MURE: MCNP Utility for Reactor Evolution. Description of the methods, first applications and results

The main aim of the MURE package is to perform nuclear reactor time-evolution using the widely used particle transport code MCNP, a Monte Carlo code that is mostly written in FORTRAN. MURE is based on C++ objects allowing a great flexibility in the use. There are three main parts in this library. Part 1: Definition of the geometry, materials, neutron source, tallies, etc.. Part 2: Construction of the nuclear tree, the network of links between neighbour nuclei via radioactive decays and nuclear reactions. Part 3: Evolution of some materials, by solving the corresponding Bateman's equations. Moreover an interface to NJOY in order to process cross-sections at the wanted temperature is provided. Part 1 can be used independently of the two others; it allows 'easy' generation of MCNP input files by providing a set of classes for describing complex geometries. The ability to make quick global changes to reactor component dimensions and the ability to create large lattices of similar components are two important features that can be implemented by the C++ interface. It should be noted that some knowledge of MCNP is very useful in understanding the geometry generation philosophy. Part 2 builds the specific nuclear tree from an initial material composition (list of nuclei). The tree of each 'evolving' nucleus is created by following the links between neighbours via radioactive decay and/or reactions until a self-consistent set of linked nuclei is extracted. Nuclei with half-lives which are very much shorter than the evolution time steps are removed from the tree and their parents and daughters re-linked in the correct way. Part 3 aims at simulating the evolution of the fuel within a given reactor over a time period of up to several years, by successive steps of MCNP calculation and numerical integration of Bateman's equations. Each time MCNP is called, the reactor fuel composition will have changed due to the reaction/decay process occurring inside. Changes in geometry, temperature, external feeding or extraction during the evolution can also be taken into account. Obviously this part is not independent of the other two. These main three parts of MURE are described in principles in the first section of this paper. The second one will then give first results obtained on precise simulation cases: FNR benchmark, PWR Th/Pu evolution and MSR temperature calculation. The paper has the following contents: Introduction; 1. Main MURE functions: geometry generation, nuclear data management and evolution calculation; Easy MCNP generation by means of a few intuitive C++ classes; Principles of the MCNP-based evolution calculations; 2. First applications and results obtained by MURE on precise reactor simulations; Benchmarking of Fast Breeder Reactors; Study of U-233 production in Th/Pu loaded PWR; Temperature calculation by coupling to a thermal-hydraulics code; Conclusions and perspectives. In summarizing, one shows that MURE consists of a powerful ensemble of utilities in C++, which are easy to use. It has been constructed for numerical simulation of reactor evolution, experimental setup modeling, and especially for evaluation of future reactor designs. Up to now, MURE contains about 18000 lines of code in 20 different classes, and is in continuous development with a constantly increasing number of users. There is still some work to do, concerning among others evolution control features, graphical interface and coupling with a thermohydraulic code like TRIO-U for safety transient studies. Next studies will include new reactor simulations (for thorium cycle, in both existing reactors and generation IV projects, and for transmutation and incineration, in FNR and ADS) as well as scenario studies involving the studied systems