An investigation for the fuel temperature of the Tehran research reactor during a complete loss of coolant accident

https://doi.org/10.1016/j.pnucene.2020.103489Get rights and content

Highlights

  • A complete core uncovering following LOCA accident in the TRR is analyzed.

  • The presence of the grid plate results in a 68 °C decrease in the hottest fuel plate temperature.

  • Thermal radiation to the surrounding walls causes a 48 °C reduction in the hottest fuel plate temperature.

  • Cooling periods cannot prevent the fuel from melting while the reactor has been operated at its nominal power for 30 days.

Abstract

Safety in a nuclear research reactor such as the Tehran Research Reactor (TRR) is a significant concern mainly after the Fukushima nuclear accident and satisfied by a continuous upgrade in the whole reactor facility and its personnel. Happening of total complete of Coolant Accident (LOCA) due to a massive rupture in the TRR primary cooling pipe is conceivable after a severe accident such as an earthquake. This accident could lead to the pool dewatering, and, finally the reactor core uncovering. Consequences of this accident are the fuel blistering, swelling, and melting depending on its severity. In this study we analyze the possibility of passively cooling the TRR reactor core by thermal radiation and conduction to the surrounding structures and natural convection to air in the complete LOCA accident. In this way, the contribution of various heat lost mechanisms are demonstrated.

Introduction

Tehran Research Reactor (TRR) is a highly used pool-type research reactor which its first criticality was in 1967. This Material Testing Reactor (MTR) was converted from the use of Highly Enriched Uranium (HEU) to Low Enriched Uranium (LEU) in 1991 (AEOI, 2009). Recently, some efforts have been made to identify the reactor safety vulnerabilities and remedy them based on using Engineering Safety Features (ESFs) such as Second Shutdown Systems (SSS) (Boustani and Khakshournia, 2017; Boustani et al., 2016; Jalili et al., 2015). One Emergency Core Cooling System (ECCS) as an ESF was not the default equipment because of the IAEA standards for medium power research reactors such as TRR (IAEA, 1991). Nowadays, due to the reasons such as aging, upgrade of safety standards and also attention to the severe accident consequences in nuclear reactors following the Fukushima Daiichi disaster, considering the role of one ECCS on the TRR safety seems necessary.

The study of LOCA has been conducted in both tank and pool-type research reactors considering safety aspects. The possibility of core damage following LOCA was anticipated in reactors with a power of 1.5–2 MW in some reports such as the IAEA documents (IAEA, 1991). When any cooling period is considered for loss of coolant, this safety level for core intact would be 5 MW. The cooling period is the time duration from occurring accident and reactor shutdown to the core uncovering. According to a study done in the Oak Ridge National Laboratory (ORNL) on the Low Intensity Test Reactor (LITR), for the LEU plate of this tank type reactor with power more than 1500 kW, the existence of an ECCS for inhibiting from the core damage is necessary if an accident occurs in which the core uncovering starts 2 min after the reactor shutdown (Cox and Webster, 1964). In another study done for the NUR reactor of Algeria the fuel temperature variations for this 1 MW reactor was studied. If this reactor is operated for 2 days and the coolant discharge lasts 250 s, the clad temperature will reach 500 °C (Meftah et al., 2006). Another study was carried out experimentally for the Kyoto University Research Reactor (KUR) which is a light water moderated tank type MTR reactor with a nominal power of 5 MW. Following the occurrence of LOCA, the decay heat of fuel after 155 days of operation at full power would be removed with the natural convection of air. Continuing the heat removal for at least 3 weeks would be necessary for preventing core melting (Ito and Saito, 2016). In another work done upon 5 MW pool-type IEA-R1 reactor, this reactor is operated for the infinite time and uncovered after 300 s following an accident. The reactor core temperature will exceed the safety limit of 500 °C if the coolant is not injected to the core during 13.5 h after the accident occurrence (Torres et al., 1999). A research was done for Greek Research Reactor No. 1 (GRR-1) core conversion from the HEU to the LEU in which the LOCA was studied for various cooling periods. The reactor with 5 MW power was assumed to be operated for the infinite time. If the cooling period after the accident is less than 16 min, the core melting will occur in 135 min after the core uncovering (Housiadas, 1999). Furthermore, the possibility of a core meltdown for this reactor was investigated in another study for a variety of operational conditions of this reactor (Chatzidakis and Ikonomopoulos, 2013). Also, another research work was done considering the 10 MW IAEA MTR type reactor examining the core meltdown possibility for reactor powers higher than 4 MW (Hamidouche and Si-Ahmed, 2011). After upgrade of the Pakistan Research Reactor-1 (PARR-1) from the initial steady state power level of 5 MW–10 MW, the ECCS was installed in it, and the evaluation of this ESF effectiveness was carried out (Bokhari and Mahmood, 2008). Relating to this research case, Hedayat et al. studied the LOCA in the TRR (Hedayat et al., 2007). In this study, the minimum time for initiating the core uncovering is 28 min, and the complete core uncovering will occur in 31.4 min in which the fuel temperature will not reach the aluminum melting point. There was no mention of the core operating time as an important parameter which is a drawback to this study.

In this paper, the hottest fuel plate temperature for a complete LOCA accident in the TRR core is estimated in order to investigate the exigency of an ECCS for it. If the fuel temperature reaches 400 °C or 660 °C, the fuel will be subject to serious damages of blistering or melting, respectively (IAEA, 2008). The core uncovering of these two conditions gives rise to substantial radiological hazards. This work aims to demonstrate the necessity of an ECCS in preventing this phenomenon from happening.

Section 2 is devoted to the introduction of the TRR, the accident scenario considered. Also, the heat transport model is presented in this section. The results of the hottest fuel plate temperature calculations are provided for the given scenario in section 3. Finally, we conclude in section 4.

Section snippets

General description of the TRR

TRR is a 5 MW light water cooled and moderated pool-type research reactor. The pool is divided into two sections, the narrow stall end, which contains the experimental facilities, and the open end that is designed for bulk irradiation. The open end is separated from the narrow stall end of the pool by a concrete wall, which has a tapered opening that may be closed or opened by a removable water-tight Aluminum gate.

The reactor cooling system consists of the primary, secondary and purification

Radial peaking factor

The reported fuel temperatures in this study are those of plates of the hottest SFE located in the position D5 of the given equilibrium core in Fig. 5. The written numbers in each position of the core refer to the radial peaking factor for the fuel element located there. Hence, the fuel element located at the position D5 having the maximum radial peaking factor in the core must be considered for the fuel temperature analysis.

Required parameters

The required coolant data such as density (ρc), kinematic viscosity

Conclusions

This study concentrates on the complete core uncovering following LOCA accident in the TRR according to the anticipated scenarios in the SAR of TRR considering natural air convection and thermal radiation from fuel plate along with conduction from the underneath core grid plate and plenum. According to this investigation, remarkable parameters determining this accident severity and consequences are operating power, operating time and cooling period. Exceeding limits assigned to these 3

Credit author statement

Ehsan Boustani: Writing all equations, solving all calculation using Matlab and also writing the manuscript were done by E. Boustani. Samad Khakshournia: Checking all written texts and equations, guidance about work and used procedures were done by S. Khakshournia.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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