ML20155F165
| ML20155F165 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 10/31/1998 |
| From: | Beahm E, Carl Weber OAK RIDGE NATIONAL LABORATORY |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| CON-FIN-L-1918 NUREG-CR-6599, ORNL-TM-13555, NUDOCS 9811050329 | |
| Download: ML20155F165 (25) | |
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..
e NUREG/CR-6599 ORNL/TM-13555 l Iodine Volatility and pH Control l in the AP-600 Reactor W3
.'l 4
Prepared by C.F. Weber, E.C. Beahm I
Ork Ridge National Laboratory
.I
>b Prepared for U.S. Nuclear Regulatory Commission M
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NUREG/CR-6599 ORNL/FM-13555 Iodine Volatility and pH Control in the AP-600 Reactor Manuscript Completed: September 1998 Date Published: October 1998 Prepared by C.F. Weber, E.C. Beahm Oak Ridge NationalLaboratory Managed by lockheed Martin Energy Research Corp.
Oak Ridge, TN 37831-6285 J.Y. Lee, NRC Project Manager i
Prepared for Division of Reactor Project Management Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Wcshington, DC 20555-0001 NRC Job Code L1918
%...../
Abstract Two design-basis accidents for the AP-600 reactor are formulated and evaluated, in which significant bypass of the principal
. pH control system occurs. Some iodine released from the reactor primary system is retained in the incontainment Refineling l~
Water Storage Tank (IRWST) water, never entering the contamment, where trisodium phosphate produces a high pH. Some of this iodine is volatilaed and is transported into the reactor containment airspace. In the worst case, a small fraction i
is released to the environment at design-basis leak rate, yielding a total cumulative iodine release at 30 days of 0.0352 mol (0.023% of core iodme inventory) due to the iodine volatilization bypassing the pH control system. No fission product removal in the contamment atmosphere (i.e., natural deposition sprays) is considered.
4 I~
iii NUREGICR-6599
Cent;nts i
Page j
4
- Abstract......
....... iii j
Executive Sununary.......................
,,,,... vii.
1 Introduction............................................
..:1 1.1 ' Accident 2.....................................................
.......1
' 1.2 Accident 3..............................
1 I
i 2 Thermal Hydraulic CondiOns...........
............2 2.1. Pressure and Temperature.........................
.........,2 2.2 IRWST Drain...............................................................2 2.3 IRWST Venting......................................
........3 2.4 Centainment Leakage.........
......3
.3 Cnat=iament Chenustry............................,
a.......3 3.1 Borates.......
.4 3.2 Phosphates.....
.............4 3.3 Acids....
...4 3.4 Fission Product Sources................
.......4 3.5 Iodine Chemistry...........
........... 5 3.5.1 Hydrolysis...
5 3.5.2 Radiolysis....
...5 3.5.3 ' Gas / Liquid Partitioning........
.....5 3.5.4. Organic lodides..........................
5 3.6 Radiation Doses.........
6 4. Computation............
.6 i
4.1 Sources.......
.7 1
4.2 Convective Flow 7
4.3 Determination ofpH.....
...7
)
4.4 Control Volume Chenustry...................
.7 1
5 Results and Discussion...
9 6 Conclusions 15 1
7 References..........
15 Figures
' 2.1 Water flow from IRWST to containment..............
3 4.1.a Addition of trisodium phosphate (as anhydrous Na3PO ) to containment water...
8 4
4.1.b Total acid production from radiolysis 8
y NUREG/CR-6599 i
. 5.1 Levels of pH in containment volumes....................................................,.. 10 1:
5.2
. ' Aqueous iodine in contamment water (Accident 2)..................
...........................10
- 5.3 Aqueous iodine in the RWST water (Accident 2)................................................... I 1
..........I1 5.4 Total gas-phase iodine (Accident 2)................................................
5.5 Environmental release for Accident 2...............................................
............12 5.6-Aqueous iodine in principal contamment (Accident 3)............................................... 13 5.7 Aqueous iodine in the RWST water (Accident 3)................................................ 13
..5.8.. Total gas-phase iodine (Accident 3).....................,.......
......................14 5.9 Environmental release (Accident 3)............................................................. 14 Tables 1.1 Fission product inventories released into RWST....................
................ 2 3.1 Radiation energy outputs to water pool.,..........
......................................... 6 4.1 Control volumes used in accident sequence geometry.................
.........................7 5.1 Cumulative environmental releases ofiodine.....................
........................12 NUREG/CR-6599 vi
l Executive Summary Two design-basis accidets for the AP 600 reactor are formulated and evaluated, in which significant bypass of the principal
' pH control system occura Some iodine released fium the reactor prunary system is retamed in the Incontainment Refueling Water Storage Tank (RWST) water, never entermg the ma'aia aaa* where trisodium phosphate produces a high pH.
' This study is a companion to an earher one, both of which are intended to evaluate pH control and iodine volatilization in the AP 600 reactor under design basis accident conditions. The previous study considered a variation of the 3BE (smallbreak LOCA) sequence (herender described as " Accident l'), and was designed to present a maxmium challenge to the capabilities of the matamment pH control system.
kaiant 2 Accident 2 was origmally conceived so as to present the maxunum possibility of bypassing the pH control system.
Accident 3 Subsequent to tht. evaluation ofAccidents 1 and 2, Westmghouse personnel suggested that even greater bypass of the pH control syt.em asight occur if RWST drain down began before 5ssion products were deposited. Hence, Accident 3 h:
conceived 1 judicious combination of Accidents I and 2. As in Accident 1, the fission product source from the primary system 6
is airborne, and assumed to deposit on wetted contamment surfaces. As in Accident 2, drainage gutters route the fission products into the RWST, rather than into the principal contamment.
- These accident sequences were fannulated so as to bypass the pH control system in the AP-600 contamment. Most iodinc is
- eventually transported to the flooded matamnet, where tnsodium phosphate raises the pH high enough to prevent =
votahination. However, a substantial fraction (about 20% in Accident 2 and 29% in Accident 3) of the iodine remams in the RWST, which never Adly drains. This water does not access the tnsodium phosphate, but instead is the repository for acids
. c produced by radiolysis in contamment. The continual lowenng ofpH in this volume does lead to some production of1, which 2
' is vented to the containment airspace, and a small amount vented to the atmosphere.
It should be noted that no credit was taken for deposition processes, which may remove some of the gaseous I,. In the IRWST, and to a lesser extent in contamment, the maha==* ion of water on walls would both impede permanent deposition and return 1 2 to the RWST pool. Here,it would be revolatilized and returned to the contamment atmosphere, although the time lag would tend to lower the overall envuonmental source.
. It should also be noted that the temperatures assumed for this sequence (Ill-120'C) are beyond the range for which some of the models have been validated. The hydrolysis reactions have been measured at 25, 50, and 90*C, and extrapolation to slightly higher temperatures is not unreasonable. Data for the radiolysis reactions are sparse at temperatures above 25'C, and indicate that the model slightly overpredicts conversion to I, (this is conservative). The chemical equilibrium model is reliable C3 least to 100*C, and slight extrapolation should be acceptable. The production of hcl is more pronounced at higher temperatures, although this effect is not signi5 cant below 150*C. Thus. uncertamties due to temperature extrapolation are not large, and are probably not nearly as great as uncertamties due to sequence formulation.
In the worst' case, a small fraction is released to the environment at design-basis leak rate, yielding a total cumulative iodine release at 30 days of 0.0352 mol (0.023% ofcore iodine inventory) due to the iodine volatilization bypassing the pH control system. No fission product removal in the contamment atmosphere (i.e., sprays) is considered.
Vil NUREG/CR-6599 4
Introdnetten I
l 1 Introduction This study is a companion to an earlier one,' both of which are intended to evaluate pH control and iodine volatilization in the AP-600 reactor under design-basis accident conditions. The previous study considered a variation of the 3BE (small-break LOCA) sequence (hereafter described as " Accident 1") and was designed to present a maxunum challenge to the capabilities of the containment pH control systen Two additional accident sequences are described in the present study.
1.1 Accident 2
'Ihis accident was origmally conceived so as to present the maxunum possibility of bypassing the pH control systen C
-W floodmg leaves a residual amount (about 20%) of water in the In-containment Refueling Water Storage Tank (IRWST), where it is not accessible to pH control chemicals. Typically, under normal operation, containment dramage gutters are set to route all wall ces.ki, don (including radiolytically produced acids)into the RWST, furtherlowering the pH. lodine in this water would be subject to volatilization and would be unaffected by the contamment pH control system The actual accident sequence, designated " Spurious ADS," involves the following features:
(1) Automatic Depressurization System (ADS) Stages 1,2, and 3 work as intended, but Stage 4 fails.
(2) Injection works for two Core Make-up Tanks (CMT), two accumulators,' the Chemical Volume and Control System (CVS), and the Boric Acid Tank (BAT).
(3) RWST injection to core fails, but drainage to contamment works fully.
(4) IRWST gutter works.
Under this sequence, hasion products are deposited directly into the RWST, where no pH control occurs. For simplicity, this source is assumed to be instantaneous, composed of the gap releases and in-vessel releases specified in NUREG-1465.8 These amounts, which are identical to those used in the earlier study,' are shown in Table 1.1. The sequence begins at this point for purposes ofpH and iodine evaluation. Also at initial time, IRWST dramage to the contamment begins and subsequency moves most of the iodine (and other fission products) to the containment. Mixing of the water with trisodium phosphate (TSP) raises the pH well above 9, virtually eliminating iodine volatilization. However, the residual water in the IRWST contains about 20% of the iodine released fmm the primary system; and since no pH control exists, the potential for iodine volatilization is high.
1.2 - Accident 3 Subsequent to the evaluation of Accidents I and 2, Westmghouse personnel suggested that even greater bypass of the pH control system might occur if RWST drain-down began before fission products had been deposited. Hence, Accident 3 was conceived by thejudicious combination of Accidents 1 and 2. As in Accident 1, the fission product source from the primary system is airborne and is assumed to deposit on wetted contamment surfaces. As in Accident 2, drainage gutters route the fiamon pmducts into the RWST, rather than into the principal contamment.
In practice, this sequence is modeled similar to Accident 2, except for a source term that is time dependent rather than inst-emaous The tumng of fission product release from the primary system is exactly that of Accident 1, consisting of gap and in-vessel releases from Reference 2. These are deposited directly into the RWST, simulating the processes of surface deposition and drainage in condensate. While detailed modeling of the actual processes would involve some lag time,it is expected to be short compared with the accident duration. All other thermal hydraulic conditions in Accident 3 are assumed to be identical to those of Accident 2 and are described in the following section. In addition, all additives and chemicals that would affect pH are also assumed to be identical to those in Accident 2.
, Thermal Hydraulic Conditions Table 1.1 Fission product inventories released into IRWST AP-600 Fission product group inventories Fission Total core Released to IRWST product inventory Mass group (g)
Fraction (g) 1 Xe 41,1900 1.00 411,900 2
I 18,360 0.40 7,344 -
3-Cs 237,600 0.30
' 71,280 4
Te 34,250 0.05 1,712 5
Sr 70,700 0.02 1,414 6
Mo 243,600 0.0025 609 7
Ba 107,800 0.02 2,156-8 La 566,200 0.0002 113 9
Ce 200,800 0.0005 100 10 Sb 2,037 0.05 102 11 U
64,360,000 0.0005 32,180 12 Ru 612,000 0.0017 1,040
- See Reference 1.
2 Thermal Hydraulic Conditions In the early stages, venting of the primary system to the IRWST would raise its level to a maximum and its temperature to asturation. Continued steaming eventually creates a small, but significant, flow from the IRWST gas space to the main containment. However, this gas flow oces not actually begin until the water drainage to mein containment is complete (since water drainege itselfproduces a much larger gas inflow). Contauunent leakage to the environment represents an additional gas flow.
2.1 Pressure and Temperature According to Westinghouse estunates, the containment pressure will increase to 29 psia within 2 h of ADS operation (Stages 1, 2, and 3) and will decrease to 22 psia after 30 days. Thus, we assume 29 psia for the first 24 h of the sequence, followed by a linear decrease to 22 psia over the remauung 29 days. Assummg the presence of saturation conditions implies temperatures in all gas and water phases of 120*C at the start, decreasing to 111 *C after 30 days.
2.2 IRWST Drain l
'Ibc flow rate of water from the IRWST to the containment was taken from plots of such flow in the AP-600 design document.'
The drain flow, as illustrated in Figure 2.1, takes about 5 h to complete.
Containment Chr.aistry ORNt.DWG 97C-130000 2.0 c.E 1.8 M
) 1.6
@ 1.4 N 1.2 3;
1.0 0.8 0.6 0.4 3
8 0.2 0-O 1
2 3
4 5
6 TIME (h)
Figure 2.1 Water flow from IRWST to containment 2.3 IRWST Venting As stated in Section 2, steaming of the IRWST produces a small gas flow to containment after the water drain is complete.
The steaming is caused by continued heat generation due to decay heat of the fission products, coupled with condensation of steam on the containment shell. Westinghouse provided estimates $ of 250 cfm at 2 h,100 cfm at 24 h, and 40 cfm at 6 days.
These three values were fit to the empirical fann flow rate (m%) = A,exp (At + B),
(1) resulting in values ofA = 8.54 x 10 * (s*'), B - 2.53 x 10. The constant A, = 1 m% allows for changing to other units. The Gow in Equation (1) was applied after 5 h and lasted throughout the entire 30 days, although it became negligibly small by the end of the accident period.
2.4 Containment Leakage Westinghouse provided an esumate' that the enntainment leakage to the atmosphere was 0.12% per day. Use of this estunate resulted in flow rate of 6.62 x 10
- m% throughout each accident sequence.
3 Containment Chemistry Most iodine enters enntain-t as Cal aerosol,88which is readily dissolved in sumps or surface condensate. Even ifreleased into the contamment straosphere, most iodine will quickly settle onto wetted surfaces and will be washed into sumps or pools.
In accident sequences 2 and 3, Calis released directly into IRWST water. It has been well established *-'that the volatility of iodine is closely related to the pH of water pools in various contamment volumes.
l l
l Containment Chemistry l
Even highly soluble iodide (1-) can be converted to other forms (e.g.,1 and organic iodides) that more easily evaporate from 3
water and could be subsequently vented to the environment. At 25*C, this conversion is negligible for pH levels greater than 7, but dommant for pH levels less than 3. Thus, the presence of various chemicals that affect water pH is quite important in determmmg the extent ofiodine volatility. In each eccident sequence, we consider three such chemicals present in the AP-600 (i.e., borates, phosphates, and hydrochloric acid) that are expected to dominate the calculation of pH in various locations.
3.1 Borates Most water in these sequences originally contains various fonns of borates, which, regardless of initial form, hydrolyi.es to form boric acid and various polyborate species? The concentration in the IRWST is 2700 ppm (0.25 mol/L) as boron. The concentration in the reactor primary system varies, depending on additions from various tanks and accumulators; however, the average is not significantly different from that of the IRWST. While the processes of steaming and condensation produce surfaces wetted with nonborated water, the latter is expected to quickly drain into pools. Thus, all water is assumed to have a boron concentration of 2700 ppm (0.25 mol/L).
3.2 Phosphates The AP-600 design includes the placement of baskets of trisodium phosphate (TSP) in the lower containment, which will dissolve and increase the pH in the flooded containment. The design specification is for a muumum of 7830 lb of TSP (43%
as anhydrous salt, corresponding to the dodecahydrate Na3PO 12H O), which translates to 9309 mot. Invariably, TSP contai 2
small amounts of NaOH (true even of reagent grade TSP); he.ever, this excess caustic was not considered. (It is conservative to neglect the excess NaOH.) In these accident sequences, the TSP begins dissolving when the water icvel in contairanent reaches the floor of the steam generator rooms (83-11 elevation) and is completely dissolved after rising 3 m (9.8 ft) above the floor. It is assumed to be distributed uniformly and instantly throughout all water in the flooded containment (i.e., a well-mixed volume is assumed).
3.3 Acids Irradiation tends to produce various acids that would decrease pH; therefore, their effects must be considered in assessing overall iodine volatility. Small amounts of nitric acid are produced from the irradiation of air-water mixtures; however, this effect is assumed to be negligible and, therefore, was not included in the present analysis. The irradiation and heating of electrical cable insulation have been shown to produce signi'bnt quantities of hydrochloric and sulfuric acids," which are subsequently dissolved in condensate and collected in sumps. In the present sequences, the calculation follows the proc outlined in Appendix B of Reference 10. Since containment drainage gutters are set to route wall condensation to the IRWS all acids are deposited there as well.
3.4 Fission Product Sources in Accident 2, all fission products are assumed to be released through spargers into the IRWST at the start of the accident sequence. Because there is very little gas space in the IRWST at this time, noble gases are assumed to vent instantly into containment atmosphere. All iodine is assumed to be initially Csl and to be immediately dissolved in IRWST water. In reality, a small quantity of volatile iodine might be present in the source from the primary system; however, our goal is to predict volatilization of that iodine which was initially nonvolatile. The assumption that all iodine occurs initially as iodide allows a clearer assessment of this process. Finally, all other fission products are assumed to be nonvolatile for the entire accident. They are released into water and *.cavel with it either in solution or in suspension. The primary reason for considering noble gases and nonvolatile fission products is to allow calculation of the radiation doses to water pools and gas spaces.
For Accident 3, the release is initially into containment airspace, but all fission products except noble gases become quickly entrained in condensate and deposited in the IRWST. This is true of both Csl and less volatile fission products. Noble gases, l
Containment Chemistry however, are assumed to be instantly released into the containment, as in Accident 2. However, in Accident 3, the releases are timed (see Reference 2), rather than instantaneous 3.5 Iodine Chemistry 3.5.1 Hydrolysis
- In aqueous solution, iodine can undergo hydrolysis, which is described well by the following reaction set?"
I, + H O
- I + H01 + W, (2a) 2 2 HOI ++ l' + HIO + W, (2b)
HOI + HIO
- I + HIO + W.
(2e) 3 ne end-point oxidation states that I and loi (or HIO ) are both extremely soluble and, therefore, highly desirable. The H01 3
and HIO species are reaction intermediates, which may be volatile but are regarded as having short lifetimes. However, molecular iodine,1, is stable and sparingly soluble. It can be seen in the above reactions that low pH levels (i.e., a large W 2
concentration) will result in more 1 and less f and 10 ". Each fonvard and reverse reaction gives rise to a single rate equation, 2
3 as described in Reference 11.
3.5.2 Radiolysis In a radioactive envuunment, the presence of free radicals induces additional aqueous reactions. The overall result can be described empirically by the equilibrium"
[#']'[T]2= a + b [Ir],
(3)
[1,]
where a and b are known coefficients depending only on temperature. The actual rate equations used in this study are based on the catalytic decomposition of hydrogen peroxide, one of the stable by-products ofirradiating water. The peroxide inventory is estunated empirically, based on cumulative dose, pH, dissolved oxygen content, and temperature. The forward rate coefficients are then based on the data of Liebhafsky, and the reverse coeflicients are chosen to move steadily toward equilibrium (3).
3.5.3 Gas / Liquid Partitioning ne same equation is used to describe the evaporation of volatile iodine species from water (1 or CH 1) and their dissolution 2
3 from gas back to the water. The model is based on the equilibrium partition coefficients (inverse of Henry's Law constant) lbr each species and rate equations obtained from naturel convection correlations."
3.5.4 Organic Iodides Organic iodides are dc xibed empirically by the formation of CH I from I, in the gas phase. The model used is described fully 3
in Reference 11. Analogous to 1, CH 1 is sparingly soluble in water - an effect which is negligible in the accident sequences 2
3 considered here.
5 NUREG/CR-6599 l
l
Cosaputation 3.6 Radiation Doses l
Several of the models depend on the prediction of decay energy in air or water. Energy outputs for beta and gamma radiation are given in Table 3.1 for each fission product group at various ti:aes. These were calculated by using ORIGEN" results for) fission product inventories, together with ICRP data" for nuclide decay energi.:s.
The doses to water assume that all energy from beta and gamma decay is absorbed in water. They are calculated by combining
- information from Table 3.1 (interpolated to the correct time) with the water inventories of each fission product group at each time step, as obtained from MAAP reactor analysis code output.
The rate of hcl formation in cable sheathing is based on the energy production rates (both beta and gamma) due to fission productsthatare airbomeordepositedonsurfaces. The latter rates are calculated using the procedure described in Reference 10.
Table 3.1 Radiation energy outputs to w ster pool l-Beta. energy output by MAAP fission product group [(MeV/s)k8 x 10'"l
[
T'une Xe Cs!
- TeO, Sr0
- Moo, CaOH Ba0 te,0
- CeO, Sb Te, 11 0 3
(h)
I 2
3 4
5 6
7 8
9 10 12 0
12.470 230.89 134.70 38.138 10.040 34.251 69.500 57.823 8365 650.10 168.00 0.337 1
5.946 185.28 53.26 35.527 4.500 17.302 29.253 34.856 8.292 232.70 66.43 0.169 2
4.729 152.24 29.82 33.271 4.134 9.956 19.828 30.953 8.186 141.80 37.19 0.138
.5 3.481 109.61 1539 28.209 3.987 4.114 11.146 25.192 7.881 87.83 19.20 0.128 l
10 2.883 88.071 11.891 23.071 3.782 1.564 8.876 19.865 7.405 5639 14.83 0.120 24 2.068 61.725 8.980 16.831 3.266 0.549 8.418 14.752 6300 31.67 11.20 0.101 48 1.362 41.878 7.093 13.990 2.537 0.512 7.985 13.030 5.002 24.700 8.846 0.076 96 0.954 24 966 4.869 13.117 1.533 0.505 7.184 11.974 3.698 17.820 6.073 0.043 Gamma-energy output by MAAP fission produd 8roup {(MeV/s)18 x 10'"]
Time Xe Cal
- TeO, Sr0
- Moo, Csoll Bao La,0,
- CeO, Sb Te, 13 0,
(h) 1 2
3 4
5 6
7 8
9 to 11 12 0
19.470 75332 235.01 60.098 15.169 52.232 56.141 41.486 4.597 2253.0 293.10 0.119 1
8.780 615.56 97.90 49.627 2386 21.006 10.097 27.007 4.549 793.10 122.10 0.097 2
6.729 500.61 54.26 41.258 1.620 9.718 7.156 22.746 4.481 466.50 67.67 0.092 5
4.005 347.38 25.68 24.911 1.525 4.518 6.473 17.590 4.280 270.70 -
32.03 0.088 10 2.298 278.13 21.33 12.608 1.447 3.654 6317 15.348 3.971 161.00 26.85 0.083 24 1.084 199.53 17.62 3.498 1.249 3317 6.153 14.045 3.255 77.95 21.98 0.070 s
48 0.529 144.10 12.98 0.592 0.970 3.257 5.900 13.243 2.418 59.510 16.190 0.052 96 0317 9137 7.51 0.018 0.586 3.164 5.432 12.067 1.587 44.590 9368 0.030 4 Computation The gennetry for the sequences in Accidents 2 and 3 involves only the three control volumes listed in Table 4.1. A compotational model was constructed to describe each relevant physical and chemical process in the entire system. Rate egastions were developed for each process and solved together to give a transient description of convective transport and chemical interactions for the duration of the accident. The sequences are divided into discrete time steps, and at each step the following actions are taken (in the order indicated): (1) sources of fission products or pH-influencing chemicals are introduced, (2) convective flow transfers between different control volumes are computed; (3) the pH within each control volume is NUREG/CR-6599 6
l Computation Table 4.1 Control volumes used 15 accident sequence geometry Control Total volume Max. water volume (m')
volume (m')
Containment 47694 1862 IRWST 2425 2265 Environment calculated; and (4) the speciation, chemical interactions, or phase change within each control volume is calculated. Each subcalculation is discussed in more detail in the subsections that follow.
4.1 Sources In addition to fission products, there are sources of pH-influencing chemicals. Borate is present in all wer, as mentioned in Section 3.1. TSP is dissolved during the first few hours, consistent with the water level increase in containment (see Section 3.2). Acid is added as hcl according to the modelin Reference 10. The transient additions used for TSP and hcl are shown in Figures 4.1.a-4.1.b. (The production of hcl is due almost entirely to radiation from noble gases, which is the same in each of the sequences considered here.)
4.2 Convective Flow Transfers of gas or liquid inventories between control volumes are based on the flows discussed in Section 2. The volume fraction of fluid that leaves the control volume during a time step is assumed to carry that same fraction of components. 'Ihus, for a 1-h time step, containment venting to the atmosphere moves.12/24, or.005%, of the containment gas volume (see Section 2.4). Then, in the same time step,.005% of current gaseous iodine and noble gas inventories would also be transferred to the environment.
4.3 Determination of pH Inventorier of all aqueous constituents are updated in the source and flow steps above. The calculation of pH assumes that equilibrium is reached rapidly The equilibrium calculation model uses the principal subroutine "OASOL" of the code SOLOASMDC." The calculation uses an extended Debye-Hockel form for activity coetlicients and includes multiple
_ phosphate and borate species. The model itself and its validation with actual data are described in Reference 11.
4.4 Control Volume Chemistry The most complicated part of the calculation is the simultaneous salution of all rate equations arising in Section 3.5 relative to iodine hydrolysis, radiolysis, gas / liquid partitioning, and methyl iodide formation. This set ofordinary differential equations is solved by using the routine LSODE and related routines."
_u
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Figure 4.1.s Addition of trisodium phosphate (as anhydrous Na3POJ to containment water i
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I i-Figure 4.1.b Total acid production from radiolysis l
NUREG/CR.6599 8
Results and Discussion 5 Results and Discussion The model described in Section 4 was nm for a 30-day transient in the case of both Accidents 2 and 3. The pH levels in the IRWST and contairanent water were virtually identical in each sequence (see Figure 5.1). The pH of borated water, which is somewhat below 6, is changed considerably by cources of other chemicals (water is above 100*C). Due to the hcl source, the pH of the IRWST water declined steadily for the first 8 h and, after 30 days, had decreased to almost 3.0. While the pH of the contaimnent water declined briefly, it rose abruptly when phosphate dissolution began and stayed above 9.5 for the remainder of the 30-day transient.
For Accident 2, iodine speciation in the two water pools is depicted in Figures 5.2 and 5.3, which show that a gradual oxidation of I to IOi began quickly in each case. Note that the plotted quantity " fraction of containment total" includes all iodine released from the reactor coolant system, which is 40% of the total core inventory. In the IRWST water, the process continued throughout the 30-day accident transient, eventually yielding almost complete conversion to 10.
This is consistent with experimental results at 25'C and is caused by the tandem processes of radiolysis and hydrolysis. In the former, I is converted to l, in the latter, la is converted-primarily to I, but also some fraction to IOi, Over time, accumulation of IOi is the i
inevitable result for moderate pH values. However, in the containment sump, the process was abmptly terminated once the water level rose sufficiently to dissolve TSP (at about 30 min). At this point, the inventories ofI, and HOI decreased drastically, and the destruction of1 was effectively ended. In Figure 5.2, inventories of all species increased due to additions from IRWST drainage; however, they stabilized at about 5 h and changed very little after that point. The inteimediates HO! and HIO continued to oxidize slowly, and the inventory ofIOi eventually exceeded that of HIO (at about 17 days). Note also that the laconcentration was much higher in the IRWST water (where the pH was low). While complete oxidation to IOf would occur in a closed volume, the presence of a significant aqueous 1 inventory in a vented control volume would allow evaporation 2
and escape of gaseous I. This effect can be seen in Figure 5.4, where I, has evaporated from the IRWST water into the IRWST airspace and is subsequently vented into the containment airspace. From there, some 1 is redissolved in the containment water 2
(see Section 3.5.3), while a small amount is actually released to the environment.
The cumulative environmental releases at various times are shown in Table 5.1. The molar amounts are direct output of the calculations. At each time step, the rate of activity released is calculated by D = P, k,
where D is tl e rate of activity release during time step 1, k = total iodine leak rate (mol/s) at step I, and P = P(t,) (Ci/mol total !) is deten: tined from ORIGEN calculations at various time steps. A good approximation for the AP-600 is 1 -3.5089 - 0.09885 t + 0.002420 t 2
0 < t < 24 In(P) =
, --4.4876 - 0.01424 t + 0.000013 t2 24 < t < 720 where t is in hours. The cumulative activity is, then.
Q=
Q (t') dt' = [ D, A t, The speciation of volatile gaseous iodine, as shown in Figure 5.5, confums that I, is the dominant form, as expected.
The iodine inventories for Accident 3 (see Figures 5.6-5.9) are very similar to the analogous results for Accident 2. Since ohnost 50% more iodine ends up in the IRWST water, greater volatilization would be expected. This is borne out in Table 5.1, where roughly 50% more iodine is released to the environment at times exceeding 24 h.
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Figure SJ Aqueous lodine in the IRWST water (Accident 2)
ORNL DWG 97C.130006 10-1 i
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ENVIRONMENT
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Figure 5.4 Total gas-phase lodine (Accident 2)
I1 NUREG/CR.6599
Resuks and Cisenssism Table 5.1 ' Cumulative environmental releases oflodine Accident 2 Accident 3 Time mot Ci mot Ci 2h 10
O.013 1 0 -82 10-8 8h 10-*
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2 Id 0.00015 174 0.00022 245 4d-0.00500 3247 0.00770 4993 30 d 0.02250' 5981 0.03519 9326 ORNL DWG 97C-130007 10-3 i
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Figure 5.5 Environmental release for Accident 2 NUREGICR-6599 12
Rssults mad Discusslos ORNt. DWG 97C-130008
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Figure 5.7 Aqueous lodine in the IRWST water (Accident 3) 13 NUREGICR-6599
Resuks and Discussion ORNL DWG 97C-130010 4
10 i
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' Figure 5.8 - Total gas phase iodine (Accident 3)
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Figure 5.9 Environmental release (Accident 3)
Conclalons/Ref,rences 6 Conclusions These accident sequences were formulated so as to bypass the pH control system in the AP-600 containment following a design-basis accident. Most iodine is eventually transported to the flooded containment, where TSP raises the pH to a level sufficiently high enough to prevent volatilization. However, a substantial fraction (about 20% in Accident 2 and 29% in Accident 3) of the iodine remams in the IRWST, which never fully drains. This water does not access the TSP; instead, it is the repository for acids produced by radiolysis in the containment. The continual lowering of pH in this volume leads to the production of some 1, which is vented to the containment airspace, a small amount is released to the environment.
2 It should be noted that no credit was taken for iodine deposition processes, which may remove some of the gaseous 1. In the 2
IRWST, and to a lesser extent in the containment, the condensation of water on walls would (1) impede permanent deposition and (2) return 1 to the IRWST pool. Here, it would be revolatilized and returned to the containment atmosphere, although the 2
time lag would tend to lower the overall environmental source.
It should also be noted that the temperatures assumed for these accident sequences (111-120*C) are beyond the range for which some of the models have been validated. The hydrolysis reactions have been measured at 25,50, and 90*C, and extrapolation to slightly higher temperatures is not unreasonable. Data for the radiolysis reactions are sparse at temperatures above 25'C and indicate that the model slightly overpredicts conversion to 1 (which is conservative)." He chemical 2
equilibrium model is reliable at least to 100*C, and slight extrapolation should be acceptable. The production of hcl is more pronounced at higher temperatures, although this effect is not significant below 150*C. Thus, uncertainties due to temperature extrapolation are not large - and probably not nearly as great as uncertamties due to sequence formulation.
7 References 1.
Beahm, E. C., C. F. Weber, and T. A. Dillow," Technical Assistance in Review of Source Term Related issues of Advanced Reactors," NUREO/CR-6408 (ORNLffM-13144), dran, Oak Ridge National Laboratory, August 1998.
2.
Soffer, L. et al.," Accident Source Terms for Light Water Nuclear Power Plants," NUREG-1465, February 1995.
3.
Reference deleted.
4.
Nuclear Regulatory Commission AP 600 documentation (FAX) to ORNL, October 30,1996.
5.
Regulatory Guide 1.4," Assumptions Used for Evaluating the Potential Radiological Consequences of a Loss of Coolant Accident for Pressurized Water Reactors," June 1974.
6.
Beahm, E. C. et al., " lodine Chemical Forms in LWR Severe Accidents," NUREG/CR-5732 (ORNLffM-11861),
Oak Ridge National Laboratory,1992.
7.
Lin, C.-C.," Chemical Effects of Gamma Radiation on lodine in Aqueous Solution,"J. Inorg. Nucl. Chem. 42, 1101-7 (1980).
8.
Weber, C. F. et al., " Optimal Determination of Rate Coeflicients in Multiple-Reaction Systems," Computers and Chem.16(4),325-33 (l992).
9.
Mesmer, R. E. et al.," Acidity Measurements at Elevated Temperatures. VL Boric Acid Equilibria," Inorg. Chem.
11(3),537-43 (1972).
10.
Beahm. E. C. et al.," lodine Evolution and pH Control," NUREG/CR-5950 (ORNUfM-12242), Oak Ridge National Laboratory,1992.
Referomees 11.
Weber, C. F. et al.. "Models of Iodine Behavior in Reactor Contamments," ORNIITM-12202, Oak Ridge National Laboratory, October 1992.
12.
Liebhafsky, H. A.. "The Catalytic Decomposition of Hydrogen Peroxide by the Iodine-Iodide Couple at 25*C,"
' J. Amer. Chem. Soc. 54,1792-1806 (1932).
13.
Croff, A. O.,"ORIGEN 2 - A Revised and Updated Version of the Oak Ridge isotope Generation and Depletion Code," ORNL-5621, Oak Ridge National Laboratory, July 1980.
14.-
NUCDECAY - Nuclear Decay Data for Radiation Dosimetry, Calculations for ICRP and MIRD, DLC-172.
Radiation Shielding Information Center, Oak Ridge National Laboratory, May 1995. See also K. F. Eckerman et al,
" Nuclear Decay Data Files of the Dosimetry Research Group," ORNI/rM 12350, Oak Ridge National Laboratory, 1993, and K. F. Eckerman et al., Health Phys. 67(4), 338-45 (1994).
15.
Enksson, G.," Thermodynamic Studies of High-Temperature Equilibria. XII. SOL-OASMIX, A Computer Program for Calculation of Equilibrium Compositions in Multiphase Systems," Chemica Scripta 8,3 (1975).
16.
Hmdmarsh, A. C.,"ODEPACK. A Systematized Collection of ODE Solvers," R S. Stepteman et al., eds., Scientific Computing, pp.55-64, North-Holland, Amsterdam (1983).
NUREGICR-6599 16
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- 2. nTLE ANo SusTiTLE ORNL/TM-13555 Iodine Volatility and pH' Control in the AP-600 Reactor 3.
oATE REPORT PUBU5dio uo r-
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October 1998
- 4. FIN oR GR ANT NUM6s A L1918
- 5. AUTHOR ($1
- 6. TYPE oF REPORT C. F. Weber and E. C. Beahm Technical A PERioo CovEREo teaave.c oms
- 8. PE.X.Poeo ING o.amsRGANIZATioN - NAME ANo AcoR E53 tit anc. seer.as, osv. men. Otr.re er Aspaa, v.& Nurdea #epue##ery Cesunseses, med meeng assessi si so FN##r. #,9'*
RM mer o Oak Ridge National Laboratory Oak Ridge, TN 37831-6285
- 9. SPONSORING oRG ANIZATioN - NAME ANo AcoRESS tst anc syse 1ene se see.e */ # awarersu. pre we adC c ressa. Offsv er Aepen. M1 Mussar A#pvasvery Camalvered een neenag eenrease Division of Reactor Program Management Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001
- 10. SUTPLEMENTARY NOTES J.Y. Lee, NRC Project Manager
- 11. A33TR ACT (200 more se mass Two design-basis accidents for the AP-600 reactor are formulated and evaluated, in which significant bypass of the principal pH control system occurs.
Some iodine released from the reactor primary system is retained in the Incontainment Refueling Water Storage Tank (IRWST) water, never entering the containment, where trisodium phosphate produces a high pH.
Some of this iodine is volatilized and is transported into the reactor containment airspace.
In the worst case, a small fraction is released to the environment at design-
' basis leak rate, yielding a total cumulative iodine release at 30 days of 0.0352 mol (0.023% of core iodine inventory) due to the iodine volatilization bypassing the pH control system. No fission product removal in the containment atmosphere (i.e., natural deposition sprays) is considered.
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