Jacob Ladd with Cade Memmott, Chemical Engineering

Introduction

The goal of the Integral, Inherently Safe Light Water Reactor (I2S-LWR) Research Project is to design a more inherently safe reactor. This reactor is capable of cooling the core with no external power and no operator action.[1] In the I2S-LWR design, micro-channel heat exchangers (MCHX) facilitate heat transfer from the reactor to the steam generation system (SGS), where steam is used to produce electricity.^{1, 2}

The I2S-LWR design requires the use of a “flashing” drum to produce steam, and the flashing drum and its impact on the SGS must be evaluated for safety before it can be incorporated into future designs. If it can be demonstrated that using a flashing drum can increase the safety of a nuclear reactor, other future nuclear designs may also include a flashing drum in their SGS.

Methodology

RELAP5 is the industry standard computer simulation tool for the analysis of reactor safety and advanced reactor designs. The first step to performing accident simulations with RELAP5 is to create a working steady-state model of the steam generation system, complete with the dimensions of the pipes, flash drum, heat exchangers, and pumps. All system components must also include flow rates and temperature and pressure parameters. The flow process diagram of the I2S-LWR SGS is shown in figure 1.³

For the purposes of this study, only the first loop of the steam generation system needed be input into the model, i.e. the MCHX, the flash drum, the forward water heater (FWH), and the recirculation pump. The rest of the steam generation system, including the feed to and from the turbines and the feed into the MCHX from the primary (reactor) loop, could be modeled in RELAP5 using “time dependent volumes.” Time dependent volumes provide a flow into and out of the system of interest with user-specified temperature, pressure, and flow rate. The system of interest is shown in figure 2; pipes 3, 8, and 25 connect the system of interest to the rest of the steam generation system.

After its construction and successful tests, the steady-state model can be used to create a Restart Plot (RSTPLT) file, which is then used to run tests under “transient,” or quickly changing conditions. The transient of most interest to the researchers is station blackout, where all power to the reactor and the steam generation system is lost.

Results

Graduate student Paul Wilding calculated thermodynamic properties of the steam generation system and estimated pipe dimensions, while I and graduate student Nathan Murray worked on the RELAP5 model.

So far we have only been able to successfully model the MCHX and the flash drum. I have been working to add the FWH, recirculation pump, and time dependent volumes for pipes 8 and 25, but we are still working to get these components functioning without errors.

The RELAP5 model was experiencing a number of bugs which hindered progress. The last run which did not terminate immediately only included the components shown above, in Figure 3, and still terminated a day later, after having simulated only 300 seconds. I am still debugging the model to determine what is slowing down the program.

Discussion and Conclusion

This project did not progress along the timeline I had originally anticipated, but I learned some valuable lessons throughout the research process. Factors contributing to the extended timeline include underestimating the difficulty of learning RELAP5, the program’s long running times, and a scarcity of resources for troubleshooting models.

In order to fully test whether the flashing drum enhances safety, the RELAP5 model must be completed, the RSTPLT file must be created, and the RSTPLT file must be used in transient simulations. If the flashing drum SGS provides increased cooling relative to standard systems during station blackouts, it can be concluded that the flashing drum enhances safety.

Bibliography

1. B. Petrovic “Integral inherently safe light water reactor (I2S-LWR) concept: Extending SMR safety features to large power output”, Proceedings of ICAPP 2014, Charlotte, N.C., April 6-9, 2014.
2. M. J Memmott, A. Manera “The Use of Flashing Drums and Microchannel Heat Exchangers to Generate Steam in Large Integral Light Water Reactors”, Nuclear Technology, Vol. 191, 2015.
3. P. R. Wilding, M. J Memmott “Optimization of Nuclear Steam Generation Systems via Multi-Parameter Sensitivity Analysis”, Proceedings of 2015 American Nuclear Society Meeting, Washington, D.C., November 8-12, 2015.