ML24222A207
| ML24222A207 | |
| Person / Time | |
|---|---|
| Issue date: | 07/31/2024 |
| From: | Shawn Campbell, Hossein Esmaili, Michael Salay, Yuan Z NRC/RES/DSA/FSCB |
| To: | |
| Zhe Yuan 310-415-4005 | |
| References | |
| RES/FSCB 2024-01 | |
| Download: ML24222A207 (15) | |
Text
RES/FSCB 2024-01 Analysis of the Source Term in the Main Steamlines of BWRs after Early In-Vessel Phase Zhe Yuan, Shawn Campbell, Michael Salay and Hossein Esmaili July 2024 Fuel and Source Term Code Development Branch Division of Systems Analysis Office of Nuclear Regulatory Research United States Nuclear Regulatory Commission
ii TABLE OF CONTENTS 1.0 Objective and Approach............................................................................................. 1 2.0 RCS Nodalization and Selected MELCOR Scenarios............................................... 1 3.0 Results....................................................................................................................... 2 3.1 Vessel Breach Time and Phase Duration......................................................... 2 3.2 Maximum MSL Pipe Wall Temperature............................................................. 3 3.3 Fractions of Airborne Fission Product Inventory in MSLs................................. 4 3.4 Time-averaged Airborne Concentrations of Fission Products in MSLs............. 6 3.5 Airborne Removal Coefficient and Estimated Source Term after Early In-vessel Phase......................................................................................................................... 8 4.0 Summary.................................................................................................................... 9 5.0 References............................................................................................................... 10
iii LIST OF FIGURES Figure 1 Reactor Coolant System Nodalization........................................................... 2 Figure 2 Maximum pipe wall temperatures in MSL-A for 7 cases............................... 4 Figure 3 Fraction of halogens airborne inventory in MSL-A for 7 cases...................... 5 Figure 4 Fraction of alkali metals airborne inventory in MSL-A for 7 cases................. 5 Figure 5 Total halogens time-averaged airborne concentration.................................. 7 Figure 6 Total alkali metals time-averaged airborne concentration............................. 7 Figure 7 Regression of airborne concentrations of halogens and alkali metals in MSLs inboard of the MSIVs.............................................................................................. 8 Figure 8 Halogens fractional inventory in MSLs of BWRs during each accident phase.
9
iv LIST OF TABLES Table 1 DC power and system availabilities of the selected cases (Ref [4]).............. 2 Table 2 Times of vessel breach and durations of early in-vessel phase of selected cases............................................................................................................. 3 Table 3 Time-averaged airborne concentration of total halogens and total alkali metals fission products in MSLs of selected BWR cases.............................. 6 Table 4 BWR MSL source terms (release fraction) of halogens during each accident phase............................................................................................................. 9
v LIST OF ACRONYMS BWR Boiling Water Reactor CV Control Volume FLEX Flexible Coping Strategy FP Fission Product MSIV Main Steam Isolation Valve MSL Main Steamline RG Regulatory Guide RCS Reactor Cooling System RPV Reactor Pressure Vessel SBO Station Blackout VB Vessel Breach
1 1.0 OBJECTIVE AND APPROACH In accordance with the current regulations outlined in Regulatory Guide (RG)1.183, Revision 1
[1], the distribution of the containment source term has been assumed to have no variability across different regions. As a result, a single-region, representative and homogenized containment source term for all release pathways has been used in regulatory consequence analyses such as SAND2023-01313 [2]. However, recent studies on multi-region source terms in boiling water reactors (BWRs) [3] have indicated that accounting for variation in fission product (FP) distribution along different release pathways, including the main steamline (MSL) pathway due to leakage through main steam isolation valves (MSIVs), can lead to a more representative source term for BWRs [3]. Therefore, reference [3] proposes multi-region tabular source terms for the containment, MSL and liquid release pathways.
The source terms can be used by downstream codes, such as RADTRAD, for dose consequences assessment. These analyses are typically carried well beyond the end of the early in-vessel phase (up to vessel breach) that is the endpoint for the MELCOR results in references [2] and [3]. The objective of this analysis is to support the assessments of multi-region source terms [3] by estimating realistic time-averaged airborne concentrations of FPs, airborne removal rates and the source term in the MSLs of BWRs for an extended period after the early in-vessel phase. The analysis of the removal rate of the source term in the containment after the early in-vessel phase will be considered in separate analyses.
For this analysis, no additional MELCOR simulations were carried out. Instead, the results are derived directly from the simulations performed for the study presented in NUREG-2206 [4]. The MELCOR model used for these simulations is the same as that used by SOARCA [5] and is the predecessor to the model used in SAND2023-0131 [2]. The simulations were carried out over a 72-hour period, providing insights into the behavior of source term over an extended timeframe.
2.0 RCS NODALIZATION AND SELECTED MELCOR SCENARIOS Peach Bottom Unit 2, which features a Mark I containment, is selected as a representative BWR plant for this analysis. Figure 1 (reproduced from Ref [4]) illustrates the nodalization of its reactor cooling system (RCS). In this model, one steam line (Line A) inboard of the MSIV is represented by two control volumes, CV373 and CV 375, while the remaining three steam lines (Lines B, C and D) inboard of the MSIVs are combined into a single control volume, represented by CV 370.
A subset of the calculation matrix from Table 3-1 of NUREG-2206 [4] is used for this analysis.
The calculation matrix includes various sensitivities to investigate uncertainties in equipment availability. For this analysis, seven (7) cases of the calculation matrix, focusing on either short-term or long-term SBOs, are selected and presented in Table 1. These cases include two (2) cases without water injection, highlighted in yellow, and five (5) cases with continuous water injection, highlighted in orange. Flexible coping strategy (FLEX) operation is initiated upon vessel breach (VB), with FLEX water being injected into the RPV shroud dome (CV345).
2 Figure 1 Reactor Coolant System Nodalization Table 1 DC power and system availabilities of the selected cases (Ref [4])
Note:
Two cases with no water injection (highlighted in yellow) and five cases with water injection (highlighted in orange) are selected.
3.0 RESULTS 3.1 Vessel Breach Time and Phase Duration In the context of the regulatory source term, the early in-vessel phase starts with the release of 5% of the initial total Xe inventory from the fuel and continues until the vessel breach. Table 2 presents the times of vessel breach and the durations of the early in-vessel phase for the seven (7) selected cases. In short-term SBO cases, the vessel breach is predicted to occur at ~16.6 Option Case 0
4 72 0
4 16 SP CST 230 240 No Yes 15 5
21' Throttle
@ 21 '
Continuo us Thermal seizure -
fraction open Seizure on #
cycles?
Open Cycle (10/20 psid)
Initial Switchov er PCPL PSP 1
1 X
X X
X X
X X
100%
Enabled X
WW X
1 3
X X
X X
X X
X No Disabled X
WW X
2A 10 X
X X
X X
X X
X X
100%
Enabled X
WW DW X
2A 11 X
X X
X X
X X
X X
100%
Enabled X
WW DW X
2B 18 X
X X
X X
X X
X X
100%
Enabled 10/10 WW DW X
2B 16 X
X X
X X
X X
X X
100%
Enabled 20/-
WW DW X
2A 42 X
X X
X X
X X
X X
No Disabled X
WW DW X
Notes 0 DC power means there is no RPV pressure control, so should start like a SBO and remains so 10/10 means both WW and DW cycle at 10 psid 20/- means allow WW cycling at 20 psid but DW is not cycling and remains open WW Level Control Injection Allow SRV stuck Mode Setpoint (psig)
Allow after RCIC Injection @ LH Location Setpoint Availability (hr)
RCIC Availability (hr)
RCIC Suction Failure Temp (F)
Open SRV after RUN MATRIX REV 9 (10/15/2014) - Mark I Pre Core Damage Post Core Damage DC Power RCIC Operation Anticipatory Venting Flex Operation SRV Operation Containment Venting
3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />, while in long-term SBO cases, the vessel breach is predicted to occur between 23.0 and 25.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />.
Table 2 Times of vessel breach and durations of early in-vessel phase of selected cases Case #
Time of vessel breach (hours)
Duration of early in-vessel phase (hours) 1 22.98 8.93 3
16.62 8.05 11 23.43 9.95 16 25.52 11.47 18 23.85 9.78 42 16.62 8.05 10 23.83 9.76 3.2 Maximum MSL Pipe Wall Temperature Figure 2 illustrates the maximum pipe wall temperatures in MSL-A for two cases without water injection and five cases with water injection. In each case, the maximum pipe wall temperature is derived from the temperatures of seven heat structures representing the pipe walls of the MSL-A adjacent to CV 373 and CV 375. The results indicate that in all five cases with water injection, the maximum pipe wall temperatures begin to decrease after ~48 hours. In contrast, in the two cases without water injection, the maximum pipe wall temperatures continue to rise and are likely to reach the melting point shortly after 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, creating an open pathway from the MSL to containment. This would result in the concentration of FPs in the MSLs being the same as in containment. Therefore, only the water injection cases are considered to characterize the source term removal rate in the MSLs inboard of the MSIVs (see Section 3.5), although the water injection cases are still included in the analyses below for comparison.
4 Figure 2 Maximum pipe wall temperatures in MSL-A for 7 cases.
3.3 Fractions of Airborne Fission Product Inventory in MSLs Figures 3 and 4 show the fractions of airborne inventories of halogens and alkali metals in MSL-A, respectively. These fractions include both aerosols and vapors. All seven cases, both with and without water injection, are represented in the figures, using red for cases without water injection and blue for cases with water injection.
The fraction of halogens airborne inventory is defined as the ratio of airborne iodine mass (in classes I2 and CsI) to the initial iodine inventory. Similarly, the fraction of alkali metals airborne inventory is the ratio of airborne cesium mass (in classes Cs, CsI and CsM) to the initial cesium inventory. According to Reference [4], the initial inventories of halogens and alkali metals are 19.94 kg and 323.04 kg, respectively.
The figures reveal that the airborne fractions of both halogens and alkali metals vary significantly over time in all cases, particularly in the early stage of the transient. Generally, cases with water injection into the RPV at vessel breach exhibit a long-term reduction in the airborne fractions of both halogens and alkali metals inventories. In contrast, cases without water injection show an increase in these airborne fractions over time. These trends align with the maximum pipe wall temperatures observed in Figure 2. The elevated temperatures in the steamline lead to the revaporization of FPs and limit the retention of FPs on the MSL pipe walls.
0 200 400 600 800 1000 1200 1400 1600 1800 0
10 20 30 40 50 60 70 80 Temperature [K]
time [hr]
Cases with water injection Cases with no water injection
5 Figure 3 Fraction of halogens airborne inventory in MSL-A for 7 cases.
Figure 4 Fraction of alkali metals airborne inventory in MSL-A for 7 cases.
1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0
10 20 30 40 50 60 70 80 Fraction of inventory [-]
time [hr]
Total halogens Cases with water injection Cases with no water injection 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0
10 20 30 40 50 60 70 80 Fraction of inventory [-]
time [hr]
Total alkali metals Cases with water injection Cases with no water injection
6 3.4 Time-averaged Airborne Concentrations of Fission Products in MSLs Table 3 presents the time-averaged airborne concentration of total halogens (I) and total alkali metals (Cs) in the MSLs for all selected cases. Because these airborne fission product fractions vary significantly over time, as illustrated in Figures 3 and 4, representative time-averaged airborne concentrations are provided for three distinct periods: the early in-vessel phase, from vessel breach to 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the accident initiation, and from 48 to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> after the accident initiation. These concentrations are calculated based on the total volume of the four MSLs (35.8 m3) [4]. The results are also shown in Figures 5 and 6, respectively, where hollow markers indicate cases without water injection during the accident. The results clearly show a decline in the time-averaged airborne concentrations of both halogens and alkali metals over time when water is injected into the RPV, whereas the airborne concentrations remain high in cases without water injection (due to revaporization). Notably, on a logarithmic scale, the results exhibit a near-linear reduction in the concentration of both halogens and alkali metals when water is injected.
Table 3 Time-averaged airborne concentration of total halogens and total alkali metals fission products in MSLs of selected BWR cases Case #
Phase Time duration (hr)
Time-averaged airborne concentration in MSLs (fraction of inventory/m3)
Halogens Alkali metals 1
Early in-vessel phase (1) 8.93 7.984E-06 4.062E-06 From VB to 48 hrs 25.02 1.037E-06 1.331E-07 From 48 to 72 hrs 24 3.074E-05 2.269E-06 3
Early in-vessel phase 8.05 2.327E-05 5.794E-06 From VB to 48 hrs 31.38 9.426E-05 5.996E-06 From 48 to 72 hrs 24 3.117E-04 1.900E-05 11 Early in-vessel phase 9.95 5.491E-06 4.403E-06 From VB to 48 hrs 24.57 3.959E-07 3.351E-08 From 48 to 72 hrs 24 5.034E-07 2.362E-08 16 Early in-vessel phase 11.47 8.413E-06 3.375E-06 From VB to 48 hrs 22.48 3.416E-06 2.719E-07 From 48 to 72 hrs 24 7.738E-08 1.112E-08 18 Early in-vessel phase 9.78 8.507E-06 3.941E-06 From VB to 48 hrs 24.15 1.882E-06 1.414E-07 From 48 to 72 hrs 24 2.830E-07 1.371E-08 42 Early in-vessel phase 8.05 2.327E-05 5.795E-06 From VB to 48 hrs 31.38 3.608E-06 2.396E-07 From 48 to 72 hrs 24 2.583E-06 1.499E-07 10 Early in-vessel phase 9.76 7.254E-06 3.595E-06 From VB to 48 hrs 24.17 3.814E-07 4.667E-08 From 48 to 72 hrs 24 2.027E-07 9.901E-09 (1) Starting from release of 5% of initial, total Xe inventory from fuel to time of vessel breach (2) Total volume of the four MSLs is (35.8 m3) [4]
7 Figure 5 Total halogens time-averaged airborne concentration.
Figure 6 Total alkali metals time-averaged airborne concentration.
1.0E-10 1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 Concentrations (fraction of inventory/m3)
Total Halogens Time-Averaged Airborne Concentration in MSLs, early in-vessel phase in MSLs, VB to 48 hrs in MSLs, 48 to 72 hrs 1.0E-10 1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 Concentrations (fraction of inventory/m3)
Total Alkali Metals Time-Averaged Airborne Concentration in MSLs, early in-vessel phase in MSLs, VB to 48 hrs in MSLs, 48 to 72 hrs
8 3.5 Airborne Removal Coefficient and Estimated Source Term after Early In-vessel Phase Figure 7 presents the results of a regression analysis of airborne concentrations of both halogens and alkali metals in the MSLs. This analysis was performed using Excels build-in regression tool and was also manually verified. The time-averaged airborne concentrations for both halogens and alkali metals were taken over three different durations (refer to Table 3) from the 5 cases that credit water injection into the RPV.
The airborne removal coefficient,, is expressed by the equation: =
Which can be rewritten as:
=
- Where C(t) is time dependent airborne concentration and is the initial airborne concentration, both in kg/m3.
In this regression analysis, the reference point is set at the time of vessel breach (i.e., tVB = 0 sec). The subsequent time intervals (t-tVB) are then defined from 0 to the midpoint between VB and 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />, and from 0 to the midpoint between 48 and 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. The regression yields a single airborne removal coefficient () of 3.4E-5 (1/s) (0.12 /hr), applicable to both halogens and alkali metals, with an R2 value of 0.81. This is an acceptable long-term removal rate in the cases where the MSLs are cooled sufficient to prevent revaporization.
Figure 7 Regression of airborne concentrations of halogens and alkali metals in MSLs inboard of the MSIVs.
The regressed airborne removal coefficient can be utilized to reasonably predict the variation in the regulatory source term (release fraction) in the steamlines (inboard of the MSIVs) of BRWs over time after the early in-vessel phase. To clarify, Table 4 and Figure 8 illustrate the proposed release fraction of halogens in MSLs of BWRs after the early in-vessel phase.
-10.0
-8.0
-6.0
-4.0
-2.0 0.0 2.0 0.0E+0 2.0E+4 4.0E+4 6.0E+4 8.0E+4 1.0E+5 1.2E+5 1.4E+5 1.6E+5 1.8E+5 ln (Conc/Conc0) (-)
Time (sec) g Halogens Alkali Metals Regression: ln(conc/conc0) =-3.4E-5*t, (R square = 0.810)
9 Table 4 BWR MSL source terms (release fraction) of halogens during each accident phase Accident phase Phase duration (hr)
Release fraction [-]
Gap release (1) 0 - 0.7 5.6x10-6 Early in-vessel (1) 0.7 - 7.4 5.1x10-5 After early in-vessel 7.4 -
5.1x10-5xexp[-3.4x10-5xt(s)]
(1) These values are taken from Table 4 of Ref [3]
Figure 8 Halogens fractional inventory in MSLs of BWRs during each accident phase.
4.0
SUMMARY
To support the multi-region source terms for BWR containment design leakage assessments conducted by SNL [3], which covered the early in-vessel phase, this analysis utilizes the simulations presented in NUREG-2206 [4] over a 72-hour period. It offers a representative source term and airborne removal coefficient specifically for the main steamlines of BWRs inboard of the MSIVs after the early in-vessel phase, providing insights into the behavior of source term over an extended timeframe.
1.0E-44 1.0E-40 1.0E-36 1.0E-32 1.0E-28 1.0E-24 1.0E-20 1.0E-16 1.0E-12 1.0E-08 1.0E-04 1.0E+00 0
1 10 100 1000 Fraction of inventory (-)
Time (hr)
Halogens Fractional Inventory in MSLs During Each Accident Phase Gap release phase Early in-vessel phase After early in-vessel phase: Airborne removal coefficient = -3.4E-5 (1/s)
10
5.0 REFERENCES
[1]
United States Nuclear Regulatory Commission, Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors, Regulatory Guide 1.183 Revision 1, U.S. Nuclear Regulatory Commission, Washington, D.C., 2023.
[2]
Sandia National Laboratories, SAND2023-01313, High Burnup Fuel Source Term Accident Sequence Analysis, Albuquerque, New Mexico, April 2023 (ML23097A087).
[3]
Lucas I. Albright, David L. Luxat, and Kenneth C. Wagner, Multi-region Tabular Source Terms for BWR Containment Design Leakage Assessments, Sandia National Laboratories, May 2024.
[4]
Jonathan Barr, Sudhamay Basu, Hossein Esmaili and Martin Stutzke, Technical Basis for the Containment Protection and Release Reduction Rulemaking for BWRs with Mark I and Mark II Containments, Office of Nuclear Regulatory Research, US NRC, NUREG-2206, March 2018.
[5]
Nathan E. Bixler, Randall O. Gauntt, Joseph A. Jones, Mark T. Leonard, State-of-the-Art Reactor Consequence Analyses Project Volume 1: Peach Bottom Integrated Analysis, US NRC, NUREG/CR-7110, Volume 1, May 2013.