Page 170 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

142 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

3.4.10 Cleaning of the facility/test section

The thermal-hydraulic behavior of HLM facilities may be affected by occlusions/
plugging forming over time in test sections or heat exchanger/steam generator sections
as a consequence of the deposition of Pb/Bi oxides, corrosion products, and other
impurities. Such occlusions or surface deposits may affect the results of the experi-
mental thermal-hydraulic campaign. Therefore, the cleaning of components or even
of the whole facility could be required before any repeated operation. Frequent visual
analysis (e.g., via endoscopy) of piping and components is advisable to check the sta-
tus/conditions of the facility and to consider whether to undertake any cleaning
measures.
Components working in a HLM environment are easily cleaned using suitable
chemical etchants able to remove residual Pb/Bi metals and Pb/Bi oxides. Typical
etchants for Pb/Bi metal and oxides are aqueous solutions of acetic acid
(CH 3 COOH) or nitric acid (HNO 3 ), able to form soluble Pb/Bi acetates or nitrates that
are rapidly removed away in the aqueous solution (Pruksathorn and Damronglerd,
2005; Gholivand et al., 2010; Saito et al., 2004). In general, the employment of acetic
acid is accompanied by the use of hydrogen peroxide (H 2 O 2 ) to speed up the leaching
in reasonable times. As an example, the reactions involving the leaching of Pb using
acetic acid or nitric acid aqueous solution are described as follows:
1. 2Pb+O 2 (air) +4CH 3 COOH!2Pb(CH 3 COO) 2 +2H 2 O (very slow)
2. Pb+H 2 O 2 +2CH 3 COOH!Pb(CH 3 COO) 2 +2H 2 O (fast)
3. H 2 O 2 !H 2 O+½O 2 (gas) (secondary reaction)
4. PbO+2CH 3 COOH!Pb(CH 3 COO) 2 +H 2 O (fast)
5. 3Pb+8HNO 3 !3Pb(NO 3 ) 2 +2NO (gas) +4H 2 O (fast if HNO 3 is diluted)
6. PbO+2HNO 3 !Pb(NO 3 ) 2 +H 2 O (fast)
The cleaning of large components (e.g., valves, test sections, and expansion vessels) is
carried out in safety inside dedicated pools containing the acidic aqueous solution
(CH 3 COOH+H 2 O 2 or HNO 3 ). An example of a dedicated system for the cleaning
of components working in liquid Pb/Bi is represented by the facility PLAnt for
Cleaning of large Equipment (PLACE) at Agenzia nazionale per le nuove tecnologie
(ENEA) in Brasimone (see Fig. 3.4.7). The facility is equipped with a pool, con-
structed from 316 stainless steel, for the containment of components during the
cleaning; two tanks in high-density polyethylene (HDPE), one for the fresh solution
and the other tank for the exhaust solution; and an external pump between the exhaust
tank and the cleaning pool for the liquid transfer. The exhaust tank is used to store the
liquid that has already worked but is not completely exhausted and can be reused. The
cleaning pool is also equipped with nozzles for rinsing components with water. In gen-
eral, diluted solutions of CH 3 COOH/H 2 O 2 (20+20wt%+balance water) or HNO3
(10–20wt%+balance water) are used in PLACE facility at ENEA to avoid too much
heat generation and gas formation. Indeed, reactions 2, 4, 5, and 6 are highly exother-
mic, and the heat promotes the decomposition of H 2 O 2 , when CH 3 COOH/H 2 O 2 is
used, according to reaction 3. For the cleaning of small components in a laboratory
(e.g., small oxygen sensors), the classic mixture CH 3 COOH/H 2 O 2 /ethanol with