FIG. 7.11. Comparison of reactivity worth of top beryllium shim for HEU and LEU fuel of PARR -2 (Courtesy of Pakistan Institute of Nuclear Science and Technology (PINSTECH), Pakistan).
185 7.3.2. Results and discussion
7.3.2.1. HEU core
The axial power profile obtained from the neutronic analysis is given in Fig. 7.12. The computed axial temperature distribution in the hot and average channels is shown in Fig. 7.13.
The results agree well with the reported data [7.2]. A maximum clad temperature of 55C at a steady state power level of 30 kW was calculated. The average values of the heat transfer coefficient are 1274 W/m2s and 1232 W/m2s in the hot and average channels respectively.
The calculated temperature difference across the core is 19.5C, and the measured value is 20.5C, which agrees reasonably well [7.2]. Maximum temperatures are also computed at various power levels. These results are shown graphically in Fig. 7.14.
During commissioning of PARR-2 with HEU fuel, power rising experiments and calculations were performed. Peak power was determined after insertion of a ramp reactivity of 4 mk, equal to the cold core excess reactivity. The intent of the work was to demonstrate the reactor’s inherent safety. This has been simulated by PARET. Mass flow rate, which is a design characteristic, was taken from the Final SAR [7.2]. Computed results are shown in Tables 7.9 and 7.10. Calculations of the 4 mk transient analysis indicate a computed peak power of 78 kW for the existing HEU core, which matches reasonably well with the measured value of 87 kW [7.2]. The heat flux increases from 2.2 W/cm2 at 30 kW to 5.8 W/cm2 at 78 kW. These values of heat flux are well below the critical heat flux of 151 W/cm2. Also the maximum cladding surface temperature in the existing HEU core remains 13.5C below the water saturation temperature. Power and temperature histories for the existing HEU core are illustrated in Figures 7.15(a) and 7.15(b).
7.3.2.2. LEU core
Three potential LEU cores were considered:
— Core 2: enriched to 12.6% U235 and a fuel pin diameter of 5.5 mm;
— Core 3: enriched to 12.3% U235and a fuel pin diameter of 5.1 mm;
— Core 4: enriched to 12.46% U235 and a fuel pin diameter of 5.5 mm, increased thickness of the control rod absorbing material and Zircaloy-4 as a control rod guide tube and grid plate material.
Power peaking factors computed through neutronic analysis are presented in Fig. 7.16.
In order to achieve the same level of neutron flux, increasing the operating power level of LEU cores to 33 kW has been suggested. Axial temperature distributions of the fuel centre line, cladding and coolant in the hot channels of the three cores are illustrated in Figs 7.17 (a)—(c). Results show that at the increased steady state operating power level, the cladding temperature increases when compared with the existing HEU core, but would remain far below the temperature at which boiling occurs in the core. Fuel centre line temperatures are also very mild.
For core 2 with a fuel pin diameter of 5.5 mm, the values of the heat transfer coefficient are 1250 W/m2s and 1209 W/m2s in the hot and average channels respectively. The temperature difference across the core is 21.2C. Peak fuel and clad surface temperatures are 69.8C and 61.0C respectively. Results of a 4 mk reactivity insertion transient analysis show that power peaks at 68.1 kW. An LEU core would attain a lower peak power due to the large Doppler coefficient compared to the HEU core. The maximum cladding surface temperature for core 2 is 88.6C, less than 97.9C for the HEU core. Both the temperature and power history in a 4 mk transient are shown in Figs 7.18 (a) and (b).
For core 3 with a fuel pin diameter of 5.1 mm, the values of heat transfer coefficient ranges are 1266 W/m2s and 1221 W/m2s for the hot and average channels respectively. The temperature difference across the core is 20.48C. Peak fuel and cladding surface temperatures are 70.2C and 61.4C respectively. The power for a 4 mk reactivity insertion transient peaks at 70.27 kW. The maximum cladding surface temperature is 90.9C. Both the temperature and power history in a 4 mk transient are shown in Figs 7.19 (a) and (b).
In a thermal-hydraulic and transient analysis of core 4, the temperature difference across the core is 22.6C. Peak fuel and clad surface temperatures are 74.9C and 60.1C respectively. Results of a 4 mk reactivity insertion transient analysis show that power peaks at 66.2 kW. The LEU core 4 would attain a lower peak power due to the large Doppler coefficient compared to the HEU core. The maximum cladding surface temperature is 86.2C, compared to 97.9C for the HEU core. The decrease is due to a less pronounced power peak in the LEU core 4 with a higher Doppler coefficient of reactivity. Both the temperature and power history in a 4 mk transient are shown in Figs 7.20 (a) and (b).
TABLE 7.8. KINETIC PARAMETERS AND REACTIVITY FEEDBACK COEFFICIENTS
Core 1
(existing HEU core)
Core 2 (proposed
LEU core) Core 3 (proposed
LEU core) Core 4 (proposed LEU core) Prompt neutron
generation time (s) 57.00 47.00 50.50 47.00
Effective delayed
neutron fraction (βeff) 0.00850 0.00845 0.00832 0.00845 Water temperature
coefficient (%k/k/°C) 6.278 × 10-2 3.966 × 10-3 4.198 × 10-3 4.276 × 10-3 Void/density coefficient
(%k/k /%void) 0.326 0.356 0.348 0.344
Doppler coefficient
(%k/k/°C) 1.546 × 10-4 1.395 × 10-3 1.342 × 10-3 1.395 × 10-3
187 TABLE 7.9. RESULTS OF STEADY STATE THERMAL-HYDRAULIC ANALYSIS OF PARR-2
Parameter Core 1
(existing HEU core)
Core 2 (proposed LEU core)
Core 3 (proposed LEU
core)
Core 4 (proposed LEU
core)
Fuel pin diameter (mm) 5.5 5.5 5.1 5.5
U235 enrichment (%) 90.2 12.6 12.3 12.46
Operating power (kW) 30 33 33 33
Power peaking factors
Axial 1.1271 1.1333 1.1389 1.3352
Radial 1.1234 1.1311 1.1388 1.0754
Pressure at core inlet (kPa) 1.5 1.5 1.5 1.5
Saturation temperature in core (C) 111.4 111.4 111.4 111.4 Steady state temperatures (C)
Coolant temperature rise across core 19.5 21.24 20.48 22.59
Peak clad surface temperature 54.97 61.01 61.37 60.06
Peak centreline temperature 55.20 69.82 70.22 74.94
Average heat flux (W/cm2) 1.833 2.016 2.603 2.016
Peak heat flux (W/cm2) 2.321 2.584 3.376 2.895
Critical heat flux (W/cm2) 151.2 151.2 151.2 151.2
Margin to critical heat flux 65.1 58.5 44.8 52.2
TABLE 7.10. RESULTS OF 4 mk TRANSIENT ANALYSIS OF PARR-2 Parameter
Core 1 (existing HEU
core)
Core 2 (proposed LEU
core)
Core 3 (proposed LEU
core)
Core 4 (proposed LEU
core) Fuel pin diameter
(mm) 5.5 5.5 5.1 5.5
U235 enrichment (%) 90.2 12.6 12.3 12.46
Peak power (kW) 78.2 68.10 70.27 66.2
Peak temperatures (C)
Fuel centreline 98.5 106.74 110.17 116.4
Cladding surface 97.9 88.58 90.93 86.2
FIG. 7.12. Axial power profile in the HEU core (Reproduced from Ref. [6.3] with permission courtesy of Pakistan Institute of Nuclear Science and Technology (PINSTECH), Pakistan ).
FIG. 7.13. Axial distribution of temperature in hot and average channels of HEU core at a steady state power level of 30 kW (Courtesy of Pakistan Institute of Nuclear Science and Technology (PINSTECH), Pakistan).
0 50 100 150 200 250
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Peaking factor
Distance from bottom (mm)
0 50 100 150 200 250
20 25 30 35 40 45 50 55 60 65 70 75 80
Average channel Average channel
Hot channel Hot channel
Coolant Clad surface
Hot channel Average channel
Fuel centerline
Temperature (C)
Distance from bottom (mm)