WEBVTT 00:00.000 --> 00:02.500 Hello, and welcome to today's presentation. 00:03.400 --> 00:07.660 This lecture will provide an overview of the NuScale concept, along 00:07.660 --> 00:11.560 with a discussion on the transient scenario chosen for this reactor. 00:13.700 --> 00:17.220 First, I will start with the description of the NuScale concept. 00:17.580 --> 00:19.680 So NuScale is a small modular reactor. 00:20.000 --> 00:22.920 It is based on the well-established PWR technology. 00:23.260 --> 00:27.240 This SMR is being developed by NuScale Power, with headquarters in 00:27.240 --> 00:28.020 Portland, Oregon. 00:28.020 --> 00:33.140 NuScale is the first SMR concept which was licensed by the NRC for the 00:33.140 --> 00:34.300 use in the United States. 00:34.940 --> 00:38.980 So far, the design with thermal power of 160 MW was approved. 00:39.660 --> 00:43.680 The uprated design with thermal power of 250 MW is currently under 00:43.680 --> 00:44.000 review. 00:46.520 --> 00:50.540 One of the important features of the NuScale reactor is a modular 00:50.540 --> 00:51.140 design. 00:51.140 --> 00:55.600 Its primary system is integrated within the reactor pressure vessel, 00:56.160 --> 01:00.100 encompassing the reactor core, steam generators, and pressurizer. 01:00.640 --> 01:05.080 This integral design eliminates the need for external piping to 01:05.080 --> 01:08.240 connect the steam generators and pressurizer to the reactor pressure 01:08.240 --> 01:08.600 vessel. 01:09.380 --> 01:13.020 The latter is housed in a steel containment partially immersed in 01:13.020 --> 01:17.220 water, providing a passive heat sink for long-term decay heat removal. 01:18.620 --> 01:23.300 The NuScale reactor is designed to operate using natural circulation 01:23.300 --> 01:27.720 as the means of providing reactor coolant flow, eliminating the need 01:27.720 --> 01:29.240 for reactor coolant pumps. 01:30.480 --> 01:35.680 The NuScale core, adopted in the MEXSAFER project, comprises 37 fuel 01:35.680 --> 01:37.760 assemblies at six different enrichment levels. 01:38.620 --> 01:42.360 Those were adjusted to approximate batch-wise fissile uranium content 01:42.360 --> 01:44.780 at the beginning of the equilibrium fuel cycle. 01:44.780 --> 01:48.780 Four fuel assemblies contain gadolinia burnable poison. 01:49.580 --> 01:52.980 The core is surrounded by a heavy steel reflector and bounded by a 01:52.980 --> 01:54.120 cylindrical core barrel. 01:55.800 --> 01:59.900 NuScale has two independent means of reactivity control, control rods 01:59.900 --> 02:01.140 and soluble boron. 02:02.160 --> 02:06.580 The control system is organized in two banks, a regulating bank and a 02:06.580 --> 02:07.300 shutdown bank. 02:07.300 --> 02:11.580 Each bank consists of two groups with four control rod assemblies, 02:12.100 --> 02:16.660 with 24 individual control rods containing boron carbide and AIC in 02:16.660 --> 02:17.240 the rod tip. 02:17.420 --> 02:21.380 The regulating bank is used during normal plant operation to control 02:21.380 --> 02:22.080 reactivity. 02:22.720 --> 02:26.220 The shutdown bank is used during shutdown and reactor trip events. 02:27.340 --> 02:32.000 All NuScale fuel assemblies have a typical 17x17 pin lattice. 02:32.000 --> 02:35.920 The fuel rod pitch inside the assembly is 1.26 cm. 02:36.440 --> 02:40.500 The fuel assembly pitch inside the core is about 21.5 cm. 02:41.200 --> 02:45.780 The fuel assemblies comprise 264 fuel rods and 24 guide tubes into 02:45.780 --> 02:47.260 which control rods can be inserted. 02:47.700 --> 02:50.160 The central guide tube is used for instrumentation. 02:50.780 --> 02:54.160 To cope with initial excess reactivity, some fuel assemblies are 02:54.160 --> 02:58.620 loaded with 16 integral burnable poison rods containing a homogeneous 02:58.620 --> 03:01.080 mixture of urania and gadolinia. 03:02.200 --> 03:03.980 So how small is NuScale? 03:04.480 --> 03:07.840 To give a perspective, this slide compares the radial layouts of 03:07.840 --> 03:10.760 NuScale and a large Westinghouse PWR core. 03:11.460 --> 03:14.140 It's important to note that the figures are to scale. 03:20.700 --> 03:25.660 NuScale has 37 fuel assemblies instead of 193 fuel assemblies and 03:25.660 --> 03:30.700 designed to produce 160 MW power instead of 3400 MW. 03:30.700 --> 03:35.920 NuScale has much shorter fuel assemblies compared to the large PWR, 2 03:35.920 --> 03:38.200 m vs 3.7 m. 03:39.580 --> 03:43.640 However, it is interesting to note that both reactors have practically 03:43.640 --> 03:48.940 identical assembly lattice parameters such as 17x17 assembly lattice, 03:49.380 --> 03:52.000 fuel rod pitch, and fuel assembly pitch. 03:53.500 --> 03:57.160 Okay, so now let's move to the description of a transient scenario. 03:58.100 --> 04:03.480 So the transient scenario to be modeled in the McSafer project is a 04:03.480 --> 04:07.460 control rod ejection accident, which can be caused by a failure of 04:07.460 --> 04:08.980 control rod drive mechanism. 04:09.860 --> 04:14.440 The rod ejection introduces positive reactivity in a very short period 04:14.440 --> 04:14.960 of time. 04:15.660 --> 04:19.320 This results in a power excursion in the region near the affected fuel 04:19.320 --> 04:23.040 assembly as well as in a highly asymmetric power distribution. 04:23.040 --> 04:27.160 This subsequently leads to possible overheating of the affected fuel 04:27.160 --> 04:27.740 assemblies. 04:28.500 --> 04:32.280 The inserted positive reactivity is quickly countered by the negative 04:32.280 --> 04:33.780 fuel reactivity feedback. 04:34.720 --> 04:38.880 The probability of a rod ejection accident is very low and it is not 04:38.880 --> 04:41.260 expected to occur during the life of the plant. 04:41.860 --> 04:45.620 The initial conditions for the transient are shown on slide. 04:45.620 --> 04:50.520 It is assumed that at steady state prior to transient, the reactor 04:50.520 --> 04:53.540 operates at 75% of nominal power. 04:53.980 --> 04:57.400 It is also assumed that all fuel is fresh with zero xenon. 04:57.920 --> 05:00.120 Soluble boron concentration is critical. 05:01.980 --> 05:06.500 The position of control rod assemblies prior to transient is quite 05:06.500 --> 05:06.980 important. 05:07.520 --> 05:11.500 In the assumed scenario, all control rod assemblies from the shutdown 05:11.500 --> 05:15.340 bank and the RE1 group are fully withdrawn from the core. 05:15.620 --> 05:20.260 Four control rod assemblies from RE2 group are partially inserted to 05:20.260 --> 05:23.540 the core according to the power dependent insertion limits. 05:26.240 --> 05:30.400 Next few slides will show a more detailed sequence of event for the 05:30.400 --> 05:32.080 control rod ejection accident. 05:33.740 --> 05:37.660 A single regulating control rod assembly is ejected at time zero 05:37.660 --> 05:39.320 within 0.1 sec. 05:41.360 --> 05:44.860 No other changes are applied to the core up to two seconds. 05:47.040 --> 05:51.440 Then, the SCRAM is activated due to high power signal and all 05:51.440 --> 05:55.680 withdrawn control rod assemblies are inserted into the core within one 05:55.680 --> 05:59.740 second except one stuck control rod assembly shown on the slide. 06:02.160 --> 06:06.180 After that, the transient continues for another second without any 06:06.180 --> 06:07.480 additional changes applied. 06:08.360 --> 06:10.600 The total simulation time is four seconds. 06:12.180 --> 06:16.660 This slide shows the expected results to be produced by simulation 06:16.660 --> 06:19.540 codes for steady state and transient conditions. 06:20.120 --> 06:24.060 They include several important operating and safety related parameters 06:24.060 --> 06:27.700 which can be used for cross comparison between the codes. 06:29.360 --> 06:33.020 The results will be produced on three different fidelity levels. 06:33.020 --> 06:37.760 This includes Traditional Nodal Approach, Advanced Pinwise Diffusion 06:37.760 --> 06:42.800 or SP3 with sub-channel thermal hydraulics and finally High Fidelity 06:42.800 --> 06:46.220 Monte Carlo also with sub-channel thermal hydraulics. 06:46.900 --> 06:50.200 The corresponding results will be presented in the follow-up lectures. 06:52.140 --> 06:54.320 This summarizes my presentation. 06:54.860 --> 06:55.840 Thank you for your attention.