Determination of the Deuterium Concentration From Heavy Water

Progress of Cryogenics and Isotopes Separation Volume 14, issue 1/2011

DETERMINATION OF THE DEUTERIUM CONCENTRATION FROM HEAVY WATER USING VIBRATIONAL DENSIMETRY METHOD

Gabriela Raducan1, Iuliana Piciorea

1National Research and Development Institute for Cryogenics and Isotopic Technologies - ICIT Rm. Valcea, code 240050 - Rm. Valcea, Uzinei 4, CP7 Raureni, Valcea, Romania,phone:0040 250 736979, fax:0040 250 732746; e-mail: [email protected]

ABSTRACT

The vibrational densimetry method was used to determin the heavy water density with the deuterium concentration less then 99% D2O mass, and an empirical relationship between heavy water density and the deuterium concentration was determined and verified. The deuterium concentration of heavy water samples was obtained using the gravimetric method. A vibrational densimeter was used to determin the density of the samples. This device is able to bring the sample at a preset temperature. The sample’s density was measured at least 5 times and an average value was computed. The relationship between the deuterium concentration and the heavy water density is assumed to be a nonlinear equation which is of the form y = a - b/x. Building an equations system, the parameters were determined. Also, the uncertainity of the method was estimated. This value is too high compared with the uncertainity estimated with the IR spectroscopy method, so we can’t use this method in the range of deuterium concentrations higher than 99% D2O mass, where it is necessary a very good measurement accuracy.

KEYWORDS: D2O; heavy water; deuterium concentration, vibrational densimeter, uncertainty.

1. INTRODUCTION

The CANDU reactor was designed by Atomic Energy Canada Limited (AECL) as an alternative to other reactor designs which use slightly enriched uranium (2-5% U-235). The CANDU design consists of a horizontal vessel which has tubes for the fuel rods and cooling water (heavy water). Around these tubes is heavy water, which acts as the moderator to slow down the neutrons (Transactions of the American Nuclear Society). Heavy water consists of 2 atoms of deuterium (a non-radioactive isotope of hydrogen) and 1 atom of oxygen. Deuterium is much more efficient as a moderator than light water, thus allowing the use of natural uranium as a fuel. Special processing plants are used to separate heavy water from natural water (Bennet 1972).

1Corresponding author: Gabriela Raducan, E-mail: [email protected]; 11

mailto:[email protected]

mailto:[email protected]


AECL, the manufacturer of CANDU reactors, has provided these reactors to Canada and also to Argentina, India, Korea, Pakistan, and Romania (INTERNATIONAL ATOMIC ENERGY AGENCY, 1970).

Fourier Transform Infrared (FT-IR) spectroscopy is an excellent analytical method to measure D2O concentration. It shows a very good accuracy and it is very stable. FT-IR spectroscopy (Choi et al. 2003) is also a non-invasive and non-destructive technique without using any chemical reagents. The Infrared Spectrometer could measure D2O concentrations in water from 0.01% to almost 100%.

In this study the vibrational densimetry method (Instruction manual DMA 5000, 2007) was used, to determin the heavy water density and an empirical relationship between heavy water density and the deuterium concentration was establish, using heavy water standards obtained by the gravimetric method.

2. EXPERIMENTAL

2.1 Sample preparation

A total of 14 samples were prepared over the concentration from 0,797 to 91,096 % D2O, using D2O primary standard of 99.961% D2O obtained from AECL and appropriate amount of ultrapure water. To minimize the analytical error in sample preparation, all the samples were weighed by electronic balance with four decimals, Sartorius BP 210 S, which is quite sensitive. Heavy water must be handled so as to avoid direct contact with air because it depreciates very quickly, making isotopic exchange. Therefore, the samples, that are actually secondary standards, were loaded into syringes, were very good mixted and their density was immediately measured.

2.2 Work method

We make the secondary standards by the gravimetry method and compute the concentration of the secondary standards that have resulted using the formula:

aterultrapurewdardprimarys

aterultrapurewaterultrapurewdardprimarysdardprimariysdardondarysHeavywater mm

cmcmc

+×+×

=tan

tantantansec

The relationship between the deuterium concentration and the heavy water density was assumed to be a nonlinear equation which is of the form y=a-b/x where y is the sample’s deuterium concentration (% D2O mass) and x is the sample’s density (g/cm3). Making the secondary standards by the gravimetric method we can compute their deuterium concentrations.

Also, using the vibrational density method, we can measure the secondary standards’ density. The device which allowed to measure the sample’s density is a vibrational densimeter which has a tube with a known volume. The tube vibrates and the vibration period is measured, then the mass of the sample is computed.

12

Gabriela Raducan, Iuliana Piciorea

Knowing the volume of the sample, the device’s software computes the sample’s density, which is shown on the display.

Knowing the concentration and the density of the secondary standards, the two parameters, a and b, could be computed. Actually, we need only two equations (the equations between the deuterium concentration of the sample and its density for two secondary standards). The other 12 equations are used to validate this relationship.

3. RESULTS

The equation’s parameters which are determined and verified for 20 Celsius degrees are:

a=1027,91 %D2O and b= -1026,09265512 %D2O*g/cm3

So, the relationship between the deuterium concentration in heavy water and its density is of the form:

c(% D2O mass) = 1027,91% D2O mass – 1026,09265512 %D2O*g/cm3/ρ

where ρ is measured in g/cm3, and it is plotted in the figure 1.

The relationship between the deuterium concentration and the sample's density

0

10

20

30

40

50

60

70

80

90

100

0.999 1.001 1.0027 1.0057 1.0087 1.018 1.0263 1.0329 1.0399 1.0489 1.0611 1.0739 1.0812 1.0953

The density (g/cm 3)

Th

e d

eute

riu

m c

on

cen

trat

ion

(%

D2O

mas

s)

Figure 1. The relationship between the deuterium concentration of the secondary standards and their density.

The graph below (figure 2) shows a very good correlation betweem the deuterium concentration of the secondary standards and their density, the correlation coefficient being 0,9994.

13


The deuterium concentration against the densityR2 = 0.9994

0

10

20

30

40

50

60

70

80

90

100

0.98 1 1.02 1.04 1.06 1.08 1.1 1.12

Density (g/cm 3)

De

ute

riu

m c

on

ce

ntr

ati

on

(%

D2

O m

as

s)

Figure 2. The deuterium concentration of the secondary standards against their density

4. THE UNCERTAINTY ESTIMATION

To estimate the uncertainty of the measurement method we had to take into account the possible sources of uncertainty which could be measured: the secondary standard, the accuracy, the repeatability and the calibration of the device (EA -4/02 Guide, EURACHEM guide, SR Ghid ISO/CEI 98-3 :20100).

We achieved a heavy water secondary standard of 7,5211g primary standard with a concentration of 99,961% D2O mass and 11,2319g ultrapure water with a concentration of 0,0144 % D2O mass, using the gravimetric method. Computing the secondary standard concentration we obtained a concentration of 40,099 % D2O mass.

We calculated the uncertainty due to the secondary standard:The primary standard uncertainty which was of 0,005 % D2O mass for k=2 and 0,0025 % D2O mass for k=1, so the relative uncertainty was of 0,0025% D2O mass / 99,961 % D2O mass = 0,000025The primary standard derive in time: the primary standard had a concentration of 99,977 % D2O mass in 1988, then a concentration of 99,961 % D2O mass in 2009, which means that it was a mass variation of 0,016% D2O mass within 21 years, that is 0,0007 % D2O mass. We applied a rectangular distribution and we obtained

0004,03

0007,0 = % D2O mass

The relative uncertainty was 0,0004 % D2O mass / 99,961 % D2O mass = 0,000004.The uncertainty due to the balance was computed taking into account the calibration of the balance and the repetability, which is of 0,00034959g. The relative uncertainty is of 0,00004648, so that the total relative uncertainty due to the 14

Gabriela Raducan, Iuliana Piciorea

secondary standard was of 0,00005292. We measured the density of the heavy water secondary standard using the vibrational densimetry method and we calculated the deuterium concentration. We did this ten times and we computed the average value. Then we calculated the BIAS which was of 0,001 % D2O mass and the relative BIAS was of 0,0000249.

Also, we computed the standard deviation which was 0,025 % D2O mass and the relative uncertainty was 0,0006234.

The device calibration uncertainty was 0,00005 g/cm3 at k=2 so it was 0,000025 g/cm3 at k=1. The relative uncertainty was of: 0,000024.

The relative total uncertaintyThe factors that enter into the determination of relative total uncertainty are:

Description The relative uncertaintySecondary standard 0,00005292Accuracy (BIAS) 0,0000249Repetability (STDEV) 0,0006234The device calibration 0,0000240

The relative total uncertainty was 0,000788 for the value concentration of 40,0979, so the total uncertainty was of 0,025101 % D2O mass for k=1.

The total uncertainty for k=2 was:

Uc= 0,025101 % D2O mass x 2 = 0,050 % D2O mass.

5. SUMMARY AND CONCLUSIONS

The present study indicates that the deuterium concentration within a heavy water sample could be determined using an empirical relationship between heavy water density and the deuterium concentration.

The relationship between the deuterium concentration and the heavy water density is assumed to be a nonlinear equation of the form:

c(%D2O mass) = 1027,91%D2O mass – 1026,09265512%D2O mass*g/cm3/ρ

where ρ is measured in g/cm3.To determin the heavy water density, the vibrational densimetry method

was used. Also, the uncertainity of the method were estimated of 0,050 % D2O mass

for k=2. This value is too big, so the concentration of a heavy water sample could be determined with low accuracy.

There are two issues to be improved. The first issue is the empirical relationship between the deuterium concentration and the heavy water density. The second is the vibrational densimetry method which must determine the density of the sample with eight decimals instead of six.

15


AcknowledgeThis work is supported by the Research-Development Department, National

Research and Development Institute for Cryogenics and Isotopic Technologies – ICIT Rm. Valcea,

REFERENCES

1. Anton Paar Company, 2007, Instruction manual DMA 5000, Graz, Austria. 2. Bennet, L. W.,1972, Advanced HWR power plants, 15, Transactions of the

American Nuclear Society, p 51.3. Choi S.Y., Choo J., Chung H., Sohn W., Kim K., 2003, Feasibility of

Fourier Transform (FT) Infrared spectroscopy for monitoring heavy water concentration in pressurized heavy water reactor, 31, Vibrational Spectroscopy, 251–256.

4. EURACHEM guide, 2000, Uncertainty cuantification for the analytical measurements, The 2nd edition, United Kingdom.

5. International Atomic Energy Agency, 1970, Heavy-water reactors: bibliographical series, 37. Vienna, IAEAIX-XIII.

6. ISO/IEC Guide, 1989, Uses of Certified Reference Materials, 33, ISO, Geneva

7. SR Ghid ISO/CEI 98-3, 2010, Measurement uncertainty, 3 (GUM :1995)

16

Documents

Determination of the Deuterium Concentration From Heavy Water