BNL-114257-2017-JA

Impact of thermal spectrum small modular reactors on performance

of once-through nuclear fuel cycles with low-enriched uranium

Submitted to the Journal Annals of Nuclear Energy

August 29, 2017

Nuclear Sciences & Technology Department

Brookhaven National Laboratory

U.S. Department of Energy USDOE Office of Science (SC), Nuclear Physics (NP) (SC-26)

Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No.DE-SC0012704 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

N. R. Brown, M. Todosow

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Impact of thermal spectrum small modular reactors on performance ofonce-through nuclear fuel cycles with low-enriched uraniumq

q This manuscript has been authored by UT-Battelle LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Govern-ment retains and the publisher, by accepting the article for publication, acknowl-edges that the United States Government retains a non-exclusive, paid-up,irrevocable, world-wide license to publish or reproduce the published form of thismanuscript, or allow others to do so, for United States Government purposes. TheDepartment of Energy will provide public access to these results of federallysponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).⇑ Corresponding author.

E-mail address: brownnr@ornl.gov (N.R. Brown).

Nicholas R. Brown a,⇑, Andrew Worrall a, Michael Todosowb

aOak Ridge National Laboratory, Oak Ridge, TN 37830, USAbBrookhaven National Laboratory, Upton, NY 11973, USA

a b s t r a c t

Keywords:Small modular reactorFuel cycle performanceEvaluation and screeningNeutron leakage

Small modular reactors (SMRs) may offer potential benefits relative to large light water reactors, such asenhanced flexibility in deployment and operation. However, it is vital to understand the holistic impact ofSMRs on nuclear fuel cycle performance. The focus of this paper is the fuel cycle impacts of light waterSMRs in a once-through fuel cycle with low-enriched uranium fuel. A key objective of this paper is todescribe preliminary neutronics and fuel cycle analyses conducted in support of the US Department ofEnergy, Office of Nuclear Energy, Fuel Cycle Options Campaign. The hypothetical light water SMR exam-ple case considered in these preliminary scoping studies is a ‘‘cartridge type” one-batch core with slightlyless than 5.0% enrichment.The high-level issues identified and preliminary scoping calculations in this paper are intended to

inform decision makers regarding potential fuel cycle impacts of one-batch thermal-spectrum SMRs. Inparticular, this paper highlights the impact of increased neutron leakage and a reduced number ofbatches on the achievable burnup of the reactor. Fuel cycle performance metrics for the simplified exam-ple SMR analyzed herein are compared with those for a conventional three-batch light water reactor(LWR) in the following areas: nuclear waste management, environmental impact, and resource utiliza-tion. The metrics performance for such an SMR is degraded for the mass of spent nuclear fuel andhigh-level waste disposed of per energy generated, mass of depleted uranium disposed of per energy gen-erated, land use per energy generated, and carbon emissions per energy generated.

Finally, it is noted that the features of some SMR designs impact three main aspects of fuel cycle per-formance: (1) small cores, which mean high leakage (there is a radial and an axial component); (2) a heterogeneous core and extensive use of control rods and burnable poisons; and (3) single-batch cores. But not all SMR designs have all of these traits. The approach used in this study is an example bounding case, and not all SMRs may be impacted to the same extent.

1. Introduction

One objective of this paper is a qualitative identification ofissues related to the impacts on fuel cycle performance of thermalspectrum light water-cooled small modular reactors (SMRs). In

addition, the paper documents preliminary neutronics analysesconducted in support of the US Department of Energy Office ofNuclear Energy (DOE–NE) Fuel Cycle Options Campaign study ofmodularity impacts.

The DOE-NE Fuel Cycle Evaluation and Screening (E&S) identi-fied nuclear fuel cycle options with significant potential benefitsrelative to the present once-through fuel cycle in thermal spectrumlight water reactor (LWRs) in the United States (Wigeland et al.,2014a). The E&S evaluated possible fuel cycles with respect to ninehigh-level criteria, including natural resource utilization andnuclear waste management. Analysis examples for each fuel cyclein the E&S were evaluated based on peer-reviewed reactor physicsand inventory analyses to quantitatively document performance.These analysis examples were evaluated based on twenty-fivemetrics that correspond to the nine high-level criteria. The metric

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values were placed in alphabetized bins to accommodate uncer-tainties. The bins used in the E&S represent performance, with‘‘A” representing the highest-performing bin and ‘‘F” representingthe least-performing bin.

The E&S and follow-on studies have yielded several contribu-tions in the literature. One important contribution is a review ofthe impact of various nuclear fuel cycles on the E&S nuclear wastemanagement metrics (Stauff et al., 2015). Stauff et al. (2015) over-views the nuclear waste management metrics performance of eachof the fuel cycles in the E&S, as well as the nuclear waste manage-ment metric definitions, which are relevant to the present paper.Other literature related to the E&S has explored performancetrade-offs in various sustainable fuel cycles by examining theimpact of technology choices (Brown et al., 2015) and the perfor-mance of externally driven systems versus critical reactors in ura-nium fuel cycles (Heidet et al., 2015) and thorium fuel cycles(Brown et al., 2016a). Similar to the present paper, Brown et al.(2015), Heidet et al. (2015), and Brown et al. (2016a) explore therelative performance of fuel cycle options or technology choicesand the underlying physics reasons the performance is different.Additional studies have examined the potential transition fromthe present once-through fuel cycle to a promising future fuel cycle(Feng et al., 2016; Brown et al., 2016b).

The analysis example for the reference fuel cycle in the E&S wasbased on a large pressurized water reactor (PWR) with 4.21%enrichment, 3% neutron leakage, and 50 GW day per metric tonof heavy metal (GWd/t) discharge burnup. SMRs will have differentfuel cycle performance compared with large LWRs because of dif-ferences in enrichment variation in the assemblies and becauseof neutron leakage, which will impact the achievable dischargeburnup for a given average enrichment and fuel batch loading.

We identified SMR characteristics that may impact fuel cycleperformance, including differences in SMRs compared with largerconventional LWRs. Some key differences that have the potentialto impact the fuel cycle performance of SMRs versus large LWRsare fuel design information, reactivity control approaches, and coreloading and design. Specific issues identified in each of these areasare outlined below.

1.1. SMR reactivity control impacts on fuel cycle performance

Some SMR design concepts, for example the mPower and Holtecdesigns, do not use boron in the coolant for global reactivity con-trol (Worrall, 2014). The aim of this design is to reduce capital costand reduce the amount of waste consisting of boron chemistryresins. However, using control rod insertion to control the reactormeans stronger axial and radial power variations, which will yieldmore heterogeneous fuel assembly designs. One challenge forPWRs without boron is the strongly coupled nature of hot excessreactivity, critical control rod insertions, and the resultant impacton the reactor power shape. Additionally, concepts without boronmay have a more negative moderator temperature coefficient ofreactivity. This will impact the shutdown margin and the axialpower distribution for those designs with a large temperaturechanges in the core. Note that this design feature was not explicitlyinvestigated for this study, which focused on a hypothetical SMRthat uses boron for reactivity control.

1.2. SMR fuel assembly design impacts on fuel cycle performance

SMR core designs typically use fewer fuel assemblies (Vujicet al., 2012), which makes core design more challenging, as thereare fewer degrees of freedom. Worrall (2014) provides a compre-hensive overview of some of the challenges associated with coredesign in SMRs. Typically, SMRs have somewhere between 37and 89 assemblies (Liu and Fan, 2014; Worrall, 2014); whereas a

large generation III+ PWR, for example, the WestinghouseAP1000 (Westinghouse, 2009), has 157 assemblies.

SMR fuel assembly designs use shorter assemblies than designsin large LWRs (Haugh and Mohamed, 2012; Liu and Fan, 2014;Worrall, 2014), roughly half the height (2–2.5 m) of the assemblyfor a typical large LWR (3.5–4.25 m). A smaller axial and radial corewill yield significantly increased neutron leakage versus a largecore, resulting in lower fuel utilization. One of the objectives of thispaper is to quantify this impact.

The small core size of SMRs and resultant increase in neutronleakage means there may be a need to control power peaking viaheavier burnable poison loadings compared with LWRs. Anotherpotential approach is more enrichment zoning in the reactor core.This design characteristic is more relevant to those designs with nocritical boron and hence uses burnable poisons and, in particular,control rod clusters for global reactivity control. These design char-acteristics yield a more heterogeneous reactor core. Additionally,because the maximum enrichment is 5%, and SMR designs mayleverage varying enrichments to control power peaking, the aver-age enrichment may be less than in a large LWR core; this willyield lower discharge fuel burnup.

1.3. SMR core loading impacts on fuel cycle performance

Some SMR concepts are focused on single-batch reloads and‘‘cartridge” fuel loadings. Although single-batch cores limit thenumber of degrees of freedom (in particular, burnup distribution),which can make the core design more challenging, it does simplifythe core design process in the long term, as the same design can beused repeatedly. Additionally, extended cycles of 3–4 years havethe potential to reduce outages compared with a large LWR.Assuming a 3-year refueling cycle (versus 1.5 years for a largeLWR), a 1-month outage duration, and a 60-year reactor lifetime,the reduction in outages equates to approximately 600 more full-power days over the reactor lifetime. However, single-batch coreloadings do not use the fuel as efficiently as multi-batch loadings(Driscoll et al., 1990). In terms of the results of the E&S study, coreloadings with enhanced complexity impact the technology readi-ness of SMRs and the development and deployment risks of thefuel, the reactor, and the spent fuel disposition.

1.4. Summary of potential SMR impact on fuel cycle performance

SMRs would impact a variety of assumptions made in the E&Sanalysis of the reference LWR once-through fuel cycle with low-enrichment uranium fuel. These include assumed enrichments,discharge burnup, core neutron leakage, and spectral effects dueto heterogeneous heavy burnable absorber loadings. The effectsof neutron leakage, single-batch cores, and heavy burnable absor-ber loadings will impact uranium utilization. The hardened spec-trum due to heavy burnable absorber loading and heavy controlrod insertion will result in differences in plutonium ingrowth, aswell as plutonium isotopics. The harder spectrum may also impactthe reactor vessel and core barrel lifetime. However, the fast flu-ence experienced by the vessel will also be impacted by the factthat some SMR designs feature linear power that is de-rated byapproximately 50% versus large LWRs (Haugh and Mohamed,2012). Each of these factors will impact the discharged isotopicsand therefore the decay heat, radioactivity, and radiotoxicity perunit of energy generation.

Fuel assembly designs with greater heterogeneity may reducethe fuel plant throughput, and may have increased quality assur-ance and quality control requirements, which could increase fuelcosts. Since there is the possibility of co-locating more than onereactor at a single site (Locatelli et al., 2014), the spent fuel poolmay be shared by several units and may be a more challenging

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optimization problem; but sharing the spent fuel across a numberof reactor cores could provide some benefit in terms of optimizedutilization of irradiated fuel. Additionally, the different fuel assem-bly designs and discharge isotopics may impact the potential recy-cling of spent fuel.

SMRs will also have environmental impacts, including footprint,water use, surface-to-volume ratio effects on building volume perunit of electricity generation, and reduced amounts of waste fromresins during operation for no-boron cores. In addition, decommis-sioning and disposal will be impacted by building volumes and thenumber of components and associated volumes per unit of elec-tricity generation.

A summary of each of the issues identified in this paper isshown in Table 1. It also shows the E&S criteria that each parame-ter and identified issue is expected to impact.

2. Scoping fuel cycle performance calculations

The qualitative identification of potential fuel cycle perfor-mance impacts of SMRs was used to inform an example quantita-tive study. The calculations in this paper focus on thermalspectrum light water SMRs. The SMR considered in these prelimi-nary scoping studies is a ‘‘cartridge type” one-batch core withslightly less than 5% enrichment. Some core parameters, such asthe size of the reactor and the general assembly layout, are similarto published information about the NuScale PWR concept (Haughand Mohamed, 2012). The calculations in this paper are intendedonly to provide information on potential fuel cycle impacts of

Table 1SMR fuel cycle performance issues identified and criteria impacted.

Category Parameter

Fuel Number of Assemblies

Fuel dimensions, including height

Neutron leakage

Burnable poison loadings

Assembly burnups

Heterogeneous fuels

Reactivity control Boron-free coolant

Heavy control rod insertion

Core loading Single vs. multibatch

Other Technology readiness: fuel (fresh and irra

Irradiated fuel management across reacto

Water and land useDecommissioning and disposal volumesSpecific power density

one-batch thermal spectrum SMRs. In particular, these calculationshighlight the impact of the reduced number of batches on theachievable burnup and on the metrics performance.

The SMR model developed here is based on a modified(reduced-size) version of a generic PWR model developed in sup-port of the DOE–NE Advanced Fuels Campaign’s evaluation of thereactor performance and safety characteristics of advanced fuelconcepts (Brown et al., 2014); the version studied has a reducedcore height and reduced core radius. The SCALE lattice physics toolTRITON/NEWT was used to generate two-group constants (DeHartand Bowman, 2011). A conventional two-step approach wasrecently shown to be adequate for the DIMPLE S06 benchmarkproblem (Kim et al., 2015), a critical assembly relevant to SMRs.The PARCS core simulator (Downar et al., 2002) was used withtwo-group parameters to calculate full-core performance. The cal-culations of SMR performance in this document assume thefollowing:

� Nodal neutron diffusion using a hybrid analytical nodal methodand nodal expansion method solver.

� Reactivity feedback for fuel temperature, moderator tempera-ture, moderator density, boron concentration, and control rodsusing thermal and hydraulic models appropriate for PWR coreanalysis.

� A 17 by 17 Westinghouse PWR assembly model for the genera-tion of few-group parameters.

� A generic cross section set for reflector regions representative ofa large Westinghouse PWR (not necessarily an SMR).

Criteria impacted

Nuclear waste managementEnvironment impactResource utilizationNuclear waste managementEnvironment impactResource utilizationNuclear waste managementEnvironment impactResource utilizationNuclear waste managementEnvironment impactResource utilizationNuclear waste managementEnvironment impactResource utilizationFinancial risk and economicsInstitutional issues

Nuclear waste managementEnvironment impactResource utilizationNuclear waste managementEnvironment impactResource utilizationFinancial risk and economics

Nuclear waste managementEnvironment impactResource utilizationFinancial risk and economics

diated) and reactor Development and deployment riskInstitutional issues

rs Development and deployment riskInstitutional issuesEnvironment impactNuclear waste managementDevelopment and deployment riskNuclear waste management

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2.1. Review of a simplified reference core model for a large PWR

The large PWR core configuration used as a starting point fordevelopment of the SMR model was calculated with a single burn-able poison configuration in a multi-cycle calculation. An identicalburnable absorber configuration was used for all fuel assemblies.The burnable poison used in all but one case was a standard West-inghouse ZrB2 coating (Simmons et al., 1988), referred to as inte-gral fuel burnable absorbers (IFBAs). The assembly geometryused in the generation of the few-group parameters is shown inFig. 1.

The reference three-batch core model of a large PWR was devel-oped using PARCS. The PARCS multi-cycle capability was used tocalculate equilibrium cores with a discharge burnup convergencecriterion of 0.1 GWd/t and a fixed fuel management scheme.

In each cycle, the relevant fuel assemblies were shuffled accord-ing to a prescribed fuel management scheme. In general, the freshassemblies were located near the periphery of the core, and theonce- and twice-burned assemblies, throughout the central coreregion. The fuel management scheme was octant symmetric. Onlyone fuel type was used in this simplified core model; in reality,multiple fuel types with varying 235U enrichments and burnablepoison content would be used in a reactor core. Soluble boron

Fig. 1. One-quarter-assembly geometry used for generation of the few-groupparameters for the reactor core model (Brown et al., 2014).

0.96

43.4 BOC 1/8-Core UO2

1.15 1.00

24.7 43.4 1.39 Relative power

1.04 1.33 1.16 18.3 Burn-up (GWd/t)

45.3 0.0 40.4

1.30 1.39 1.36 1.28

24.7 18.3 24.5 32.9

1.06 0.99 1.16 1.36 1.14

34.1 45.2 34.5 12.9 14.2

0.89 1.12 1.18 0.84 0.61

40.4 0.0 11.6 35.2 0.00.79 0.91 0.73 0.4732.9 11.9 0.0 0.00.49 0.41

0.0 0.0

Fig. 2. Equilibrium one-eighth core radial fuel assembly power and burn

was used for reactivity control throughout the cycle (e.g., controlrods were not used during power operations). The critical solubleboron concentration was calculated iteratively within each burnupstep. As the fuel depleted, the core simulator calculated the boronletdown curve for the core configuration. Within the fuel assem-blies, 112 of the fuel pins used ZrB2 IFBAs for reactivity control.

The equilibrium core search was performed using a prescribedfuel management scheme with fuel temperature, moderator den-sity and temperature, and soluble boron feedback. The enrichmentwas slightly less than 5% and the average discharge burnup wasapproximately 56.1 GWd/t. The equilibrium core search conver-gence criterion (0.1 GWd/t) was the difference between the maxi-mum end-of-cycle (EOC) burnup in the current cycle versusmaximum EOC burnup from the previous cycle. The beginning-of-cycle and EOC core power and burnup distributions are shownin Fig. 2, where each ‘‘box” represents a fuel assembly. The valuesin ‘‘Relative power” and ‘‘Burn-up” boxes at the top/right of theradial power distributions are examples.

Note that the core design used in this study for the large PWR isan out–in loading pattern (fresh fuel on the outside of the core,moved more toward the center in subsequent reloads). Therefore,this form of loading pattern has higher radial neutron leakage thanmodern low-leakage-loading patterns that purposefully put moreburned fuel on the periphery of the core to reduce leakage andpeak pin burnup. This form of loading pattern was chosen to be asimplified bounding case for the large PWRs against which theSMR results can be compared.

2.2. Development of a simplified thermal spectrum SMR model

The NuScale SMR concept (Haugh and Mohamed, 2012) wasused as a general guiding concept in the development of the SMRscoping model. The same assembly configuration was assumed tobe a 17 by 17 Westinghouse assembly with a reduced radial andaxial core size and one-batch fuel. The relative radial core sizesare shown in Fig. 3 and other key parameters in Table 2. Two reac-tor heat outputs were selected for the SMR calculations, which areintended to span the parameter space of potential SMR designs.The layout of control rod clusters in the SMR model is shown inFig. 4. In the core models, the assembly pitch was 21.44 cm withfour x-direction and y-direction mesh points per assembly. Therewere 20 axial meshes of 10.675 cm in the fuel region and two axialmeshes in the reflector regions, each 21.44 cm. It is noted that theloading pattern shown in Fig. 3 loads twice burnt fuel assemblies inthe center of the core, this would slightly exceed peak pin burnup

0.76

58.7 EOC 1/8-Core UO2

0.92 0.85

43.3 60.3 1.35 Relative power

0.86 1.23 0.91 24.7 Burn-up (GWd/t)

62.6 24.5 59.1

0.99 1.05 0.99 0.94

45.3 40.4 45.2 52.4

0.91 0.86 0.94 1.11 1.05

52.1 62.2 53.5 35.3 34.1

0.96 1.35 1.22 0.88 0.79

58.2 24.7 34.5 51.1 12.91.03 1.20 1.12 0.7650.7 32.9 18.3 11.60.98 0.8214.2 11.9

up distributions for a large PWR with UO2 fuel (Brown et al., 2014).

Fig. 3. Comparison of radial core size and layout for three-batch large PWR (left) and one-batch SMR (right). Each colored ‘‘box” is a fuel assembly. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)

Table 2Large PWR vs. SMR core parameters (differences are bolded).

Parameter Large PWR SMR

Reactor core heat output, MW(th) 3400 160 or 400System pressure, nominal, MPa 15.51 15.51Nominal inlet coolant temperature, �C 279.4 279.4Fuel assembly design 17 � 17 17 � 17Active fuel height, cm 426.7 213.35Number of fuel assemblies 157 37Fuel assembly pitch, cm 21.44 21.44Uranium rods per assembly 264 264Number of control clusters 53 16Control rod material Ag-In-Cd Ag-In-CdNumber of fuel batches 3 1

Bank 2

Bank 1 Bank 2

Bank 1 Bank 2

Bank 2

Fig. 4. Control rod bank configuration, one-quarter core (Haugh and Mohamed,2012).

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limits and perhaps exceed fuel performance limits as well (due to ahigh linear heat rate at high burnup). This simplified loading pat-tern is intended to inform on physics behavior on a scoping basis,not directly represent a realistic commercial PWR core design.

Key simplifications versus the NuScale reference design (Haughand Mohamed, 2012) included the following:

� Only one enrichment zone: NuScale does not report the exactenrichment scheme used, only that the maximum enrichmentis slightly less than 5% (Haugh and Mohamed, 2012; Worrall,2014).

� Simplified reactor and reflector geometry: NuScale does notreport the exact reactor geometry, only the radial layout (37assemblies), assembly layout (17 by 17), and other generalinformation.

� ZrB2 is used for reactivity control instead of Gd2O3: NuScaledoes not report the exact intra-assembly burnable absorberconfigurations.

� Assembly height is assumed to be half of AP1000, and axialdepleted uranium blankets are neglected: NuScale does notreport the exact active core height but does state that the heightis approximately half that of a large PWR.

The core calculations in this document are expected to yieldconservative (high) estimates of neutron leakage from fuel regionsdue to a uniform enrichment of slightly less than 5% (includingnear the periphery) and the omission of axial depleted uraniumblankets. Although the model developed in this study is simplified,reasonable agreement was achieved between the reported NuScaledesign value of hot zero power (HZP) critical boron concentration(1303 ppm) and the calculated HZP boron concentration usingthe simplified model employed here (1456 ppm). Specific designlimits were not considered in the development of the SMR model.The SMR model should not be considered an optimized design, butrather is a hypothetical example used for scoping calculations.

3. Fuel cycle performance results and analysis

The one-batch discharge burnups in the 37 fuel assemblies arereported for two assembly average powers. One assembly powercorresponds to the AP1000 average linear power (5.72 kW/ft or18.7 kW/m) and the other to the NuScale average linear power(2.28 kW/ft or 7.48 kW/m). The higher heat generation rate(AP1000) corresponds to a reactor thermal power of 400 MWt,and the lower heat generation rate (NuScale) corresponds to a

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

0 25 50 75 100 125 150 175 200 225

Axi

al r

elat

ive

pow

er a

t EO

C

Axial height (cm)

AP1000 linear power (5.72 kW/ft)NuScale linear power (2.28 kW/ft)

Fig. 5. EOC axial power distribution for the simplified SMR: AP1000 linear powerand NuScale linear power.

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reactor thermal power of 160 MWt. The EOC axial and radial powerdistributions are shown in Figs. 5 and 6, respectively. The corre-sponding radial burnup distributions are also shown in Fig. 6. Inthe radial distributions, each ‘‘box” represents a fuel assembly.The values in ‘‘Relative power” and ‘‘Burn-up” boxes at the top/right of the radial power distributions are examples. Note thatthe AP1000 average linear power is consistent with some recentscoping analyses of SMRs (Mart et al., 2014).

The average discharge burnup at a linear power of 5.72 kW/ftwas 30.5 GWd/t and at 2.28 kW/ft linear power was 32.4 GWd/t.The key driver of the difference was the different amounts ofbuildup of fission product poisons, particularly xenon and samar-ium, associated with the different flux levels in the two cores.The difference in burnup between the central fuel assemblies andthe peripheral fuel assemblies was apparent. The cycle lengthwas approximately 24 months with slightly less than 5% 235Uenrichment and a 5.72 kW/ft linear power, and approximately48 months with slightly less than 5% 235U enrichment and a2.28 kW/ft linear power. The discharge burnup was consistent withestimates in the literature for cores of similar size (Mart et al.,2014).

The reduced discharge burnup versus the reference large LWRonce-though fuel cycle using uranium dioxide fuel (referred to asthe EG01 analysis example) cascaded throughout all of the E&Smetrics and negatively impacted fuel cycle performance. The E&Smetrics (Wigeland et al., 2014b) are compared with the referenceonce-through fuel cycle with low-enrichment uranium in LWRsin Table 3. The radioactivity quantities in the nuclear waste man-agement metric values from Table 3 were calculated using theMonte Carlo neutronics tool Serpent (Leppänen et al., 2013) for asingle reflected fuel assembly, assuming 3% leakage for the largeLWR, using the linear reactivity model. The specific metrics calcu-lated using radioactivity data from Serpent include: the activity ofspent nuclear fuel and highlevel waste (at 100 years), and the

1.16

46.3 EOC 1/8-Core UO2

1.18 1.19 5.72 kW/ft

44.5 42.2 1.19 Relative power

1.19 1.12 0.81 42.2 Burn-up (GWd/t)

37.6 33.8 21.4

0.90 0.73

22.4 17.6

Fig. 6. EOC one-eighth core radial fuel assembly power and burnup distrib

activity of spent nuclear fuel and high-level waste (at100,000 years).

All of the metric values are energy normalized per gigawatt-electric year (GWe-year) based on the discharge burnupdetermined from the full core PARCS calculations and assuming athermal efficiency of 33% and a 90% capacity factor. The assump-tions for the metrics calculations for the large LWR in Table 3 areconsistent with the assumptions made for EG01 in the E&S. Inthe E&S, the WIMS code and ORIGEN-2 were utilized by ANL tocalculate the metrics for EG01. A comparison was conducted whichshowed that the radioactivity metrics derived from the Serpentcalculation are in good agreement with those from the E&S. Adetailed discussion of the E&S metrics is outside the scope of thispaper. More information about the metric calculations is found inWigeland et al. (2014b) and Stauff et al. (2015).

The metrics that are appreciably different for the SMR case arethe: mass of spent nuclear fuel and high-level waste disposed of,mass of depleted uranium disposed of, land use per energy gener-ated, and carbon emissions—CO2 released per energy generated.The mass of spent nuclear fuel and high-level waste disposed aswell as the mass of depleted uranium disposed of are inverselyproportionally to the discharge burnup for this once-through fuelcycle. In the E&S, land use was defined to include land not availablefor other purposes as a result of the fuel cycle processes containedin the nuclear fuel cycle such as mining, reactors and fuel fabrica-tion plants (Wigeland et al., 2014b). It is noted that the E&S landuse metrics do not account explicitly for the potential for severalSMRs to be located together. However, the reactor operation landuse factor scales linearly with capacity in GWe-yr. The E&S metricsalso indicate that the higher requirement for natural uranium perunit energy generated and greater mass of spent nuclear fuel dis-posed per unit energy generated will increase the land requiredfor the front-end and back-end of the fuel cycle, respectively.

The uranium natural resource utilization was significantlyworse in the simplified SMR (�345 t/GWe-year) than in the EG01(�190 t/GWe-year) analysis example and was lower than in theleast-performing evaluation groups in the E&S (�305 t/GWe-year). The mass of depleted uranium was higher than in any anal-ysis example in the E&S. The performance of all metric values wasdegraded. As noted in the introduction, the E&S used alphabetizedbins to gauge the relative performance of fuel cycles and accom-modate uncertainties. The metrics where bin boundaries werecrossed included mass of SNF + HLW disposed of per energy gener-ated, mass of depleted uranium disposed of per energy generated,land use per energy generated, and carbon emissions per energygenerated. Although neither the large PWR nor the SMR coredesign was optimized (either pin/assembly powers or cyclelength), doing so would change the results marginally but wouldnot change the overall observations and conclusions. However, amore efficient loading pattern design (using a split batch approachto a single batch core) may marginally increase discharge burnupversus this simplified example.

1.12

49.1 EOC 1/8-Core UO2

1.15 1.17 2.28 kW/ft

47.2 44.8 1.19 Relative power

1.19 1.13 0.81 40.0 Burn-up (GWd/t)

40.0 35.9 22.6

0.91 0.74

23.7 18.6

utions for the simplified SMR: 5.72 kW/ft (left) and 2.28 kW/ft (right).

Table 3One-batch SMR scoping impact on E&S metrics (metrics where bin boundaries were crossed are in bold typeface). The letters after the slash (‘‘/”) denote the alphabetized binsfrom the E&S study.

Criterion Metrics Large LWR SMR

5.72 kW/ft 2.28 kW/ft

Nuclear waste management Mass of spent nuclear fuel and high-level waste disposed of, t/GWe-year 22.13/E 36.28/F 34.16/EActivity of spent nuclear fuel and highlevel waste (at 100 years), MCi/GWe-year 1.34/C 1.40/C 1.35/CActivity of spent nuclear fuel and high-level waste (at 100,000 years), 10�4 MCi/GWe-year 15.1/C 16.9/C 16.9/CMass of depleted uranium disposed of, t/GWe-year 167.98/E 329.67/F 310.34/FVolume of low-level waste, m3/GWe-year 399.6/C 470.6/C 462.04/C

Environmental impact Land use per energy generated, km2/GWe-year 0.175/B 0.263/C 0.251/CWater use per energy generated, ML/GWe-year 23,893/B 24,067/B 24,046/BCarbon emissions—CO2 released per energy generated, kt CO2/GWe-year 44.4/B 72.4/C 68.8/CRadiological exposure, Sv/GWe-year B B B

Resource utilization Natural uranium required per energy generated, t/GWe-year 190.16/D 366.04/D 344.58/DNatural thorium required per energy generated, t/GWe-year 0/A 0/A 0/A

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Fig. 7. Neutron leakage (%) from 3-dimensional core models for a one-batch SMRand three-batch large PWR.

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Fig. 8. Impact of one-batch core size on neutron leakage and achievable dischargeburnup.

172

The overall reduced performance of the thermal spectrum SMRwas driven by the higher core neutron leakage and the reducednumber of fuel batches. The leakage from the 3-dimensional coremodel is shown in Fig. 7 throughout the burnup cycle. Fig. 7 alsoshows the representative core leakage from a three-batch largePWR core model. Although this is not an entirely consistent com-parison, it provides an indication of the overall impact of neutronleakage on neutron economy in the SMR versus that in a largePWR. Fig. 7 also illustrates the need for full-core calculations ofthermal spectrum SMR reactor physics: the leakage from the fuelregion was considerable, and it varied significantly throughoutthe burnup cycle.

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Fig. 9. Discharge burnup and uranium resource required as a function of radial coresize.

4. Parametric study of core size, neutron leakage, and single-batch discharge burnup

It is well understood that multi-batch fuel loading yields betterfuel utilization (Driscoll et al., 1990), and the best fuel utilization isachieved in a continuously refueled system (Wigeland et al.,2014c). However, it is useful to quantify the relationship betweenone-batch core size and resource utilization in the context of thisstudy. This relationship shows the impact of leakage on achievableburnup in a one-batch LWR SMR. The calculated leakage and dis-charge burnup are shown in Fig. 8 for light water SMRs as a func-tion of increasing core size. These calculations were performed as ascoping study. This simplified parametric study shows one-batchSMR discharge burnup for radial core sizes of 12, 21, 32, 45, 60,and 77 assemblies. The calculated discharge burnup and uraniumresource requirement are shown for the radial cores sizes in Fig. 9.

Also calculated was the maximum discharge burnup for a one-batch LWR with less than 5% enrichment and infinitely reflectedwith no leakage. This value is approximately 44 GWd/t, signifi-cantly less than the three-batch large LWR example (50 GWd/t at4.2% enrichment). This discharge burnup corresponds to a mini-mum natural resource requirement of 254 t/GWe-year versus thelarge LWR example value of 190 t/GWe-year.

173

5. Summary

The discharge burnup of SMRs will be different from thatof large LWRs because of varying enrichments, especially insingle-batch cores. The effects of neutron leakage, single-batchcores, and heavy burnable poison loadings will affect uraniumutilization.

Heavy use of control rods and burnable absorbers will hardenthe neutron energy spectrum; that will impact the buildup oftransuranic isotopes, including the production of additional pluto-nium, which could increase the cycle length slightly. In addition,the harder spectrum could limit vessel/barrel lifetime. Fuel costsper bundle will be higher because of increased complexity due tothe additional radial and axial heterogeneity.

In an example scoping calculation, metric values from theDOE-NE Fuel Cycle E&S study were negatively impacted. The E&Smetrics negatively impacted in SMRs include the mass of depleteduranium disposed of per energy generated, the mass of spentnuclear fuel and high-level waste disposed of per energy gener-ated, and the natural uranium required per energy generated. Thisimpact is due to single-batch fuel loadings and increased neutronleakage from the reactor core, both of which act to reduce the dis-charge burnup of an SMR. In addition, environmental metrics willbe impacted, including land use, water use, and carbon emissions.It is noted that an SMR with a multi-batch fuel cycle would beexpected to perform better than a cartridge core design.

Finally, note that some SMR designs impact three main aspectsof fuel cycle performance: (1) small cores, which mean high leak-age (there is a radial and an axial component); (2) a heterogeneouscore and extensive use of control rods and burnable poisons; and(3) single-batch cores. But not all of the SMR designs have all ofthese traits. The approach used in this study is therefore a bound-ing case, and not all SMRs may be impacted to the same extent. Themain findings of this work should be interpreted conceptually andthe example calculations in this paper are not representative of aparticular optimized SMR design. A systematic evaluation of thedischarge burnup of different SMR core designs, similar to thestudy by Secker et al. (2005) for large LWRs, would better definethe fuel cycle performance envelope for thermal spectrum SMRsin a once-through fuel cycle.

Acknowledgments

Jeff Powers and Ben Betzler are gratefully acknowledged fortheir excellent internal review comments. Comments and ques-tions from Greg Borza of Holtec International are greatly appreci-ated. The authors also thank the two anonymous reviewers fortheir comments. This work was supported by the US Departmentof Energy, Office of Nuclear Energy, Fuel Cycle Options Campaign.The Fuel Cycle Options Campaign SMR activity was led by BrentDixon at Idaho National Laboratory.

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