TRACE CONTAN Model. TRACE User Problems by Jay Spore, ISL. NRC Notes

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1 A Sponsored by the U.S. Nuclear Regulatory Commission dequate simulation of containment pressure and temperature responses is important for nuclear power plant safety analysis. During loss of-coolant accidents (LOCAs) in conventional pressurized water reactors (PWRs), containment pressure significantly affects the reflood hydraulic and heat transfer processes, and consequently the peak cladding temperature. For LOCAs in conventional boiling water reactors (BWRs) the reactor pressure vessel and containment are strongly coupled because the suppression pool is a TRACE CONTAN Model TRACE User Problems by Jay Spore, ISL Volume 12 / Number 1 / April 2012 source for emergency core coolant injection. Suppression pool temperatures are controlled by the flows of steam/air mixtures from the containment drywell through vents into the torus or wetwell. Because many of the advanced nuclear plants employ gravity driven safety injection systems, the containment behavior is even more influential for safety than it is for the conventional plants. (Continued on page 2) T his is a report on TRACE user problems and resolutions. Open trouble reports identified in the previous newsletters with no new progress are not discussed, but can be found in the TRACE trouble report system (TRACEZilla) on the TRACE website ( For the time period between December 17, 2011 and March 31, 2012, 16 new trouble reports were submitted to TRACEzilla. Twenty-four trouble reports were either resolved or closed during this same time period. As of March 31, 2011, there are 599 trouble reports in the TRACE bug reporting system; 27 of those are open, with the remainder resolved, closed, or duplicates. (Continued on page 4) A ssessment of the Patch3 version of TRACE5 is now underway. Several critical bug fixes, such as eliminating/reducing inexact restart problems, have been addressed in the candidate release version. Also, modifications have been made to improve code robustness. The planned release date for Patch3 is late Spring or early Summer Users or organizations unable or unwilling to wait for this release date may send a request to Antony Calvo to obtain the latest development code version. NRC Notes current TRACE development. TRACE is being used extensively for confirmatory analysis of both current design and advanced plants. Much of the recent attention has been focused on modeling of coupled thermal-hydraulic/neutronic phenomena during transients such as over-cooling events, control rod bank withdrawal, rod drop, rod ejection, flow boiling instabilities, and Anticipated Transients Without SCRAM (ATWS), and on features needed to model small modular reactors. Current and anticipated applications of TRACE and TRACE/PARCS at the NRC are driving much of the Thermal-Hydraulic Code News Committed to the support of the T/H Codes User Community 1 Contents: TRACE CONTAN Model TRACE User Problems NRC Notes Recent RELAP5 User Problems Website: (Continued on page 3)

2 TRACE CONTAN Model continued from page 1 For example, some proposed integral pressurized-water reactor (ipwr) designs, such as mpower by Babcock & Wilcox, have a reinforced concrete, steel-lined, dry containment that functions in a fully passive manner. Therefore, the NRC is in the process of improving the containment modeling capability in TRACE. Basic simulation capabilities for representing containment behavior were previously introduced into TRACE with addition of the CONTAN component. The TRACE CONTAN component is a lumped parameter containment model similar to other lumped parameter containment codes such as CONTAIN and CONTEMPT. Having the CONTAN component integral to TRACE eliminates the need for coupling with a containment code, a sometimes inefficient process in terms of both CPU and user "care and feeding" time. This feature is particularly useful when the main thrust of the analysis is the NSSS response, with the containment parameters serving primarily as boundary conditions. Efforts have been initiated to replace the PIPE, VESSEL and BREAK components used in plant models with CONTAN components for a more consistent containment modeling approach. During the verification and validation process of the CONTAN component, several deficiencies were identified. A project is now underway to: (1) develop improved condensation heat transfer models for containments, (2) assess the performance of these new models through comparisons with relevant experimental data, (3) implement new capabilities for control of CONTAN model features using TRACE control blocks, signal variables and trips, (4) develop CONTAN capabilities for modeling containment engineered safety features, such as fan cooler and spray systems, and (5) identify remaining deficiencies in the CONTAN component and recommend additional work to improve TRACE containment simulation capabilities. Prior assessments of the TRACE CONTAN component model generally indicated underprediction of condensation within compartments, leading to overprediction of the containment pressure during LOCAs. Liquid in the improved CONTAN model is either in the pool, the film or the droplets (as shown in the accompanying figure). A new CONTAN film condensation heat transfer model determines film thicknesses on vertical walls and the rates of film flows into pools at the bottoms of compartments. The film 2 break film pool drops condensation calculation is based on the existing TRACE film condensation heat transfer model. A droplet field was also added to the CONTAN compartment fluid model. The liquid mass flow from a break that does not flash as the effluent depressurizes to the containment pressure is assumed to be in the form of droplets. Drift flux models are used to determine droplet size and settling velocity. Values of these quantities are needed for calculating interfacial heat transfer rates between the droplets and the surrounding steam/air mixture. To assess the new CONTAN models, TRACE calculations were performed and compared with relevant data from blowdown tests in four experimental facilities: Carolinas Virginia Tube Reactor (CVTR), Heiβdampfreaktor (HDR), Marviken, and the Pressure Suppression Test Facility (PSTF). These facilities simulated large dry PWR containments (CVTR and HDR) and the drywell and wetwell regions of BWR containments (Marviken and PSTF). Overall, the assessments showed reasonable to excellent agreement with the test data for containment pressures and temperatures. However, some of the comparisons indicated large calculated flow oscillations among containment compartments that were not exhibited in the test data. There are plans to implement improved numerics for the flows of mass, momentum and energy among compartments. This implementation is expected to successfully address the unphysical calculated flow oscillations among compartments observed in some of the current assessments. CONTAN component capabilities are also being upgraded to provide simulation flexibility and more realistic models for prototype containment systems. TRACE signal variables were added for CONTAN compartment problem variables, such as pressures, temperatures and densities. Capabilities were added for controlling CONTAN component model features (such as flow and temperature boundary conditions) using TRACE signal variables, control blocks and trips. A new, more physically based model is being added to the CONTAN component for simulating containment fan cooler system performance. A model for simulating

3 containment spray system performance is also being added. An additional droplet field, similar to that used for the break liquid discharge, is included to represent the containment spray liquid discharge. Work on the new containment fan cooler and spray system models is in progress at this time. Following completion of the project and implementation of the improved numerics, the CONTAN component will be placed into a version of TRACE so that it can be subjected to testing by the user community. Recent RELAP5 User Problems by Dan Prelewicz, ISL R ELAP5 user problems reported or resolved are summarized in each issue of the newsletter. If you encounter a problem with RELAP5, please report it to Joseph.Staudenmeier@nrc.gov. The complete list of RELAP5 user problems, including a description of the problem, status (resolved, in work, on hold or unresolvable) and, if resolved, the manner of resolution, is on the web site. Since the last TH newsletter was published two new user problems were submitted, of which one was resolved and one is in work. The section below informs users on the problems that have been submitted and, if resolved, discusses the manner of resolution. No (1/30/2012) Code Versions Affected: RELAP5 3.3 Patch04 A user had a simple model with a TDV/TDJ on the inlet end, a pipe in the middle, and a single junction/tdv on the outlet end. In looking at superheated conditions at a constant pressure, the gas temperature was being increased some arbitrary amount every 10 seconds in the TDVs from approximately saturation to 5,000 K. However, the results showed the temperature stopped increasing at 500 K. The problem exists when using the old H2O properties, but works for the new H2O properties file. A problem was identified in tstate() for (P,T) calls. The vapor property wasn't being set for T > Tcrit and P < Pcrit, so the gas temperature (tempg) stayed at the last table value below the critical temperature (Tcrit), in the user s case, 500 K. A simple change was implemented to allow the code to set vapor properties when T > Tcrit. The user s problem now has the appropriate values for the TDV, but the code eventually fails (properly) when T > Tlimit ( K). This problem is resolved. No (04/02/2012) Code Versions Affected: RELAP5 Patch04 3 A user reported a discrepancy between the sonic velocity at the choking plane (sonicj) compared to the upstream homogeneous equilibrium sound speed (sounde). The results were obtained with the Henry- Fauske model for single phase superheated vapor. Differences observed were on the order of 30 to 40%. Calculations performed with the Ransom-Trapp model showed only a 3 to 4% difference. Investigation of the problem revealed that the Henry- Fauske model in RELAP5 uses the formula by Shapiro for isentropic flow of a perfect gas, which is based on the stagnation pressure and temperature. These conditions are considerably different than the throat conditions used by Ransom-Trapp and the homogeneous equilibrium model. For the pure vapor Henry-Fauske model, Section 7.3 in Volume IV of the RELAP manuals discusses the difference between the stagnation and throat condition (pg. 405) and includes a plot showing the discrepancy at various pressures for the saturated vapor case. At low pressure, the discrepancy is negligible. However, for superheated vapor it s not clear how much of a discrepancy there exists at low pressure. More investigation is required. This problem is in work. NRC Notes from page 1 Detailed modeling of the power distribution and its interaction with thermal-hydraulic phenomena is important for these events. Nuclear data including cross sections is interpolated at current fluid property states through the coupling. Much of the current work involves resolution of problems and issues that arise as NRC users develop and exercise models to simulate events in current generation and advanced plants, especially BWRs. This interaction between the code development and applications is resulting in continuous improvement in code capabilities, automation of routine tasks, and expansion of the assessment cases that exercise coupled TRACE/ PARCS capabilities. Work is also progressing on adding features needed to model small modular reactors (integral PWRs). Preapplication reviews are underway for a number of designs and additional applications are expected. Current work includes adding the capability to model helical coil steam generators and improvements to the CONTAN model (see article in this newsletter). If you haven t yet visited the new SharePoint site, you might want to take a look at the features and information that have been included. The SharePoint site has replaced both the

4 and sites. Users were sent an with instructions on how to login and then change their password, both for the SharePoint login and for Bugzilla. Antony Calvo is the contact point for the SharePoint site. If you have questions or would like additional information, on the NRC TH codes, please contact Chris Murray at or TRACE User Problems from page 1 A resolved trouble report indicates that it has been addressed with an update or documentation change that is pending. A closed trouble report indicates that it has either been addressed by successfully re-running the test problem(s) of interest with a current version of TRACE or it has been closed by incorporating a pending update into the NRC developmental version of TRACE. A pending update implies that it has been tested, documented, reviewed, and submitted to the NRC, but has not been included in the developmental version of TRACE. Inclusion of a pending update into TRACE typically requires additional review and testing by the NRC staff. The most current developmental version of TRACE is Version as of March 31, Updates included in Version can be found on the TRACE user support web site ( New Trouble Reports Still Open The following new trouble reports are still open and have not been resolved. Trouble Report 599 Crossflow tube bank wall drag. A developer reported that the FxCrossFlowHT update currently in the TRACE Holding Bin did not include models for pressure drop or flow loss for cross flow across a tube bank. Available models for cross flow across a tube bank are typically given in terms of a flow loss coefficient that is a function of the Reynolds number for the flow through the tube bank. Although TRACE has this capability, the available function for this model goes as K-loss = A * Re ** B. The available data is not accurately fit by this function and a table for K-loss as a function of the Reynolds number is proposed. This trouble report is still open. Trouble Reports Closed During the time period from December 17, 2011 to March 31, 2012 the following trouble reports have been closed with updates included into the NRC developmental version of TRACE. Alternatively, these trouble reports have been closed by rerunning the test 4 problems of interest with the most current version of TRACE and/or by modifying TRACE documentation to address the issue. Trouble Report 496 "Non-stopping" flow with no constant driving force in VESSEL component. A user reported that for a MASLWR TRACE model static test the long term flows in the VESSEL component were not approaching zero as expected. This trouble report was initially marked as closed in 2010 because turning on the level model in the VESSEL component resulted in elimination of the oscillations that were occurring in 3D VESSEL cells where subcooled liquid was in contact with steam. The pressurizer is essentially inside of the 3D VESSEL component and turning on level tracking where the two-phase level is located is necessary to keep interfacial heat transfer areas consistent with a vertical stratified level. This trouble report was reopened when a user reported oscillations in the steam generator model. It was determined that these oscillations were due to flow regime and heat transfer regime oscillations during steady-state that could be effectively eliminated by renoding the once through steam generators. For a feedwater transient with version of TRACE some unexpected changes were observed in pressure as the level in the VESSEL component changed. This behavior was traced to errors in the interfacial heat transfer model for 3D cells with levels. These errors were fixed with update Fx3DLevIHT which went into version This trouble report is closed. Trouble Report 570 Built-in heat structures need capability to input PDRAT (i.e., pitch to diameter ratio). A user reported that warnings concerning the setting of a default PDRAT for spawned heat structures (HSs) could not be eliminated by input changes, since the HSs were spawned and there is no input available for PDRAT for spawned HSs. PDRAT, the pitch to diameter ratio for rod bundles or steam generator tube banks, etc. is an input available for HTSTR component input. However, a PIPE component with nodes > 0 will spawn a HS and the PDRAT is set to a default. Current coding in TRACE will write a warning indicating that PDRAT has been set to a default value for this spawned HS. This can be frustrating to the user since there is no input capability for PDRAT for spawned HSs. In general, the default for PDRAT for cylindrical geometry HS is 1.0 for the inside surface and 1.33 for the outside surface. If the user does not wish to use these defaults for a spawned HS, then the user can change the HS to a HTSTR input, where the PDRAT for the outside surface is an available user input. A request to allow for the conversion of spawned HS to HTSTR input via the SNAP Modeling Editor was made to the SNAP development team. Update FxPDRATWarn modifies logic to eliminate the PDRAT warning for spawned HSs to reduce user frustration. In addition, update FxPDRATWarn contains

5 documentation updates to make users aware of the defaults for PDRAT for spawned HSs. This update went into version and closed this trouble report. Trouble Report 572 Using SJC VALVE for small break modeling. A user reported that significantly different results were obtained when a SJC VALVE was used for small break modeling as compared to using a SJC PIPE. The recommendation for small break modeling is to use a SJC PIPE and not a SJC VALVE. Internally calculated losses within a TRACE VALVE component can result in small break mass fluxes smaller than the choked flow mass fluxes. When update FxValveAddL went into version 5.220, the internal calculated losses for a VALVE component could be replaced with a flow loss table. Note that the user must set namelist input INTLOSSOFF to.true., and supply the flow loss table in the VALVE component input. In addition, the SJC VALVE in this plant model was different from the SJC PIPE because of the following items: 1) The SJC VALVE had a trip set point at seconds into the transient. However, the first time step was 0.1 seconds, so the VALVE did not start to open until 0.1 seconds into the transient. 2) The SJC VALVE table opens fully in seconds, but because of some old TRAC-P valve limiting logic the SJC VALVE was not fully open until approximately 0.5 seconds into the transient. 3) As the VALVE was opening there was one time step where the VAVLE component was not choked but the velocity was approximately 700 m/s with a very small valve open flow area. The velocity through the VALVE should never have been over 150 m/s. 4) The choked flow model limiting logic required approximately two seconds for the velocities through the VALVE to go from ~700 m/s down to ~150 m/s. The SJC PIPE first time step now starts out with choked velocities at ~150 m/s as expected. The FxValveTrans update eliminates the old TRAC-P VALVE component limiting logic. This logic in TRAC-P was there in an attempt to stop VALVE chatter. Users can now stop VALVE chatter with appropriate inputs for the RVMX (i.e., maximum rate at which a VALVE component can open or close). With the FxValveTrans update the small break results for this plant are very similar for the SJC VALVE and the SJC PIPE. With the FxValveTrans update and the SJC VALVE there is a spike in the choked flow mass flux as the valve starts to open, but the mass flux quickly relaxes to the values consistent with the SJC PIPE small break model. The FxValveTrans update went into version and closed this trouble report. 5 The recommendation for small break modeling is not to use a SJC VALVE component, even if TRACE contains the FxValveTrans update. There is no advantage to using a SJC VALVE component rather than an SJC PIPE component. Each small break simulation with different size breaks requires modification of the SJC PIPE or SJC VALVE input. Trouble Report 576 Heat Structure (HS) temperatures do not change during transient. A user reported that while running a CVTR test facility TRACE model with version 5.440, the heat structure temperatures were not changing. This was traced to logic errors in the TRACE HS to CONTAN compartment communication logic that were introduced when the TRACE HS general solution logic was modified to be based on heat fluxes. FxHSCONT, which went into version 5.509, fixed this logic error and closed this trouble report. Trouble Report 581 Code failure for version but not for 5.0 Patch 2. A user reported that a TRACE model of the MASLWR test facility executed successfully for a 6,000 second initialization run with version 5.0 Patch 2, but failed with version This trouble report could not be duplicated. This test problem was run with versions 5.333, 5.505, 5.508, and However, the test problem did not fail with any of these versions. A number of input changes were recommended to improve the accuracy and robustness of this input model, including turning on level tracking in the 3D VESSEL. This closes this trouble report. Trouble Report 583 Equation of the theory manual does not seem to agree with Figure A user reported that Eq for the two-phase multiplier of (1 alpha)**(-1.72) was not consistent with Figure There is no inconsistency, since the equation is for phi**2 and the plot is for phi, where phi is the two-phase multiplier. This closes this trouble report. Trouble Report 586 TRACE/PARCS does not couple when the fine mesh heat transfer heat structure (HS) model is active. A user reported that attempts to run a TRACE/PARCS coupled calculation with the fine mesh model turned on resulted in input errors for version The TRACE/PARCS communication logic for TRACE HSs in version did not support HSs with fine mesh turned on. This was originally reported in Trouble report 249, which was closed based on the assumption it would not be necessary to have a TRACE/PARCS coupled calculation with the fine mesh model turned on. However, this user had a specific problem where fine

6 mesh was needed since axial conduction was expected to be an important phenomenon during an ATWS calculation. The FxFinePARCS update added logic to allow for TRACE/PARCS coupled calculations with fine mesh turned on. This update went into version and closed this trouble report. Trouble Report 587 CVF Developer Studio project files need to be cleaned up. A user reported that starting with version files were removed from TRACE, but were still in the CVF Developer Studio project files. In addition, other update between version and added new files to the TRACE source files and these new files were not added to the CVF project files. Changes to the project files were made to version 5.521, which closes this trouble report. Trouble Report bit Lahey debug compile on LINUX operating systems is resulting in a number of failures due to undefined variables. A developer reported that version with the LINUX 64 bit Debug Lahey compiler resulted in a number of Regression test problems failing with undefined variables, when these test problems were expected to run. Update Fx64bLahey, which resolved these failures, went into version This closes this trouble report. Trouble Report 594 Regression testing scripts were failing on the LINUX2 operating system. A developer reported that the Regression testing perl scripts were failing on the LINUX2 operating system. It appeared that the first line of the scripts (i.e., #!/usr/bin/env perl) was working as expected and telling the operating system the rest of this file is a perl script to be executed by perl. For this operating system in the runtests.pl script it was necessary to make the following change from: system (../maketestmake.pl ) To system( perl../maketestmake.pl ) This is not a general and portable fix to the runtests.pl perl script and should only be included if necessary. This trouble report is closed. Trouble Report 595 Significant NCG mass error with accumulator in ROSA model. A user reported that a significant non-condensable gas (NCG) mass error was observed as the accumulator discharged in a ROSA test simulation. For this TRAC input model the user was using the TRAC-P thermo fits. 6 The lower limit to these thermo fits is K (i.e., triple point of water). During discharge for this accumulator the gas phase temperature in the accumulator should drop below the triple point as the NCG expands. TRACE would not allow the gas phase temperature to go lower than the triple point. This resulted in the mass and energy error that was observed for this calculation. The TRACE steam tables allow for gas phase temperature as low as 140 K. When the equation of state (EOS) model for this input was changed to the TRACE steam tables (i.e., USE_IAPWS_ST =.TRUE., then the NCG mass error was eliminated and the gas phase temperature in the accumulator went down as low as 210K during the discharge phase of the accident scenario. This closes this trouble report. Trouble Reports Resolved with Updates Pending During the time period from December 17, 2011 to March 31, 2012 the following Trouble reports have been resolved with updates or documentation modifications pending. They are waiting for NRC review and a decision to implement or not to implement into TRACE. Trouble Report 403 Signal variables for CONTAN component. A user requested that signal variables be added to the TRACE CONTAN component. The CONTANSV update was originally developed with a limited number of CONTAN signal variables. The CONTANSV update was expanded to include all anticipated signal variable needs for the CONTAN component and moved up to be consistent with version FxFilmHTCont. The modified update was uploaded. This update is pending. In addition the CONTANCTRL update was uploaded to the TRACE Holding Bin. This update adds control block capability to the CONTAN component. This update is pending. Trouble Report 520 Problem with D2O properties at pressures greater than 100 bars. A user reported the need to simulate D2O properties at pressures above 100 bars. The original implementation of the D2O properties in the TRAC-P code was for reactors operating at or near to atmospheric pressure and therefore the TRAC-P D2O thermo fits covered a limited range of pressures. The maximum allowed pressure for these D2O thermo fits was 100 bars. On , update FxHeavyWater went into version 5.410, resolving this trouble report by including the capability to read and use RELAP5 D2O steam tables. On this trouble report was reopened because the logic in two RELAP5 D2O routines (i.e., std2x1 and std2x3) was found to be failing. In order to be consistent with the TRACE programming guidelines it was necessary to re-write the Fortran 77 versions of

7 std2x1 and std2x3 using modern programming standards. For certain conditions the modernized versions of std2x1 and std2x3 were failing to return the correct D2O properties. Update FxD2oSTLT was submitted to the TRACE Holding Bin and resolved this trouble report by fixing the logic errors in the modernized versions of std2x1 and std2x3. Trouble Report 577 Signal Variable (SV) type 105 (enthalpy) does not work for FILL and BREAK components. A user reported that signal variable type 105, mixture enthalpy, did not work for FILLs and BREAKs. In version the FILL/BREAK logic for calculating mixture enthalpy was changed from a static enthalpy to a flow enthalpy. Variable names were modified, however the signal variable type 105 logic still pointed to the old variable names. The user could use the generic signal variable luflowenthalpy for FILL and BREAK components. Update FxSV105 fixes the logic for signal variable type 105 so it points to the appropriate variable for FILLs and BREAKs. This resolves this Trouble report. The update is pending. Trouble Report 582 Heavy Water steam table minimum temperature should be K. A user reported the minimum temperature for RELAP5 D2O steam tables should be K, but is K. This causes some run time problems with RELAP5 because the D2O minimum temperature is not the same throughout the code. The TRACE implementation of the RELAP5 D2O steam tables obtains the minimum allowed temperature from the steam table files and there is no internal TRACE coding that assumes some other minimum allowed temperature. However, the D2O steam tables for TRACE were regenerated with a minimum allowed temperature of K. The FxD2oSTLT update was uploaded to the TRACE Holding Bin. This update resolves this trouble report. In addition, this update fixes some problems identified with the standalone equation of state (EOS) programs in the UnitTests and Tools sub-directory. Trouble Report 584 Enable side junctions for PRIZER. A user requested that the PRIZER component have the capability to simulate side junctions to allow simulation of PORVs, sprays, and SRVs. The FxPRIZERSJ update was developed to resolve this trouble report. This update adds a PIPETYPE = 9 option to the PIPE component that results in a pressurizer model. This allows pressurizer models to have side junctions or any other capability available with a PIPE component. This update is pending. Trouble Report 585 Enhancement to build-in heater model in PRIZER component. 7 A user requested that a PRIZER component heater model have trip capability. With the FxPRIZERSJ the PRIZER component heater models are moved over to be used with the PIPE component. With PIPETYPE = 9, the power to fluid model includes trip capability. This update resolves this trouble report. This update is pending. Trouble Report 588 Suggested improvement to PIPE wall model (Spawned HTSTR components). A user suggested adding the capability for variable noding for heat structures (HS) spawned by PIPE components. PIPE component input with nodes > 0 results in a spawned HS component that represents pipe wall heat transfer. However, the current logic in TRACE uses uniform node sizes in the radial direction across the pipe wall. The outside surface of the pipe wall is typically adiabatic, or a relatively small heat transfer coefficient for natural circulation in an atmospheric environment. This implies that the largest temperature gradient will be at the inside surface. The recommended noding in this case is to have relatively small nodes on the order of millimeters on the inside, with node size increasing as the outside surface is approached. It was decided that SNAP should be used to provide this user modeling option and generate the input for a HTSTR component which allows for non-uniform noding. During investigation of the trouble report it was noted that when ISOLVEDEF = 1, HSs spawned by fluid components still have ISOLVE = 0. For the HS component, ISOLVE = 0 implies a finite volume conduction solver and ISOLVE = 1 implies a 2 nd order finite element conduction solver. The 2 nd order finite element conduction solver can have the same level of accuracy with a larger radial node spacing than the finite volume conduction solver. Update FxFESpawn was developed to add logic to HS spawning so that when ISOLVEDEF = 1, then the fluid component HSs are spawned with ISOLVE = 1. This update resolves this Trouble report and is pending. Trouble Report 590 Plot variables tsatp and tsats are incorrect in British units. A user noted that CONTAN component plot variables tsatp and tsats were incorrect when plotted with British units. It was determined that in the EngUnits Module in TRACE plot variables tsatp and tsats were labeled with units of temperature difference rather than absolute temperature. Conversion from SI to English units for temperature difference goes as 1.8*dK = df, where dk is temperature difference in Kelvin and df is temperature difference in Fahrenheit. Conversion from SI to English units for absolute temperature goes as 1.8*K = F, where K is absolute temperature in Kelvin and F is

8 temperature in Fahrenheit. Fixes for the units labels for tsats and tsatp were included in the FxValveOpenChoke update, which resolves this trouble report and is pending. Trouble Report 591 Potential error with direct moderator heating. A user reported that steady-state fuel rod peak heat flux was not consistent with expectations when the POWER component promheat was varied from 0.0 to 1.0, with decaheat = 0.0. However, when promheat = decaheat, then results were consistent with expectations. Update FxPKPowSource that went into version made some documentation and coding changes in an attempt to reduce the confusion that may occur for the promheat and decaheat POWER component input. Specifically, when the total reactor power is specified during a transient there is an attempt to split the user specified reactor power up into power due to fissions and power due to decay heat. The total direct moderator heating is given by promheat * power due to fissions, plus decaheat * power due to decay heat. When decaheat is zero, the user must know the estimated fission power to determine the total direct moderator heating. A decay heat fraction is typically on the order of 6%, which implies that 94% of the reactor power is due to fissions at steady-state. However, following a scram the decay heat is still on the order of 6% while the fission power is approaching zero. For the case with total reactor power user specified, use of constants for promheat and decaheat is not appropriate. If the reactor power was calculated from the point kinetics model and the decay heat model, then the split of total reactor power between fission power and decay heat power is known and promheat and decaheat can be used to simulate direct moderator heating. Update FxDirModHeat was developed in an attempt to address the continued confusion about the direct moderator heating model. With this update promheat input is renamed fissheat and is still the fraction of the fission power that is direct moderator heating if a point kinetics model is used (i.e., IRPWTY = 1, 2, 3, 4, 11, 12, 13, or 14) and decaheat is the still the fraction of decay heat power that appears as direct moderator heating. It should be noted that typically decaheat is small or zero, since most direct moderator heating is associated with the slowing down of neutrons. If the user specifies the transient power (i.e., IRPWTY = 5, 6, 7, 15, 16, or 17), then fissheat is the fraction of the total reactor power that is direction moderator heating and the user input for decaheat is ignored. In addition, the user can provide tables to make fissheat and/or decaheat a function of time. This allows the user to simulate direct moderator heating when a reactor is scrammed. For example, if the direct moderator heating fraction during steady-state and based on the total reactor power is ~2.5%, then after a scram the direct moderator heating fraction will approach zero. This behavior can now be simulated 8 using the table input when the user specifies the transient power. The FxDirModHeat update is pending. Trouble Report 592 Constrained steady-state models that save control block indices during initialization may be in error. A developer noted that in some cases the indices of signal variables and control blocks may change between input processing and the end of the initialization. Some of the constrained steady-state (CSS) models save signal variable and control indices to be used during the transient. The FxCSSInit update moves the logic for saving these indices into separate routines and calls these routines after ConSysInit but before the end of the initialization phase. This update resolves this trouble report and is pending. Trouble Report 596 Behavior of ISNOTB in point kinetics model turns off all boron reactivity feedback. A user noted that ISNOTB = 1 input for the point kinetics model effectively turned off the boron reactivity feedback, while the Users Guide documentation indicated that it takes into account reactivity feedback from boron in the control rods and burnable poison rods. The coding and documentation for ISNOTB are not consistent. The FxDirModHeat update includes coding and documentation changes to remove ISNOTB from TRACE. The simulation of burnable poison rods is via the user input for BPP0 and BPP1 and these two parameters can be input as zero to turn off this contribution to the boron reactivity feedback. The simulation of boron contribution due to control rod insertion can be simulated with BCR0 and BCR1, which can be input as zero to turn off this contribution to the boron reactivity feedback. Note that changes in core average boron concentration are given by the solute boron concentration, plus the change in burnable poisons given by BPP0 + BPP1 * Tc plus BCR0 + BCR1 * kprog, where Tc is the core average coolant temperature and kprog is the k infinity multiplier change due to control rod insertion. The FxDirModHeat update resolves this trouble report and is pending. Trouble Report 597 Suggestion to add core average parameters in reactivity feedback model to XTV graphics. A user requested that core average parameters used in the reactivity feedback models be provided as XTV graphics variables. The following signal variables are currently available: SV Type 51 Core Average Fuel Temperature, SV Type 52 Core Average Coolant Temperature, SV Type 53 Core Average Void Fraction, and SV Type 54 Core Average Boron Concentration. Core average parameters were also added to the POWER component graphics variables with the FxDirModHeat update. Update FxDirModHeat resolves this trouble report and is pending.

9 Trouble Report 598 Valve not choking for first time step when it opens. A user reported that when a plant ATWS model SRV bank starts to open, for at least one time step, TRACE calculated velocities were significantly larger than the expected critical flow velocities. Review of the choked flow logic indicated an error that resulted in no check for choked flow for the first time step as a VALVE starts to open. The FxValveOpenChoke logic fixes the logic error so that potential VALVE choking is accounted for as a VALVE starts to open. This update resolves this trouble report and is pending. 9