Relaxation of the requirements on loop height and heat transfer area of a passive heat removal system in integral SMR using a self-powered booster

Highlights

• Height and area requirements of natural circulation system are relaxed.

• TEG based efficiency boosters are used to realize the relaxation.

• Numerical simulation and experiment are conducted to valid the concept.

• Height of natural circulation system can be reduced by 90% with the booster.

• Heat exchanger area can be reduced by 25% comparing with the original system.

Abstract

In this study, compactness improvement capacity of a self-powered efficiency booster is analyzed for natural circulation systems used in integral small modular reactors (SMR). Based on thermoelectric principle, the booster converts heat energy into electricity directly, which is used to produce additional driving force for natural circulation flow and heat transfer. Thus, flow and heat transfer are enhanced and such systems do not rely on gravity solely. Consequently, requirement on loop height and heat transfer area can be relaxed. Numerical simulations and experimental tests have been carried out. The results indicate that, using the booster, loop height is no longer a determining factor for natural circulation systems, and lower height can be used for the same performance. Furthermore, if the desirable heat transfer rate was kept, the heat exchanger area can be reduced by a factor of 25%–78%. With these results, compactness improvement capacity of the booster is proved.

Introduction

In most of integral reactor designs, reactor core, coolant pumps, steam generators and pressurizer are all packed inside the reactor pressure vessel (Buongiorno et al., 2012, Fetterman et al., 2011, Wu et al., 2016, Wang et al., 2020a, Wang et al., 2020b) to form a compact system, as shown in Fig. 1. Such configuration makes it possible for pre-fabrication and testing at factories, which simplifies on-site installation process, reduces the number of auxiliary systems, and eliminates many pipe connections, thus, reducing the potential risks sources of loss of coolant accidents. Due to these unique advantages, the integral reactors have been chosen as a preferred configuration by several small modular reactor (SMR) types (Yang et al., 2008, Butt et al., 2016, IAEA, 2016), including reactors installed in limited spaces, such as onboard ships and submarines (Saunders, 2015, Yamaji and Sako, 1994, Zverev et al., 2013).

To improve safety of the reactors, passive heat removal systems, based on gravity driven natural circulation, have been adopted in many designs, such as MRX (Kusunoki et al., 2000), SMART (Kim et al., 2016) and WSMR (Buongiorno et al., 2012, Smith and Wright, 2012). As it is well known, in natural circulation systems, the fluid moves through a loop using inherent gravity force generated by the density differential between the cold and hot water in vertical pipes of the loop. This allows the residual heat to be removed out of the reactor core even during a complete loss of power.

On the other hand, among other things, such as temperature differential and the heat transfer areas, the gravity generated driving force is directly proportional to the loop height. Unfortunately, this height-dependence conflicts with the philosophy of compactness in integral SMRs. Furthermore, the gravity induced driving force is much weaker than pump induced force (IAEA, 2005, IAEA, 2009, IAEA, 2013). For a certain designed heat removal rating, a larger heat exchange is often needed in a natural circulation based system (Vijayan and Nayak, 2010) as compared with an externally powered forced flow system. This larger area requirement is again undesirable when one tries to make the overall system compact. In the current design of integral reactor systems, significant efforts have been made to improve compactness of the plant (Shirvan et al., 2016), by using reactor fuels with higher power density (Shirvan et al., 2012), compact steam generators (Zaman et al., 2017, Xiao et al., 2018) and supercritical CO₂ Brayton cycle with reduced size, for different types of SMRs, including the IRIS (Carelli et al., 2004), and SMART (Yoon et al., 2012).

Despite the above efforts, the study on reduction of the physical size of the natural circulation systems in a passive heat removal system has not been found. As a result, the total space occupied by the natural circulation based residual heat removal system is also disproportionally large as compared to other sub-systems, considering that the decay heat often counts for only a few percent of the total unit power (Agnew, 1981, Ragheb, 2011, Yan and Hino, 2016).

To reduce the size of the heat removal system, a novel technique is developed, in which a self-powered efficiency booster is adopted. The booster converts part of the heat energy of working fluid in the loop itself into electrical power with thermoelectric generator (TEG). The electrical power is then used to generate drive force and enhance the circulation flow. Power conversion of the booster is independent of the gravity difference generation. As a result, one can potentially reduce the height of the natural circulation column and adopt smaller heat exchanger area without jeopardizing safety of the reactor.

In previous study of the authors’ research group, design and analysis of the booster in heat transfer capacity enhancement of natural circulation systems have been performed (Wang et al., 2017, Wang et al., 2020a, Wang et al., 2020b). While the current paper focuses on compactness improvement of the natural circulation system in integral SMRs with the booster.

To prove the concept of compactness improvement with the booster, both numerical simulation and experimental tests have been conducted. Special attentions have been paid to examine the relationships between the heat removal performance of the system and the loop height, and the heat exchanger area. The results have conclusively shown that, by using such a booster, the loop height of the heat removal system can be reduced by as much as 90%, and the heat exchanger area can be reduced by a factor of 25%–78%.

The rest of the paper is organized as follows. In Section 2, the problem under investigation is formulated and the solution process is proposed. The influence of the booster on the loop height and heat exchanger area has been numerically analyzed in Section 3. In Section 4, the results of experimental studies are presented. Finally, some conclusions are drawn in Section 5.