Repeated operation of fuel assemblies in the VVER-1000 core after their holding in the spent fuel pool
During reactor plant operation sometimes situations occur when the fuel assembly is required to be replaced because of the flaws having appeared during operation throughout the operating period. As a rule, the fresh FA with a smaller enrichment or the burnt out assembly are used as replacement. An assembly with the required parameters is selected at the NPP by means of a complex of neutronic codes KASKAD /1/ implemented for justification of a choice of the current loading at all Russian VVER Units. As the codes included in complex KASKAD do not allow us to calculate a change in isotope composition during holding (with the exception of isotopes Pm - Sm and I – Хe transformations), the question naturally arises on application of this code package for justification of FA reuse after their long-term holding in the pool.
The present report deals with the matters related to a change in isotope composition and multiplying properties of the FAs having held in the spent fuel pond for a long time and used preliminarily in the core during one and two operating periods of 18 months' fuel cycle. To obtain a conservative evaluation Kalinin NPP fuel grid of U49G6 type was accepted as the FA having maximum k∞ out of all the used fuel assemblies in the base 18 months' cycle An assessment of change in multiplying properties of fuel assembly during irradiation and holding was performed using code TDMCC which has been verified for criticality calculations.
The preliminary comparative calculations (without considering the FA holding in the pool) of multiplying properties by codes TDMCC /2/ and SAPFIR_95.1/3/ certified for calculation of VVER fuel grids were performed and a good agreement on k∞ was shown.
We considered the versions of FA loadings into VVER-1000 RP core after one and two fuel cycles in the mode of standard refuelling and with holding of 10 years.
As a result it was shown that for the most conservative version of return of the FA (irradiation during two operating periods and 10 years of holding, difference in value k∞ ≤ 3 %) a change in power of this FA in the reactor caused by its holding in the spent fuel pool is within a passport error of BIPR-7A and within an allowable deviation established in the operational documentation. Thus, loading into VVER-1000 FA core from the pool with holding till 10 years does not require for the additional conditions to be introduced from the viewpoint of applicability of software package KASKAD for calculation of fuel cycles with the FA used at the NPP.Full version (russian)