Experimental and Theoretical Investigations of Ferro Boron as In-vessel Shield Material in FBRs

Transcription

1 Experimental and Theoretical Investigations of Ferro Boron as In-vessel Shield Material in FBRs R. S. Keshavamurthy, S. Raju, S. Anthonysamy, S. Murugan, D. Sunil Kumar, V. RajanBabu, S. C. Ravi Chandar, C. N. Venkiteswaran, S. C. Chetal Indira Gandhi Centre for Atomic Research Kalpakkam, , India

2 In-Vessel Shielding in Fast Reactors Leaking neutrons from core and blankets of Fast Reactors have energies that peak around kev, In pool type of fast reactors, Intermediate Heat Exchangers (IHXs) are immersed in the pool of liquid sodium contained in the reactor vessel itself. Therefore, sodium becomes radioactive and as a result, the secondary sodium passing through IHXs also gets activated. Without adequate shielding around the core, this activation can lead to unacceptable dose in the steam generator building. Therefore fast reactors are generally characterized by large in-vessel shields Therefore the fuel and blankets are surrounded by a large quantity of shield materials, to the extent of % of the core volume in fast reactors.

3 Shield Materials used in different Fast Reactors Stainless steel was used as shielding material in a variety of fast reactors, such as RAPSODIE, JOYO, FBTR, BOR-60, FFTF and FERMI EBR-II used graphite and borated graphite, SS/borated graphite was used in DFR. In PHENIX Graphite/SS was used in the radial direction. In addition, B 4 C was added in the axial direction because of the superior neutron absorption properties of boron. In SUPER PHENIX too, B 4 C shielding was provided in the axial direction. B 4 C was also considered for radial shielding in the reactors SPX2, EFR and DFBR. The prototype fast breeder reactor (PFBR), presently under construction in India has been designed with 3 outermost rows of B 4 C, in addition to 6 rows of SS.

4 PFBR CORE PLAN Important radiological criterion for radial shields: Secondary sodium activity should be less than 90 Bq/cc Dose in Steam Generator building less than 25 µsv/h 6 rows of SS + 3 rows of B 4 C; 1026 SAs 609 SS B 4 C SHIELD SAs

5 Considerations for an alternate In-Vessel Shield Material The optimal design of shield materials with regard to their volume and weight, indigenous development cum testing capability and overall cost economy is one of the major objectives of future FBRs of India. The alternative, when realised should lead to potential cost saving both due to a reduction in vessel size and in the number of shield assemblies. PFBR shield is taken as the reference point, for evaluating alternate shielding options

6 Choice of shield material Several probable candidate materials were considered. Neutron attenuation experiments in the KAMINI reactor and detailed shield analyses for Fast Reactor Reactors formed the basis for choice of material. Ferroboron(Fe-B), an alloy of boron and iron with controlled amounts of impurities, is identified as a promising alternate shield material for possible use in future Fast Breeders Reactors.

7 Ferroboron Typical Composition of commercial Ferro-Boron (Wt%) : Boron: 10-18, Si: 0.9, C: 0.3, Al: 0.2, S: 0.006, P: 0.05 Commercially available Ferro-Boron has up to 18 wt% boron. The material is available in the form of lumps, granules and powder Typically, the density of Ferro-Boron in granular form is around 4 g/cm 3 Melting Range: o C Ferro-Boron commercially available in tonnage quantities. Much cheaper than boron carbide

8 Neutron Attenuation Experiments in KAMINI Reactor Boron Carbide filled in Aluminum boxes at south beam of KAMINI Ferro-boron granules filled in Aluminum boxes at south beam of KAMINI

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10 PFBR WITH SS AND B 4 C RADIALSHIELDS (9 rows) FBR WITH FERROBORON RADIAL SHIELDS (8 rows) 609 SS B 4 C SHIELD SAs 881 FERRO BORON SHIELD SAs

11 DORT code used 2D FBR (R-Z) Model for Transport Calculations 175 neutron energy group cross sections in the VITAMIN-J structure Several configurations with Ferro- Boron tried Secondary sodium activity is lower than the PFBR case. Current calculations indicate that IHX can be moved inside by ~20 cm with borated steel/b 4 C shielding with secondary sodium activity same as in PFBR. Detector flux can be maintained at the same level Radioactive waste generation is also lower Co-60 production from Fe-B does not happen Ferro Boron (8 row) Case

12 X-Ray studies XRD pattern of Fe-B alloy powder No free boron or Fe is present The Fe-B contains a mixture of three intermetallic, namely FeB, Fe 2 B and Fe 3 (B,C). No free iron or boron are present in Fe-B; in other words the entire boron is present in the combined form. The orthorhombic FeB constitutes the major phase, the other two being present in smaller volume fractions, especially the Fe 3 (B,C) - boro cementite phase. This latter phase is present due to the presence of carbon (0.3 wt.%), which together with boron forms the cementite phase [13]. The alloy with about 15 to 17 wt.% B contains some amount of Fe 2 B, the lower boride phase. However, this phase dissolves back into the FeB phase at high temperatures. In a similar manner, the Fe 3 (B,C) boro cementite phase is also unstable at high temperatures, and it dissolves into the FeB and Fe 2 B at high temperatures

13 Thermal Properties of Fe-B The Curie temperature (T c ) was foundtobe 725 o C. Dissolution of boro cementite Fe 3 (B,C) occurs at about 1104 o C. Dissolution of Fe 2 B in the γ- austenite phase was found to occurat 1227 o C. Fig. 4. DTA thermogram of Ferro boron powder recorded during heating and cooling. S The melting of Ferro boron beginsat 1359 o C. It is found to be almost complete at 1450 o C. This later temperature may be taken as the melting point or the liquidus point of Ferro boron.

14 Microstructure of Fe-B Fig. 5 (a). Microstructure of fine Ferro boron powder IGCAR Fig. 5(b). Microstructure of bulk Ferro boron block FR-13 Paris March

15 High temperature compatibility studies Fig Schematic of the diffusion jig

16 Diffusional jig used for simulating the high temperature.

17 The estimated attack layer depth is about 250 microns at 550 o C for a period of 60 years. Microstructural collage of the reaction layer formed at the interface between 304L stainless steel and Fe-B powder for various time durations at 973 K (700 ( o C). Microstructural collage of the reaction layer formed at the interface between 304L stainless steel and Fe-B powder at various temperatures for 600 h exposure.

18 Chemical compatibility Extensive chemical compatibility experiments were carried out by equilibrating Ferroboron with SS304L in presence of sodium at various temperatures. SS 304L samples used were analysed using SEM and EDX after several equilibration experiments in sodium for 1000, 2000, 3000, 4000 and 5000 h. Microhardness of the above specimens was also measured using a Vickers micohardness tester. It was observed that the reaction layer is clearly delineated (Next slide). It is found that the zone of chemical interaction increased with time. The clad thinning is seen to be around 250 microns at this elevated temperature of 973 K over 5000 h. It has been established that at the normal service temperatures, the general and in-sodium compatibility is good and do not pose constraints for the use of Fe-B up to about 1000 K. It must also be mentioned here that sodium is extremely unlikely to come into contact with the material as it is provided with double containment.

19 Chemical Compatibility studies-reaction layer Chemical compatibility Experiments 3000 h 5000 h equilibration show good compatibility at 700 C.

20 Ferroboron Irradiation Test in FBTR An irradiation test of the material was planned, keeping in view good compatibility shown by Ferroboron with SS clad and sodium in out of pile tests. This is to ensure that good out of pile compatibility is matched with the actual in reactor experience, under an irradiation test in FBTR. To simulate the fluence, it was found necessary to load the capsule in the 4 th ring location of FBTR and irradiate it for 45 effective full power days at a flux level of 1.0x10 15 n/cm 2 /s.

21 Fabrication of Ferroboron capsule The irradiation capsule consists of inner capsule with OD 14 mm and ID 12 mm containing fully packed ferroboron granules in five partitions along with an outer tube (OD: 16 mm and ID:14.5 mm). The outer tube acts as secondary containment preventing the release of FeB powder even in the unlikely event of breach in the inner capsule. The central three partitions of irradiation capsule are in line with the fuel column of driver fuel pins. The fabrication involved intricate thin section welds under argon glove box atmosphere, and devising suitable weld qualification procedures. These welds were subsequently qualified by neutron radiography as well.

22 Ferro-Boron Capsules Type I Prototype CFBR capsule Type I Idesigned for irradiation test in FBTR

23 Fabrication of FeB capsules at MINEX Metallurgicals, Nagpur

24 Secondary containment of Ferroboron Capsule The welding of secondary containment was done at site in IGCAR followed by due quality inspection protocols. High purity helium was filled in the annular gap between the inner capsule and the outer containment tube for ensuring good heat transfer characteristics. For filling helium, an innovative helium filling arrangement was designed and fabricated. An intermediate plug was welded to the outer containment tube with provision to evacuate and fill the annular gap with helium and close the filling path. Leak-tight ring type containment with separate connections to vacuum pump and the helium gas cylinder was designed and fabricated to facilitate evacuation and gas filling operations

25 Fabricated Ferro-Boron capsules The period of 45 days in the 4 th ring location of FBTR corresponds to maximum fluence seen by FeBshield assemblies in the planned FBRs IGCAR and MINEX Metallurgicals have carried out the work IGCAR Ferro-Boron Capsule fabricated with secondary containment filled with He FR-13 Paris March Fabricated FeBcapsule in 1:1 scale of that in CFBR as part of technology development

26 Neutron radiography of Pre-Irradiated Capsules N-Radiography was undertaken in KAMINI reactor on three Ferroboron capsules i) to evaluate the homogeneity of Ferroboron packing & ii) evaluate the stack consolidation after irradiation Neutron radiograph of Ferroboron capsule before irradiation

27 Conclusions Neutron attenuation characteristics of Ferroboronhas been investigated both theoretically and experimentally and Fe-B has been found to be favourablefor use in FBRs. Its use will result in significant savings in cost, without impairing shielding capabilities. Extensive and in-depth out of pile characterization thermophysical properties and high temperature metallurgialcompatibility tests with SS 304L clad were carried out. These as well as effects studied of interaction of sodium and Fe-B together on clad at high temparatures show excellent compatibility with clad. Design of two types Ferro-Boron capsules was done for conducting an irradiation test in FBTR simulating 60 years of neutron fluencein FBRs, Fabrication of the capsules was successfully carried out. The capsule was loaded in FBTR and the capsule has been discharged after the intended 45 days of irradiation. Post Irradiation Examination will be carried out to look for any possible effect due to irradiation.

28 Thank you for your attention