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ASTM E2956-25

Standard Guide for Monitoring the Neutron Exposure of LWR Reactor Pressure Vessels
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ASTM E2956-25

Standard Guide for Monitoring the Neutron Exposure of LWR Reactor Pressure Vessels

PUBLISH DATE 2025
PAGES 12
ASTM E2956-25

1.1 This guide establishes the means and frequency of monitoring the neutron exposure of the LWR RPV throughout its operating life.

1.2 The physics-dosimetry relationships determined from this guide may be used to estimate RPV damage through the application of Practice E693 and Guide E900, using fast neutron fluence (E > 1.0 MeV and E > 0.1 MeV), displacements per atom (dpa), or damage-function-correlated exposure parameters as independent exposure variables. Supporting the application of these standards are the E853, E944, E1005, and E1018 standards, identified in 2.1.

1.3 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.

1.4 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.

1.5 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.

1.1 This guide establishes the means and frequency of monitoring the neutron exposure of the LWR RPV throughout its operating life.

1.2 The physics-dosimetry relationships determined from this guide may be used to estimate RPV damage through the application of Practice E693 and Guide E900, using fast neutron fluence (E > 1.0 MeV and E > 0.1 MeV), displacements per atom (dpa), or damage-function-correlated exposure parameters as independent exposure variables.

Supporting the application of these standards are the E853, E944, E1005, and E1018 standards, identified in 2.1.

1.3 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.

1.4 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.

1.5 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.

4.1 Regulatory Requirements— The USA Code of Federal Regulations (10CFR Part 50, Appendix H) requires the implementation of an RPV materials surveillance program for all operating LWRs. Other countries have similar regulations.

The purpose of the program is to (1) monitor changes in the fracture toughness properties of ferritic materials in the RPV beltline7 resulting from exposure to neutron irradiation and the thermal environment, and (2) make use of the data obtained from surveillance programs to determine the conditions under which the RPV can be operated with adequate margins of safety throughout its service life.

Practice E185, derived mechanical property data, and (r, θ, z) physics-dosimetry data (derived from the calculations and reactor cavity and surveillance capsule measurements (1) using physics-dosimetry standards) can be used together with information in Guide E900 and Refs. 4, 11-18 to provide a relation between property degradation and neutron exposure, commonly called a “trend curve.”

To obtain this trend curve at all points in the RPV wall requires that the selected trend curve be used together with the appropriate (r, θ, z) neutron field information derived by use of this guide to accomplish the necessary interpolations and extrapolations in space and time.

4.2 Neutron Field Characterization— The tasks required to satisfy the second part of the objective of 4.1 are complex and are summarized in Practice E853.

In doing this, it is necessary to describe the neutron field at selected (r, θ, z) points within the RPV wall. The description can be either time dependent or time averaged over the reactor service period of interest.

This description can best be obtained by combining neutron transport calculations with plant measurements such as reactor cavity (ex-vessel) and surveillance capsule or RPV cladding (in-vessel) measurements, benchmark irradiations of dosimeter sensor materials, and knowledge of the spatial core power distribution, including the time dependence.

Because core power distributions change with time, reactor cavity or surveillance capsule measurements obtained early in plant life may not be representative of long-term reactor operation. Therefore, a simple normalization of neutron transport calculations to dosimetry data from a given capsule is unlikely to give a satisfactory solution to the problem over the full reactor lifetime.

Guide E482 and Guide E944 provide detailed information related to the characterization of the neutron field for BWR and PWR power plants.

4.3 Fracture Mechanics Analysis— Currently, operating limitations for normal heat-up and cool-down transients imposed on the RPV are based on the fracture mechanics techniques outlined in the ASME Boiler and Pressure Vessel Code.

This code requires the assumption of the presence of a surface flaw of depth equal to one fourth of the RPV thickness. In addition, the fracture mechanics analysis of accident-induced transients (Pressurized Thermal Shock, (PTS)) may involve evaluating the effect of flaws of varying depth within the RPV wall (4).

Thus, information is required regarding the distribution of neutron exposure and the corresponding radiation damage within the RPV, both in space and time (4).

In this regard, Practice E185 provides guidelines for designing a minimum surveillance program, selecting materials, and evaluating metallurgical specimen test results for BWR and PWR power plants. Practice E2215 covers the evaluation of test specimens and dosimetry from LWR surveillance capsules.

4.4 Neutron Spectral Effects and DPA— Analysis of the neutron fields of operating power reactors has shown that the neutron spectral shape changes with radial depth into the RPV wall (2, 3).

The ratio of dpa/Ď•t (where Ď• is the fast (E > 1.0 MeV) neutron fluence rate and t is the time that the material was exposed to an average fluence rate) changes by factors of the order of 2.0/1.0 in traversing from the inner to the outer radius.

Although dpa, since it includes a more detailed modeling of the displacement phenomenon, should theoretically provide a better correlation with property degradation than fluence (E > 1.0 MeV) (1, 19), this topic is still controversial and the available experimental data does not provide clear guidance (19, 20).

Thus it is recommended to calculate and report both quantities; see Practice E853 and Practice E693.

4.5 In-Vessel Surveillance Programs:

  • 4.5.1 The neutron dosimetry monitors used in RPV surveillance capsules provide measurements of the neutron fluence and fluence rate at single points on the core midplane within the reactor, and near the RPV wall; that is, at the surveillance capsule locations (1).
  • In actual practice, the surveillance capsules may be located within the reactor at an azimuthal position that differs from that associated with the maximum neutron exposure (or that differs from the azimuthal and axial location of the assumed flaw); and at a radial position a few centimeters or more from the flaw and the RPV wall (4, 5).
  • Although the surveillance capsule dosimetry does provide points for normalization of the neutron physics transport calculations, it is still necessary to use analytical methods that provide an accurate representation of the spatial variation (axial, radial and azimuthal) of the neutron fluence (refer to Guide E482).
  • It is also necessary to use other measurements to confirm the spatial distribution of RPV neutron exposure.
  • 4.5.2 Given that surveillance capsules are located radially closer to the core than the surface of the RPV, they may be shifted azimuthally away from the peak exposure location in order to limit the magnitude of the surveillance capsule lead factor.
  • The lead factor is defined as the ratio of the fast neutron fluence at the center of the surveillance capsule to the peak fast neutron fluence at the clad–base metal interface of the RPV.
  • One adverse effect of this azimuthal shift away from the peak is that the surveillance capsule dosimetry does not “see” the part of the core that produces the peak exposure of the RPV. As a result, the surveillance capsule is unable to monitor the effect of changes in the core power distribution that are made to reduce the peak RPV neutron exposure.
  • Another adverse effect is that with larger lead factors, the capsules are rapidly exposed to a high neutron fluence. For example, with a lead factor of five, a surveillance capsule will receive an exposure in as little as twelve years that is equivalent to what the RPV peak may see in 60 years of operation.
  • Practices E185 and E2215 suggest not exceeding twice the maximum design fluence (MDF) or twice the end-of-license fluence (EOLF). In this example, this would require withdrawing any remaining surveillance capsules after 24 years of operation. Thus, without taking other steps, the reactor would be operated for the remaining 36 years (of a 60 year life) with no dosimetry present.
  • 4.5.3 New or replacement surveillance capsules should recognize and correct operating deficiencies by using improved capsule dosimetry.
  • For example, for one class of PWR, the copper wire is cadmium shielded to minimize interference from trace amounts of cobalt. In about one third of the measurements the copper has become incorporated into the cadmium preventing separation and further processing.
  • A simple solution to this problem is to use stainless steel hypodermic tubing to contain and separate the radiometric monitor wire inside the cadmium tubing.
  • Example dimensions include:
    • Typical radiometric monitor wire outside diameter = 0.5 mm (0.020 in.)
    • Typical stainless steel tubing is 1.07 mm (0.042 in.) outside diameter by 0.69 mm (0.027 in.) inside diameter, 0.19 mm (0.008 in.) wall thickness.
    • Typical cadmium tubing is 2.29 mm (0.090 in.) outside diameter by 1.27 mm (0.050 in.) inside diameter, 0.51 mm (0.020 in.) wall thickness.
  • 4.5.4 Guide E844 states that radionuclides with half-lives that are short compared to the irradiation duration should not
SDO ASTM: ASTM International
Document Number E2956
Publication Date Feb. 1, 2025
Language en - English
Page Count 12
Revision Level 25
Supercedes
Committee E10.05

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