Fusion Engineering and Design 88 (2013) 2235– 2239
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Fusion Engineering and Design
journa l h om epa ge: www.elsev ier .com/ locat e/ fusengdes
xperimental and numerical analyses of a hydrogen and deuteriumtorage system
usebiu Ilarian Ionetea,∗, Bogdan Moneaa, Christoph Plusczykb, Roxana Elena Ionetea
National R&D Institute for Cryogenics and Isotopic Technologies, Rm. Valcea, RomaniaInstitute for Technical Physics, Tritium Laboratory, Karlsruhe, Germany
i g h l i g h t s
Experimental data to develop a real-time tritium storage control system.Simulation (3D modeling) of real working conditions for long term tritium storage on titanium.No major differences between the reaction of hydrogen and deuterium on titanium sponge were revealed.
r t i c l e i n f o
rticle history:eceived 13 September 2012eceived in revised form 23 May 2013ccepted 27 May 2013vailable online 2 July 2013
eywords:ritium storage
a b s t r a c t
This paper conducts an experiment and a three-dimensional (3D) modeling study for the absorption ofhydrogen and deuterium on a storage tank with titanium sponge bed in order to simulate the real workingconditions of a tritium storage system prior to tritium service. The 3D model is further numericallyimplemented and experimentally validated. The model is composed of an energy balance, mass balanceand momentum balance and hydriding reaction kinetics. These differential equations are solved usingfinite element method. The experimental consisting in absorption of hydrogen and deuterium gas wasmade in batch made, under vacuum condition. Before absorption, an activation of the titanium bed was
umerical simulationbsorptionitanium
performed. A number of loading and releasing operations typically required in tritium handling loopswere conducted using one bed containing a well determined quantity of titanium sponge. A comparisonbetween theoretical results and experimental data has found that the gas was not uniformly absorbed onthe metal bed volume. This work provides an important platform to understand the phenomena duringtritium absorption on a titanium storage bed and the development of a real-time tritium storage controlsystem.
. Introduction
The long term storage of tritium resulted from a CANDU typeuclear Power Plant operation, such as that one located in Cer-avoda, Romania, is receiving a lot of attention lately due to safetynd public concerns. The presence of tritium in the heavy waterystems of CANDU reactors is a major source of operator exposureo radiation [1]. At National Institute for Cryogenics and Isotopicechnologies – ICIT Rm. Valcea we developed a proper technol-gy for the tritium extraction from heavy water used as a primaryoolant and moderator.
For the storage of the resulted tritium several candidate metalydride materials (uranium, La–Ni–Al alloys, intermetallic com-ounds and titanium) that can store large quantities of tritium
without taking-up a large volume, have been assessed for long termtritium storage [2]. In order to better understand the phenomenonof tritium absorption on titanium storage bed, a series of experi-ments were conducted simultaneously with a three-dimensionalmodeling to simulate the real working conditions of a tritium stor-age system prior to tritium service. Since the chemical propertiesof tritium are virtually identical to those of hydrogen/deuterium,the experiments were performed using hydrogen/deuterium.
2. Mathematical modeling
During the absorption process, hydrogen is injected to a storagevessel and transported via the interstitial space of the packed tita-nium particles, reacting with the unconsolidated metal particles.
To describe the dynamic process taking place in the reactor, theflow and heat transfer theory of porous media have been applied[3–5]. The mathematical model that describes the absorption pro-cess consists of differential equations for the system (energy, mass
As specific surface area (m2/m3)C kinetic constant (s−1)Cp specific heat capacity (J/kg K)E activation energy (J/mol)h heat transfer coefficient (W/m2 K)�H reaction enthalpy (J/mol H2)K permeability (m2)MH2 molar mass of hydrogen (kg/mol)p pressure (bar)r radius (m)Rg universal gas constant (J/mol K)S source term�S reaction entropy (mol−1 K−1)t time (s)T temperature (K)v velocity (m/s)
Subscriptsa adsorptiond desorptione equilibriumg gas phasem mass0 initials solid phasesf between solid and gas phase
Superscripts saturation
a[
cvotmmpl
2
me
ae
nd momentum) and the ones referring the kinetics of adsorption6,7].
The model was applied to a reactor consisting in a cylindricalontainer filled with sponge titanium. The theoretical model wasalidated by comparing the results of the simulation performedn the reactor with the experimental data. The following assump-ions have been made and where taken into account to simplify the
odel: (a) the flow bed can be treated as a homogeneous porousedia with uniform morphological properties, such as porosity and
ermeability; (b) the ideal gas law holds true in the gas phase; (c)ocal solid and fluid are in thermal equilibrium [3].
.1. Model equations
The differential equations governing the hydriding process areass balance of the hydrogen and metal, momentum and energy
quations.
Mass balance: The metal hydride tank is typically characterized
s a porous media packed with titanium sponge (hydride bed). Massquation for metal is:
∂((1 − �)�s)∂t
= Sm (1)
d Design 88 (2013) 2235– 2239
Gaseous hydrogen flows through the bed and reacts with thehydride metal. The mass conservation equation governing the gasphase (hydrogen) is:
∂��g
∂t+ ∇(�gvg) = −Sm (2)
Momentum balance: Momentum equation describing the hydro-gen flow is in the form of the Darcy’s low:
vg = − K
�g∇pg (3)
The kinetic equations that describe the absorption/desorptionprocess are in the form of:
Sm = Ca exp
(− Ea
RgT
)ln
(pg
pe
)(�s
s − �s) (4)
Sm = Cd exp
(− Ed
RgT
)(pg − pe
pe
)(�s − �0) (5)
The Van’t Hoff relationship can be used to calculate the equilibriumpressure pe [3].
Energy balance: Energy equation in the porous bed can beexpressed as:
∂((1 − �)�sCps Ts)
∂t= ∇((1 − �)�s∇Ts) + hsf As(Tg − Ts)
+ Sm
(�H
MH2
+ Cpg Tg
)(6)
The used energy equation for gas phase is:
∂(��gCpg Tg)
∂t+ ∇(�gCpg vgTg) = ∇(��g∇Tg) + hsf As(Ts − Tg)
− SmCpg Tg (7)
2.2. Boundary and initial conditions
Initial conditions: The gas and the solid are initially at the sametemperature, the pressure and hydride density being assumed tobe constant.
Ts = Tg = T0; pg = p0; �s = �0 (8)
Inlet boundaries: The inlet conditions are set according to thespecific temperature, flow rate and pressure. Considering the sym-metry of the reactor, along the z-axis can be imposed the followingconditions:
∂p
∂r(z, 0) = 0;
∂T
∂r(z, 0) = 0 (9)
The boundary conditions on the wall are set as follows [7–10]:
∂p
∂r(z, r) = 0;
∂p
∂z(z, r) = 0 (10)
3. Numerical algorithm
The ANSYS CFD software Fluid Flow (Fluent), using 3D double-precision, serial processing, transient pressure-based solver wasused for all the numerical calculation performed, taking appro-priately specified conditions of the metal hydride reactor under
investigation in order to analyze the hydrogen absorption pro-cess on titanium bed. The numerical simulation model is based onSIMPLE (semi-implicit method for pressure linked equations) algo-rithm. The source terms generated by the chemical reaction are
E.I. Ionete et al. / Fusion Engineering and Design 88 (2013) 2235– 2239 2237
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i
Table 1Physical, geometrical and operating parameters.
Parameter Value
Tank dimension, height/diameter 50/30 mmTitanium sponge bed height 20 mmInitial/inlet temperature, T0/Tin 20/20 ◦CInlet pressure, P 90,000 PaInitial pressure, P0 0.1 PaPorosity of the hydride bed, � 0.4Thermal conductivity of the bed, �g/�s 0.167/2.0 W/m KSpecific heat capacities, Cpg /Cps 14.89/0.419 kJ/kg KViscosity of Ti, � 0.826 × 10−3 kg/msDensity of Ti, �s 4.506 g/cm3
Kinematic viscosity of H , � 1.03 × 10−4 m2/s
ig. 1. Geometry and computational domain of the reactor: (1) porous media and2) free space.
nserted into the energy, mass and momentum equations using UDFUser Defined Function). The coupled set of equations was solvedteratively, with fixed time step of 0.01 s, until the relative errorn each field reached a specific convergent standard, smaller than0−5.
The geometry and computational mesh of the hydrogen storageessel is shown in Fig. 1. Only a part is filled with titanium sponge,s a hydride material. The region filled with titanium sponge has0 mm height and the free space 30 mm. The mass of sponge tita-ium used was well determined by weighting with an accuratecale. The mesh was created using tetrahedrons method containing58,000 nodes and 678,000 cells. The reactor was modeled usingymmetry properties, only a quarter of the vessel being consid-red. To simplify the current process and experiment the expansionechanism of the constituent particles was excluded. On the cylin-
er we had foreseen provisions, in the form of empty space, for thebove mentioned expansion.
Several materials properties, operating parameters and geomet-ical characteristics of the storage tank used in the simulation,ssumed based on real data from our experiments are summarizedn Table 1.
. Results and discussions
Experimental installation [11] was built together with propernstrumentation systems and titanium sponge like a storage
Fig. 2. The experimental data: pressure and temperature evolu
2 g
Sphericity of the particles making the medium, ϕ 0.9Diameter of the particles making the medium, D 4 mm
material and hydrogen and deuterium like stored gas was tested[12]. Due to the fact that at room temperature the non-activatedtitanium (Ti) its unable to absorb hydrogen, several proper acti-vation procedures, consisting of heating the Ti bed to hightemperature (at 510 ◦C, 520 ◦C and 530 ◦C) under high vacuum con-ditions were performed in order to desorbs all the gases trapped onthe Ti surface. Fig. 2 presents the experimentally measured pressureand temperature evolution inside the sponge bed in the physi-cal reactor after a number of batch loading operations, typicallyrequired in hydrogen (tritium) handling loops.
To deeper in the hydrogen/deuterium storage process wefocused our attention on the numerical simulation of pressure andtemperature evolution inside the reactor.
The pressure profile inside of the reactor and reactor spongebed has few characteristics. At the very beginning when a batch isdelivered, the pressure inside the sponge bed is almost uniform and,as the time is running and the hydride process evolve, it decreasesrapidly from the original values [3]. The results obtained for the evo-lution of the pressure distribution within the reactor, reproducedat 1, 5, 10, 30, 50 and 60 s are highlighted in Fig. 3.
It is noted that on bed area there is a fast pressure evolutionwhich denotes the presence of the active material. Also, a rela-tive good corellation with the experimental obtained values can
be observed at almost each stage of the storage process. Fig. 4compares the experimental data with model prediction, highlight-ing that model underpredicts the pressure values due to the fastevolution of absorption process in the first 60 s.
tions after Ti bed activation at 510 ◦C, 520 ◦C and 530 ◦C.
2238 E.I. Ionete et al. / Fusion Engineering and Design 88 (2013) 2235– 2239
Fig. 3. The pressure (kPa(a)) distribution in the reactor after (a) 1 s, (b) 5 s, (c) 10 s, (d) 30 s, (e) 50 s and (f) 60 s.
btaphsAbe
Fig. 4. Validation with experimental data in terms of pressure evolution.
The computed evolution of temperature inside of the Tied during the hydridisation process is ilustrated in Fig. 5. Athe beginning of the process the temperature distribution islmost uniform. As the absorption process evolves, the tem-erature distribution became non-uniform, the Ti bed havingigh temperature because of the reaction. The area above theponge material remain much colder than the adsorption bed.
t 60 s after the batch was delivered the temperature distri-ution from the metal bed on the metallic reactor walls isvident, due to the strong exothermic process. However the
Fig. 5. The temperature (◦C) distribution in th
Fig. 6. The speed (m/s) of gas inside the reactor.
temperature distribution depends on the thermal conductivitiesof the adsorbtion bed as well as reactor boundary condi-
tions.
Fig. 6 displays the gas speed resulted from the simulation andfrom the figure one can see the simulation magnitude. On the bed
e reactor at (a) 1 s, (b) 30 s and (c) 60 s.
ring an
ab
5
itgt
tetnermrd
[version and Management 47 (2006) 3632–3643.
E.I. Ionete et al. / Fusion Enginee
rea the velocity is smaller than the one at upper part of the reactorecause of the reaction that is taking place.
. Conclusions
A three dimensional model of a storage system was developedn order to simulate the real working conditions of a long termritium storage system (and vessel). This system is used in order toain information for the design of a storage container, prior to anyritium service.
The simulation results indicated that the pressure drop betweenhe Ti clusters is not uniform during the absorption. This fact is notasy deductible from the experiment because of the relative spa-ial location of the pressure gauge. The temperature distribution isot uniform on the whole volume of the sponge bed and it remainslevated, in the range of tens to hundreds degrees, for a long time,
equiring a cooling device or proper provisions for an optimal ther-al design of storage vessel. During the experiments we did not
eveal major differences between the reaction of hydrogen andeuterium on titanium sponge.
[
[
d Design 88 (2013) 2235– 2239 2239
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