Chapter 5
PROCEDURE AND LIST OF PERFORMED CALCULATIONS
5.1 Procedure
For all the calculations executed, a complete procedure was adopted in order to achieve the following results:
• Obtain the radially averaged peak fuel enthalpy for the FA that experienced the rod ejection and for its neighbour FA.
• Obtain the time trend of the clad and fuel centerline temperatures for the FA more affected by the transient
• Obtain the time trend of the main plant parameters, e.g. reactor power, reactivity, pressurizer pressure
The calculation chart, as thoroughly described in Chapter 4, contemplates:
• a preliminary calculations of the cross section library, by the 2D neutron transport code HELIOS
• a processing of the cross section library in order to adapt its format to the PARCS input • the execution of a steady state calculation by the thermal-hydraulic code RELAP5 in a
stand-alone mode
• the execution of a steady state calculation by the coupled thermal-hydraulic code RELAP5 / 3D neutron kinetics code PARCS, using the cross section library calculated before
• the execution of the transient by the coupled RELAP5 / PARCS codes
• the data processing of the transient, using ‘ad-hoc’ Fortran programs and commercial software
Transient duration was fixed for every case to 8.0 seconds. It should be noted that this implied a running time of roughly one hour for each transient calculation by a PC equipped with a 800 MHz Celeron™ processor and a Windows™ XP™ operating system.
5.2 Reference cases
The reference cases studied, as described in Chapter 3.3, were the HFP and HZP, each of them with the fuel at BOL and EOC. The main plant parameters assumed are there described. Thus, using the previous cross section libraries calculated for the BOL and the EOC of the fuel, there were executed the first 4 reference calculations in order to understand the importance of plant status, of the fuel burn-up and to select the more severe condition for the application of successive sensitivity analyses. It can be seen from the Table 5.2.1 that the maximum value for the peak fuel enthalpy was found in the HZP- EOC case. Therefore the successive deep analyses were focused on this last case.
Table 5.2.1 - Reference case peak fuel enthalpy Reference cases Cross section library
used
Peak Fuel Enthalpy (cal/g) HFP – BOL Xsec_20_BOL 63.9 HZP – BOL Xsec_20_BOL 26.2 HFP – EOC Xsec_20_EOC 59.5 HZP – EOC Xsec_20_EOC 81.0
Further information about the transient results of the reference cases are reported in Chapter 6.
5.3 Sensitivity analyses
Sensitivity analyses were performed on HZP-EOC case in order to understand the effects of: • fuel characteristics variation
• plant flow regime variation
• cross section library resolution variation
• PARCS neutron kinetics code nuclear parameters variation • Xenon concentration variation
So, the full list of performed calculations executed is reported in the Table 5.3.1. A complete description of all the physical and geometrical models implemented is reported in Chapter 4.
Regarding to fuel characteristic variations there were executed two transients:
• in the first were changed in the HELIOS and in the RELAP5 input the fuel geometric characteristics, assuming a gap closure consequently to the fuel burnup. Contemporary, it was changed also the fuel density to simulate a restructuring (see Chapter 4.)
• in the second were decreased, on the RELAP5 input, the heat conductivity values of the FA that experienced the rod ejection (see Chapter 4.)
Regarding to plant flow regime, it was changed the RELAP5 input in order to simulate a plant operation with a decreased core flow value. This was obtained shutting down 2, 3 and all main circulation pumps. The importance of the calculation this type of transients was particularly stressed in the IAEA guidelines [13] because normally the HZP in a pressurized reactor is maintained with reduced core flow.
Regarding to cross section library variations, it was performed a first calculation using in the PARCS code a cross section library with an increased number of reference points (see Chapter 4.). The great ranges of values that fuel temperature and moderator density could assume in this type of transient, justified that attempt to obtain a better interpolation
A second calculation was run using the standard cross section library for the EOC with delayed neutron fractions calculated for each of the 280 fuel type. This increased resolution for the calculation was completely justified by the great importance that the delayed neutron played in the nuclear reaction kinetic.
Regarding to PARCS code nuclear parameter variation, it was run a calculation changing the Doppler weighting factor
w
from 0.1 to 1.0. Doppler weighting factor is a parameter used by PARCS to calculate Doppler temperature in the PARCS-specific Data Map Routine [20]. In particular, a linear combination of the fuel center-line temperature and of the fuel surface temperature is done by the code for obtain a Doppler temperature average value. The linear combination is done in the following way:SURFACE fuel CL fuel DOPPLER
T
T
T
=
(
1
−
ω
)
+
ω
Putting
w
parameter to 1.0 caused a decrease of the Doppler effect during the transient calculation because of TDOPPLER became equal to TSURFACE, notably a much lower value than TCL. The resultingdecrease of this important feedback parameter resulted, as shown in Chapter 6, in a greater energy quantity released to the fuel.
A Xenon transient was the last effect investigated for the HZP-EOC case. As described in Chapter 4. , an appropriate set cross section library was created, simulating a Xenon poisoning of the core. In particular was assumed that:
• the Xenon was at its highest concentration (i.e. after 9 hours)
• the FA that would have to experience the rod ejection was not poisoned; therefore it was supposed the CR inserted by a lot of the time before the scram that caused the Xenon poisoning
The events originating this type of transient for the NPP were these:
• Reactor operating at Full Power; CR where it will happen the accident inserted • Reactor scram
• Reactor stand-by at HZP; starting of the Xenon poisoning • REA 9 hour after scram
As it will be shown in the Chapter 6, this was found as one of the more severe transient for the fuel.
5.4 Worst REA scenario
In order to maximize the energy released, the temperature of the fuel and the temperature of the clad, there were run several transients combining a variation of all the parameters discussed above in the sensitivity analysis. It resulted that the worst REA scenario from the combining of:
• the effect of the Xenon poisoning
• the increase of the Doppler weighting factor from 0.1 to 1.0
• the reduction of flow regime, turning off 2 of the 4 main coolant pumps The results are shown in details in Chapter 6.
Table 5.3.1 – Cross Section Library for Sensitivity analyses of the HZP – EOC case
Calculation Cross Section Library
Fuel characteristics
Gap closure & fuel restructured XSec_20_EOC_GapClosure
Heat conductivity variation XSec_20_EOC
Plant flow regime
2 pump in operation XSec_20_EOC 1 pump in operation XSec_20_EOC Natural circulation XSec_20_EOC
Cross section library
Increase of reference points from 20 to 42 XSec_42_EOC Delayed neutron fractions calculated for every fuel type XSec_20_EOC_Beta
PARCS nuclear parameters
Doppler weighting coefficient increase from 0.1 to 1.0 XSec_20_EOC
Xenon concentration
Xenon Poisoning of whole core excluding the FA where the REA happens XSec_20_EOC_Xenon
