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that solutions containing oxalic acid under some conditions induce fntergranular attack on
sensitized type 304 stainless steel. A cautious period followed, when sufficient corrosion
evaluation was conducted to understand and avoid conditions which could potentially cause a
problem. The LOHI process, which does not contain oxalic acid, was first used on a BWR in
1984, and was selected by most BWR utilities carrying out decontaminations from 1985 on,
Including Monticello and Quad Cities BWRs in 1989. Aside from the encouraging corrosion
data, LOHI offered at least comparable DFs and required short application times. CITROX
has continued to be used at the Brunswick BWR. Meanwhile a new process, CANDEREH, which
does not contain oxalic acid, was developed from CAN-DECON, and is now available and was
recently used on a heat exchanger at Indian Point 2 PWR and on the steam generator channel
heads at Beaver Valley I PWR. CANDEREM and LOMI are given equal prominence at this time in
the development of corrosion data necessary for full system decontamination in PWRs.
2. TECHNICAL BACKGROUND
2.1 Concentrated reagents
The 1984 de contamlnatlon of the complete primary system (with the fuel removed) of the
Dresden-I BWR, using the NS-I process, was the only recent U.S. application of concentrated
chemical reagents. This decontamination was unusual in several respects. Dresden-l, an
early BWR, was shutdown in 1978 and Is unlikely to operate again. The decontamination was
originally scheduled for 1980, but environmental impact and safety issues were mainly
responsible for a delay of four years.
Decontamination factors exceeded 11.3, and over 750 curies of activity was removed. The
total activity removed was extremely close to the 1979 calculation of total activity in the
system, when decay was taken Into account. The use of an organic inhibitor to control
corrosion in concentrated reagents tended to slow down the cleanup process, and the return
of the total organic carbon to reactor water speclffcatlon took considerable tlme.
The detailed report of this project (9) reveals both the strengths and the problems of
concentrated (or "hard") decontamlnatlon--processes. High removal of activity was achieved,
but waste disposal was a major difficulty, and concern about corrosion effects necessitated
a large and expensive materials qualification program. Recently-developed dilute
decontamination reagents ("soft" processes) achieve as much activity removal, with greatly
reduced concerns about corrosion damage and waste disposal. With the current Industry
emphasls on avoiding corrosion and reducing radwaste volumes, it seems unlikely that
concentrated processes will be used on operating plants again, although localized
applications on components such as pumps will continue. This review focuses on system and
subsystem decontamination dilute chemical processes.
2.2 Dilut e reasents - BWR
There are two types of dilute chemical reagents. In the early '80's, the organic acld
reagents based on citric and oxalic acids (e.g., CITROX), using a chelating agent such as
EDTA to retain dissolved corrosion products In solution (e.g., CAN-DECON), were used for
the majority of plant applications. Since 1985, however, the low oxldatlon-state metal ion
(LOMI) process has been the most widely used process In the United States. Table 2-I lists
primary coolant system decontamlnatlons (IO).
The organic acid reagents dissolve oxides by simple acidic dissolution
8H + + Fe304 ~ Fe 2+ + 2Fe 3+ + 4H20 (I)
acid oxide metal ions
in solution
and reductlve dissolution:
8H + + 2e- + Fe304 ÷ 3Fe 2+ + 4H20 (Most likely the major
pathway for oxide
acld reducing oxide metal ions destabillzatlon)
agent in solution
The radioactive Impurities, such as Co-60, Co-58, Fe-59, Mn-54, are released at the same
time. A corrosion inhfbltor may be necessary for some applications, depending on the
materials In the system, the process temperature, duration, and reagent strength. Because
3+
Fe solubility is small, a chelating agent needs to be present at the site of dissolution
in order to retain the iron in solution.
(2)