Open Access
Issue
EPJ Nuclear Sci. Technol.
Volume 2, 2016
Article Number 14
Number of page(s) 9
DOI https://doi.org/10.1051/epjn/2016012
Published online 25 March 2016

© B. A. Lindley et al., published by EDP Sciences, 2016

Licence Creative Commons
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

The majority of nuclear reactors operating in the world today and similarly the majority of near-term new build reactors will be LWRs. These currently accommodate traditional Zr clad UO2/Pu fuel designs which have an excellent performance record for normal operation. However, the events at Fukushima culminated in significant hydrogen production and hydrogen explosions, resulting from high temperature Zr/steam interaction following core uncovering for an extended period. These events have resulted in increased emphasis towards developing more accident tolerant fuels (ATFs), particularly for current and near-term build LWRs.

Candidate advanced fuel materials include uranium nitride (UN) and uranium silicide (U3Si2), both of which have higher thermal conductivity than UO2, leading to improved margins under accident conditions, and also have the benefit of higher heavy metal density leading to the possibility of increased core heavy metal loading [1,2]. Candidate cladding materials include advanced stainless steel (FeCrAl), silicon carbide (SiC), and the possibility of adding a coating to Zircaloy clad [3]. Stainless steel cladding exhibits a lower oxidation rate under accident conditions than Zircaloy [4] and is relatively easy to fabricate [5], but has the disadvantage of introducing a large reactivity penalty [4]. SiC cladding can withstand much higher temperatures than Zircaloy, but is expensive and difficult to fabricate [5]. R&D programmes are underway in the US and elsewhere to develop ATFs, encompassing fabrication and testing of UN, U3Si2, SiC and coated Zr rods [6].

This paper presents the core analysis performed with the ANSWERS reactor physics code suite WIMS/PANTHER [7,8] for the Integral Inherently Safe Light Water Reactor (I2S-LWR). The I2S-LWR concept [9] is a Gen III+ large scale (i.e. 1 GWe) reactor. The design stage is being carried out by a consortium of universities (Michigan, Virginia Tech, Tennessee, Florida Institute of Technology, Idaho, Morehouse College, Brigham Young University, Cambridge, Politecnico di Milano, Zagreb), Idaho National Laboratory, Westinghouse and Southern Nuclear Company. The project is led by the Georgia Institute of Technology.

This innovative PWR includes: an integral primary circuit, a fully passive decay heat removal system that provides indefinite cooling capability, and the use of new materials. The types of materials that were originally chosen for this design include U3Si2 fuel pellets within advanced steel cladding.

The equilibrium cycle core analysis was performed using the WIMS/PANTHER codes and the results were verified in a code-to-code comparison. In the first stage, the 2D results obtained with WIMS [2] were compared against the Monte Carlo code Serpent [10], and a good agreement was observed. In the second stage, the full 3D core results obtained with the WIMS/PANTHER codes were compared with results form the literature for the I2S-LWR [11]. This cross-comparison of results provides enhanced confidence in the reliability and accuracy of the results.

2 UK context for accident tolerant fuel

The UK has a long history in industrial fuel manufacture and fabrication for a wide range of reactor systems including LWRs. This is supported by a national infrastructure to perform experimental and theoretical R&D in fuel performance, fuel transient behaviour and reactor physics.

The UK is seeking to engage with international programmes on ATF research to “strengthen international collaboration opportunities and establish the UK as a centre of expertise for advanced fuel fabrication R&D, and consequently commercial manufacture of such fuels” [12]. Such fuels could be utilized in nuclear new build plants, and also potentially in small modular reactors (SMRs), in which the UK has expressed a strategic interest [13]. The UK Nuclear Industry Research and Advisory Board (NIRAB) recently recommended that the UK perform research on manufacturing advanced cladding materials in order to enable future manufacture of ATF on a commercial scale [14]. Opportunities for ATF use are identified to include Generation III reactors and SMRs.

3 Modelling of accident tolerant fuel with ANSWERS software

The ANSWERS lattice code WIMS and core simulator PANTHER are used to support the operation of existing PWRs, including in the UK and Belgium [15]. WIMS-PANTHER has recently been validated for analysis of part-MOX-fuelled PWRs. In academia, WIMS and PANTHER have also been applied to a range of PWR configurations including SMRs [16], seed-blanket-fuelled PWRs [17,18], PWRs loaded with transuranic fuels [19,20]. Modelling of ATFs is a natural extension of these capabilities and can largely be performed using existing calculation routes.

Challenges of modeling ATFs include:

  • validation of software for different fuel types. This includes validation of the relevant nuclear data libraries. For stainless steel, an extensive amount of validation has been performed as steel is commonly used in fast and thermal reactors. For other isotopes/elements, a reasonable amount of experimental data is available, but further validation may be required for use in new applications;

  • modelling of non-standard isotopes. An example is the presence of 15N in UN fuel. The most abundant isotope of nitrogen, 14N, has a large (n,p) cross-section which adversely impacts the neutron economy. It is therefore commonly proposed to increase the 15N content of the nitrogen in the UN fuel through enrichment [1]. While limited experimental data on 15N cross-sections is available, it is not usually considered in isolation and hence further experimental validation may be necessary for thermal reactor applications;

  • some candidate ATFs may have the capability to be driven to higher burnups than existing Zircaloy clad UO2 fuels. Both stainless steel [4] and SiC [21] have superior performance when irradiated compared to Zircaloy. This leads to the need to validate the reactor physics code for higher enrichments and high burnups, and account for a wider range of actinides.

WIMS10, the most recent release of WIMS, contains nuclear data for high burnup applications, including cross-sections and delayed neutron fraction data for a wider range of isotopes including 246Cm, 247Cm and 248Cm. Use of higher enrichment fuel, being driven to high burnups, leads to increased reactivity swings, which requires use of novel burnable poison arrangements and core loading strategies [22]. PANTHER contains inbuilt multi-objective optimization algorithms which facilitate PWR [23] and VVER [24] core design. These have recently been applied to the non-standard case where PWRs are highly loaded with Pu [25,26] and have been shown to facilitate low power peaking core design under challenging circumstances.

4 Use of WIMS/PANTHER to model I2S-LWR

4.1 I2S-LWR core description

The I2S-LWR core contains 121 assemblies with 144-in active fuel height as shown in Figure 1. The I2S-LWR is designed to achieve 40% higher power rating than a typical 2-loop Westinghouse core (∼2850 MWt vs. ∼2000 MWt). The major modification to achieve this objective was transitioning from a typical 16 × 16 assembly array to a 19 × 19 square pitch lattice. The increased number of fuel rods in the 19 × 19 lattice counterbalances the higher power density in the I2S-LWR thereby benefitting DNB performance and, also thanks to the high thermal conductivity of U3Si2, fuel temperature. The larger number of fuel rods in the 19 × 19 lattice leads to approximately same average linear power, 5.8 kW/ft, and only about 3% higher heat flux at the rod surface, 62 kW/ft2, for the I2S-LWR relative to a 5% uprated 4-loop PWR with 17 × 17 lattice. It must be pointed out that the H/HM atomic ratio for the 19 × 19 is lower, i.e. 3.5, than a typical PWR 17 × 17 lattice with H/HM of 3.9 due to the higher HM density of the U3Si2 fuel. Although under-moderated in terms of neutronic performances, both the 19 × 19 and 17 × 17 designs have similar moderator to fuel volumetric ratio of ∼2, and therefore the 19 × 19 lattice design poses no issues in normal and accidental operations. The main geometric parameters and fuel design characteristics are shown in Table 1.

The 3-batch I2S-LWR core loading pattern as shown in Figure 1 is identical to the one adopted by reference [11]. There are 40 fresh assemblies per reload out of 121 assemblies. The twice-burnt assemblies are positioned at the outermost peripheral locations to create a low leakage core. The I2S-LWR features 45 reactivity control clusters assemblies with 24 control rods (Ag-In-Cd) in the assembly.

The U3Si2 core design includes fresh and burned assemblies as shown in Figure 1. Fresh assemblies exploit different enrichments (i.e. 4.65, 4.45 and 2.6 w/o). The active core height of the I2S-LWR fuel axial stack is presented in Figure 2. In fuel assemblies with integral fuel burnable absorber (IFBA) rods (Fig. 2), only the middle portion (120-in) contains ZrB2 burnable poison, which is surrounded by 6-in non-IFBA top and bottom layers carrying the same fuel enrichment. Finally, 6-in top and bottom axial blankets are used to create the fuel stack. Lower enrichment (2.6 w/o) is used in the blankets in order to decrease the axial leakage of neutrons.

The 10B concentration used in the IFBA rods for the I2S-LWR, with U3Si2, fuel design, is 2.5 mg/in. Four assembly loading patterns are used to flatten the core power distribution and were investigated here, as depicted in Figure 3.

thumbnail Fig. 1

I2S-LWR equilibrium cycle core loading pattern (bottom right quadrant of the core).

Table 1

Main fuel assembly design parameters.

thumbnail Fig. 2

I2S-LWR fuel axial stack.

thumbnail Fig. 3

I2S-LWR IFBA loading patterns – the top right quadrant of the assembly is shown; IFBA rods are indicated in green.

4.2 Methods

The current work was divided into the following stages:

  • verification of the 2D WIMS assembly models against the reference solutions obtained with the Monte Carlo (MC) code Serpent. Serpent is a continuous-energy MC reactor physics code recently developed for reactor physics applications at VTT Technical Research Centre of Finland. Serpent can be used for 2D fuel lattice calculations as well as for 3D full core simulations. JEFF-3.1 cross-section libraries were used for WIMS and Serpent to minimize discrepancies in neutronic parameters (e.g. kinf) that could arise from the use of different nuclear data evaluations;

  • the core physics analysis of the I2S-LWR core design was performed with the core physics package PANTHER. WIMS10 was used for lattice data generation by employing a 172-group JEFF-3.1-based library. WIMS10 utilizes a multicell collision probability method to form 22-group cross-sections, followed by a method-of-characteristics solution to generate data for PANTHER. Results were compared to those reported in reference [11], which use deterministic lattice calculates to provide data for a 3D core analysis [27,28]. PANTHER used the same 3-batch self-generating reloading scheme that was iteratively applied to the U3Si2 core design until the main core parameters converged and a 12-month equilibrium cycle was reached.

4.3 Results

4.3.1 WIMS vs. Serpent comparison

This section presents the single-assembly comparison between WIMS and Serpent for different fuel assembly layouts (i.e. different numbers of IFBA rods). Figure 4 shows criticality curves for the different cases examined. Zero buckling hypothesis was adopted in the current comparison. The difference in reactivity, between Serpent and WIMS, for each of the cases is presented in Figure 5. In addition, Figure 6 shows the maximum difference in within-assembly power (pin-by-pin) between Serpent and WIMS. It must be pointed out that the average absolute difference in the assembly power between the codes is much lower (<0.15%). Figure 7 presents the pin-by-pin power distribution for an assembly that carries 156 IFBA rods.

thumbnail Fig. 4

Criticality curves for different IFBA loading patterns (note that k-inf initially increases with burnup as the burnable poison burns out).

thumbnail Fig. 5

Difference in reactivity (WIMS vs. Serpent) for different IFBA loading patterns.

thumbnail Fig. 6

Maximum relative difference (%) in assembly radial power distribution (WIMS vs. Serpent).

thumbnail Fig. 7

Top right octant assembly (4.45 w/o and 156 IFBA rods) radial power distribution (WIMS vs. Serpent) at zero burnup; IFBA rods are indicated in green.

4.3.2 Equilibrium core analysis

The representative burnup (MWD/tHM) distribution at the beginning of the equilibrium cycle is presented in the octant-core map in Figure 8. Figure 9 shows the required boron concentration to maintain criticality over the equilibrium cycle. The radial and total power peaking factors, which represent the quarter-wise assembly values, are depicted in Figure 10, which also presents the time-dependent axial offset. Results are in good agreement with the values reported in reference [11] (e.g. assembly burnups within around 1%). This cross-comparison of results provides enhanced confidence in the reliability and accuracy of the results.

thumbnail Fig. 8

I2S-LWR equilibrium burnup in PANTHER.

thumbnail Fig. 9

Comparison of the critical boron concentration (ppm) as a function of burnup in PANTHER.

thumbnail Fig. 10

Axial Offset (AO) (left) and radial and total power peaking factors (right) for I2S-LWR calculated using PANTHER.

5 Conclusions

The UK has a long history in industrial fuel manufacture and fabrication for a wide range of reactor systems including LWRs. This is supported by a national infrastructure to perform experimental and theoretical R&D in fuel performance, fuel transient behaviour and reactor physics. The ANSWERS lattice code WIMS and core simulator PANTHER are used to support the operation of existing PWRs, including in the UK and Belgium. Modelling of ATFs is a natural extension of these capabilities and can largely be performed using existing calculation routes. Reactor physics modelling of the I2S-LWR equilibrium cycle core was performed with the WIMS-PANTHER codes. The results were compared to reported results for the equilibrium cycle of the I2S-LWR and indicate that there is a reasonable agreement between the codes. One possible source for the observed deviations between the codes is the different cross-section library employed in WIMS to generate lattice parameters. For this study, the JEFF-3.1 libraries were used in WIMS, whereas ENDF BVII.0 was used in reference [11]. Future work could consider using the ENDF BVII.0 library in WIMS to allow for a more consistent comparison. It may also ultimately be necessary to validate the reactor physics codes against experimental data.

Acknowledgments

We are grateful to our colleagues in the ANSWERS team for providing advice and guidance during the preparation of this paper.

This research was in part funded by the UK Engineering and Physical Sciences Research Council (EPSRC) under grant EP/K033611/1 and by the US Depart of Energy (DOE) Office of Nuclear Energy's Nuclear Energy University Programs (NEUP).

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Cite this article as: Benjamin A. Lindley, Dan Kotlyar, Geoffrey T. Parks, John N. Lillington, Bojan Petrovic, Reactor physics modelling of accident tolerant fuel for LWRs using ANSWERS codes, EPJ Nuclear Sci. Technol. 2, 14 (2016)

All Tables

Table 1

Main fuel assembly design parameters.

All Figures

thumbnail Fig. 1

I2S-LWR equilibrium cycle core loading pattern (bottom right quadrant of the core).

In the text
thumbnail Fig. 2

I2S-LWR fuel axial stack.

In the text
thumbnail Fig. 3

I2S-LWR IFBA loading patterns – the top right quadrant of the assembly is shown; IFBA rods are indicated in green.

In the text
thumbnail Fig. 4

Criticality curves for different IFBA loading patterns (note that k-inf initially increases with burnup as the burnable poison burns out).

In the text
thumbnail Fig. 5

Difference in reactivity (WIMS vs. Serpent) for different IFBA loading patterns.

In the text
thumbnail Fig. 6

Maximum relative difference (%) in assembly radial power distribution (WIMS vs. Serpent).

In the text
thumbnail Fig. 7

Top right octant assembly (4.45 w/o and 156 IFBA rods) radial power distribution (WIMS vs. Serpent) at zero burnup; IFBA rods are indicated in green.

In the text
thumbnail Fig. 8

I2S-LWR equilibrium burnup in PANTHER.

In the text
thumbnail Fig. 9

Comparison of the critical boron concentration (ppm) as a function of burnup in PANTHER.

In the text
thumbnail Fig. 10

Axial Offset (AO) (left) and radial and total power peaking factors (right) for I2S-LWR calculated using PANTHER.

In the text

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