Phenomena identification and ranking table exercise for thorium based molten salt reactor-solid fuel design

Abstract

Thorium based molten salt reactor-solid fuel (TMSR-SF) design is an innovative reactor concept that uses high-temperature tristructural-isotropic (TRISO) fuel with a low-pressure liquid salt coolant. In anticipation of getting licensed applications for TMSR-SF in the future, it is necessary to fully understand the significant features and phenomena of TMSR-SF design, as well as its transient behavior during accidents. In this paper, the safety-relevant phenomena, importance, and knowledge base were assessed for the selected events and the transient of TMSR-SF during station blackout scenario is simulated based on RELAP/SCDAPSIM Mod 4.0.

The phenomena having significant impact but with limited knowledge of their history are core coolant bypass flows, outlet plenum flow distribution, and intermediate heat exchanger (IHX) over/under cooling transients. Some thermal hydraulic parameters during the station blackout scenario are also discussed.

Keywords

phenomena identification and ranking table (PIRT) / thorium based molten salt reactor-solid fuel (TMSR-SF) / safety analysis / RELAP/SCDAPSIM

Introduction

Thorium based molten salt reactor-solid fuel (TMSR-SF) design is an innovative reactor concept that uses high-temperature tristructural-isotropic (TRISO) fuel with a low-pressure liquid salt coolant [¹,²]. It can achieve excellent performance on safety and economy with a high coolant outlet temperature output. The TMSR-SF has an outstanding suitability for the comprehensive utilization of nuclear power based on hydrogen production and the application of small modular reactor [³]. The TMSR-SF is being planned for construction by the Shanghai Institute of Applied Physics (SINAP) [⁴]. In anticipation of getting licensing applications for TMSR-SF, it is very necessary to fully understand the significant features and phenomena of TMSR-SF design, as well as its behavior during accidents.

Prior to construction of the TMSR-SF, several important phenomena should be pointed out to know the details of the reactor design and the priority of future study. Phenomena identification and ranking process is the basis for licensing approach of TMSR-SF. Therefore, a phenomena identification and ranking table (PIRT) exercise was conducted for the TMSR-SF design. Safety-relevant phenomena, importance, and knowledge base were assessed for normal operation, protected transient overpower event, anticipated transients without scram (ATWS), station blackout, loss of heat sink (LOHS), loss of forced circulation (LOFC), overcooling, and small break loss of coolant accident (SBLOCA). The transient of TMSR-SF in the station blackout scenario is simulated based on RELAP/SCDAPSIM Mod 4.0. The judgment of the importance ranking of a given phenomenon was based on the effect it had on one or more figures of merit or evaluation criterion. The major phenomena of concern that were identified and categorized as high importance combined with low knowledge are core coolant bypass flows, outlet plenum flow distribution, and intermediate heat exchanger (IHX) over/under cooling transients. The PIRTs developed in this study can provide technical support for further experiment plan for TMSR-SF design as well as safety analysis.

Schematic structure of TMSR-SF

Figure 1(a) shows the schematic structure of the TMSR-SF, which is based on a pebble bed test reactor design proposed by University of California Berkeley (UCB) [⁵]. The reactor uses spherical fuel elements with TRISO particles containing UO2 as shown in Fig. 1(b), and is cooled by the binary molten salt system of 7LiF-BeF₂ (66.7–33.3 mol%), or FLiBe. The primary loop consists of a reactor core, pipes and a molten salt-to-molten salt IHX [⁶].

The goal of the TMSR-SF is to realize the integration, construction, operation, and maintenance of the TMSR system, to verify the physical behaviors, thermal-hydraulic and intrinsic safety characteristics, and to provide a comprehensive experimental platform for the design of future commercial reactors. The general design parameters of TMSR-SF plant are summarized in Table 1.

PIRT exercise for TMSR-SF

The purpose of PIRT is to identify the phenomena that are important to the thermal-hydraulic behavior of a specified plant in a particular accident scenario and it is a structured elicitation process designed to support decision making [⁹]. The process consists of nine distinct steps, as described in Table 2.

Identification of the issue

In this step, the issue that is driving the need for a PIRT should be identified. The primary issue for TMSR-SF is to ensure that a sufficient experimental and analytical database exists to support the licensing process.

Objectives

The case study focuses on the characterization of the thermal-hydraulic behavior of the primary coolant system in TMSR-SF. The specific objectives of this exercise are establishment of evaluation criteria for the thermal-hydraulic phenomena of TMSR-SF (FoMs); establishment of a decomposition scheme for the system; identification and ranking of the safety-related phenomena, component, and process; and establishment of evaluation criteria for knowledge level.

The decomposition scheme for TMSR-SF was established in Step 3 and the evaluation criteria in Step 4.

Hardware-scenario

The relative importance of phenomena/processes is dependent on plant design. Therefore, it is necessary to identify the hardware, equipment, and scenario of TMSR-SF. Table 3 summarizes the TMSR-SF systems. A series of selected event categories were selected which is recommended for the early development stage of the FHRs [⁵], such as normal operation, protected transient overpower events, anticipated transient without scram, station blackout, loss of heat sink, LOFC, overcooling, and small break loss of coolant accident.

FoMs

FoMs are those criteria against which the relative importance of each “phenomenon” is judged. These FoMs are preliminary and will need to be updated when more analyses and results become available. The FoMs identified in this process are top level—the dose received by the pubic due to fission products release; second level—worker dose; and third level—fuel failure fraction during events (accident); lower level criteria—peak coolant outlet temperature—directly related to the peak temperature of metal structural materials, peak fuel temperature—correlates with the fraction of failed TRISO particles, and minimum primary coolant temperature and margin to freezing.

It should be noted that the lower level criteria were primarily considered in subsequent quantitative analysis as the TMSR-SF which is at the stage of preliminary design and it is unnecessary to talk about the severe accident.

Identification and compilation of current knowledge base

In this step, all available materials for TMSR-SF like detailed design information need to be identified, obtained, and reviewed.

Identification of plausible phenomena

This step involves identification of all plausible safety-related phenomena/processes that have some significance to the TMSR-SF behavior. The objectives are to develop a preliminary but comprehensive list of phenomena, which have relevance to safety [⁹].

Ranking of phenomena

This step is the core of the development of the PIRT. All the judgment recommended is based on the safety criteria of interest (FoMs) which may need to be modified with the development of the TMSR-SF project. The scale described in Table 4 was adopted, as recommended by the Next Generation Nuclear Plant (NGNP), to rank the phenomena. A ranking breakdown of high, medium, and low (H, M, and L) was employed to describe the phenomena.

Identification of knowledge level

The recent PIRT includes the “state of knowledge.” This process significantly emphasizes on processes or phenomena that are flagged as highly important with a low state of knowledge. The scale adopted was described in Table 5.

Documentation of PIRT results

The subject evaluation and assessments for all plausible phenomena and their states of knowledge level were completed. All the safety-relevant phenomena ranking and knowledge level are listed in Table 6, which uses “HM” to identify high importance ranking (H) with medium knowledge level (M).

During the PIRT process, three important phenomena were obtained with high importance (H) and low knowledge level (L), coolant bypass flow/upper plenum mixing/molten salt freezing and melt, which indicates that further research and development is needed to fully understand the three important phenomena.

Coolant bypass flow observably affects the peak fuel temperature—correlates with the fraction of failed TRISO particles during the accident and is difficult to predict.

Upper plenum mixing affects the peak coolant outlet temperature-directly related to the peak temperature of metal structural materials and there is scarcity of benchmarks which can be used.

Molten salt freezing and melt affects minimum primary coolant temperature and margin to freezing, which may lead to blocking in the heat exchanger. Only few experimental data are available. Therefore, the process and the phenomena could not be completely understood.

Simulations of TMSR-SF based on RELAP/SCDAPSIM Mod 4.0

To quantitatively evaluate the importance of the phenomena, a more detailed analytical method is needed to verify the key phenomena in any specific scenario.

To fully understand the phenomena and scenario of the TMSR-SF, a station blackout simulation was conducted based on system code RELAP/SCADAPSIM Mod 4.0 developed by Innovative System Software (ISS) for the analysis of nuclear power plants (NPPs) [⁴] cooled by light water and heavy water. However, it could also be modified to use molten salt as coolant material. In this paper, the modified RELAP5 Mod 4.0 [¹⁰] code was used to simulate the TMSR-SF behavior in the station blackout scenario. The development and validation of the code can be found in Ref. [¹⁰].

Modeling of TMSR-SF heat transfer system

Figure 2 is an overview of the RELAP5-Mod 4.0 nodalization of the TMSR-SF. The modeling information of the TMSR-SF1 is described in Ref. [¹¹]. The main principle of nodalization is that the different geometry structure and physical boundary conditions of the core and pipes will be divided into different volumes [¹²]. There are two loops within the TMSR-SF system. Components 190/100/106/140 are the main components of the core while component 100 represents the average channel and 106 represents the hot channel, which is based on the assumption that a reactor core would have one channel and its heat flux is greater than that of any other channel in the core [¹²]. The object 190 shows the downcomer and 140 shows the upper plenum, while heat structure options are used to model heat exchanger. Table 7 tabulates the initial condition for the simulation.

Results and discussion

The TMSR-SF operates under a full power for 30000 s as steady-state simulation. At the time= 30000 s, the station blackout occurred. All the pumps, including primary loop pump, secondary loop pump, air loop pump were switched off instantly and safety shutdown system responded 1 s later. The primary coolant flow rate decreased sharply after the AC power lost and after 60 s of the accident, due to the opening of the valve, the PRHS starts to operate. The main transient parameters for station blackout are listed in Table 8.

The variation of thermal-hydraulic parameters during a station blackout accident is explained in this section.

Figure 3 shows the inlet and outlet coolant temperature variation in the station blackout scenario. Station blackout accident is initiated at time= 30000 s, the outlet coolant temperature rises sharply and the maximum temperature is 655.49°C. With the PRHR system putting into operation 60€s later, the outlet coolant temperature descends gradually.

Figure 4 displays the temperature distribution in the hot channel. It can be seen that the temperature rises as the height increases and the maximum temperature of the highest location is 674.26°C, which is lower than the allowable temperature of structural material of 700°C.

From Figs. 3 and 4, it can be concluded that the upper plenum mixing phenomena mentions in 3.9 is safe in the condition of station blackout scenario because the maximum outlet coolant temperature is lower than the structure failure temperature.

Figure 5 shows the air mass flow rate of PRHS while Fig. 6, the inlet and outlet air temperature of PRHS. The PRHS system came into service 60 s after the station blackout accident occurred and the mass flow rate increased to 2.82 kg/s rapidly. The inlet and outlet temperature of PRHS declined and remained at about 94.3°C and 40.3°C for the natural circulation built up, as shown in Fig. 6.

Figure 7 demonstrates the temperature of TRISO fuel at the different heights, which indicates that the temperature of the upper position fuel is higher than that of the lower one and the maximum fuel temperature is 801.3°C.

It can be concluded from Fig. 7 that the coolant bypass flow phenomena mentioned in 3.9 is safe in the condition of station blackout scenario because the maximum temperature of the TRISO fuel affected by bypass flow is lower than the safety limited temperature, which is required in the safety criteria mentioned in Step 4 of the PIRT process.

The station blackout simulation based on RELAP/SCDAPSIM provides a quantitative method to evaluate the phenomena importance in PIRT process. It suggests that TMSR-SF has a reasonable safety margin in the station blackout scenario and TMSR-SF design has favorable inherent safety features.

Conclusions

A PIRT exercise for TMSR-SF were developed and the major phenomena of concern that were identified were categorized as high importance having very limited knowledge about them were core coolant bypass flows, outlet plenum flow distribution, and IHX over/under cooling transients. The station blackout accident was simulated based on RELAP5 MOD 4.0. The maximum coolant temperature in hot channel was 674.26°C, which was lower than the required structure temperature of 700°C, and the maximum temperature of the TRISO fuel was 801.3°C, which as lower than the safety limited temperature of 1250°C. It indicated that the coolant bypass flow and upper plenum mixing phenomena identified and ranked in PIRT process had a large safety margin under SBA and the preliminary design of TMSR-SF was conservative. The PIRTs developed in this study can provide technical support for further experiment plan for TMSR-SF design as well as analysis.

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