Providing Answers

27 July 2017

Amec Foster Wheeler’s John Lillington says that software codes that were first developed decades ago have remained an essential tool for designers of new reactors.

The engineers, scientists and software engineering experts behind Amec Foster Wheeler’s Answers Software Service have been at the forefront of radiation transport modelling for more than 30 years. The Answers computer codes are used to simulate nuclear processes, creating models which enable customers to solve problems in core design, fault studies, reactor shielding, dosimetry and transport flask assessments, waste management and decommissioning.

Considerable effort goes into keeping Answers up to date and relevant, with improvements in areas such as computational methods development, high performance computing, automated processing, adaptivity, advanced graphics, multi-physics, uncertainty quantification and optimisation.

This has helped to ensure that the software is extensively validated for current and future reactor designs.

Pressurised water reactors

The Answers physics codes have been utilised for many Gen III and Gen III+ reactor applications on currently operating plant; for loading pattern optimisation, fuel design and safety studies. This includes the UK PWR Sizewell B and other reactors that have been in operation for many years. Much of the recent work has focused on validation of the Answers WIMS and MONK codes’ validation for modelling high burn-up fuel.

Answers is also used for new water reactor designs that are at various stages of development in the UK, and codes applications take full advantage of the experience gained from these earlier activities.

Small modular reactors (SMRs)

Since SMRs are smaller than established power generating reactors, costs can be reduced by taking advantage of increased modularisation and increased production volumes. There are also more opportunities for load following and new non-electricity generating applications. This makes SMRs an attractive part of the future energy mix. In using Answers to model SMRs we can take advantage of experience gained from work on earlier small research and prototype reactors with various technologies. In particular this is true for modelling and operational experience. It also includes development work carried out for early UK SMR designs such as the Safe Integral Reactor. Answers codes have been developed and validated against many small-scale experiments and small reactors. This background makes them ideally suited to current SMR applications.

Currently, Answers codes are being utilised in the development of a UK PWR SMR core design led by Rolls-Royce.

Future naval propulsion plants

The development of a new ‘Whole Core 3D Reactor Physics Code’ for analysis of future naval nuclear propulsion plants is a multi- year collaboration between Rolls-Royce and Amec Foster Wheeler.

The modelling development is based on the Answers WIMS physics code, including new 3-dimensional fine-mesh whole-core solution methods based on transport theory. The new code will provide better accuracy, greater justifiable safety margins and better core performance than current methods.

Tractebel MOX fuel route

Two codes – LWRWIMS and the EDF-owned Panther – are currently used in combination for licensing and reload studies analysis in Belgian PWRs. The goal of the programme is to provide Tractebel with a modern route at the state of the art of current industrial methodologies. This would ensure enhanced accuracy and be a step towards producing high-fidelity methods in terms of pin power reconstruction and depletion evolution. These capabilities are beneficial for new-build projects and are mandatory where MOX cores are concerned.

This is a multi-year programme of collaborative work between Tractebel Engineering, EDF Energy and Amec Foster Wheeler.

Boiling water reactors

In the UK, there has been considerable investment in developing BWR modelling capability, following Hitachi’s decision to seek a licence for its UK ABWR. The design is currently in the last phase of its Generic Design Acceptance process in the UK. Horizon Nuclear Power has plans to build two units on Anglesey in North Wales (a progress update is on p15-17).

BWR technology has been utilised previously in the UK but mainly focused on pressure tube boiling water reactors. Considerable effort has been devoted to extending the Answers core physics codes to ABWR technology. In particular, Amec Foster Wheeler has been developing the Answers WIMS/ PANTHER code methodology (previously developed for PWRs) for BWR applications.

The original PWR code methodology has been modified to meet the challenges in core modelling for BWRs. The codes can potentially be used for fuel cycle design and other purposes so they accommodate:

  • Bulk core boiling and heterogeneous axial power profile;
  • Control rod insertion from the bottom of the reactor vessel;
  • Reactivity control practices (burnable poisons, deeper control rod insertion, coolant flow variation);
  • More heterogeneous fuel assemblies etc.

High temperature reactors

The UK has amassed a great deal of experience in gas-reactor technology through experience of operating first the Magnox reactors and kater the AGR fleet. Much of this experience is relevant to potential future new-build HTR technology.

In particular, Amec Foster Wheeler has applied the EDF Energy NUMEG code for whole plant AGR steady state modelling, optimisation and boiler transients. Re- engineered from earlier versions, Model Driven Architecture (MDA) methodology generates code from design and there are versions in C, F90, F95 and F2003. Amec Foster Wheeler is now responsible for the ongoing maintenance and development of this software.

The Dragon reactor, which operated at Winfrith during the last century, was one of the first HTRs to operate with high- temperature carbon layered fuel. Experience with this technology has been useful in the UK’s contributions to the PBMR project and for support work on high-temperature TRISO fuel modelling.

A current interest is the UK U-Battery helium-cooled micro-reactor, a graphite- moderated high-temperature reactor with
a capacity of about 4MWe, which is being developed by Urenco in a consortium with Amec Foster Wheeler and others (see p18-19).

A goal for Answers HTR activities has been to develop WIMS and MONK models for advanced HTR concepts. The reactor physics and fuel performance of HTRs is difficult to model and the ‘double heterogeneity’ geometry (small spherical TRISO fuel particles in a graphite matrix, within larger cylinders) presents unique modelling challenges. Modelling small reactors is also challenging as they are ‘leaky’, with neutron losses at the peripherals. 

HTR pebbles, cylinders and plates are analysed using double-heterogeneity modelling. If necessary, a lattice calculation can then be used to model complicated assembly geometries. This allows equivalent cross sections to be generated for multigroup core diffusion theory calculations, or alternatively from a full 3D multi-group transport solution of the core.

R&D has focused on modelling high temperature fuel, and validation of the Answers MONK code performed against the PROTEUS experiments and other data.

Fast reactors

The UK has long history of operating fast reactors and the Answers codes have been validated against of number of experiments in UK and international programmes. Currently, the main interest is in the fast reactor Gen IV systems. Amec Foster Wheeler is participating in European activities including the SNETP ESNII+ programme, covering liquid sodium fast reactor (SFR), gas cooled fast reactor (GFR), lead cooled fast reactor (LFR) and lead-cooled ADS technologies. These are also being developed in underpinning national programmes, Astrid, Allegro, Alfred and Myrrha, respectively.

Interest in these Gen IV applications has expanded from electricity generation to include plutonium disposition, actinide recycling and waste transmutation interests. There is also some level of interest in fast-spectrum molten salt reactor (MSR) technology, in regard to enhanced safety, and actinide and fission product management through the life of the fuel cycle.

The Answers reactor physics codes have been extended to liquid-metal and gas- cooled fast reactors and also molten salt reactors (MSRs). The WIMS code can be used in hexagonal grid geometry (in addition to rectilinear geometry usually used for thermal reactors). Core solutions can be obtained using various methods, using diffusion theory or multi-group Monte Carlo. The Answers codes have also been validated for application to more advanced fuels; the Answers fuel performance code TRAFIC can also model nitride and carbide fuels.

Back-end fuel cycle applications

The QSS and RTM codes have been developed for Radioactive Waste Management (RWM), a wholly-owned subsidiary of the UK’s Nuclear Decommissioning Authority (NDA), to model hypothetical criticality transients – quasi steady state (QSS) and rapid transients (RTM) – in a geologic disposal facility once it has been sealed up.

QSS and RTM are owned by RWM and are not part of the Answers software but Answers has been used to provide data for RWM’s models.

MONK and WIMS have both been used to model neutronics effects, thermal-hydraulics and structural response of the planned Geological Disposal Facility and surrounding rock. They have been used to analyse a wide range of scenarios, encompassing various packaging and disposal concepts, and have been subject to verification, validation and benchmarking studies as well as international peer review. They have also been used for extensive sensitivity and uncertainty studies.

The QSS model has been compared favourably with measured data from one of the Oklo natural reactors that operated for hundreds of thousands of years in West Africa two billion years ago.

Dr John Lillington is a Fellow of the Institutes of Physics and Mathematics and a Chartered Engineer with more than 40 years’ experience in the nuclear industry. He has worked on all major reactor systems as a theoretical physicist, safety analyst, and technical and project manager. 

AFW WIMS supercell model (Reproduced courtesy of Horizon Nuclear Power)
AFW Answers has been used at the UK’s fleet of AGRs, as well as the Sizewell B (credit: EDF Energy)
AFW High temperature reactor multiscale modelling approach in WIMS

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