Maria Teresa
Porfiri and
Luigi Di Pace
246
E
XPERIMENTAL
E
XPERIENCES ON
T
RITIUM
’
S
I
NTERACTION
S
WITH
M
ATERIALS
Experiments conducted to
study tritium
’s
permeation
of stainless steel
at ambient and
high temperatures revealed that HT converts relatively quickly to HTO. Further, the HTO
partial pressure contributes
in a substantially equal way with
the
elemen
tal tritium gas in
driving permeation through the stainless steel. Such permeation appears to be due to the
dissociation
of the water
molecule on the hot stainless steel surface. The uptake process of
tritium from the car
rier gas, causing the permeation effect, involves both surface adsorption
and isotopic exchange
s
with surface
-
bound water [
9
].
In ASDEX [
10
],
the divertor
tiles, consisting of a fine
-
grain graphite
substrate
coated
with a 0.5 mm thick layer of tungsten
, were covered
in a 20
m W/Re
-
multilayer in order to
prevent the formation
of carbides
within the main tungsten layer
,
thus inhibi
t
ing
its
embrittlement. The solution
was not optimum for the different
behaviors
of the two materials
under
a
high heat load,
causing
the transformation
of the sandwich Re
–
W interlayer (diffusion
barrier) into a mixed sigma phase [
11
]. The material used in the reprocessing
system of the
tritiated water
is generally 316L austenitic stainless steel
, which presents good corrosion
resistance
. Nevertheless, several cases of corrosion have been detected in the reprocessing
plants
,
caused by
the decomposition
of polymers
in valves and vacuum
pump oils, induced by
particles. Tritiated water, initially at pH
7, after sticking in
the
presence of air
,
will have a
pH 4. In this environment
, the tritiated water
becomes acidic
and contains hydrogen
peroxide
,
aggressive for corrosion.
To the purpose of studying the effect of temperature
on corrosion
of t
he 316L stainless
steel
in tritiated water
containing chloride and hydrogen
peroxide
at pH
4, experiments have
been carried out by means of corrosion tests, performed by electrochemical analysis methods
and visual
ly
inspect
ing
the surface of
the
stainless steel [
1
].
As a result, the 316L stainless steel
subjected to tritiated wat
er
containing chloride,
hydrogen
peroxide
at acid
pH
and at different temperatures shows a well
-
defined
,
localized
corrosion
region over a wide range of potentials,
ranging
from the corrosion potential to
trans
-
passivity. The temperature
increase facilitates defective oxide formation
, pitting and
crevice corrosion.
T
ECHNICAL
S
OLUTIONS FO
R
T
RITIUM
D
ELIVERY
/P
ROCESS
,
H
ANDLING AND
S
TORAGE
Tritium can be delivered, handled and stored in different forms: as a gas in metal tritide
and in
the
form of tritiated water
. Each form presents some advantages and some counter
-
indications.
The tritium delivery/process, handling and storage
practices have been evolving over
several decades. The objective has been
to
accomplish the
required tritium work while
minimizing and controlling the exposure
of workers
, the public, and the environment
to
tritium [
8
]. Gaseous tritium at ambient pressure is easily handled by most gas
-
handling
systems.
本书版权归Nova Science所有
Tritium in Tokamak Devices
247
Figure 3. Pressure versus time in a container of tritium (reprinted from Reference [
8
])
.
In relation to
its delivery, handling and storage, gaseous tritium at low pressures and
temperatures [
8
] does not penetrate deeply into the container wall. Helium and tritium
embrittlement of the container wall is no significant issue
at low pressures, even after several
years of exposure
. The problem derives from the decay
of tritium generating
3
He
which
causes the pressure increase within the container: in Figure 3, the behavior of tritium and He
partial
pressures
,
as well as the total pressure vs.
time
, is reported.
If tritium is stored at high pressure, the container’s wall
embrittlement due to T and
3
He
increases the probability of leaks. Tritiated water
(T
2
O) may be difficult to store for long
periods, due to its corrosive properties and is therefore not common
ly
used for bulk shipment.
The third storage option, the metal tritide, reduces the overall volume of the stored
tritium, but some of the finely divided metals used are pyrophoric. Among the metal tritides,
Uranium is the most used material for tritium storage beds, together with titanium, palladium
and zirconium.
Each of them has some advantages and some disadvantages:
-
The uranium
metal bed has the advantage
of being reu
sed
many times, but
has
the
following
disa
dvantages
: the uranium forms a less stable tritide, which can be
released in a controll
ed manner by heating it to temperatures around 400°C
when
it
is
pyrophoric
; the generation of significant tritium pressure requires a high temperature
,
which
results in tritium permeation
through the vessel wall;
and
its capacity is
permanently reduced by exposure
to active impur
e gases
.
-
The use of the palladium
bed reduces the time and the temperature
to which the bed
has to be heated. This reduces any diffusion
losses which might occur through the
stainless steel
body of the getter bed.
On
the other side,
tritium has a high partial
pressure (6667Pa)
over the palladium powder at room temperature
.
-
The titani
um
metal is one of the preferred material
s for
the
long
-term storage
of
tritium gas. When tritium comes into contact with titanium, it adsorbs onto the metal
and reacts to form a solid compound, the titani
um tritide. In this form, tritium is a
stable solid. When tritium is required in its gaseous form, it can be released by
0. 0
0. 5
1. 0
1. 5
2. 0
0. 000
12. 323
24 .646
36. 969
49. 292
61 .615
73. 938
Time in Yea r
s
Time Period Shown = 6 Half-Lifes
Pres sure A ny U nit s
T2 Parti al P ressure
He3 Partial Pressure
T2 Parti al P ressure + He3 P ar ti al
P re ssu re
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