Primary circuit contamination

The deposition of fission products on primary circuit surfaces and, in particular, on the reformer tube walls causes difficulties during maintenance and catalyst refilling procedures, if the activity is intolerably high. Cesium and silver isotopes released during reactor operation are of major concern. Particularly silver diffuses easily out of the fuel elements at operating temperature conditions into the coolant and migrates easily into metal surfaces and is difficult to remove in decontamination operations [32]. 

In a gas turbine version of the HTTR, half of the radiation dose is estimated to originate from the silver isotope Ag-llOm. Countermeasures could be either a fuel temperature reduction which would require a change of the core design, or the use of advanced fuel with the most efficient barrier against fission product release, the SiC coating layer, to be substituted by a ZrC layer. The higher retentivity of ZrC against Ag release, however, needs further confirmation [40]. 

Numerous experimental and theoretical efforts have been made to examine the plateout distribution of fission products in the primary circuit both under normal operating and accident conditions. Different in-pile and out-of-pile deposition loops were operated in Germany, Japan, France, the UK, and the USA to study systematically the ad-/desorption behavior of fission product on metallic surfaces as a function of temperature and gas 

In Japan, the Oarai Gas Loop No. 1, OGL-1, installed in the JMTR, was used for cesium and iodine plateout distribution measurements basically under the normal operating conditions of the HTTR. Within a joint German/French effort, another plateout experimental series was conducted in the in-pile loop SAPHIR in the French PEGASE reactor. Also extensive plateout distribution data were obtained in the UK by examining components removed from the Dragon HTGR 

Furthermore impurities in the helium coolant, mainly the air constituents, can cause corrosion effects on the outside reformer tube walls which eventually change its properties. Measurements of impurity contents in Dragon and AVR revealed a large scattering of the data. Experimental results obtained within the Dragon project indicate a strong corrosion of aluminum and titanium, i.e., the formation of Cr-, Mn-, Si-, and Ti-oxide layers, and an increased corrosion rate in moist helium compared with a dry atmosphere [26]. 

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From the different topics discussed in the preceding sections it can be concluded that there are different possible ways of keeping the buildup of primary circuit contamination low or of reducing already existing contamination levels. Their common aim is to keep the production of radioactive cobalt isotopes low and/or to minimize the transport of the radionuclides produced from the reactor pressure vessel to other regions of the primary circuit and, in addition, to minimize their plate-out on the surfaces there. Because of the importance of low radiation levels for an undisturbed operation and maintenance of the plants, different measures have been attempted with quite varying success. 

The investigations reported on by Clinton and Simmons (1987) also showed that the presence of dissolved oxygen in the solution results in an iodine partition coefficient lower by a factor of about 10, apparently due to the formation of volatile iodine species. This effect might be of interest in a steam generator tube rupture accident which happens shortly before a planned shutdown of the plant, after hydrogen had been removed from the coolant and/or H2O2 had been added to reduce the primary circuit contamination and radiation dose rates. In order to cover such conditions as well, in the US Guidelines a total iodine partition coefficient of 2 10 has been specified for tube rupture accidents. 

The principal barriers against fission product release into the environment are the high quality TRISO fuel, the reactor pressure vessel, and the reactor building. The calculation of the fission product release during normal operation of the reactor (Fig. 3-5) which determines the contamination of the primary circuit and thus the source term in case of a depressurization or a water ingress accident has again identified silver to be the nuclide with the largest release fraction. 

As a result of that operation, uncontrollable accumulation of significant masses of lead oxides in the primary circuit h pened, they could have formed when the pipelines of the primary circuit gas system, which were necessary for its repair, were depressurized, and thus the air penetrated into the primary circuit. Besides that, the primary circuit was contaminated by products of oil pyrolysis, which was the working medium for seals of rotational shafts of pumps that provided the gas leak prooihess of the primary circuit. Masses of oil were spilled into the primary circuit because the oil seals had not been reliable enough. 

The major reason for its radiation danger is the formation of radioactive polonium aerosols when hot LBC contacts with air. It could happen under conditions of emergency tightness loss of the primary circuit and coolant spillage. In this case, as the RI operation experience at the NS has displayed, the yield of Po aerosols and air radioactivity (according to the thermodynamics laws) reduce quickly with temperature decreasing and spilled alloy solidifying. Fast solidification of spilled LBC restricts the area of radioactive contamination and simplifies its removal in the form of solid radioactive wastes. 

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). 

Other studies are being conducted to develop methods for dismantling problem - raising structures such as those comprising irradiated steels and graphites. An arc furnace at Marcoule made an industrial demonstration on melting more than 5000 tons of slightly contaminated steel from CO2 primary circuit. The... 

The water of the primary circuit always contains trace amounts of fission products. These may stem from a small outer contamination of the fuel rods by traces of uranium. (The total surface area of the fuel rods exposed to the primary coolant amounts to about 7,000 m, equal to the size of a soccer field.) The fission products may also stem from pinholes in a fuel rod. The measurement of the fission product spectrum and the nuclidic composition allows the radiochemist to decide between the two possibilities. Consequently, the defective fuel rod can be isolated and removed. 

The primary circuit is isolated fi-om the secondary ones by using an intermediate heat exchanger, so the heat (energy) is transferred fi-om liqttid metal to the same liquid metal at another side. Intermediate loop is designed as such, if there is a leakage, the primary liquid metal will never come to the intermediate loop, so that the contamination of the intermediate liquid metal is expected not to occur. 

To ensure the water quality of the primary and secondary circuit water, a purification system is deployed. In the primary circuit the system consists of pumps and ion-exchange filters. The maximum temperature of the water intended for purification is up to 80°C. The secondary circuit does not have a permanently acting purification system. Once every half year, partial replacement of the secondary circuit water is performed. The water is replaced by makeup water from the fresh water storage tanks. Contaminated water from the secondary circuit is later forwarded through the evaporator to tanks as pure condensate. 

It can be assumed that the radioactive cesium isotopes are present in the coolant as Cs" ions, but as yet there is no direct experimental proof of this assumption. Very low cesium activities which are occasionally detected in filtered suspended corrosion products are probably attached by occlusion or by adsorption at the surfaces of the solids. Since cesium is not able to form insoluble compounds in the primary coolant, it does not participate noticeably in the contamination buildup on the primary circuit surfaces. 

At the PWR primary coolant pH of 7 to 8, the fission product isotopes of the tri- and tetravalent elements show strong hydrolysis, resulting in very low solubilities. This macrochemical behavior is consistent with the observations made in coolant analyses that these radionuclides can be almost quantitatively isolated together with the suspended corrosion products by filtration. However, this behavior does not necessarily indicate the presence of particular oxides or hydroxides of these fission products, since due to their very low element concentrations in the coolant their solubility limits are probably not exceeded. Presumably, these element traces are attached to the corrosion product oxides either by adsorption onto their surfaces or by formation of mixed crystals. A significant fraction of the longer-lived tri- and tetravalent fission products, as well as of the actinides, is incorporated into the contamination layers which cover the primary circuit surfaces. However, because of the usually very low actiAuty concentrations of these radionuclides in the coolant and, consequently, in the contamination layers, their contribution to the contamination dose rates is negligible. 

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-... 

There is no doubt that the generation of radionuclides from the corrosion product elements can only occur in the neutron field, i. e. inside the reactor pressure vessel (RPV). On the other hand, the radionuclides which cause the radiation fields which potentially complicate work during plant normal operation as well as during inspection and repair work are those deposited on the inner surfaces of the out-of-RPV primary circuit piping and components, regions they are transported to by the primary coolant. This means that contamination buildup in the PWR primary circuit is a complex process. It can be roughly divided up into three stages (see Fig. 4.26.), each of which raises its particular questions ... 

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