Monitoring of a Radiation Embrittlement by a Reliable Fluence Evaluation With a Neutron Dosimetry in a Reactor Pressure Vessel

The assurance of a nuclear reactor pressure vessel integrity plays an important role in achieving a safety and extending the life of nuclear power plants. In the assessment of the state of an embrittlement of a pressure vessel in a pressurized light water reactor, an accurate evaluation of the neutron exposure of each of the materials comprising the beltline region of the vessel is required. The evaluation is performed through a neutron dosimetry analysis where a fluence calculation is done by both measurement of the dosimeter materials removed from surveillance capsules and a neutron transport calculation. Now that all the capsules have been completely removed from the reactor vessel and analyzed by a periodic monitoring schedule, four ex-vessel sensor sets are installed as a substitute capsule in an axial direction in the reactor cavity and then removed for an analysis in order to meet the regulation requirements. The results showed that the differences between the measurements and calculations are less than 20% for each capsule, which means these analyses satisfy the acceptable criterion required by Regulatory Guide 1.190, and they also provide an assurance that such an evaluation including an ex-vessel neutron dosimetry can be used to predict the fluence of a nuclear reactor vessel with a good reliability.