Monte Carlo depletion codes (like MONTEBURNS [1] or MCNPX 2.6.a+ with its internal depletion capability [2]) have the advantage of being flexible in geometry and their application areas (from simple 1D to complex 3D geometries, from normal critical systems to external fixed sources). The use of Monte Carlo also allows for multi-particle physics, complex interaction laws and detailed energy-angle descriptions. These capabilities could make Monte Carlo burn-up codes very powerful, especially for hybrid and advanced nuclear systems.
Still, continuous energy Monte Carlo depletion codes have had limited success mainly due to the rather long CPU time required to carry out very detailed and accurate calculations, even with modern computer technology. To work around this issue, users often have to reduce the number of nuclides in the evolution chains or to consider either longer irradiation time steps and/or larger spatial burn-up cells, jeopardizing the accuracy of the calculation in all cases.
These are the basic issues that we wish to address with VESTA, a Monte Carlo depletion interface code that is currently under development at IRSN. With VESTA, the emphasis lies on both accuracy and performance, so that the code will be capable of providing accurate and complete answers in an acceptable amount of time compared to other Monte Carlo depletion codes.
From its inception, VESTA is intended to be a “generic” interface code so that it will ultimately be capable of using any Monte-Carlo code or depletion module and that can be tailored to  the user’s needs. For now, VESTA allows for the use of any version of MCNP(X) and a modified version of ORIGEN 2.2. Support for other Monte-Carlo codes (like the MORET 5 code developed at IRSN [4]) and/or depletion modules is foreseen in the near future.