Sections 3.3.1 and 3.3.2 present the measured test data recorded by the data acquisition system during SHRT-17 and SHRT-45R, respectively. Of the available measurements described in Section 3.2, these measurements collectively represent the system behaviour during the two transients, and they were identified as the data best suited for comparison against the CRP participants’ simulation results. Section 3.4 discusses the accuracy of these measurements and potential concerns for comparisons with systems code predictions. Section 10 compares the measured test data against the participants’ final simulation results. These measurements represent an appropriate collection of parameters for comparison against results from the systems codes used by the benchmark participants.
3.3.1. SHRT-17
Primary system flow rates coasted down gradually during the first minute of SHRT-17 until the pumps stopped and natural circulation was established. Due to the rapid large negative reactivity insertion from the simultaneous control rod scram, fission power decreased much more quickly and the normalized power to flow ratio dropped to 0.2 after one second. The power to flow ratio then began to gradually increase as the core mass flow rate decreased faster than the heat generated by delayed neutrons and decay heat.
Over the first minute of the test, the normalized power to flow ratio continued to increase, exceeding the nominal value after forty seconds. Elevated core temperatures provided the necessary driving head to maintain relatively constant flow rates before the two pumps stopped at 42 and 51 seconds. The increased resistance of the stopped pumps caused flow rates to decrease again and the normalized power to flow ratio peaked at 2.4 after one minute.
As the system transitioned to natural circulation, the core outlet temperature remained elevated, and temperatures in the Z-Pipe began to increase. The core flow rate gradually
increased over the next few minutes and total power continued to decrease. Three minutes after the test began, the power to flow ratio decreased below its nominal value.
FIG. 6 illustrates mass flow rate measurements for the high and low pressure piping following pump #2 during the transition to natural circulation. These flow rates were measured with electromagnetic flowmeters as normalized flow rates and were renormalized to the initial flow rates specified in the benchmark. Because SHRT-17 was not representing station blackout conditions, the auxiliary EM pump in the Z-Pipe was disabled for the test. SHRT-17 flow rates were also lower than those during SHRT-45R due to lower system temperatures in the core, outlet plenum and Z-Pipe. After ten minutes, SHRT-17 flow rates were less than half of the SHRT-45R flow rates at ten minutes into that test. Analysis performed during the CRP suggests that after both pumps coasted down, they locked up due to the low flow rates.
Locked pumps provide additional flow resistance that would further contribute to the lower flows measured during SHRT-17 than SHRT-45R.
FIG. 7 illustrates the power level during the SHRT-17 test. The neutron flux intensity measurements described in the previous section recorded the normalized fission power during the test. The American Nuclear Society (ANS) decay heat standard [4] was then used to calculate the initial fraction of power generated by decay heat at the start of the test. Then, decay heat generation during the test was calculated with the decay heat standard based on the fission power measurements.
FIG. 6. SHRT-17 pump #2 high and low pressure mass flow rates.
The total power shown in FIG. 7 is the sum of the measured fission power and the calculated decay heat generation.
FIG. 8 illustrates the measured core inlet and outlet temperatures during SHRT-17. Because the pumps stopped during the loss of flow test, sodium in the cold pool was not well-mixed, and it took a long time for hot sodium leaving the IHX to propagate to the pump inlets.
Consequently, temperatures in the inlet plena were nearly constant during the fifteen-minute test. It should be noted that the low pressure inlet plenum temperature shown in FIG. 8 is the average of the three thermocouple measurements from the plenum. All three low pressure inlet plenum thermocouples recorded very similar temperatures during the test.
The core outlet temperature decreased quickly at the beginning of the test following the control rod scram. Over the next two minutes the core outlet temperature increased due to the core flow rate decreasing faster than the total power level. The core outlet temperature peaked approximately one minute after the power to flow ratio peaked due to the heat capacity of the upper core structure and colder sodium already present in the outlet plenum. Although a thermocouple was installed at the Z-Pipe inlet, the measured core outlet temperature for SHRT-17 was not recorded by this instrument. It is believed that this measurement was a combination of several subassembly outlet temperature measurements, although the exact combination of those measurements is not known for certain. Section 3.4 discusses this further.
FIG. 7. SHRT-17 total power, fission power and decay heat.
FIG. 8. SHRT-17 core inlet and outlet temperatures.
FIG. 9 illustrates the SHRT-17 measured primary and intermediate IHX temperatures. The primary side inlet temperature was measured by a thermocouple installed behind an impact baffle plate in the diffuser region at the top of the IHX. The primary side outlet temperature shown in FIG. 9 is the average of four temperatures measured just outside the IHX outlet window. The primary side inlet and outlet temperature measurements are considered to be unreliable representations of the average temperature of sodium entering and exiting the IHX.
Section 3.4 discusses the IHX primary side temperature measurements further.
Intermediate side temperatures were measured by thermocouples installed within pipes upstream and downstream of the IHX. The reliability of these measurements has been verified by other temperatures measured further upstream and downstream within the intermediate system.
FIG. 9. SHRT-17 IHX temperatures.
FIG. 10 illustrates the flow rate and temperature measurements for the XX09 fueled instrumented subassembly during SHRT-17. The XX09 measured flow rate displayed a similar trend as the measured high pressure flow rate, reaching a minimum around sixty seconds before flow rates increased slightly as natural circulation was established.
Over the first 100 seconds, the thermocouples installed at the XX09 flowmeters at the bottom of the subassembly measured a temperature increase followed by a gradual decrease over the next 300 seconds. Neither of these temperature changes was recorded by the thermocouples in the inlet plena. It is believed that the temperature increase was due to gamma heating decreasing more slowly than the average core power level and undercooling as the core flow rate decreased. The flowmeter temperatures then began to decrease as gamma heating continued to decrease and natural circulation was established.
Within the pin bundle region of XX09, 22 thermocouples measured temperatures at one of three elevations: mid-core (MTC), top of core (TTC), and above core (14TC). Two thermocouples were installed at the outlet of the subassembly (OTC) and two more were installed at the top of the subassembly in the annular thimble region (ATC). Each of these 26 temperature measurements followed the power to flow ratio. Temperatures decreased very rapidly at the beginning of the transient before increasing to a maximum around 100 seconds.
Temperatures began to decrease again after natural circulation was established and power continued to decrease. The TTC temperatures were higher than the 14TC, OTC and ATC temperatures due to the heat capacity of the above core structure.
FIG. 10. SHRT-17 XX09 mass flow rate and temperatures.
FIG. 11 illustrates the flow rate and temperature measurements for XX10 during SHRT-17.
Flowmeters and thermocouples were installed in similar locations in the XX10 non-fueled instrumented subassembly. The flow rate and temperature measurements for XX10 followed the same trends as XX09. While the initial power generated in XX09 was more than 25 times larger than the initial power generated in XX10, flow through XX09 was only 7 times larger.
The lower power to flow ratio for XX10 resulted in lower peak temperatures than for XX09.
FIG. 11. SHRT-17 XX10 mass flow rate and temperatures.
3.3.2. SHRT-45R
Primary system flow rates coasted down gradually at the beginning of SHRT-45R after the pumps tripped. The two biggest differences between SHRT-45R and SHRT-17 were whether or not the control rods scrammed and the behaviour of the auxiliary EM pump, both of which contributed to higher flow rates for SHRT-45R. Because the control rods did not scram during the unprotected loss of flow test, power decreased more gradually and the power to flow ratio increased immediately at the start of the SHRT-45R test. The power to flow ratio peaked at
2.8 times the nominal value after one minute, producing higher temperatures in the core, upper plenum and Z-Pipe, which led to a larger driving head to help establish natural circulation. SHRT-45R represented a station blackout, so the auxiliary EM pump was left on battery power, providing a small head that also contributed to higher flow rates than for SHRT-17.
FIG. 12 illustrates the mass flow rate measurements for the high and low pressure piping for pump #2 during SHRT-45R. Due to the elevated temperatures at the beginning of the test, the pumps did not stop until 95 seconds into the test, after which the elevated temperatures in the system provided most of the driving head. Primary system flow rates remained relatively constant until 600 seconds into the test, when the current to the auxiliary EM pump was increased by 50% and flow rates increased by approximately 20%. The natural circulation flow rate during SHRT-45R was more than double the natural circulation flow rate during SHRT-17 because of the head provided by the auxiliary EM pump and the higher temperatures during the test. Additionally, the pumps may have locked up during SHRT-17 but not during SHRT-45R, so the flow resistance of the pumps would have been lower during the unprotected SHRT-45R test.
FIG. 12. SHRT-45R pump #2 high and low pressure mass flow rates.
With temperatures in the core increasing, the net reactivity quickly became negative, driving power down. After natural circulation was established, power continued to decrease due to the negative reactivity feedbacks. Based on the CRP participants’ simulation results, the sodium density and radial core expansion reactivity feedback effects provided most of the negative reactivity. Axial core expansion also provided a negative reactivity feedback. The Doppler and control rod driveline effects were much smaller and did not play as significant a role in SHRT-45R as for other SFRs.
The power to flow ratio dropped below the nominal value around 400 seconds, as core flow rates were nearly constant but power was still decreasing gradually. The power to flow ratio stabilized at 0.8 times the nominal value around 600 seconds, just before the auxiliary EM pump current increased. Higher flow rates led to lower core temperatures and a small power increase. FIG. 13 illustrates the power level during the SHRT-45R test. As with SHRT-17, total power was the sum of the measured fission power and calculated decay heat. However, unlike SHRT-17, fission represented more than half of the total power for the entire test, hence the SHRT-45R natural circulation flow rates were more than double the SHRT-17 natural circulation flow rates.
FIG. 13. SHRT-45R total power, fission power and decay heat.
FIG. 14 illustrates the measured core inlet and outlet temperatures during SHRT-45R. As with SHRT-17, a lack of mixing in the cold pool after the pumps tripped led to relatively flat temperatures in the high and low pressure inlet plena. The inlet plena temperatures began to diverge during the second half of the test due to increased stratification in the cold pool and because sodium in the low pressure inlet pipes was more exposed to the colder sodium at the bottom of the cold pool. The portion of the low pressure inlet pipes that traveled horizontally at the bottom of the cold pool was 63 cm longer than the horizontal portion of the high pressure inlet pipes at the bottom of the cold pool. Additionally, the high pressure inlet pipes were nearly twice as thick as the low pressure inlet pipes, providing greater insulation from the cold sodium.
FIG. 14. SHRT-45R core inlet plena and Z-Pipe inlet temperatures.
Figure 14 also shows the Z-Pipe inlet temperature measurement for SHRT-45R. As the power to flow ratio increased, hotter sodium leaving the core mixed with the sodium already in the upper plenum before flowing into the Z-Pipe. The Z-Pipe inlet temperature continued to increase after the power to flow ratio began to decrease because the sodium leaving the core was still hotter than the average upper plenum temperature. There was an issue with the measured Z-Pipe inlet temperature between 75 and 200 seconds, which is discussed in the following section, that led to this data being removed for comparisons with simulation results.
After 200 seconds, the Z-Pipe inlet temperature decreased for the remainder of the test as the power level continued to decrease.
FIG. 15 illustrates the SHRT-45R measured primary and intermediate IHX temperatures. The primary side inlet and intermediate side outlet temperatures followed the Z-Pipe inlet temperature, increasing during the first few minutes of the test and then decreasing as the power level continued to drop while flow was relatively constant. Heat lost through the Z-Pipe to the cold pool along with the position of the primary side inlet thermocouple contributed to lower temperatures at the IHX inlet than the Z-Pipe inlet. As with SHRT-17, the primary side temperature measurements are considered to be unreliable representations of the average temperature of sodium entering and exiting the IHX. Reasons why the primary side measurements were not as accurate as the intermediate side measurements are discussed in the following section.
FIG. 16 illustrates the flow rate and temperature measurements for the XX09 fueled instrumented subassembly during SHRT-45R. While the pumps were coasting down, the XX09 measured flow rate displayed a similar trend as the other flow measurements in the primary system. But after the pumps stopped around 100 seconds, the XX09 measured flow rate behaved differently from other flow measurements in the system, particularly the XX10
and pump #2 high pressure flow rates. Because of the large flow rate changes that were recorded by the XX09 flowmeter but not the other flowmeters, it is believed that the XX09 measured flow rate was not accurate for SHRT-45R after the first minute of the test. This is discussed further in Section 3.4.
FIG. 15. SHRT-45R IHX temperatures.
Neither the XX09 flowmeter temperature nor the mid-core temperature measurements were available for SHRT-45R.
The TTC, 14TC, ATC and OTC thermocouples, which were installed at the top of or above the pin bundle region, all recorded temperatures that were very similar to the Z-Pipe inlet temperature. For each thermocouple type, the peak temperature was lower and occurred slightly later the higher up the thermocouple was installed. This was due to the heat capacity of the upper core structure drawing heat from the hotter sodium flowing out of the XX09 pin bundle region. At 650 seconds, all of these temperatures decreased due to the increased flow through the subassembly after the auxiliary pump head increased.
FIG. 17 illustrates the flow rate and temperature measurements for XX10 during SHRT-45R.
The flow rate and temperature measurements for XX10 followed the same trends as XX09.
Even though the core inlet temperature did not increase during SHRT-45R, the XX10 flowmeter thermocouples, which were installed at the bottom of the subassembly, recorded a 20 K increase. As with SHRT-17, this was likely caused by gamma heating and lower flow rates. The MTC, TTC, 14TC, ATC and OTC thermocouples all displayed very similar behaviour as the other temperature measurements described above. Due to the lower power to flow ratio in the non-fueled instrumented subassembly, lower peak temperatures were measured for XX10 than for XX09.
FIG. 16. SHRT-45R XX09 mass flow rate and temperatures.
3.4. INTERPRETATION OF THE EXPERIMENTAL DATA USED IN THE