Chinese integrated pressurized water reactor (CIPWR) has compact configuration and high inherent safety, which is appropriate for nuclear power plants of small and medium scale. Heat balance model has been adopted widely in thermal power calibration of PWRs because of its advantage of accuracy. In this paper, a package based on FORTRAN language is developed and added into RELAP5 to calculate the heat loss value needed in heat balance analysis. The steadystate operation of CIPWR is modelled correctly by RELAP5. The heat loss of CIPWR is calculated by the package, and the comparison of the main values of parameters needed in the heat loss calculation between RELAP5 and the package has been done. It shows that the package has high calculation accuracy and can be applied in reactor design and monitoring.
Reactor Excursion and Leak Analysis Program (RELAP5) is a reactor analysis program developed by Idaho National Engineering Laboratory (INEL), approved by Nuclear Regulatory Commission (NRC) for engineering review of thermal hydraulic calculation of PWRs [
To make reactor operate more safely and economically, the accurate calibration of real thermal power is important for design, analysis, and monitoring of the reactor, which has been applied in PWRs currently [
Integral pressurized water reactor (IPWR) is one of the hot issues in the field of smallsize nuclear systems, which has high inherent safety. The structure of IPWR is more compact for the reason of the less use of pipes. The characteristic of small size makes IPWR more easily used in different conditions than other reactors, such as ships and remote places. There are various IPWR designs in the world with the same characteristic [
Our study aims at evaluating the performance of CIPWR in stable and transient operations. In this paper, RELAP5/MODE3.4 is used to model the stable condition of CIPWR and calculate the heat loss needed in heat balance analysis with the package we developed. The series of transient accidents of CIPWR will be studied in the future.
The package added to RELAP5 is used to calculate the heat loss in heat balance analysis. All the input values in the package can be obtained from the calculation result of steady condition by RELAP5, such as the value of temperature, pressure, and vapour fraction. Figure
Structure of the heat loss package.
Heat balance method is based on the steam generator approximation enthalpy balance [
As is shown in Figure
Heat balance principle diagram of CIPWR.
In the second loop, unsaturated feedwater with the mass flow rate
The enthalpy of water and steam included in the heat balance calculation can be computed by the thermodynamic properties subroutine according to IAPWSIF97 [
The range of IAPWSIF97 has five subareas, each of which has a basic equation that can be used to calculate the water and steam properties by derivation. As one of the thermophysical properties, enthalpy can be obtained directly from the pressure and temperature values by the basic equations except region 3. The feedwater in the entrance of steam generator is unsaturated water belonging to region 1, whose thermophysical properties can be computed by (
The enthalpy of feedwater can be computed by
The steam in the export of steam generator loop is wet saturated steam consisting of saturated steam and saturated water with different enthalpy. The total enthalpy of steam can be calculated according to the vapor fraction
The enthalpy of saturated steam
As shown in Table
Comparison of calculation results between package subroutine and IAPWSIF97.
Region  Parameters  Given by 


Region 1 

IF97  115.331273 

Package  115.33127302  

IF97  184.142828  

Package  184.14282773  


Region 2 

IF97  2549.91145 

Package  2549.9114508  

IF97  6571.22604  

Package  6571.2260386 
The whole reactor system can be modelled by different modules of RELAP5, which contains hydrodynamic components, branches, heat structures, reactor kinetics, control system, and so on. Hydrodynamic components consist of the parts where the fluid flows through, such as pipes, annulus, valves, pumps, and timedependent volumes. Heat structures simulate the heat transfer in the system by setting the thermal conductivity and heat capacity of materials. Control system can set the change with time of physical quantities and trips and also can be used in selfinitialization. Reactor kinetics consist of multigroup neutron transport model and point reactor model, which calculate the power of reactor by setting the initial power and reactivity feedback coefficient of various influent factors.
Figure
Sketch of CIPWR.
Figure
Node map of CIPWR.
Sketch of core materials.
OTSG has high power density and can be adjusted rapidly, which makes it conventionally applied in IPWR. Figure
Node map of OTSG.
Steady state of CIPWR has been modelled by RELAP5. Figure
Pressure of reactor core, upper plenum, low plenum, steam, and pressurizer in steady state.
Temperature of reactor core, upper plenum, low plenum, steam, and second side of SG U tube in steady state.
Figure
Table
RELAP5 calculation values of pressure and temperature in different positions.
Parameter  Position  Values 

Pressure (MPa)  Core  7.1127 
Upper plenum  7.0989  
Low plenum  7.1216  
Pressurizer  7.07127  
SG feedwater  3.0 MPa  
SG steam  2.943 MPa  


Temperature (K)  Core  0.9667 
Upper plenum  557.59  
Low plenum  515.17  
SG primary side  547.92  
SG feedwater  423.15  
SG steam  505.93 
When steadystate operation finished, the heat loss calculation package started, using the value in the RELAP calculation result. The calculation result is shown in Table
Values of heat loss calculation.
Belong to  Parameters  Values 

RELAP5  Feedwater temperature  423.15 K 
Feedwater pressure  3.0 MPa  
Steam temperature  506.05 K  
Steam pressure  2.943 MPa  
Vapor fraction  0.9667  
Reactor power  96.58 MW  
Feedwater flow rate  43.9 kg/s  


Heat loss calculation package  Feedwater enthalpy  633.813 kJ/kg 
Steam enthalpy  2802.974 kJ/kg  
OTSG power  95.226 MW  
Heat loss  1.354 MW 
As is shown in Table
Difference and error of calculation results between RELAP5 and package.
Parameter  RELAP5 calculation results  Package calculation results  Difference  Error (%) 

Feedwater enthalpy (kJ/kg)  633.602  633.813  0.211  0.0333 
Steam enthalpy (kJ/kg)  2798.343  2802.974  4.631  0.1655 
OTSG Power (MW)  95.032  95.226  0.194  0.2041 
Heat Loss (MW)  1.548  1.354  0.194  — 
The comparison between the simulated value and design value shows that the steady state of CIPWR has been modelled by RELAP5 correctly. According to the distribution of simulated values of pressure in primary loop, the upper pressure vessel space can act as pressurizer practically with the advantage of eliminating lots of pipes which reduces the risk of some accidents effectively.
The heat loss calculation package which is added to RELAP5 was developed based on heat balance method. The comparison of the main values needed in the heat loss calculation between RELAP5 and the package shows that the heat loss package can be applied in reactor design and monitoring. Further study will aim at calculating the series of transient accidents of CIPWR based on the steady state by RELAP5.
The authors declare that there is no conflict of interests regarding the publication of this paper.