Basis for revision 2 of the U.S. Nuclear Regulatory Commission's Regulatory Guide 1.99

Regulatory Guide 1.99. ''Radiation Damage to Reactor Vessel Materials,'' is being updated to reflect recent studies of the physical basis for neutron radiation damage and efforts to correlate damage to chemical composition and fluence. Revision 2 of the Guide contains several significant changes. Welds and base metal are treated separately. Nickel content is added as a variable and phosphorus removed. The exponent in the fluence factor is reduced, especially at high fluences. And, guidance is given for calculating attenuation of damage through the vessel wall. This paper describes the basis for these changes in the Guide