken in IRIS is to use IFBA in the MOX core, in place of erbia which is employed in the UO 2 design. Due to the larger absorption cross-section of IFBA with respect to erbia, and its relatively large concentration, low soluble boron concentration is obtained in the MOX design as in the UO 2 design, notwithstanding the reduced soluble boron worth in the MOX spectrum. Moreover, the slower depletion rate of 10 B in the MOX spectrum extends the IFBA reactivity control capabilities to a higher burnup than in UO 2, while reducing the power swing following IFBA consumption. The smoother depletion of IFBA in the MOX spectrum can be appreciated in Fig. 4, comparing K-inf vs. BU of respectively MOX (solid line) and UO 2 (dotted line) fuel assemblies with relatively high IFBA content. Fig. 5 compares the rod worth of B 4 C and Ag In Cd banks for the IRIS MOX (solid lines) and UO 2 (dotted lines) fuel assemblies. The rod worth in the MOX design is considerably lower. In order to offset this rod worth reduction, more absorptive materials need to be employed in the control rodlets of the MOX design with respect to their counterpart in the UO 2 core, with more details to be given in Section Benefits of IRIS enhanced moderation The softening of the neutron spectrum fostered by the IRIS enhanced moderation can be appreciated in Fig. 6, showing the ratio IFBA-UO2 IFBA-MOX 1.10 K-inf Fig. 4. K-inf vs. burnup of IFBA fuel in UO 2 and MOX. 60% 55% B4C-UO2 Ag-In-Cd UO2 50% B4C_MOX Ag-In-Cd_MOX Rod Worth (%) 45% 40% 35% 30% 25% 20% Fig. 5. Rod worth vs. burnup in UO 2 and MOX (selected materials).

5 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) MOX - 40 GWd/tHM 1.6 Normalized Flux Ratio UO 2-40 GWd/tHM MOX - 0 GWd/tHM UO 2-0 GWd/tHM E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Log Energy (ev) Fig. 6. Ratio of IRIS to standard PWR lattice flux in UO 2 and MOX fuel. of normalized fluxes for IRIS and standard PWR lattice with either MOX or UO 2 fuel as a function of the energy. These factors in the thermal range are between 1.26 and 1.37 for MOX, and between 1.58 and 1.76 for UO 2. This may be interpreted that, due to increased moderation, flux in the thermal range for IRIS lattice is 30% softer for the UO 2 fuel and over 60% softer for the MOX fuel with respect to a standard PWR lattice. This spectrum softening improves neutron economy for fuel enrichments and discharge burnups typical of commercial PWRs. Already in the UO 2 core this translates into a 5% increase in the cycle length, which yields significant economic benefits (Franceschini, 2007). However, the benefits of the enhanced moderation for the MOX core are of even greater importance. The first beneficial effect is the increased fuel utilization (discharge burnup), as for the UO 2 core, but of a larger magnitude, gained from the further enhanced moderation in IRIS MOX fuel with respect to standard PWR (V m /V f is increased from 1.7 to 2.3). The increase in fuel utilization is promoted by more efficient usage of the fuel fissile material, but also by more complete depletion of burnable absorbers in the softer spectrum. The relative importance of these contributions is illustrated in Fig. 7, showing the reactivity of MOX fuel with and without IFBA, for IRIS and a standard PWR. The fuel containing IFBA features relatively high 10 B concentration, needed to control reactivity over the long IRIS cycle. The estimated K-eff corresponds to the K-inf of the assembly reduced to account for the neutron leakage. Although the predictions are only qualitative, the assembly study shows a significant improvement in fuel utilization as a consequence of the increased moderation. For example, one can compare the burnup corresponding to k-eff = 1 for different fuel types. Moreover, the burnable absorber depletion is considerably more effective than it would have been in a standard PWR fuel lattice, where the high concentration of BA proposed in this study would lead to unacceptable penalty on cycle length. The enhanced burnable absorber depletion of IRIS expands the range of BA types and loadings that can be employed, ultimately leading to more flexible operation. Namely, it allows incorporating heavy BA loading to obtain low soluble boron concentration in the MOX core as well as in the UO 2 core, with benefits from reduced primary system corrosion and volume of liquid wastes, and more favorable reactivity coefficients. Achieving low soluble boron concentration with MOX core is more difficult than with UO 2 due to the generally reduced boron worth in the harder spectrum. However, due to the enhanced moderation, the reduction of boron IRIS- No IFBA Estimated K-eff Std.- No IFBA IRIS- IFBA Std.- IFBA Fig. 7. K-eff of MOX fuel assembly with and without IFBA (IRIS vs. standard PWR lattice).

6 1592 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) worth is mitigated in IRIS, facilitating the low soluble boron concentration. Another important beneficial effect of the enhanced moderation, and of the increased lattice pitch, concerns the worth of the neutron absorbers contained in the control rodlets of the control banks. Due to the IRIS increased pitch, the space available for the guide tubes of the control rods is larger, ultimately enabling increased diameter of the neutron absorber pellets contained in the control rodlets. The effect of the larger pellets favorably combines with the increased effectiveness of neutron absorbers due to the enhanced moderation to yield a higher control rod worth. Rod worth in percent is calculated as: CRworth ¼ 100% kunrodded 1 k rodded 1 Fig. 8 shows the control rod worth of a MOX fuel assembly for IRIS and standard PWR fuel lattice with either B 4 C or Ag In Cd rodlets. IRIS control banks have larger worth than standard PWR banks for the two analyzed absorber types, with more significant increase at higher burnup. For the reference two-batch reloading scheme, once-burned fuel assemblies (with higher burnup) are mainly placed under the control rods, with the corresponding larger rod worth increases (right side of Fig. 8). The benefit of the enhanced moderation lattice on rod worth can be exploited to ease the transition when moving from the UO 2 to full-mox core and to improve the reactor control capability during load follow maneuvers. 4. Core analysis 4.1. Methods used and analyses performed Westinghouse standard core physics analytic tools have been used for this analysis, including the PHOENIX and PARAGON codes for 2-D fuel assembly analysis and cross-section generation and the ANC code for 3-D core analysis (Nguyen et al., 1988; Ouisloumen et al., 2004; Liu et al., 1986). Analysis of several successive cycles starting with cycle 1 and leading to an equilibrium cycle has been performed for both MOX and UO 2 cores. All main reactor physics parameters have been evaluated in each case and compared, including the critical boron concentration, discharge burnup, peaking factors (F DH,F z,f q ), reactivity coefficients and shut-down margin. A preliminary assessment of the MOX core design load follow capability has also been undertaken Results Critical soluble boron concentration and reactivity coefficients The critical soluble boron concentration (CBC) for the equilibrium UO 2 and MOX cores is shown in Fig. 9. CBC is initially higher in the MOX core due to reduced effectiveness of neutron absorbers (burnable and soluble) in harder spectrum. The slope later becomes flatter due to conversion of fertile Pu isotopes into fissile Pu isotopes. The higher reactivity for MOX fuel at higher specific burnup (GWd/tHM) is offset when converted to equivalent full power days by its lower heavy metal loading due to the smaller fuel rods, resulting in approximately equal cycle length. Fig. 9 shows that CBC is below 1000 ppm for most of the cycle for both the UO 2 and the MOX core. This is rather low level of soluble boron, especially when considering the long cycle of operation of 3.5 years combined with the full MOX core, which significantly reduces the worth of boron in harder spectrum. In fact, as shown in Fig. 10, boron worth in the IRIS MOX core is roughly half of that in the UO 2 core. Beside the benefits from reduced primary system corrosion, the low soluble boron concentration results in lower (typically more negative) values of the reactivity coefficients, and in particular of the moderator temperature coefficient, MTC. Fig. 11 shows that MTC is already negative at HZP over the entire cycle for both the UO 2 and the MOX core. Since MTC becomes lower as the power level increases, it ensues that MTC in IRIS is negative over the entire power range, with benefits on safety and reactor control. The Doppler power coefficient, DPC, for the UO 2 and MOX cores is shown in Fig. 12. Two burnups representative of core depletion are chosen for the comparison, i.e., BOC and EOC with equilibrium xenon. The DPC is negative throughout the entire power range in both core designs, again benefiting safety and reactor control. The total power defect, TPD, defined as the reactivity difference between HZP and HFP, is shown in Fig. 13. As expected from the MTC and DPC trends, the TPD is also negative throughout the depletion, with the more negative values pertaining to the MOX core. Its behavior with burnup is similar to that of MTC, since its DPC component remains comparatively constant during the irradiation. Accordingly, the TPD, following an increasing trend during 35% B 4 C IRIS B 4 C Std. PWR Rod Worth (%) 30% 25% Ag-In-Cd IRIS Ag-In-Cd Std. PWR 20% Fig. 8. Rod Worth in MOX for IRIS and standard PWR fuel lattice.

7 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) CBC (ppm) Fig. 9. Critical soluble boron concentration for the equilibrium UO 2 and MOX IRIS cores. -3 Boron Worth (pcm/ppm) Fig. 10. Boron worth vs. burnup for the equilibrium UO 2 and MOX IRIS cores. 0-5 MTC (pcm/degf) Fig. 11. Moderator temperature coefficient at HZP for the equilibrium UO 2 and MOX IRIS cores. the first part of the irradiation, peaks at 6 GWd/tHM and decreases afterwards. The fact that the TPD is more negative in the MOX core implies that larger reactivity variations need to be compensated during

8 1594 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) UO 2 -BOC (eq. Xe) -6.0 DPC (pcm/%pow) MOX-BOC (eq.xe) UO 2 -EOC MOX-EOC Relative Power Fig. 12. Doppler power coefficient for the equilibrium UO 2 and MOX IRIS cores (BOC, EOC) TPD (pcm) Fig. 13. Total power defect vs. burnup for the equilibrium UO 2 and MOX IRIS cores. reactor transients, including load follow maneuvers and plant shut-down. The implications on the control bank design are discussed in Section Nuclear peaking factors and power distribution The radial and total peaking factors F DH and F q are shown in Figs. 14 and 15, respectively. As suggested by these figures, the harder spectrum in MOX fuel tends to make depletion-related changes more gradual and reduces the spatial relocation of peaking factors over the cycle, reducing the cycle-wise maximum values as well. For the same reason, the maximum F DH and F q in the MOX core occur at higher burnups than in the UO 2 core. In this specific case, the maximum value of the radial peaking factor F DH is lowered from 1.52 to 1.42 and the total peaking factor F q is reduced from 1.92 to The axial power distribution is frequently represented by a single parameter, axial offset. The axial offset, AO, is defined as the relative difference between the power produced in the top and bottom core halves, P t and P b : AO ¼ P T P B P T þ P B The variation in AO over the equilibrium cycle is shown in Fig. 16. The low absolute value of the AO and the minor variation shown with irradiation reflect a well distributed power sharing between the top and bottom halves of the core for both the reference UO 2 and MOX IRIS core designs. After the burnable absorbers are depleted, and soluble boron concentration is reduced, the more negative MTC of the MOX core typically leads to a bottom peaked power distribution with respect to the UO 2 core Control bank design for MSHIM operation The control bank design in IRIS is optimized to enable maximum flexibility in load follow operations and ensure safe shutdown of the reactor. The traditional approach to load follow is to compensate for the reactivity change due to power variation by adjusting the soluble boron concentration and moving a limited number of control rod banks. This leads to large volume of liquid effluent, in particular toward the end of cycle when the soluble boron concentration is low and it is difficult to dilute. To address this, Westinghouse has developed the innovative mechanical shim (MSHIM) strategy (Morita et al., 1988, 1991, 2003), where the control rods and control banks are designed to allow load follow using only control rods.

9 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) FdH Fig. 14. Radial peaking factor F DH vs. burnup for the equilibrium UO 2 and MOX IRIS cores UOx-eq.cycle 1.90 Fq Fig. 15. Total peaking factor F q vs. burnup for the equilibrium UO 2 and MOX IRIS cores AO (%) Fig. 16. Axial offset (%) vs. burnup for the equilibrium UO 2 and MOX IRIS cores.

10 1596 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) AO MA MC 2 MB SD2 SD1 3 MA AO M1 4 SD2 MD 5 MC M1 6 SD Fig. 17. Control bank placement in IRIS. Gray locations represent burned fuel assemblies (quarter core map). Table 2 Burnable absorber and reactivity worth of the control banks for UO 2 and MOX IRIS core design Control bank Number of RCCAs Rodlets absorber Worth (cycle-average, pcm) UO 2 MOX UO 2 MOX AO 8 Ag In Cd B4C MA 4 Ag In Cd Ag In Cd MB 4 Ag In Cd Ag In Cd MC 4 Ag In Cd Ag In Cd MD 4 Ag In Cd Ag In Cd M1 8 Ag In Cd B4C SD1 8 Ag In Cd B4C SD2 8 Ag In Cd B4C Banks MA through MD feature Ag In Cd pellets in both UO 2 and MOX but the diameter is different and smaller than that of the other banks. The boron for the AO, M1, SD1 and SD2 banks in MOX is enriched in 10 B at 70 a/o. Implementation of MSHIM in IRIS has been already demonstrated for the UO 2 core (Franceschini and Petrović, submitted for publication). In order to enable MSHIM operation, a design of control rods is required which is more complex than that of a standard PWR, necessitating black and gray control rods and axial power shaping bank, plus a certain set of requisites to be satisfied by each bank. For the IRIS MOX core, a control bank design complying with the requirements of MSHIM operation has been developed and is presented here. The complete assessment of the MOX core performance during MSHIM maneuvers is under way and will be reported in the future. In MSHIM two independently operable groups of control banks are employed during the maneuver to achieve simultaneous control of reactivity and power distribution. The reactivity control is provided primarily by the so called M banks, which consist of several control banks, including subgroups of light, gray banks. After insertion of this light group of banks is completed, heavier M banks may follow with a predetermined overlap to compensate the larger reactivity defects entailed by the more demanding maneuvers. Axial power distribution control is provided through a dedicated, heavy (high worth), AO bank. The SD group is not used to perform any load follow maneuver; its reactivity worth must be adequate to ensure that safe shut-down can be accomplished under all conditions. The control bank assignment in the IRIS UO 2 core is shown in Fig. 17; the rodlets absorber material and worth associated with each bank are shown in Table 2. The adopted bank design proved effective in allowing MSHIM maneuvers for the UO 2 core (Franceschini and Petrović, submitted for publication) on a set of challenging load follow scenarios, including those proposed by EPRI for advanced LWRs (EPRI, 1993). The validity of the control bank assignment shown in Fig. 17 was examined for the MOX core. It was found that there is no need to change position or functional assignment of any bank as the result of the transition to MOX fuel. However, the neutron absorber of the control rodlets has to be adjusted as shown in Table 2 to reproduce worth similar to that of the corresponding banks in the UO 2 core. More specifically, the following two changes are introduced: (1) the cylindrical absorber of the gray banks, MA through MD, in the UO 2 core is Ag In Cd with reduced diameter with respect to the other banks. While the material is not changed, its diameter is increased in the MOX core to compensate for the otherwise reduced worth in the harder spectrum; (2) the remaining banks employ Ag In Cd rodlets at nominal diameter in the UO 2 core. Their counterparts in the MOX design also feature rodlets of nominal diameter, but a more absorptive material is used, i.e., B 4 C with enriched boron, having 10 B concentration of 70 at.%. The rationale for using enriched instead of natural boron is twofold: first, enriched boron enables placing more solid 10 B in the core and thus provides the additional negative reactivity needed to obtain worth similar as in the UO 2 design; secondly, the highly enriched 10 B concentration results in increased self-shielding of the inner part of the rodlets with considerable reduction of its depletion rate and thus increases the control bank lifetime. This is an important consideration for MSHIM since the M1 and AO banks in MSHIM maneuvers may be partially inserted into the core at power, and therefore exposed to high neutron flux and consumption of their absorber content. The combined M-banks worth resulting from the above design is nearly 3000 pcm for both the UO 2 and MOX core, more than enough to accommodate the range of power defects observed throughout the power changes analyzed (see also Fig. 13). The AO bank has the required worth and proper position to allow an effective control of the axial power distribution during the maneuvers. Moreover, it has been demonstrated by performing the standard Westinghouse shut-down analysis that the overall negative reactivity of the control banks is adequate to ensure a safe shutdown of the reactor at all conditions for both the UO 2 and MOX core design. Additionally, the following considerations suggest that performing MSHIM load follow maneuvers may actually be simpler with the MOX than UO 2 IRIS core: the lower xenon worth in the MOX spectrum should simplify controlling its variations during the xenon transients originated by the maneuvers; the well-behaved nuclear peaking factors at all-rods-out conditions in MOX (even lower than in the UO 2 core; see Section 4.2.2), will likely result in a more benign power redistribution following control banks insertion and xenon transient during load follow operation. The above considerations provide confidence that the control bank design developed for this study will be capable to support MSHIM operation in the MOX core Fuel rod performance A preliminary assessment of fuel rod performance indicates that the main requirements are met with the considered MOX core design. Specifically, the discharge burnup is within the experience database limits, and the longer plenum has been provided to accommodate increased fission gas release. A detailed analysis, however, is beyond the scope of this paper. From a fuel manufacturing perspective, while the use of IFBA burnable absorber in UO 2 fuel is a consolidated practice in Westinghouse, its viability for the MOX rods needs to be confirmed in future studies.

11 F. Franceschini, B. Petrović / Annals of Nuclear Energy 35 (2008) Summary of the MOX core design assessment While the exhaustive safety analysis has not been completed, all main safety and operational requirements have been met. The results obtained so far confirm that a 100% MOX core is a viable option in IRIS, moreover, its enhanced moderation lattice makes the use of MOX fuel less challenging relative to its use in a typical present PWR. 5. Conclusion In addition to the previously developed reference UO 2 core design, a MOX core design has been developed for IRIS and presented in this paper. IRIS fuel with its somewhat larger lattice pitch, and thus increased moderator-to-fuel ratio, facilitates implementation of a 100% MOX core. Comparison of a 2-batch equilibrium cycle MOX core to the reference equilibrium cycle 2-batch UO 2 core has shown that a MOX core may be designed to provide a comparable performance to the UO 2 core, and satisfy all main safety and operational requirements. Moreover, the control rod banks design has been modified as required by the MOX core reactor physics parameters to allow extended load follow capability via implementation of the Westinghouse MSHIM strategy. Thus, while the reference first IRIS design features UO 2 based core, in the future IRIS may also offer an attractive design for a 100% MOX core, should recycling and use of MOX fuel become the preferred option. References Carelli, M.D., IRIS, a global approach to nuclear power renaissance. Nuclear News 46 (10), Carelli, M.D., Petrović, B., Here s looking at IRIS. Nuclear Engineering International 51 (620), Carelli, M.D. et al., The design and safety features of the IRIS reactor. Nuclear Engineering and Design 230, Electric Power Research Institute (EPRI), Advanced Light Water Reactor Requirements, vol. III. Revisions 5 and 6. Franceschini, F., Advanced Fuel Cycles for Light Water Reactors. VDM Verlag Dr. Mueller EK. Franceschini, F., Petrović, B., Fuel management options in IRIS reactor for the extended cycle. In: Proceedings of the 5th International Conference on Nuclear Option in Countries with Small and Medium Electricity Grids, Dubrovnik, Croatia, May Franceschini, F., Petrović, B., submitted for publication. Advanced operational strategy for the IRIS reactor: load follow through mechanical shim (MSHIM). Nuclear Engineering and Design. Liu, Y.S. et al., ANC-A Westinghouse Advanced Nodal Computer Code, WCAP Morita, T. et al., Load follow operation with the MSHIM control system. Trans. ANS Topical Meeting No. 2, vol. 56. Morita, T. et al., MSHIM load follow operation in the westinghouse AP600 plant design. In: Proceedings of the Topical Meeting on The Next Generation of Nuclear Power Plants: A Status Report, San Francisco, USA. Morita, T. et al., Application of MSHIM core control strategy for westinghouse AP1000 nuclear power plant. In: GENES4/ANP2003, Kyoto, Japan. Nguyen, T.Q. et al., Qualification of the PHOENIX-P/ANC Nuclear Design System for Pressurized Water Reactor Cores, WCAP A. Ouisloumen, M. et al., Qualification of the Two-Dimensional Transport Code PARAGON, WCAP P-A.