FINAL REPORT OF THE WORK DONE IN ARGENTINA FOR AN IAEA CRP AND A REGIONAL PROJECT FOR LATIN AMERICA ON “CORROSION OF RESEARCH REACTOR ALUMINIUM CLAD SPENT FUEL IN WATER” R. HADDAD, R. GUASP Comisión Nacional de Energía Atómica (CNEA) Argentina Abstract. In the frame of two different IAEA programmes, a Coordinated Research Project (CRP) “Corrosion of research reactor aluminium-clad spent fuel in water - Phase II” and a Regional Project for Latin America RLA-4/018, an ample experimental plan was established to asses the corrosion condition of MTR type spent fuel kept in the various storage sites in Argentina. Twenty three racks, nineteen vertical (horizontal coupons) and four horizontal (vertical coupons) assembled with discs manufactured with different aluminium alloys, were immersed in several water basins. After testing times ranging from one to three years, they were extracted and analysed. Corrosion effects such as oxidation, pitting, galvanic attack, etc., are reported. All these are correlated with environmental variables: water chemical composition, occurrence of crevices or galvanic couples, surface orientation, etc. Complementary actions were taken in order to evaluate the incidence of dust deposition on the metal surfaces. 1. INTRODUCTION Table 1 lists the several experimental nuclear reactors that have been put into service in Argentina since the early sixties. Several of them use Aluminium clad fuel, either rods or plates. RA3, located at the Ezeiza Atomic Centre (some 40 Km off Buenos Aires city) is the biggest one, now running at power levels of up to 10 MW. This open pool reactor started up burning 90% enriched Uranium with fuel plates made of pure (99.7%) Aluminium. At the end of the 80’s it was converted to use 20% enriched Uranium, with the new fuel plates made of AA 6061 aluminium alloy. Some of the early spent fuel elements were moved to RA6 research reactor, located in Bariloche (1700 Km SW from Buenos Aires), an open pool reactor that runs at 500 KW. RA1 reactor, a low power research reactor located in the Constituyentes Atomic Centre in Buenos Aires, uses a 90% enriched uranium fuel embedded in tar contained in aluminium tubes. These three reactors were selected as “case studies” given the demanding conditions for the storage of their spent fuel. Table 1. Research, experimental or production reactors in Argentina RR LOCATION START POWER FUEL TYPE (clad-meat) RA0 UNC (Córdoba) 1967 0.001 KW Al rods – 20% enriched U RA1 CAC (Bs. As.) 1958 120 KW Al rods – 20% enriched U3O8 RA3 CAE (Ezeiza) 1967 10000 KW Al MTR – 20% enriched U RA4 UNR (Rosario) 1969 0.001 KW 20%U dispersed in polyethilene RA6 CAB (Bariloche) 1982 500 KW Al MTR – 90% enriched U* RA8 CTP (Pilcaniyeu) 1997 10 KW Zry pins – 5% enriched UO2 * Reactor fully converted to LEU on 2008 The nuclear fuel has to sustain long periods of immersion in water, whether it is in use or has been stored in a decay pool after being exhausted in the reactor core. RA1 fuel rods have been in use for about 50 years and are still working. In RA6, the fuel has been in service for more than 20 years; the 59 bundles that are not in use (either “fresh” or exhausted) are kept into a basin pool sited in the same building. RA3 spent fuel elements are normally maintained into a decay pool situated in the same reactor area for a period of time, which could range from one to several years, before they are transferred to a Central Storage Facility (CSF), in a nearby location, where they have been stored for more than 30 years. Fig. 1 shows a view of CSF. It is composed by an array of vertical buried steel tubes, distributed in lines and sharing the same cooling water. These channels are surrounded by a compact selected soil enclosed in an underground brick wall precinct, in order to prevent water leakage. There are two sectors, each one with six lines of 17 tubes, totalizing 204 channels. FIG. 1. Central Storage Facility (CSF) located in Ezeiza, Argentina. Two spent fuel elements can be accommodated in each position, one on top of the other, as illustrated in Fig. 2, but one tube is left empty at the end of every line (as channels #97, #130, #147, #164, #181 and #198, marked in Fig. 1), for service purposes. FIG. 2. Schematic drawing of a spent fuel storage tube. Although provisions were made for water circulation and purification, the maintenance procedure was halted in the past, and the water remained stagnant. After the experience obtained from a first CRP that took place between 1996 and 2000, a gradual improvement of the quality conditions has been implemented. The first step was to establish a periodic water cleaning procedure for the line that 60 includes position #97. Hence, it was decided to perform corrosion tests in all the mentioned channels. This was done with the purpose to make evident the importance of water maintenance in keeping aluminium spent fuel elements free from corrosion. 2. EXPERIMENTAL 2.1. Test racks During the experimental work, twenty three racks were tested: nineteen vertical racks (horizontal coupons) and four horizontal racks (vertical coupons). Seventeen of them belonged to the RLA project and were sent to the following destinations: three vertical racks to each of RA6 Reactor Pool (RP), RA6 Basin Pool (BP), RA1, RA3 Decay Pool (DP) and CSF (one in each of channels #130, #147 and #164); and two horizontal racks were set at RA1 and RA3 DP. Six more racks were employed on the CRP project, all in the CSF, of which four vertical racks were used in channels #97 and #198 (two in each) and two horizontal racks in #164 and #181. A schematic description of the CRP vertical racks can be seen in Fig. 3. Coupons of two alloys were included: SZAV-1 and AA 6061. One coupon of each material was pre-treated to grow an oxide layer and a scratch was mechanically marked on their surfaces; these are marked as “POS” (pre-oxidised and scratched). The whole assemblies included an isolated non pre-treated coupon, the POS, a pair of coupons of the same alloy stacked together to form a crevice, a pair of one disc of each alloy in contact with a stainless steel disc, to form a galvanic couple, and two locally manufactured samples on top: one isolated and the other coupled with stainless steel. These coupons were made of Argentine 6061 alloy of the same kind used to manufacture the fuel plates. FIG. 3. Composition of CRP vertical racks and coupon order. The horizontal CRP racks and all RLA racks are slightly different, because they included material manufactured in two different ways: rolled and extruded. The corresponding description can be seen in Fig. 4. Table 2 contains the chemical composition of the Argentine alloy 6061 used. Racks assembling and installation were performed, following as strictly as possible the recommendations of the Agency’s Supervisory Group. Only stainless steel cables were used for hanging. Table 3 contains all the information referent to racks distribution, test dates, etc. FIG. 4. Composition of CRP horizontal and RLA racks, showing coupon order. 61 Table 2. Chemical composition of Argentine aluminium alloy Element Mg Si Fe Cu Cr Mn Zn Ti Al Wt.% 0,95-1,10 0,55-0,65 0,15-0,45 0,2-0,4 0,10-0,2 0,1 0,25 0,03-0,07 Rest Table 3. Test schedule SITE PROGRAMME RACK RACK IMMERSION WITHDRAWAL Nº TYPE DATE DATE RLA 13 V 2002/12/30 2004/09/30 RLA 14 V 2002/12/30 2005/10/17 RA1 RLA 15 V 2002/12/30 2003/09/10 RLA RLAV-2 H 2005/07/26 2006/03/02 RLA 01 V 2002/10/30 2005/10/20 RLA 02 V 2002/10/30 2003/09/08 RA3 DP RLA 03 V 2002/10/30 2004/10/22 RLA RLAV-1 H 2005/04/21 2006/02/27 RLA 10 V 2002/11/06 2004/10/17 RA6 RP RLA 11 V 2002/11/06 2005/10/22 RLA 12 V 2002/11/06 2003/09/15 RLA 07 V 2002/11/06 2004/10/17 RA6 BP RLA 08 V 2002/11/06 2005/10/22 RLA 09 V 2002/11/06 2003/09/15 CSF #130 RLA 06 V 2003/05/21 2004/10/06 CSF #147 RLA 05 V 2003/05/21 2003/09/09 RLA 04 V 2003/05/21 2005/10/20 CSF #164 CRP CRPV-1 H 2004/11/02 2005/11/24 CSF #181 CRP CRPV-2 H 2004/11/02 2005/11/24 CRP CRP-1 V 2003/05/21 2004/05/20 CSF #198 CRP CRP-2 V 2003/05/21 2005/11/24 CRP CRP-3 V 2003/12/03 2004/05/20 CSF #097 CRP CRP-4 V 2003/12/03 2005/11/24 In the RA1 reactor, the three racks were immersed in the reactor pool near the top, in a zone where no interference with reactor operations is produced but the water with the fuel elements is shared, as seen in Fig. 5. FIG. 5. Distribution of racks in RA1 (different views). 62 In the open pools of RA3 DP, RA6 RP and RA6 BP there is much more room available, what permitted to locate the racks in the vicinity of the fuel. In the RA6 RP it was possible to hang them by the reactor wall at the same height of the core; in this position there is a fair water movement produced by the toroidal diffuser that surrounds the core.