1. Introduction

System decontamination applied after nuclear power plants are permanently ceased is the technology to remove contaminated metal oxide films, or metal oxide deposits from the interior surfaces of systems or equipment such as RPV, SG, PZR, pipes, pumps, valves and heat exchangers, etc.. This technology is known to have a close relation to the decontamination effect of the circulation flow formed in RCS [1]. There are various methods of providing circulating flow rate to the system decontamination. Generally, the circulation flow rate is supplied by the reactor coolant pump, the residual heat removal pump, and vendor’s pump as shown in Table 1 [2, 3]. The pumps to provide the circulation flow rate are selected according to the system decontamination scope.

The decontamination scope of Maine Yankee NPP was the full system excluded RPV and SG [2] as shown in Table 1. For Maine Yankee system decontamination, it was not possible to remove the contaminated materials of the entire planned decontamination scope with the vendor’s pump (300~650 gpm) even if excluded RPV and SG. So, Maine Yankee NPP classified the decontamination scope to effectively decontaminate the contaminated system or equipment and conducted the system decontamination in first and secondary.

The decontamination scope of Connecticut Yankee NPP was similar with Maine Yankee NPP but only excluded RPV, and the RHR pumps were used to supply the chemical agents and the circulation flow rate [2]. Connecticut Yankee NPP was able to conduct the decontamination operation by single flow path using RHR pump (~2,000 gpm) with large capacity than Maine Yankee NPP.

Unlike Maine Yankee and Connecticut Yankee NPP, Jose Cabrera NPP included RPV and SG in the decontamination scope [3]. Jose Cabrera NPP used to the RCPs to supply the chemical agents and circulation flow rate in the decontamination scope. In this plant, nitrogen was pressurized in the PZR to provide the minimum pressure (~430 psig) required for RCP operation and the design was changed so that the demineralized water could be supplied separately to protect the RCP seals.

In this paper, the flow characteristics in Kori-1 NPP reactor coolant according to RHR pump operation were evaluated. In order to evaluate the flow characteristics, RCS, CVCS, RHRS, Safety Injection System (SIS) and piping were considered as the range of system decontamination.

2. Flow characteristics in reactor coolant system

It is essential that inner circulation flowrate be required for system decontamination operation and if the RCP is not operated, another pump should be used to supply the circulating flow. In this paper, the flow characteristics are evaluated based on the operation of RHR pump. Fig. 1 shows a schematic diagram of the WH 2-Loop PWR RCS [4], and Fig. 2 shows the circulation flow for operating RHR pump.

As shown in Fig. 2, the circulation flow in RCS with RHR pump operation is run into the RHRS from the hot leg for each loop which passes through the RHR pump, and then flows into the cold leg. Table 2 delineates the pressure drop characteristics in RCS during normal operation. The values shown in Table 2 are based on Kori-3&4 NPP, WH 3-Loop PWR, data because there is no Kori-1 NPP data available [5].

The pressure drop characteristics shown in Table 2 are the values of the conditions under which the RCP is operated and the fuel is present in the core. In system decontamination, the pressure drop formed in RPV is formed to be very low compared with the case in which the fuel is present. Even if two RHR pumps are operated, flow rates of up to 4,000 gpm (2,000 gpm / pump×2 pumps) are only possible, and the pressure drop formed in RPV is less than 0.1 psi even under fuel conditions. Therefore, it can be assumed that there is no pressure drop through the RPV in case of system decontamination using the RHR pump since it is a negligible value in the fuel withdrawn condition. In addition, it is assumed that the pressure drop in the hot and cold legs is distributed by 50% based on the connection point with RHRS. Fig. 3 shows the flow path formed in the RCS when the RHR pump is used as the driving force of the circulating flow.

In Fig. 3, the branch points P1 and P2 have arbitrary pressure values, and the pressure drop (ΔP1) through flow path 1 and the pressure drop (ΔP2) through flow path 2 are assumed the same during RHR pump operation. That is, ΔP1 = ΔP2, and ΔP is proportional to KQ², so that it can be expressed as follows.

where K is resistance coefficient and Q is flow rate (89,000 gpm).

The pressure drop for each path is calculated as follows.

ΔP1 = 0.7 + 1.7 = 2.4 psi

ΔP2 = 0.7 + 41 + 3.4 + 1.7 = 46.8 psi

ΔP1 = K1×Q1², K1 = ΔP1/ Q1² = 3.03×10^-10

ΔP2 = K2×Q2², K2 = ΔP2/ Q2² = 5.91×10^-9

K1/K2 = Q2/Q1=0.22645,

Q2 = Q1×0.22645

The flow rates of Q1 and Q2 are 1,630 gpm and 370 gpm, respectively. In case of no fuel in the core, 1,630 gpm of 81.5% of the 2,000 gpm RHR pump flow is formed through the flow path 1, and the flow path 2 is formed of the flow rate of 370 gpm of 18.5%.

2.1 Flow Characteristic of Flow Path 1

As shown in Fig. 3, the flow path 1 is formed as RHRS → Cold Leg part → RPV → Hot Leg part → RHRS and it was estimated that a flow rate of 1,630 gpm, which is 81.5% of the RHR pump flow rate of 2,000 gpm, was formed. Table 3 is on the flow characteristics at flow path 1.

In order to prevent impurity deposit in the system piping, the flow velocity inside the piping is generally designed to be more than 2 ft·s^-1 (0.6 m·s^-1). Such a flow velocity is a value considering complex shapes such as vertical piping, valves and orifices. In case of horizontal piping, it is considered that impurities are not likely to be deposited in the flow velocity condition in Table 3, but it could not excludes the possibility that the impurities are deposited in the lower RPV region because the flow rate of RPV has a very low.

2.2 Flow Characteristic of Flow Path 2

Flow path 2 is circulated as RHRS → Cold Leg part → RCP Suction Leg → SG → Hot Leg part → RHRS and the flow rate of 370 gpm is formed, which is 18.5% of RHR pump flow rate. Table 4 shows the flow characteristics at flow path 2. The flow rate formed in the flow path 2 is about 370 gpm, which is considered to be a very low flow rate considering the piping size. If the flow rate through the system piping is low, there is a high possibility that impurities are deposited in the region that is structurally capable of forming a trap or below the U-shaped piping. As shown in Fig. 4, the areas where impurities may be deposited in the flow path 2 include the RCP suction U-shaped pipe, the SG chamber bottom area, and the RCP. Table 5 shows the volume of piping and equipment located at flow path 2 [1]. The concentration change of the fluid in the system due to the feed and bleed is shown in Eq. (3).

where α is a ration of flow and volume.

Fig. 5 is shown the evaluation results of the decontamination performance of flow path 2 using Eq. (3). As shown in Fig. 5, it was estimated that it took 65 minutes to reduce the initial impurity concentration to 10% or less. However, since the Eq. (3) is derived from the assumption that all the fluids in the system are uniformly mixed at each time step, the concentration changes over time when the homogeneous mixing is not formed due to low circulation is shown in Fig. 5, and it is expected to take more time.

2.3 Decontamination Characteristics of PZR

If the RCP is not running, the main spray flow rate is not formed and the separate piping must be connected to use the auxiliary spray flow rate to inject the chemicals or decontamination the PZR, or to pass a part of RHR pump flow rate through the PZR. The flow rate of auxiliary spray is supplied by charging pumps in CVCS and the charging pump is reciprocating pump and installed 3 pumps in CVCS. The design flow rate of each pump is 60.5 gpm. The internal decontamination characteristics of the PZR are related to the flow rate through the PZR and can be evaluated using Eq. (3). According to Eq. (3), it was evaluated that the residual concentration of impurities in the PZR with the charging pump operation for supplying the auxiliary spray flow rate was 61.6% of the initial concentration after 60 minute, 37.9% for two charging pumps operation and 23.3% for three charging pumps operation, respectively. Fig. 6 showed the decontamination characteristics of the PZR with auxiliary spray flow rate.

2.4 Flow Distribution of Path 1 and 2 According to Flow Resistant Installation

When system decontamination is performed with only the RHR pump, there is an unbalance in the circulating flow rates of the flow path 1 and 2. In order to solve this unbalanced circulation flow, a separate facility is required, and a method of adding a facility (spider) for connecting a pipe to the inside of the RPV or a method of adding a flow-limiting resistor to the RPV can be considered [2]. When equipment are added to distribute the RHR pump flow rate equally, it is possible to distribute a flow rate of about 1,000 gpm. Table 6 shows the flow characteristics for the case where the flow resistors are added to the core area to change the flow rates of both flow path 1 and 2 to 1,000 gpm.

As we shown in Table 6, even if the flow rate of the RCP suction leg increases, it is still considered to be a low flow rate. As the flow rate of the flow path 2 increases, the flow rate of the flow path 1 decreases and the flow rate at the RPV also decreases. Therefore, the possibility of accumulating impurities at the bottom of the RPV will be relatively high. Fig. 7 shows the impurity concentration change with time of the fluid in the flow path 2 at a circulating flow rate of 1,000 gpm. It took 25 minutes for the circulation flow rate of 1,000 gpm.

3. Conclusion

This paper dealt with the flow characteristics in Kori- 1 NPP reactor coolant according to RHR pump operation. It was demonstrated that sufficient circulation flow rate was required in the RCS to efficiently decontaminate systems and equipment, and the decontamination effect was improved as the circulation flow rate was increased. This paper showed that system decontamination using RHR pump could not be effective due to impurities deposited in piping and equipment, and the extreme flow unbalance in RCS caused the deposition of impurities as well. A separate facility can be considered to distribute the flow rate equally to solve the unbalance of the flow rate. But since the circulating flow is formed only by the RHR pump, the flow rate unbalance is still generated. As a result, it was confirmed that the use of the RHR pump as the driving force of the circulating flow was very limited.