Resolution of Generic Safety Issues: Issue 164: Neutron Fluence in Reactor Vessel (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
To calculate the value of RTPTS as required in 10 CFR 50.61 and 10 CFR 50, Appendix G, licensees must determine the value of the fast neutron fluence on the inside surface of their pressure vessels. Through a number of reviews, NRR found1515 a non-conservative computational bias in the W methodology which, in one instance (Yankee Rowe), was determined to be 13%, while in WCAP-11815 (Indian Point 3 Surveillance Capsule Z Report) it was reported to be 20%. Several publications suggest that the iron inelastic scattering cross-sections in ENDF/B-VI yield higher fluence values for transmissions through iron; thus, ENDF/B-IV (currently in use) may not be conservative.
Licensees are required to determine applicable uncertainties in the measurements and calculations for reactor cavity dosimetry. In Regulatory Guide 1.99,841 Revision 2, the staff assumed a fluence uncertainty of 20% in determining trend curves. Thus, this level of uncertainty at the inside surface of the pressure vessel should be supported. The bias in the W calculations may exist in other vendor or licensee methodologies, or it may pertain to the iron cross-sections. Thus, the staff believed that this issue could affect all PWRs.
The safety concern of this issue is being addressed in the Surveillance Data Base, Analysis, and Standardization Program (FIN B04152). Concurrent with this program, the staff developed Draft Regulatory Guide DG-1025, "Calculational and Dosimetry Methods for Determining Pressure Vessel Neutron Fluence." Thus, the issue was DROPPED from further consideration as a new and separate issue.1516 In an RES evaluation,1564 it was concluded that consideration of a 20-year license renewal period did not change the priority of the issue.