c b 1 *h!r338-QW3 c S A N n - T g - O g O q G

The Effect of the Number of Condensed Phases Modeled on Aerosol Behavior During an Induced Steam Generator Tube Rupture Sequence*

Sandia National Laboratories Albuquerque, New Mexico 87185-0739

and J. H. Schaperow Nuclear Regulatory Commission

Washington, DC 20555

N. E. Bixler ~0~6-480 6 +@ --

ABSTRACT

VICTORIA is a mechanistic computer code designed to analyze fission product behavior within a nuclear reactor coolant system (RCS) during a severe accident. It provides detailed predictions of the release of radioactive and nonradioactive materials from the reactor core and transport and deposition of these materials within the RCS. A recently completed independent peer review of VICTORIA, while confii ing the overall adequacy of the code, recommended a number of modeling improvements. One of these recommendations, to model three rather than a single condensed phase, is the focus of the work reported here. The recommendation has been implemented as an option so that either a single or three condensed phases can be treated. Both options have been employed in the study of fission product behavior during an induced steam generator tube rupture sequence. Differences in deposition patterns and mechanisms predicted using these two options are discussed.

0 a3 0

1.0. Introduction

Release of radionuclides into the atmosphere is the main concern in the event of a nuclear reactor accident. The consequences of a severe accident depend on the quantity, characteristics, and timing of the releases of radionuclides from the reactor coolant system (RCS) into containment and finally into the atmosphere. In a bypass accident, such as the induced steam generator tube rupture (ISGI'R) sequence considered here, releases bypass containment and go directly from the RCS into an auxiliary building or into the atmosphere. As a result, accurate determination of the quantity of fission products that are retained in the primary and secondary circuits and the extent of any leakage of fission products into containment is central to the assessment of risk.

The physical processes that influence the quantity and timing of a release'are complex. In order to predict the outcome of a severe nuclear accident, it is necessary to accurately model all

*This work was supported by the U.S. Nuclear Regulatory Commission and was performed at Sandia National Laboratories, which is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U. S. Department of Energy under Contract DE-AC04-94AL85000.

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, exprey or implied. or assumes any legal liabiiity or responsibility for the accuracy,. completeness, or use- fulness of any infomation, apparatus. product, or proces~ disclosed, or represents that its w would not infringe privately owned rights. Reference hmin to any spe- cific commercial product, process, or tcnricc by trade name, trademark, manufac- turn. or otherwise do# not necessarily constitute or imply its endorsement, ncom- mmd&tion, or favoring by the United States Government or any agency thereof. The views and opinions of authors e x p d henin do not neassariiy state or reflect thosc of the United States Government or any agency thereof.

the important physical processes. VICTORIA [l] is a mechanistic computer code designed to model such processes so that the magnitude, chemical speciation, physical properties, and timing of fission product releases can be predicted.

VICTORIA does not predict thermal hydraulics, but requires such information as input. The strength of the code is in its mechanistic treatment of fission product releases from &el, chemical interactions, aerosol processes, vapor and aerosol transport, and decay heating of surfaces from deposited and suspended fission products. The coupled treatment of these phenomena makes VICTORIA unique in its predictive capabilities.

VICTORIA recently underwent an independent peer review for the US Nuclear Regulatory Commission (NRC), which was completed in April 1997 [2]. The purpose of the peer review was to assess the VICTORIA code and its documentation against a set of design objectives and targeted applications, and to make recommendations on ways the code and documentation could be improved. The positive results of the peer review are viewed as a confirmation of the overall adequacy of the VICTORIA code; however, a number of recommendations for model improvements were made by the committee. One of these recommendations was that three separate condensed phases should be modeled: an oxidic phase, a metallic phase, and a third phase consisting only of cesium iodide. As reviewed, VICTORIA treated a single condensed phase in which all condensed-phase species were assumed to form an ideal solution. To implement the recommended three-condensed-phase model, each condensed-phase species in the VICTORIA thermochemical database is assigned to one of the three phases [3]. The ideal solution assumption is retained for each of the phases, as recommended by the committee.

The reasoning behind the recommendation to treat separate metallic and oxidic phases was based on the modeling approach used in the VANESA model for corekoncrete interactions [4]. The reasoning behind the recommendation to treat cesium iodide as a separate phase appears to be novel. A number of questions remain to be answered. For example, are other salts that would form during a severe accident, especially other iodides, soluble in the cesium iodide phase and are the relatively small quantities of cesium iodide soluble in the relatively large oxidic phase? It is clear that both the single- and three-condensed-phase models are approximations to the complex phase equilibria that would prevail during a severe accident; which approximation is better remains to be determined.

Analyses of an ISGIX sequence, using thermal-hydraulic data from a SCDAP/RELAP5 calculation [5 ] , were conducted in support of a recent NRC initiative on steam generator tube integrity. The objective of the VICTORIA analyses was to acquire a best estimate of fission product releases to the environment during this important bypass sequence. These analyses include calculations to assess the sensitivity of predicted releases to the environment to the number of condensed phases modeled.

2.0. Recent Code Modifications

Modifications to the VICTORIA code have recently been made in response to recommendations from the VICTORIA Independent Peer Review Committee [2]. At this time, the highest priority recommendations have been addressed. These include the following: (1)

removing three outdated or unused options from the code, which involve (a) highly mechanistic fission product release modeling [6], (b) treating chemical interactions between fission products and the he1 grain lattice, and (c) an option to directly input mass-transfer boundary layer thicknesses rather than using standard mass transfer correlations; (2) making treatment of mechanical resuspension of deposited aerosols optional; (3) adding warnings when the user- selected time-step size is too large; (4) treating three condensed phases; and ( 5 ) treating the solubility of fission products in the fuel. One medium priority recommendation, revising the thermochemical treatment of the UOz+x fuel matrix, has also been addressed. The revised UOz+x thermochemistry model is based on the work of Blackburn [7]. These modifications will be included in a future code release, which will be called VICTORIA 2.0. Other medium- and low- priority recommendations will be addressed in the future.

The third of the recommended modifications to the VICTORIA code provides useful information to the code user but does not affect predictions at all; the others could affect predictions, depending on the options chosen by the code user. From investigations to date, implementation of the fourth recommendation, which is to treat three condensed phases [3], appears to have a stronger effect on predicted fission product behavior than implementation of any of the other recommendations. This recommendation has been implemented as an option-- either one or three condensed phases can be modeled with the current code version--because it is not yet clear which of the options is a better approximation of the phase behavior that would prevail during a severe accident. The current hope is that assessment against data from the Phebus experimental program [8] will serve as a guide for choosing between these options.

The general effect of the three-condensed-phase option, as compared with the single- condensed-phase option, is to increase fission product volatilities. This is especially so for iodine under conditions where cesium iodide is the dominant iodine species because this species is assumed to form a nearly pure phase in the three-condensed-phase model. The other elements are also predicted to be more volatile, but to a lesser extent than iodine. Even cesium is affected to a lesser extent than iodine because it is about 10 times more abundant; thus, a maximum of about 10% of the cesium can form cesium iodide. The effect of higher fission product volatilities is that the fission products condense at cooler temperatures, which usually means they condense farther away from the reactor core.

The effect of the number of condensed phases modeled on predicted vapor pressures can be understood from the relationship between the equilibrium vapor pressure of a pure species and that of the equilibrium partial pressure in a mixture with other species. For ideal phases, the following relationship governs the vapor pressure of a species that exists in both the vapor and condensed phases:

p ( i ) = x(i)p'(i)

where p(i) = equilibrium partial pressure of vapor species i p a ) x(i) p'(i) = equilibrium vapor pressure of pure species i (Pa)

= mole fraction of species i in the condensed phase (-)

The mole fraction of condensed-phase species i is calculated differently under the two options. When a single condensed phase is treated, all condensed-phase species are assumed to form an ideal mixture and the mole fractions are calculated accordingly. When three condensed phases are treated, each species is assumed to be soluble in only one of the three phases. Table 1 shows which species are assigned to which condensed phase. Mole fractions are calculated separately for each phase.

- ~

CsOH u409

cs20 u3°8

Table 1. Assignment of Species to the Three Condensed Phases Recommended by the VICTORIA Peer Review Committee

~~

Sb203

EuO

I Species Contained in Each Condensed Phase

cs2uo4

c%?u207

Phase 3: Iodides

uo3 EhO3

FeO c%?u4012

Phase 2: Oxides

Ba02H2

BaO

Phase 1: Metals

F%O4 Cs2Zr205

Fe02H2 cSzzr307

SrO2H2

SrO

I CS I CrTe

CSBO~ C%Te04

HBO2 ?iTe40 12

CSI

Ba12

z f l 2

SnO

InZo3 csqcr04

MnO TeH204

Sn02

Te02

W304 UH405

NiO

SrI,

+p SnTe

C S ~ M O O ~ Fer2

Zr14

Te14

Ru BaZr03 M o I ~

Sb Mo13

M012 Sb2Te3

A d CdI2

FeTe

FeTe2

Eu

C%Te

SrUO4 I H3B03 I cs2cr207 Cr12 Mo

Ag

BaH2

SrH2 SrZrO, 1 CdO 1 cs2cro4 In1

ZrTe2 SrMo04 Ni12

B

ZrB, RUT%

,

In the three-phase model, the metallic phase includes the tellurides, borides, and hydrides, which are thought of as being alloys (for the tellurium) or as being solutions in a predominantly metallic phase (for hydrogen and boron). Phase 2, the oxidic phase, contains all oxygen bearing species, even those that might be classified as salts and so might have been included in phase 3. Phase 3, the iodide phase, was chosen to contain all the iodides, rather than just cesium iodide, as recommended by the VICTORJA Independent Peer Review Committee 121. This choice seemed more reasonable than placing most of the iodides in phase 2. However, the abundance of cesium iodide is generally predicted to dominate over those of all other iodides by several orders of magnitude under conditions that are typical of the RCS, so the recommendation of the peer review committee has been implemented in effect.

The focus of the study reported here is on the effect of the option to model one or three condensed phases on predicted fission product behavior during an ISGlR sequence, and especially on predicted releases to the environment. This study is conducted as a sensitivity analysis. Predicted deposition mechanisms and patterns are examined. At the time of the analyses reported here, the revised treatment of the thermochemistry of UOz+x had not been implemented, so the effect of this modeling change is not considered in the following discussions. Other code modifications should have little or no effect on the predicted fission product behavior for this sequence.

3.0. Modeling of the ISGTR Sequence

SCDAPREiLAPS [SI analyses were performed to evaluate the potential for an ISGTR [5] during the early stages of a station blackout sequence. In reference 3, seven possible ISGTR sequences are documented. Sequence 6 was chosen as the basis for this work. The specific characteristics of sequence 6 are as follows: (1) the secondary side of loop C, shown in Figure 1, hereafter denoted the faulted loop, depressurizes early in the sequence because of a stuck-open atmospheric dump valve (ADV) and (2) the primary remains at full system pressure even after the surge line and hot leg nozzle are predicted to fail. Table 2 shows the timing of major events for sequence 6, as predicted by SCDAPRELAPS. In Reference 3, the reported calculation was terminated at the point of surge line failure; however, the actual calculation used as the basis for this study was extended to lower head failure. The thermal-hydraulic data were transmitted by Darrel Knudson of Idaho National Engineering & Environmental Laboratory to Sandia National Laboratories. VICTORIA was used to analyze the release of fission products from core and their transport through the primary circuit, secondary circuit, and out through the stuck-open ADV [lo].

Figure 1 shows a schematic representation of the VICTORIA nodalization of the Surry reactor vessel and primary circuits. A total of 48 nodes was used to represent the domain. The unfaulted loops, A and B, are represented as a single loop because the thermal-hydraulic behavior in these two loops is almost identical. The faulted loop, C, which is the one with the pressurizer, is represented individually because its thermal-hydraulic behavior is quite different from that of the other two loops.

During this sequence, periodic accumulator injection maintains a water level that fluctuates between the bottom of the core and about 40% of the core height. This water vaporizes, providing

the source of steam to the core and primary circuits. Steam losses occur through the surge line and pressurizer, and out through the pressure-operated relief valve (PORV) or through the broken steam generator tube, as indicated in Figure 1. The losses through the PORV and broken steam generator tube are distinct in time. Early losses occur through the PORV, which cycles until the steam generator tube rupture occurs; after that time, the system depressurizes and PORV cycling ceases.

Event Station blackout initiates sequence ADV sticks open on loop C Onset of PORV cycling Hot leg natural circulation of steam begins Fuel rod oxidation begins Surge line fails by creep rupture, but failure is ignored Faulted loop steam generator tube fails by creep rupture Faulted loop hot leg nozzle melts, end of VICTORIA calculation

Steam Generator Tubes

Intact Loops Faulted Loop (C)

Csre (A&B)

Time (s) 0

20 1,960 9,090

11,620 13,730 14,960 33,750

Figure 1. Schematic of the VICTORIA representation of the Surry reactor vessel and primary circuits.

Figure 2 shows the VICTORIA nodalization of the faulted secondary circuit. Four nodes represent this region: one for the steam separators, one for the steam dryers, and two for the long steam line. The coolant and fission products exit from the faulted primary circuit into node 1 of

the secondary circuit; they exit from node 4 through the stuck-open ADV and release into the environment. Coupling the primary and secondary circuits was accomplished via a subdomain coupling technique [ 111.

A model for deposition of aerosols on the exterior surfaces of the steam generator tubes adjacent to a broken tube has been developed recently [ 121. This model accounts for impaction of aerosols against steam generator tubes by inertial effects. Although this deposition mechanism could have reduced the predicted fission product release to the environment in this investigation, it is not currently modeled by VICTORIA. However, for reasons explained below, it is unlikely that fission product deposition by this mechanism would be very large for the ISGI'R sequence studied here; thus, the omission of a model for this mechanism should not have a significant effect on the predicted results.

Steam Line ADV - 3 4

To Environment

rt- Steam Dryers A 1 rt- Steam Separators

From Ruptured Tube

Figure 2. Schematic of the VICTORIA representation of the Surry faulted secondary circuit.

4.0. Predicted Fission Product Behavior

Two cases are presented in this section. In the first, which is described in subsection 4.1, a single condensed phase is modeled; in the second, which is described in subsection 4.2, three condensed phases are modeled. Subsection 4.3 compares the results of the two cases and explores the mechanisms that are responsible for fission product deposition.

4.1. Results for Single-Condensed-Phase Case

Figure 3 shows the VICTORIA-predicted fission product release histories for the case in which a single condensed phase is modeled. Also shown in this figure is the peak core temperature history predicted by SCDAP/RELAPS. Sudden reductions in core temperature

correspond to accumulator injections. By the end of the transient, peak core temperatures exceed 2700 K while the bottom 40% of the core remains sufficiently cool that little fission product release occurs from this region. Thus, even for the noble gases, integral releases from he1 in core are only about 60% at the end of the sequence. Integral releases of iodine, cesium, and tellurium at the end of the sequence are only about 46%, 28%, and 32%, respectively. Figure 3 also shows that releases from core of the noble gases occur very early during the transient; releases of iodine, cesium, and tellurium are more protracted.

0.8-- E Ctl 0

-+I--- Noble Gases - $. -Iodine -f-Cesium -A- Tellurium ..... (3 ..... Temperahre

Time (s)

3500 cd

E 8 B

1500 53

3000 0

2500 4

cd 2000 g

8 CD

U

1000

Figure 3. SCDAPRELAPS-predicted peak core temperature and VICTORIA-predicted fission product release histories fiom fuel for ISGTR sequence using single-condensed-phase model.

VICTORIA-predicted releases from the faulted secondary into the environment are shown in Figure 4. Noble gases are released quite rapidly to the environment; the other fission products are released more gradually. Fission product releases into the environment occur over a four- to five-hour interval.

Table 3 provides details on the fractions of the core inventory of fission products that are retained in each region of the reactor vessel, primary circuits, and faulted secondary circuit. It also includes the fractions that exit through the PORV into containment and the fractions that are released from the secondary circuit into the environment. The integral noble gas release into the environment is predicted to be less than 13%; the balance of the noble gases that are released from core are predicted to exit from the RCS through the PORV into containment. (Fission product behavior in the surge line and pressurizer were not modeled in this analysis because the primary focus was on releases to the environment. For simplicity of discussion, fission products that exit through the surge line are considered to be released into containment.)

c, c 2 0.40 c 2 0.35 + 8 0.30 ..-(

g 0.10 .C( * 0 e 0.05 cr,

0 0.25 .&

-- --

a

Q)

3 0.20

6 0.15 d

Core 0.393

UP’ H L ~ S G ~ H L ~ SG= 0.000 0.000 0.000 0,000 0.000

1.5

0.715

2.0 2.5 Time (s)

0.056 0.001 0.037 0.001 0.041

3.0.104

0.676 0.005 0.001

Figure 4. VICTORIA-predicted fission product releases to the environment for ISGTR sequence using single-condensed-phase model.

0.056 0.000 0.098

Table 3. Fraction of Fission Product Inventory Retained by Region and Losses through Pressurizer into Containment and to Environment Using Single-Condensed-Phase Model

0.990

0.932

Elem.

N G ~

0.002 0.000 0.001 0.000 0.003

0.010 0.000 0.009 0.000 0.015

I

Cs

Te

Sr

Ba

Ru

Mo -

Reactor Vessel I Intact Loops I Faulted Loop

Secon. 0.000

0.539 I 0.001 I 0.000 I 0.096 I 0.000 I 0.046 0.004

0.019

0.036

0.001

0.007

0.999 1 0.000 I 0.000 I 0.000 I 0.000 I 0.000 I 0.000 . I I

0.879 I 0.004 I 0.000 I 0.021 I 0.000 I 0.023 I 0.014

Losses

0.049 I 0.264

0.001 1 0.002

0.005 1 0.021

0.000 I 0.000

0.001 I 0.057

a. Upper plenum. b. Hot leg. c. Steam generator. d. Noble gases.

Iodine release to the environment is predicted to be about double that for the noble gases, about 26% compared with 13%. There are two reasons for this difference: (1) more of the noble gases are released from core during the early phase of the sequence while the PORV is cycling and (2) some of the iodine released early in the sequence is deposited and is later revaporized, as shown in Figure 5 beginning at about 2.5*104 s. These factors both result in greater fractional releases of the noble gases into containment and less into the environment than are predicted for iodine. Revaporization of cesium and tellurium occurs during the sequence, but is not clearly demonstrated in Figure 5, which only shows the total deposits throughout the RCS.

a .d 22 0.4 B 6 0.3 5

$ 0.2

g 0.1

F=; 0.0

a

0 E

E cu 0

c.,

U

.H c., 0 crl I * / I

Figure 5. VICTORIA-predicted fission product deposition history for ISGI’R sequence using single- condensed-phase model.

Cesium and tellurium are retained in greater fractions within the RCS than iodine; their releases to the environment are predicted to be only about 8% and 12% of the fission product inventory, respectively. Other less volatile fission products are predicted to be released in even smaller fractions, largely because they are retained in core.

Contrary to expectations based on an earlier investigation of a spontaneous steam generator tube rupture sequence [ 131, little retention of the volatile fission product elements--iodine, cesium, and tellurium--is predicted to occur in the faulted secondary circuit for the ISGTR sequence. This is because temperatures throughout the secondary circuit exceed 1250 K by the end of the transient, and total gas pressures are well below ten bars throughout the sequence; under these conditions, the more volatile species, such as cesium iodide, remain primarily in the vapor phase and so do not deposit efficiently.

4.2. Results for Three-Condensed-Phase Case

Figure 6 shows the VICTORIA-predicted fission product release histories for the case when three condensed phases are modeled. The same peak core temperature history shown in Figure 3 is also included in Figure 6 as a point of reference. For this case, integral noble gas and iodine releases from core are both about 60% at the end of the sequence. Integral releases of cesium and tellurium from core are about 35% and 41% at the end of the sequence, respectively. As in the single-condensed-phase case, the noble gases are released very early during the transient; releases of cesium and tellurium are more protracted. Release of iodine is the most sensitive to the number of condensed phases modeled because cesium iodide, which is predicted to be the dominant form of iodine, is treated as a nearly pure phase. Because of the volatility of pure cesium iodide, the predicted iodine release history for this case is nearly identical to the one for the noble gases.

-E+ Noble Gases - e -Iodine -T-Cesium *Tellurium

1.0 '

0.8 - - .....e..... Tempemare

1.0 1.5 2.0 2.5 3.0.10

Time (s)

3500

3000

25 00

2000

1500

1000

Figure 6. SCDAPRELAPS-predicted peak core temperature and VICTORIA-predicted fission product release histories from fuel for ISGTR sequence using three-condensed-phase model.

VICTORIA-predicted releases from the faulted secondary to the environment are shown in Figure 7 for the three-condensed-phase case. The noble gases are released quite rapidly to the environment; the other fission products, including iodine, are released more gradually. Because the release histories from core for the noble gases and iodine are nearly identical, the significant difference between the release histories to the environment can only be explained by chemistry. As described above, some of the iodine is deposited during the early portion of the transient, while structural surfaces are still relatively cool; later, as these surfaces warm, some of the deposited iodine revaporizes. Predicted iodine revaporization for the three-condensed-phase case is much more dramatic than for the single-condensed-phase case, as seen by comparing Figures 8 and 5. Revaporization of some of the deposited cesium is also demonstrated in Figure 8. At least

part of the revaporized cesium is in the form of cesium iodide. Revaporization of tellurium also occurs during this sequence but is not clearly demonstrated in the figure, which only shows the total deposits throughout the RCS.

c,

E - * -Iodine

2 0.30 -*-----e- w

*Tellurium & .CI

0.25 .L.l t 3 0.20 3 22 0.15 0 & c 0.10

8 0.05 & 0.00

0 .CI c,

5l 1 .o 1.5 2.0 2.5 3.0 -10 Time (s) 0-

C .c,

U

Figure 7. VICTORIA-predicted fission product releases to the environment for ISGTR sequence using three-condensed-phase model.

Figure 8. VICTORIA-predicted fission product deposition history for ISGTR sequence using three-con- densed-phase model.

Table 4 is the analogue of Table 3 for the three-condensed-phase case. The integral noble gas release to the environment is predicted to be the same as for the single-condensed-phase case, about 13%. The balance of the noble gases released from fuel again is predicted to exit into containment. In this case, iodine, cesium, and tellurium releases to the environment are about 30%, 8.5%, and 13%, respectively. These values are only slightly higher than those for the single- condensed-phase case reported in Table 3.

Table 4. Fraction of Fission Product Inventory Retained by Region and Losses through Pressurizer into Containment and to Environment Using Three-Condensed-Phase Model

Mo 0.844 0.004 0.000 0.028 0.000 0.027 0.015 0.002 0.079

a. Upper plenum. b. Hot leg. e. Steam generator. d. Noble gases.

4.3. Comparison of the Two Cases

This section considers the differences in predictions for the one- and three-condensed-phase cases in each region of the RCS. Differences in the underlying mechanisms are the focus of the discussions. Differences between the two cases are summarized in Tables 5 and 6.

Releases from Core As expected, predicted releases from core are generally greater for the three-condensed-

phase case than for the single-condensed-phase case because the fission product volatilities are predicted to be higher for the three-condensed-phase case. The one exception is for noble gas releases, which are predicted to be independent of the number of condensed phases modeled, also as expected. Iodine is most affected by the number of condensed phases modeled; its release fraction, which is one minus the fraction retained in core, increases from 46% for the single- condensed-phase case to over 60% for the three-condensed-phase case, an increase of about 15%

of the fission product inventory and a relative increase of about one-third. Releases for the other fission products increase by less than 10% of the fission product inventory.

H L ~ same

Table 5. Fission Product Retention for the Three-Condensed-Phase Case as Compared with the Single-Condensed-Phase Case. Values Are Rounded to Nearest 1% of Fission Product

Inventory

S G ~ Secon. Cont. Envir.

same same same same

Reactor Vessel

same same

same

same

same

same

7 15%

same same same same

2% same 1% 1% more more more same same same same

same same same 2% more

. I

9% same less 1% same

less

Te

Sr

Ba I I Same

4% same MO I less I

Intact Loops

same less *

same

same more

Faulted ~~ Loop __ I ~ Losses

same 2% same 22% 3% 1 less I I more I more

same 1 sa;; 1 sa;; I same 1 more more

more more more more

a. Upper plenum. b. Hot leg. c. Steam generator. d. Noble gases.

Retention in the Upper Plenum Fission product retention in the upper plenum of the reactor vessel is small for all the fission

products except cesium because the temperatures in this region are predicted by SCDAPI RELAP5 to exceed 1700 K by the end of the transient. The dominant mechanism for cesium retention in this region is chemisorption of cesium hydroxide onto the stainless steel structures. About 2% more of the core inventory of cesium is deposited in this region for the three- condensed-phase case than for the single-condensed-phase case; the depositional difference results from the greater release of cesium from core and the enhanced volatilities of cesium species. These two factors result in about 50% greater concentrations of cesium hydroxide vapor throughout the RC S .

Gravitational settling is the dominant aerosol deposition mechanism in the upper plenum over most of the transient; thermophoretic deposition dominates later in the transient. However, vapor condensation directly onto structure surfaces dominates over aerosol deposition in this

region for all the species in the single-condensed-phase case and for all but iodine in the three- condensed-phase case. For the three-condensed-phase case, the iodine that deposits as an aerosol revaporizes later in the transient; thus, the net iodine retention in the upper plenum is negligible.

Elem.

I - 1 Cond. Phase I - 3 Cond. Phases

c s

Te

Sr

Ba

Ru

Table 6. Predicted Mechanisms for Fission Product Retention

Vessel

UP*

VCd GS 8~ TP

CSh

vc vc vc vc

I vc i vc

Mo I vc

Intact Loops

GS & TP

I GS cs

I GS vc

I GS vc

I GS vc

~

Faulted L

cs I G S & T P

vc I G S & T P vc I G S ~ T P vc I G S L T P

I vc vc

O P

Secondary

TP & GS & SCg TP & GS & SC

TP & GS & SC

vc & sc TP & GS & SC

TP & GS & SC

TP&GS&SC

vc & sc a. Upper plenum. b. Hot leg. c. Steam generator. d. Vapor condensation. e. Gravitational settling. f. Thermophoresis. g. Sudden contraction. h. Chemisorption.

Retention in the Hot Legs Very little fission product retention is predicted to occur in the three hot legs, regardless of

the number of condensed phases that are modeled, because the surface areas for deposition are relatively small in these regions and the temperatures are relatively hot. The faulted loop hot leg nozzle is predicted by SCDAPRELAPS to exceed the melting point of stainless steel, which is over 1700 K, by the end of the sequence. Even the intact hot legs reach temperatures of more than 1600 K on the upper side and more than 1200 K on the lower side by the end of the sequence. At these temperatures, the released fission products exist predominantly as vapors and so deposition is minor. The little deposition that does occur in these regions is mainly by vapor condensation for most of the fission products, by chemisorption for cesium, and by gravitational settling and thermophoresis for iodine when three condensed phases are modeled.

Intact Steam Generators For the single-condensed-phase case, more than 2% of the fission product inventories of

iodine, cesium, tellurium, and molybdenum is retained in the two intact steam generators at the end of the sequence; in fact, nearly 10% of the iodine is retained in this region. Most of the retention is on the large surfaces of the steam generator tubes, which are relatively cooler than the rest of the RCS, with the exception of the steam generator outlet plenum.

The primary deposition mechanism in the intact steam generators is gravitational settling, regardless of whether one or three condensed phases are modeled. Gravitational settling acts in the horizontal portions of the steam generator U tubes, where surfaces are in an orientation for settling to occur. However, for iodine the dominant deposition mechanism is vapor condensation directly onto surfaces when a single condensed phase is modeled. In this case, condensation occurs on the surfaces of the steam generator tubes. When three condensed phases are modeled, most of the condensation occurs in the steam generator outlet plena, where temperatures are the coldest of all locations in the RCS. Because the outlet plena have much smaller surface areas than the steam generator tubes and the residence time of gases in this region is short, less iodine is retained in the three-condensed-phase case than in the single-condensed-phase case.

Faulted Steam Generator Somewhat surprisingly, more fission products are retained in the single faulted steam

generator than in the two intact steam generators combined, regardless of the number of condensed phases that are modeled. This increased retention occurs because, once steam generator tube rupture occurs, there is about three times as much mass flow into the faulted loop as into the two intact loops combined. The difference in flow rates more than compensates for the fact that the intact loops remain cooler than the faulted loop.

Iodine, cesium, tellurium, and molybdenum are all retained in the faulted steam generator at fractions greater than 2% for the case when a single condensed phase is modeled; in addition, barium is retained at a fraction greater than 2% when three condensed phases are modeled. Tellurium is retained in the greatest fraction in this region: almost 10% of the core inventory is predicted to be retained when a single condensed phase is modeled; about 12.5% of the core inventory is predicted to be retained when three condensed phases are modeled.

Dominant aerosol deposition mechanisms are thermophoresis and gravitational settling, depending on the time during the sequence and on the location within the faulted steam generator. These mechanisms prevail regardless of the number of condensed phases that are modeled. For tellurium and molybdenum, vapor condensation dominates over aerosol deposition; for all the other fission products, aerosol deposition dominates.

Faulted Steam Generator Secondary Relatively small fractions of the fission products are retained on the secondary side of the

faulted steam generator and in the steam line leading to the stuck-open ADV. For the single- condensed-phase case, only tellurium is retained at a fraction greater than 2% of the fission product inventory; for the three-condensed-phase case, cesium and tellurium are both retained at 2% or more of the fission product inventory. The primary reason for the low retention in the region is that temperatures throughout the faulted secondary reach peaks of about 1250 K by the

end of the sequence. At such high temperatures, and at the relatively low gas pressures of the faulted secondary, many of the fission products remain in the vapor phase and thus do not deposit.

The dominant mechanisms for fission product deposition are thermophoresis in the steam separators and dryers, gravitational settling in the steam line, and deposition at the abrupt contraction of the ADV. On the other hand, most of the tellurium and molybdenum deposition in the steam separators and dryers and in the steam line, excluding the vicinity of the ADV, results from condensation of vapors directly onto structural surfaces. The same mechanisms prevail regardless of whether one or three condensed phases are modeled.

L S Note that, although predicted losses to the atmosphere are greater for the case where three

condensed phases are treated than for the case where a single condensed phase is treated, the predicted losses for the two cases are within 3% of the fission product inventory of each of the elements. This slight difference in predictions is inconsequential for risk analysis. However, the maximum difference in the fractions released into containment is not so small; 22% more of the core inventory of iodine is released into containment for the case in which three condensed phases are treated than for the case in which a single condensed phase is treated. Only iodine and the noble gases are released in large fraction into containment; the other elements are not released in such large fraction into containment because their release from core is slower and, as a result, only small fractions of these fission products have been released from core at the point when steam generator tube rupture occurs (about 15,000 s).

5.0. Summary

Analyses of an ISGTR sequence were performed using the mechanistic computer code VICTORIA [l]. Thermal-hydraulic data used as input for these analyses were taken from a previous SCDAPAELAPS [9] analysis of a station blackout sequence that led to an induced steam generator tube rupture [ 5 ] . The primary objective of the VICTORIA analyses was to determine the magnitude and timing of fission product releases to the environment for this hypothetical accident sequence.

A recently completed peer review of the VICTORIA code has led to some modifications to the VICTORIA models. One of the recommended modifications, an option to treat three condensed phases instead of a single condensed phase as in previous code versions, was used as the basis for a sensitivity study. The sensitivity of release fractions, deposition patterns, and deposition mechanisms to the number of condensed phases modeled is the major focus of this paper.

Using the single-condensed-phase option leads to predictions that more than half of the fission products, other than the noble gases, remain in the reactor core at the end of the sequence. However, significant fractions of the fission products that are released from core are also released to the environment. For example, about 26% of the iodine, 8% of the cesium, 11% of the tellurium, and 6% of the molybdenum inventories are predicted to be released to the environment.

Predicted releases from core and to the environment using the three-condensed-phase option are larger than those using the single-condensed-phase option. Iodine release from core is about 33% larger using the three condensed-phase option; nevertheless, iodine release to the environment is only predicted to be slightly larger because a significant fraction of the iodine is lost through the pressurizer and PORV into containment before steam generator tube rupture. Releases to the environment are only slightly larger for the other fission products; the maximum difference in the predicted release fractions is only about 3%.

Even though the magnitudes of deposited fission products are predicted to be different, dominant deposition mechanisms using the two options are very similar. The main differences occur because fission product volatilities are predicted to be greater using the three-condensed- phase option than using the single-condensed-phase option. The difference in predicted vapor pressures is greatest for iodine, which is predicted to deposit in the steam generator tubes using the single-condensed-phase option and to deposit in the steam generator outlet plena using the three-conden sed- p hase option.

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