EIR-Bericht Nr. 257

EIR-Bericht Nr.257

Eidg. Institut flr Reaktorforschung Wirenlingen
Schweiz

The possibility of continuous in-core gaseous extraction
of volatile fission products in a molten fuel reactor

M. Taube

=

Wirenlingen, Mai 1974

EIR-Bericht Nr. 257

The possibility of continuous in-core gaseous extraction

of volatile fission products in a molten fuel reactor

M. Taube

May 1974
Abstract

A system of continuous gas purging of a reactor core for
removal of volatile fission products from the molten chloride
fuel is discussed and calculations based on a computer pro-

gramme are shown. The results indicate:

a) the precursors of the delayed neutrons nearly all

remain

b) d1odine-13%1 in the steady-state core is reduced approx
by a flactor 1000

¢) caesium-137 concentraticn in the core decreases
only by a factor 7, but the veolatility is relativ

low due to chloride

d) caesium isoftopes 135 and 133 can be separated from
Cs-137 by a factor of which make casier the Cs-137

management

e) xenon isotopes are extensively extracted

All this results in a significant improvement in the safety

of this reactor type in the case of a core accident.
1. General Remarks

"The aim of this paper is to discuss the possibility of de-
creasing the concentration of volatile fission products in
the steady state core as a counter measure in the event of

a core accident.

The design philosophy of a safe reactor is guided by the
statements of the following type: (WASH 1250, 1973)

"The measures agalinst the escape of radiocactivity from nuclear
facilities (nuclear power plants, fuel reprocessing waste
disposal facilities, shipping, storage etc.) should use design
features inherently favourable to safe operation e.g. by se-
lection of fuel, coolant and core structural materials which

will have inherent stability and safe characteristics.™

In this paper a molten fuel fast breeder is discussed which
could realise both these requirements through: - minimising
the hazard of the escape of radioactive by substances by

continuous extraction of the most volatile fission products

from the molten fuel so that the steady state amounts of these
are reduced by one or more orders of magnitude compared to
the classical case of solid fuel periodically discharged and

reprocessed.

It is well known that in the case of a reactor accident the
hazards are centered around the volatile fission products

(F.P.). For example according to Farmer (1973) a 1500 MW(th)
9

reactor contains 4 x 10” curies of volatile and gaseous
fission products of which I-131 equals 50 Megacuries. The
hypothetical release having a very low probability of causing
harm to the general public (unlikely to cause one case of
fatal illness within the ensuing ten or more years) is of the
order of 5 kilocuries of I-131. In a hypothetical low pro-
bablility accident they discussed a release of 5 Megacuries

of I-131 and 0.5 Megacuries of Cs-137.

2. Continuous gas extraction

The continuous in core gas extraction of volatile fission
products from the liquid fuel is based on the following
data and/or roughly estimated calculations (using the appro-

priate "GASEX" programme)

a) the reactor is fuelled with molten salt fuel - in this

case molten chlorides PuCl3 - UC1l, - NaCl etc. (fig. 1)

b) the chemical properties of the fission products in
this molten media are characterised by the scheme

shown in (fig. 2)

¢) the balance of the fission products - especially for

chlorine is given in (fig. 3)

d) the thermodynamic stability of all the important

irradiated fuel components are given in fig. 3

e) the volatility (partial pressure) of some selected cru-
cial fission products: e.g. caesium in the form of ele-

ment, oxide and chloride is given in fig. 4
t breeder

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Fig., 2

'ission Products in Molten Chlorides Media

36  Kr 54 Xe
35 Br >3 1
34 Se 5 Te
435 As 51 Sb
32 Ge 50  &n
31 Ga hg In
50 Zn hg cda
L7 Ag
he  Pg
45 Rh
L  Ru
bz Te
bz Mo
41 Nb
4o Zr
64 Gd
63 Eu
62 Sm
6l Pm
60 Nd
b9 Pr
58 Ce
39 Y 57 La
38 Sr {50 Ba
57 Rb |55 Cs

fast
Gas 1
Extraction slow
very slow
<:§%latile chlorides
Low volatile
chlorides

Non
volatile
metals

Low
volatile
chlorides

Non

volatile

chlorides

Low volatile chlorides

/\

Fig. 3 Free Energy of Formation

=
n g
© 0
0 T g°fl"Noble metals
B »| Noble gases
0 &
o o
Q, o
o
oo
o g
H oo
n
a° MoCl
g © X
-
- NbC1l
5
-100 T — AgCl
— SbCl
3
CdCl2
AG = SnCl
-1
(KJ mol = InCl
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-200 =
- - 7ZrCl
UCl3 3
P 5
E - NdCl3
« -
o < PrCl3
e — CeCl,
o
HoL - LaCl3
—BOO T - NaCl = RbCl
= CsC1
- SmCl2
- Sr'Cl2
= BaCl

Fission products in form of chloride

Fig, & Volatility Pressure of Caesiumemetall (Caesium Oxide)

and Caesium Chlorides

1000 =

100 -

bar Cs-met __ —
—

1 - ‘o .
boeiling point 500 ¢

CsCL

! ! I | I |
900 1000 1200 1400

Fig. 5 Scheme of the independent Yield Calculation
A = 137 §

Extraction rate

Sb Te 1 Xe Cs Ba

B A
. . /’\t B—‘ A
Filssioned

plutonium o 7
——t -
avom Q“—\‘Q\\&gt B
100 =
51.77 %
50
20
10
yield
%
accordingl
(Crouch,
1969)
0,1
107
106
time
seconds
lOLl
10°
102
10
1

Fig. 6 Scheme of Gas-Extraction

Liquidjdrop H,O HBO
* Separalor \eparatibn
? e,Te

Aepaatio

o o =

o o o ‘

Br,T
jeparatign T

[ Kr ,Xe

deparatipn

!

| e— |

Cq R

10

f) +the history of each individual fission product is
calculated on the basis of the independant yields:

see fig. 5 (according to Crouch 1973)

g) the in-core molten salt medium is purged by means of
heliumhydrogen gas bubbles which greatly influence the
chemical behaviour of some fission products (parti-
cularly iodine). The assumptions concerning the size

of this stream is given in Appendix 1

5., Scheme of gas extraction and possible technology of

gas separation

In order to give a basis for discussion on the gas extraction
systems fig. 6 gives a simplified schematic. It must be
stressed that this is a preliminary suggestion only without

any basic studies.

4., Scheme of calculation

For calculation of the gas extraction system refered to here
a computer programme "GASEX" has been prepared (using Fortran
IV for the CDC 6500/6700). The principal layout on which the

calculation is based is given in fig. 7.
11

Fig. 7 The Scheme of Calculation

Independent yield: n

A(z—l)
Iradiation Irradiation
(Al)z
(A+1)
f
(HJ ’Y)
Neutron
irradiation
A A
(NA1)
(n,y)
Neutron
irradiation
f_l
Z

(Z-1) (Z) (Z+1)
12

The only assumption arbitrarily made was the gas extraction

rate of the volatile fission products.

The calculation was made for the follocwing four assumptions,

given in table 1.

Table 1 Core dwell time - (seconds)
Variant
1 2 ) Y
Se and Te 10° 10° 10" 10°
Br and 1 106 lOLL lO3 102
Kr and Xe 106 10° 10° 10t
A1l other elements *) 106 106 106 106

* ) 106 seconds equals 11.57 days which 1s postulated as the

reprocessing period of the total liquid fuel

5. Results of the calculations

The most important results are:

1. Stable bromine isotopes A-79 and A-87 (fig. 8) are extrac-
ted with high efficiency.
The short lived isotopes (t 1/2 ~ hours or minutes) (fig. 8)
are extracted in rather small quantities (logarithmic ver-

tical scale).
5

13

Short lived bromine isotopes A = 88, 89, 90, being also
the probable precursors of delayed neutrons are in
practice not extractable (no loss of delayed neutrons)

(fig. 9 =~ 1linear vertical scale)

The very high retention of the heavy bromine isotopes
in the molten fuel is almost independant of the rate of
gas extraction. For the highest gas 'extraction' only

a small extraction of bromine occurs (fig. 9).

The main problem for krypton isotopes is the Kr-85
(fig. 10). The gas extraction is successful for the
lowest case (2) and results in a reduction of the amount

of Xr-85 by approx 1000 times.

By increasing the gas extraction rate the amounts of this

isotopes decreases by a factor 10

5. The short lived krypton

isotopes are very efficiently extracted (fig. 11).:

5.

The gas extraction has only a small effect on the two
radioactive strontium isotopes (fig. 12). They are de-

creased only by a factor of 7.

The case of the iodine isotopes (fig. 1% and 14) is cri-

tical for two reasons

I) the iodine-131 is an important factor in the core acci-

dent hazard.
14

The ligures 8-17 are made directly by the CDC 6500/6700 prin-

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TOPES OF

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24

II) The isotopes I-136, I-137, I-138, I-139 and I-140
are probably the most efficient precursors of de-

layed neutrons.

Fig. 13 shows that the gas extraction of I-131 is very
efficient but rather strongly dependant on the postu-
lated gas extraction rate. For the lowest rate the I-131
decreases by a factor of approx 10, but for the middle
case - variant 3% this factor increases to more than 100
and for the highest rate (4) reaches approx 1000. For the
same gas exftraction rates the loss of iodine isotope de-
layed neutron precursors is negligible (see lodine-136 -
140, fig. 14).

The xenon isotopes are also an important factor in the
core accldent hazard. Gas extraction decreases the amount
of xenon in the core significantly including the long
lived Xe-133 and Xe-13%5 (fig. 15 and 16). The short 1life
xenon isotopes are only slightly influenced by the gas

extraction.

The next controlling radionuclide is Cs-137. From fig. 17
it is seen that the gas extraction does not aid in the
removal of this nuclide from the molten fuel, a factor
of 7 is all that can be obtained. But the amount 1is not
the only important factor, the volatility of caesium is

also of interest.

From fig. 4 it is clear that the caesium chloride has a
much lower partial pressure than that of caesium metal

or caesium oxide in the same temperature range.

A very important problem arises with the possibility of

the nuclear transformation of fission products.
ALL

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27

One of the methods especially for Cs-137 and Sr-90 is their
irradiation in a fast high flux reactor. (The use of fast
reactors 1is important because the absorption cross sections

for thermal neutrons are relatively small - 0.1 and 0.8 barns.)
It is important that the nuclide in question 1is more or less

isotopically pure for two reasons
I) the neutron absorption in other isotopes
ITI) the increased volume.
In the case of caesium there are three isotopes of importance

for neutron irradiation techniques they are: {(yield for fast
fission of Pu-239)

Cs=135 stable yield 6.91 atom %
Cs-13%5 t 1/2 - 2.1 x 106 years yield 7.54 atom %
Cs-137 t 1/2 - 30 years yield 6.69 atom %

If isotopic separation is not possible the amount of caesium
for neutron irradiation must be approx 3.15 times bigger than
for pure Cs-137 and the macroscopic absorption cross section

is probably more than 20 times bigger.

With the continuous in-core gas extraction system proposed
here the separation of caesium isotopes - particularly Cs-137
from Cs-135 (long lived) and Cs-133 (stable) is possible, as

is described elsewhére (Taube 1974).

8. Other nuclides with 'mean 1life times' greater than 0.3
years and shorter than 3 years as shown in fig. 18 - that
is Ru-106, Sb=-125, Ce-144, Pm-147 and Eu-155 are all little

affected by the gas extraction.
28

9. Fig. 19 shows the most hazardous nuclides together Kr-85,
Sr-30, Xe-135, Cs-137.

10. Fig. 20, 21, 22 show the estimated values of radioacti-
vity for some selected nucliides in a 2000 MW(th) fast

reactor for different cases

a) without continuous gas extraction and with 85 days

irradiation period.
b) with gas extraction at the highest rate

c) with gas extraction at the lowest rate.

Conclusions

The model of a possible continuous in-core gas-extraction process
described here for a core fuelled by molten chlorides has the

following features.

1. The extraction rate of the precursors of the delayed
neutrons can be reduced to an insignificant value - even

if these are very volatile. e.g. Kr, Xe, Br, I etc.

2. The extraction rate of I-131, the most hazardous fission
product, prominent in any severe core-accldent, is so high
that the steady state concentration of this nuclide 1s
reduced by more than 1000 times which significantly impro-

ves the reactor safety.
29

FPig. 20 Krypton-85
107 1
10° i
lO5 b
Activity
Curie
4 discharging
10 4 of solid
fuel
10° ,
102 4
liquid fuel
lO1 i infcore gas extraction
1
0
10 T T 4 T T T T
10° 10° 10" 10° 10° 107 108 107

Time of fuel irradiation

Seconds
30

Fig. 21 Iodine-131

decay only
lOlO-

solid fuel

(@e)

107 - discharging

1

107
Activity
Curie

~6.9 |MCi gas-extraction

1

rate 10

10° -

76.2 KCi  gas-extraction

3

rate 10

107 -+

0
10 T T
10 lO5 106 10

Time of fuel irradiation

Second
51

Fig, 22 Xenon-135
10~ 4 decay only

Rate of Extr: 10

Activity
Curie

lOb 7

Rate of Extr! 10

lO T T  § I Ll
10 iol

Time of fuel irradiation

Seconds
32

3. The extraction rate of Cs=137 is rather small and reSults
in the diminishing of the caesium concentration by only
7 times when compared to the normal liquid fuel repro-
cessing having a time constant of 106 seconds or
= 10 days mean dwelling time of the ligquid fuel in the
reactor. In the so0lid fuel case the same amounts of Cs-13T7
are accumulated after approx 100 days that is ~3.5 months
dwell time in the core. A special feature of the molten
chlorides fuel is that the vapour pressure of CsCl is
much lower than CSO2/CS metal in solid oxide or carbide
fuels, decreasing the amount of vapourised caesium in
the case of a core accident. Another important point is
that it appears possible to separate the caesium isotopes
135 and 137.

4, Further hazardous fission products such as Xe-133 and
Xe-135 can also be effectively extracted and the steady
state level is >100 times lower than in the unextracted

core.

5. The gas extraction rate influences also the oxygen ex-
traction from the molten fuel which considerably improves
the corrosion behaviour of metallic molybdenum and also
the stability of the uranium and plutonium chlorides
(Taube 1974).

Acknowledgements

The author thanks Mr. S. Padiyath for help in the writing and
computation of the programme 'GASEX' and Mr. R. Stratton for
polishing the English.
33

Appendix I

Calculation of the gas stream rates.

The 2000 MW(th) reactor has a fission rate of about

2 X 109 Wx 3.1 X lOlO fiss/s = 1 = 100 uM Pu/g4
W 6£.02 x 1023
(UM = micro Mol = 6.02 x 102 % 107 = 6.02 x 10%7 atoms)

The relative amounts of volatile nuclides is estimated very

roughly here as 0.2, that is 2 x 20 uM/  of gaseous volatile

elements. The volume of these elements at ~950 °C and ~10 bars

pressure 1is

40 x 10—6 mol/S X 22.2 X% 103 cm3 x 12563 K x 1 = 0.4 cmB/S
mol 275 K 10

This amount of 0.4 cmB/S must be removed from the core by means

of continuous gas extraction. If we postulate a helium stream

5

(with probably 1 mol % of hydrogen) of 4 ecm /S then it seems

the volatile elements can be removed. The assumed dwelling

time of the gas bubbles in the core is 10 to 1000 s. The
5

volume of the gaseous phase in the core is from 40 to 4000 cm

The total volume of fuel 1n the core equals

m3 2
8.75 x 0.3%386 fuel fraction = 3.377 m
core
The volume of gas bubbles given above equals from 10—5 to 10_3

of the fuel volume. This should not give rise to problems of

criticality particularly in the steady state.
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