Orphan processes in killed NWChem 6.1.1 job

We are running NWChem 6.1.1 (Jan 2012) on CentOS-6.3-x86_64 using openmpi 1.5.4 on a 16-core node. I notice that after a multi-processor job is killed, the system still claims the cpu activity and the %idle does not return to 100% as it does after a completed job, according to the output of a sar command. These cores are then not available for subsequent multi-processor jobs. The only way I know to reclaim these cores is to restart the node.