Influence of Input Neutron Spectrum Covariances on Results of Pressure Vessel Neutron Spectrum Adjustments

One of the most critical problems in practical applications of spectrum adjustment in pressure vessel dosimetry is the insufficient knowledge of the input spectrum covariance matrix. Usually very crude approximations for this covariance matrix are used in adjustment procedures and, moreover, it is not clear to which extent these approximations influence the final results. Therefore it appeared useful to calculate this matrix as carefully as possible for a typical system and in positions for which a sufficient amount of activation measurements and precise transport calculations was available. The paper presents and discusses results of such calculations for a VVER-1000 type reactor. They base on the one hand on sensitivity coefficients estimated in transport calculations by means of the ANISN code and on the other hand on cross section covariance data taken from the Obninsk ABBN-93 data library. The results of spectrum adjustments based on the estimated covariance matrix are compared with results based on formerly used matrices. The obtained differences are discussed.