L - KTH

42 C.J. WOOD
With the suction piping filled, circulation and heatup proceed in the same manner as for
the discharge side. For this phase, flow enters the system in the same location but
travels up the suction piping of the first loop, into and around the annulus and back down
the suction llne of the second loop. Unlike the discharge side, all parts of this
circulating loop are exposed to full system flow. Chemical injection, process monitoring,
and subsequent cleanup are all controlled in the same manner as the discharge side. Flow
reversals, although not as important, are still performed at regular intervals.
If the annulus cannot be included in the flow path, the approach for performing the
suction decontamination is almost identical to that described for the discharge side
without a cross-tle llne. A "sloshing" technique is used to move fluid from one loop to
the other and hold it at the maximum elevation for a short period of time. One additional
problem with the suction side is the accuracy of the level control. If the fluid cannot
enter the annulus it is important that the equipment be able to control the maximum fluid
levels very accurately. Otherwise, much of the benefit of the application can be lost
(from levels which are too low) or problems "can be encountered with fluid ~n the annulus
(from levels which are too high).
3.1.2 Single Phase RWR Decontaminations For decontaminations that are coincident with
major modifications or complete piping replacements, a single phase application may be more
appropriate. In order to accomplish this, however, additional connection points must be
made available. A number of potential connection points is possible depending upon the
specific needs and concerns of the individual utility. Usually twelve additional
connections are necessary. One is located on each of the ten discharge risers as close to
the penetration into the vessel as possible. The remaining two are located at the suction
nozzles. With these additional connections both the suction and discharge sides can be
decontaminated simultaneously and fresh reagent supplied to virtually all portions of the
system. A typical flowpath is shown in Fig. 3.
The major advantages of this approach are the potential for critical path savings since
only one heatup, injection, circulation, and cleanup step is required and the improved DFs
achieved in the risers from direct injection of fresh reagents. This must, however, be
balanced against the additional effort, time, expense and radiation exposure required to
prepare for this approach.
3.2 PWR steam generator channel head applications
Decontaminations are performed in the channel heads of PWRs to reduce the amount of
radiation exposure necessary to complete major maintenance activities at the tubesheet on
the primary side (e.g. sleeving, plugging etc.). To maximize the benefit, two to three
feet of the tubes above the bottom of the tubesheet need to be exposed to the reagent.
Fig. 3. BWR reactor water recirculatlon system two
phase decontamination flowpath