IEC TR 61361 Nuclear Reactors — Pressure Vessel Surveillance Dosimetry – TNLab

IEC TR 61361 is a technical report that provides complementary guidance to IEC TR 61359 on neutron dosimetry methods for reactor pressure vessel (RPV) surveillance. While IEC TR 61359 focuses on the overall surveillance capsule program design and implementation, IEC TR 61361 delves deeper into the dosimetry methodology itself, covering both in-vessel and ex-vessel neutron dosimetry techniques. The report addresses the need for accurate neutron fluence determination at multiple locations — not only at capsule positions but also at the RPV inner wall, quarter-thickness, and outer wall positions where embrittlement predictions are required. The relationship between these standards is hierarchical: the general principles and capsule program design are covered in IEC TR 61359, while the specialized dosimetry calculations, measurement techniques, and data analysis methods are detailed in IEC TR 61361. Both reports work together to provide a comprehensive framework for RPV surveillance that supports regulatory compliance and plant life extension decisions.

IEC TR 61361 provides extensive guidance on neutron dosimetry techniques used in RPV surveillance. The report covers: (1) Activation dosimeter selection — detailed guidance on choosing dosimeter materials (Fe-54, Fe-58, Ni-58, Cu-63, Nb-93, Co-59, Ti-46, Ti-47, Ti-48, Al-27) with specific attention to their energy-dependent cross sections and coverage of the neutron energy range from thermal to >10 MeV; (2) Multiple foil method — using multiple dosimeters with different energy responses to unfold the neutron energy spectrum through techniques such as the SAND-II, STAYSL, or LSL-M2 adjustment codes; (3) Ex-vessel dosimetry — methods for placing and analyzing dosimeters outside the reactor vessel to provide supplementary data on neutron leakage and vessel fluence; (4) Computational dosimetry — using Monte Carlo codes (MCNP, TRIPOLI) and deterministic transport codes (TORT, DORT) to calculate the full 3D neutron flux distribution in the reactor cavity and vessel. A key technique described is spectrum adjustment, where measured dosimeter reaction rates are used to adjust calculated neutron spectra through least-squares fitting, reducing uncertainties in the final fluence determination.

Uncertainty quantification is a central theme of IEC TR 61361. The report provides methodologies for estimating and combining uncertainties from multiple sources: (1) Dosimeter cross section uncertainties — typically 3-10% depending on the energy range and dosimeter material; (2) Measurement uncertainties — gamma counting statistics, detector efficiency calibration, and sample mass determination, typically 2-5%; (3) Calculation uncertainties — nuclear data, geometric modeling approximations, and statistical precision of Monte Carlo calculations, typically 5-20% depending on location; (4) Covariance information — the report emphasizes the importance of using full covariance matrices for cross sections and calculated fluxes in the adjustment process to avoid underestimation of uncertainties. Best practices include using redundant dosimeter materials (at least 4-6 different reactions), cross-checking calculations with measurements at multiple positions, and performing sensitivity studies to identify the most influential parameters. The report also addresses the increasingly important topic of RPV integrity assessment beyond 60 years of operation, where dosimetry uncertainties must be well below 10% (1σ) to support the reduced safety margins inherent in long-term operation.