Performanceindices of hot liquid sodium exposed sacrificial surface layers in fast breederreactorsK. Mohammed Haneefa1*, ManuSanthanam1, F.
C. Parida21 Department of CivilEngineering, IIT Madras, Chennai, India {mhkolakkadan, manusanthanam}@gmail.com2 Radialogical Safety& Environment GroupIndira Gandhi Centre for Atomic Research,Kalpakkam, [email protected].
We Will Write a Custom Essay about Performance rankings were influenced by composition of
For You For Only $13.90/page!
order now
inAbstract. Leakage accidentsin sodium cooled fast breeder reactors trigger various thermo-chemicaldegradations of structural materials used for their construction. Theinteractions of hot liquid sodium with concrete at around 550 °C and above areinvestigated in this paper.
Potential materials were designed and tested.Degradation mechanisms and extend of damages were identified; and subsequentlyperformance indices were developed. Four different types of cements, eightdifferent w/c ratios and geopolymer composites were investigated in thisstudy. Comprehensive mechanical, physical,chemical and microstructural characterizations were performed before and afterexposure.
Micro-analytical tools such as SEM (SE and BSE), TG/DTA, XRF, XRD andthin section petrography were used for characterizing the degradationbehaviour. Study revealed that the performance rankings were influenced bycomposition of concrete and water to cement ratios employed for conventionalcement-based systems. Performance indices for geopolymer composites weresuperior to the conventional cement-based systems in hot liquid sodium hostileenvironment. Keywords: Fast breederreactors, hot liquid sodium interactions, sacrificial layer, limestone mortars,geopolymers 1 IntroductionLiquidsodium is used as a coolant in Fast Breeder Reactors (FBRs).
Sodium leakageaccidents in FBRs results in formation of a pool of hot liquid sodium or sodiumspray on Structural Concrete (SC). These interactions (at around 550°C andabove) elicit various thermo-chemical degradations of concrete structuresemployed in inert atmosphere (equipment cells or reactor cavity) or in air (containmentand steam generator building). To prevent the SC from deterioration in FBRs, asacrificial surface layer is employed over it.
Even though various researchershave studied the hot liquid sodium and concrete interactions, there are stilluncertainties persist in quantifying the fundamental degradation mechanisms 1-9.The present study summarizes the investigations of Haneefa et al. 10-20 andreports performance indices for 24 different mixes for hot liquid sodiumexposed sacrificial surface layers in FBRs. 2Experimental DesignsFourtypes of cements were used in the study namely Ordinary Portland Cement (OPC),Portland Pozzolana Cement (PPC), Portland Slag Cement (PSC) and High AluminaCement (HAC). Studies were conducted with a range of water to cement ratios(w/c) 0.4 to 0.
6 for OPC. At a w/c of 0.55, performances of different types ofcements were studied. River sand and limestone aggregates were used in thestudy to evaluate their suitability for hot sodium hostile environment.Different geopolymer composites with varying molarities of NaOH (8M, 12M, 16Mand 18M), fixed solid sodium silicate to NaOH ratio (s/n) of 1.5 and varyingactivator fly ash ratios (0.
45 and 0.50) were tested to check their suitabilityfor sacrificial surface layer. Class F fly ash was used for making geopolymers.The experimental program was divided in to three phases. In the first phase,the constituent materials used were tested for physical properties. Apart fromthat, mineralogical and microstructural characterizations were performed usingmicro-analytical techniques. In the second phase, all the constituent materialsand mixes were tested for thermal performance. The specimens were exposed to 550 °C (ideal operation conditions of FBRs)for duration of 30 minutes.
The duration was a deemed one based on a maximumtime required to flow away the accidently spilled hot liquid sodium and reachthe collection pits in FBRs 13-15. Thermal performance test was conducted in a maffle furnace with anaverage rise in temperature of 0.60 °C/s (Fig.1). Hot liquid sodiumexposure studies were conducted at Indira Gandhi Centre for Atomic Research(IGCAR), Kapakkam, India. Specially designed carbon steel vessels equipped withdismountable thermal insulations of aluminium cladding (Fig.
2) were used with1200 W electric surface heater. The temperatures were monitored and controlledby the help of long and flexible thermocouples. After the required sodium fireexposures, the samples hung from the top were removed and allowed to cool inambient temperatures (Fig. 3 and Fig.
4). Post-test specimens were stored in anelectronic desiccator after cleaning with ethyl alcohol and drying. Compressive strength, flexural strength as per ASTM C348 21, mass loss behaviour and abrasion resistance conforming to IS1237-1980 22 were assessed understand the thermal effects. Scanning Electron Microscopy (SEM) with backscattered imaging on polished specimens and secondary electron images onfracture surface, thin section petrography on 30µm slides using plane andcrossed polarized light, X-Ray Fluorescence (XRF), X-Ray Diffraction andthermogravimetric differential thermal analysis (TG/DTA) were used to perform acomprehensive forensic analysis of different mixes after exposure to 550 °Cwith and without sodium.
3Results and Discussions3.1Thermal performance Indices Table1 provides the mixes and their compositions used in the study. Performanceindices were developed based on performances in compressive strength, flexuralstrength, mass loss and abrasion resistance after exposure to 550°C for 30minutes 10-20.
Mixes are then rankedon a scale 1 to 8, for 1 being the best and 8 being the worst for limestone andriver sand mortars. For the geopolymer mixes the ranking was from 1 to 4 forpastes and mortars separately. For both the types of aggregate, the cementmortars with lower w/c ratios performed well in thermal in ranking. Similarly,among the different types of cement, the performance of Portland pozzolanacement was better in most of the cases. However, the relative performance ofriver sand mortar was inferior to limestone mortars. The reductions incompressive strengths after thermal exposure were 8.
4, 9.1, 11.5, 16.1, 17.
4,12.9, 15.9 and 13.2 % for limestone mortars LS1 to LS8. The correspondingvalues for river sand mortars were 12.3, 15.3, 18.
1, 23.4, 22.4, 19.0, 19.1 and17.
6% respectively for the mixes RS1 to RS8. Similar trends were observed forflexural strength and mass loss. However, abrasion resistance of river sandmortars was better compared to limestone. This effect was resulted from themineralogy of river sand and limestone (calcite has a Mohs hardness scalenumber of 3, whereas quartz is 7).
Geopolymerpastes exhibited increase in compressive strengths upon heating at 550°C for 30minutes. Hence, for the geopolymerpastes strength indices were based on absolute values. Among the differentpastes, the mix GM3 with 16 M ranked first. Increment in strength ofgeopolymers may be due to the attainment of high level polymerization upon heattreatment.
Similar trends were observed for other indices, except for abrasion.Further, geopolymer mortars with limestone aggregate were tested. The mixeswith lower molarities ranked higher indicating mixes becoming more brittle withhigh molarities of NaOH. Moreover, the mix 18 M was severely cracked and brokenupon thermal exposure.
3.2 Indicesbased on sodium fire performance Basedon performance indices from thermal study, limestone mortars with differentcement types and w/c, geopolymer pastes and geopolymer mortars were consideredfor sodium fire tests. For cement based mortars calcium depletion and sodiumenrichment indices were developed based on XRF data. Apart from these indices,aluminium depletion index was developed for geopolymer composites. Theseindices were calculated as (Reduction/increment in elemental composition aftersodium fire) i / (Average elemental composition before exposure).
Calcium depletion index and sodium enrichment index for limestone aggregate(LS) were found to be 0.436 and 802.5 respectively. Figure 5 depicts calciumdepletion indices for cement based limestone and limestone mortars after sodiumfire. The indices after sodium fire corroborate the observations from thethermal performance indices.
As the w/c ratio increases, the calcium depletionindices showed an increasing trend. Similar trends were observed in sodiumenrichment indices. The mixes with higher w/c might have resulted in morerelease of free water and subsequent formation of NaOH and hydrogen gas. Theseconsequences intensify the reaction kinetics during the sodium fire associatedwith more calcium depletion and sodium enrichment by possible cationicexchanges. Since the present studysimulates the worst case of sodium accidents, the maximum w/c ratio for FBRssacrificial surface layer should be equal or less than 0.4 to extend thereinstatement period of sacrificial surface layers.
Table1Mixes used in the study and performance indices based on 30 minutes exposure to550 °C 10-20 Figure7 represents calcium depletion, sodium enrichment and aluminium depletionindices for geopolymer composites. Unlike the cement based systems, geopolymersystem contains more sodium content. As illustrated in the Figure 7, geopolymerpastes were less affected by sodium fire. However, the mortars exhibitedsignificant changes upon sodium fire compared to geopolymer pastes. Thedeveloped indices pronounced that the performance of higher molaritygeopolymers were deficient to the ones with lower concentrations. The elementaldepletion indices after sodium fire of geopolymers revealed that the reactionkinetics of hot liquid sodium with geopolymer composites were less andeventually the degradation was minimal compared to conventional cement based systems.
Additionally, From the XRD study; Na4SiO4, Na3SiO3,Na2CaSiO4, NaOH, Na2CO3 NaAlO2,Ca(OH)2, Na (Hexagonal) and Na (Cubic) were found as reactionproducts. Fig.7 Major elemental depletion indices for geopolymer compositesTable 2provides glimpses of TG/DTA analysis (a typical pattern is presented in Fig.
8)for all sodium fired specimens along with shape retaining indices ({mass ofunaffected inner core after sodium fire i / mass of the specimenbefore any exposure} ×100)) and residual strength indices (absolute values ofresidual strength after sodium fire in MPa). Among the cement-based systems,only the mixes with 0.40 and 0.45 w/c ratios exhibited enthalpy changescorresponding to the decomposition of calcite (CaCO3); whichindirectly portrays that most of the calcite was decomposed in the mortars withhigher w/c upon sodium fire. Theseresults resembled that the degradation of mortars was worst at higher watercement ratios due to transport of hot liquid sodium into the inner core.Similarly, the formation of Ca (OH)2 was high at higher w/c. Corroboratingtrends were observed in shape retaining indices calculated based on absolutevalues of mass retained after sodium fire. There were no inner cores present inthe mixes with 0.
50, 0.55 and 0.60 w/c. Due to high level of disintegration residualstrength calculation was not possible for cement based system. The mix with flyash blended PPC showed a sign of minor DTA peak 751.
7°C related tocalcite decomposition which implies presents of unaffected limestone in themortar after sodium fire. Table2 Indices based on hot liquid sodium fire Mix Enthalpy Change of Calcite (CaCO3) decomposition Enthalpy Change of Ca(OH)2 decomposition Shape Retaining Index Residual Strength Index LS1 51.75 J/g 23.90 J/g 91.5 Disintegrated LS2 12.75 J/g 39.44 J/g 43.
2 Disintegrated LS3 No complex peak 34.33J/g No inner core Disintegrated LS4 No complex peak 42.36J/g No inner core Disintegrated LS5 No complex peak 55.93J/g No inner core Disintegrated LS6 Minor peak at 751.7°C 35.
55J/g 83.7 Disintegrated LS7 No complex peak 45.91J/g 49.7 Disintegrated LS8 No complex peak 5.
88J/g 41.4 Disintegrated GP1 Not performed Not performed 92.3 10.7 GP2 Not performed Not performed 91.8 17.7 GP3 Not performed Not performed 90.
7 18.8 GP4 Not performed Not performed 89.1 23.5 GM1 81.
53J/g No complex peak 57.4 22.3 GM2 98.76 J/g Minor peak at 451.3°C 61.0 23.8 GM3 Not performed Not performed 46.6 Disintegrated GM4 Not performed Not performed Cracked Disintegrated Geopolymersdisplayed superior indices based on CaCO3 and Ca(OH)2enthalpy changes, shape retaining indices and residual strength indices.
Amongthe TG/DTA performed (the geopolymer mortars with 8M and 12 M NaOH); both themixes exhibited distinct complex peaks of 81.53 J/g and 98.76 J/g respectively.
The result proved that the hot liquid sodium had not intruded into the mortarsand decomposes the limestone aggregates. Moreover, there were no any sign ofenthalpy changes in TG/DTA corresponding to Ca(OH)2 decomposition.Corroborating inferences were drawn from the shape retaining indices andresidual strength indices of geopolymer composites. The minimum residual masswas 89.
1 % for paste phase and 46.6% for mortars. The mix 18M was completelydisintegrated. Meanwhile the maximum changes in strengths were 23.5 % and 22.
3%for paste mortars respectively. Due to the high level of damage, residualstrengths were not able assess for the mortars mixes with 16M and 18M of NaOH. 3.3General discussions on influence of microstructure changes on performanceindices Thissection describes how the microstructural alterations influence the performanceindices. Fig. 9 represents a thin section image of fire damaged river sandmortar.
Indian river sands are generally composed of weathered granite. Thehypidiomorphic and interlocked texture of granite microstructure was crackeddue to differential thermal expansion and contraction. Ferric oxidation and subsequentcleavage staining disrupts the mineral assemblage in river sand as seen the inthe Fig. 9. These mechanisms result in more reduction in compressive strengthof river sand mortars compared to the limestone aggregate mortars. Limestone isa mono-mineral rock with very less accessory impurity minerals in it. Itsthermal stability at 550°C is intact.
Figure 10 to 13 provide SEM- back scatterelectron images of polished sodium fired specimens. The geopolymermicrostructure (Figure 10) was less affected upon sodium fire compared to theconventional cement-based systems (Fig. 11). Moreover, the formation of cracksin paste phases was more intensive in cement-based systems (Fig. 12) comparedto the geopolymers (Fig. 13). These observations were directly reflected onperformance indices of sodium fired specimens.
4. Conclusions The current study considered 24 differentmixes for sacrificial surface layer to protect structural concrete from hotliquid sodium fire in FBRs and performance indices were developed baseddegradation behaviour. The study recommends the following;a) Use of limestone over river sand orgranite for the sacrificial surface layer b) Use of low w/c ratio concretes forsacrificial surface layer preferably less than or equal to 0.4c) Effective deployment of geopolymertechnology for sacrificial layers in FBRs AcknowledgementPartial financial support from Indira Gandhi Centrefor Atomic Research, Kalpakkam, India, for the project is gratefullyacknowledged.Reference1Chasanov, M.
G., Staahl, G.E.
: High temperature sodium–concrete interactions. J.Nucl. Mater. 66, 217–220 (1977).2Barker, M.G.
, Gadd, P.G.: A chemical study of the sodium–concrete reactions.In: Proceedings of the LMFBR Safety Topical Meeting, Lyon-Ecully, France,PartIII, p. 91 (1982).3Fritzke, H.W.
, Schultheiss, G.F: An experimental study on sodium concreteinteraction and mitigating protective layers. In: 7th International Conferenceon Structural Mechanics in Reactor Technology, 135–142 (1983).4Muhlestien, L.D., Postma, A.
K.: Application of sodium–concrete reaction data onbreeder reactor safety analysis. Nucl. Safety. 25 (2), 212–22 (1984).5 Chawla, T.
C., Pederesen, D.R.: A review of modelingconcepts for sodium–concrete reactions and a model for liquid sodium transportto the un reacted concrete surface. Nucl. Eng. Des. 88, 85–91 (1985).
6Schultheiss, G.F., Minden, C.
V., Fritzke, H.W.: Method for avoiding or reducingthe interactions and their consequences from contact of hot liquid metallicsodium with concrete, United States Patent No. 4642300 (1987).7 Bae,J.H.
, Shin, M.S., Min, B.H., Kim, S.M.: Experimental study on sodium–concretereactions. J.
Korean Nucl. Soc. 30, 568–580 (1998). 8Premila, M., Sivasubramanian, K., Amarendra, G., Sundar, C.S: Thermo chemicaldegradation of limestone aggregate concrete on exposure to sodium fire.
J.Nucl. Mater.
375, 263–269 (2008).9 Das,S.K., Sharma, A.K., Parida, F.C., Kashinathan, N.
, 2009. Experimental study onthermo-chemical phenomena during interaction of limestone concrete with liquidsodium under inert atmosphere. Constr. Build. Mater.
23, 3375–338110 Haneefa, K.M., Santhanam, M., andParida, F.C.: Performance evaluation of sodium resistant mortars as sacrificiallayer in fast breeder reactors,9th fib International PhD Symposium in CivilEngineering, Karlsruhe Institute of Technology (KIT)-University, Karlsruhe,Germany, July 22-24, 2012, 715-721(2012). 11 Haneefa, K.
M., Santhanam, M., andParida, F.
C.: Performance evaluation of limestone mortars for elevatedtemperature application in nuclear industry, 3rd International conference onrepair, rehabilitation and retrofitting, ICCRRR-2012, University of Cape Town,South Africa, September 12-15, 2012, 111-116 (2012). 12Haneefa, K.M., Santhanam, M., and Parida, F.
C.: Review of Concrete performance at ElevatedTemperature and Hot Sodium Exposure Applications in Nuclear Industry, NuclearEngineering and Design, 258, 76-88(2013). 13Haneefa, K.M., Santhanam, M., and Parida, F.
C.: Thermal performance oflimestone mortars for use in sodium cooled fast breeder reactors, IndianConcrete Journal, 87(12), 25-41(2013). 14Haneefa, K.M., Santhanam, M.
, Ramaswamy, R., and Parida, F.C.: Hot sodiumtriggered thermo-chemical degradation of concrete aggregates in thesodium-resistant sacrificial layers of fast breeder reactors, NuclearEngineering and Design, ,265, 654-667 (2013). 15Haneefa, K.M., Santhanam, M.
, and Parida, F.C.: Performance characterization ofgeopolymer composites for hot sodium exposed sacrificial layer in fast breederreactors, Nuclear Engineering and Design, 2013, 265, pp.
542-553 (2013).16 Haneefa, K.M., Santhanam, M., and Parida, F.C.: Deteriorationof limestone aggregate mortars by liquid sodium in fast breeder reactorenvironment, Nuclear Engineering and Design, 2014, 275, 287-299 (2014).
17 Haneefa, K.M., Santhanam, M., and Parida, F.
C.:Studies on Hot Liquid Sodium and Concrete Interactions in Fast BreederReactors, 2nd International Congress on durability of Concrete, NorwegianConcrete Association, December 4-6, 2014b, Paper 29 (2014).18 Haneefa, K.M., Santhanam, M., andParida, F.
C.: Performance characterization of hot sodium exposed sacrificialconcrete layer for fast breeder reactors, Proceedings of the InternationalConference on Advances in Civil Engineering and Chemistry of InnovativeMaterials, ACECIM’14, Department of Civil Engineering and Department ofChemistry, SRM University, Chennai, India, March 13-14, 2014, pp. 842-847 (2014).19 Haneefa, K.M., Santhanam, M.
, andParida, F.C.: A study on potential materials for hot sodium exposed sacrificiallayer in Fast Breeder Reactors, Proceedings of the National Conference onAdvances in Civil Engineering, ACE2K15, 23rd March, 2015, SSNCollege of Engineering, Kalavakkam, Tamilnadu, India,173-181 (2015). 20 Haneefa, K.M., Santhanam, M.
, andParida, F.C.: Development of a forensic methodology for investigation ofconcrete structures affected by sodium fires in fast breeder reactors, National Conference on ForensicStructural Engineering, Vellore Institute of Technology, Chennai, Volume:Session 1, Paper No.1, 1-18 (2016). 21ASTM C 348: Standard test method for flexural strength of hydraulic-cementmortars, ASTM International, U.S.A.
(2014)22 IS 1237:Cement Concrete Flooring Tiles – Specification, Bureau of Indian Standards, NewDelhi, (1980)