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    Experimental and Theoretical Neutron Flux Estimations at the Surveillance Chain, at the Pressure Vessel Inner Surface, and in the Cavity of a WER-440 PWR

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    Neutron flux measurements and calculations have been performed for VVER 440 reactors at the Loviisa nuclear power plant (NPP) at several locations and for three core configurations.The neutron fluxes within the specimens and the mean axial distributions were measured by using the irradiated steel specimens themselves in the surveillance chains (SC) as dosimeters. The neutron spectra (SC, cavity) were checked and adjusted based on dosimeter sets with Fe(nat), 54Fe, Ni, Ti, Cu, Nb. 238U, Co, and Ag (232Th, 237Np).

    The quality of the neutron dose estimates could be significantly improved by using reactor structure materials and especially scraped samples (Fe, Ni, Nb) from the inner and outer surfaces of the reactor pressure vessel (PV) as dosimeters, by applying standardized and relative (PV/SC) methods, which cancel or strongly suppress the influence of systematic (correlated) errors in the cross sections and in other quantities, by developing and using the Nb technique, and by applying a dosimeter rack in the cavity outside the PV.

    Accurate flux ratios (lead factors PV/SC) were derived from these measurements.

    Nb was separated and purified by chemical methods.

    The computed estimates agree well with the measurements even at the distant radial positions at the PV. The calculations were performed using the REPVICS program system (mainly SN method, BUGLE-80 cross-section library). Three-dimensional flux distributions were synthesized from one-dimensional ANISN and two-dimensional DOT 3.5-E calculations ((r,θ) and (r,z)) with additional verification using Monte Carlo calculations.

    Together with the material embrittlement measurements of irradiated SC specimens the obtained unique set of measured and calculated flux estimates provide an improved basis for the prediction of pressure vessel embrittlement.

    A summary of the neutron flux estimations is presented.


    neutron fluence, pressure vessels, embrittlement, activation detectors, PWR-type reactors, accuracy, neutron reactions, niobium 93 target, Loviisa reactors

    Author Information:

    Bäars, LB
    Technical Research Centre of Finland, Espoo,

    Serén, TO
    Technical Research Centre of Finland, Espoo,

    Wasastjerna, F
    Technical Research Centre of Finland, Helsinki,

    Committee/Subcommittee: E10.05

    DOI: 10.1520/STP10068S