The four CASL strategic goals are long-term overall accomplishments CASL endeavors to achieve for success. The CASL strategy, consisting of six key elements, is the method CASL employs to bring its goals to a successful conclusion. Each of the six CASL strategic elements is briefly described below. Associated with each element of the CASL strategy is one or more strategic roadmaps outlining the logical steps that build upon one another in progressing toward CASL’s strategic goals.

Challenge Problems for PWR Core Phenomena

To achieve the key CASL objectives of enabling power uprates, increasing fuel burnup and cycle length, and achieving life extension for U.S. nuclear plants, there is a need to better understand operational and safety margins. CASL is focused on a set of specific Challenge Problems (CPs) that encompass the key phenomena currently limiting the performance of PWRs, with the recognition that much of the capability developed will be broadly applicable to other types of reactors. CASL defines a Challenge Problem as one whose solution is (1) important to the nuclear industry and (2) amenable to or enabled by modeling & simulation (M&S). The currently defined Challenge Problems are shown in Table 1.

Table 1: CASL Challenge Problems for First Phase

CASL Challenge Problems for First Phase

The Challenge Problems were selected to address three key issues for nuclear energy: cost, reduction in amount of used nuclear fuel, and safety. All three can be enabled by power uprates, lifetime extension, and higher burnup through predictive simulation, accomplished in a fashion that maintains or improves nuclear safety. Table 2 indicates how the CASL challenge problem strategy supports these industry drivers.

Table 2: CASL Challenge Problems directly support key nuclear industry drivers

Industry Driver Industry Driver Performance Measures Supporting CASL Challenge Problems Simulation Impact Goals
Power Uprates
  • Augment planned U.S. reactor fleet power uprates beyond current value of 2694 MWe thru 2016
  • Increase proportion of stretch (>2%) and extended (>7%) power uprates in U.S reactor fleet
  • Cladding Integrity (DNB, LOCA, RIA)
  • Reactor Integrity (vessel, internals)
  • Advanced Fuels
  • Reduce or eliminate restrictions to uprates associated with current challenge problem
  • Contribute capability to support one or more license uprate applications
Lifetime Extension
  • License remaining ~30% of U.S. reactor fleet to 60 years where appropriate before 2015
  • Safely extend lifetimes of U.S. reactor fleet to 80 years where appropriate before 2020
  • Reactor Integrity (vessel, internals)
  • Advanced Fuels
  • Provide reliable estimates of reactor environment experienced over lifetime
  • Contribute capability to support one or more lifetime extension applications
Higher Fuel Burnup
  • Increase maximum fuel assembly burnup of 55 GWd/MTU in U.S. reactor fleet
  • Increase allowable peak fuel rod burnup of 62 GWd/MTU in U.S. reactor fleet
  • Cladding Integrity (LOCA, RIA)
  • Advanced Fuels
  • Provide capability to support increase of average fuel cycle length
  • Provide capability to support increase NRC fuel burnup limits
  • Facilitate advanced fuel design
Improved Safety
  • Maintain coolable fuel geometry and minimize fuel hydrogen production during/after LOCA
  • Minimize fuel damage and fission gas release during accidents
  • Maintain fission product barriers during anticipated accidents
  • Cladding Integrity (LOCA, RIA)
  • Reactor Integrity (vessel, internals)
  • Advanced Fuels
  • Facilitate advanced fuel design
  • Improve understanding and quantification of margins

Each CASL Challenge Problem is by definition impactful to industry, yet whose solutions currently possess uncertainties and a lack of understanding that prohibit a more aggressive pursuit of the performance goals given in Table 2. Amelioration of these uncertainties follows by addressing the M&S gaps and drivers for each CP and meeting specific capability targets for the CASL M&S tools to ensure closure of these gaps.

VERA: Virtual Environment for Reactor Applications

In order to address the Challenge Problems previously described, CASL is collectively developing a suite of simulation capabilities known as the Virtual Environment for Reactor Applications (VERA). While VERA development is guided by the CASL Challenge Problems, its capabilities will be broadly applicable for other PWR analyses, and designed to be extensible, to allow future application to other reactor types and to challenges beyond normal operation.

As shown in the figure below, VERA includes many physics and software infrastructure components required to support the simulation of the CASL challenge problems. These areas include neutronics being developed by the Radiation Transport Methods (RTM) Focus Area, Thermal-Hydraulics including Computational Fluid Dynamics methods being developed by the Thermal-Hydraulics Methods (THM) Focus Area, Subchannel methods being developed by the Physics Integration (PHI) Focus Area, and fuel performance and coolant chemistry methods being developed by the Materials Performance Optimization (MPO) Focus Area. An integrated subset of VERA, called VERA-CS (Core Simulator), as shown with the red line in the figure below, is needed to follow normal steady-state operation and is being developed by the RTM and PHI focus areas. Physics coupling, data transfer, input output and other activities are also being developed to provide the integrated VERA capability. The Validation and Uncertainty Quantification (VUQ) Focus Area is developing methods to support verification and validation of VERA. The Advanced Modeling Applications (AMA) Focus area are the early users, applying VERA and its components to challenge problems and performing other applications to inform development.


Figure 3: VERA is a toolkit of components for scalable simulation of reactor core behavior

VERA is not a single simulation tool but rather a collection of capabilities for scalable simulation of nuclear reactor core behavior. It is a usable and flexible toolkit of components partitioned into two primary categories (see Figure 3):

  • Baseline: Existing industry codes in use today, in most cases proprietary. These tools provide a baseline for comparison against new CASL-developed capabilities, and provide useful insight into current industry analysis practices – in addition, they can be used for sensitivity analysis in order to guide development priorities. While CASL is not extending the core functionality of these tools, we are in some cases coupling them in new ways so as to explore feedback effects. These efforts have the benefit of quickly providing industry partners with improved tools they are already familiar with.
  • Advanced: These are capabilities developed both within VRI and in other FAs (THM, RTM, MPO) and integrated into the common VERA software environment, where they are algorithmically coupled, share input/output, and subjected to sensitivity analysis and UQ tools provided by VUQ.

A specific subset of advanced VERA capabilities can be grouped to encompass functionality similar to current industry tools often referred to as “core simulators”, though typically of higher-fidelity, increased feedback between physics components, and designed to be scalable from small clusters to the largest supercomputers. The VERA core simulator components, denoted VERA-CS (see Figure 3), represents a code system used to model quasi-steady-state LWR conditions and depletion, and includes models for neutron transport, cross sections, thermal-hydraulics, fuel temperature, and depletion. It can link to other physics (e.g., CFD, fuel performance, CRUD models, structural, reactor system) but its principal function is to provide reactor conditions and distributions needed to solve the CASL Challenge Problems.

The VERA-CS distinguishing capability is to deplete the reactor core on a pin-by-pin basis with actual reactor conditions as shown in Table 3.

Table 3: Comparison of CASL’s core simulator (VERA-CS) with industry core simulators

Physics Model Industry Practice VERA-CS (in progress)
Neutron Transport 3D diffusion (core), 2 energy groups (core), 2D transport on single assemblies 3-D transport, 23+ energy groups
Power Distribution nodal average with pin power reconstruction methods explicit pin-by-pin*
Thermal Hydraulics nodal average (1D) subchannel with crossflow
Fuel Temperatures nodal average pin-by-pin*
Xenon / Samarium nodal average with corrections pin-by-pin*
Depletion reconstructed pin exposures with infinite-medium cross sections quadratic burnup correction, history corrections, spectral corrections pin-by-pin* with actual core conditions
Reflector Models 1D cross section models actual 3-D geometry
Target Platforms workstation (single processing core) 1K – 300K processing cores

*pin-homogenized or pin-resolved, depending upon the application

Objectives for VERA by 2015 include:

  • Version 5.0+ released and in use with supporting documentation and tutorials
  • Release and support model implemented
  • Functionalities integrated and coupled (varying degrees of maturity)
    • Industry Baseline: neutronics (nodal diffusion) + T-H (assembly subchannel) + chemistry
    • T-H: pin-by-pin subchannel, single- and multi-phase CFD
    • Neutronics: pin-homogenized transport, pin-resolved transport, hybrid transport
    • Fuel performance: advanced and upscaled capabilities
    • Coolant chemistry: CRUD formation and evolution
    • SA/UQ: advanced DAKOTA-based capabilities integrated and accessible
    • Structural mechanics: accessibility to legacy capabilities, prototype new capability
    • System: accessibility to legacy and advanced capabilities
    • Infrastructure: build/test system, LIME and coupling model evaluators, data transfer and coupling toolkits, linear/nonlinear solvers (Trilinos), reactor aware I/O
  • Core Simulator: full functionality for all Benchmark Progression Problems (core follow and reload)

Enabling R&D

Three (RTM, THM, MPO) of the six CASL focus areas (FAs) are dedicated to developing fundamental models of each relevant physical phenomenon linked through VERA, and one focus area (VUQ) is developing and utilizing Verification and Validation (V&V) and UQ capabilities.

Each of these science focus areas has an expected evolution of R&D accomplishments and technology capabilities. Specific 2015 capability goals are given in Table 4.

Table 4: 2015 capability goals for the CASL science focus areas

Focus Area

2015 Capability Goal


Robust 3D pin-resolved transport and prototype hybrid Monte Carlo transport with modern cross section/shielding treatments and coupling to T-H, fuel, and corrosion chemistry capabilities


Robust 3D steady-state/transient turbulent multi-phase capability with subcooled boiling models, an initial assessment of DNB, and complementary with a modern subchannel capability


Mature 3D fuel performance capability with full assessment against CRUD/PCI/GTRF problems. Validated fuel performance models inform assessments of safety margin (PCI) and best operational practices (CRUD, GTRF). Functional capability and partial assessment for RIA- and LOCA-based transient problems


Mathematical tools and methodologies integrated and accessible to enable quantifying sensitivities and uncertainties in full-scale multi physics PWR simulations


Technology Delivery

A key strategy for CASL is the proactive delivery of its technology to current and potential customers, clients, and users throughout the nuclear energy enterprise. In keeping with an agile philosophy and project management approach, CASL is committed to early and regular delivery of its technology (e.g., VERA) on fixed schedules, with the functionality of this technology evolving based on iterative feedback with those receiving and applying/testing the technology. This regular technology export is especially useful and appropriate for accommodating changing requirements and research-driven projects typical of many CASL efforts. It also tends to amplify learning through iterative development and refinement based on customer feedback and assessments. CASL has four principal strategic paths for delivery of its technology:

  • Release and support of VERA - regular (e.g., annual) software releases of selected, ready (robust) portions of VERA to both CASL partners and the nuclear community at large;
  • Test Stands - early deployment of VERA into actual engineering/design environments at CASL core partner institutions for the purpose of rapid and enhanced feedback and testing, use, and ultimate adoption of VERA in supporting real-world LWR applications;
  • Pilot Project - early demonstration of CASL-developed capabilities (VERA and/or VERA simulation results) to a problem of interest brought before the CASL Industry Council (IC) by a nuclear industry institution outside of the CASL consortium; and
  • Collaborative Applications - use of VERA in close association with but external to CASL to demonstrate VERA’s usability and value.

Each of these technology delivery mechanisms is discussed in turn below.

Release and Support of VERA

In accordance with its vision, namely that VERA will be deployed and applied broadly throughout the nuclear energy community, CASL envisions two strategic paths for release and support of VERA:

  • Continued availability and limited support of stable (static) or beta (exploratory) versions of VERA through Radiation Safety Information Computational Center (RSICC) distribution at ORNL and through open source distribution of the VERA infrastructure in accordance with practices established during active CASL execution; and
  • Periodic availability and full support of evolving, stable, industry-hardened, and user-friendly versions of VERA through distribution by a CASL core partner or a third party entity.

Beyond CASL lifetime as an Energy Innovation Hub (2015 or 2020), subsequent nuclear technology federal programs (in the DOE or elsewhere) supporting M&S technology development and application would be encouraged to leverage and evolve the VERA releases. It is envisioned that a stable and user-friendly version of VERA will be available through one or more CASL core partners, or a third party entity, after the CASL Program is complete.

VERA updates that are released after the CASL Program is complete would be compliant with the CASL VERA licensing model and IP Management Plan. These releases would typically be the result of DOE-funded R&D efforts at DOE laboratories and university institutions, and would likely focus on improved or new physical model and numerical algorithm development, rather than on usability or application. The releases would therefore be more exploratory in nature and their full support would be limited, or at least not guaranteed.

Test Stands

CASL Test Stands are functional prototype VERA installations at a CASL industry partner or Industry Council member site. The principal objective of the CASL Test Stands, ideally deployed during active VERA development, is to provide constructive feedback to CASL through critical exercise of CASL virtual reactor capabilities while addressing issues that are important to users. The Test Stands involve early deployment of VERA into an actual engineering/design environment supporting to give the Test Stand host improved modeling and simulation capabilities and to give CASL enhanced feedback and testing.

Examples of how the CASL Test Stands could be used to address industry issues include:

  • Use of VERA to solve Challenge Problems for plant applications associated with operational and safety scenarios;
  • Use of VERA to support power uprates, lifetime extension, and higher fuel discharge burnup.

The test stand deployment process is subdivided into five stages: development, release, application, post analysis and refinement. The development, release and refinement stages encompass activities mainly within CASL that support the deployment of test stands. The application and post analysis stages encompass activities that the CASL Partners using the test stands will undertake, and activities that are performed as part of the CASL defined scope related to the physical reactor and Challenge Problem applications projects within the AMA FA.

Pilot Projects

Pilot Projects are early demonstrations of CASL-developed capabilities to a problem of interest to industry. They are identified and implemented as a result of joint efforts by the AMA and VRI FAs and the IC. Here, the expertise of IC member organizations is solicited to inform CASL staff and provide feedback for the development of CASL products. The pilot projects will provide information that will help ensure VERA meets the needs of the nuclear industry. Key additional areas considered during the pilot project development process are system usability in an engineering environment, quality assurance, output of results, computational runtime, and integration into existing engineering analysis processes.

Collaborative Applications

Collaborative Applications will be performed to further evaluate the usability of VERA. Investigators who are external to CASL with support from CASL staff will generally develop these applications. The purpose of these applications is to ensure VERA can be used to solve problems that are not strictly scoped within the CASL development process. Collaborative Applications will generally be externally funded and will be initially sponsored through the IC.

Education and Training

The CASL education and training objective is to educate today’s reactor designers in the use of advanced M&S tools, and to develop the next generation of nuclear engineers and scientists through development of new curricula.

The CASL education and training effort is the first of its kind to develop new curricula and programs in nuclear energy at the graduate level based on the M&S technologies developed by CASL. The CASL education and training strategy is being carried out with the following elements:

  • Promoting diversity and excellence among the nuclear engineering student population with active recruiting of top students at the undergraduate and graduate levels, underrepresented minorities and women to higher levels, and interdisciplinary students;
  • Mapping the CASL Challenge Problems into existing graduate course curricula;
  • Developing new chapter modules for courses based on CASL Challenge Problem course materials;
  • Developing and executing an industry education knowledge transfer plan;
  • Developing a template of courses for a new certificate in nuclear systems design; and
  • Executing annual programs for undergraduate scholars and graduate student interns.

Collaboration and Ideation

CASL’s virtual one-roof objective wholly relies upon the development and maturation of its Virtual Office, Community, and Computing (VOCC) venture. The VOCC delivers a unified collaboration platform and creative work environment to support CASL’s mission (see Figure 4), and it has two primary goals. The first goal is to bring together under one virtual roof the best scientists, researchers, students, and design and applications engineers. The second goal is to successfully operate and mature a state-of-the-art scientific collaboration space (the VOCC Laboratory) that supports all CASL R&D use cases: business and program management; agile code development; VERA application and data analytics (of VERA simulation datasets); and lectern environment and research partnerships. The VOCC uses human-centered, immersive, and visual analytic design techniques and principles to build a physical workspace for the specific purpose of unifying geographically distributed CASL staff and computational (e.g., HPC) resources.

Figure 4: CASL VOCC Laboratory visual venues