Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Fo 2314
A method for processing absorber rods
from water-cooled nuclear reactors
The invention relates to a method for processing
absorber rods with respect to the compact and safe final stor-
age of radioactive materials, these rods being shaped as steel
tubes filled with boron compounds.
In water-cooled nuclear reactors, for example boiling
water reactors, the reactivity control is carried out by means
of rods made of steel tubes, for example of the type AISI 304
or 306, which are filled with boron compounds, in particular
B4C (boron carbide).
This method uses the neutron absorption features of
the 10B-iqotope. In interactions wi-th thermal neutrons, this
isotope then creates 7L,i, which its~l reacts with fast neu-
trons and forms tritium 3H.
In order to simplify the descrlption, tritium will be
referenced hereinafter T and tritium water, in which part of
the normal hydrogen atoms are replaced by tritium atoms, is
referenced HT0.
The isotope 18 itself reacts also with fast neutrons,
thus directly creating ~. The following nuclear reactions are
the result:
IB ~ ln (thermal) ~ 7Li + 4He
7Li + ln (fast) -, T ~ 4He ~ n
B ~ ln (fast) ~ T ~ 24He
Typical dimensions of such rods in a boiling water
reactor are:
inner diameter 4,7 mm
thickness of wall . 0,67 mm
total length ca. 2900 mm
material SS AISI 304
filling 47 g of B4C
Due to a vibrating system, this filLing reaches a
7 ~ l~
density of about 70~ of the theoretical density of boron car-
bide. After the filling, the tube is evacuated and sealed. In
a typical boiling water reactor there are 4140 rods, i.e. a
total amount of 194 kg boron carbide.
After three years of operation in a boiling water
reactor, such a rod contains, besides the radioactive activat-
ing products usually resulting from neutron bombing of the
metal, a tritium contents equivalent to of 0,85 Ci.
This tritium is mainly present in the boron carbide
matrix. About 6 per mille of the total tritium quantity is
contained in the envelope material of the rod and an even
smaller part~of about 0,2 per mille is present in the residual
gas volume inside the rod.
Since the crystalline structure of B4C has remained
unchanged in spite of the interactions with the neutrons, the
tritium thus created can be bound inside the structure in
several manners:
The most probable are depositions in the interspaces
of the ikosaedric crystals of the boron carbide and an intrin-
sic strong covalent chemical bonding. This fact confers to the
tritium an extraordinary stability, so that it is only
released by the boron carbide at temperatures above 700C in
an oxygen-free atmosphere. In fact, the presence of oxygen,
even with concentrations of only 50 ppm, influences the
release of tritium, since a partial oxidation of the carbide
occurs, which leads to the superficial formation of a vitreous
boron hydride layer, which finally retards the release.
In practice, these absorber rods cannot be prepared
for final storage like a normal metal radioac-tive waste prod-
uct, for example by mechanical crushing and/or compacting.
Firstly, the boron carbide has become very brittle by the
interactions with the neutrons and thus has become very frag-
ile. Secondly, the container tubes have hair cracks whic
result on the one hand from the interactions of the boron
carbide with the metallic material of the rods and on the
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other hand from the pressure created by the gas contained
inside the rods, or finally also from a deposition of the
boron carbide during the neutron impact and thus from a swell-
in~ inside the rod connected therewith. Furthermore, the waste
removal is particularly complicated by the not negligible
presence of tritium.
Up to now, no appropriate solution has been found for
compacting such absorber rods with respect to a terminal stor-
age, since the risk of a propagation of the boron carbicle and
thus a release of the radioactive products connected therewith
could not be excluded. In fact, the boron carbide is so
britt-le after irradiation and so little water-resistant, that
the decomposition of boron carbide leads to a fine dispersion
of powdery radioactive material.
It is thus the aim of the invention to indicate such a
method which leads to a waste product which can be finally
~tored with a significantly reduced volume. Furthermore, this
method is supposed to reduce the risk of an uncontrolled dis-
semination of the radioactivity contained in the rods, due to
by the fact that mechanical steps like sawing are not necess-
ary.
This problem is solved by the method according to
claim 1. As for features of preferred embodiments of the
invention, which refer in particular to the simultaneous sep-
aration and collection of the total tritium contents, refer-
ence is made to the sub-claims.
The invention will now be described more in detail
with reference to several embodiments of this method.
Example 1
In an electrically heated furnace a melt of iron is
obtained in a melting pot made of sintered alumina. The gas
volume above the melt is continuously swept with an inert gas,
for example argon. The temperature of the melt is brought to
at least 1500C and is maintained at this temperature. Then
the end of an absorption rod to be processed of the type
7 ~ ~
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referred to above is introduced into the melt and is advanced
in accordance with the melting speed. The temperature of the
melt is chosen high enough so that the envelope material of
the rod melts and thus the boron carbide can be exposed to the
influence of the iron of the melt, a low melting eutecticum
being thus formed. The sweeping with argon avoids oxidation of
the boron, because boron oxide would be volatile at the tem-
perature of the melt.
For the separation of tritium, which mounts in elemen-
tary form (HT) as well as in oxide form (HTO) and which is
conveyed away by the sweeping gas, the method can be adopted
which is also in use in the technology of fusion reactors for
totally converting HT into HTO by means of a catalyst at tem-
peratures of some hundred degrees Celsius, followed by a sep-
aration of water molecules at ambient temperature by means of
molecular sieves (or zeolites). This step is known per se, for
example from Proceedings of the International Conference on
Tritium Technology in Fission, Fusion and Isotopic Applica-
tion~, Toronto/Canada, 1 to 6 May 1988, see "Europ. Tritium
Handling Experimental Laboratory" E. Vassallo, J. Bourdon, H.
Dworschak, D. Pugh.
After cooling the melt, a compact monolithic mass is
obtained in the pot which, during examination by means of X-
ray fluorescence analysis, shows the presence of eutectic
ternary iron compounds like Fe3C, ~e2B, FeC, FeB, as well as
the presence of carbides and borides of different
stoichiometry of nickel, chromium and iron. This monolith can
be finally stored without problem.
Example 2
The melt is prepared in a pot of zirconium oxide which
is placed in an electromagnetic induction furnace. The furnace
is made of a quartz tu~e which is surrounded by an induction
winding and connected at its two ends to a sweeping circuit.
The temperature of the melt is 1550C. A few minutes after the
introduction of the rod to be melted, a homogenous mass is
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practically excluded.
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obtained which, after cooling of the furnace, is converted
into a monolithic compact material, the analysis of which
resembles that of example 1.
Example 3
Instead of iron, the same amount of nickel is used for
the original melt. In this case, the result is preponderantly
nickel boride, but also iron and chromium borides respectively
carbides occur.
ExamPle 4
The iron of the original melt is replaced by cobalt.
The result is preponderan-tly cobalt boride as well as the
usual Fe-Ni-borides respectively carbide.
The speed of the cooling ~f the melt after introduc-
tion of the rod has almost no influence on the result, which
was confirmed by a comparison of a fast cooling phase (a few
minutes under increased sweep gas flow) with a natural cooling
phase over more than 12 hours.
Also other pot materials, in particular pots made of
graphite are well adapted, but also pots of other high melting
point metals such as for example Ti, V, Zrj Mo, Hf, Ta, W, Pt,
Pd, which are covered with an inner layer of 5 mm of graphite
or alumina or zircona.
With the described method, also integral absorber
elements can be processed, which consist of a central guiding
rod made of steel SS AISI 304 and double-walled plates sol-
dered thereto in cross-shape. Between these plates are dis-
posed in adjacent longitudinal rows of the rod fifteen of the
above mentioned absorber rods. In order to obtain a better
ratio between the material to be introduced into the ~urnace
and the diameter of the furnace, it can be useful to cut the
plates off the guiding rod before melting them. Due to the
fact that the first rod close to the guiding rod does not
contain in general any boron carbide, with an appropriate
choice of the cutting device, damaging of boron carbide con-
taining rods and thus undesirably releasing this material is
