Applied Mechanics and Materials Vols. 201-202 (2012) pp 545-548 © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.201-202.545
Study on the Effect of Containment Performance on Iodine Source Term Khurram Mehboob1, a, Cao Xinrong2, a, Majid Ali3,a, Rehan Khan4,b a
College of nuclear Science and Technology, Harbin Engineering universality, Harbin, China b
College of Underwater acoustics, Harbin Engineering universality, Harbin, China
1,a
[email protected], 2,a
[email protected],3,a
[email protected] 4,b
[email protected]
Keywords: TMI-2; PWR; Containment; Spray System; LOCA; Core Damage; Lodine; Source Term
Abstract. In this paper, the iodine source term is studied with the containment performance in normal, isolation, and emergency state of containment. For this study, a MATLAB computer-based program is developed, which simulates the iodine source term with time. The environmental iodine source term is determined with time normal, isolation and emergency state of containment. With the operation of Engineering Safety Features ESFs, the effect on the iodine source term has been observed. From the results, it is observed that the iodine is strongly dependent on the exhaust rate and significantly reduced with ESFs operation. Introduction In last few decades, substantial work has been done on source term evaluation. The TMI-2 accident initiates the research on the source term evaluation. U.S.NRC funded different research laboratories and organizations, i.e. BCL, BNWL ANL, ORNL, SNL ANS, and etc. for the development of severe accident and source term codes packages to analyze the nuclear accidents [1]. These efforts result in the development of effective computer codes, e.g. MAAP, ASTC, which not only evaluate the source term but also simulate the severe accident's scenarios [2]. A considerable research has been carried out in sixties to develop mathematical models for fission product behavior in containment and later verified by containment system experiments (CSE) [3] [4]. The results of these tests are in good agreement with the theoretical models [5]. Different techniques are used to analyze the source term. Some researcher suggests evaluating the source term with single code method, and some uses coupled code techniques. Lee and Yu have studied the source term of Washington 3-loops NPP assuming NUREG-1465, TID-14844, and WASH-1400 source term [6]. Huang et al, (2010) have investigated the In-vessel source term of Chinese 600MW PWR for SBO, LOFW and large break LOCA [7]. In the Indian pressurized heavy water reactor (PHWR), the source term was estimated by B. Chatter jee. He adopted coupled technique using ORIGEN2 for core inventory, SCDAP/RELAP5 for thermal hydraulics, ASTAC for FP transportation, CONTAIN for containment source term and COSYMA for dose calculations [8]. In this work, we have evaluated the iodine source term for two loops PWR. The TMI-2 reactor is considered as the reference rector. The source term has been evaluated concerning the containment performance. Modeling and simulation has been carried out by MATLAB developed code. This code takes iodine inventory, decay constants, accident conditions, and ESFs operation perimeters as its input. Dependency of radioiodine in different states of containment is also considered. Mathematical Model The Source term is defined as the amount of air born radioactivity which in term depends upon the burn up, fuel cycle, fissile contents, power level and operating schedule, etc. [9]. The source term depends upon the exhaust rate and containment ESFs. Thus, the release of radioactivity is governed by the performance of ESF of containments plus some natural phenomena i.e. decay deposition and re-suspensions, etc. The containment model described in WASH-1400 is as follows [10]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 221.212.116.50-14/08/12,13:43:16)
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dCi (t ) dt
dCir (t ) dt
ij i Ci (t ) rCri (t ) P (t ) j
uit Ci (t ) rCi r (t )
(1)
(2)
Where λij are removal factors P(t) is the precursor production, Ci (t) airborne concentration of ith particle (curie/m3), αi is leakage rate (m3/s), r is the re-suspension rate (s-1), uit is the deposition terminal velocity (m/s), Cir(t) is the activity due to re-suspension of ith particle. Initial boundary condition at the time of release is Cir = 0 at t = 0 sec; Ci= C0 at t = 0, where C0 is the release to the containment at the time of the accident. If we consider FL as the exhaust rate and ɳ the efficiency of the exhaust filtration system then the activity of the ith radioisotope released to the environment will be [9].
Ci (t ) Ci (t ) FL (1- ) The total activity released to the environment at the time t is given by the following equation t C (t ) FL (1- ) Ci (t )dt o
(3)
(4)
Methodology In order to evaluate the iodine source term, core inventory has been evaluated for 1100 reactor operation days. A MATLAB code is developed based on the mathematical model described above. The TMI-2 post accident conditions are considered. TMI-2 is 2720MWt two loops PWR. In this study, we consider the followings assumptions; The release fraction for molten fuel is taken from NURE-1465. Single volume containment is considered for source term evaluation. The recirculation is ignored throughout the simulation since, the reduction in iodine due to recirculation is extremely small and removal half time merely extends to several hours [10]. The iodine deposition velocity considered is 5.1×10-2cm/sec. Whereas the effective deposition velocity on the concrete is 6.7×10-4cm/sec while the iodine deposition velocity on concrete is 5.5×10-2cm/sec [11]. The re-suspension of iodine is taken as 2.3×10-5 per sec [12] while it is ignored during spray operation. In the isolation state, leakages occur due to penetration through cracks. Results and Discussion A reactor operating for several hundred hours contains almost all possible radionuclides. However, all these radionuclides are not needed to explore in case of release. The core inventory is directly proportional to the reactor operation schedule, fissile contents, power level, etc. Therefore, the iodine core inventory was calculated for 1100 operation days to get the maximum inventory [13]. The iodine source term is evaluated in normal, emergency and isolation state at different pH value of spray. Source term in normal emergency and isolation state of containment is shown in Fig 1. In the normal state, without spray operation, the radioiodine rapidly released through the ventilation stack because radioiodine diffuses towards the ventilation stack with higher rate. The saturation source term is achieved almost after 60 post accident days. In this state, 2.53E7Ci, 4.74E7Ci, of I-131 and I-133, are observed to release into environment. However, in emergency and isolation state, a linear leakage has been observed for iodine 131 and 133. 1.53E7Ci and 2.02E7Ci of iodine131 and 133 is observed for post three months of the accident. The iodine source term has also been studied with boric acid (pH 5.0) and caustic acid (Ph9.5) in determined states of containment. In
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the isolation state with boric acid spray (pH 5.0) significant decrease in environmental source has been observed. With boric acid spray the saturation source term has been achieved after 30 days. In this state, maximum release of I-131 and I-133 are 3.13E2Ci and 6.0E2Ci respectively. While, with caustic spray operation (pH 9.5) removal rate significantly increases. Maximum release of iodine has been observed between 2 to 10 hours after the release. In this phase, 17Ci of iodine131 and 34Ci of 133 has been accounted. After 12 days of release, the iodine 131 and 133 found to decrease to 7.0Ci and 10.2Ci respectively. Since the maximum release time of iodine is the in-vessel and Ex-vessels phase, at the end of these phases environmental release drastically decrease due to spray removal process. The iodine source term in the isolation state is shown in Fig 2.
b
a
c
Fig.1 Iodine source term due to (a) normal state of containment, (b) emergency state of containment, (c) isolation state of containment.
pH 5.0
pH 9.5
Fig. 2 Iodine source term due to isolation state of containment with ESFs operation at pH 5.0 and pH 9.5
pH 5.0
pH 9.5
Fig.3 Iodine Source term due emergency state of containment with ESF operation at pH 5.0 and pH 9.5
In emergency state with boric acid spray, the 2.2E4Ci and 4.2E4Ci of iodine131and133 has been observed. In this situation, the iodine source term is saturated after 48 post accident days. However, with caustic acid spray a rapid decrease in release is observed after 12 hours. The maximum release phase is found between 2 hours to 10 hours after the accident. During this phase 1.25E4Ci and 2.4E4Ci of iodine 131 and 133 has been observed respectively. The iodine source term due to emergency operation of containment is shown in Fig 3. Since in the normal state of containment the exhaust rate is higher than the emergency state, thus the diffusion of iodine towards the ventilation stack is greater as compare to diffusion in emergency operation of exhaust fans. This results a higher release of iodine to the environment. During removal
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with boric acid spray maximum releases of iodine131 and 133 observed to be 5.2E5Ci and 1.0E6 respectively. Similarly, with caustic acid spray maximum released is observed during 2.0 to 11.0 hours of release. In the normal state of containment, the release is significantly higher due to high ventilation rate. 3.73E4Ci and 7.23Ci of iodine 131 and 133 are observed in this phase. The iodine source term due to normal operation of containment is shown in Fig 4.
pH 5.0
pH 9.5
Fig. 4 Iodine Source term due normal state of containment with ESF operation at pH 5.0 and pH 9.5
Summary In this paper iodine source term in normal emergency and isolation state is compared. The effect of containment spray has also been studied in confined states of containment. The release of iodine-131 in containment isolation state with caustic acid spray is verified with the amount of iodine released in the TMI-2 accident which is 15Ci declared by NRC. Significant decrease in iodine source term has observed with caustic and boric acid spray. The iodine is found strongly dependent on the ventilation exhaust rate and spray system. References [1] K. Mehboob, X.R. Cao: “U.S.NRC Progress In Source Term Evaluation” proceeding of Asian Pecific power and energy Engineeinrg (APPEEC 2012) (under press) [2] K. Mehboob, X.R. Cao: ICCSIT 2011, vol.8, (2011) No. CFP1157E-PRT. [3] BNWL-268: “A review of mathematical models for for predicting spray removal of fission product in reactor conatainment vessel” battelle northwest laboratories 1973. [4] BNWL-1244: “Removal of Iodine and particles from containment atmosphere by sprays” containment system experiment interium report, Battele North West Liboratories, 1970. [5] BNWL-1457: “natural transport Effects on the fission producat behaviour in the containment system experiments” ACE research and development report, 1970. [6] M. Lee, Y.C. Ko: Nuclear Engineering and Design, vol.238 (2008), pp.1080–1092 [7] G.F. Huang, L.L. Tong, J.X. Li, X.W. Cao: Nuclear Engineering and Design, vol. 240 (2010), pp. 3888-3897. [8] B. Chatter jee et al: Nuclear Engineering and Design, vol. 240 (2010), pp.3529-3538. [9] International Atomic Energy Commission: IAEA Safety Report Series,(2008) No.53. [10] Wash- 1400, “Ractor Safety Study” appendix V , U.S.NRC, 1965 [11] ORNL-NSIC-4 “Behavior of iodine in the reactor building” ORNL, 1965 [12] J.E. Cline & associates, inc. “MSIV leakage iodine transport analysis” contract nrc-03-87-029, task order 75, U.S. nuclear regulatory commission, 1991. [13] K. mehboob, X.R. Cao: Advanced Materials Research, Vol. 1115, pp.482-484.
