Significance and Use
4.1 A pressure vessel surveillance program requires a methodology for relating radiation-induced changes in materials exposed in accelerated surveillance locations to the condition of the pressure vessel (see Practice ). An important consideration is that the irradiation exposures be expressed in a unit that is physically related to the damage mechanisms.
4.2 A major source of neutron radiation damage in metals is the displacement of atoms from their normal lattice sites. Hence, an appropriate damage exposure index is the number of times, on the average, that an atom has been displaced during an irradiation. This can be expressed as the total number of displaced atoms per unit volume, per unit mass, or per atom of the material. Displacements per atom is the most common way of expressing this quantity. The number of dpa associated with a particular irradiation depends on the amount of energy deposited in the material by the neutrons, and hence, depends on the neutron spectrum. (For a more extended discussion, see Practice .)
4.3 No simple correspondence exists in general between dpa and a particular change in a material property. A reasonable starting point, however, for relative correlations of property changes produced in different neutron spectra is the dpa value associated with each environment. That is, the dpa values themselves provide a spectrum-sensitive index that may be a useful correlation parameter, or some function of the dpa values may affect correlation.
4.4 Since dpa is a construct that depends on a model of the neutron interaction processes in the material lattice, as well as the cross section (probability) for each of these processes, the value of dpa would be different if improved models or cross sections are used. The calculated displacement cross section for ferritic iron, as given in this practice, is determined by the procedure given in . The currently recommended iron displacement cross section in this practice ( ) was generated using the ENDF/B-VI iron cross section (. ) A recent calculation using ENDF/B-VII.0 produced identical results (. The iron cross section data in ENDF/B-VII.1 does not differ from ENDF/B-VII.0. Although the ENDF/B-VI based iron displacement cross section differs from the previously recommended ENDF/B-IV iron displacement cross section , )( by about 60 % in the energy region around 10 keV, by about 10 % for energies between 100 keV and 2 MeV, and by a factor of 4 near 1 keV due to the opening of reaction channels in the cross section, the integral iron dpa values are much less sensitive to the change in cross sections. The update from ENDF/B-IV to ENDF/B-VI dpa rates, when applied to the H. B. Robinson-2 pressurized water reactor, resulted in “up to )∼4 % higher dpa rates in the region close to the pressure vessel outer surface” and in “slightly lower dpa rates ... close to the pressure vessel inner wall” (. , ) presents a comparison of a previous edition (Practice –94) and currently recommended dpa estimates for several neutron spectra.
1.1 This practice describes a standard procedure for characterizing neutron irradiations of iron (and low alloy steels) in terms of the exposure index displacements per atom (dpa) for iron.
1.2 Although the methods of this practice apply to any material for which a displacement cross section σd(E) is known (see Practice ), this practice is written specifically for iron.
1.3 It is assumed that the displacement cross section for iron is an adequate approximation for calculating displacements in steels that are mostly iron (95 to 100 %) in radiation fields for which secondary damage processes are not important.
1.4 Procedures analogous to this one can be formulated for calculating dpa in charged particle irradiations. (See Practice .)
1.5 The application of this practice requires knowledge of the total neutron fluence and flux spectrum. Refer to Practice for determining these quantities.
1.6 The correlation of radiation effects data is beyond the scope of this practice.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.8 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.