qualification of a new insulation material concept for

1 Copyright © 2012 by ASME

QUALIFICATION OF A NEW INSULATION MATERIAL CONCEPT FOR NUCLEAR POWER PLANTS BASED ON AEROGEL

Sören Alt University of Applied Sciences Zittau/Goerlitz

Zittau, Germany

Wolfgang Kästner University of Applied Sciences Zittau/Goerlitz

Zittau, Germany

Josep Badelles RWE Power Aktiengesellschaft

Essen, Germany

Aerogel is an amorphous silica gel based insulation material of CABOT corporation

ABSTRACT During loss of coolant accidents (LOCA) the release of

insulation material attached to pipes and vessels may lead to disturbances of the long-term residual heat removal from the reactor core during sump recirculation operation. Investigations on the behavior of insulation material in LOCA-conditions were particularly performed for fibrous insulation materials in the past.

Analyses and experiments were carried out to demonstrate the appropriateness of using aerogel as insulation material for the use in nuclear power plants, which is differently designed compared with fibrous insulation materials. The investigations included: • thermal conductivity behavior of cassettes filled with

aerogel of various bulk densities (purely and with additives) at a hot pipe compared to mineral wool filled cassettes

• thermography of aerogel • fragmentation behavior of non-calcined and calcined aerogel

during steam jets of 110 bar • complex tests with integral character concerning transport,

accumulation and head loss behavior in the sump and during flow-through a single FA-dummy at the test facility ‘Zittau Flow Tray’ (ZFT) under consideration of the fluid composition (boric acid, floor grating, dust, paint, rust)

• investigations on activating aerogel with a neutron source • tests on the mechanical stability of the cassettes • γ-irradiation testing • determination of the halogen content (esp. chlorides) with

AOX

• durability in boron during increased temperatures • viscosity analyses of dissolved aerogel in boric acid.

Aerogel filled cassettes (bulk density: 100 kg/m³) surrounding on a horizontal pipe at 320 C showed an insulation factor of W = 1.13 and W = 1.25 (for 110 kg/m³) in comparison to standard mineral wool-filled cassettes.

Two variants of non-calcined and calcined aerogel were fragmented at the “Fragmentation facility” with steam pressures of 110 bar. After the fragmentation tests non-calcined aerogel (hydrophobic type) was observed as swimming masses with huge surface-volume-ratio. The steam fragmentation of calcined aerogel (hydrophilic type) showed as result a suspension consisting of deionized water and aerogel.

Complex experiments were carried out concerning the behaviour of steam fragmented suspensions of calcined aerogel during sump operation at ZFT. An 18x18 PWR-fuel assembly dummy was used downstream of a 3x3 mm sump strainer. Input masses of released insulation material, concentration of boron acid, the strainer surface and velocity as well as the flow through the FA-dummy were scaled accordingly based on the German PWR “Biblis B” plant conditions. The steam-fragmented calcined aerogel did not show any deposit on the sump strainer, on the debris filter at the FA-bottom nor on the spacers in a PWR-FA-dummy. Accordingly, at these positions no significant head loss build-up was detected.

AEROGEL – STRUCTURE AND PROPERTIES

Aerogel was discovered by Steven Kistler between 1929 and 1931. Chemical companies tried to commercialize the

Proceedings of the 2012 20th International Conference on Nuclear Engineering collocated with the

ASME 2012 Power Conference ICONE20-POWER2012

July 30 - August 3, 2012, Anaheim, California, USA

ICONE20-POWER2012-54431


aerogel from 1950 to 1990 but failed due to the high manufacturing costs, limited batch production, and safety risks associated with supercritical drying. In the mid 1990´s patents of direct silation routes were developed to control gel shrinkage. Cabot began the commercial production of aerogel in 2003. The aerogel granulate is translucent granulate consisting of 95 % air and 5 % solid matter with pore sizes of 20 nanometers on the basis of silicon dioxide (Figure 1).

Figure 1: Chemical constitution of Aerogel

Figure 2 shows the steps from the production to the application of aerogel in insulation cassettes. The small pores between the silica aerogel structure (grey structures) enclose air molecules (red dots) and disable the interaction of the air molecules. Because of this nano-structures and the restricted molecular interaction the aerogel extremely reduces the thermal heat transport which leads to the excellent insulation values and high insulation efficiency. For the application of pipe insulation the translucent granular Aerogel was filled into stainless steel half cassettes.

Figure 2: Application of aerogel inside insulation cassettes

The following basic aerogel properties are specified by the manufacturer and visualized in Figure 3: • Particle size range: 0,01 to 4 mm • Bulk density: 65-110 kg/m³ • Translucency: up to 80 %/cm • Thermal conductivity (compacted bulk density at 20 C):

0,018 W/(m•K)

• Combustibility and smoke production (calcined): no

Moreover, Cabot provided a material safety data sheet according to EU-guideline 2001/58/EC concerning aerogel. Cabot Corporation specifies equal thermal conductivity for both states of surface chemistry:

• non-calcined (completely hydrophobic) and

• calcined (hydrophilic) granulate.

Figure 3: Properties of aerogel (figure by CABOT) REQUIREMENTS FOR QUALIFIED INSULATION MATERIALS AND OVERVIEW OF INVESTIGATIONS

During LOCA the release of insulation material attached to vessels and pipes into the sump of PWR´s or the wet well of BWR´s may lead to disturbances of the long-term residual heat removal from the reactor core during sump recirculation operation. Hitherto, investigations on the behavior of insulation material under LOCA-conditions were particularly performed for fibrous insulation materials [2]. According to [1] ‘qualified insulation material in terms of protected sump suction is an insulation material which was tested with regards to its moistening and sedimentation behavior as well as to the head loss behavior at sump strainer (including an investigation to of the long-term head loss build-up – e.g. a minimum of 3 days) at test facilities….’. The aims of the analyses and experiments were to proof the appropriateness of using aerogel as insulation material in nuclear power plants instead of the differently designed fibrous insulation cassettes. Exceeding the requirements according to [1] the following investigations concerning the use of aerogel were carried out:

at Zittau/Goerlitz University of Applied Sciences:

• thermal conductivity behavior of cassettes filled with pure aerogel of various bulk densities at hot pipes and


with additives (MgO, Graphit) compared to MD2-mineral wool filled cassettes of same size

• thermography analyses • viscosity analyses of water-aerogel-boric acid solutions • fragmentation behavior of non-calcined and calcined

aerogel impacted by steam jets of 110 bar • complex tests with integral character concerning

transport, accumulation and differential pressure behavior in the sump and during flow-through of a single FA-dummy at test facility ‘Zittau Flow Tray’ under consideration of the fluid composition (boric acid), installation of corrodible materials (hot dip galvanized floor gratings) and injection of other debris (dust, paint, rust)

• investigations on activating aerogel with a neutron source

by the RWE group: • tests on the mechanical stability of the boxes • γ-irradiation testing • determination of the halogen content (esp. chlorides)

with AOX • durability investigations on boron during increased

temperatures. CONFIRMATION OF INSULATION PROPERTIES

Investigations on the insulation effect of variously filled and geometrically equal proportioned insulation material cassettes at a horizontal heating pipe (NPS: 80 mm) were carried out at the pressurizer test facility (PTF) (dimensioning: 16 MPa, 350 C). This heating pipe (Figure 4) was continuously connected to the pressurizer (Figure 5) which worked as steam source. Condensate resulting from the condensation process was drained into a bypass pipe of NPS 25 mm back into the pressurizer. Thermocouples were directly installed at the heating pipe outer surface as well as at the outer surface of the insulation half-cassettes at the 0:00, 3:00, 6:00 and 9:00 o´clock positions in the longitudinal centre of the cassettes (Figure 6). As a comparison cassette an original MD2-cassette (one of the standard used materials for German NPP) was installed at position M (Figure 6). Non-calcined aerogel cassettes of various bulk densities and additives were located at the longitudinal positions A and K (Figure 6). The de-aeration of the pressurizer and heating pipe system and the heating-up procedure to 110 bar of saturated steam in the pressurizer and thus in the heating pipe was performed in a time-period of about 2 h. Subsequently, the long-term test over 50 h was carried out concerning the insulation effect.

Figure 4: Construction of the horizontal heating pipe

Figure 5: Installation of test cassettes on the horizontal

heating pipe at the pressurizer test facility

Figure 6: Arrangement of the test cassettes on the heating

pipe and positions of the thermocouples (TC)

Cassettes at the heating pipe

Conventional insulation of other heating pipe sections


Figure 7 shows the preheating, de-aeration and level adjustment at T > 100 C and the heating-up in the pressurizer on the saturation steam line up to 110 bar (319 C).

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Time / sp_PTF T_amb_1125 T_PTF_water T_PTF_steam T_amb_3000 T_sat = f(p_PTF) l_PTF

Figure 7: Parameters during start up procedure

The heat up of the heating pipe and the cassettes are

exemplary shown in Figure 8. The labeling of the charts in the following Figures corresponds to the specifications shown in Figure 6, e.g.: T_A_6i means (Temperature)_(cassette position A)_(6:00 o´clock)_(inside cassette), T_K_110_0a means (Temperature)_(cassette position K)_(110 kg/m³ bulk density)_(0:00 o´clock)_(a-outside cassette).

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T_A_0i T_A_100_0a T_A_6i T_A_100_Graphit_6a

T_M_0i T_M_0a T_M_6i T_M_6a Figure 8: Temperatures on heat pipe and cassettes

surfaces during start up procedure The charts in Figure 9 show the long-term behavior of the

temperature distribution at the heating pipe surface (resp. the inner side of the cassettes) of the half-cassettes at position M (MD2-insulation). Due to a small leakage through a valve at PTF the level in the facility slowly decreased (see level in Figure 7). The short-term deviations depicted in Figure 9 (temperature instabilities) occurred at that time when the sinking level was balanced by an additional feeding with a colder medium by a high pressure pump. Otherwise, there was performed a stabile operation. The temperature distribution in

Figure 9 can be explained in the following manner. The highest temperature was measured at the upper position (position 0i, Figure 6) in comparison to the lower position (position 6i, Figure 6). At positions 3i and 9i the stainless steel faces of the half cassettes were pushed together by locking devices (Figure 5, Figure 6). The heat losses at these positions were larger and lead to decreased inside temperatures.

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T_M_0i T_M_3i T_M_6i T_M_9i Figure 9: Long-term TC-measurements charts of inner

temperatures of cassette Position M

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Figure 10: Long-term TC-measurements charts of outer

temperatures of cassette Position M and ambiance

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T_A_100_0a T_A_100_graphite_6a T_K_110_0a T_K_100_MgO_6a T_amb_3000 Figure 11: Long-term TC-measurements charts of outside

cassette temperatures at positions A and K (aerogel filled)


Long-term TC-measurements charts are shown in Figure 10 for the outer temperatures of cassette Position M (standard insulation material) and ambiance. Best insulation results were observed at positions 0 and 6. The higher temperatures at positions 3 and 9 could lead back to the same reasons described before.

The insulation values of a mineral wool (MD2) cassette and two aerogel filled cassettes with bulk densities of 110 and 100 kg/m³ (for pure aerogel, and with additives of 3 mass percentages MgO or 3 mass percentages graphite) were calculated considering the temperature values shown in Figure 10 and Figure 11. Moreover, it becomes clearly, that the outer temperatures at the MD2-cassette had higher values than at the aerogel cassettes which is an indication for better heat insulation of the aerogel cassettes. Table 1 contains the average values of temperature measurements and the standard deviations of the single measuring positions. The precision of the TC type K complies with DIN IEC 584-2. The standard deviation values of the temperature measurement average values were also influenced by the cyclic feeding processes in the pressurizer. Table 1: Average values of temperature measurements

and standard deviations during stationary test operation

100 h 0:00 o'clock 3:00 o'clock 6:00 o'clock 9:00 o'clockoperation position position position position material

mean value T_PTF [C] saturated steamstandard deviation [K]mean value T_M_i [C] 318,8 313,6 317,3 311,7 MD2standard deviation [K] 0,29 0,29 0,58 0,29mean value T_M_a [C] 49,8 72,2 43,2 75,0 MD2standard deviation [K] 0,98 0,68 0,65 0,61mean value T_K_i [C] 318,3 318,4 319,9 316,1 above: aerogel 110 kg/m³standard deviation [K] 0,31 0,32 0,63 0,31 below: aerogel 100 kg/m³ + 3m%_MgOmean value T_K_a [C] 45,8 67,1 40,5 65,9 above: aerogel 110 kg/m³standard deviation [K] 1,01 0,99 0,91 1,01 below: aerogel 100 kg/m³ + 3m%_MgOmean value T_A_i [C] 319,2 316,1 319,9 309,4 above: aerogel 100 kg/m³standard deviation [K] 0,32 0,31 0,56 0,30 below: aerogel 100 kg/m³ + 3m%_Cmean value T_A_a [C] 47,5 67,2 34,7 66,3 above: aerogel 100 kg/m³standard deviation [K] 1,20 0,92 0,90 0,94 below: aerogel 100 kg/m³ + 3m%_C

mean value T_amb_3000 [C] ambient airstandard deviation [K] 0,58

319,60,39

29,0

Assuming the heat transfer from cassette outside wall to the

ambient: ( )ambientCSambient TTAQ −⋅⋅= a (1)

with heat flux ambientQ , heat transfer coefficient a , cassette

surface area A, cassette surface temperature CST and ambient

temperature ambientT and taken into account equal cassettes

areas the spezific heat flux q ′′ becomes:

TqA

Qambient ∆⋅=′′= a

(2).

The consideration of equal boundary flow conditions gives equal heat transfer coefficients. However, the calculation of an insulation factor W against the heat transfer at MD2-cassettes results:

WT

Tq

q

cassette

MDcassette

cassette

MDcassette =∆

∆=

′′′′

insulationother

2

insulationother

2

(3).

The following statements could be derived by the measured parameters: • In the undisturbed insulation range, the insulation factor W

for pure aerogel compared with MD2 at bulk densities of 100 kg/m³ was calculated to W = 1,13 and at 110 kg/m³ to W = 1,25.

• Compared to MD2 the insulation factor was W = 1,25 with dosage of 3 mass percentage white magnesium oxide into aerogel with a bulk density of 100 kg/m³.

• Compared to MD2 the insulation factor was W = 2,49 with dosage of 3 mass percentage black graphite into aerogel with a bulk density of 100 kg/m³.

THERMOGRAPHY Weight loss analyses were carried out up-heating aerogel specimens in a TGA 701 thermogravimetric analyzer. The results are shown in Figure 12.

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Figure 12: Results of TGA 701 thermogravimetric analyses

of aerogel Up-heating to 320 C caused a medium weight loss of aerogel of about 2 percent. The main effect occurred at a temperature of about 100 C. However, in this temperature region the air humidity was lost. The weight loss of about 2% up to 320 C should not influence the insulation properties of aerogel. FRAGMENTATION BEHAVIOUR OF AEROGEL DURING LOCA

Both variants of aerogel (non-calcined, calcined) were fragmented at the fragmentation facility of Zittau/Goerlitz University [2] with a vapour pressure of 110 bar.

After the fragmentations, in both, the primary and in the secondary container swimming masses with high surface-volume-relation were observed for non-calcined aerogel (hydrophobic type, Figure13). The fragmentation of calcined aerogel (hydrophilic type) caused a suspension consisting of deionised water and aerogel.


Condensate in the primarycontainer (dry well)

Condensate in the secondarycontainer (wet well)

Non-calcinedaerogel

calcinedaerogel

Figure13: State after fragmentation of non-calcined

(figures above) and calcined aerogel (figures below)

DYNAMIC VISCOSITY ANALYSES OF CALCINED AEROGEL

These analyses were carried out to investigate the influence of suspended calcined aerogel in boric acid on the dynamic viscosity of the fluid. Table 2: Overview about boundary conditions for

viscosity analyses of aerogel

No. concentration boric acid concentration

aerogel [mg/l(deionized water)]

[ppm] [g/l(deionized water)]

1 deionized water 0 0

2 2300 13.25 0 3 2300 13.25 130 4 2300 13.25 260 5 2300 13.25 390 6 2300 13.25 520

A Hoeppler viscosimeter was used to estimate the dynamic viscosities of the specimen shown in Table 2 at 27 C and 50 C. The density of the fluids did not essentially differ between the various concentrations. A mean value of 1.0049 g/cm³ was measured at 20 C for specimen-no. 2 - 6. The behavior of the dynamic viscosities is shown in Figure 14. Increased aerogel concentrations caused an increasing of the dynamic viscosity of the boric acid fluid. Doubling the aerogel concentration (260mg/l up to 520 mg/l) led to an increment of the viscosity of about 10 percent.

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Fluid temperature: 50 C Fluid temperature: 27 C

Figure 14: Dependency of the dynamic viscosity on aerogel concentration and temperature of the fluid in boric acid with 2300 ppm boron

BEHAVIOUR OF AEROGEL IN THE SUMP AND IN THE FUEL ASSEMBLY DUMMY (FA)

At the test facility ‘Zittau Flow Tray’ (ZFT) [3] complex tests with integral character were carried out concerning the behavior of aerogel in sump operation. In test V03 an 18x18 PWR-fuel assembly dummy was used downstream the ZFT. As a sump strainer a 3x3 mm screen of stainless steel was installed. The fluid temperature of 60 C was constant during the experiment. The input mass of aerogel, concentration of boron, strainer size and area, velocity at the strainer and the FA-dummy volume flow were scaled accordingly, based on the facility conditions of Biblis plant B NPP, which are also representative for plant A regarding the investigated effects. The behavior of the main parameters is shown in Figure 15. The fluid was transported through the facility only as a slightly cloudy suspension. There was no debris agglomeration on the sump strainer screen or in the FA (Figure 16). Accordingly, no significant head losses were detected. Sedimental debris at the bottom of ZFT contained sedimented aerogel, particles of paint, dust and rust.

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Figure 15: Long-term behavior of significant parameters

during the complex test V03


Figure 16: Status of the strainer screen (above) and the

screen as FA-debris filter at the FA-bottom

CONCLUSION

Resulting from the investigations concerning the application of aerogel insulation material in NNP the following statements are derived: • Aerogel filled cassettes with bulk densities of

100…110 kg/m³ in geometrically similar insulation cassettes showed equal or higher insulation values in comparison to insulation with MD2-cassettes.

• Using non-calcined aerogel (hydrophobic), steam fragmentation led to a swimming fluid with large surface-volume-relations. However, calcined aerogel appears as a suspension in deionised water.

• In complex tests with integral character steam-fragmented calcined aerogel did not show any deposit on the sump strainer, on the debris filter at the FA-bottom nor on the spacers in a PWR-FA-dummy.

• Accordingly, at these positions no significant head loss build-up was detected considering long-term corrosion processes.

Therefore, aerogel is an appropriate insulation material for nuclear power plants.

ACKNOWLEDGEMENT

The investigations depicted in this paper were supported by research projects of RWE Power AG at Zittau/Goerlitz University.

REFERENCES [1] VGB PowerTech e.V.: „Empfehlung der VGB-AG

„Gesicherte Sumpfansaugung““, VGB-M 999, 2004, Essen

[2] S. Alt, et al.: „Experiments for CFD-modeling of cooling water and insulation debris two-phase flow phenomena during loss of coolant accidents”, CD-proceedings, NURETH-12, No. 22, 2007, Pittsburgh, Pennsylvania, U.S.A.

[3] S. Alt, W. Kästner, A. Kratzsch: „Behaviour of mineral wool in the sump and the reactor core - generic experiments at “Zittau Flow Tray (ZSW)””, CD-proceedings, KTG-Fachtag, 2010, Forschungszentrum Dresden-Rossendorf

FA-bottom after the test

Sump strainer at the end of the test

qualification of a new insulation material concept for

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