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46 C.J. Wood surface area are in the same ballpark as equivalent decontaminations for BWR reclrculatlon systems. It is very likely, therefore, that decontamination equipment currently used in the industry could also be utilized for this work. Further investigations of thls approach are planned. 3.5 Discussion Many routine decontaminations have been performed in both BWR reactor water reclrculatlon systems and PWR steam generator channel heads. All major decontamination vendors have portable equipment that is capable of being used for either application. It is also capable of being used with either the LOMI process or dilute organic acid processes. Oxidizing treatments, which are mandatory for PNR applications and optional for BWR applications, are also handled routinely. In order to achieve the required circulation of the decontamination reagent, RWR system decontaminations are usually performed in two phases--dlscharge piping followed by the suction piping. If the piping is to be replaced, temporary connections can be made at the top of each riser and in the suction nozzles to ensure flow in all parts of the system. This enables the decontamination to be performed in a single application wlth a net saving in critical path time. PWR channel head decontaminations require the installation of nozzle dams to isolate the generators from the piping. Specially modified manway covers are used to connect the decontamination equipment to the channel heads. Reagent level is usually controlled at 2-3 feet up the tubes. This gives maximum dose reduction in the bowl while minimizing waste volumes. Apart from these fundamental differences, operational and chemical techniques associated with channel head decontaminations are very similar to those employed in RWR system decontaminations. Later sections will address techniques to reduce recontamlnetion. ~ LEVEL FttMO DE(X~N r, ~mWAL FLOW : sou: : ms~c.oN : E(~,P : ! LEVEL ! I I lo fq~)M E~.~. ,'2" :~,P. L_ ols~c.o..__j NOTE~ ,. S~,CE CCX~ SHOVM r-c~ cu~nY. ~CT~UEb.X~C~ BE OECONTAMINATED SIM~TANEOUELY. 2. SYSTEM FILLED TO APPI:IOXIMATE BLEVATION OF STEAM ~d~IERATOR 1UBE SHEE'I'. FLOW FEVERSN.S .~OOOla~JSH DECONTAMNAllON OF BOTTOtl ENDS OF ~SES ON BOTH HOT AND COLID LEGS AS SHOWN IN DETAIL "A'. & JU~ ASSEMBLY I~STALLEO V~ REAGiOH ~.~ @ ll=M REkW~VAL OF FUEL ANO CORE 8NqflEL QF'IIIeE ,#~ DETAIL 'A' ~~__/TIGN Fig. 6. ~ partial reactor coolant system decontamination flowpath
Chemical decontamination technology -t7 4. CORROSION 4.1 Introduction Corrosion has always been recognized as an area of potential concern to decontamination users. Corrosion is generally minimized by using dilute concentrations of relatively noncorrosive organic acids, and by including corrosion lnhibitors in the formulation. These measures have been successful in avoiding significant corrosion damage during the decontamination. A second issue concerns the need to demonstrate that the decontamination will not increase susceptibility to intergranular stress corrosion cracking (IGSCC) after return to power. It is against this more severe criterion that decontamination solvents have been evaluated. As part of a comprehensive program to qualify currently-available decontamination processes, extensive testing has been carried out in recent years. These tests include general corrosion measurements, constant extension rate tensile (CERT) tests, pipe crack tests and crack growth rate measurements. Although carried out under BWR conditions in almost every case, these tests results are generally applicable to PWR channel head decontaminations. For PWR full system decontamination, however, further data on the effects of reagents in the presence of boric acid are desirable. Results from the corrosion programs have been published and/or discussed extensively in EPRI seminars (4-8,12). In this section, corrosion consequences are reviewed and the available test results are summarized and discussed. 4.2 BNR decotamlnatlon flowpath and wetted materials An earlier section discussed the two basic approaches to applying decontamination solution to a BWR recirculation system, which are illustrated in Fig. 1. As noted in this figure, the two choices consist of applying the decontamination solution only to the piping system (A) or to the piping system plus the annulus between the reactor pressure vessel and the core shroud (B). Gordon (13) has provided a detailed review of BNR corrosion issues, which is summarized here. If the decontamination process is limited to the piping system, then the piping, pumps, and valves will be wetted by the solution. The reclrculation piping, pumps, and valves in most operating BWRs are primarily fabricated from welded Type 304 stainless steel. Experience has demonstrated that the pipe weld heat-affected-zones (HAZs) are susceptible to intergranuler stress corrosion cracking (IGSCC) as will be discussed in detail later. Partial mitigation against IGSCC has been obtained in some Type 304 stainless steel piping by the application of various stress reduction techniques, such as induction heating stress improvement (IHSI) and last pass heat sink welding (LPHSN), that place the wetted surface of the weld HAZ in compression. Newer BWRs have more extensive application of these and other IGSCC remedies, including the use of Type 316 Nuclear Grade stainless steel which is highly resistant to IGSCC. Since there exists an extensive body of laboratory and field data that any lntergranular attack (IGA) produced by a decontamination or passivation treatment increases the probability of IGSCC during subsequent service, this type of localized attack must be avoided. The pumps and valves which are also wetted in this decontamination option contain wear resistant surfaces, low alloy steel wedges, other small parts, and valve packing material. Accelerated general, crevice and galvanic corrosion of these parts must be prevented to avoid premature replacement or repair of these components, which are fabricated from a wide variety of materials. Consider the case of allowing the decontamination solution into the annulus between the reactor pressure vessel and the core shroud, Figure 2-1 (B). Not only is there the same concern for the components discussed above, but with this option, a number of reactor internals will also be wetted by the decontamination solution. The range of material concerns will thus increase. For example, Alloy 182 weld metal stainless steel safe ends and nozzles, as well as the crevices between the recirculation system thermal sleeves and safe ends, are regions where only limited field cracking incidents have been observed. Decontamination-solution produced IC~ could significantly increase the predisposition of these components to IGSCC. Jet pump parts fabricated from welded stainless steel and Alloy-X-750, the welded stainless steel core shroud, and the heavy section weld between the shroud support and the shroud all have had good service experience. The good performance
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