The present paper concerns a novel type of integral database generated in the course of the LWR-PROTEUS Phase I experiments, viz. the relative reactivity effects of removing individual fuel pins from a highly heterogeneous BWR assembly. The measurements reported were conducted in the central assembly of a 33 test-zone configuration of SVEA-96+ assemblies in PROTEUS, under full-density water moderation conditions. Calculations of the pin-removal reactivity worths have been carried out using two different LWR assembly codes, viz. CASMO-4 and BOXER. The discrepancies observed between experiment and calculation have been analysed in detail, on the basis of an extended reactivity decomposition methodology. This has allowed quantification of the different phase-space contributions (in terms of reaction rate types, energy groups and spatial regions) of a given calculated pin-removal reactivity worth, thus providing useful insights regarding the most important sources of error in each of the assembly codes investigated.[All rights reserved Elsevier].