ISSN 1066-3622, Radiochemistry, 2010, Vol. 52, No. 6, pp. 592–595. © Pleiades Publishing, Inc., 2010. Original Russian Text © A.M. Aleshin, B.A. Gusev, I.S. Orlenkov, 2010, published in Radiokhimiya, 2010, Vol. 52, No. 6, pp. 497–499.
Optimization of the Decontamination of Primary Circuits of Nuclear Power Installations A. M. Aleshin, B. A. Gusev*, and I. S. Orlenkov Alexandrov Research Institute of Technology, Federal State Unitary Enterprise, Sosnovy Bor, Leningrad oblast, 188540 Russia; * e-mail:
[email protected] Received June 18, 2010
Abstract—Two conflicting problems are solved in the course of chemical decontamination of the primary circuit of nuclear reactors: removing the maximal amount of radionuclides from the surfaces of the primary circuit equipment and minimizing the impact of decontaminating solutions on structural materials. A criterion for reliable determination of the completion of dissolution of surface corrosion deposits is suggested and substantiated. Since the criterion suggested is not related to the specific decontamination solutions, it can be applied to any decontamination procedure and chemical washing of the equipment in nuclear and thermal power industry. Keywords: nuclear reactor, primary circuit, water coolant, activated corrosion products, chemical decontamination, process completion criterion DOI: 10.1134/S1066362210060056
Chemical decontamination of the primary circuit equipment of a nuclear reactor is the most efficient way to improve the radiation situation and to reduce the personnel radiation exposure [1]. The main decontamination stages are injection of the appropriate chemical reagents into the circuit, dissolution of loose corrosion deposits containing corrosion-produced radionuclides and fission products, in-circuit circulation of the decontaminating solution at the predetermined temperature, and its replacement in the circuit by highpurity water [2, 3].
number of common features in the behavior of activated corrosion products (ACPs) which commonly contain 54Mn, 59Fe, 60Co, 51Cr, 95Zr, 95Nb, and 58Co radionuclides. Figure 1 shows the typical trend in variation of the ACP activity in decontaminating solu-
This paper considers the decontamination completion criteria based on the results of chemical decontamination of the primary circuits of ship nuclear reactors [4, 5] and of multiple forced circulation circuits (MFCCs) of the RBMK-1000 power units [6]. Prior to our investigations it has been assumed that in-circuit circulation of the solution leads only to dissolution of loose surface deposits on primary circuit equipment and to their release into water. After completion of dissolution of the deposits whose chemical and phase compositions are known [7], the specific activities of the radionuclides in the circulating solution reach their maximum values and remain at this level for a long time. The obtained experimental data were useful to gain new insight into the generally accepted concepts.
Fig. 1. Variation of the total specific activity in the course of chemical decontamination of MFCC of the RBMK reactor (standard procedure) in 1995: (1) injection of KNO3 + H2С2O4 solutions, (2) partial drainage, (3) injection of Н2О2, (4) repeated injection of KNO3 + H2С2O4 solutions, (5) partial drainage, (6) repeated injection of Н2О2, and (7) start of displacement of the decontaminating solution. Dashed line: approximation by the equation A = Amaxe–Oτ.
Analysis of the investigation results revealed a 592
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tions for standard procedure of chemical decontamination of MFCCs at the RBMK-1000 reactor of the 3rd power unit at the Leningrad Nuclear Power Plant (LNPP) in 1995. The relative changes in the total activity of all ACPs starting from the moment of injection of the first chemical reagents into MFCC are presented for comparison. The results obtained can be accounted for by assuming the occurrence of two opposite processes in the circuit after injection of the chemical reagents: release of loose deposits into the coolant with their partial dissolution and deposition of undissolved suspended ACPs on the internal surfaces of the circuit equipment. Depending on the rates of these processes, the monitored coolant parameters can increase, remain constant, or decrease. Hence, displacement of the decontaminating solution from the circuit should be started before the deposition will become prevalent over release of ACPs into the coolant. Figure 1 shows that the first injection of KNO3 and H2C2O4 solutions as components of the decontaminating solutions into MFCC leads to a sharp increase in the specific activities of the ACPs. Rapid increase in the specific activities of the radionuclides is difficult to explain only by dissolution of the deposits containing them. Rapid dissolution of loose deposits is in contradiction with the known mechanisms of the dissolution kinetics, even taking into account the high ability of oxalate ions to form complexes with ions of different chemical elements. It can be supposed that even partial dissolution of loose deposits distorted the continuity of their layer and led to its loosening and formation of separate suspended particles which were washed off by the coolant stream and became suspended. The above assumption is confirmed by trends in variation of the specific activity of the decontaminating solution. A decrease in the activity is in this case due to the fact that shutdown of the main circulation pumps (MCPs) leads to changes in the hydrodynamic conditions and to accelerated deposition of suspended particles on primary circuit surfaces. When the MCPs are switched on, the system returns practically to the initial state. Under constant conditions (solution composition, pump operation mode), the specific activities of the radionuclides monotonously decrease, as clearly seen from Fig. 1. If we exclude the time interval from 50 to 70 h (partial drainage period), all the experimental data up to the moment of the second partial drainage are satisfactorily fitted by a straight line (dashed line in Fig. 1). This fact can be naturally explained by deposition of suspended particles on the surfaces of the RADIOCHEMISTRY Vol. 52 No. 6 2010
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MFCC equipment with the deposition constant of 0.016 h–1 (Fig. 1), which can be considered as an average value over the different sections of MFCC. The data obtained fit in the following scheme of the decontamination process: detachment of loose deposits with their partial dissolution during injection of the decontaminating solution into MFCC; the subsequent removal of ACPs in the course of partial drainage and displacement of the decontaminating solution against the background of continuous deposition of the suspended particles on the equipment surfaces. Such scheme indicates that the process time is too long, so that favorable conditions are created for the formation of secondary deposits. As seen from Fig. 1, after the total specific activity of ACPs reaches its maximum value it remains the same for 10–20 hours, i.e., it is possible, to a first approximation, to speak of a certain steady state corresponding to the decontamination completion. In the ideal case, the specific activities of all the ACP radionuclides in several (e.g., three) successively taken coolant samples should be constant and hence the following equation should be satisfied: Ai+1/Ai + Ai+2/Ai+1 = α = 2,
(1)
where Ai, Ai+1, and Ai+2 are the specific activities of a radionuclide chosen as a reference for monitoring the decontamination, determined in three successively taken samples. Under real conditions, the Аi values are determined with a certain uncertainty δ, i.e., the obtained specific activity is not Ai but Ai ± Аiδ, where δ is the relative uncertainty of the determination method. Correspondingly, for a steady state in three successive measurements the coefficient α calculated by Eq. (1) will lie in a certain interval depending on the uncertainty. In our case, the maximal value of α is as follows: αmax = 2/(1 – δ).
(2)
Thus, the criterion of the completion of the chemical decontamination is as follows: k = Ai+1/Ai + Ai+2/Ai+1 ≤ αmax,
(3)
or, for several (n) reference nuclides monitored simultaneously, n
k = n–1∑(Ai+1/Ai + Ai+2/Ai+1)j ≤ αmax. j=1
(4)
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ALESHIN et al. Table 1. k values for reference radionuclides obtained in the course of chemical decontamination of MFCC of the 3rd power unit of LNPP (2001)
Fig. 2. Variation of the specific activities of reference nuclides in the course of chemical decontamination of MFCC of the RBMK reactor (suggested scheme) in 2001: (1) injection of KNO3, (2) injection of H2C2O4, (3) start of displacement of the decontaminating solution, (4) interruption of the displacement of the decontaminating solution because of failure of the coolant pump, (5) resumption of the displacement of the decontaminating solution, (6) switch of the displacement scheme, and (7) completion of the displacement of the decontaminating solution.
τ, h
Sample no.
0.35 1.25 3.10 5.55 7.10 8.25 9.15 10.05
1 2 3 4 5 6 7 8
51
Cr – – 44.3 57.1 15.6 2.3 1.9 2.2
54
Mn – – 6.6 34.0 33.2 2.7 1.5 2.1
k Fe 58Co – – – – 7.9 8.9 43.1 27.1 38.1 22.6 2.1 2.5 1.8 2.0 2.1 2.1 59
60
Co – – 9.9 24.2 20.6 2.3 2.0 2.1
95
Zr – – 23.6 26.0 5.0 2.2 2.0 2.2
Table 2. k values for reference radionuclides obtained in the course of chemical decontamination of MFCC of the 4th power unit of LNPP (2003) k
τ, h
Sample no.
3.5 9.1
1 2
Cr – –
10.4
3
1.8
2.3
2.3
2.3
2.3
3.3
12.6
4
2.8
2.6
2.7
2.8
3.0
2.0
14.8
5
2.0
2.1
2.1
2.1
2.9
2.1
16.3
6
1.8
1.9
1.9
1.8
2.0
2.1
18.0
7
2.1
1.9
2.0
1.9
1.9
2.0
51
54
Mn – –
59
Fe – –
58
Co – –
60
Co – –
95
Zr – –
In the latter case, determination of the moment of chemical decontamination completion becomes more representative. Figures 2 and 3 show the diagrams of chemical decontamination performed using the suggested criterion [7] and the curves reflecting variation of the specific activities of ACPs in the course of decontamination of MFCC of the 3rd power unit at LNPP in 2001 and of the 4th power unit in 2003. The results of calculating k for different radionuclides are given in Tables 1 and 2, where τ is the time elapsed from the start of injecting KNO3 into the circuit. Fig. 3. Variation of the specific activities of the reference nuclides in the course of chemical decontamination of MFCC of the RBMK reactor (suggested scheme) (2003): (1) injection of KNO3, (2) injection of H2C2O4, (3) start of displacement of the decontaminating solution, (4) switch of the displacement scheme, and (5) completion of the displacement of the decontaminating solution.
Taking into account the fact that the relative methodical uncertainty is about 20% (δ = 0.2), αmax obtained by Eq. (2) is 2.5. The k value calculated by Eq. (3) for all the radionuclides indicated in the table demonstrates that, ~10 h after the start of the acid decontamination, the transfer of loose deposits into the decontaminating solution was completed. RADIOCHEMISTRY Vol. 52 No. 6 2010
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The data obtained show that use of the suggested criterion decreases the time for MFCC treatment with the decontaminating solution by approximately an order of magnitude, on the one hand, and allows reaching the γ-radiation dose rates close to those attained using standard chemical decontamination procedure, on the other hand. The suggested method for determination of the moment of chemical decontamination completion [7] is not associated with the specific solutions used. It can be applied to any similar processes for decontamination of NPI primary circuits and for chemical washing of the equipment of nuclear and thermal power industry, with the only difference that in the latter cases the k value is calculated not from the activity of reference radionuclides but from the concentration of corrosion products. REFERENCES 1. Itige, H. and Takiguti, H., At. Tekh. Rubezhom, 2001, no. 6, pp. 10–24. 2. Brusov, K.N., Krutikov, P.G., Osminin, V.S., et al., Produkty korrozii v konturakh atomnykh stantsii (Corrosion Products in Circuits of Atomic Power Plants),
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Moscow: Energoatomizdat, 1980. 3. Ampelogova, N.I., Simanovskii, Yu.M., and Trapeznikov, A.A., Dezaktivatsiya v yadernoi energetike (Decontamination in Nuclear Power Engineering), Sedov, V.M., Ed., Moscow: Energoizdat, 1982. 4. NITI im. A.P. Aleksandrova 40 let (40 Years of the Alexandrov Research Institute of Technology), St. Petersburg: Morintekh, 2002. 5. Gusev, B.A. and Aleshin, A.M., Ekol. At. Energet., 2007, no. 1 (20), p. 29. 6. Aleshin, A.M., Gusev, B.A., Epikhin, A.I., et al., in Sbornik dokladov Mezhdunarodnogo nauchno-tekhnicheskogo soveshchaniya “Vodno-khimicheskii rezhim AES” (Coll. of Reports at the Int. Scientific and Technical Meet. “Water-Chemical Conditions at NPPs”), Smolensk NPP, October 13–17, 2003, Moscow, 2005, pp. 255–261. 7. Sedov, V.M., Nechaev, A.F., Doil’nitsyn, R.A., and Krutikov, P.G., Khimicheskaya tekhnologiya teplonositelei yadernykh energeticheskikh ustanovok: Uchebnoe posobie dlya vuzov (Chemical Technology of Coolants of Nuclear Power Installations: Textbook for Higher Schools), Sedov, V.M., Ed., Moscow: Energoizdat, 1985. 8. Aleshin, A.M., Gusev, B.A., Krasnoperov, V.M., and Orlenkov, I.S., RF Patent 2 331 125.
