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atw Vol. 63 (2018) | Issue 4 ı April DECOMMISSIONING AND WASTE MANAGEMENT 242 1 Introduction The ENEA RB3 (Reattore Bologna 3) reactor was a 100 Wth research installation owned and operated by ENEA in its center of Montecuccolino, near Bologna, from 1971 to 1989. It consisted of a cylindrical aluminium vessel, about 4.3 m high and 2.9 m in diameter, which could host various types of fuel elements suspended from the top of a special adjustable rack, and submerged into heavy water serving both as moderator and coolant. Principal aim of the reactor was to provide neutronics data for the CIRENE NPP, a SGHWR that was being designed, and then partially built in Latina, starting from 1979. The specific RB3 core, surrounded by a graphite reflector and housed inside a concrete biological shielding, allowed to test easily very different fuel element configurations by changing their pitches and by regulating the heavy water level inside the vessel. The reactor design, similar to that of the ZED-II Canadian research facility, was originally developed by CEA for its Aquilon facility in Saclay in 1956; in fact, through a special arrangement between ENEA and CEA, parts of the Aquilon facility were ultimately donated to ENEA at the end of the 60s for the construction of RB3. In 1989, after more than 18 years of operation, the RB3 reactor was shut down, and in the late 2010, after waiting for the entry into force of Legislative Decree (L.D.) 230/1995 [1], which introduced new laws for the decommissioning of NPPs, ENEA received by ministerial decree the authorization to its dismantling, with the aim of reaching the “green field” status and with the unconditional release of its building, including the reactor concrete biological shielding, which is actually owned by the University of Bologna. In fact the site of Montecuccolino, some 3.5 km to the South of downtown Bologna, hosted three research reactors: RB1, owned and operated by the University of Bologna, RB2, owned and operated by AGIP Nucleare, and RB3, owned and operated by ENEA. RB1 and RB2 were decommissioned up to the green field status well before the entry into force of L.D. 230/1995. Figure 1 shows an aerial view of the Montecuccolino research center, with the area hosting RB3 contoured in red. Figure 2 shows a plan of the main reactor hall, with in red the area once occupied by the reactor vessel, surrounded by the hectagonal graphite reflector and encased within a thick concrete biological shielding. Figure 3 shows a vertical section of the RB3 building; the lowermost floor hosted 4 large tanks for a total of 20,000 L (in red) for the storage of the heavy water which was daily pumped up into the vessel to reach criticality and then drained after the conclusion of the experiments. Three floors are present in the building: floor +6.0 m corresponding to the ground level, floor +0.0 m, corresponding to the level of the reactor vessel, and floor -3.0 m, with the heavy water storage tanks, heating and cooling systems, and other auxiliaries. The control | | Fig. 2. Plan of main hall of RB3. room was located at floor +0.0 m. While allowed to operate up to 100 Wth, operations at RB3 were always conducted at 50 Wth. Between 1991 and 1992, all the fuel elements used at RB3 were either restituted at their owner (JRC Euratom Ispra) or sent to the ENEA Research Center of Saluggia or to the fuel fabrication plant of Fabbricazioni Nucleari at Bosco Marengo. Between 1992 and 1993 all the heavy water was transferred to the ENEA Research Center of Borgo Sabotino, and before the end of 1997 all the sealed radioactive sources used at the plant were disposed of. 2 Regulatory Requirements and Classification of Components and Materials In the late 2010, ENEA received, by decree of the Italian Ministry of Economic Development, the authorization [2] to proceed with the dismantling of RB3; included in the | | Fig. 1. Aerial view of the Montecuccolino site; the RB3 building is inside the red square. | | Fig. 3. Section of the RB3 building. Decommissioning and Waste Management The Decommissioning of the ENEA RB3 Research Reactor in Montecuccolino ı F. Rocchi, C. M. Castellani, A. Compagno, I. Vilardi, R. Lorenzelli and A. Rizzo
atw Vol. 63 (2018) | Issue 4 ı April Radionuclide Metals Concrete Other materials Reused Recycle Both reuse and recycle Demolition Surface (Bq/cm 2 ) decree were also the requirements imposed by the Italian Nuclear Regulatory Body ISPRA. According to these requirements, the components and materials of RB3 were classified by ENEA, taking into account the various areas of the plant and the history of its functioning, into four main categories: A) materials and components which were both in contact with possibly contaminated or activated process fluids and subject to neutron flux; B) materials and components which were in contact with possibly contaminated or activated process fluids but not directly irradiated by neutrons; C) materials and components which were irradiated by the neutron flux but which never went into contact with possibly contaminated or activated process fluids; D) s.c. “exempt” materials, which were never irradiated and never went into contact with possibly contaminated or activated process fluids. The only component classified in the A category was the aluminium vessel; the only components in the B category were the heavy water distribution pipings. Exempt materials, given their unirradiated and uncontaminated status, were subject only to a general screening through CANBERRA In Situ Object Counting Systems (ISOCS) to estimate any possible level of presence of 60Co and 137Cs; if the measured levels were below the decision threshold of the measuring system in terms of mass concentration levels, then Surface (Bq/cm 2 ) | | Tab. 1. Surface or mass activity concentration levels for clearance. Mass (Bq/g) these materials were automatically discarded from the plant without any further radiological analysis. This demonstrates the “instrumental” zero of this category of materials hence the “exempt” classification. All materials which had been classified as “exempt” were released unconditionately, for a total mass of about 30 tons, between March 2013 and May 2015. For all the other three categories, the clearance levels imposed by the Regulatory Authority are summarized in Table 1. These were derived either from the Italian L.D. n. 230/95 or from RP 89 [3] and RP 113 [4] publications. In presence of more than one radionuclide, the sum of the ratios of the measured concentrations to the respective levels must be lower than 1. The components and materials were further grouped by ENEA into 12 s.c. “homogeneous groups” using material and historic criteria; homogeneous groups are therefore constituted by components (or parts of them) made by the same material and possibly with a homogeneous and uniform activity content. 3 Radiological Characterization of Homogeneous Groups Before the radiological characterization of the batches of materials from the various homogeneous groups started, a preliminary, special campaign was conducted to exclude the presence of various isotopes among those given in Table 1, expecially in the most potentially activated or contaminated materials (category A). Surface (Bq/cm 2 ) Mass (Bq/g) 3 H 10,000 100,000 1 10,000 1 1 14 C 1,000 1,000 1 10,000 1 1 Mass (Bq/g) 54 Mn 10 10 1 10 0.1 0.1 55 Fe 1,000 10,000 1 10,000 1 1 59 Ni 10,000 10,000 1 100,000 1 1 60 Co 1 10 1 1 0.1 0.1 63 Ni 1,000 10,000 1 100,000 1 1 90 Sr 10 10 1 100 1 1 125 Sb 10 100 1 10 1 1 134 Cs 1 10 0.1 10 0.1 0.1 137 Cs 10 100 1 10 1 1 152 Eu 1 10 1 10 0.1 0.1 154 Eu 1 10 1 10 0.1 0.1 Generic Alfa 0.1 0.1 0.1 0.1 0.1 0.01 241 Pu 10 10 1 100 1 1 In particular 54Mn, 59Ni, 90Sr, 125Sb, 134Cs, 137Cs, 239Pu, 240Pu and 241Pu were excluded from further analyses finalized to the unconditional release of materials. Then, for each homogeneous group, a precharacterization measurement campaign was con ducted with a three-fold aim: 1) to verify if the hypothesis on the homogeneity of activity for that given group held; 2) to evaluate the minimum number of samples to be analized for the subsequent characterization phase; 3) to evaluate the value of isotopic ratios of 55Fe to 60Co and of 63Ni to 60Co, so to limit the next analyses only to the research of 60Co contents. After that, and using typically 13 multiple measurements for each batch of each homogeneous group, summations of the ratios between measured activity concentrations and limits (Table 1) over all the relevant isotopes were carried out. If these summations resulted
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