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ORNL-TM-4210.txt
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ORNL/TM-4210
Dist. Category UC-76
Contract No. W-7405-eng-26
CHEMICAL TECHNOLOGY DIVISION
MRPP - MULTIREGION PROCESSING PLANT CODE
C. W. Kee and L. E. McNeese
SEPTEMBER 1976
NOTICE
This report was prepared as an accoumt of work
sponsoted by the United States Government. Neither
the United States nor the United States Energy
warranty, express or implied, or assumes any legal
liability or responsibility for the agcuracy, completeness
or usefuiness of any information, Apparatus, product or
process disclosed, or represents that its use would not
infringe privately owned rights,
0AX RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830
operated by
UNION CARBIDE CORPORATION
for the
N
ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATIO
07 WTHCLHMETZE iy
ABSTRACT
INTRODUCTION
iii
TABLE OF CONTENTS
EQUATIONS AND ASSUMPTIONS .
2.1 Model for the Reactor
2.2 Model for the Processing Plant .
NUMERICAL METHODS EMPLOYED
3.1
3.2
Equations .
3.3
3.4
3.5
3.6
LIMITATIONS AND SPECIAL CONSIDERATIONS
4.1
4.2
4.3
System of Units
Coefficients .
4.4
REFERENCES
APPENDIX A:
APPENDIX B:
APPENDIX C:
DESCRIPTION OF SUBROUTINES USED
INPUT
»
OUTPUT .
-
»
*
Iteration with Reactor Code
Calculation of Molar Volumes .
Uses Requiring Modifications .
.
Correction of Distribution Coefficients
Limitations of Steady State Calculation
-
Solution of Reactor Material Balance Equations .
Solution of the Processing Plant Material Balance
Investigation of Faster Solution Methods .
Description of Particular Items Using Mass Transfer
. 13
13
. 14
15
. 16
. 21
. 22
. 22
. 25
. 27
42
. 60
MRPP - MULTIREGION PROCESSING PLANT CODE
C. W. Kee and L. E. McNeese
ABSTRACT
This report describes the machine solution of a
large number (v 52,000) of simultaneous linear algebraic
equations in which the unknowns are the concentrations
of nuclides in the fuel salt of a fluid-fueled reactor
(MSBR) having a continuous fuel processing plant. Most
of the equations define concentrations at various points
in the processing plant. The code allows as input a
generalized description of a processing plant flowsheet;
it also performs the iterative adjustment of flowsheet
parameters for determination of concentrations throughout
the flowsheet, and the associated effect of the specified
processing mode on the overall reactor operation,
1. INTRODUCTION
In a reactor such as a Molten~Salt Breeder Reactor for which contin-
uous processing is planned, the ability to compare alternate proceésing
methods is essential in determining the effect of small changes in proces-
sing method on the steady-state operation of a reactor and processing plant.
Because of the degree of interaction between the reactor performance and
that of the processing plant, it is necessary to consider the reactor and
the processing system simultaneously. The short cooling times resulting
from continuous processing cause appreciable decay of nuclides in the
processing system and result in high decay heating rates and time-dependent
chemical compositions. Most processing plant flowsheets of interest, when
coupled closely with a reactor, produce numerous feedback streams that
complicate material balance calculations and lead to accumulation of
materials over long time periods. To obtain an accurate representation
of the performance of the reactor and processing plant in such cases,
a flowsheet must be represented in detail, and a larger number of
nuclides than considered previously must be taken into account.
Computer programs have been developed and are discussed that allow
a processing plant and the associated reactor to be represented in such
a manner that the programs can be used independently to represent either
the reactor or processing plant in detail or in combination to obtain a
detailed repregentation of the reactor and processing plant system.
Although each code solves a large system of simultaneous linear algebraic
equations, because of the different characteristics of the two systems,
different methods of solution are used; both methods were adapted for
use with sparse matrices. The reactor code uses a Gauss-Seidel iteration
to solve approximately 750 equations. The processing plant code solves
approximately 52,500 equations. However, a special ordering scheme allows
a direct solution in which the equations are considered in blocks of 70
equations each.
The calculations are carried out in an iterative manner between the
reactor and the processing plant for determination of flowsheet parameters.
These parameters include the molar density of phases in each holdup vol-
ume and the distribution coefficients for each mass transfer operation
in the flowsheet; this feature allows the simulation of complex steps
in a flowsheet by using either an equilibrium or first-order rate
mechanism.
Sections 2 through 4 of this report discuss the general problem
and the set of equations employed in its solution, the method used for
solution of the equations, and the extensions of the code to similar
problems. The appendixes essentially form a user's manual and include
a description of the subroutines, the input, and the output from both
the reactor and processing plant codes. A code listing is available
from the authors.
2. EQUATIONS AND ASSUMPTIONS
The multiregion processing plant code is based on a model in which
a MSBR and the associated processing plant are represented by separate
regions of uniform composition; allowance is made for continuous flow
between regions, radioactive decay of materials in each region, and trans-
fer of materials between regions in order to represent common mass transfer
effects. Allowance is also made for fission and neutron capture reactions
in regions representing the reactor. The equations which describe regions
in the processing plant are similar to those for the reactor; howefier, the
reactor is represented by a separate code, MATADOR,2 which receives input
from a single region of the processing plant (the region from which proc-
essed salt is returned to the reactor). The equations for the processing
plant are described separately from those for the reactor to accommodate
the different assumptions that are required.
2.1 Model for the Reactor
A previously developed computer program, MATADOR, is used for cal-
culation of the concentrations of nuclides in the primary circuit of a
MSBR under steady-state conditions in which fuel salt is continuously
circulated between the reactor and a processing system. As such, the pro-
gram serves as a subroutine that calculates the input for the processing
plant program (in the form of concentrations and flow rate of the salt to
be processed) based on output from the processing plant (in the form of
concentrations and flow rate of the processed salt that is returned to
the reactor). From the standpoint of the processing plant program, the
reactor is treated as a single region; however, the reactor program treats
the reactor as a multiregion system. The equations permit the use of
terms which are not necessary for every region; for example, flow of
material between any two regions can be specified, but this seldom occurs.
The equations have been described previously,2 although much of that
description is repeated here.
The accumulation rate of species i in the fuel salt in the reactor
is the input rate of species i by feed, fission, radiocactive decay, and
neutron capture in the fuel salt, graphite, and circulating bubbles minus
the disappearance rate of species i due to radioactive decay, neutron
capture, deposition on exchanger surfaces, and chemical processing. At
steady state, this rate of accumulation is‘zero, so that:
0 = E:(Yeij Aj + ¢ vcfij Gj + Ag gij + Abhij + Sij + ¢ chijgf%)cj
.
J
+ Fcio - (D\iv + oiqavc + F + Ag Gi + AbHi + Si + Pi)ci, (1)
where
surface area of circulating bubbles, cmz,
surface area of graphite, cmz,
volumetric flow rate of fuel salt to the reactor, cc/sec,
coefficient for loss of species i by diffusion into
graphite, cm/sec,
coefficient fbr loss of species i by migration to bubbles,
cm/sec,
effective chemical processing rate for species i, cc/sec,
rate at which species i plates on the heat exchanger
surfaces, cm/sec,
volume of fuel salt, cc,
volume of fuel salt in core, cc,
concentration of species i, moles/cc,
feed concentration of species i, moles/cc,
fraction of disintegrations by species j which leads to
formation of species i,
fraction of neutron captures by species j which leads to
formation of species i,
coefficient for production of species i by diffusion of
species j into graphite, cm/sec,
coefficient for production of species i by migration of
species j to gas bubbles, cm/sec,
rate of production of species i from species } plated on
the heat exchanger surfaces, cc/sec,
fission yield of species i1 from fission of species j,
. -1
Ai = radioactive disintegration constant of species i, sec 7,
. 2
Ui = average neutron-capture cross section of species 1, cm ,
Ufi = gaverage fission cross section of species i, cm ,
-2 ~1
¢ = average neutron flux, cm = sec .
Thus, for I nuclides, this equation is a system of I simultaneous alge-
braic equations and I unknowns. Two other sets of I equations and I
unknowns are considered by: (1) allowing for a volume of gas bubbles and
a holdup for materials plated out in the reactor flux, and (2) allowing
a region in which the evolving gas bubbles and the materials plated out
outside the reactor core are held in contact with fuel salt so that sol-
uble decay products may be returned to the reactor.
All three sets of I equations are solved by the Gauss—Seidel
successive substitution algorithm, with iteration occurring over each
of the successive sets of I equations. Because of the size of the
diagonal terms, the system of equations converges rapidly. Direct sol-
utions were not used because of the storage required for remembering a
coefficient matrix.
The treatment of diffusion of noble gases into and decay products
out of the graphite uses a model developed by Kedl and Houtzeel.3 This
model assumes that the graphite moderator is replaced by semi-infinite
solid cylinders with the same surface-~to-volume ratio, and that the
graphite is coated with a low permeability material to a depth of 1 mil.
Diffusion of noble gases into the graphite occurs through a liquid film
and the coating, and is simulated by a lumped resistance model. The
model developed for the migration of noble gases and noble metals to
circulating helium bubbles is an extension of the model proposed by Kedl
and Houtzeel.3 Both of these models are described in ref. 2.
Once the set of concentrations is obtained, it is possible to cal~
culate heat generation rates, fission product poisoning, production rates
of the materials in the reactor, inventory in moles of the material in
the reactor, the uranium mole fraction, and the breeding ratio. The
breeding ratio is calculated by assuming that it varies linearly with
small changes in the fission product poisoning. Thus, the breeding ratio
is determined from the difference between the reference fission product
poisoning (from R.OD4 calculations) and the calculated fission product
poisoning.
In calculating the fission product poisoning, the poisoning from
135Xe is excluded because it was also excluded in the reactor design code,
ROD, where it was always assumed to be one-half of 1Z. While MATADOR
calculations of fission product poisoning include absorptions by the
noble gases and noble metals in the gas bubbles, noble metals plated
onto surfaces outside the reactor core are not assumed to absorb neutrons.
Neptunium absorptions are included, because the reference fission product
poisoning used by ROD from previous calculations included the neptunium
absorptions with fission product poisoning.
2.2 Model for the Processing Plant
The processing plant code calculates steady-state concentrations
based on material balance equations, and constant reactor effluent concen-
trations as determined by the last MATADOR calculation. The multiregion
code assumes that the processing plant is composed of a number of regions
(or holdup volumes) connected by flowing streams. A region consists of
a well-mixed volume containing one or two phases in equilibrium. The
equilibrium is specified by a proportionality constant that varies with
atomic number and may be changed between successive flowsheet calcula-
tions. Any two regions may be linked by flowing streams or by rate
expressions, and each region may have feeds or discards. (These are
streams which do not connect two regions in the plant.)
2.2,1 Material balance equations
The rate of accumulation of nuclide i in a region (0 at steady
state) is the rate of appearance of 1 in the region from feed streams,
from flowing streams from other regions, from mass transfer from other
regions, and from production of other nuclides by radiocactive decay
minus the rate of disappearance of i from the region by flow out, by
mass transfer to other regions, and by radioactive decay. Thus, at
steady state in region n:
0 = A.e,.(V + K, V C.
)j: ] 13( 5,n j.n B,n) j,n (2)
3#
+ +
i FSm,n Ci,m + é FBm,n Ki,m Ci,m FIi,n
m¥n m#n
+ 2: [(kia)m,n Ci,m - (kia)n,m ci,n]
where
e' *
1]
A
i
subscripts
discard rate of phase S from region n, cm3/sec,
flow rate of phase S from region m to region n, cm3/sec,
feed rate of nuclide i to region n, moles/sec,
distribution ratio for nuclide i in region n,
volume of phase S in region n, cc,
concentration of nuclide i in first phase of region n,
moles/cec,
fractions of disintegrations of species j which lead to
formation of species i,
mass transfer rate constant for transfer of species i
from region m to region n, cm3/sec [the ratio of (kia)m,n
and (kia)n,m is a distribution function, as discussed
in Sect. 2.2.2],
- - —l
decay constant for nuclide i, sec ~, and
B and S = phases
i and j = nuclides
m and n = regions.
Hence, there is one equation and one unknown for each nuclide in each
region. There are 52,500 equations for 750 nuclides in 70 regions.
Because of the number of equations and the number and size of recycle
streams present in a flowsheet, the Gauss-Seidel iteration is too time
consuming.
Since neutron captures are neglected, it is possible to
arrange the nuclides so that each nuclide precedes all of its decay
daughters.
For N regions, there is one set of N equations with N unknowns
10
for each nuclide. By arranging the nuclides in the proper order and
solving each set of equations, a direct solution for the entire set of
equations is obtained. Solutions in the processing plant are alternated
with calls to MATADOR to obtain a converged solution for a reactor
coupled with continuous processing.
The program used has been designed to solve a large system of
equations in which the nontriangular coefficient matrix may be expressed
as a lower triangular coefficient matrix whose elements are matrices.
Each matrix on the diagonal is solved directly, with substitutions being
made for unknowns that were calculated previously. In this case, the
lower triangular matrix has off-diagonal terms in row i and column j of
the form:
-A,e.. V., ,
j il =i
where
Efi = diagonal matrix with the nth term on the diagonal being
V.. +
Sn Kj,n VBn ’
The terms on the diagonal (i=j) are matrices of the form:
R. + A, V., .
~1 141
P
'51 is a matrix whose element in row n and column m is
ot
(én,m - [%Sm,n + Kim FBm,n + (kia)m,%] + ;% sn;m [?Sn,m + Ki,n FBn,m
+
(kia)n,m] + DSn t Ki,n DBnt ?
O
il
1 if n=m, and
o
"
0 if n#m.
11
The resulting system of equations, when used with a constant vector
indicating feed streams, is the set of Egs. (2).
2.2.2 Mass transfer coefficients
Three models were used for mass transfer between liquid and gas
phases. TFor transfer of noble gases to gas bubbles, the overall liquid
mass transfer coefficient was estimated from the Hughmark correlation
given by Schaftlein and R.ussell.5 This is substituted into the relation
for mass transfer:
N; = Kop2gley — Heg) = Kypagey — KopAgh ¢,
where
bubble surface area, cmz,
o
H = Henry's law constant, moles/cc (liq) per mole/cc (gas),
Ky = overall liquid mass transfer coefficient, cm/sec,
Ni = rate of mass transfer, moles/sec,
cy and c, = concentrations in liquid and gas, moles/cc.
This equation is broken into two contributions to matrix terms analogous
to a flow rate so that:
(kja)) m = Koo 4B
(kia)m,n = KOL AB H = H(kia)n,m’
where region n is the liquid phase, and region m the gas phase.
In addition to this transfer, the rate of mass transfer at the
surface between the liquid and cover gas must also be considered. For
volatile components, the mass transfer resistance is assumed to be across
a liquid film of thickness XR:
12
N, = DQAS (c, - Hc ) = DQAg c, — DQAS He |, (5)
L X % 8 X ¢ X g
% 2 2
where
DQ = diffusivity in liquid, cmz/sec,
2
A.S = surface area, cm ;
thus,
(kia)n m (DQAS)/XR
(kia)m,n (DRAS H)/XQ = H(kia)n,m . (6)
The same model is used to describe transfer of nonvolatile decay products
of volatile fission products across a gas film of thickness Xg; however,
since the transfer from liquid to gas is zero, there is only one term.
All of these terms must be added to obtain the matrix coefficients that
are used.
In both models the design criterion is based on some consideration
other than the removal of volatile nuclides. TFor example, the gas rate
might be based on the amount of reductant necessary for a hydrofluorinator
or a hydrogen-sparged vessel, or on the amount of argon needed for a level
probe in a surge tank. In a fluorinator, however, the design is chosen
to achieve a specified performance such as per cent removal of uranium.
For zero inlet concentration in the gas, a material balance gives:
F.c. =F.c.+ F c = PRFLCI + FLCL,
or
cp = (1 - PR) cyi Cp = L . (7)
13
where
FL’FG = flow rates in liquid and gas, cc/sec,
CIfCL’CG = concentrations in inlet liquid, outlet liquid, and
outlet gas, moles/cc,
P = percent removal;
R
thus, the transfer across the interface is P_ F. c Substituting for
R L T
CI:
P
N, =T R . (8)
i L-zzuzefigy cL
In the simulation, FL was included as a parameter independent of the
element, and PR/(l - PR) was listed as a constant that depended on the
element number.
3. NUMERICAL METHODS EMPLOYED
3.1 Solution of Reactor Material Balance Equations
The number of equations to be solved by the reactor code is equal
to the number of nuclides, ®739, The coupling of the equations used for
the salt in the drain tank with those used for the gas bubbles adds two
additional sets of the same number of equations. The direct solution
of such a set of equations would require a method that would take advan-
tage of sparseness and might need only limited pivoting to minimize f£ill.
These problems are eliminated by the use of an iterative solution, pro-
viding the solution is obtained in a reasonable number of iterations.
The number of iterations has always been less than 30.
14
3.2 Solution of the Processing Plant Material Balance Equations
The number of equations and unknowns for a flowsheet of 70 regions
and a nuclear library of 739 nuclides is about 52,000. Storage of the
coefficient matrix of this system of equations is completely impractical,
and storage of even the nonzero terms could easily exceed the storage
capacity of machines available; therefore, a direct Gauss reduction
could not be used. Various approaches were made by using a Gauss-Seidel
iteration. The best method was found to be a series of solutions for
all nuclide concentrations in the flowsheet:; the nuclides were considered
one at a time so that some nuclides required only a few iterations. Even
with the rearrangement of nuclides, which necessitated only one solution
for each species, the time requirements were still excessive; this was
because the diagonal element of the coefficient matrices was comparable
in magnitude to the off-diagonal terms, except for nuclides with short
half-lives.
For each successive nuclide, a 70 by 70 matrix was solved to obtain
the concentration of that nuclide in each of 70 regions. The time
requirement for a direct solution for all concentrations in the processing
plant by this method was quite reasonable — less than a minute for the
IBM 360/91. In addition, the memory requirements were essentially the
same as those for the iteration technique. The storage of the solution
vector is a significant fraction of the storage requirement, and for
larger flowsheets (the IBM 360/91 at ORNL was able to handle 250 regions)
the solution vector uses most of the storage.
15
3.3 TIteration with Reactor Code
The existence of this reasonably rapid solution for the processing
plant calculation enabled iterations with the MATADOR routine which
simulated the reactor performance. MATADOR was coupled to the processing
plant calculation by using removal times defined as the reactor inventory
of a species divided by its net removal rate by chemical processing.
Fairly rapid convergence of concentrations was achieved for most nuclides
if the parameters that passed between routines were averaged with their
previous values to provide damping. All concentrations converged within
20 iterations. However, the variation of more parameters between itera-
tions, and the consideration of more difficult flowsheets required
improvement to the code. The use of reactor inlet concentrations rather
than removal times resulted in comparable performance, but permitted a
different set of nuclides to converge rapidly. Hence, the concentrations
in the stream returning to the reactor can be described as the sum of
the amount remaining after processing and the amount due to production
in the processing plant.
At the same time the solution is found for the processing plant
concentrations, a solution is also found for the first derivative of
the reactor inlet concentration with respect to the outlet concentra-
tion of the same nuclide. This is possible because the same coefficient
matrix is used for both sets of equations. The constant vector in the
solution for the derivative is a vector with unit concentration of each
nuclide in the reactor and no terms for production by decay. Hence,
with little increase in calculational effort and complexity, provision
16
can be made for small changes in concentration to allow the reactor
code to predict the processing plant performance. In addition, an even
greater saving of time is made by considering only those nuclides whose
concentrations are slowly converging, and by periodically reconsidering
all nuclides.
3.4 Calculation of Molar Volumes
The steady-state performance of a processing plant depends on the
rates of decay and, therefore, the molar inventories of radioactive
nuclides throughout the processing plant. It is thus necessary for the
molar inventories and molar volumes to be consistent with the volumes
specified. It is also necessary to know the ratio of the molar volumes
of the two phases in certain regions for use as a conversion factor to
convert the distribution coefficients from ratios of mole fraction to
the ratio of concentrations in moles per cubic centimeter. This problem
is not alleviated by use of molar flow rates and mole fractions, because
the parameters must be such that the mole fractions add up to 1.0,
Between iterations, each flow rate from a region is expressed as
a fraction of the total flow from that region. The molar volume of each
stream for which a molar volume correction is to be made is determined
by assuming additive molar volumes. The corrected flow rate is given
by the product of this molar volume; the molar flow rate and the ratio
of molar volumes for regions containing salt and bismuth provides a
conversion factor for the distribution coefficients.
17
3.5 Correction of Distribution Coefficients
For a given region n, the set of concentrations of that region is
the solution of:
[ S + Aivs,n + Ki,n(FB + AiVB,n{}Ci,n = § Ajeij(vs,n + K'j,nVB,n)cj,n
+ (NPUT), (9)
where
FS = L FSn,m + Dn’ and
(INPUT)i 0 = total input of nuclide i to region n from feeds or other
b
regions.
From a table of valences, it is possible to calculate an equivalent
density in the second phase in equivalents per cubic centimeter. It is
this number which must remain constant through any series of reductive
extraction operations to satisfy the condition of an equivalent balance.
The equivalent density, Eo’ in the second phase that enters the region
may be a calculated variable or an input variable. In either case, the
variable DLi = KLi,n must be determined so that
E-E =0,
0
where
E=LE N = E(VAL)i K = total number of equivalents per
1“1, 1,n °i,n
milliliter in the bismuth phase, and
(VAL)i = valence of i in salt.
18
We have also defined:
K, =A D . (10)
since
log K = A log D., + A , (11)
i,n Li c
where A.n and AC are constants.
The proper value of D ., can be found iteratively by Newton's method
Li
if we have a means for evaluating
d
— (E - E ) .
dDLi 0
Although the specific derivative may be found, an approximation is given
because it requires less time and memory, involves terms all of the same
sign, and still arrives at the proper DLi' It is convenient to redefine
the constant terms:
A F,+ A, V
i
1 S S,n
A2 = FB + Ai VB,n
(PROD)i = § Ajeij(vs,n + Kj,nVB,n)Cj,n + (INPUT)i,n .
so that
(PROD)i
c, = —
i,n Al + Ki,nAZ
PROD), K, VAL) .
. ] ( )4 1,n( )¢ (12)
i,n Al + Ki,nAZ
By obtaining the derivative of E - E0
d d d
(E-E) = E=3%—+— (B, ) (13)
dDLi o dDLi i dDLi i,n
from Eq. (12):
19
dE, K. A dK
i,n _ 1 _ i,n 2 i,n , (14)
ap, . - (PROD), (VAL), | 3= % % 7| ~ap
Li 1 i,n 2 (A1 + Ki nAZ) Li
assuming that (PROD) is independent of D While this assumption is
Li®
Afl
d Ki n nAcDLi n
_L= -
iD D 5 %o (15)
Li Li Li
for
D ;#0,
then
aE; , (PROD) (VAL), K, o A, A,
db; 5 Ay K af A+ K o Dy
A A
E 1 a_ (16)
1 i,n 2 DLi
Thus, as calculations are made for E = % Ei 0’ calculations are easily
s
made for
dE d
— = — (E, ). (17)
dDLi i dDLi i,n
The proper DLi for use in the next iteration can be obtained by allowing
convergence of
d
(DLi)m+l - (DLi)m + (E - Eo)/dDLi
(E-E) . (18)
Since'(PROD)i is dependent upon DLi’ two terms have been left out.
Some provision must also be made for the failure of this scheme to converge.
The calculation procedure examines the sign of E - EO in order to contin-
uously specify upper and lower bounds for DLi' Any new DLi value that
is outside this interval is replaced with the value at the midpoint of
the interval. This value of DLi might tend to overcorrect in a flowsheet,
20
especially if these regions are in series. As a means of providing a
damping effect, the value returned for the next iteration is a weighted
average of this value and the old wvalue.
3.6 Investigation of Faster Solution Methods
When flowsheet parameters such as flow rates (i.e., molar volume)
and distribution constants are not changed between iterations, the solu-
tion for all but the first iteration can be speeded up. The solution
by reduction and back substitution is a series of row operations on an
augmented matrix; much of the calculational time is used to perform
operations on the coefficient matrix. For double precision calculations,
the information required for making the necessary row operation on only
the constant vector is a double precision constant and two indexes which
indicate the rows involved. This information can be stored easily in
the eliminated matrix positions during the first iteration so that they
may be used on subsequent iterations. The calculations after the first
iteration are manipulations performed on a vector rather than operations
on a matrix; in addition, the matrix, which is not required, need not
be determined in these subsequent iterations.
Before the introduction of changing flowsheet parameters, the
number of row operations required for a 50 by 50 flowsheet matrix was
determined. Between 200 and 280 row operations were performed with
about 807% of the calculational time required for operations on the
coefficient matrix. The number of constants needed for all nuclides
requires more memory than is available as fast memory; however, because
the constants need only be accessed sequentially, storage on a direct
access device is sufficient, and only a small buffer space is required,
21
4, LIMITATIONS AND SPECIAL CONSIDERATTIONS
4,1 Limitations of Steady State Calculation
A number of problems arise from the consideration of a steady state
process (i.e., flowsheet steps that are designed to be intermittent). A
good example is a waste tank that is slowly filled with waste salt over
a period of about 1 year, after which the salt is held up for a decay
period and fluorinated for uranium recovery. In addition, a number of
materials are accumulated over the lifetime of a reactor. As a result,
some materials must be removed by a discard stream so that the residence
time is about one reactor lifetime if the nuclides (e.g., lead) are to
be discarded.
It is also desirable to treat some nuclides (e.g., transuranium
isotopes) in a steady state concentration, even though steady state
requires several reactor lifetimes to achieve. Although the code makes
a steady-state material balance calculation, the convergence is slowed
down by this kind of calculation, because the code requires time to
permit the concentrations of this material to build up during successive
iterations.
A steady state calculation for processing plants with near total
recycle of any species requires user caution. The existence of any
material that cannot be removed from some region or group of regions
causes the system of equations to be singular, since the concentration
of this material in those regions is undefined. Iterations with the
reactor code do not alter the concentrations of major salt components,
and provision is made in the processing plant code for the concentration
of any nuclide to be defined by the input at any point.
22
4,2 System of Units
In the system of units that was used, the code calculated concen-
trations in moles per cubic centimeter by using flow rates in cubic
centimeters per second, volumes in cubic centimeters, etc.; however,
this is not necessarily implied by the equations, since they are just
as valid in other systems of units (the nuclear library uses seconds
as the unit of time). The most logical alternate set of units is the
description of volume in moles, rates in moles per second, etc., which
results in concentrations in mole fraction. Some calculations might
be changed by the user who prefers this system of units. For example,
the calculation of mole fractions is redundant and might be replaced
by the calculation of concentrations in moles per cubic centimeter.
The greatest alteration required in such a change of units is the
replacement of the output headings. By such label changes, it would
be possible to treat any system of equations of this form.
4,3 Description of Particular Items Using
Mass Transfer Coefficients
In most simulations, the use of mass transfer coefficients was
limited to gas—liquid contacts requiring one region for the gas phase
and one region for the liquid phase containing only one or two liquids.
It was assumed that the noble gas concentrations were small enough so
that no appreciable error would occur by treating the gas bubbles as if
they had the same concentrations as the bulk gas. This is not an essen-
tial assumption, however, since by using more regions, the same case
might be described as having more than one gas region. The bulk gas
23
would be a separate region from the gas bubbles, and the gas bubbles
in separate liquids could easily be separate regions, If the gas con-
centration changes as it rises through the liquid and is not small
rélative to its equilibrium concentration, the gas at wvarious levels
in the liquid would be in différent regions and would represent a
column.
Careful consideration has also been given to simulation of mass
transfer-limited transfer rates as a replacement of equilibrium stages
in a liquid-liquid contactor. For turbulent flow when transfer is lim-
ited by eddy diffusivity, the only difficulty arises in specification
of the distribution coefficients at the interface. This case can be
described by three regions: the bulk liquid phase S, the bulk liquid
phase B, and the interface contact of both phases. The flows between
the interface region and the bulk liquid region are the rates of eddy
transport in each phase. The product of eddy diffusivity and concentra-
tion driving force in each phase is implicitly obtained in two flow rate
terms for each phase in the same manner described in the section on mass
transfer coefficients. 1In this case, the distribution coefficients can
still be determined iteratively by the same technique used for equilib-
rium stages even though the distribution coefficients are not expected
to be the same.
4.4 VUses Requiring Modifications
It is possible to consider flowsheets involving batch processes
that might not be represented by either an equilibrium process or a rate
limited process, because the amount of material transferred between the
24
phases was dependent on the concentrations of other elements as well
(e.g., oxide precipitation). The rate coefficients that are used have
always remained constant, but this is not essential to the code since
iteration is required to converge other parameters in the flowsheet.
By using a reasonable estimate for percent removal (i.e., rate constants)
for various components, a set of concentrations would be obtained.
Subroutine VOLUME would then be used to calculate the proper removal
rates on the basis of these concentrations, and it would modify the rate
constants accordingly for the next iteration. This system for treating
more general processing steps should cause no greater convergence prob-
lem than the iterative determination of distribution parameters described
later.
An alternative to this approach is to provide concentration or feed
rate links to such a subroutine as is done with MATADOR, or even to
replace MATADOR with a routine simulating some section of the flowsheet.
However, the most likely substitution for MATADOR is either a subroutine
that reads and stores entering processing plant concentrations or one
that simulates a different reactor type. In this last case, the reactor