Measurement and analysis of in-vessel component activation and gamma dose rate distribution in Joyo (2)

In the experimental fast reactor Joyo, the damaged upper core structure (UCS) was received into the cask in May 2014 for its replacement as a part of the restoration work of the incident concerning with the irradiation test sub-assembly named MARICO-2 (material testing rig with temperature control). The dose rate on UCS surface was quite high due to the activation for over 30 years operation. In order to attain the optimum safety design, manufacture and operation of equipment for UCS replacement, the method to evaluate UCS surface dose rate was developed on the basis of C/E obtained by the in-vessel dose rate measurement in Joyo. In order to verify the evaluation method, the axial gamma-ray distribution measurement on the surface of the cask, which contained UCS, was conducted using a plastic scintillating optical fiber (PSF) detector. This paper describes the comparison results between calculation and measurement as follows. (1) The measured axial gamma-ray distribution on the cask surface had a peak on proper location with considering the cask shielding structure and agreed well with the calculated distribution. (2) The C/E of axial gamma-ray distribution on the cask surface was ranged from 1.1 to 1.7. It was confirmed that the calculation for UCS replacement equipment design had a margin of conservatively. Then, the results showed that the applied evaluation method for UCS replacement equipment design was sufficiently reliable. In the in-vessel repair work for sodium cooled fast reactors (SFRs), the design and work condition estimation are important. The accumulated experience provides valuable insights into further improvements for in-vessel repair techniques in SFRs. (author)