L - KTH

38 C.J. Wo~l~
Good decontamination factors are generally obtained for BNR rectrculation systems wlth
either a single or double LOMI application. In low flow areas, a second LOMI step is
sometimes required to deal with excessive oxide burden, because catalytic decomposition of
reagent occurs before it contacts surface oxides. Thus the need for a second step can be
minimized by improving reagent circulation (e.g., by use of spray devices for low flow
areas). In most cases, the addition of an oxidizing step, typically nitric acld/potasslum
permanganate (NP) has given only a small further reduction in residual fields. However,
for some components, such as the reclrculation pumps, use of NP has proved beneficial.
When a permanganate step is used, oxalic acid Is added stolchlometrlcally to dissolve
manganese dioxide (MnO2). Adding insufficient oxalic acid will cause some of the
subsequent vanadous reagent to be used up in dissolving MnO 2. Adding excessive oxalic acid
will lead to the presence of oxalic acid briefly in the system. To avoid possible
corrosion, this should be minimized by terminating the addition when samples taken of the
decontamination solution are clear.
The chemical reagents can be added in the solid form (as with CAN-DECON) or as liquids
(LOMI Is a mixture of dilute solutions of vanadous formate and plcollnlc acid). Depending
on the type of circuit to be decontaminated, feed-and-bleed or fill-and-draln methods are
used. In most cases, the radioactive waste Is removed on Ion-exchange resin.
Two different flow paths have been used in recent reclrculatlon piping
decontaminations. Use of the annulus between the shroud surrounding the core internals and
the pressure vessel provides a convenient flow path (Fig. I), allowing effective reagent
circulation. However, concern about corrosion damage to in-core materials has resulted In
restricting the reagents to out-of-core regions for a number of plant applications. In one
case, the piping was cut and capped before decontamination, which can be costly in both
time and radiation exposure. In other plants, fluid levels were kept below the pressure
vessel inlet level, wlth raising and lowering of the liquid used to provide reagent
replenishment. This approach is operationally more complicated and less satisfactory than
permitting fluid flow through the annulus. Flow paths are discussed further in Section 3.
A general conclusion drawn from recent experience Is that differences in decontaminating
power of different solvents is less significant than the differences between the oxide
films in the various plants. Almost any decontamination factor can be achieved with any of
the processes, but longer application times may be required in some cases. LOHI proved to
be the quickest process currently in use. Decontamination factors (DF) or residual
radiation fields in designated areas are often specified in the decontamination contract.
A utility chooses between a lower DF with minimum waste and shorter downtime, or multl-step
applications giving higher DFs at higher cost. The choice depends on the magnitude of
special maintenance work to be done.
2.3 PWR applications
There have been fewer P~/R decontaminations, because of higher cost and because the
maximum incidence of repair work associated with secondary side corrosion problems
(denting, wastage, pitting and intergranular attack) had passed before sufficient
confidence was generated in dilute processes for PWR applications. The decontamination
reagents used for PWR applications are slmilar to those described above. In all cases, one
or more permanganate steps have been used to remove chromium from oxide films.*
In contrast to BWRs~ an oxidizing step is essential for PNR decontamination. Alkaline
oxidation yields higher DFs when applied in the presence of high NI alloyed materials,
whereas acidic oxidation is preferred (higher DFs) in the presence of stainless steel
surfaces. The recent CAN-DECON and LOMI applications have each used alkaline permanganate
(AP) followed by nitric acld/permanganate (NP) to achieve maximum chromium removal from
oxide films initially present on both Inconel and stainless steel. This pH switching
technique was developed by CEGB (11) to optimize DFs in channel heads.
Table 2 compares the DFs obtained on different materials with NP and AP preoxldatlon.
One important factor observed in all PWR decontaminations, and in the BWR applications
using permanganate, is the need to remove a loosely-adherent radioactive residue
particularly with the organic acid reagents. In some applications, high pressure
hydrolazlng has been used, whereas In other cases, repeated flushing proved adequate. If
*For a detailed discussion of surface oxide character in P~R and BWR environments see
Reference 11.