Verifiactions of the Accuracy of the Neutron Fluence Calculation

 

 The accuracy of the neutron fluence calculation is limited by the uncertainties of the nuclear data,plant constructional and operational data (geometry dimensions and material content, neutron source et.) as wells calculation method approximation which are used in the neutron transport calculations. That is because a verification of the neutron fluence based on comparison of calculated with measured flux-related quantities is necessary. In according to the quality management documents ISO 9001:2001 and IAEA 50-C/SG-q the verification is a process by which the calculations methodology is approved in regard to determined requirements.

The calculated reactor pressure vessel (RPV) neutron fluence of Kozloduy NPP is verified and validated by measurements of the induced activity of threshold neutron detectors in assemblies and induced activity of threshold neutron foil-detectors placed in the air cavity behind the RPV. The measured activity of detectors placed in the surveillance assemblies together with induced with induced activity of metal samples serve for verification and validation of neutron fluence into the samples.

Same of the problems verification and validation neutron fluence based of the actual surveillance programs of Kozloduy NPP Unit 5 and 6 are:

-          The irradiation conditions respectively the fluence of the surveillance capsules and specimens can be calculated respectively determined only if the exact position of the surveillance capsules in the core zone restrictor in the reactor pressure vessel is known with respect to axial and radial coordinates;

-          The exact radial location of the specimens in relation to the core is not known for the irradiation period due to the rotation of the assemblies and as a result, the fluence can not be determined exactly;

-          Generally, the number of fluence detectors is a too small to enable complete characterization of rhe neutron flux distribution within the assembly as well as within the specimens;

-          The irradiation temperature is assumed to be non-uniform due to above mentioned varying irradiation conditions.

In accordance with the General Time Schedule as laid down in the Main Contract of the modernization of Units 5 and 6 (Contract No. AEZ-765/99), following tasks were assumed.

-          Implementation of the surveillance assemblies for Units 5 and 6 as well as for Unit 6 during the outage of the respective Unit in 2003;

-          Withdrawal of one standard surveillance assembly of Unit 6 after 9 cycle during the respective outage in 2004;

-          Follow-up disassembling, testing and evaluation of the surveillance assembly to determine the irradiation conditions with respect to fluence and irradiation temperature is Unit 6.

References

[1] Rhoades, W.A., Childs, R.L., “TORT Three Dimensional Discrete Ordinate Neutron/Photon Transport Code with Space-Depedment Mesh and Quadrature”, ORNL-6268 (Nov. 1987).

[2] BUGLE-96, Coupled 47 Neutron, 20 Gamma-Ray Group Cross Section Library Derived from ENDF/B-VI for LWR Shielding and pressure Vessel Dosimetry Applications, RSICC, ORNL, DLC-185.

[3] The International Reactor Dosimetry File (IRDF-2002), IAEA-INDC(NDS)-448.

[4] U.S. Nuclear Regulatory Commission, Regulatory Guide 1.99, Revision 2, “Radiation Embrittlement of Reactor Vessel Materials”, May 1988.

[5] Desing and Construction Rules for Mechanical Components of PWR Nuclear Islands (RCC-M), AFCEN, Paris, 1993 Edition.

[6] In-Service Inspection Rules for Mechanical Equipment of PWER Nuclear Islands (RSEM) 1990 Edition.

[7] Regulations and Calculation Norms for Equipment and Tubing Strength of the Nuclear Energy Facilities (PNAE G-7-002-86, PNAE G-7-008-89). Gosatomenergonadzor, Energoatomizdat, 1989.

[8] Calculational and dosimetry Methods for Determining Pressure Vessel Neutron Fluence, Draft Regulatory GuideDG-1025, U.S. Nuclear Regulatory Commission, 1993.