An Estimate of Carbon-14 Inventory at Wolsong Nuclear Power Plant in the Republic of Korea

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1 Journal of Nuclear Science and Technology ISSN: (Print) (Online) Journal homepage: An Estimate of Carbon-14 Inventory at Wolsong Nuclear Power Plant in the Republic of Korea Wook SOHN, Duk-Won KANG & Wi-Soo KIM To cite this article: Wook SOHN, Duk-Won KANG & Wi-Soo KIM (2003) An Estimate of Carbon-14 Inventory at Wolsong Nuclear Power Plant in the Republic of Korea, Journal of Nuclear Science and Technology, 40:8, To link to this article: Published online: 07 Feb Submit your article to this journal Article views: 260 View related articles Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at

2 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 40, No. 8, p (August 2003) TECHNICAL REPORT An Estimate of Carbon-14 Inventory at Wolsong Nuclear Power Plant in the Republic of Korea Wook SOHN *, Duk-Won KANG and Wi-Soo KIM Korea Electric Power Research Institute, Yuseong, Daejoen, , Korea (Received March 13, 2003 and accepted in revised form May 20, 2003) In this paper, carbon-14 inventory of Wolsong Nuclear Power Plant (NPP) which has CANDU type reactors was estimated for The carbon-14 inventory can be estimated based on a simple relation: [carbon-14 inventory]=[carbon-14 production] [carbon-14 emission]. The second term can be calculated whereas the third term should be measured. However, all the measured data for carbon-14 emissions at Wolsong NPP are not available. So an empirical relation is employed: [98.283%]=[100%] [1.717%]. From this relation, the theoretical values for the total production, emission and inventory of carbon-14 at Wolsong NPP for are estimated to be TBq, 11.9 TBq and TBq, respectively. Comparison of carbon-14 emission levels at Wolsong NPP and Pickering A NPP in terms of rates for theoretical productions and measured emissions of carbon-14 per unit power generated shows that although more carbon-14 is produced, less carbon-14 is released at Wolsong NPP except for Furthermore, comparison of the extend of carbon-14 emissions at both NPPs in terms of ratios of measured to theoretical carbon-14 emissions indicates that the control and regulation of carbon-14 emission are more effectively performed at Wolsong NPP. KEYWORDS: carbon-14 inventory, CANDU, heavy water, Wolsong Nuclear Power Plant, neutron capture reaction, pressurized heavy water reactor, airborne emission, waterborne emission I. Introduction Wolsong Nuclear Power Plant (NPP), which has four reactors of the CANDU Ra -6 type, is located in the southeastern part of the Republic of Korea (Fig. 1). The CANDU reactor is a pressurized heavy water reactor (PHWR) fueled with natural uranium and moderated and cooled by heavy water (D 2 O). The routine operation of this type reactor and its auxiliary process systems results in the production of a variety of solid, liquid and gaseous radioactive wastes. The design and layout of the plant including reactors and associated process systems ensure that releases of liquid and airborne wastes are minimized. Nevertheless, very small quantities of these wastes are released. Notable airborne radioactive emissions include tritium, carbon-14 (hereinafter designated C-14), noble gas radionuclides, radioiodines and particles. Among these, one of the principal gaseous radioactive emissions from the CANDU reactor is C-14. 1) Actually, Notification No of the Ministry Of Science and Technology (MOST) of Korea added C-14 to the items to be monitored around Wolsong NPP in Carbon-14 is produced mainly via a nuetron capture reaction of oxygen-17 (O-17) in the CANDU reactor. 1) CANDU production and emission rates of C-14 are higher than those of other types of commercial reactors such as light water reactors (pressurized water reactors and boiling water reactors) and gas-cooled reactors (see Table 1). The reasons for this are as follows. First, there is a large inventory of O-17, which arises from the large amount of D 2 O in the zone of high thermal neutron fluxes. Secondly, the isotopic abundance of O- Corresponding author, Tel , Fax , wsohn@kepri.re.kr a CANDU R (CANadian Deuterium Uranium reactor) is a trademark of Atomic Energy of Canada Limited (AECL). 604 Fig. 1 Location of Wolsong NPP in the Republic of Korea 17 in D 2 O (0.058%) is higher than in light water (0.037%) owing to the enrichment of this isotope in the distillation step of the heavy water manufacturing process (Girdler sulphide process). 1) Thirdly, the average thermal neutron flux in the CANDU reactor (order of n cm 2 s 1 ) is higher than that of light water reactors (order of n cm 2 s 1 ). Carbon-14 is a radioactive isotope of carbon which decays to nitrogen-14 (N-14) by emitting low energy β-radiation with an average energy of 49.5 kev and a maximum energy of 156 kev. The radioactive half-life of C-14 is 5,730 years. The principal pathway of the release of C-14 from the CANDU re-

3 An Estimate of Carbon-14 Inventory at Wolsong Nuclear Power Plant 605 Comparison of production rates of C-14 in various reac- Table 1 tors 2) Reactor Production rate per unit electric power TBq (GW e a) 1 LWR-PWR 0.7 LWR-BWR 1.0 CANDU (N 2 annulus gas) 50 CANDU (CO 2 annulus gas) 26 GCR-MGR 10 GCR-AGR 11 GCR-HTGR 3.0 actor is as stack emission of the cover gas in the Moderator Cover Gas (MCG) system, where the bulk of C-14 assumes the chemical form of CO 2 owing to the oxidative capacity of the recombiners in the MCG system. For this reason, C-14 imparts a radiological dose to human beings through ingestion or inhalation. At equilibrium, more than 99% of the dose contributed by C-14 comes from the ingestion pathway, and the rest from inhalation. 1) Because of the relatively long halflife and biological effects mentioned above, information on C-14 inventory is of great value for radiation protection in nuclear power plants. The purpose of this paper is to estimate C-14 inventory at Wolsong NPP for the period from April 1984 (the first year of commercial operation of Wolsong Unit 1) to April The estimate of C-14 inventory is expected to give useful information for optimizing of emission control and the management of radioactive wastes produced at Wolsong NPP. II. Process and Calculation of C-14 Production Rate Carbon-14 is produced in the CANDU reactor via neutron capture reactions such as 17 O(n,α) 14 C, 14 N(n, p) 14 C, and 13 C(n,γ) 14 C where atoms such as O-17, N-14 and C- 13 which are activated by neutrons are called target atoms. In order to calculate C-14 production rate in the CANDU reactor, it is sufficient to consider these three target atoms and their relevant neutron capture reactions. In addition, a material that includes one or more target atom is called a target material. Each target atom and its relevant neutron capture reaction may occur in multiple target materials. So, in order to distinguish reactions occurring in different target materials with the same target atom, the term stream is used instead of reaction. For example, the reaction 17 O(n,α) 14 C for heavy water, where O-17 and D 2 O are the target atom and the target material, respectively, can be expressed simply as Stream i where i stands for Stream ID number. In general, the annual prodcution rate of C-14 is esimated in terms of activity. The activity arising from C-14 produced for one year in Stream i (A i ) is calculated as follows (for the derivation see Appendix): A i = φ i σ i n i λtcf/100. (1) By summing activity arising from each stream, we obtain the total annual activity A arising from C-14 produced in the CANDU reactor: A = N T i=1 N T A i = φ i σ i n i λtcf/100, (2) i=1 where N T : Total number of streams (=17) φ i : Thermal neutron flux for Stream i (n cm 2 s 1 ) σ i : Neutron cross section of target atom in question for Stream i (cm 2 ) n i : Number of atoms of target atom in question for Stream i λ: Decay constant for C-14 (= ln(2)/t 1/2, t 1/2 : decay half-life of C-14, s) t: Time (1 year, s) CF: Annual capacity factor. b Here, φ i stands for the neutron flux for Stream i at 100% reactor power which is taken from the Analysis Report of Wolsong NPP. 3) The vaules for CF are provided by the site staff of Wolsong NPP and are listed in Table 2. The number of atoms of target atom in question for Stream i, n i, is calculated according to the given amount of target material (W i ); if given by weight (g), n i is calculated as n i = W i f i k i A v (3) 100M i and if given by volume (l), it is n i = W i 2,240 f ik i A v, (4) where W i : Amonut of target material for Stream i (g or l) M i : Molecular weight of target atom in question for Stream i (g) Table 2 Annual capacity factor (CF) data for each unit of Wolsong NPP (%) Year Unit 1 Unit 2 Unit 3 Unit b CF, which is defined as the percentage of nominal (or full) power that the reactor actually produces in a given time (here, one year), sometimes does not stand for an annual capacity factor but a capacity factor for a shorter period. In such a case, the shorter period should be reflected in the time, t. VOL. 40, NO. 8, AUGUST 2003

4 606 W. SOHN et al. f i : Isotopic abundance of target atom in question for Stream i (%) k i : Number of atom(s) of target atom in question included in one molecule of the target material for Stream i A v : Avogadro s number ( ). Observation of Eqs. (2), (3) and (4) indicates that in a given stream, all factors except for annual capacity factor are constant. Therefore, the annual production rate of C-14 in the CANDU reactor can be represented simply as a function of annual capacity factor only: A = N T i=1 A i = Const CF. (5) Seventeen streams contributing to the production of C-14 in the CANDU reactor are summarized along with other information in Table 3. Carbon-14 is produced in the moderator (MOD) system, Primary Heat Transport System (PHTS), Annulus Gas System (AGS) and fuel. In the MOD system, there are seven streams; 17 O(n,α) 14 CofD 2 O (Stream 1), 14 N(n, p) 14 CofN 2 in dissolved air (Stream 2), 17 O(n,α) 14 C of O 2 in dissolved air (Stream 3), 14 N(n, p) 14 C of nitrate (NO 3 ) (Stream 4), 17 O(n,α) 14 C of nitrate (NO 3 ) (Stream 5), 14 N(n, p) 14 C of nitrite (NO 2 ) (Stream 6) and 17 O(n,α) 14 Cof nitrite (NO 2 ) (Stream 7). 4) The isotopic abundance for O-17 of D 2 O (0.058%) is higher than that for O-17 of other target materials (0.037%) including oxygen, such as O 2,NO 2,NO 3, CO 2, and UO 2. This is owing to the enrichment of this isotope in the distillation step of the heavy water manufacturing process as mentioned previously. The amount of the target material (D 2 O) of Stream 1 ( g) is the largest of all the streams. An estimate shows that in the CANDU reactor, approximately 94.8% of the total C-14 production occurs in the moderator heavy water by the 17 O(n,α) 14 C reaction (Stream 1) because of the higher isotopic abundance and large inventory of O-17 in heavy water. 5) On the other hand, the PHTS has only one stream: 17 O(n,α) 14 CofD 2 O (Stream 8). As with Stream 1, the isotopic abundance of O-17 is 0.058% because the target material of Stream 8 is D 2 O. Although the total mass of D 2 O in the PHTS of the CANDU-6 reactor is g, only a small fraction (3.1%) of the total amount of the D 2 O resides in the high thermal neutron flux zones of the fuel channels. For this reason, only gofd 2 O is considered in the calculation of C-14 production via Stream 8. In addition, the average thermal neutron flux in the fuel channels ( n cm 2 s 1 ) is lower than in the calandria ( n cm 2 s 1 ) of the MOD system. 3) Calculation of C-14 production in the AGS depends on the kind of annulus gas used. Currently, all CANDU reactors use CO 2 as the annulus gas. However, in the early days of Pickering A NPP in Canada, N 2 gas was used. Table 3 Streams, target atoms, target materials and other data needed for calculation of C-14 production at each unit of Wolsong NPP System Stream Target Target ID No. atom material Amount of target material W I Unit 1 Unit 2 Unit 3, Unit 4 k i a) Occurrence Molecular weight Thermal Neutron cross ratio of target material neutron flux section f i (%) M i (g mol 1 ) φ i (n cm 2 s 1 ) σ i (cm 2 ) MOD 1 17 O D 2 O 264 Mg D 2 O 265 Mg D 2 O 262 Mg D 2 O N N l N b) V c) O O l O b) V c) N NO 3 20 µg NO b) 3 /kg D 2 O 20 µgno b) 3 /kg D 2 O 20 µgno b) 3 /kg D 2 O Mg D 2 O 265 Mg D 2 O 264 Mg D 2 O 5 17 O NO 3 20 µg NO b) 3 /kg D 2 O 20 µgno b) 3 /kg D 2 O 20 µgno b) 3 /kg D 2 O Mg D 2 O 265 Mg D 2 O 262 Mg D 2 O 6 14 N NO 2 20 µg NO b) 2 /kg D 2 O 20 µgno b) 2 /kg D 2 O 20 µgno b) 2 /kg D 2 O Mg D 2 O 265 Mg D 2 O 262 Mg D 2 O 7 17 O NO 2 20 µg NO b) 2 /kg D 2 O 264 Mg D 2 O 20 µgno b) 2 /kg D 2 O 265 Mg D 2 O 20 µgno b) 2 /kg D 2 O 262 Mg D 2 O PHTS 8 17 O D 2 O 205 Mg D 2 O 3.1% 207 Mg D 2 O 3.1% 205 Mg D 2 O 3.1% AGS Fuel 9 13 C CO kg CO O CO kg CO N N N N N N 16µgN b) /g U 238 g U/270 g UO Mg UO O UO Mg UO N N l N b) V c) O O l O b) V c) C C 10.4 kg C b) a) The number of atom(s) of target atom included in a molecule of the target material. For example, the number for 17 OofD 2 Ois1. b) These values are from Bagli et al. 4) c) V means that the amount of the target material is given by volume. In such a case, a value of 22.4 l mol 1 is used that corresponds to a volume occupied by one mole of target material in a gaseous state. JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

5 An Estimate of Carbon-14 Inventory at Wolsong Nuclear Power Plant 607 Streams associated with CO 2 are 13 C(n,γ) 14 C (Stream 9) and 17 O(n,α) 14 C (Stream 10) while streams associated with N 2 are 14 N(n, p) 14 C (Stream 11) and 14 N(n, p) 14 C (Stream 12). Stream 12 accounts for C-14 production from N 2 gas remaining as impurity or process gas in the AGS after converting N 2 annulus gas to CO 2. 4) The isotopic abundance (99.63%) and the neutron cross section for N-17 ( cm 2 ) are larger than those for C-13 (1.11% and cm 2 ) and O-17 (0.037% and cm 2 ). So, the production of C-14 in the AGS with N 2 is much larger than with CO 2.For Wolsong NPP, however, the annulus gases of all the units are CO 2 and thus we do not consider Steams 11 and 12 in this paper. Carbon-14 is also produced in the fuels due to 14 N(n, p) 14 C of N present as impurity (Steam 13), 17 O(n,α) 14 CofUO 2 (Steam 14), 14 N(n, p) 14 C of N 2 filling air (Steam 15), 17 O(n,α) 14 CofO 2 filling air (Steam 16), and 13 C(n,γ) 14 C (Steam 17). 4) The filling gas of the fuel elements contains up to 20% air (79% N 2 and 21% O 2 ) per current fuel bundle manufacturing specifications. Carbon-13 is present in the graphite coating of the fuel. Recently, it has been found that typical CANDU fuel pellets contain nitrogen impurity at the average level of about 9 16 µg/gu. 4) The flowchart for the calculation of C-14 production via all the streams mentioned above is given in Fig. 2. Using the data and information in Tables 2 and 3, we have calculated the annual production rates of C-14 at each unit of Wolsong NPP according to this flowchart. Calculation results of C-14 in each stream are given in Table 4. III. Estimate of C-14 Inventory and Its Implications The inventory of C-14 can be defined as the amount of C-14 that remains in a plant after any emission to the environment Yes Calculate the number of atoms for target atom n i = W i k i f i A v /100M i Start i (stream ID No.) = 1 Is W i Calculate production rate of 14 C A i = i i n i t CF/100 No (W i by Volume) Calculate the number of atoms for target atom n i = W i k i f i A v /2240 i = i + 1 during the period concerned. Thus, C-14 inventory in a reactor can be formulated simply as: [C-14 inventory] =[C-14 production] [C-14 release]. (6) The term [C-14 production] can be theoretically calculated provided that the needed data are given, as described in the previous section. However, we need to measure [C-14 release]. The releases of C-14 from the CANDU reactor occur via two main pathways; airborne and waterborne emissions. The waterborne emission, which may occur due to activities such as the deuteration-dedeuteration process, fuel exchanging, and storing spent IX resin columns, however, accounts for only 1% of the total C-14 emission (0.017% of the total C-14 production 4) ). For this reason, the measurement of C- 14 release from Wolsong NPP, which started in 1998, is carried out via the implementation of stack emissions monitoring only. c Actually measured data for the airborne emissions of C- 14 from each unit of Wolsong NPP are presented in Table 5. Actually measured data for C-14 emissions from Unit 1 and Unit 2 are not available for the periods of , and 1997, respectively. In such a case, we need to find an alternative method to estimate them. For this we investigate the state of the production, release and inventory of C-14 in the CANDU reactor under normal operation conditions. 1. Carbon-14 Source and Pathways to Release d (1) Moderator System The production of C-14 in the CANDU reactor occurs in the MOD system, PHTS, AGS, and fuel. Among the four systems, the MOD system is the largest contributor to C-14 production as stated before (approximately 94.8%). In the high neutron and photon fluxes of the calandria, C-14 is converted by radiolysis to a combination of D 14 2 CO 3 /D 14 CO 3 /14 CO 2 3 species. Of these species, the bicarbonate form is predominant because the ph of the MOD system is 7.0 under normal operation conditions. 1) Almost the 100% of anions (D 14 CO 3 and 14 CO 2 3 ) of these dissolved species in the moderator heavy water are removed on a mixed bed of stronganion and strong-cation exchange resins in the moderator purification system. However, some of the carbonate or bicarbonate species become off gases at the liquid/gas interfaces between the moderator heavy water and the helium cover gas, and may be equilibrated between these two phases. In the helium cover gas, carbon species are present mainly as carbon dioxide ( 14 CO 2 ). e Through this equilibration, C-14 may be transferred from the moderator heavy water into the cover gas. Therefore, if the C-14 concentration in the circulating A = A + A i No i > 17 Yes Output A End Fig. 2 Flowchart for calculation of C-14 production rate in the CANDU reactor c Since the airborne emission occurs via the stack of a plant, it is also called stack emission. In addition, because 14 CO 2 is the dominant species in airborne emission from the CANDU-6 reactor under normal operating conditions, it has been common practice to measure only 14 CO 2 in stack emissions to minimize costs. d This section deals with airborne emission of C-14 only. e Since D 2 and H 2 present in the helium cover gas are readily explosive, it is common practice to convert them into D 2 O and H 2 O, respectively, by adding excess O 2 in the recombiner. This oxidation also converts C-14 into 14 CO 2 and this is the reason why most C-14 released from stack emission assumes the form of 14 CO 2. VOL. 40, NO. 8, AUGUST 2003

6 608 W. SOHN et al. Table 4 Production rates of C-14 at each stream of each unit at Wolsong NPP (Bq a 1 ) JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

7 An Estimate of Carbon-14 Inventory at Wolsong Nuclear Power Plant 609 Table 5 Theoretical and measured C-14 inventories at Wolsong NPP Year Inventory (TBq a 1 ) Production rate (TBq a 1 ) Emission rate (TBq a 1 ) Theoretical Measured Theoretical Theoretical Measured Wolsong Unit E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E 03 Total for Unit E E+01 a) 4.21E E E+00 a) Wolsong Unit E E E E E E E E E E E E E E E E E E E E E E E E E E E E 03 Total for Unit E E+01 a) 1.14E E E+00 a) Wolsong Unit E E E E E E E E E E E E E E E E E E E E E E E E E 01 Total for Unit E E E E E 01 Wolsong Unit E E E E E E E E E E E E E E E E E E E E 02 Total for Unit E E E E E 01 Total E E+02 a) 6.94E E E+00 a) a) These values do not stand for true totals since some data needed for these totals are absent. heavy water is elevated, then the concentration of C-14 will also be elevated in the cover gas. Since the concentration of D 2 in the cover gas should be kept below 2 4% by volume, when the concentration of D 2 is over 4%, the cover gas is purged or vented thereby leading to release of C-14. This frequent purging and venting of the cover gas is a main pathway leading to high release rates of 14 CO 2 in the airborne emission. Furthermore, during the annual outage, there is usually a complete purging of the moderator cover gas and thus it may also be another VOL. 40, NO. 8, AUGUST 2003

8 610 W. SOHN et al. primary pathway contributing substantially to C-14 release. These two pathways substantially account for all the airborne emissions of C-14 that are estimated to be 1.5% of the total C-14 production. 5) Eventually, % of the total C-14 production, which corresponds to the C-14 production (94.8%) minus the airborne (1.5%) and the waterborne (0.017%) emissions of C-14, is immobilized by the resins as C-14 inventory in the moderator system. Besides these, heavy water may leak from the moderator system or its auxiliary system. Thus the C-14 in that heavy water will off gas to the air in the reactor building or the service building, and be exhausted from the station stack by the active ventilation system. However, its contribution is negligible under normal operating condition. (2) Primary Heat Transport System Carbon-14 production in the PHTS contributes 1.6% to the total C-14 production. This C-14 is present predominantly as carbonate ions ( 14 CO 2 3 ) at the prevailing ph of , and thus it is removed by the primary coolant purification system with a removal efficiency of nearly 100%. Consequently, the contribution of this system to the airborne emission of C-14 may be assumed to be negligible. The pressure in the PHTS is controlled by venting heavy water periodically from the main PHT circuit into the degasser condenser of its auxiliary system where the steam is condensed and gases dissolved in the heavy water are released into the condenser. The released gases are subsequently vented to the stack via vapor recovery dryers. This is a potential pathway for airborne emissions of C-14 to the environment. Carbon-14 emissions from the stack by purging and venting in the PHTS also contribute the total airborne emissions of C-14 as well as in the MOD system. However, the contribution from the PHTS is far smaller than that from the MOD system. Accordingly, we can conclude that the airborne emissions of C-14 from the PHTS are negligible and that all the C-14 produced in this system (1.6%) is captured by the resins as C-14 inventory. (3) Annulus Gas System The third contributor to C-14 emissions is the AGS where approximately 0.2% of the total C-14 production takes place. The AGS is purged to the atmosphere in cycles of once every 7 12 days to lower the dew-point and ensure the system leak detection capabilities, and also to lower radiation fields that build up during normal operation. During these purges, the entire inventory of C-14 (0.2%) produced in this system is released to the atmosphere. Since the present CANDU reactors use CO 2 as the annulus gas with added oxygen concentrations of 0.5 to 2% by volume, it is expected that most of the C-14 produced in this system remains mainly in the oxidized form of CO 2 and to a lesser extent in other forms. (4) Fuel The C-14 content in the fuel has been estimated to be 3.4% of the total C-14 production. The C-14 release as a result of fuel failure is another pathway for C-14 emission. Evidently, the release rate depends on the rate of fuel sheath failure. An estimate has shown, however, that for a fuel failure rate of 0.003% which is a normal case, C-14 released to the stack is negligible. 4) 2. Empirical Relation for C-14 Inventory Summarizing the above discussion, approximately 1.7% (1.5% mostly from the cover gas plus 0.2% from the AGS) and 0.017% (via the condenser cooling water duct) of the total C-14 production are discharged in the forms of airborne and waterborne emissions, respectively. On the other hand, approximately % of the total C-14 production is removed and retained by the liquid purification ion-exchange system (IX resin), and about 3.4% remains in the spent fuel. Accordingly, we can conclude that 1.717% of the total C-14 production is released and % remains, mostly kept by the spent resins at the plant as C-14 inventory on an average. This relation is presented graphically in Fig. 3. Thus, we substitute 100%, % and 1.717% for the terms [C-14 production], [C-14 inventory], and [C-14 release] in Eq. (6), respectively, which leads to in an empirical relation for the C-14 inventory: [98.283%] =[100%] [1.717%]. (7) This simple relation is our starting point for estimate of C- 14 emissions and inventories for the period where information on C-14 emission is not available or insufficient; it is derived for the CANDU reactor under normal conditions. Therefore, each percentage in the relation can be thought to designate the representative and normal ratio of the corresponding term. The rates for the production and emission of C-14 with the inventories of C-14 at Wolsong NPP for are listed in Table 5. The term emission rate f is divided into two categories; theoretical and measured. The former designates the emission rates calculated on the basis of Eq. (7) while the latter are the actually measured rates. Similarly, the term inventory also has two categories, theoretical and measured which stand for the C-14 inventories calculated with the values of theoretical and measured emission rates, respectively. Since all of the actually measured emission data are not available for Unit 1 and Unit 2, their totals for measured C-14 inventories are meaningless in Table 5. In contrast, for Unit 3 and Unit 4, all of the actually measured emission data for the periods concerned are available and hence their totals for measured C-14 inventories represent the true values of totals for C-14 inventories more than their totals for theoretical C-14 inventories do. Table 5 indicates that the totals for theoretical C-14 inventories are TBq for Unit 1 ( ), TBq for Unit 2 ( ), TBq for Unit 3 ( ) and TBq for Unit 4 ( ). Accordingly, it is estimated that the total for theoretical C-14 inventory at Wolsong NPP for the period of is TBq. On the other hand, the totals for measured C-14 inventories are TBq for Unit 3 ( ) and TBq for Unit 4 ( ). Two points should be noted in Table 5. First, the measured C-14 emission rates after 2000 are lower than the theoretical ones at all units with the exception of 2002 for Unit 3. If f In this paper, production, emission and inventory of C-14 are expressed in terms of both rate (Bq a 1 ) and amount (Bq). This is because we treat annual activity of C-14, which corresponds to the amount of radioactive nuclides produced for the period of one year. JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

9 An Estimate of Carbon-14 Inventory at Wolsong Nuclear Power Plant 611 Fig. 3 Schematic for the production and emission of C-14 in the CANDU reactor we assume the theoretical C-14 emission rate represents an expected value for C-14 emission rate of the CANDU reactor under normal operation conditions in the year concerned, the lower emission rates after 2000 indicate that there were some improvements in operation practices and/or conditions at Wolsong NPP in Actually, the staff of Wolsong NPP started to reduce the total service time of purification ion exchange resins from over 120 days to about 80 days in Thus, this reduction is inferred to have resulted in the improvement of effectiveness of the resins to capture C-14, and thereby lowering the C-14 emission levels. This fact suggests that the theoretical C-14 emission rates provide the criterion to determine whether or not measured C-14 emission rates are normal, and thus whether or not the operation practices and/or conditions should be improved. Furthermore, the selection of a numerical value of 1.717% as [C-14 release] in Eq. (6) seems to be reasonable. Secondly, the differences between totals for the theoretical and measured C-14 inventories are very small. This may be explained by the fact that the ratio of emission rate to the total production for C-14 is very small (1.717%) under normal operation conditions. Thus, when actually measured data for C-14 emission are not available, we may use the theoretical C-14 emission rates instead of the actually measured ones. 3. Comparison of Wolsong and Pickering A NPPs For comparison of plant capacity to produce or release C-14 at both Wolsong and Pickering A NPPs, normalized quantities are calculated: rates of C-14 production or emission per unit electrical power generated. The normalized rate of C-14 production for a specific reactor is independent of its operation conditions. In other words, the rate of C- 14 production per unit electrical power generated at the reactor is constant. This is because the processes by which C-14 is produced in the reactor are uniform throughout the operation. On the other hand, the normalized rate of measured C-14 emission may depend on the relevant operation conditions such as the frequencies of purging, venting and leaking of the moderator cover gas which are not uniform throughout reactor operation. Rates and normalized rates of theoretical C-14 productions, and of both theoretical and measured C-14 emissions at both Wolsong and Pickering A NPPs for selected years are presented in Table 6. The normalized rates of C-14 productions for Wolsong NPP cluster around 38 TBq (GW e a) 1 while those for Pickering A NPP are around 28 TBq (GW e a) 1. g This difference implies that Wolsong NPP produces more C-14 per unit electrical power generated than Pickering A NPP does. Heavy water inventories of the MOD system and the PHTS, the major sources for the production of C-14, are almost identical in each unit of both Wolsong NPP ( and g) and Pickering A NPP ( and g). On the contrary, thermal neutron fluxes at the two systems of each unit of Wolsong NPP ( and n cm 2 s 1 ) are higher than those of Pickering A NPP ( and n cm 2 s 1 ). It can be concluded, therefore, that these higher thermal neutron fluxes explain the higher normalized production rate of C-14 at Wolsong NPP. However, although more C-14 is produced per unit electrical power generated at Wolsong NPP, its measured C-14 emission rates per unit electrical power generated except for 1999 are less than at Pickering A NPP for the period concerned. Even the normalized rate of measured C-14 emission in 1999 at Wolsong g Since the conversion of annulus gas from N 2 to CO 2 was completed in 1991 in Pickering A NPP, its normalized C-14 production rate shows rather higher value in Thus we have excluded this rate from the discussion. VOL. 40, NO. 8, AUGUST 2003

10 612 W. SOHN et al. Table 6 Rates and normalized rates of theoretical C-14 productions, and of both theoretical and measured C-14 emissions at Wolsong and Pickering NPPs for selected years Plant Wolsong Pickering A b) Year Electrical power generated a) (GW e h) Production (TBq) Normalized production TBq (GW e a) 1 Emission (TBq) Normalized emission TBq (GW e a) 1 Measured Theoretical Measured Theoretical , , , , , , , , , , , , a) These values are the sums of the values for the electrical power generated (GW e h) of all the units of the plants in the year concerned. The electric power generated for a unit is calculated as capacity factor of the year concerned nominal capacity (MW e ) 365 d 24 h The nominal capacity factors for Wolsong Units 1, 2, 3, and 4 are 679, 700, 700, and 700 MW e, respectively. b) Values for Pickering A NPP are calculated using data from Bagli et al. 4) Pickering A Wolsong 7 6 ratio Year Fig. 4 Ratios of measured to theoretical C-14 emission rates Wolsong and Pickering A NPPs NPP is smaller than the average of the normalized ones of measured C-14 emissions at Pickering A NPP. The extent of the emission of C-14 is estimated in terms of ratio of the measured to the theoretical C-14 emission rates. As before, the theoretical C-14 emission rate that corresponds to 1.717% of the total production of C-14 in each year is used as the reference value to assess the extent of C-14 emission. The ratios of the measured to the theoretical C-14 emission rates calculated for both Wolsong and Pickering A NPPs are presented graphically in Fig. 4. Figure 4 implies that C-14 emission levels at Wolsong NPP are considerably lower than in Pickering A NPP, especially after This also supports the fact that the improvements in the operation practices in 1999 lowered C-14 emission levels at Wolsong NPP. Therefore, it may be concluded that the control and regulation of C-14 emissions are carried out more effectively in Wolsong NPP, especially after IV. Conclusion Carbon-14 inventory at Wolsong Nuclear Power Plant in the Republic of Korea was estimated. The C-14 inventory could be estimated based on a simple relation, [C-14 inventory]=[c-14 production] [C-14 emission] where the term C-14 production was calculated while the term C-14 emission was measured. For Wolsong, all of the actually measured data for C-14 emission rates in the period concerned were not available, and thus an empiri- JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

11 An Estimate of Carbon-14 Inventory at Wolsong Nuclear Power Plant 613 cal relation, %=100% 1.717% was employed where %, 100% and 1.717% stand for C-14 inventory, C-14 production and C-14 emission, respectively. Using this relation, the theoretical values for the total production, the total emission and the total inventory of C-14 at Wolsong Nuclear Power Plant for the period of were estimated to be TBq, 11.9 TBq and TBq, respectively. Comparison of some theoretical C-14 emission rates with some measured C-14 emission rates pointed out that the former was very useful because they could provide the criterion for assessment of actual C-14 emissions. Therefore, the introduction of a numerical value of 1.717% as the ratio of the theoretical C-14 emission to the total C-14 production seemed to be reasonable. Also, C-14 emission levels at Wolsong and Pickering A plants were compared in terms of the theoretical production rate and the measured emission rate of C-14 per unit power generated. The results showed that, although more C-14 was produced per unit electrical power generated at Wolsong, its measured rates for C-14 emissions per unit electrical power generated were lower than at Pickering except for Furthermore, the extents of C-14 emissions in both plants were also assessed by introducing ratios of the measured C-14 emissions to the theoretical ones. The ratios for Wolsong were considerably lower than those for Pickering A, indicating that the control and regulation of C-14 emission were more effectively performed at Wolsong. Acknowledgments The authors wish to thank Mr. Tae Gun Ha, Mr. Young Seock Park, Mr. Keun Won Lee, Mr. Hwa Jun Lee and Mr. Yeon Duck Han, on the staff of Wolsong Nuclear Power Plant (Korea Hydro & Nuclear Power Co., Ltd.) for furnishing valuable information on the plant. References 1) S. R. Peterson, P. A. Davis, R. R. Rao, Modeling Doses from Tritium and 14 C in the Environment, RC-1951, Atomic Energy of Canada Limited (AECL), (1997). 2) U. H. Kim, K. B. Sung, D. W. Kang, The Development of Monitoring Techniques of Radiocarbon from Heavy Water Reactor, Technical Report, TR.95ZJ14.J , Korea Electric Power Research Institute (KEPRI), (1998), [in Korean]. 3) K. Aydogdu, K. Y. Kim, Y. I. Kim, Radiation Protection and Shielding, AR-003, Analysis Report Wolsong NPP, Korea Atomic Energy Research Institute (KAERI), (1995). 4) K. S. Bagli, M. E. Brett, S. K. Sood, An Estimate of Carbon-14 Inventory at OPG, Nuclear Sites: , N-REP R00, Ontario Power Generation, (1999). 5) A. J. Elliot, S. Vijayan, Agreement for the Supply of Technical Support for Carbon-14 Removal Technologies for Wolsong Nuclear Power Plant Between Korea Electrical Power Corporation and AECL, Atomic Energy of Canada Limited (AECL), (2002). Appendix The amount of C-14 produced from Stream i for time dt, dn p,i, can be formulized as: dn p,i = φ i σ i n i dt, (A1) where φ i : Average thermal neutron flux for Stream i (n cm 2 s 1 ) σ i : Neutron cross section of target atom in question for Stream i (cm 2 ) n i : Number of atoms of target atom in question for Stream i. For time dt, C-14 also decays by the amount dn d,i : dn d,i = λn i dt, (A2) where λ: Decay constant for C-14 N i : Number of C-14 nuclides present in Stream i. Therefore, the net amount of C-14 produced for time dt by the Stream i, dn i,is: dn i = dn p,i dn d,i = φ i σ i n i dt λn i dt (A3) which can be rewritten as dn i + λn i = φ i σ i n i. (A4) dt Since this equation is a non-homogeneous partial equation, its solution is N i = φ i σ i n i (1 e λt )/λ. (A5) In order to obtain an annual activity of C-14 arising from Stream i, A i, we should multiply Eq. (A5) by the decay constant of C-14, λ, and take t as 1 year (in seconds): A i = N i λ = φ i σ i n i (1 e λt ). Since e x 1+x, replacing e λt by 1 λt: A i = φ i σ i n i λt. (A6) (A7) Equation (A7) yields the annual activity of C-14 arising from Stream i for a reactor running at full power because φ i stands for neutron flux at the 100% reactor power. However, reactors do not always operate at full power throughout the year. Thus, in order to obtain the period of time during which the reactor actually is running at its full power, we should multiply Eq. (A7) by the capacity factor (CF) that is the percentage of nominal (or full) power that the reactor actually produces in a given time (here one year): A i = φ i σ i n i λtcf/100. (A8) VOL. 40, NO. 8, AUGUST 2003