Note: Descriptions are shown in the official language in which they were submitted.
~ ~ ~37~
l~liS invention .relates to a.pr.ocess ~or the
simultaneous extraction of ~rotium and tritium from a liquid
heavy w~er stx~am. -
Nuclear power reactors of the type using
heavy w~lt.er ~D2O) as coolant and moderator incur a progressive
build-up of tritiated heavy water (DTO) in the D~O~ and this
can lead to problems of controiling radiation exposure at :
the nuclear power stations. This D2O impurity is produced
con-tinuously in the reactor as the D2O is subjected to neutron ~.
10 irradiatioll. In present Canadian nuclear ~enerating stations, .
the a~Jerage tritium levels are~the order o-f 1 curie per kg
f ~2 in the primary heat transport systems and :over 10 curies
per ky of D2O in the moderator systems and these .le~els are
~ rising. Thus the tritium, while present in.comparatively
minute qu~nti~ies, because o~ its radioactlvity should
desireably be extrac.ted from reactor systems to maintain ~ :
concentrations at current levels or lower. ~ .
TrLtium oxide (or "tritiated water") can be
concentrated by various proces~ses such as vacuum distillation
or elec~rolytic cascase (several stages of water electrolysis)~
However, these proc~sses are o-f limited use.fulness because
of high to~îcity Qf triti~m in the oxide form, the low
separation factor for water.distillation, a~d ~he high power
consumption ~or the electrolysers. A more practical method ~.
i5 to either convert the tritiated heavy water to the ele.~ental
form, for examp7e, by water electrolysis or to.extrAct tritium
from wat~r by ca~alytlc exchange with a deuterium stre m. ~he
much less ~oxic elemental form can then be enriched by known .
processes such as distillation at cryogenic temperatures~
.
. A process for removin~ protium and txitium
from heavy water by vapour-phase catalytic exchan~e is described
in United Sta~es Patent No. 3,505~0~7 issued to E. Roth on
1-
April 7, 1980~ ~lthough the process described and ~lai~ed
in this patent inclu~es the s-teps of tapping the hea~y ~ -
water containecl in a nuclear reactor and subjecting said
tapped h~avy water to an isotope exchange reaction with ga~eous
deuteri~tm~ it is obvious from the'disclosure tha~ the "tapped
hea~y water'l is heavy water vapour. Because the exch~n~e is
between water vapour and gas, -the two streams flow to the
column concurrently and the process mus-t operate at elevated
temperatures ~80 to-400C using catalystsj~ This process
involves the use of man~ stages and evaporators and condensers
at each equilibrium exchange step and this is most disad~nta-
.
~eous, both in energy consumption and the ~omplexity of theprocess.
A process for hydrogen isotope concentration
between liquid water ana hydrogen gas is described in United
States Patent No. 3,981,g76 issued September 21, 1976 to W.H~
Stevens and assigned to Atomic Energy o Canada Limi ed. This
patent points out ~hat the process may'be used to reduce the
tritium concentration, present as DTO, in heavy water that
~0 has~been used in an operating nuclear reactor. This i~
achieved by increasing the concentration of tritium in liquid
water by dona-tion ~rom gaseous d uterium dexived'~rom th~
liquid water. The deuterium is produced from heavy watex in
a deuterium gas generatorO
The following are patents on the removal
o~ extraction of tritium from heavy and light water~
4,173,620 M. Shimizu issued Nov. 6, 1979
4,190,~15 Butler et al issued Feb. 26,1980
4,191,626 Hammerli et al issued Mar~ 4, 1980`
' Thes~ three patents are concerned ~ith
systems employing a catalytic exchange column and an electrolysis
'~ cell. In ~hese systems all of ~he liquid water passing through
2-
~37Q~S
th~ cataly-tic exch~nge column is pa,ssed to the,electrolytic
cell for conversion to hydrogen and oxyyen gases. Therefore
-the electrolysis stage'n&eds to be large consuming a large
amount of electrical eneryy.
In Canadian Patent application nurnber 320,154
filed January 22, 197g, applicant disclosed a system ~or the
extraction of tritium from a liquid heavy water stream~ The
present invention is an extension of that invention that '~
provldes a wider range'of application~
' lQIt is an obj`ect of the present invention to
pro~ide a process for simultaneous extraction of protium and
tritium from liquid heavy water that operates at or near am~ient
temperatures and pressures.
It is another obj'ect of the invention to
pxQvide a process fox removal of protium and tritium from ~ ,,
llquid heavy water extracts, rather than concentrates, protium
and tritium from hea~y water.
These and other objects of the invention are ,~ ~ '
achieved by a process for the extraction of protium and
20 tr~tium from a liquid heavy water stream comprising contacting '
the liquid heavy water with a countercurrent gasebus deuterium
strea~ in a column packed with'a catalyst such that tritium "'~
is transferred'~by isotopic exchange ~rom the liquid heavy
water s-tream to the gaseous deuterium stream, passiny the gas
enriched in tritium from the column throuyh means for removing
~' txitium therefrom and returning the gas lean in tritium to
' the'column, taking a portion of the liquid heavy water
;, stream after it leaves the column and passing it throu~h an
electrolysis stage to produce'a yaseous deuterlum output which
3Q is retu~ned to the'column and a gaseous oxyyen output th,at
is sent to waste'or to other uses, obtaining the remaining ~,
fraction of the l~quid hea~y water output from the column,
-3-
~.,
:. .
~.~37~5
said hea-~ water being reduced in tritium cont~nt~ ,
In drawings ~hiGh illustrate an embodi~ent
of th~- inve~tion,
Figure l is a flow diagram of th0 process in
its si~ lest form, and
Figure 2 is a ~lowsheet of a complete
system incorporated in a heavy water moderated~and cooled
nuclear reactor.
Referring to Figure 1, a liquid heavy ~ater
feed is passea throuyh a purification stage lO. Depending
on the quality of the feedwater, the feedwater purification
~tage will include a-filtering system to~remove suspended
soli~s, an i~n-exchange~system to remove ionic compounds and
a standard degassing system to remove dissolved gases,'such
as ~2 and N2. If the fee~ater is contaminated w1th oil
or other organic materials, it will be purified by charcoal ,'
adsorp-tion or chemical metho~s. Normally, the heavy water ,
~-ithdrawn from reactor systems is relatively clean and wil~
be p~ssed onIy through a ~iltering and an ion~exchange system.
~Q The tritium to deuterium (T/D) atom ratio in the heavy water
stream is in the arde~ of part~ ~er million (typically O.l -
~ r J ~ C ~
, 1 10 ppm) a~d the ~YihffYjrh~,tr~tiu~n-deuterium species, are '
DTO and D2O. After purification5 the liquid ~tream is fed
to the top o~ a ~atalytic isotope exchange co~umn 11 in which
the tritium is extracted ~rom the liquid stream by contacting
it with a counter-~flowing gaseous stream of DT - D2 in the
co'umn pack~d with a water~repellent,catalyst~ The process ~,
is opera~ive with~any type of catal~st that i~ ~at~r-repell~r.t
but the preferred type is that~described in, United States
-30 Patent No. 3,888,974 issuPd June lQ, 1975 to WrH~ Stevens and
assigned to Atomic Energ~ of Canada Limited. This catalyst `'
consists of at least one'cataiytically active' metal 'selected
31 13~6~2~
from Group VIII of the Periodic Table having ~ substantially
liquid-~z~er-repellent organic resin or polymer c~ating
thereon which is permeable to.water vapour and hydrogen gas~
This ti-pe of catalyst is al~o described in the aforementioned ~.
Paten~ No. 3,9Bl,976 and United States Paten No. 4,025,560
issued r~1ay 24, 1977 to JOhn H. Rclston et aI and assigned to
Atomlc ~nergy of Canada ~imited. After passin~ through the ~:
column a major proportion o~ the detritiated liquid heavy
water is returned to the nuclear reactor or ot~er source~
The deutsrium gas entering the bottom of the
column 11 is lean in tritium ~DT component) and after leaving
the colum~ is enriched in tritium lDT~. This gas is purified
i~ gas purification stage 12 and sent to a cryogenic distilla~
tion stage 13 that lowers the concentration of the DT-T2 in
the gas after which i.t is returned to the bottom of the column ~`
11. rrhe feedyas purificatio~ system or the cryogenic unit
is designed to remove traces of impur~iti~s which condense
and solidi~y as the temperature o~the ~ _ ea~ drops ~ ~:
~mois~ure CO2~ N2~ 2~ CO). Typically, the ~eed~as purifica- ~
.
~ion Lrain includes molecular sieve driers, regenerative
heat ~xchangers and cryogenic silica gel or cl~arcoal absorbers.
Distillation staye 13 gives as output a conce.ntrated DT-T~
gas stream which would normally be ~ithdrawn into suita~le
container~. The cryogenic D~ distillation stage 13 may be
replaced with other isotopic separation processes such as :
the~mal dif~usion or gas chromotography.
A proportion of the upgraaea heavy water
: from catalytic exchange column 11 is passed through
~ electrolysis stage 14 and deuterium gas is returned to ~he
.~ 30 ~listillation column recycle loop either to the gas stream
- ent~ring the exchange column or alter~ati~ely to the stream
leavin~ ~he colur.~ as shown by broken line 14a.
-5-
37~
The process tr~ns~ers protium and tritium
rrom heavy water ~o the cryogenic unit by either contacting
feedwater with a carrier D2-gas over catalysts:
~iDO ~ D~ ~ D20 ~ HD . . ;
Catalyst Exchange
DT0 ~ D2 ~ D20 -~ DT
D20~1DO-DTO ~ D2-HD-DT(02) Blectxolysis
The process, in this case, performs
a three-component transPer (D~0-~ID0-DT0 to ~2-HD-DT) as ;
sho~n above. :
Por the extraction of tritium two gas~liquid :
exchange reactions occur simultaneously:
(Q) + DT(g) ~ D2(g) + DT(Q~ ~....... (1) K~ 62 (25G~ -
0(~j ~ HT~g) p ~Iq(g) + ~ITO(Q) ...... ~2)- K2 = 6~8 (25C)
~; These are in equilibriu~i with gas-phase reactions:
2 H2 ~ (3) K3 =~: 3.26
D2 ~ HT ~ HD ~:DT ~..(4) K~ - 2.20
H2 + DT ~ HD + HT .~.(5) K5 - 1.48
and liquid-phase reactions:
D20 ~ H20 ~ 2HD0 ....... ~t6) K6 = 3.8 (25 C)
D20 + HT0 ~ HD0 -~ DT0~.(7) K7
E20 ~ I)TO ~ HDO ~ HTO~.~ (8) K8 ~:
. -
~ : K6 ~: K7K~ . :
where ICl, R2 t ~Cn are the appropriate equilibrium constants.
The overall separation factor a = ~ , :~
where x - ~H ~ D + T) ~ Y rJ may be ca~culated
for the above reaction equilibirium but is difficult because
o~ ~he lar~e number o~ simultan~ou~ reactions taking place.
Tn protium extra~tion the equilibrium reactions are~
~: H~~Q) ~ D2(g) ~ D2o~Q~ ~ HD(g) Kl = 3~3 at 25C :
D20 ~ H20 ~ 2HDO K2 -- 3- 8~6
D2 ~ H7 . ~a 2HD . ~ K3 - 3. 26
~ = y (1 Yx~)- wgere x = D~H in liquid phase, and
:: ~6 ~ :
~ gL37~2~
Y = D~DH in ~as phase.
The reaction howe~er can be maintained ~nly
temporarily because deuterium is depleted in the cryogenic
unit ~s illus-trated in Figure 1, one mol o~ D2 is transferred
from gas to liquid Eor one mol of El2 stripped from the
liquid,; the product leaving the column thexe~ore contains
one e.ctca mol of ~2 compared to that enteriny the column with
the feedw~t2r.
To maintain tbese reactions indefinitely,
the "extra" D20 produced b~ the colu~n is continuously
: .
decomposed and the D2 is returned ~o the cryogenic unit to
avoid D~ depletion and thi~ is done in the electrolysis stage
14~ For reactor applications, the best method for this
par~ia~ D2O product decomposition is electrolysis. However, ~ ~,
chemical water decomposition ~such as M~ + D20 ~ MgO ~ D2), or
~ other methods, could also be used. , ,
,; As shown~in Flgure 2, two exchange columns
~ .
~la and 11~ would be employed in a~system in relation to a
nuclear reactor having a moderator system 15 and a heat
transport system 16. Leakag~ recovery from both systems
is stored in tank 17 and passed to column llb. The column
enti,tled "detritiation" removes the bu~k of tritium ~nd ~ -
rela~ively small quantity of protiumj while the column
entitled 'lupgrading" removes the bulk of protium and relatively
small quantities of tr,itium. Although theoretically one
column as shown in Figure~l can be used,-such an arran~ement
would be expensi~7e. The cxyogenic unit-catalytic exchange,
coupling must be care~ul~y designed as this effects the overall
performance of the plant to a greater extent than the other
processes~ The key function of the cryogenic unit is to
produc~ relatively pure D~, H2 and T2 streams. The D2 stream
is returned to the catalytic exchange column (to pick up more
~7~
~3~2~
proti~Lm and tr~tium), the H2 can be discharged as ~aste,
and the T2 is packaged and stored~
Each catalytic column feeds ~as to a
separa~.F cryogenic distillation col~nn: (1) the protium
d-s~ a~:ion column 18 (fed hy the upgrading exchange column)
is desiyned to produce a highly concentrated H2 stream, but
it does not concentrate trltium, the gas from the bottom
of this column, ~ogether with the feed from the detritiation
- column, is fed to (2~ the deu~erium distillation column 19
lG to produce a protlum/tritium-lean D2 s~ream in the middle,
and pre concentrated protium and tritium streams at.the top
and bo-ttom of the column/ respect.ively. The tops ~rom th1s
column are returned to the protium column and bottoms are
fed to a.smaller tritium dist.ill~tion column 20, for further
s~paration.
':. : '
~ ;~'''.
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