Development of a method for the estimation of low amountof plutonium in 200 L waste drums in presence of high amountof b c activity due to 137Cs and 60Co

Sarbjit Singh • Veena Sagar • Amol Mhatre

Received: 23 October 2012 / Published online: 17 November 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract The waste drum monitoring system based on

HPGe detector was used to study its performance for the

estimation of low amounts of plutonium in presence of

high activity of 137Cs and 60Co. The counting was carried

out by keeping amount of plutonium constant at 100 mg

level and varying the count rate for the c rays of 137Cs and60Co. Present study has shown that the estimation of low

amount of 239Pu in a waste drum can be carried out using

129 keV c ray in the presence of 137Cs up to an activity

level of 16 mCi and in the presence of 60Co up to an

activity level of 8 mCi.

Keywords Waste drum monitoring � c Ray scanning

system � Plutonium � 239Pu � 240Pu � 241Pu � a Active waste �HPGe detector � 60Co � 137Cs � Dead time correction

Introduction

The quantitative assay of plutonium in the waste produced

during its handling is essential for nuclear material

accounting as well as for the assessment of radiological

hazards. The allowed limit for disposal of a active waste as

specified by I.A.E.A. [1] is 4,000 Bq/g which is equivalent

to *87 mg of 239Pu in a waste drum of 50 kg. Therefore

each drum has to be monitored for its a activity so that it

can be classified according to this limit. The a active waste

produced in a reprocessing plant may contain high b cactivity due to the presence of long lived isotope such as137Cs. Also some of the a active waste drums were found to

contain 60Co. The high count rate due to 137Cs and 60Co

increases dead time of the system and introduces a pile-up

effect which makes it difficult to estimate plutonium

amount in presence of 137Cs and 60Co. In our earlier

experiments, the performance of the system was studied [2]

for the estimation of plutonium at 4 and 1 g level in

presence of high b c activity of 137Cs. The work was

extended for estimation of *100 mg of Pu, the amount

closer to the allowed limit as specified by I.A.E.A. Sys-

tematic studies were carried out for plutonium estimation

in the presence of high b c activity of 137Cs and 60Co and

the results obtained are reported in the present work.

Experimental

Segmented c ray scanning system using a 20 % HPGe

detector system was set up [3] for the detection and estima-

tion of radionuclides present in 200 L waste drum. The

estimation of 239Pu was carried out using 129.3, 203.5, 375.0

and 413.7 keV c rays. The amounts of 240Pu (160.3 keV) and241Pu (237U) (148.6, 208.0 and 332.4 keV) were estimated

using their respective c rays.

Estimation of plutonium in presence of high b c activity

of 137Cs

The estimation of mg amount of plutonium requires

counting of drums for a long time due to very low abun-

dances of plutonium c rays. Hence, the conditions were

simulated for the presence of 97 mg of plutonium in 200 L

waste drum by keeping a suitable standard source of

S. Singh (&) � A. Mhatre

Radiochemistry Division, Bhabha Atomic Research Centre,

Mumbai 400085, India

e-mail: sarbjitsingh@yahoo.com

V. Sagar

Radioanalytical Chemistry Division, Bhabha Atomic Research

Centre, Mumbai 400085, India

123

J Radioanal Nucl Chem (2013) 297:149–152

DOI 10.1007/s10967-012-2322-9

plutonium near the detector. Since the actual handling of

the drums with high b c activity of 137Cs is difficult in the

laboratory, therefore counting was carried out by simulat-

ing the conditions for the presence of high activity of 137Cs.

The amount of plutonium was kept constant at 97 mg

and the count rate of 661.7 keV c ray of 137Cs was varied

from 0 to 3,000 counts per second (CPS). Gamma ray

spectrum analysis was carried out using DSPG [4] program

so as to eliminate any manual bias in the detection of peaks

and calculation of their areas. Dead time correction [5] was

applied to arrive at the corrected count rate for all the crays.

Standard drum containing known amount of 137Cs was

prepared based on the random distribution studies [6]

carried out earlier. This drum was counted on the c ray

scanning system for the emission rates and attenuation

correction [3]. The count rate of 137Cs for the known

activity (20 lCi) was obtained after applying the appro-

priate corrections for the dead time and attenuation. The

count rate of this standard drum was used to calculate the

activity of 137Cs present in the waste drum.

Estimation of plutonium in presence of high b c activity

of 60Co

The c ray energies (1,173.2 and 1,332.5 keV) of 60Co are

higher as compared to the c ray energy (661.7 keV) of137Cs. Hence the effect on the c rays of plutonium due to

the Compton of 60Co c rays will be much more than due

to the c ray of 137Cs. This will make it further difficult to

estimate plutonium in the presence of high amount of 60Co.

As mentioned above in the case of studies with 137Cs,

conditions were simulated for the presence of 97 mg of

plutonium and high activity of 60Co in 200 L waste drum.

The count rate of 1,332 keV c ray of 60Co was varied from

0 to 850 CPS and amount of plutonium was kept constant.

Standard drum containing known amount of 60Co was

prepared based on the random distribution studies [6]

carried out earlier. This drum was counted in a similar

manner as in case of 137Cs. The count rate of 60Co for the

known activity (10 lCi) was obtained after applying the

appropriate corrections for the dead time and attenuation.

The count rate of this standard drum was used to determine

the activity of 60Co present in the unknown waste drum.

Results and discussion

Effect of 137Cs activity on the estimation of plutonium

The plot of count rates for the 129.3 keV c ray of 239Pu as a

function of 137Cs count rate is shown in Fig. 1 and the plot

of the deviations in the count rate of 129.3 keV c ray as a

function of 137Cs count rate is shown in Fig. 2.

The deviation in the count rate for the 129.3 keV c ray

was found to be \25 % up to 2,000 CPS of 137Cs. The

deviation was found to increase slowly as the count rate of137Cs was increasing and was becoming erratic beyond

2,000 CPS. This deviation is due to the increase in the total

count rate of 137Cs and also due to the slope of the back

scatter peak (*185 keV). However, 129.3 keV c ray peak

was found to be less affected as it does not lie on the back

scatter peak or Compton (*476 keV) of the 137Cs spec-

trum (Fig. 3).

The 203.5, 375.0 and 413.7 keV c ray peaks of 239Pu

could not be seen in presence of high count rate (CPS) of137Cs. This may be due to the reason that these peaks are

lying in between the back scatter peak and Compton peak

of 137Cs. Variation in the slope of the spectrum introduces

additional error in the calculation of the peak areas of these

c rays. The statistical deviation also increases due to the

increase in the count rate of 137Cs.

It was possible to determine the amount of 241Pu using

148.60 keV c ray up to 500 CPS of 137Cs, but the statistical

deviation was very large due to its low abundance and low

amount of 241Pu. The 160.31 keV c ray peak of 240Pu could

not be seen as it is near the back scatter peak of 137Cs.

Effect of 60Co activity on the estimation of plutonium

Figure 4 gives the count rates for 129.3 keV c ray of 239Pu

as a function of 60Co count rate. The 129.3 keV c ray was

seen up to 800 CPS for 1,332 keV c ray of 60Co. The

375.0 keV c ray of 239Pu was seen up to 600 CPS and

413.7 keV c ray of 239Pu was seen up to 100 CPS only. The

203.5 keV c ray of 239Pu was not seen above 50 CPS of60Co. Also 160.3 keV c ray of 240Pu and 148.6 keV c ray

and 208.0 keV c ray of 241Pu were not seen above 50 CPS

of 60Co.

Fig. 1 Plot of the count rates for 129.3 keV c rays of plutonium as a

function of 137Cs count rate

150 J Radioanal Nucl Chem (2013) 297:149–152

123

Plot of the deviations in the count rate of 129.3 keV cray as a function of 60Co count rate is shown in Fig. 5.

Deviation in the count rate of 129.3 keV c ray was found to

be \15 % up to 600 CPS for the 1,332 keV c ray of 60Co.

Deviation in the count rate of 375 keV c ray was found to

be \30 % up to 600 CPS of 60Co whereas for 413 keV c

ray it was found to be \10 % up to 100 CPS of 60Co. The

deviation was found to increase slowly as the count rate of60Co was increasing beyond 100 CPS.

Gamma ray spectrum of plutonium in presence of

varying count rates of 1,332 keV c ray of 60Co is shown in

Fig. 6. The deviation observed in the count rate of pluto-

nium c rays is due to the increase in the Compton and back

scatter peak (*209 keV) of 60Co. However, 129.3 keV cray peak was found to be less affected as it does not lie on

the back scatter peak or Compton of the 60Co spectrum as

seen in Fig. 6.

The 203.5 keV c ray of 239Pu could not be seen in

presence of high count rate of 60Co because it is very close

to the back scatter peak of 60Co. The 375.0 and 413.7 keV

c rays of 239Pu could not be seen in presence of high count

rate of 60Co because these peaks are lying in between the

back scatter peak and Compton peak of the 60Co.

It was observed that 800 CPS of 60Co corresponds to an

activity of 8 mCi present in a 200 L drum. The 375.0 and

413.7 keV c rays of 239Pu have not been found suitable

when the amount of plutonium is small. However, the

estimation can still be carried out using 129.3 keV c ray of239Pu with 40 % variation up to 8 mCi of 60Co at 100 mg

level.

Comparison of results for the estimation of plutonium

with varying CPS of 137Cs and 60Co

Table 1 shows the comparison of results for % variation in

the count rates for c rays of 239Pu with varying CPS of137Cs and 60Co activity. Beyond these count rate limits of137Cs or 60Co, the deviations in the count rates for c rays of

plutonium isotopes were found to increase. It was observed

[2] that during studies on the estimation of plutonium in

presence of 137Cs, the deviations at 4 and 1 g level were

lower than that at 100 mg level. Variation in the count rate

was found to increase for higher count rate of 137Cs and

Fig. 2 Plot of the deviations in the count rate of 129.3 keV c rays of

plutonium as a function of 137Cs count rate

200 400 600 800 1000 1200 14000.0

5.0x105

1.0x106

1.5x106 2052 CPS66

1.7

keV

of C

s-13

7

413

keV

375

keV

Com

pton

edg

e

1141 CPS

498 CPS

0.1 CPS

0.1 498 1141

239Pu = 91 mg

2052

Bac

k S

catt

er P

eak

208

keV

203

keV

413

keV

375

keV

129

keV

129

keV

129

keV

Co

un

ts p

er c

han

nel

Channel number

Fig. 3 Gamma ray spectrum of plutonium (*0.1 g) in presence of

varying count rates of 661.7 keV c ray of 137Cs

Fig. 4 Plot of the count rates for c rays of plutonium as a function of60Co count rate

Fig. 5 Plot of the deviations in the count rate of c rays of plutonium

as a function of 60Co count rate

J Radioanal Nucl Chem (2013) 297:149–152 151

123

with decrease in the amount of plutonium. It was observed

that 375.0 and 413.7 keV c rays are more affected as

compared to 129.3 keV c ray of 239Pu. Moreover 203.5,

375.0 and 413.7 keV gamma peaks of 239Pu were much

more affected at 100 mg level as compared to gram level.

The 375.0 and 413.7 keV c rays of 239Pu have not been

found suitable when the amount of plutonium is very small.

However, the estimation can still be carried out using

129.3 keV c ray of 239Pu with 25 % variation up to 16 mCi

of 137Cs at 100 mg level. The limit for the estimation of

plutonium in the presence of 60Co is low, which is because

of the high Compton and back scatter peak of 60Co. Further

studies are planned for the improvements in the results.

Conclusions

Present study has shown that the estimation of 239Pu (gram

level) [2] can be carried out in the presence of high amount

of 137Cs up to an activity level of 20 mCi with a variation

of ±10 % and up to an activity level of 40 mCi with a

variation of ±15 %. However, the estimation of 239Pu at

100 mg level can still be carried out using 129.3 keV c ray

of 239Pu with 25 % variation up to an activity level 16 mCi

of 137Cs. The estimation of 239Pu up to 100 mg level in the

presence of 60Co can be carried out using 129.3 keV c ray

of 239Pu with 40 % variation up to 8 mCi level of 60Co.

The detection limit in the presence of high b c activity may

improve if the amount of 239Pu is more.

Acknowledgments We are thankful to Dr. A. Goswami, Head,

Radiochemistry Division for the encouragement in carrying out this

work. We are also thankful to Equipment Electronics Services Section

of Radiochemistry Division for maintenance of the HPGe detector

system.

References

1. IAEA (1994) Classification of radioactive waste: a safety guide,

safety series no. 111-G-1.1. International Atomic Energy Agency,

Vienna, p 13

2. Singh S, Mhatre A, Sagar V (2012) J Radioanal Nucl Chem

294(1):7–11

3. Singh S, Sagar V, Mhatre A, Ramaswami A (2006) Report BARC/

2006/I/015. Bhabha Atomic Research Centre, Mumbai

4. Rattan SS, Madan VK (1994) Report BARC/1994/E/038. Bhabha

Atomic Research Centre, Mumbai

5. Singh S, Mhatre A (2009) DAE–BRNS symposium on nuclear and

radiochemistry, NUCAR-2009. Chemistry Department, SVKM’s

Mithibai College of Arts, Mumbai, 7–10 January 2009, p A1

6. Singh S, Mhatre A (2009) DAE–BRNS symposium on nuclear and

radiochemistry, NUCAR-2009. Chemistry Department, SVKM’s

Mithibai College of Arts, Mumbai, 7–10 January 2009, p E1

0 500 1000 1500 2000 2500 3000

Channel number

Co

mp

ton

ed

ge

Co

mp

ton

ed

ge

Bac

k sc

atte

r

1332

( 60 C

o)

1173

( 60 C

o)

59.5

4 ( 2

41A

m)

28 192 296 471 741

0.0

2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

1.2x106

Cou

nts

per

chan

nel

Fig. 6 Gamma ray spectrum of plutonium in presence of different

count rates of 1,332 keV c ray of 60Co

Table 1 Comparison of % variation in the count rates for c rays of

plutonium with varying CPS of 137Cs or 60Co

c, keV

(nuclide)

97 mg of plutonium 97 mg of plutonium

CPS of137Cs

%

Variation

CPS of60Co

%

Variation

129.3 (239Pu) 2,000 \25 600 \15

203.5 (239Pu) [50 Not seen

375.0 (239Pu) 500 \25 600 \30

413.7 (239Pu) [200 Not seen 100 \10

148.60 (241Pu) [50 Not seen

160.31 (240Pu) [50 Not seen

152 J Radioanal Nucl Chem (2013) 297:149–152

123