NUREG/IA-0233 

 

 International 

 Agreement Report

 

Assessment of TRACE 4.160 and 

5.0 against RCP Trip Transient in 

Almaraz I Nuclear Power Plant 

Prepared by: 

C. Queral*, A. Expósito*, G. Jiménez*, L. Valle*, and J. Carlos Martinez-Murillo** 

*Universidad Politécnica de Madrid 

Alenza 4. 28003 Madrid 

Spain 

**Almaraz-Trillo AIE 

Avenida de Manoteras 46 Bis. 28050 Madrid 

Spain 

 

A. Calvo, NRC Project Manager 

Office of Nuclear Regulatory Research 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001

July 2010 

 

Prepared as part of 

The Agreement on Research Participation and Technical Exchange 

Under the Thermal-Hydraulic Code Applications and Maintenance Program (CAMP) 

Published by 

U.S. Nuclear Regulatory Commission
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NUREG/IA-0233

Assessment of TRACE 4.160 and 5.0 against 

RCP Trip Transient in Almaraz I Nuclear 

Power Plant 

Manuscript Completed: June 2008 

Date Published: July 2010 

Prepared by: 

César Queral1

, Antonio Expósito1, Gonzalo Jiménez1, Laura Valle1 and Juan Carlos Martínez-Murillo2

1

Universidad Politécnica de Madrid 

Alenza 4. 28003 Madrid. 

Spain. 

2

Almaraz-Trillo AIE 

Avenida de Manoteras 46 Bis. 28050 Madrid. 

Spain. 

A. Calvo, NRC Project Manager 

Prepared for:

Division 

Office 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001

ABSTRACT

The Energy Systems Department of Universidad Polit ́ecnica de Madrid has developed a model

of Almaraz NPP, Westinghouse three loops PWR design, for TRACE code in the framework

of several projects sponsored by the Spanish Nuclear Regulatory Commission (CSN) and the

Electric Energy Industry of Spain (UNESA). At present, a full power and a shutdown model

are applied for sequences analysis. The model has been adjusted at several power rates and

steady state results show good agreement with plant data. In order to check the whole model,

a wide spectrum of transient simulations were performed, observing a good behavior. For

partial fulfilment of the bilateral - agreement for cooperation in thermalhydraulic activities

between the Consejo de Seguridad Nuclear of Spain (CSN) and the United States Nuclear

Regulatory Commission (US-NRC), the simulation of single RCP trip transient that took

place in Almaraz NPP on May 13 (2002) has been performed, comparing the results obtained

with plant data. The data comparison was quite encouraging when TRACE version 4.160

is used. However, the results obtained using TRACE 5.0 were not satisfactory. Wrong

predictions were obtained for the steam flow through the Steam Dump valves during the

transient. Regardless of this problem, the work made allowed performing the first validation

transient with the plant model. The simulations have been run in a Pentium IV 3.4 MHz

under Windows XP and AMD Opteron Dual Core Processors 180 & 1222 under Debian

Linux, both with 32 and 64 bits executables. This work has been sponsored by UNESA and

Almaraz-Trillo AIE.

iii
iv
FOREWORD

This report represents one of the assessment/application calculations submitted in fulfillment

of the bilateral agreement for cooperation in thermalhydraulic activities between the Consejo

de Seguridad Nuclear (CSN) and the US Nuclear Regulatory Commission (USNRC) in the

form of Spanish contribution to the Code Assessment and Management Program (CAMP)

of the USNRC, whose main purpose is the validation of TRACE code.

The CSN and UNESA (the association of the Spanish utilities), together with some relevant

universities, have set up a coordinated framework (CAMP-Spain), whose main objectives

are the fulfillment of the formal CAMP requirements and the improvement of the quality

of the technical support groups that provide services to the Spanish utilities, the CSN, the

research centers and the engineering companies.

This report is one of the Spanish utilities contribution to the above mentioned CAMP-Spain

program and it has been reviewed by the AP-28 Project Coordination Committee for the

submission to the CSN.

v
vi
CONTENTS

Page

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

2 Description of the Almaraz I NPP . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

3 Description of Almaraz I NPP TRACE Model . . . . . . . . . . . . . . . . . 3-1

4 Verification of Almaraz I NPP TRACE Model . . . . . . . . . . . . . . . . . 4-1

5 Description of the One RCP Trip Transient . . . . . . . . . . . . . . . . . . . 5-1

5.1 Evolution of Primary Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

5.2 Evolution of Secondary Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 5-6

6 Analysis of the Simulation Results with TRACE 4.160 . . . . . . . . . . . . 6-1

6.1 Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

6.2 Results from the Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4

7 Sensitivity Analysis with TRACE 4.160 . . . . . . . . . . . . . . . . . . . . . . 7-1

8 Problems Found in the Simulation Using TRACE 5.0 . . . . . . . . . . . . . 8-1

9 Execution Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1

10 Graphical Interface of the Almaraz I NPP TRACE Model . . . . . . . . . . 10-1

11 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-1

12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-1

Appendix A. TRACE 5.0 Opening Valve Flow Problem Reported . . . . . . . A-1

vii
Figures

Page

1 Almaraz I NPP general scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

2 Almaraz I NPP primary and secondary (from SGs to steam collector) schemes . 2-3

3 Almaraz I NPP secondary scheme from steam collector to high and low pressure

turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4

4 Almaraz I NPP MFWS/AFWS scheme . . . . . . . . . . . . . . . . . . . . . . . 2-5

5 Simplified nodalization of the Almaraz I NPP TRACE model . . . . . . . . . . 3-2

6 Vessel nodalization of Almaraz I NPP TRACE model . . . . . . . . . . . . . . . 3-3

7 Transit time and cross area sections for primary loop 2. Nominal flow conditions. 3-4

8 Primary loops mass flow rates (plant data) . . . . . . . . . . . . . . . . . . . . . 5-3

9 Averaged temperature (plant data) . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

10 Cold leg temperature. Loops 1, 2 and 3 (plant data) . . . . . . . . . . . . . . . 5-4

11 Hot leg temperature. Loops 1, 2 and 3 (plant data) . . . . . . . . . . . . . . . . 5-4

12 Pressurizer pressure (plant data) . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

13 Pressurizer level (plant data) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

14 Steam dump control in temperature mode with turbine trip . . . . . . . . . . . 5-6

15 MFW mass flow rate - SG1, SG2 and SG3 (plant data) . . . . . . . . . . . . . . 5-7

16 Steam generators narrow range level (plant data) . . . . . . . . . . . . . . . . . 5-7

17 Steam generators pressure (plant data) . . . . . . . . . . . . . . . . . . . . . . . 5-8

18 MFW mass flow rates - SG1, SG2 and SG3 (plant data) . . . . . . . . . . . . . 6-1

19 MFW temperature - SG1, SG2 and SG3 (plant data) . . . . . . . . . . . . . . . 6-2

20 AFW mass flow rate - SG1 (plant data) . . . . . . . . . . . . . . . . . . . . . . 6-2

21 AFW mass flow rate - SG2 (plant data) . . . . . . . . . . . . . . . . . . . . . . 6-3

22 AFW mass flow rate - SG3 (plant data) . . . . . . . . . . . . . . . . . . . . . . 6-3

23 Loop 2 hot leg water temperature during the simulation of RCP trip transient . 6-8

24 Loop 2 mass flow during the simulation of RCP trip transient. TRACE 4.160

model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9

25 Loops 1 and 3 mass flows during the simulation of RCP trip transient. TRACE

4.160 model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9

26 Loop 2 temperatures during the simulation of RCP trip transient. TRACE 4.160

model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10

viii
27 Loops 1 and 3 temperatures during the simulation of RCP trip transient. TRACE

4.160 model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10

28 Steam generators pressure during the simulation of RCP trip transient. TRACE

4.160 model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11

29 Steam generators narrow range calibrations . . . . . . . . . . . . . . . . . . . . 6-12

30 Steam generators narrow range levels during the simulation of RCP trip transient.

TRACE 4.160 model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13

31 Pressurizer level during the simulation of RCP trip transient. TRACE 4.160 model6-13

32 Pressurizer pressure during the simulation of RCP trip transient. TRACE 4.160

model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14

33 Pressure drops along primary system. TRACE 4.160 model . . . . . . . . . . . 6-14

34 Improved nodalization of the steam dump system in SNAP . . . . . . . . . . . . 7-2

35 Loop 2 RCP angular velocity variation for nominal hydraulic torque reduction . 7-3

36 Hot leg 2 first peak temperature variation for nominal hydraulic torque reduction 7-3

37 Loop 2 mass flow rate variation for nominal hydraulic torque reduction . . . . . 7-4

38 Loop 2 mass flows for different Ci par friction coefficients . . . . . . . . . . . . . 7-4

39 RCP 2 angular velocities for different Ci par friction coefficients . . . . . . . . . 7-5

40 Loop 2 hot leg higher temperature peak for different Ci par friction coefficients . 7-5

41 Loop 2 hot leg higher temperature peak variation for different nodalization schemes 7-6

42 Comparison of secondary pressure trends for RCP trip transient using TRACE

4.160 and 5.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

43 CPU time versus simulation time. TRACE versions 4.160 and 5.0 with Debian /

AMD Opteron Dual Core Processor 180 system . . . . . . . . . . . . . . . . 9-1

44 SNAP animation mask for Almaraz I NPP model . . . . . . . . . . . . . . . . . 10-1

45 CNA mask simulating RCP trip transient at 0, 292.5 and 759.6 s. . . . . . . . . 10-2

46 Valve opening for both code versions, TRACE 4.160 and 5.0 . . . . . . . . . . . A-2

47 Mass flow rates with/without choked flow model on. TRACE 4.160 and 5.0 . . . A-3

48 Mass flow rates for different valve aperture speeds. TRACE 4.160 and 5.0 . . . A-3

49 Mass flow rates with choked flow model and different discharge coefficients. TRACE

4.160 and 5.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-4

50 Mass flow rates with/without choked flow model on. TRACE versions 4.160,

4.273, 5RC1 and 5.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-4

ix
Tables

Page

1 Main operating parameters of the Almaraz I NPP . . . . . . . . . . . . . . . . . 2-2

2 Systems and controls actuated in the verification transients . . . . . . . . . . . . 4-2

3 Almaraz I NPP model steady states at different power levels . . . . . . . . . . . 4-2

4 Time sequence for relevant events in the RCP trip validation transient . . . . . 5-2

5 Comparison of steady state conditions of Almaraz I NPP plant and TRACE 4.160

model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

6 Sequence events in the RCP trip validation transient . . . . . . . . . . . . . . . 6-8

x
EXECUTIVE SUMMARY

A TRACE 4.160 simulation was conducted to assess code capability to predict RCP trip

transient behavior in Almaraz I Nuclear Power Plant. In this transient, the RCP of the loop

2 (this loop has the pressurizer surge line connection) tripped and the reactor SCRAM is

generated in a short time by the low flow in 1/3 loops coincident with permissive P8. After

its demand, the reactor protection system starts the insertion of shutdown and control banks.

After reactor SCRAM, primary temperatures decrease, leading to a decrease in pressurizer

level and pressure. The turbine trip signal demands the turbine bypass opening. There are

no reliable data about how the SD valves actuate during the transient, but it is clear that

one or more valves did not fully close (because Tavg decreases below HZP value), so some

assumptions had to be taken to reproduce the same plant behavior in secondary pressure.

The transient simulation was made using two versions of TRACE code, 4.160 and the last

released, 5.0. Meanwhile the results obtained using TRACE 4.160 were accurate, TRACE

5.0 presented a non expected behavior in the calculation of mass flow rates during valve

openings. This problem made unsuitable the transient simulation using this code version.

Despite this problem, from the run statistics it was found that TRACE 5.0 is quite promising

according to time execution. In fact, 5.0 version is 1.6 times faster than 4.160 version.

Also, no significant differences where found between the results of different runs executed in

Windows and Debian systems, and between 32 and 64 bits code versions.

xi
xii
ABBREVIATIONS

ACCS Accumulators

AFWS Auxiliary Feedwater System

BC Boundary condition

CSN Spanish Nuclear Regulatory Commission

CVCS Chemical and Volume Control System

DSE-UPM Departamento de Sistemas Energ ́eticos - Universidad Polit ́ecnica de Madrid

Department of Energy Systems - Technical University of Madrid

ECCS Emergency Core Cooling System

F&B Feed and Bleed

HPIS High Pressure Injection System

LPIS Low Pressure Injection System

MDP Motor Driven Pump

MFWS Main Feedwater System

MSIV Main Steam Isolation Valve

MSLB Main Steam Line Break

NPP Nuclear Power Plant

PORV Power Operated Relieve Valve

RCP Reactor cooling Pump

RHRS Residual Heat Removal System

RTD Thermoresistance sensor

RV / SV Relieve and Safety Valves

SD Steam Dump

SG Steam Generator

T Temperature

TDP Turbine Driven Pump

TLFW Total Loss of Feedwater

UNESA Electric Energy Industry of Spain

xiii
1 INTRODUCTION

Our group (DSE-UPM) began to work with RELAP5 code in 1997, [12]. Later, during the

Consolidated Thermal-hydraulic Code project (1999-2002), DSE-UPM group performed the

conversion of a RELAP5/MOD3 model of Almaraz I NPP to TRAC-M, [10]. This model

was applied in the simulation of several nominal and shutdown transients. Finally, along

2005 the DSE-UPM group converted the TRAC-M model to the last code developed by the

NRC, the TRACE code. The development of the Almaraz I NPP model for TRACE code

has been framed within several national and international projects (CAMP, OECD/NEA

SETH, OECD/NEA ROSA and OECD/NEA PKL-2) sponsored in Spain by Consejo de

Seguridad Nuclear (CSN) and Electric Energy Industry of Spain (UNESA). In the Spanish

national projects, one of the most important objectives is the maintenance and development

of the Spanish NPP models with the thermal-hydraulic codes that were sponsored by NRC,

such as RELAP5 and TRACE. Since 2005, a collaboration between Almaraz-Trillo AIE and

our group is undergoing. The basic aim of this collaboration is the improvement of the

Almaraz NPP model for TRACE code, and the work presented in this document is one of

the activities made during last year, [5].

Almaraz NPP is owned by Iberdrola (53%), Endesa (36%) and Uni ́on Fenosa (11%) located

in C ́aceres (Spain). This facility is a Westinghouse PWR with three loops and two turbines,

high and low pressure turbines, in tandem compound. The nominal current power is 2739

MWt (year 2008), but it was 2686 MWt when the RCP trip transient took place (year

2002). Original Westinghouse steam generators, model D3, were replaced on 1996/7 with

KWU Siemens steam generators model 61W/D3. These new steam generators do not have

preheaters and the feedwater injects directly above the downcomer, through J-tubes.

Different transient simulations have been performed using the Almaraz I NPP model: several

core power steady states, full power verification transients, simulations of loss of RHRS

sequences in low power - shutdown plant configuration and a full power validation transient

occurred in Almaraz I NPP. This document is mainly focused on the validation transient,

related with the reactor trip induced by one RCP trip and the following steam dump partial

malfunction, showing the results obtained.

The transient was initiated by an automatic trip of one RCP at about 100 % power in the

Almaraz I NPP. In this transient, the RCP of the loop 2 (this loop has the pressurizer surge

line connection) tripped and the reactor SCRAM is generated in a short time by the low

flow in 1/3 loops coincident with permissive P8. After its demand, the reactor protection

system starts the insertion of shutdown and control banks. After reactor SCRAM, primary

temperatures decrease, leading to a decrease in pressurizer level and pressure, the minimum

pressure value reached is 135.55 bar abs. The level decrease brings out a water mass flow

discharge from the pressurizer to the hot leg of loop 2. During this discharge, the angular

velocity of the tripped RCP is decreasing and then, the flow is inverted in loop 2.

In the secondary side, turbine trip signal demands the turbine bypass opening. There are no

reliable data about how the steam dump valves actuated during the transient, but it is clear

1-1
that one or more valves did not fully close (because pressure behavior of steam generators

and Tavg decreases below HZP value), so some assumptions had to be taken to reproduce

the same plant behavior in secondary pressure.

This document presents:

• a brief description of the main design aspects of the Almaraz I NPP, section 2,

• a description of the relevant NPP TRACE model parts, section 3,

• a revision of the different transients run to model verification, section 4,

• the detailed description of the one RCP plant transient, section 5,

• the simulation results obtained using TRACE 4.160, section 6,

• an analysis of the different sensitivity tests carried out for this transient using TRACE

4.160, section 7,

• a description of the problems found during the transient simulation using TRACE 5.0,

section 8, and

• a presentation of the Almaraz I NPP mask developed to perform simulations using

SNAP animation capabilities, section 10.

This validation transient is the first of a series of validation transients which will be performed

in the next future.

1-2
2 Description of the Almaraz I NPP

Almaraz NPP consists of two PWRs located in C ́aceres (Spain) and it is owned by a consortium of three spanish utilities: Iberdrola (53%), Endesa (36%) and Uni ́on Fenosa (11%).

Comertial operation started in April 1.981 (Unit I) and in September 1.983 (Unit II). Each

unit has a PWR Westinghouse with three loops and two turbines, high and low pressure turbines, in tandem compound. The nominal power is 2686 MWt and 977 MWe. It is equipped

with three steam generators Siemens KWU 61W/D3. Reactor coolant pumps are type single

stage, centrifugal model W-11011-Al (93-D) designed by Westinghouse. The AFWS consists

of one turbine driven pump and two motor driven pumps. More plant characteristics and

plant schematic diagrams are shown in Table 1 and Figures 1 to 4. LP

LP

RHRS

Spray PZR

Charge CVCS

Accumulator

High & low pressure SI

High & low pressure SI

RHRS

RHRS

Relief valves

Safety valves

Isolation valves

Seal pump

Seal pump

Seal pump

AF pump AF pump

FW pumps (mode 2)

HP

Steam bypass

MSR B MSR D

MSR C MSR A

(mode 1)

(mode 1)

High & low pressure SI

RHRS

Accumulator

Discharge

CVCS

Alternative charge

Accumulator

CVCS

Spray PZR

RHRS

FW TDP B

FW TDP A

MS Collector

LOOP 1

LOOP 2

LOOP 3

AF pump

Steam bypass

3 4 5

2

1

6

8

7

+ + AF MDP AF MDP

Check valves

Isolation valves

Control valves

AF TDP

FW Collector

Control valves

Control valves

Isolation valves Check valves

FW TDP A FW TDP B

From condensate pumps

and heaters and heaters

From condensate pumps

CST AFST

ESW

ESW

3 4 5

6 7 8

1 2

Figure 1: Almaraz I NPP general scheme

2-1
Table 1: Main operating parameters of the Almaraz I NPP

Description Value

Thermal reactor power 2686 Mwt(∗)

Fuel UO2 + GdO2

Number of assemblies 157

Number of loops 3

Reactor operating pressure 155.017 bar

Coolant averaged temperature

Zero load 564.9 K

100 % 580.8 K

Steam generators Siemens KWU 61W/D3

Number of tubes (per SG) 5130

Total tube length (per SG) 108294.3 m

Tube inner diameter 17.96 mm

Tube material INCOLOY 800

Pumps type Centrifugal model W-11011-Al (93-D)

Pump discharge head 86.26 m

Design flow rate 6.27 m3/s

Pump speed 155.509 rad/s

Primary volumes

Vessel 100.81 m3

Hot leg (x3) 3.18 m3

Steam generator (x3) 32.28 m3

Cross leg (x3) 3.6 m3

Reactor coolant pump (x3) 4.02 m3

Cold leg (x3) 3.23 m3

Surge line 1.14 m3

Pressurizer 39.64 m3

Spray lines 0.45 m3

TOTAL 280.97 m3

Number of PZR relieve / safety valves 2 / 3

Number of PZR spray valves 2

Heaters capacity (proportional/backup) (377 kW / 1023 kW)

Maximum spray flow 0.022 m3/s·valve

Steam mass flow rate at 100%

SG1 489 kg/s

SG2 486 kg/s

SG3 500 kg/s

∗ At present (2008), plant power is 2739 MWt

2-2
RHRS

Charge CVCS

Spray PZR

Accumulator

High & low pressure SI

High & low pressure SI

RHRS

RHRS

Relief valves

Safety valves

Isolation valves

Seal pump

Seal pump

Seal pump

AF pump AF pump

FW pumps (mode 2)

High & low pressure SI

RHRS

Accumulator

Discharge

CVCS

Alternative charge

Accumulator

CVCS

Spray PZR

RHRS

MS Collector

LOOP 1

LOOP 2

LOOP 3

AF pump 3 4 5

6

8

7

Figure 2: Almaraz I NPP primary and secondary (from SGs to steam collector) schemes

2-3
LP

LP

FW TDP pumps (mode 2)

HP

Steam bypass

MSR B MSR D

MSR A MSR C

(mode 1)

(mode 1)

FW TDP B

FW TDP A

MS Collector

Steam bypass

1

2

Figure 3: Almaraz I NPP secondary scheme from steam collector to high and low pressure

turbines

2-4
+ + AF MDP AF MDP

Check valves

Isolation valves

Control valves

AF TDP

FW Collector

Control valves

Control valves

Isolation valves Check valves

FW TDP A FW TDP B

From condensate pumps

and heaters and heaters

From condensate pumps

CST AFST

ESW

ESW

3 4 5

6 7 8

1 2

Figure 4: Almaraz I NPP MFWS/AFWS scheme

2-5
3 Description of Almaraz I NPP TRACE Model

Almaraz I NPP TRACE model (Westinghouse, three loops plant) is composed of 252 thermalhydraulic components (1 VESSEL, 52 PIPE, 71 TEE, 41 VALVE, 3 PUMP, 20 FILL, 27

BREAK, 36 HEAT STRUCTURE and 1 POWER component), 685 signal variables, 1532

control blocks and 47 trips, Figure 5, [4, 11, 3, 9].

With regard to the primary circuit, the following components have been modelled:

• Reactor vessel, modelled by a VESSEL component with 24 axial levels, three azimuthal

sectors and four radial sectors, which includes the core region, guide tubes, support

columns, core bypass, and the bypass to vessel head via downcomer and guide tubes,

Figure 6.

• Three loops, with its pumps, steam generators and pressurizer in loop 2 (containing

heaters, relief/safety valves and pressurizer spray system).

• Chemical and Volume Control System (CVCS).

• High pressure safety injection system and accumulators.

In reference to the secondary circuit, the following components have been modelled:

• The steam lines up to the turbine stop valves, including the relief, safety and isolating

valves.

• The steam dump system, including all the lines with the eight discharge valves to the

condenser.

• FW and AFW systems. Feed water pumps coastdown, auxiliary feedwater mass flows

and temperatures are included as boundary conditions.

The control systems and protection and engineering safeguards systems and signals that have

been modelled are:

• Pressurizer level control: CVCS isolating discharge signal, CVCS charge flow and

heaters.

• Pressurizer pressure control: including proportional and backup heaters, spray lines

and PORVs.

• Steam generators level control system.

• Steam dump control.

• Turbine control.

3-1
• Protection and engineering safeguard system-signals: Emergency shutdown system

(SCRAM); safety injection; pressurizer safety valves logic; auxiliary feedwater system

activation; logic of relief, safety and isolating valves of steam lines; normal feedwater

system isolation, turbine trip and pump trip.

Finally, a transit time and cross area sections figure for primary loop 2 is included to have

a general idea about the model topology, Figure 7.

j604

PIPE

605

TEE

604

j605 j606

BREAK

579

534

VALVE

533

VALVE

532

VALVE

531

VALVE

522 TEE 521 TEE 523

TEE 524 TEE 541 542 TEE

TEE

j582

j583

j580

j581

j578 j577

j579

j576

j573 j574

j575

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BREAK

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BREAK

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BREAK

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VALVE

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VALVE

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VALVE

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VALVE

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VALVE

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VALVE

TEE 504 TEE 503 TEE 505 TEE 500 TEE 510

PIPE

520

526 TEE 525 TEE 527

TEE 528 TEE 529 530 TEE

TEE

j591

j593

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j594

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j597

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j563 j565 j560

j598

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j567

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BREAK

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BREAK

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BREAK

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BREAK

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BREAK

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BREAK

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FILL

FILL

j802 j801

j800

PIPE 803

802

800

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TEE

j803

FILL

FILL

j801 j802

j800

803 PIPE

802

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TEE

j803

423

VALVE

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VALVE

BREAK

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BREAK

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TEE

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TEE

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TEE

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TEE

443

BREAK

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BREAK

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VALVE

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VALVE

471

VALVE

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VALVE

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VALVE

TEE 206

PUMP 240

j207

j222

j210

j211

j215 j213 j212

FILL

270

PIPE

221

PIPE

125

222

PIPE PIPE

224

TEE 251 TEE 250

FILL

261

PIPE 260

j214

j216

j219

j220

VALVE PIPE 231 262

j217

j218

TEE 253 TEE 252

280

FILL

j304

j305

PIPE 106

PUMP 140

j107

j122

j125

j110

j111

j113

j112

j114 j115 j117

TEE 135

FILL

170

FILL

192

PIPE 131

PIPE

121

PIPE

125

122

PIPE PIPE

124

TEE 151 TEE 152

FILL

161

PIPE 160

VALVE 162

j116

j118

j119

j120

TEE 153 j104

j105

j204

VESSEL 1

j121

TEE

606

j607

j606 j609

TEE

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TEE

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TEE

706

j707

j708

TEE

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TEE

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PIPE 306

j351 j313 j315

FILL

370

TEE 351 TEE 352 PIPE 353

FILL

361

PIPE 360

VALVE 362

j314

j316

j317

j318

PUMP 340

j307

j321

j322

j325

j310

j311

PIPE

321

PIPE

325

322

PIPE PIPE

324

PIPE 331

TEE

806

j807

j806 j809

TEE

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TEE

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j806

j804

PIPE

805

TEE

804

j805

j704

PIPE

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TEE

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j705

j221 j125

j709

j706 PIPE

809

j810

PIPE 609 j610

709

j710

PIPE

TEE 810 TEE 811 TEE TEE 813 TEE 815 TEE 816 PIPE 817

818

VALVE

819

VALVE

820

VALVE

821

VALVE

822

VALVE

823

VALVE

j811

j812

j819

j825 j826

j820

j813

j821

j827

j814

j822

j828

j815

j829 j830

j824

j816 j817 j818

VALVE

814

812

j823

BREAK

824

BREAK

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BREAK

826

BREAK

827

BREAK

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BREAK

829

TEE 610 TEE 611 TEE TEE 613 TEE 615 TEE 616 PIPE 617

618

VALVE

619

VALVE

620

VALVE

621

VALVE

622

VALVE

623

VALVE

j611

j612

j619

j625 j626

j620

j613

j621

j627

j614

j622

j628

j615

j629 j630

j624

j616 j617 j618

VALVE

614

612

j623

717 PIPE 716 TEE 715 TEE 713 TEE TEE 711 TEE 710 TEE

718

VALVE

719

VALVE

720

VALVE

721

VALVE

722

VALVE

723

VALVE

j711

j712

j719

j726 j725

j720

j713

j721

j727

j714

j722

j728

j715

j730 j729

j724

j718 j717 j716

VALVE

714

712

j723

BREAK

724

BREAK

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BREAK

726

BREAK

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BREAK

728

BREAK

729

j561

j603

FILL

FILL

j602 j601

j600

PIPE 603

602

600

601

TEE

PIPE 401

PIPE 402

TEE 414

PIPE

400

j400

j401

j403

j402

TEE 430 TEE 436 TEE 450

TEE 420 TEE 426

j404

j405

j406

j407

j409

j408

j413

j412 j411

j410 j414

j205

j415

j417

j418

j416

j424 j419

j423 j421

j422 j420

j428 j425

j427 j426

Figure 5: Simplified nodalization of the Almaraz I NPP TRACE model

3-2
PIPES: 8, 9, 10, 11, 12, 13

FILLS: 14, 15, 16, 17, 18, 19

ID GUIDE TUBES: 2, 3, 4, 5, 6, 7

ID BYPASS:20, 21, 22

ID SUPPORT COLUMN:

ID VESSEL: 1

 

Z0

Z1

Z2

Z3

Z4

Z5

Z6

Z7

Z8

Z9

Z10

Z11

Z12

Z13

Z14

Z15

Z16

Z17

Z18

1

2

3

4

6

5

7

8

9

10

11

12

13

14

15

16

17

18

Z19

Z20

Z21

Z22

Z23

Z24

19

20

21

22

23

24

R4R3R2

R1

Hot leg conexion

1 m

4

5 6

8

11

7

10

9

12

2 3

1

R2

R1

R3

R4

DOWNCOMER

CORE

4

5 6

8

11

7

10

9

12

2 3

1

R2

R1

R3

R4

Support column

Guide tube

DOWNCOMER

CORE

Core Bypass

4

5 6

8

11

7

10

9

12

2 3

1

R2

R1

R3

R4

BARREL

CORE

hot leg

cold leg hot leg

cold leg

cold leg

hot leg

HYDRODINAMIC COMPONENTS

Support columns

Cold leg conexion

Guide tubes

Figure 6: Vessel nodalization of Almaraz I NPP TRACE model

3-3
0

2

4

6

8

10

12

0 10 20 30 40 50 60 70

Developed lenght (m)

Transit time (s)

0

1

2

3

4

5

6

7

8

Cross section (m2)

 transit time

cross section

 Pump

 SG outlet 

Cross over leg

Cold leg

Downcomer

Lower plenum

Upper plenum

Core

Hot leg

SG inlet 

Ascending Descending 

U-tubes U-tubes

Figure 7: Transit time and cross area sections for primary loop 2. Nominal flow conditions.

3-4
4 Verification of Almaraz I NPP TRACE Model

Several simulations were performed with the model to assess its main parameters and systems. These simulations are classified in three groups.

First group consists of eight steady states at several power rates. In detail, steady state simulations for 100 %, 75%, 50%, 25%, 10%, 3%, 2% and 1% of nominal power were considered,

and the results show a good agreement with the plant data for the main parameters (RCS

temperatures and pressure and SG steam flow, dome pressure and recirculation ratio), [2].

Results of some important variables at different power levels such as primary average temperature, secondary pressure and secondary mass flow are included in Table 3.

The second group, a set of verification transients at full power, allowed to check all the

systems and signals included in the model, Table 2:

• Turbine trip (TT).

• Turbine trip without steam dump (TT1).

• Turbine trip without SD or relief valves (TT2).

• Turbine trip without relief valves (TT3).

• Turbine trip without Auxiliary Feedwater (TT4).

• Opening of a Pressurizer PORV (OPP).

• Main Steam Line Break (MSLB).

• Total Loss of Feedwater (TLFW) - TLFW without AFWS and F&B.

• Total Loss of Feedwater (TLFW) - TLFW with AFWS.

The results of the simulations show that the behavior for the transients is the expected [2].

In general, good results were obtained for all the simulations, and they allowed improving

the model, in particular SGs, SD and pressurizer models. Other transients like inadvertent

injection of ECCS (ECCS), load rejection (LR) and trip of all RCPs (3RCP), are being

performed in order to finish the verification of the plant model.

The third group, oriented to low power and shutdown plant conditions is composed of several

simulations to analyse plant behavior during a loss of RHRS, considering different primary

configurations and steam generators availability, [6]: closed primary system, [7]; two pressurizer PORVs open, [1]; pressurizer manway open, [1] and [8]; one or two PORVs open, [8];

and vessel vent valve open, [8].

The results of this work, performed in the framework of SETH and PKL-2 OCDE/NEA

projects, allow improving available times for these sequences, proposing modifications to

some abnormal operating procedures, [1] and [8].

4-1
Finally, with respect to future works, we also expect to perform SGTR and different LOCA

simulations to analyze the plant behaviour and to compare with the phenomenology observed

in the OECD/NEA ROSA and PKL experiments.

Table 2: Systems and controls actuated in the verification transients

System/control TT TT1 TT2 TT3 TT4 MSLB OPP 3RCP ECCS TLFW LR

PZR RV NO NO NO NO NO NO NO NO YES YES YES

PZR Spray NO NO NO NO NO NO NO NO YES YES YES

PZR Heat. Prop. YES YES YES YES YES YES YES YES YES YES YES

PZR Heat. Back. YES YES YES YES YES YES YES NO NO NO YES

PZR SV NO NO NO NO NO NO NO NO YES NO NO

Lev. control YES YES YES YES YES NO NO YES NO YES YES

Disch. isolat. NO NO NO NO NO YES YES NO NO NO NO

Heaters activ. NO NO NO NO NO YES YES NO NO NO NO

Heaters disact. NO NO NO NO NO YES YES NO NO NO NO

Control rods NO NO NO NO NO NO NO NO NO NO NO

ECCS HPIS NO NO NO NO NO YES YES NO X NO NO

LPIS NO NO NO NO NO NO NO NO NO NO NO

ACCS NO NO NO NO NO NO NO NO NO NO NO

RCPs NO NO NO NO NO NO NO YES NO NO NO

Turbine X X X X X YES YES YES YES YES YES

Steam dump YES X X YES YES NO YES YES YES YES YES

AFW YES YES YES YES X YES YES YES YES YES YES

SGs RV YES YES X X YES YES NO NO NO NO NO

SGs SV NO NO YES NO NO NO NO NO NO NO NO

MSIV NO NO NO NO NO YES NO NO NO NO NO

Table 3: Almaraz I NPP model steady states at different power levels

Power (%) 100% 75% 50% 25% 10% 3% 2% 1%

Steam flow (kg/s)

TRACE 1488.24 1072.6 683.1 322.2 126.3 37.6 24.6 12.6

PLANT 1489.5 1072.8 683.1 323.4 126 37.8

Primary flow per loop (kg/s)

TRACE 4643 4648 4652 4654 4654 4654 4654 4652

PLANT 4650.4 4650.4 4650.4 4650.4 4650.4 4650.4 4650.4 4650.4

Average primary temp. (oC)

TRACE 307.6 303.6 299.65 295.6 293.3 292.15 292.03 291.8

PLANT 307.63 303.64 299.6 295.6 293.3 292.8 292.05 291.9

Recirc. Ratio of SG

TRACE 3.6 4.73 6.72 11.37 22.5 45.8 58.02 85.9

PLANT 3.64 5.12 7.91 15.352 33.564

SG Dome Pressure (bar)

TRACE 68.22 69.85 71.52 73.3 74.45 75.13 75.31 75.48

PLANT 68.3 69.5 70.5 72 73.5

4-2
5 Description of the One RCP Trip Transient

First transient chosen for validation of TRACE model was the reactor trip induced by one

RCP trip, which took place in May 2002.

Almaraz I NPP was operating in stable conditions, at 100% of nominal power (2686 MWt),

supplying an electrical power of 972 MWe. The plant state was, Table 3:

• Reactor, turbine and main unit controls were in automatic mode.

• Control rods were also in automatic, with all control banks withdrawn 228 steps.

• Primary system boron concentration was 1409 ppm.

• Main operation parameters were within normal values for the power level in which the

unit was operating.

• Electric grid configuration was normal, with the 6.3 kV 1A1, 1A2, 1A5 bars connected

to the auxiliar transformer, and the 1A3 bar from 1A1, and 1A4 from the start up

transformer (T1A2).

On May 13, 2002, at 12.59 pm, the RCP of loop 2 tripped generating a transient (events

sequence is included in Table 4), which resulted in the reactor trip due to low flow signal in

one loop. The reactor trip signal originated the opening of the two reactor trip switches (RTA

and RTB) with the dropping of all control and shutdown rods, and the turbine trip signal.

Due to the turbine trip signal all stop and control turbine valves were closed. Following

plant behavior, considering the main plant parameters evolution, is described in detail in the

following sections. It is remarkable that the plant data has a 0.5 s of time interval and some

measurements were made in no IS units.

5.1 Evolution of Primary Parameters

RCP 2 trip led to reactor SCRAM. The mass flow started to decrease in loop 2 according

with RCP coastdown, while the mass flow in the other loops tended to increase, Figure 8.

During the final part of the RCP 2 coastdown, the mass flow in that loop is inverted. It is

important to clarify that loop flow meters are not configured to measure inverted flows, and

that is the reason why there are not reliable data in those conditions for the mass flow in

that loop.

After reactor scram, averaged temperature decreased to 289.4 oC approximately, changing

the trend when the operator close all MSIVs, recovering hot zero power value (291.7oC),

Figure 9. Regarding cold leg temperatures, no change is shown respect its usual behavior

during a normal reactor scram, Figure 10. On the other hand, hot leg temperatures decreased

suddenly as expected until they reached hot zero power values, with a trend similar to the

averaged temperature. In loop 2, it is shown that there is a slight difference in the hot leg

5-1
Table 4: Time sequence for relevant events in the RCP trip validation transient

Events Time (s)

RCP loop 2 trip 282.0

Reactor SCRAM induced by low mass flow in loop 2 (90 % in 1/3)+ P8 (Qnom 1 loop 4650.4

kg/s)

283.5

Turbine trip caused by reactor SCRAM 283.5

SD valves opening in turbine trip mode 284.5

Pressurizer pressure starts to decrease 285.5

Backup heaters switch on due to low pressure (155,5 kg/cm2rel. = 153.45 bar) 286.0

Averaged temperature starts to decrease to 289.4 oC 286.5

Full insertion of the control and shutdown rods 289,5

AFWS turbo and motor driven pumps starting up caused by the low low narrow range level

signal in the steam generators (17.6%)

292.5

MFW turbo driven pumps velocities start to decrease 294.5

MFW admission valves closing induced by SCRAM reactor signal coincident with low averaged temperature (295 oC)305.0

Reverse flow in loop 2 307.0

Minimum temperature reached at the loop 2 hot leg 311.0

Averaged temperature equals hot zero averaged temperature (291.7 oC), and decreasing 404.5

Operators decide to close the main steam isolation valves to mitigate overcooling transient

(MSIV bypass valves are open)

448.0

End of available plant data 840.0

temperature behavior during the SD cooling transients, due to PZR surge line discharge,

Figure 11. Hot leg temperature had two peaks that appeared when the water mass flow

discharged through the surge line, hotter than hot leg water, and the inverse flow from the

steam generator mix and pass through RTD location (RTD are located between the vessel

and the surge line connection).

After reactor SCRAM, pressurizer pressure decreased reaching a minimum value of 137.5

kg/cm2rel. Subsequently, the pressure recovered normal values due to the actuation of the

pressure control, backup and proportional heaters, Figure 12.

When SCRAM took place, pressurizer level decreased to hot zero power level values, remaining around this value for all the transient, Figure 13. The primary cooling lead to a

inventory contraction, decreasing the level to a minimum value of 14 %. As it was expected,

reference level value was reached due to level control actuation on charge flow.

5-2
0 100 200 300 400 500 600 700 800 900

Time (s)

0

20

40

60

80

100

Mass flow (%)

Loop 1

Loop 2

Loop 3

SCRAM 5a 90 %

Figure 8: Primary loops mass flow rates (plant data)

0 200 400 600 800

Time (s)

280

285

290

295

300

305

310

Temperature (oC)

Low average temperature 295 oC

Hot zero power temperature 291.7 oC

Figure 9: Averaged temperature (plant data)

5-3
0 200 400 600 800

Time (s)

288

289

290

291

292

293

294

295

Temperature (oC)

Cold leg 1

Cold leg 2

Cold leg 3

Figure 10: Cold leg temperature. Loops 1, 2 and 3 (plant data)

0 200 400 600 800

Time (s)

280

290

300

310

320

330

Temperature (oC)

Hot leg 1

Hot leg 2

Hot leg 3

Figure 11: Hot leg temperature. Loops 1, 2 and 3 (plant data)

5-4
0 200 400 600 800 1000

Time (s)

130

135

140

145

150

155

160

Pressure (kg/cm2)

Backup heaters ON (155.48 kg/cm2 rel)

Figure 12: Pressurizer pressure (plant data)

0 200 400 600 800

Time (s)

0

10

20

30

40

50

60

Level (%)

Hot zero power level (20.1 %)

15 % (Heaters switch off)

Figure 13: Pressurizer level (plant data)

5-5
5.2 Evolution of Secondary Parameters

Main Feedwater water mass flow rate to the three steam generators is reduced during the

feedwater turbo driven pumps coastdown after its trip and, finally, the mass flow is canceled

by the closing of the main feedwater isolation valves, Figure 15, induced by SCRAM reactor

signal coincident with low averaged temperature (295 oC), Figure 9.

Narrow range levels decreased suddenly due to scram, reaching values of 6.5 %, 11% and 5%

at the steam generators 1, 2 and 3 respectively, Figure 16. The starting up of AFWS turbo

and motor driven pumps is caused by the low low narrow range level signal in the steam

generators (17.6%), injecting to all SGs.

At the time of turbine trip and the consequent closing of the stop valves, there was an

increase in the pressure of the steam generators, Figure 17, reaching maximum values at

around atmospheric relief valves set point. After the initial maximum, pressure decreased

showing the same trend than averaged temperature.

Some SD valves failures were induced from the secondary pressure and the Steam Dump

behavior during the transient. This system actuated properly in the first part of the transient,

but three valves (MS1-HV-4501/4504/4505) did not fully closed after the closure demand,

Figure 14. Despite that averaged temperature was below its hot zero power value, the system

was still discharging steam to the condenser. Also, secondary pressure measurement shows a

strong evidence of a steam discharge equivalent to a SD bank capacity. As a result, operators

decided to close the main steam isolation valves, remaining open the MSIV bypass valves,

Figure 17.

∆

0

10

20

30

40 BANKS VALVES

VALVES

1o

2o

3o

4o

Close

Open

Open

Close

Open

Close

Close

100%

100%

100%

100%

100%

100%

100%

100%

Open

AND

AND

AND

AND

15,4 Turbine trip

HV−4503

HV−4507

HV−4501

HV−4505

HV−4502

HV−4506

HV−4500

HV−4504

7,69

STEAM

FLOW (%)

T (oC)

Figure 14: Steam dump control in temperature mode with turbine trip

5-6
0 100 200 300 400

Time (s)

0

100

200

300

400

500

600

Mass Flow (kg/s)

SG1

SG2

SG3

Figure 15: MFW mass flow rate - SG1, SG2 and SG3 (plant data)

0 200 400 600 800

Time (s)

0

10

20

30

40

50

60

Level (%)

SG1

SG2

SG3

Low level SG (17.6 %)

Full power reference level (50.6 %)

Figure 16: Steam generators narrow range level (plant data)

5-7
0 200 400 600 800

Time (s)

66

68

70

72

74

76

78

80

Pressure (kg/cm2)

SG1

SG3

SG2

Relief valve setpoint (78.5 kg/cm2)

Figure 17: Steam generators pressure (plant data)

5-8
6 Analysis of the Simulation Results with

TRACE 4.160

In this section an in-depth view of the boundary conditions considered for the transient

simulation and the simulation results are presented.

6.1 Boundary Conditions

For the simulation, the following boundary conditions were considered:

• Steam generators main feedwater mass flow rates and temperatures, Figures 18 and

19, taken into account the MFW isolation and TDP coastdown.

• Steam generators auxiliary feedwater mass flow rates, Figures 20 and 22. Constant

value of 25 oC was set for the inlet temperature.

• Adjustment of several steam dump valves to obtain secondary pressure trend. The

valve areas were adjusted using trial and error method because there were not reliable

plant data.

260 270 280 290 300 310 320

Time (s)

0

100

200

300

400

500

600

Mass Flow (kg/s)

SG1

SG2

SG3

Figure 18: MFW mass flow rates - SG1, SG2 and SG3 (plant data)

6-1
0 200 400 600 800

Time (s)

218

219

220

221

222

223

Temperature (oC)

SG1

SG2

SG3

Figure 19: MFW temperature - SG1, SG2 and SG3 (plant data)

0 200 400 600 800

Time (s)

0

5

10

15

20

25

30

35

40

45

50

Mass flow (kg/s)

Figure 20: AFW mass flow rate - SG1 (plant data)

6-2
0 200 400 600 800

Time (s)

0

10

20

30

40

50

Mass flow (kg/s)

Figure 21: AFW mass flow rate - SG2 (plant data)

0 200 400 600 800

Time (s)

0

5

10

15

20

25

30

35

40

Mass flow (kg/s)

Figure 22: AFW mass flow rate - SG3 (plant data)

6-3
6.2 Results from the Simulation

The simulation was run from a steady state restart, Table 5, which shows a good agreement

with plant data. A first group of simulations shown that was necessary to reproduce the

pump coastdown and secondary pressures in a very detailed way. So, in order to separate the

simulation of primary side from secondary side in a first stage, it was decided to simulate a

group of transients with secondary pressure as boundary condition. Later, when coastdown

was well adjusted, secondary pressure was also adjusted in the whole model without pressure

boundary condition. The final results obtained in the simulation are detailed below.

The transient begins with a RCP trip in loop 2 at 282.0 s. This RCP trip generates a

transient with three differentiate phases, Figure 23:

Phase 1. RCP coastdown in loop 2 from nominal mass flow to null mass flow. From t =

282 s to t ≈ 307 s.

Phase 2. Reverse flow in loop 2 with ω > 0. From t ≈ 307 s to t ≈ 550 s.

This phase begins when the flow in loop 2 is reversed, t ≈ 307 s. This time was

calculated in two ways:

1. Taking into account that loop 2 hot leg temperature starts to increase, t ≈ 307 s,

when the hot water from the pressurizer, mixed with the cold water from the

steam generator, reaches the RTD location, Figure 26.

It must be remembered that RTD location is between pressure vessel hot leg outlet

and the connection of PZR surge line with hot leg.

2. Also, this moment can be inferred from the extrapolation of the coastdown curve

from plant data, Figure 24.

Both methods are affected by uncertainty. Hot leg temperature trend change has a

transport delay, from surge line connection to RTD location, which could be corrected

considering the flow velocity. Coastdown curve extrapolation supposed linear trend for

the coastdown where no plant data is available. This last uncertainty is higher that

the previous one.

The best way to determine this moment, is obtaining the time when loop 2 hot leg

temperature starts to increase, t ≈ 307 s.

Phase 3. Reverse flow in loop 2 with ω = 0. From t ≈ 550 s to t ≈ 840 s.

It is clear that there is reverse flow because Thot-leg < Tcold-leg, Figure 23.

As it was commented above this is one of the main variables that was necessary to adjust so,

in order to obtain a good simulation of the RCP coastdown several changes and adjustments

where performed in the model:

6-4
Phase 1. This phase was well reproduced adjusting RCPs hydraulic and friction torques,

in order to obtain the right RCP coastdown in loop 2, Figure 24. This adjustment also

gives good transient mass flows in loops 1 and 3, Figure 25.

Phase 2. This phase begins when the flow in loop 2 is reversed, t ≈ 307 s.

In order to reproduce this coastdown phase it was observed in a sensitivity analysis

that the behavior of hot leg temperature in loop 2 is more sensitive to pump parameters

than mass flow in Phase 1. So, pump parameters values were tuned again in order to

obtain good results in Phase 1 and 2 simultaneously. However, only first maximum

temperature was well reproduced in Phase 2. The rest of hot leg temperature transient

in loop 2 is quite difficult to reproduce because, in the sensitivity analysis, is observed

that the results are quite sensitive to mesh size near the connection of surge line with

hot leg. This sensitivity analysis is described in detail in the next section.

This adjustment gives good simulation results for the mass flows and temperatures in

loops 1 and 3, Figures 25 and 27.

Phase 3. When loop 2 RCP is full stopped the mass flow rates in all loops do not depend on

pump parameters so, these mass flows could be adjusted with loop 2 reverse friction

factor, Figures 24 and 25.

In order to adjust these mass flows, it must be taken into account that there is no

reliable plant data of mass flow in loop 2. The right way to adjust mass flow in loop

2 is to reproduce the temperature difference between hot leg/cold leg of loop 2 during

this phase, Figures 26 and 27.

Referring to the other loops, a mass flow rate increase takes place due to flow redistribution, reaching a maximum value in accordance with plant data, Figure 25.

Due to the initial mass flow decrease a reactor SCRAM signal is generated in a short time by

low flow in 1/3 loops. A few seconds later primary temperatures start to decrease, leading to

a decrease in pressurizer level and pressure. This first stage of the transient is well simulated,

Figures 26, 27, 31 and 32. The reactor SCRAM signal generates a turbine trip signal, which

also demands the turbine bypass valves opening.

As mentioned earlier, some not fully closed SD valves led to an anomalous secondary pressure

behavior, Figure 28, which led to a primary overcooling transient, Figure 9. In order to

reproduce the secondary pressure, the steam dump model was adjusted supposing a bank

(two valves) not fully closed. In detail:

• For the first part of the transient, until several SD valves remained not fully closed,

the transient was simulated without additional considerations.

In this way, it was taken into account that the SD worked properly until t ≈ 325.0 s.

6-5
• The second part of the pressure transient, from t ≈ 325.0 s to t ≈ 440.0 s, was adjusted

with one bank stuck partially open, leading to primary overcooling during almost 115

seconds.

• After this time interval, t > 440.0 s, this bank is almost fully open, worsen the cooling

transient.

At the beginning of this transient phase, t ≈ 440.0 s, the operator isolated the steam

lines, opening the MSIV bypass to allow cooling via SD. The simulation of these manual

modifications had an important impact in the primary temperatures, whose simulation

show a good behavior in this phase of the transient.

During the different simulations performed, the SGs narrow range level was always underestimated by 4% when compared with plant data. This deviation respect to plant data led

to consider a 4% calibration offset. Three calibrations were tested to correct this problem,

Figure 29:

1. Adding 4 % to the whole range (4% - 104%).

2. Adding 4 % to the origin keeping 100 % point (4% - 100%).

3. Adding 4 % to the origin keeping 50.6 % point, which corresponds to the narrow range

level value for 100 % power (4% - 96%).

The best results were obtained using the last one, Figure 30.

Respect to the pressure and level in the pressurizer, the level trend is quite good

compared with plant data, Figure 31. However, pressure tends to be higher during the

transient and, at the final part, the overestimation is 2 bars above the plant pressure, Figure

32. This problem has not been solved, but several tests and sensitivity analyses have been

performed to establish the cause of this behavior:

• Several pressurizer nodalizations were tested. Future work is going to be performed

to include new pressurizer models, including 2D models for the pressurizer, in the full

plant model to analyze the influence in the transient simulation.

• The spray mass flow behavior during the transient has been analyzed, including pressure differences between loops with and without pressurizer, Figure 33. This parameter

has not strong influence in pressure trend in this transient.

• Different heaters models have been tested in order to quantify the response time in

transient conditions. These models were a direct heat deposition in the fluid and

several heat structure models. It was concluded that the response time of the heaters

has not a strong influence on the pressure trend after reactor SCRAM.

In general, despite the primary pressure difference, it is observed that event simulation

sequence match quite well with plan data, Table 6.

6-6
Table 5: Comparison of steady state conditions of Almaraz I NPP plant and TRACE 4.160

model

Items Plant (loop w/wo PZR) TRACE (loop w/wo PZR)

Core power 2696 MWt 2696 MWt

Primary loops

Hot leg liq. temperature (K)

Loop 1 598.5 K 597.3 K

Loop 2 597.9 K 596.8 K

Loop 3 597.6 K 597.4 K

Cold leg liq. temperature (K)

Loop 1 563.5 K 563.0 K

Loop 2 563.7 K 563.0 K

Loop 3 563.7 K 563.1 K

Averaged temp. (K)

Loop 1 581.0 K 580.2 K

Loop 2 580.8 K 579.9 K

Loop 3 580.7 K 580.2 K

Mass flow rate (kg/s per loop)

Loop 1 100% 4615 kg/s

Loop 2 100% 4685 kg/s

Loop 3 100% 4634 kg/s

Pressurizer (PZR)

Pressure 155.04 bar abs 155.04 bar

Liquid level (%) 56.3% 55.6%

Steam generators

Secondary pressure (MPa)

SG 1 67.8 bar abs 68.0 bar

SG 2 68.2 bar abs 67.9 bar

SG 3 67.6 bar abs 68.0 bar

Secondary liq. level (%)

SG 1 50.2% 50.6%

SG 2 49.6% 50.6%

SG 3 49.6% 50.6%

Main feedwater mass flow rate (kg/s)

SG 1 489 kg/s 492.3 kg/s

SG 2 486 kg/s 491.6 kg/s

SG 3 500 kg/s 494.4 kg/s

6-7
Table 6: Sequence events in the RCP trip validation transient

Events Time (s)

Plant Simulation

RCP #2 trip 282 282

SCRAM / Turbine trip signals 283.5 283.7

MFW isolation valve closing signal 284 284.0 (BC)

SD demand start 284.5 285

Backup heaters switch on 286 288.9

Demand signal for AFWS 292.5 292.5 (BC)

MFW TDP angular velocity decreases 294.5 294.5 (BC)

Inverse flow in loop 2 ∼ 307 305.8

Regulating bank (#1) stuck open — ∼ 325.0∗

Bank stuck (#1) fails full open — ∼ 440.0∗

MSIV manually actuated (MSIV bypass valves are opened) 448 448 (manual)

End of plant data 840

∗ Events inferred from plant data trend of the secondary pressure

Figure 23: Loop 2 hot leg water temperature during the simulation of RCP trip transient

6-8
Figure 24: Loop 2 mass flow during the simulation of RCP trip transient. TRACE 4.160

model

Figure 25: Loops 1 and 3 mass flows during the simulation of RCP trip transient. TRACE

4.160 model

6-9
0 200 400 600 800

Time (s)

280

290

300

310

320

330

Liquid temperature (oC)

Loop2 (plant data)

tln-206A01

tln-250A02

Figure 26: Loop 2 temperatures during the simulation of RCP trip transient. TRACE 4.160

model

0 200 400 600 800

Time (s)

280

290

300

310

320

330

Liquid temperature (oC)

Hot leg loop 1

Hot leg loop 3

Cold leg loop 1

Cold leg loop 3

tln-351A02

tln-151A02

tln-306A01

tln-106A01

Figure 27: Loops 1 and 3 temperatures during the simulation of RCP trip transient. TRACE

4.160 model

6-10
200 300 400 500 600 700 800

Time (s)

6,8e+06

7e+06

7,2e+06

7,4e+06

7,6e+06

7,8e+06

pressure (Pa)

SG1 (plant data)

SG2 (plant data)

SG3 (plant data)

pn-708A06

pn-608A06

pn-808A06

Figure 28: Steam generators pressure during the simulation of RCP trip transient. TRACE

4.160 model

6-11
0

10

20

30

40

50

60

70

80

90

100

0 5000 10000 15000 20000 25000 30000 35000 40000

Hydrostatic Pressure (Pa)

Narrow Range (%)

Standart 4% - 104% 4% - 100% 4% - 96%

Figure 29: Steam generators narrow range calibrations

6-12
200 400 600 800

Time (s)

0

10

20

30

40

50

60

Narrow range level (%)

SG1 (plant data)

SG2 (plant data)

SG3 (plant data)

cb4514

cb4543

cb624

Figure 30: Steam generators narrow range levels during the simulation of RCP trip transient.

TRACE 4.160 model

0 200 400 600 800

Time (s)

0

10

20

30

40

50

60

Level (%)

Plant data

cb718

Figure 31: Pressurizer level during the simulation of RCP trip transient. TRACE 4.160

model

6-13
0 200 400 600 800

Time (s)

1,3e+07

1,35e+07

1,4e+07

1,45e+07

1,5e+07

1,55e+07

1,6e+07

pressure (Pa)

Plant data

pn-414A07

Figure 32: Pressurizer pressure during the simulation of RCP trip transient. TRACE 4.160

model

-3

-2

-1

0

1

2

3

4

5

6

SG outlet

box

Cross over

leg

Pump Cold leg Downcomer Upper

plenum

PRIZER SG inlet

box

Ascending

U-tubes

Descending

U-tubes

Caractheristic points

Pressure(bar)

Loop 1 4100(s)

Loop 2 4100(s)

Loop 1 6600(s)

Loop 2 6600(s)

Figure 33: Pressure drops along primary system. TRACE 4.160 model

6-14
7 Sensitivity Analysis with TRACE 4.160

Several sensitivity tests were performed to analyze the impact in the plant main parameters:

• RCP model parameters: nominal hydraulic and friction torque constants.

• Nodalization of the hot leg at the pressurizer surge line connection.

• Measurements uncertainty in plant data.

• Simulation of the short term part of the transient (2-3 seconds) with no reactor SCRAM

or no turbine trip signals.

RCP nominal hydraulic torque and friction torque constants

These parameters are important to adjust the time when the hot leg flow in loop

2 is inverted and the first peak in temperature appears, Figures 35 to 40. For the

nominal hydraulic torque, a 30% reduction was necessary to obtain the higher hot leg

temperature peak trend. Considering for each friction factor a value of Ci=4000 N·m,

it was possible to obtain the peak trend for the friction factors C2 and C3.

Also, the parameters of torque friction model for ω ≤ ω0/10 were made equal to the

ω > ω0/10 ones. It was noted that this assumption has no coastdown effect in this

transient.

Nodalization of hot leg near the pressurizer surge line connection

The results show that due to TRACE numerical diffusion, the size of the nodalization

in this part of the model has an important impact in the value and shape of the

temperature peaks, Figure 41. Due to the nature of this transient (RCP trip), Courant

number variation during the transient is within a large interval, so it is not possible

to avoid the numerical diffusion during the whole transient changing the nodalization.

Therefore, hot leg nodalization was not changed.

Measurements uncertainty in plant data

The sensitivity analyses on steam collector pressure, MFWS and AFWS mass flows

and temperatures shown that plant behavior only presents a high sensitivity on MFW

mass flow rate. Therefore, accurate boundary conditions for MFW mass flow rate was

set obtaining a better simulation of steam generators narrow range levels.

Simulation of the short term part of the transient

Two tests where run, one with no reactor SCRAM and the other one with no turbine

trip signals, to study the behavior of the pressure and level of the pressurizer during

the short term part of the transient (2-3 seconds). The main objective of this analysis

was to establish the dominant effects in the first seconds of the transient.

7-1
It can be remarked that during the validation process were obtained some results which led

to some nodalization changes in parts of the model (steam lines, pressurizer, ...), Figure 34,

and some adjustments were made (RCS loop reverse fiction factors, RCP coastdown, ...)

getting improvements in the results. During this stage of the work, individual models were

used to study its behavior during the transient. For example, SG, 3D vessel and pressurizer

models were tested individually. Also, some tests were run with the secondary pressure as

boundary condition in the steam line header for the whole plant model. Theses tests were

useful to study separate effects in model parameters.

Figure 34: Improved nodalization of the steam dump system in SNAP

7-2
4280 4300 4320 4340 4360 4380 4400

Time (s)

0.2

0.4

0.6

0.8

1

impeller rotational speed (normalized)

Reference case

-30%

Figure 35: Loop 2 RCP angular velocity variation for nominal hydraulic torque reduction

4300 4350 4400 4450

Time (s)

570

580

590

liquid temperature (K)

-30%

Reference case

Thl L2 CNA

Figure 36: Hot leg 2 first peak temperature variation for nominal hydraulic torque reduction

7-3
0 200 400 600 800

Time (s)

-20

0

20

40

60

80

100

total mass-flow (%)

-30%

Reference case

Figure 37: Loop 2 mass flow rate variation for nominal hydraulic torque reduction

300 350 400

Time (s)

-20

0

20

40

60

80

100

total mass-flow (%)

Only C0

Only C1

Only C2

Only C3

Reference case

Figure 38: Loop 2 mass flows for different Ci par friction coefficients

7-4
4280 4300 4320 4340 4360 4380 4400

Time (s)

0.2

0.4

0.6

0.8

1

impeller rotational speed (normalized)

Reference case

Only C1

Only C2

Only C3

Only C0

Figure 39: RCP 2 angular velocities for different Ci par friction coefficients

4300 4350 4400 4450

Time (s)

570

580

590

liquid temperature (K)

Only C0

Only C1

Only C2

Only C3

Reference case

Thl L2 CNA

Figure 40: Loop 2 hot leg higher temperature peak for different Ci par friction coefficients

7-5
4320 4330 4340

Time (s)

570

575

580

585

590

liquid temperature (K)

Right node - sim.

Center node - sim.

Left node - sim.

Right node - wo left

Center node - wo left

Left node - wo left

Right node - wo right

Center node - wo right

Left node - wo right

Plant data

Right node - ref.

Center node - ref.

Left node - ref.

Figure 41: Loop 2 hot leg higher temperature peak variation for different nodalization

schemes

7-6
8 Problems Found in the Simulation Using

TRACE 5.0

During the works performed for the transient simulation a new version of TRACE was

available, TRACE 5.0 (oficial release). However, when this version was used, an unexpected

behavior in the calculation of the vapor mass flow during the opening of the steam dump

valves was observed. In this case, the secondary pressure decrease due to steam dump bypass

actuation is more accused, Figure 42.

In order to study this problem a small model ”break-valve (2 nodes)-break” was run using

several TRACE versions (TRACE 4.160, 4.273, 5RC1 and 5.0 RC3). The conclusions of this

work are included in the appendix 6.2.

0 200 400 600 800

Time (s)

6,8e+06

7e+06

7,2e+06

7,4e+06

7,6e+06

pressure (Pa)

SG1 (plant data)

SG2 (plant data)

SG3 (plant data)

SG1 T5.0

SG2 T5.0

SG3 T5.0

SG1 T4.160

SG2 T4.160

SG3 T4.160

Figure 42: Comparison of secondary pressure trends for RCP trip transient using TRACE

4.160 and 5.0

8-1
9 Execution Statistics

The transient simulation was made using two versions of TRACE code, 4.160 and the last

released, 5.0. Despite the problem described in section 8, related with the mass flow calculation during the few seconds after a valve opening, TRACE 5.0 is quite promising regarding

time execution. In fact, 5.0 version is 1.6 more times faster than 4.160 version, Figure 43.

Both codes have a slight calculation overload during turbine trip and steam dump demand,

and manual MSIV closing.

The simulations have been run in Pentium IV 3.4 MHz under Windows XP and AMD

Opteron Dual Core Processors 180 & 1222 under Debian, both with 32 and 64 bits precompiled executables provided by NRC. No significant differences where found between runs

executed in Windows and Debian systems, and between 32 and 64 bits code versions.

0 100 200 300 400 500 600 700 800 900

Time (s)

0

250

500

750

1000

1250

1500

Total CPU time (s)

TRACE 4.160 - 32 bits

TRACE 5.0 - 64 bits

Figure 43: CPU time versus simulation time. TRACE versions 4.160 and 5.0 with Debian /

AMD Opteron Dual Core Processor 180 system

9-1
10 Graphical Interface of the Almaraz I NPP

TRACE Model

SNAP tool availability has supposed an advantage in the graphical representation of the

results of the nuclear plants simulations. This application, developed with the sponsorship of

Nuclear Regulatory Commission (NRC), is outlined as a good complement for the engineering

analysis.

One of its main characteristics is the possibility of creation of masks in two dimensions for

its use in the way of animation, allowing to the user to draw the part of the model that

is considered of interest. This characteristic is complemented with the choice of chromatic

ranges for the termal-hydraulic components based on the variable of interest. In addition,

it is possible to add online plotting of these variable while the transient is running in the

animation mask.

Our previous works include several analysis of transients for isolated components, as well as

the modelization of the whole Almaraz I NPP mask, useful for verification of this transient,

Figure 44. To give an example, three snapshots of the simulation video are shown in Figure

45. The animation mask only works properly when using last versions of both programs,

SNAP 26.6 and TRACE 5.0.

Figure 44: SNAP animation mask for Almaraz I NPP model

10-1
Figure 45: CNA mask simulating RCP trip transient at 0, 292.5 and 759.6 s.

10-2
11 CONCLUSIONS

The TRACE model developed for Almaraz I NPP has been verified considering a wide

spectrum of conditions, being robust and showing good behavior. The results obtained in

the simulation of a real transient corresponding to one RCP trip transient in Almaraz I NPP

were in good agreement with plant data.

In this transient, the RCP of the loop 2 (this loop has the pressurizer surge line connection)

tripped and the reactor SCRAM is generated in a short time by the low flow in 1/3 loops.

After its demand, the reactor protection system starts the insertion of shutdown and control

banks. After reactor SCRAM, primary temperatures decrease, leading to a decrease in

pressurizer level and pressure. Turbine trip signal demands the turbine bypass opening.

There are no reliable data about how the SD valves actuate during this transient, but it is

clear that one or more valves did not fully closed (because Tavg decreases below HZP value),

so some assumptions were taken to reproduce the same plant behavior in secondary pressure.

It was found that adequate reproduction of secondary pressure was specially challenging,

and only it was achieved when assuming different steam discharges to the condenser.

The transient simulation was made using two versions of TRACE code, 4.160 and the last

released, 5.0. Meanwhile the results obtained using TRACE 4.160 were accurate, TRACE 5.0

presented a non expected behavior in the calculation of mass flow rates during valve openings.

These problems made unsuitable transient simulation using this code version. Despite this

problem, from the run statistics it was found that TRACE 5.0 is quite promising regarding

with time execution. In fact, 5.0 version is 1.6 more times faster than 4.160 version. Also, no

significant differences where found between runs executed in Windows and Debian systems,

and between 32 and 64 bits code versions.

More validation transients will be necessary to improve the quality of the model. In this

way, it is expected that next years the validation process will go on with the simulation of

new transients.

This work was supported by UNESA and Almaraz - Trillo AIE. Their support is gratefully

acknowledged.

11-1
12 REFERENCES

[1] I. Gonz ́alez, C. Queral, and A. Exp ́osito. Phenomenology during the loss of residual

heat removal system at midloop conditions with pressurizer PORVs open: Associated

boron dilution. Annals of Nuclear Energy, 34(3):166, 2007.

[2] I. Gonz ́alez, C. Queral, A. Exp ́osito, and M. Novales. TRACE Model of Almaraz Nuclear

Power Plant. In Proc. Of the International Conference Nuclear Energy for New Europe,

Bled, Slovenia, 2005. Nuclear Society of Slovenia.

[3] A. Lopez, C. Queral, and I. Gallego. Conversion of Almaraz NPP Model from RELAP5

into TRAC-M. Transactions of the American Nuclear Society, 89:408, 2003.

[4] J. Mulas, C. Queral, I. Collazo, A. Concejal, N. Burbano, A. L ́opez, and I. Tarrega.

Conversion of control systems, protection and engineering safeguard system-signals of

Almaraz NPP model from RELAP5 into TRAC-M. In Proc. Of the International Conference Nuclear Energy for New Europe, Kranska Gora, Slovenia, 2002. Nuclear Society

of Slovenia.

[5] C. Queral, A. Exp ́osito, G. Jimenez, L. Valle, and Martinez-Murillo J. C. Verification

and validation of Almaraz NPP TRACE model. In Proc. Of the International Congress

on Advances in Nuclear Power Plants, Anaheim, California, USA, 2008. American Nuclear Society.

[6] C. Queral, I. Gonz ́alez, and Exp ́osito. Simulation of loss of RHRS sequences in the Almaraz NPP during midloop operation using TRACE code. In Proc. Of the International

Congress on Advances in Nuclear Power Plants, Reno, USA, 2006. American Nuclear

Society.

[7] C. Queral, I. Gonz ́alez, and A. Exp ́osito. Analysis of the cooling capability of steam

generators during the loss of residual heat removal system at midloop operation with

closed primary system. Annals of Nuclear Energy, 33(13):1112, 2006.

[8] C. Queral, I. Gonz ́alez, and A. Exp ́osito. Analysis of abnormal operation procedures

in sequences of loss of the RHRS at midloop operation. Annals of Nuclear Energy,

35:1321–1334, 2008.

[9] C. Queral, I. Gonz ́alez, A. Exp ́osito, I. Gallego, and A. Concejal. Conversion of the

steam generator model of Almaraz NPP from RELAP5/MOD3 into TRAC-M, TRAC-P

and TRACE. In Proc. of the Mathematics and Computation, Supercomputing, Reactor Physics and Nuclear and Biological Applications, Avignon, France, 2005. American

Nuclear Society.

12-1
[10] C. Queral, J. Mulas, I. Collazo, A. Concejal, and N. Burbano. Problems found in the

conversion of Almaraz NPP model from RELAP5 into TRAC-M. Document sent to the

NRC in April, 2003, DSE-UPM, 2002.

[11] C. Queral, J. Mulas, I. Collazo, A. Concejal, N. Burbano, I. Gallego, and A. L ́opez.

Conversion of the Thermal Hydraulics Components of Almaraz NPP Model from RELAP5 into TRAC-M. In Proc. Of the International Conference Nuclear Energy for New

Europe, Kranska Gora, Slovenia, 2002. Nuclear Society of Slovenia.

[12] C. Queral, J. Mulas, and C. G. de la R ́ua. Analysis of the RELAP5/MOD3.2.2beta

Critical Flow Models and Assessment Against Critical Flow Data From the Marviken

Tests. NUREG/IA-0186, NRC, 2000.

12-2
Appendix A. TRACE 5.0 Opening Valve Flow

Problem Reported

In this appendix it is described the problem that was found during the simulations of the

transient with TRACE 5.0 (oficial release), see Section 8. Here it is shown the report text

as it was sent to NRC: TRACEZilla Bug 370 ”Unexpected behavior of critical flow model

during the opening of a valve”.

It has been noticed an unexpected behavior in the calculation of choked flow during the

opening of the steam dump valves of a plant model using TRACE 5.0 (released in August

2007). We noticed this behavior because we were working with a previous version, TRACE

4.160, and the results fitted quite well with plant data.

A small model ”break-valve (2 nodes)-break” was made and several tests were performed

to study the problem. The reference model is a break with a constant pressure similar to

pressure in NPP SGs at full power and the other break with 1 bar pressure (to simulate

steam dump discharge conditions). The transient starts when the valve is opening, Figure

46. The tests carried out were:

• Behavior of mass flows with/without choked flow model with both code versions, Figure

47:

1. With choked flow model off: same results for TRACE 5.0 and TRACE 4.160.

2. With choked flow model on:

– Phase 1): during the valve opening TRACE 5.0 calculates a mass flow 150 %

higher than TRACE 4.160.

– Phase 2): when the valve is opened at steady state conditions both codes

calculate the same mass flow.

– Phase 3) when the valve is closing both codes calculate the same mass flow.

• Comparison and sensitivity analysis between different models and parameters values:

– It is remarkable, that TRACE 5.0 choked flow is higher than noncritical mass flow

calculated during phase 1.

– Comparison between mass flows with choked flow model ”on” and with different

aperture speeds, Figure 48: the mass flow overestimation is greater as faster the

valve opens. Also, it is remarkable that TRACE 4.160 shows a similar overestimation behavior (with lower mass flow values) when the valve opens with very

high velocities (aperture fraction from 0.0 to 1.0 in 0.1 ms).

– Comparison between mass flows with choked flow model on and different values of

discharge coefficients, Figure 49: the mass flow overestimation remains unaltered

in phase 1 (the discharge coefficient doesn’t affect its value), but the mass flow

obtained for both code versions in phase 2 and 3, changes as expected.

A-1
• Platform execution: the results are the same in SUN and PC Linux and Windows

platforms (Debian and Windows XP) using 32 and 64 bit precompiled executables of

TRACE 5.0 and TRACE 4.160 supplied by NRC.

More tests were performed using another two TRACE versions, so the test has been performed for TRACE 5.0 official release, 5.0 RC1, 4.273 and 4.160 with and without choked

flow model on, Figure 50. It seems that the problem is more important in the TRACE 5

versions (RC1 and 5.0 official). Additionally, it could be remarked that some differences have

found in the results for RC1 and 5.0 official release. The tests run with choked flow model

”off” show different maximum mass flow rates.

285 290 295 300 305 310

Time (s)

0.002

0.004

0.006

0.008

Area (m^2)

T4

T5

Figure 46: Valve opening for both code versions, TRACE 4.160 and 5.0

A-2
285 290 295 300 305

Time (s)

0

50

100

150

200

Mass Flow (kg/s)

w chocked flow T5.0

w chocked flow T4

wo chocked flow T5.0

wo chocked flow T4.160

Figure 47: Mass flow rates with/without choked flow model on. TRACE 4.160 and 5.0

0 5 10 15 20

Time (s)

0

100

200

300

400

500

Mass Flow (kg/s)

T5.0 0.1 ms

T4.160 0.1 ms

T5.0 2.0 s

T4.160 2.0 s

T5.0 4.0 s

T4.160 4.0 s

Figure 48: Mass flow rates for different valve aperture speeds. TRACE 4.160 and 5.0

A-3
0 5 10 15 20

Time (s)

0

50

100

150

200

250

300

Mass Flow (kg/s)

Default (1.0) coefs T4.160

Default (1.0) coefs T5.0

1.0 coefs T4.160

1.0 coefs T5.0

1.1 coefs T4.160

1.1 coefs T5.0

0.9 coefs T4.160

0.9 coefs T5.0

1.2 coefs T4.160

1.2 coefs T5.0

Figure 49: Mass flow rates with choked flow model and different discharge coefficients.

TRACE 4.160 and 5.0

0 5 10 15 20

Time (s)

0

50

100

150

200

250

300

Mass Flow (kg/s)

T5.0 official w/cf

T5RC1 w/cf

T4.273 w/cf

T4.160 w/cf

T5 official wo/cf

T5rc1 wo/cf

T4.273 wo/cf

T4.160 wo/cf

Figure 50: Mass flow rates with/without choked flow model on. TRACE versions 4.160,

4.273, 5RC1 and 5.0

A-4