Neutron irradiation of RPV steels results in an in­crease in the yield stress (sY) and an upward shift in the DBTT. The increase in the DBTT is generally determined from the shift in the Charpy impact curve at a specific energy level, typically 41J (AT41j). Historically, most data on effects of irradiation on the DBTT arose from irradiation and testing of Charpy specimens, although frequently increases in yield stress were also reported. The specimen geometries in both cases were convenient for irradiations in the restricted space available for either surveillance capsules1 or rigs inserted into the cores of material test reactors (MTRs).1 More recently, with the advent of the Master Curve tech — nique,16 there has been greater interest in acquiring

Figure 1 Effect of irradiation on the Charpy-V USE of the JRQ (A508 cl.3 forging) irradiated in a materials test reactor to doses of 15.7 and 27 mdpa (1 mdpa = 0.001 dpa).

data directly on fracture toughness (see, e. g., Viehrig and Lucon17) (see also Chapter 4.14, Fracture Tough­ness Master Curve of bcc Steels). Correlations have been established between the different measures; for example, see Sokolov and Nanstad18 for experimental data on the relationship between Charpy and static fracture toughness shifts and between Charpy and yield strength shifts. Williams et a/.19 have published hardness/Charpy shift correlations for A533B plate and weld.

From the early test programs in the 1960s to the present day, great reliance has been placed on measur­ing the shift in the Charpy transition from lower energy to the upper shelf energy (USE) as a means of determining the effect of irradiation damage on bulk mechanical properties. Figure 1 shows an example for an RPV forging; it can be seen that not only is there a shift in the transition temperature (usually measured at the 41 j level), but there is also a drop in the upper shelf. Note that the decrease in the USE is accompa­nied by a decrease in the slope of the Charpy curve in the transition region; this can create issues with esti­mating the Charpy shift if the upper shelf level approaches the indexing level. The greatest attention has been given to the shift in the transition region.

Table 4	Parameter range of interest for different reactor types
Reactor type	Temperature range	Dose range (n cm 2) > 1 MeVa	Dose range (mdpa)	Dose rate (n cm 2) E > 1 MeVs1
Magnox		160->390°C	1016-2 x 1018fast ncm~2 (Ni doses)	0.02-4
PWR		270°C-296 °C	6-8 x 1019	60	~1 x 1011
BWRs		Predominantly 270 °C	<1-2 x 1018	<3	~1 x 1010

aIn the LWR, community fluence (or equivalently dose) is generally expressed in terms of neutron exposure (e. g., ncmT2, E> 1 MeV); this reflects the fact that the neutron spectrum does not vary significantly from location to location or plant to plant. In contrast, in the United Kingdom a key feature of Magnox reactors is that the neutron spectrum varies significantly with location, and displacements per atom, dpa, is a more appropriate measure.

The change in mechanical properties is required as a function of irradiation fluence (dose), dose rate (flux), irradiation temperature, steel type and composition, product form (plate, forging, or weld), and material heat treatment. Insight into the ranges of the irradia­tion variables of interest can be seen from Table 4. At the design stage of the early reactors, an allowance for the potential irradiation-induced embrittlement was included. The magnitude of this allowance was based on the judgment of potential irradiation effects. At the time of the design of Magnox reactors, the shift allowance for SMA welds was ^40 °C.20 Such allow­ances were frequently exceeded once data relevant to plant conditions became available.

The intent of this section is not to provide an exhaustive description of the exact mechanical prop­erty response of individual steels to specific irradiation conditions; rather the intent is to identify the main parameters that control the radiation response of fer­ritic pressure vessel steels and to give an indication of the potential changes in mechanical properties that may occur through typical in-service conditions and lifetimes.

The fluence dependence of RPV embrittlement has been studied since experimental programs were initiated in the 1950s. Most data available refer to the case in which embrittlement is determined by irradiation-induced hardening (rather than nonhar­dening embrittlement — see Section 4.05.4) at tem­peratures greater than 150 °C and less than 300 °C. Indeed, the available data are dominated by experi­ments that investigated the embrittlement of MnMoNi steels at an irradiation temperature of 270-295 °C. It should also be recognized that after over 50 years of experimentation, a significant quan­tity of data has been obtained. For example, the US surveillance database arising from BWR and PWR reactors now encompasses ^800 individual data points on the embrittlement observed from various steels irradiated at a range of fluences (and a restricted range of irradiation temperatures).21,22 Data on the results of French surveillance programs have also been published.23 In parallel, well- controlled experiments in MTRs, frequently making use of steels with well-controlled compositional varia­tions, have been performed. These have provided incredibly valuable information that has helped develop an understanding of the radiation damage processes in RPV steels. Indeed, early irradiation pro­grams by Odette and coworkers,24 and Hawthorne and coworkers,25 focused on a restricted fluence range but on a number of steels with well-controlled composi­tional variation. Other notable programs were the irra­diations performed as part of the Heavy-Section Steel Irradiation (HSSI) program at Oak Ridge (see, e. g., Taboda etal26 and Nanstad and Bergen27) and a num­ber of IAEA coordinated programs (see, e. g., Interna­tional Atomic Energy Agency28). The most recent program by Odette and coworkers at the Ford Nuclear Reactor, University of Michigan, focuses on a range of fluxes, fluences, and material compositions. The resulting IVAR irradiation database encompasses 57 alloys that were irradiated at three different fluxes and three different temperatures over a range of flu­ences, giving in total 1537 different alloy/irradiation conditions. Irradiations were at 270, 290, and 310°C, fluxes between 5 x 1010 and 1 x 1012 ncm~2s_1, and fluences between 0.004 x 1019 and 4 x 1019ncm~2, E > 1 MeV (see Heatherly et al.15 and Eason et al29). The irradiation temperature of the irradiation rig was restrained to <±5 °C and the uncertainty in fluence was ~±7%. (It should be noted that there are well — defined techniques for establishing the neutron spectra in specific locations in reactors and for evaluating the resultant damage in the irradiated material; see Heatherly etal.15 and ASTM Standard 185 on ‘Practice for Conducting Surveillance Tests for Light-Water Cooled Nuclear Power Reactor Vessels’ and related ASTM Standards for a discussion of these techniques.) It is to be noted that typical MTR dose rates are x 1012—1 x 1013ncm~2, E > 1 MeV s-1, that is, one or two orders of magnitude higher than the PWR surveillance dose rates given in Table 4.

Overall, it was found in all RPV steels of interest that embrittlement increases with increasing fluence, but the rate of embrittlement may decrease (with increasing fluence). Furthermore, embrittlement does not saturate in the fluence range of interest to power reactor applications. These trends are illustrated in Figure 2 for Magnox CMn steel SMA weld transition shifts and for a MnMoNi plate HSST-02.

Irradiations in the 1960s demonstrated that com­position was a major factor controlling the response of the ferritic low alloy steels employed in operating reactors. By the mid-1960s, it was thought likely that residual elements in steels could be responsible for much of the observed scatter in the irradiation embrittlement response.3 , The work suggested that reducing the residual element content of A302-B steel would markedly improve the resistance

0 1 X 1019 2 X 1019 3 X 1019 4 X 1019 5 X 1019 (b) Fluence (n cm-2, E >1MeV) Figure 2 (a) Magnox submerged-arc weld transition shift data. Reprinted with permission from Buswell, J. T.; Jones, R. B. In Effects of Radiation on Materials, 16th International Symposium, ASTM STP 1175; Kumar, A. S., Gelles, D. S., Nanstad, R. K., Eds.; American Society for Testing and Materials: Philadelphia, PA, 1994; pp 424-443. Copyright ASTM International and (b) HSST-02 reference plate irradiated in surveillance schemes in a number of US LWRs.22

to irradiation embrittlement at typical service tem­peratures of 550 oF (288 oC).

In a pioneering set of studies, Hawthorne and coworkers undertook studies that explored the effect of material composition on irradiation embrittlement in a systematic manner. Studies were undertaken on materials irradiated at a controlled temperature of 288 °C in the Union Carbide Research Reactor (UCRR) and in the light water-cooled and moder­ated test reactor, UBR, at the Buffalo Materials Research Center.25 A series of small (150 kg) labora­tory melts were produced to the nominal plate steel specifications using pure elements. These were then split, generally into three blocks, two of which were remelted and selected residual element additions added, while the third was kept to provide a low residual element reference. Each steel was also com­pared to material obtained from normal commercial production.

Potapovs and Hawthorne33 demonstrated that additions of Cu, and Cu and Ni, to a laboratory melt containing low level of residuals greatly increased the observed embrittlement (see Table 5). This must be regarded as a landmark paper in the understanding of the factors that control radiation damage in RPV steels. Note that it was ^15 years before the underlying mechanisms were elucidated (see Section 4.05.4). The effect of different Cu and Ni levels in steels irradiated as part of US surveil­lance programs is illustrated in Figure 3. The effect of increased levels of Cu in steels of the same Ni level and the effect of increased levels of Ni at constant Cu level is clear (data taken from Eason et a/.33).

Table 5 Comparison of shift in 30ft-lbs (41 J) transition temperature (AT30ft-lb) due to irradiation at 288 °C for experimental and commercial weld deposits and the A543 reference plate studied by Potapovs and Hawthorne33 Material Composition (wt%) Fluence (101sncm~2) [E > 1 MeV] DT41J (DT30ft-lb) C Cu Ni Expt. weld 1934 0.06 0.77 3.5 53 Expt. weld 1938 0.07 1.62 3.5 111 Expt. weld 1938 0.07 1.62 3.5 200 Expt. weld 1948 0.03 1.56 3.5 110 Commercial Filler (0.24Cu) 0.24 1.58 3.5 415

0

0 1X1019 2X1019 3X1019 4X1019 5X1019 6X1019 7X1019

(b) Fluence n cm-2 (E >1MeV

Figure 3 Charpy shift (AT41 J (°C)) for (a) a US weld and a US forging containing 0.25 and 0.06 wt% Cu, respectively, and (b) US welds and a US plate containing ~0.2 wt% Cu and varying levels of Ni.

observation from Figure 4 is that tin additions (0.023% vs. <0.004% Sn) to the high phosphorus alloy did not affect embrittlement. They further established that an arsenic content of 0.035% does not have an observable effect on the radiation sensi­tivity of plates containing ^0.020% P and <0.18% Cu. Alloying with 0.50% chromium also did not alter the radiation resistance of plates having a copper content of ^0.30% Cu.

In the United Kingdom, there was considerable interest in the effect of irradiation temperature because of the wide variation of temperatures in Magnox pressure vessels. Barton et al34 reported the irradiation temperature, the dose rate, and the dependencies of the yield stress increase for CMn steels following irradiation in the PLUTO MTR. The work focused on EN2, a Si-killed mild steel and an Al-killed grain-size-controlled mild steel. The Cu contents of the steels were 0.14, 0.18, and

0.13%, respectively, by weight. Irradiation tempera­tures were reported to have been controlled to ±1 °C34 and the fast neutron doses were restricted to a maximum of 2.5 x 1017ncm-2 (as measured by Ni monitors). The results between 100 and 350 °C exhibited a simple linear dependence of yield strength increase as a function of irradiation temper­ature. Jones and Williams35 carried out an analysis of the Barton data34 and another dataset on the irradia­tion temperature dependence of similar steels from Grounes,36 and pointed out that the combined data form a homogeneous data set with relatively little variability. The least squares regression is

FT(As) = 1.869 — 4.57 x 10-3 T [1]

This parameter has been important for the correla­tion of data from similar steels irradiated at different irradiation temperatures.2

Understanding the effect of flux, or dose rate, on radiation damage of ferritic steels has proved particularly important in the formulation of mecha­nistically derived DDRs. There was particularly strong interest in the United Kingdom in under­standing the effects of flux on bulk properties as Magnox RPVs operated within a range of fluxes.2 A number of experimental investigations have examined the dose rate dependence of hardening.37 In the absence of precipitation effects, no influ­ence of dose rate on irradiation hardening has been detected. Data obtained from over five orders of magnitude change in dose rate for C-Mn plate steels at an irradiation temperature of 200 °C, typical of Magnox applications,37 demonstrated no dependence on dose rate.

However, there is agreement that there is a strong effect of flux on the embrittlement of Cu-containing steels.38 In these steels, it was found that at doses before the ‘saturation’ of embrittlement (see Section 4.05.4) the rate of embrittlement with fluence increased with decreasing dose rate. Williams etal. studied the effect of dose rate on the embrittlement in low Ni welds at preplateau doses19 (1 x 1019ncm-2, E> 1MeV is approximately 1.5 x 10-2dpa). They reported the irradiation-induced shift in the 41 J transition temper­ature of a number of Mn-Mo-Ni SMA welds after irradiation in MTRs at dose rates between 6 x 10-10 and 2 x 10-8dpas-1 and doses of less than ^30mdpa. It was observed that for the welds SD and SL, in which the Cu levels are low (<0.15 wt%), there is no apparent effect of dose rate (Figure 5). At higher copper levels (0.56 wt% for weld SH, 0.36% for SG, and 0.24% for SF), there was a marked effect of dose rate at low

Figure 5 Comparison between results obtained at different dose rates and in different irradiations of quenched and tempered low alloy welds (DIDO, HERALD, and OSIRIS are all MTRs in which samples were irradiated). Reproduced from Williams, T. J.; Ellis, D.; Swan, D. I.; etal. In Proceedings of the 2nd International Symposium on Environmental Degradation of Materials in Nuclear Power Systems — Water Reactors, Sept 1985; ANS: Monterey,

CA, 1986.

doses (<0.01 dpa), where low dose rate produced a significantly higher shift (Figure 5). At higher doses, no such effect was observed.

In studies where both Charpy specimens and ten­sile samples were irradiated, it was found that radia­tion damage in RPV steels generally caused an increase in both Charpy shift and yield strength. This is significant, as irradiation can also lower the fracture strength of materials (see Section 4.05.4) and cause nonhardening embrittlement. As will be shown, none of the DDRs developed for MnMoNi steels have found it necessary to account for the embrittlement due to nonhardening embrittlement. It is only in the case of CMn steels that evidence has been found for nonhardening embrittlement in irradiated SMA welds.2 There have been investiga­tions of nonhardening embrittlement in experiments mounted in MTRs; for example, McElroy eta/.39 and Nanstad eta/.40 investigated nonhardening embrittle­ment of simulated coarse-grained heat-affected zones (CGHAZ). A review of intergranular embrittlement in RPV steels can be found in English et a/.41

4.05.3.1 Summary

In contrast to the assumptions made at the design stage, it was found that radiation damage in ferritic RPV steels could cause a significant change in bulk properties, primarily an increase in DBTT (as man­ifested by an increase in Charpy 41 J level) and an increase in yield strength. Evidence of radiation

damage causing nonhardening embrittlement was also found in CMn steels.

There are proven mechanical test techniques for determining the change in bulk properties and large irradiation programs have been performed. These have made use of both surveillance programs and MTRs. The level of the measured embrittlement depends on the fluence, flux, and irradiation temper­ature. The most important discovery was the sensi­tivity to steel composition, in particular a strong dependence of embrittlement on the levels of Cu, Ni, and P.