Document 6528107

An Automated Sample-Processing and Titration
System for the Determination of Uranium
J. E. Harrar, W. G. Boyle, J. D. Breshears, C. L. Pomernacki,
H. R. Brand, A. M. Kray, R. J. Sherry, and J. A. Pastrone
Lawrence Livermore Laboratory, University of California,
Livermore, California 9^550
ABSTRACT
The system comprises a computer-controlled automatic wet
chemical analyzer, and a scheme for handling all of the data generating operations associated with the assay of solid or solution
samples containing uranium. The analytical measurement technique
is based on the Davies-Gray/New Brunswick Laboratory method, and
involves controlled-current coulometry with potentiometric endpoint detection. To increase the credibility of the analytical results and minimize the probability of processing large numbers of
samples incorrectly, the analyzer includes an extensive faultmonitoring network. This guards against (a) off-normal conditions
that might result in analytical errors, and (b) unsafe operating
conditions. There is provision for analyzing standards along with
samples, and for automatically comparing the standard results with
allowable control limits.
INTRODUCTION
Among the numerous methods proposed for the accurate assay
of uranium in nuclear materials, the titrimetric method of Davies
and Gray (!_) as modified by workers at the New Brunswick Laboratory (2_), has emerged as the most selective and hence the most
versatile. It has been thoroughly characterized at the New Brunswick
Laboratory and elsewhere, and found to be especially applicable to
uranyl nitrate solutions, samples such as aluminum-, stainlesssteel-, and Zircaloy-clad fuels, and samples containing niobium,
hydrofluoric acid, and nitric acid.
The original Davies-Gray method involved the reduction of
U(VI) to U(IV) by Pe(II) in strong H^POn, oxidation of the excess
Fe(II) with UNO3 in the presence of Mo(VI) catalyst and sulfamic
acid, dilution of the solution, and then titration of the U(IV)
with KpCrpOy to a visual end point. The method was improved and
refined at the New Brunswick Laboratory by the addition of V(IV) to
the solution to markedly speed up the attainment of equilibrium,
which allowed the use of potentiometric end-point detection. The
basic method has also been extended by Goldbeck and Lerner (3.) to a
coulometric titration, in which the titrant is V(V), electrogenerated at constant current by the oxidation of V(IV). This procedure,
*
Work performed under the auspices of the U.S. Energy Research and
Development Adminstration, Division of Safeguards and Security.
Fall 1976
199
compared to the volumetric one, has the advantages of not requiring a titrant solution, having greater precision and lower
systematic error in the measurement of the quantity of titrant,
being easier to adapt to changing quantities of uranium titrated,
and being more amenable to automation.
Because of the large number of assay samples processed by
many nuclear laboratories, there has been a continuing need for the
automation of a general-purpose method such as that based on the
Davies, Gray, and NBL procedures. Other benefits that usually accompany automation, such as more timely analyses, freedom from
operator bias, reduced clerical time and errors, increased precision, and reduced operator intervention in the processing of
radioactive samples, could also be realized. However, to achieve
the maximum benefits of the automation, all operations performed on
the samples, riot Just the titratlon itself, should be incorporated
into the automatic system. Pour basic steps are involved in the
complete analysis: sample dissolution, sample solution preparation,
titratlon, and calculation and presentation of the results. All of
the work done thus far has been directed toward mechanizing the last
three of these steps.
Progress in the automation of the Davies-Gray-NBL method
thus far has included the adaptation of the volumetric potentlometric titration to a Fisher Scientific Company Titralyzer (20,
Radiometer Company titration equipment (4_), and a Metrohm-Brinkmann
titrator with programmable calculator control '(5.) • The coulometric
titration has also been carried out with the Fisher Titralyzer (6.,2_) •
AUTOMATIC URANIUM TITRATION
SYSTEM
The design of the automatic analysis system developed at
the Lawrence Livermore Laboratory departs from previous systems in
several respects. A simplified block diagram of the system is shown
in Figure 1 and Figure 2 is a photograph of the system. The system
is controlled by a Digital Equipment Corporation PDP-8/e minicomputer. To reduce the systematic errors in the analysis, all
sample solution aliquoting is carried out on a weight basis. The
two principal operations — the weighing of the solid and solution
samples, and the titration of the uranium In the solutions — have
been integrated so that previously entered weight information, with
appropriate identification numbers, can be extracted from a computermemory file and used with the titration results in an automatic
calculation of the quantity and weight concentrations of uranium in
the analyzed samples. Weight information can be stored semiautomatlcally via the electronic balance or entered from the keyboard.
A commercially-available sample changer (8_) having a capacity of
MOO 300-ml beakers is used, together with a Mettler optical-reader
electronic subsystem that reads coded labels denoting the positions
of the beakers in the sample changer<
2M
Nuclear Materiab Management
The apparatus for delivering the pre-titration reagent
solutions and discarding the waste solution is based on the use of
pressurized solution containers and an appropriate valving arrangement, rather than a system of pumps. Titrations are performed by
controlled-current coulometry using a specially-designed electrolysis
cell and electronic modules for programming the current and measuring
the electrode potentials (9.).
BASIC STEPS IN OPERATION OP SYSTEM
Weighing and Storage of Weights. There are three basic
steps in the operation of this system: the weighing operations, the
loading of the sample changer, and the analytical measurement.
First, a series of weights, from which the weight concentration of
uranium will be calculated, are entered into the computer memory
file along with appropriate identification numbers. An example computer printout illustrating the entry format for a sample, followed
by a standard is shown in Figure 3. For unknown samples, three
weights are entered: the Subsample Weight, or portion of the solid
sample to be analyzed, which is then dissolved in appropriate solvents; the Subsample Solution Weight, which is the solution resulting
from the dissolution of the subsample; and the Subsample Solution
Aliquot Weight, which is the portion of the subsample solution
placed in the beaker for titration. In the case of known standard
solutions, the weight of the aliquot of standard solution is first
entered, the operator then enters the concentration of the standard
solution, and finally the computer calculates and stores for later
use only the quantity of uranium in the standard aliquot, along with
its I.D. number.
Loading of Sample Changer. After a group of samples and
standards have been weighed and prepared for titration, the beakers
are placed in the sample-changer magazines. The operator, interacting with the computer, then enters into the computer memory file
the I.D. number of each subsample solution aliquot or standard
quantity of uranium, along with the number of the position in the
magazine into which he has placed each beaker. The input format for
this operation is illustrated in Figure 4. During the operation of
the sample changer, the sample identification subsystem identifies
each beaker by position number as its solution is being analyzed.
After the titration has been completed and the quantity of uranium
found has been calculated, the computer links the quantity found viathe position number to the previously-entered I.D. number and weight
information.
Analysis and Data Readout. After a series of samples and
standards have been weighed, loaded on the sample changer, and their
position numbers indexed to the I.D. numbers, the automatic analysis
system is started. After each beaker is processed, the result of
the analysis is printed out, along with the previously entered weight
information and information on the characteristics of the titration
that may be useful for diagnostic purposes. This is illustrated in
Figures 5 and 6. In the case of the standard, as shown in Figure 6,
an error is calculated; this value is compared with a previously-set
FalM 976
201
control tolerance, and a system shutdown will be initiated if the
error is too large. Because of the positive sample identification
features of this system, samples and standards can be loaded and
processed in any proportion and in any order.
FAULT MONITORING SYSTEM
In addition to monitoring errors in the analytical results
themselves, a number of other conditions of the system operation are
also sensed; a summary these is given in Table I. The faultdetection system is "intelligent," i.e., if any of the fault conditions should occur, the system is programmed to give an alarm,
indicate and record the fault, and shutdown, hold or carry out
certain actions that depend on the nature of the fault and the
point in the analytical cycle during which the fault occurred.
TABLE I.
Sensor Points and Off-Normal Conditions
Monitored by Fault-Detection System.
Spills or Leaks:
in Reagent Containment System
in Reagent Valve System
at Sample Changer
Flows:
Sulfamic acid
FedD-HoPOjj
HNOo-Mo(VI)
Diluent
Reagent-Line H20
Rinse H20
Waste solution
Cell Gas
Pressures:
Reagent Solution - Low or High
Water Supply - Low or High
Air Supply - Low
Cell - Low or High
Power Outage
Digital Panel Meter Overload
Titration Characteristics:
Electrolysis Current Error
Current-Source Voltage Limiting
High Generator-Electrode Potential
Quantity of Uranium over Maximum Limit
Failure to Find Inflection Point in Titration Curve
Control Standard Error
202
Nuclear Materials Management
SYSTEM PERFORMANCE
The prototype of the system was developed for use at the
New Brunswick Laboratory of U.S. E.R.D.A. and Is being evaluated
there. Preliminary results with uranium-metal standards, UoOg, and
U02 indicate that the precision of the system is ^0.03 mg U (A.S.D.)
in the range of 12-l8o mg U. Absolute accuracy based on electrical
calibration of the instrumentation varies from -v-0.15 to +0.20 mg U
as the quantity of uranium decreases. Types of samples tested thus
far, and analyzed by the system without difficulty are 11303, UOg,
UF6, U-A1, and U-stainless steel.
ACKNOWLEDGMENT
The encouragement, advice, and cooperation of C.D. Bingham
and the personnel of the New Brunswick Laboratory during this work
are gratefully acknowledged.
REFERENCES
(1) W. Davies and W. Gray, Talanta 11, 1203 (1964).
(2) A. R. Eberle, M. W. Lerner, C. G. Goldbeck, and C. J. Rodden,
Titrimetric Determination of Uranium in Product, Fuel, and
Scrap Materials After Ferrous Ion Reduction in Phosphoric Acid,
U.S. Atomic Energy Commission, New Brunswick Laboratory
Rept. NBL-252, (1970).
(3) C. G. Goldbeck and M. W. Lerner, Anal. Chem. 4JJ_, 59^ (1972).
(4) L. Z. Bodnar and J. M. Scarborough, in Annual Progress Report
for the Period July 1972 through June 1973» U.S. Atomic Energy
Commission, New Brunswick Laboratory Rept. NBL-267 (1973),
pp. 22-28.
(5) J. V. Bender, Paper presented at 27th Pittsburgh Conference on
Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio,
March, 1976.
(6) C. G. Goldbeck and M. W. Lerner, in Annual Progress Report for
the Period July 1971 through July 1972, U.S. Atomic Energy
Commission, New Brunswick Laboratory Rept. NBL-265 (1972),
pp. 5-20.
(7) C. G. Goldbeck, M. W. Lerner, and G. E. Peoples, in Reference 4,
pp. 29-31.
(8) Metrohm Ltd.-Brinkmann, see HJ. Keller, Neue Teknik, 14_ (2),
31 (1972).
(9) J. E. Harrar and C. L. Pomernacki, Computer-Compatible Instrumentation for Automated Cdntrolled-Potential Coulometry,
Fotentiometry, and Galvanostatic Measurements. Lawrence Llvermore
Laboratory Rept. UCRL-78059 Preprint (1976).
Fall 1976
203
FIGURES
Figure 1.
Block diagram of automatic uranium titration system.
Figure 2.
Automatic uranium titration system.
Figure 3-
Entry of weight data for a sample and a standard.
Figure 4.
Entry of sample-changer position numbers and identification numbers for a standard and two samples.
Figure 5.
Data readout for the analysis of an unknown sample.
Figure 6.
Data readout for the analysis of a standard.
"Reference to a company or product
name does not imply approval or
recommendation of the product by
the University of California or the U.S.
Energy Research & Development
Administration to the exclusion of
others that may be suitable."
204
Nuclear Materials Management
Minicomputer
Keyboard
Control
panel and
interface
1
Programmable
constantcurrent source
Electronic
balance
Water
Electrode
potential
measurement
interface
subsystem
Solution and
gas
distribution
subsystem
Post-titration
waste
distribution
subsystem
Figure 1.
8
Sample
identification
subsystem
pressure
regulation
Block diagram of automatic uranium titration system,
Figure 2. Automatic uranium titratIon system.
«
M
Minicomputer
Keyboard
Control
panel and
interface
I
Programmable
constantcurrent source
Electrode
potential
measurement
Sample
identification
subsystem
Electronic
balance
Water
pressure
regulation
interface
subsystem
Solution and
gas
Post-titration
distribution
subsystem
waste
distribution
subsystem
Figure 1. Block diagram of automatic uranium titration system.
S
W»
DRTE ENTERED: 04-81-76
I. D. NUMBER: E2849-1-2
SUBSRMPLE WT. , GM. : 099. 32521
SUBSRMPLE SOL. WT. , CM. : 925. 21899
SUBSflMPLE SOL. flLIQUOT WT. , GM. : 919. 28549
DflTE ENTERED: 94-91-76
I. D. NUMBER: R9919
STD. flLIQUOT WT. > GM. : 999. 43429
STD.
STD.
CONC. , MG. URflNIUM/GM. SOLN. : 19. 352
QUflNTITV OF URflNIUM, MG. : 193. 32399
Figure 3.
5
Entry of weight data for a sample and a standard.
DRTE: 84-01-76
SfiMPLE POS. N U M B E R : : 2 1
I. D. N U M B E R : R0811
DflTE: 94-01-76
S R M P L E POS. N U M B E R : : 2 2
I. D. N U M B E R : E2849-1-1
DfiTE: 94-01-76
SflMPLE POS. N U M B E R : : 23
I. D. N U M B E R : E2849-1-2
Figure 4.
nu
2
V
n
v^
2
u
3
U
$
n
Entry of sample changer position numbers and identification
numbers for a standard and two samples.
DATE: 04-92-76
TITRflTION PARAMETERS :N.
SflMPLE POS. NUMBER: 22
I. D. NUMBER: E2849-1-1
SUBSRMPLE WT. , GM. : 000. 32521
SUBSAMPLE SOL. WT. , GM. : 925. 21390
SUBSAMPLE SOL. flLIQUOT WT. , GM. :809. 59358
MG. NORMAL URANIUM IN SflMPLE ALIQUOT=
30. 5172
GM. NORMAL URANIUM PER GM. SUBSAMPLE= 9. 24668
SMALL SAMPLE
OPEN-CIRCUIT POTENTIALS, MV. =IND. ELEC. :27.91-3
GEN. ELEC. :23. 70
FINAL PRE-ENDPOINT INTERVAL POTENTIALS, MV. =IND. ELEC. :GEN. ELEC. :
IND.
Cl-~
ELEC. ENDPOINT POTENTIAL, MV=
93.29
25 C2=
67 E. P. -•=
261. 234 C3=
354
Figure 5.
Data readout for the analysis of an unknown sample.
13.30
60S. 10
DflTE: 93-31-76
TITRflTION PflRflMETERS:N.
SflMPLE
POS. NUMBER: 27
I. D. NUMBER: R0005
STD. QUflNTITV OF URfiNIUM, MG. : 035.93192
MQ. NORMflL URflNIUM IN STD. flLIQUOT, MG. ERROR: 0. 0112
OPEN-CIRCUIT POTENTIflLS, MV. =IND. ELEC.
:GEN.
ELEC.
:-
85.
9912
46.29
47. 80
FINflL PRE-ENDPOINT INTERVflL POTENT IflLS, MV. = IND. ELEC. :GEN. ELEC. :
IND. ELEC. ENDPOINT POTENT IflL, MV=
100.34
Cl=
0 C2= 113 E. P. =
364. 886 C3= 457
Figure 6.
Data readout for the analysis of a standard.
20.50
635. 50