Primary circuit materials

Main isolations these isolations are designed to separate the residual heat evacuation systems and the primary circuits materially and functionally, and allow secondary sodium draining from the intermediate exchangers. These isolations will also be used to set up the carbonation installations for treatment of the circuits residual sodium. The main operations are as follows ... 

In the course of the operation of pressurized as well as of boiling water reactors, a contamination layer consisting of the oxides of primary circuit materials is formed on all surfaces which are in contact with reactor coolant. In high-temper-... 

Production and transport of corrosion products are the main reasons for contamination buildup in the PWR primary circuit. Basically, in a PWR the generation of radionuclides from elements present in the primary circuit materials can proceed according to two different mechanisms, as is schematically shown in Fig. 4.26. 

Table 4.7. Fe Co ratios in PWR primary circuit materials and in primary coolant corrosion products...

Radiochemistry during normal operation of the plant Table 4.11. Primary circuit materials and areas... 

The highly beneficial effect of Stellite replacement in the newer Siemens PWRs was confirmed by a comparative study performed by Garbett (1992). In this study it was emphasized that cobalt impurities in the primary circuit materials are only minor contributors to the radiation fields in the areas surrounding the piping, components and systems. 

As has been discussed above in detail, °Co and Co are usually the radionuclides that determine the radiation dose rates at the primary circuit surfaces. In isolated cases, however, other radionuclides, usually activation products of other primary circuit materials, may also be of significance. Fission products normally do not contribute to a significant extent to contamination buildup. 

In both PWRs and BWRs, corrosion of the primary circuit materials is an essential factor in the buildup of contamination layers on the surfaces of the pipes and the components. The materials used in BWRs which are in contact with the reactor water and, therefore, are potential sources of radionuclides are mainly stainless steels wear-resistant hardfacing alloys such as Stellite are also present in most of the plants. Zircaloy as the material of fuel rod claddings, spacers and fuel assembly casks need not be considered in this context, because of the extremely small release of activated constituents from this material. Due to differences in temperature and environment, the mechanisms of the corrosion process and the resulting metal release rates, which contribute to the input of corrosion products into the region of the reactor core, may show differences in different regions of the plant. Thus, corrosion of materials in the water-steam cycle exhibiting H2O phase transformations and considerable temperature differences will proceed differently than in the recirculation lines and the reactor water cleanup system, which are in contact with liquid water exclusively and show comparatively small variations in operating temperature. 

Plant Primary circuit material hot leg (for pool reactors, there is no hot leg piping) Primary circuit material cold leg (for pool reactors, this is the piping coimecting the primary piunps to the diagrid)... 

After a few years of operation there was a significant increase in radiation fields from the primary circuit piping in the Douglas Point generating station. Other water-cooled reactors around the world experienced similar effects. The principal source of the radioactivity was traced to cobalt-60, formed by neutron absorption in the natural cobalt-59 which arose from hard-facing alloys and was also present as an impurity in boiler materials such as HMDnel, and in carbon steel and other structural materials. The mechanism of this radioactivity transport was found to be corrosion of the cobalt bearing materials, transport... 

Sea disposal could be carried out in the following way, similar to the approach used during the Soviet period as discussed in section 2. The reactor vessels and the primary circuits are drained for water and filled with a material like furfurol or a low melting point metallic alloy to fix the damaged fuel and the control rod and to decrease the release of activity to the environment. If need be, additional material, e.g. concrete may be added to the reactor compartment to reduce the radiation level around the submarine. At the same time the floatability of the submarine must be ensured. After these preparations the submarine is transported out to the sea to a proper place, where the submarine is disposed of by sinking. Depending on the state of the submarine, it may by towed to the disposal area or it may have to be transported in a floating dock or by some other means. 

Reverse osmosis preceded by microfiltration or ultrafiltration is considered as an option for the treatment of radioactive wastes from Romanian nuclear centers. Effective studies are carried on at Research Center for Macromolecular Materials and Membranes, Bucharest and at Institute of Nuclear Research, Pitesti aiming in employing these pressure-driven techniques for cleaning the wastes from decontamination of nuclear installations and reactor primary circuit [34,35]. 

A. Bertuch, J. Pang, D, D. Macdonald, The Argument for Low Hydrogen and Lithium Operation in PWR Primary Circuits, in the Seventh International Symposium on Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors, Breckenridge, CO, August 6-10, 1995. 

Later, it was proposed to use redox potential for monitoring the materials of the primary circuit of PWRs as well [7]. 

The direct measurement of corrosion was formerly obtained by inserting material specimens in autoclaves connected with the primary circuit. This method requires periodic weighings performing a visual controls and metallographic investigations. 

Variation of the eutectic volume during its solidification and comparability of its mechanical properties with those of structural materials leads to additional loading of the primary circuit components. 

During the last 10 years of NPIs operation there have been no problems with either structure materials corrosion in the primary circuit or the deviations from specifications on the circuit purity. 

After three years preparation, the TV inspection of the primary circuits internals has been successfully issued in April 2001. The Novatome division of Framatome-ANP, completed the extensive non-destructive inspection program requested by the Phenix plant with an inspection of the primary circuit internals. Included in its service package were the setting up of the TV inspection — which required the partial draining of the primary-side sodium — the supervision of the supply of various materials needed to be developed and qualified, and the actual carrying out of die inspection. Justification studies for the inspection were carried out at the same time focusing on the thermalhydraulic, safety and radiation protection aspects affected by the lowering of the sodium level in the primary vessel. 

In 1985, the sodium was transferred to another sodium storage. During this transfer, the primary sodium had a purification campaign from caesium ( Cs and " Cs are the main radio contaminants in primary circuit of fast reactors). This purification campaign consisted in passing the liquid sodium through caesium traps. These caesium traps are made with carbonaceous solid material where the trapping of caesium is made by adsorption phenomenon [13, 14]. Thus the 37 tons of primary sodium of RAPSODIE was purified from around 1.85 1012 Bq of Cs. The contamination of the primary sodium that was initially of 42 kBq/g of sodium was lowered to 5.8 kBq/g of sodium (reduction factor of more than 7). 

In any water-cooled reactor account must be taken of the remote possibility that a failure of the primary water circuit could occur. The result of this would be a rapid release of steam from the primary circuit. This steam would carry with it radioactive material. In order to prevent this being released to the environment, it is normal practice for water reactors to be snrronnded by a containment which is a strong structnre, designed to prevent the release of radioactive steam to the environment. 

Possible remedies for PTS events are automatic overpressure protection which disable systems that can pressurize the RPV, adnodnistrative measures to isolate high pressure systems from the primary circuit, and dedicated outage safety valves. It is also possible to mitigate PTS by recovering the material properties of the RPV through thermal annealing. 

HTR-10 uses spherical fuel elements with ceramic coated particles, graphite as the core structure material and helium as the coolant The fuel elements are charged from the core top and removed from the core bottom via a discharge tube with multi-pass recycling Fig 4 shows the cross section and primary circuit of the HTR-10 The primary system consists of a reactor pressure vessel and IHX-SG vessel are arranged in... 

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