Surveillance capsules

Typical surveillance capsule arrangement (Steele, 1975). (Reprinted, with permission, from ASTM STP 784, Status of USA Nuclear Reactor Pressure Vessel Surveillance for Radiation Effects, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428). 

The location of the capsule is important because the flux profile varies both axially and vertically in the vessel. Rgure 4.4 shows the typical placement of surveillance capsules in the reactor vessel. The capsule will experience a higher neutron flux than the reactor vessel wall, depending on how close to the core the capsule is positioned.The term used to characterize the difference in flux is lead factor, usually defined as the ratio of the neutron flux E> MeV) at the location of the capsule specimens to the peak neutron flux E> MeV) at the RPV inside surface. Sometimes, lead factor is expressed as the ratio of the capsule flux to the flux in the vessel wall at the 1/4-thickness (1/4-T) position (not at the inside wall), so it is important to specify which definition is used in order to avoid confusion. 

ASTM E185-82 also provides the option of including correlation monitor material in the capsules. Correlation monitor material provides an independent check on the irradiation conditions because other specimens of the same correlation monitor material have been irradiated in other reactors and its response to irradiation (embrittlement characteristics) is well documented. An example of a common correlation monitor is Heavy Section Steel Technology (HSST) Plate 02 (an SA533 Grade B, Class 1 material). Correlation monitor material is also included in the surveillance program to provide a means of validating neutron fluence estimates for the surveillance capsules. The data obtained from correlation monitors should... 

The surveillance program is specified in AFCEN RSE-M code (RSE-M, 2005,2010), and methods and practices are similar to those used in the US. The materials generally included in the surveillance capsules are those of... 

Table 4.5 Example of surveillance capsule content and irradiation durations of French 900 MWe reactors...

Table 4.5 gives an example of the content of capsules of 900 MWe reactors. Charpy V-notch tests are used to monitor all the materials. Additional tensile and fracture toughness tests are carried out for base and weld metals. Most reactors have four surveillance capsules to cover their design lifetime (40 years), except the first series of six 900MWe reactors known as CPO which have eight capsules. Generally, capsules are planned to be removed so that irradiation at quarter, half, three-quarters and completion of the component design end-of-life fluence can be achieved (Brillaud and Hedin, 1992 Chas et al, 2004). Table 4.6 shows the lead factors, defined by... 

Table 4.6 Lead factors in design of surveillance capsules of French reactors...

In 900MWe units, the capsules are irradiated along the outer surface of the thermal shield assembly. In 1300 and 1450 MWe units, the capsules are directly attached to the core barrel. In the case of Chooz-A, the thermal shield assembly was removed in 1970 and the surveillance capsules were then located under the core. The capsules are equipped with neutron dosimeters and thermal monitors, with some variations according to the reactor series. In all cases, activation dosimeters of nickel, copper and cobalt, as well as fissile dosimeters of uranium-238 and neptunium-237 are used. This instrumentation is complemented by iron and cadmium-shielded cobalt dosimeters in most cases and, in the most recent plants, also by niobium dosimeters. Each capsule contains temperature detectors based on eutectic alloys with melting points generally of 304 and 310 °C. 

The Code specifies that surveillance capsules shall be located within the reactor vessel so that the specimen history duplicates as closely as possible the neutron spectrum, the temperature history and the maximum neutron flux experienced by the reactor vessel. A sufficient number of surveillance capsules shall be provided to monitor the effect of neutron radiation on the reactor vessel materials, that is, the transition temperature shift, ARTndt and the decrease in USE throughout its operating period. A minimum number of capsules is specified depending on the predicted ARTndt value of each testing material at the inside surface of the beltUne of the reactor vessel. In this section, the Japanese surveillance tests program is reviewed. The details of the JEAC 4201 can be found elsewhere (Tomimatsu et al., 2006). Table 4.8 summarizes major revisions of the JEAC 4201. 

Table 4.10 Example of the content of Japanese PWR surveillance capsules...

The Charpy impact specimens from HAZ material are inserted in each surveillance capsule except the recent surveillance program. Although ARTnot values of HAZ material due to neutron exposure are not shown in the previously mentioned paper by Yamashita et al., ARTndt of HAZ material versus that of base material is given by Soneda et al. (2007) and is shown in Fig. 4.14. From this figure, ARTndt of HAZ material is almost the... 

In the majority of PWR plants each surveillance capsule has Charpy impact specimens made from the Japanese PWR correlation monitor material. The contents of copper, nickel and phosphorus in this material are 0.09 %, 0.62 % and 0.007 %, respectively. Some of the surveillance data for this material are shown by Tomimatsu et al. (1996). Figure 4.15 shows ARTndt and USE for the correlation monitor material as a function of neutron fluence. As there are a lot of transition temperature shift data as a function of neutron fluence for one specific material, the effect of fluence... 

In each surveillance program, fracture toughness specimens are usually optional and the number of specimens is still limited, even if these specimens are inserted in each surveillance capsule. Fracture toughness data can be... 

ASTM E 2215-02 (2002), Standard Practice for Evaluation of Surveillance Capsules from Light-Water Moderated Nuclear Power Reactor Vessels , American Society... 

With the alternative indexing parameter RTt, surveillance capsule Charpy specimens can be used by fatigue pre-cracking the V-notch specimen... 

One of the merits of test reactor irradiation is the higher dose irradiation at higher flux compared to irradiation at vessels or surveillance capsules in commercial power reactors. As of the end of 2013, the maximum fast neutron fluence of surveillance data was around 9 x lO n/cm E> MeV) while the maximum fluence at the inner surface of a RPV may exceed 1 x 10 °n/cm E > IMeV) for a 60-year operation in a pressurized water reactor (PWR). Test reactor irradiation is the only way to obtain experimental data for embrittlement behaviour at fluences relevant to 60 years or longer operation, together with an understanding of neutron flux effects on embrittlement behaviour. 

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