[en] In recent years, the SP_L (simplified spherical harmonics) equations have received renewed interest for the simulation of nuclear systems. We have derived the SP_L equations starting from the even-parity form of the S_N equations. The SP_L equations form a system of (L+1)/2 second order partial differential equations that can be solved with standard iterative techniques such as the Conjugate Gradient (CG). We discretized the SP_L equations with the finite-volume approach in a 3-D Cartesian space. We developed a new 3-D general code, Pensp_L (Parallel Environment Neutral-particle SP_L). Pensp_L solves both fixed source and criticality eigenvalue problems. In order to optimize the memory management, we implemented a Compressed Diagonal Storage (CDS) to store the SP_L matrices. Pensp_L includes parallel algorithms for space and moment domain decomposition. The computational load is distributed on different processors, using a mapping function, which maps the 3-D Cartesian space and moments onto processors. The code is written in Fortran 90 using the Message Passing Interface (MPI) libraries for the parallel implementation of the algorithm. The code has been tested on the Pcpen cluster and the parallel performance has been assessed in terms of speed-up and parallel efficiency. (author)