RADIOACTIVITY LEVELS IN WATER AND PARAFFIN SAMPLES FROM THE DECOMMISSIONG VVR-S NUCLEAR REACTOR BY GAMMA-RAY SPECTROMETRY I. IORGA1, 2*, A. SCARLAT1, A. PANTELICĂ1, M. DRĂGUŞIN1 1
Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH), 30 Reactorului St., Magurele, Ilfov county, P.O. BOX MG-6, RO-077125, Magurele, Romania E-mail:
[email protected],
[email protected],
[email protected],
[email protected] 2 Faculty of Physics, University of Bucharest, P.O.B. MG-11, 077125, Bucharest-Magurele, Romania * Corresponding author: Ioan Iorga. E-mail:
[email protected] Received September 10, 2015
In view of radiological characterization of water from the reactor ponds and paraffin from the biological shield of the 2 MW VVR-S nuclear research reactor undergoing decommissioning in IFIN-HH, activity concentrations of 60Co, 134Cs, 137 Cs, 152Eu, and 241Am artificial radionuclides, 234Th (238U decay), 214Pb and 214Bi 238 ( U-226Ra series), as well as 228Ac, 212Pb, and 208Tl (232Th series), and 40K natural radionuclides have been determined by gamma-ray spectrometry. Their values were found to be lower than the clearance levels given by the radiological safety norms, except for 137Cs in a water sample of about four times higher radioactivity. Key words: gamma-ray spectrometry, nuclear reactor, radiological characterization, water, paraffin.
1. INTRODUCTION
The 2MW VVR-S nuclear research reactor from IFIN-HH at BucharestMagurele, Romania was built between 1955 and 1957 and operated until December 1997. During its life time, it was functional for 113,467 h, including 2,000 h at 3.0– 3.5 MW power. The total thermal energy produced was 9,510 MWd. The reactor decommissioning started in 2010 and will be completed in three phases until 2020 [1, 2]. The chosen decommissioning strategy was “immediate dismantling” [3]. The decommissioning activities of a nuclear reactor involve a large volume of radioactive wastes, which have to undergo to radiological characterization, in order to decide on their treatment, recycling, or disposal in conformity with the radiological safety norms [4–9]. Rom. Journ. Phys., Vol. 61, Nos. 5–6, P. 1079–1086, Bucharest, 2016
1080
I. Iorga et al.
2
In this paper, we proposed to examine by gamma-ray spectrometry the radioactivity levels of water from the reactor ponds and paraffin from the biological shield coming from the 2MW VVR-S nuclear research reactor undergoing decommissioning in IFIN-HH, in view of their radiological characterization. Studies on radiological characterization of the reactor block and reactor effluents pipelines in IFIN-HH were also reported in the literature [10, 11]. Figure 1 presents a general view of the VVR-S reactor hall, showing the reactor block, experimental assemblies, lead bricks and paraffin boxes.
Fig. 1 – General view of the 2MW VVR-S nuclear research reactor undergoing decommissioning in IFIN-HH.
3
Radioactivity levels in water and paraffin from the decommissiong VVR-S nuclear reactor 1081 2. EXPERIMENTAL 2.1. WATER SAMPLES
The investigated water samples were collected from six ponds of the VVR-S nuclear research reactor of IFIN-HH, as follows: four reactor storage ponds for the spent nuclear fuel (RSP1-RSP4), reactor cooling pond (RCP), and reactor tank for contaminated water (RCT). The RSP water radioactivity has arisen from the spent fuel assembles (S-36 and EK-10 types) before their return to the Russian Federation (in 2009, and 2012, respectively). The RCP water radioactivity resulted from the spent fuel assembles subjected to cooling for 1-2 years, before storage in RSP, while RCT water radioactivity consisted of radioactive effluents from decontamination or cleaning operations insight the reactor building (e.g. hot cells, pumps rooms, primary circuit) [1, 2, 7]. In view of the radioactivity analysis, water samples of 0.5 kg each were collected near the bottom of the pond, with a sludge deposit, to determine relatively higher radioactivity levels (Fig. 2). Water level Water sample Sludge level
Fig. 2 – Schematic view of the water collection from a nuclear research pond.
2.2. PARAFFIN SAMPLES
During the nuclear reactor operation, paraffin was used as biological shield for scientific experiments developed at the reactor. At the VVR-S nuclear research reactor, the paraffin, encapsulated in OLC steel boxes was placed in the reactor hall around the nuclear reactor block. In the cleaning phase of the reactor decommissioning process, a scanning over the paraffin boxes, using portable devices for contamination monitoring, revealed a light radioactive contamination. After a surface decontamination, the
1082
I. Iorga et al.
4
steel boxes were cut and the paraffin was removed and stored for several months on wood box-pallets in the reactor hall. Finally, the paraffin was transferred to the temporary warehousing tent for its radiological characterization [1, 2]. In view of the radioactivity analysis, paraffin samples, with mass around 0.5 kg each, were collected from the box-pallets as shown in Figure 3.
Up Back
Left Right Front
Down
Fig. 3 – Schematic view of the paraffin samples collection from the box-pallet.
2.3. RADIOACTIVITY ANALYSIS
The radioactivity levels of the water and paraffin samples were measured by high resolution low background gamma-ray spectrometry in the GamaSpec Laboratory of IFIN-HH [12–13]. Marinelly geometry for water samples and cylindrical geometry for paraffin samples were considered for the spectrometric counting. The experimental set-up was based on HPGe Ortec detector of 2.3 keV FWHM at 1332.5 keV of 60Co, and 30 % relative efficiency. The low background shield consisted in 10 cm thickness of Pb, coated with Sn and Cu foils of 1 mm, and 1.5 mm thickness, respectively; in addition, lead bricks of 10 cm thickness were put under the detector to reduce the 40K contribution from the floor. The background count rate was of 1.51–1.86 cps in the energies range 20–2700 keV. GAMMAW software (Dr. Westmeier, Mölln, Germany, Version 18.03/ Feb. 2007) was used for the spectra processing. Activity concentrations of 60Co, 137Cs, 134Cs, 152Eu, and 241Am artificial radionuclides, 234Th (238U decay), 214Pb and 214Bi (238U-226Ra series), 228Ac, 212Pb, and 208Tl (232Th series), and 40K natural radionuclides were determined by using
5
Radioactivity levels in water and paraffin from the decommissiong VVR-S nuclear reactor 1083
appropriate certified standards of similar matrix and geometry with the measured samples. In the case of 208Tl, the gamma-ray intensities were corrected for 212Bi decay branching ratio (35.93 %) [14]. Analytical uncertainties in the tables were calculated from statistical counting and detection efficiency uncertainties. Detection limits (LD) were assessed according with the formula: ALD = 2.71+4.65·σb, where ALD represents the peak area for LD calculation, and σb =
Ab ( Ab is the background area under the peak
of interest in the gamma-ray spectrum) [15, 16].
3. RESULTS AND DISCUSSION
Table 1 presents the activity concentrations (in Bq·kg-1) determined for Cs, 134Cs, and 60Co artificial radionuclides in the water samples collected from six ponds of the VVR-S reactor, as well as the clearance levels from the radiological safety norms in Romania [5]. The highest 137Cs activity concentration (about four times over the clearance level) was determined in the RSP-3 spent fuel storage pond, due to leakage from a cracked fuel assembly. Concerning the cooling water pond, 137Cs activity concentration slightly exceeds, in the uncertainty limits, the clearance level. For the rest of samples, 137Cs radioactivity was found to be lower than the clearance level. 60 Co and 134Cs presented the highest activity concentrations in the cooling water pond, but their values are lower than the clearance levels. A typical gamma-ray spectrum for the radioactively contaminated water from the VVR-S nuclear reactor is given in Figure 4. 137
Table 1 Activity concentrations of the artificial radionuclides determined in water samples collected from VVR-S nuclear reactor, Bq·kg-1 Water Water Spent Fuel Spent Fuel Radionuclide Pond 1 Pond 2 (RSP-1) (RSP-2)
Water Spent Fuel Pond 3 (RSP-3)
Water Water Spent Fuel Cooling Pond 4 Pond (RSP-4) (RCP)
Water Reactor Tank (RCT)
Clearance Level [5]
137
22 ± 2
25 ± 2
3320 ± 130
116 ± 5
770 ± 40
45 ± 3
8·102
134
Cs
< 0.7
< 0.7
< 3.7
< 1.2
15 ± 2
< 1.1
5·102
60
Co
< 1.2
0.8 ± 0.2
1.2 ± 0.4
2.8 ± 0.4
Cs
690 ± 20 8.5 ± 0.7
1·103
1084
I. Iorga et al.
6
Fig. 4 – Gamma-ray spectrum of a reactor cooling pond water sample.
Tables 2 and 3 present the activity concentrations (in Bq·kg-1) of the artificial and natural radionuclides determined in 60 paraffin samples (minimum, maximum, mean and standard deviations (SD) values). Table 2 contains the results obtained for 60Co, 137Cs, 152Eu, and 241Am artificial radionuclides, as well as 40K natural radionuclide. Table 3 shows the results obtained for gamma-emitting radionuclides from the 238U-226Ra series (234Th, 214Pb, 214Bi) and 232Th series (228Ac, 212 Pb, 208Tl). For most of the samples, detection limits (LD) could only be determined, their number and ranges being included in the tables. For mean value calculation, 0.5·LD was considered in such of the cases [17]. The tables also present the corresponding clearance levels given by the Romanian and IAEA radiological safety norms [5, 6]. A typical gamma-ray spectrum of the paraffin samples is given in Figure 5. Table 2 Activity concentrations of the artificial radionuclides and 40K natural radionuclide determined in paraffin samples, Bq·kg-1 Radionuclide Min. Max. N n LD LD Mean ± SD Clearance level [5]
60
Co 0.2 ± 0.1 7.2 ± 0.9 39 21 0.4–1.3 1.08 ± 1.39 1·103
137
Cs 0.2 ± 0.1 10 ± 2 45 15 0.4–0.9 1.04 ± 1.54 8·102
152
Eu 0.4 ± 0.2 2.9 ± 1.5 9 51 0.8–3.0 0.81 ± 0.37 7·103
241
Am 1.1 ± 0.5 11 ± 3 19 41 1.4–3.3 1.93 ± 1.65 5·101
40
K 35 ± 8 35 ± 8 1 59 8.1–29 8.4 ± 4.5 2·103
7
Radioactivity levels in water and paraffin from the decommissiong VVR-S nuclear reactor 1085 Table 3 Activity concentrations of the natural radionuclides from 238U series and 232Th series determined in paraffin samples, Bq·kg-1 238
Radionuclide Min. Max. N n LD LD Mean ± SD Clearance level
234
Th 8±6 12 ± 10 3 57 7.5–22 7.0 ± 2.3 3·105 [5]
U series 214 Pb 1.4 ± 0.7 14 ± 4 42 18 1.3–3.3 3.5 ± 2.5 1·103 [6]
232 214
Bi 1.3 ± 0.9 13 ± 3 43 17 1.3–3.3 3.5 ± 2.5 1·103 [6]
228
Ac 0.8 ± 0.2 3.7 ± 1.0 7 53 1.4–6.4 1.6 ± 0.7 1·104 [5]
Th series 212 Pb 0.8 ± 0.5 3.8 ± 1.0 23 37 0.6–2.8 1.1 ± 0.6 2·103 [5]
208
Tl 0.7 ± 0.5 3.2 ± 0.8 18 42 0.8–4.4 1.2 ± 0.5 1·103 [6]
Fig. 4 – Gamma-ray spectrum of a paraffin sample.
4. CONCLUSIONS
Activity concentration values of the artificial and natural radionuclides determined by low background high resolution gamma-ray spectrometry in water and paraffin samples from the 2MW VVR-S research nuclear reactor of IFIN-HH, currently in decommissioning process after forty years of operation, were found to be situated below the clearance levels given by the radiological safety norms, except for 137Cs in a spend fuel storage pond. The results obtained in this study provide valuable information to be used for the radiological characterization in the reactor decommissioning process.
1086
I. Iorga et al.
8
REFERENCES 1. IFIN-HH, Final Safety Report for VVR-S Reactor, 2005. 2. IFIN-HH, Decommissioning Plan of VVR-S Nuclear Research Reactor, Rev. 11, 2013. 3. IAEA, Decommissioning of Research Reactors: Evolution, State of Art, Open Issues, TRS 446, Vienna, 2006. 4. IAEA, Radiological Characterization of Shut Down Nuclear Reactors for Decommissioning Purposes, TRS 389, Vienna, 1998. 5. CNCAN, Norms of Radiological Safety (NSR-01), Bucuresti, Romania, 2002. 6. IAEA, Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards, General Safety Requirements, Safety Standards Series No. GSR Part 3, Vienna, 2014. 7. IAEA, Managing low radioactivity material from the decommissioning of nuclear facilities, (Technical reports series, ISSN 0074–1914); no. 462, STI/DOC/010/462, ISBN 978–92–0– 109907–5, Vienna, 2008. 8. M. Dragusin, D. Stanga, D. Gurau, E. Ionescu, Radiation Monitoring under Emergency Conditions, Rom. Journ. Phys. 59, 891–903 (2014). 9. Tz. Nonova, D. Stankov, Al. Mladenov, K. Krezhov, Radiological Characterization Activities During the Partial Dismantling of the IRT – Sofia Research Reactor Facilities, Rom. Journ. Phys. 59 (9–10) 976–988 (2014). 10. E. Ionescu, D. Gurau, D. Stanga, O. G. Duliu, Decommissioning of the VVR-S research reactor radiological characterization of the reactor block, Rom. Rep. Phys. 64, 387–398 (2012). 11. I. Iorga, D. Gurau, O. Sima, Analysis of radioactive effluents pipelines for contamination / activation, Rom. Journ. Phys. 59, 1043–1047 (2014). 12. http://www.nipne.ro/facilities/laboratories/gamaspec.php. 13. A. Ene, A. Pantelica, Characterization of Metallurgical slags using Low- level Gamma-ray Spectrometry and Neutron Activation Analysis, Rom. Journ. Phys. 56 (7–8), 1011–1018 (2011). 14. BIPM-5, Table of radionuclides Vol 7, 2013, http://www.nucleide.org/DDEP_WG/ DDEPdata.htm. 15. L.A. Currie, Limits for qualitative detection and quantitative determination, Application to radiochemistry, Anal. Chem. 40 (3), 586–593 (1968), DOI: 10.1021/ac60259a007. 16. ISO/FDIS 11929:2010 (E), Determination of the characteristic limits (decision threshold, detection limit and limits of confidence interval) for measurements of ionizing radiation – Fundamentals and applications. 17. R.C. Antweiler, H.E. Taylor, Evaluation of Statistical Treatments of Left-Censored Environmental Data using Coincident Uncensored Data, Sets: I Summary Statistics, Envir. Sci.Technol. 42 (10) 3732–3738 (2008).
