Influence Study of Actuation Conditions of Passive Safety

INFLUENCE STUDY OF ACTUATION CONDITIONS OF PASSIVE SAFETY SYSTEM ON SGTR

Zijiang YANG School of Nuclear Science and Technology, Xi’an

Jiaotong University Xi’an, Shaanxi Province, China

Jianqiang SHAN School of Nuclear Science and Technology, Xi’an


Junli GOU School of Nuclear Science and Technology, Xi’an


ABSTRACT As the development of the nuclear industry, passive

technology turns out to be a remarkable characteristic in the

design of advanced nuclear power plants. Since the 20th

century, much effort has been given to the passive technology,

and a number of evolutionary passive systems have developed.

The CPR1000 plant, which is one kind of mature

pressurized water plants in China, is proposed to be improved

with some passive systems to enhance safety. The proposed

passive systems include: (1) the RMT (reactor makeup tank);

(2) the A-ACC (advanced accumulators); (3) the IRWST (in-

containment refueling water storage tank); (4) the PEFS

(passive emergency feed water system), which is installed on

the secondary side of SGs; (5) the PDS (passive

depressurization system).

In designing the passive safety system, we found that the

actuation conditions of the passive safety system bring a great

effect on the accidental mitigation. It was necessary to study the

influence induced by them. In this paper, the transient

phenomenon of SGTR was analyzed to show the method in

modification of the actuation conditions. The reason of

choosing SGTR was to demonstrate the great impact induced by

the PEFS.

KEYWORDS passive safety system; CPR1000; modification; actuation

conditions

1. INTRODUCTION Passive safety systems are widely used nowadays to

enhance the safety of the reactors. According to IAEA report,

the passive technologies are those utilize natural forces, such as

gravity and natural circulation. And the passive safety systems

should be composed entirely of passive components and

structures(Juhn et al., 2000). The advantages of the passive

safety systems can be summarized as (1) independent on

pumps or external power (Nayak and Sinha, 2007); (2) ruling

out human errors; and (3) more economical.

The passive safety systems are widely applied on advanced

reactor designs such as the AP1000 in the USA, the Next

Generation PWR in Japan, the WWER-1000 (Timofeev and

Karzov, 2006) in Russia, the ESBWR in the Europe and so on.

Among them, the AP1000 (Sutharshan et al., 2011) adopts an

entirely passive safety system. Its technology is relatively

mature. The Nest Generation PWR (Tujikura et al., 2000) is

equipped with a passive safety system which acts as backups to

prevent core damage. The APR1400 (Kim and Kim, 2002) in

Korea applied several passive systems including advanced

accumulator. Unlike AP1000, the APR1400 adopts a secondary

passive heat removal system, which is also used to improve the

CPR1000 technology (Zhang et al., 2011). As for the passive

containment cooling system, C.S. Byun (Byun et al., 2000)

designed a semi-passive containment cooling system for a large

concrete containment and Mirela Gavrilas (Gavrilas et al.,

Proceedings of the 2016 24th International Conference on Nuclear Engineering ICONE24

June 26-30, 2016, Charlotte, North Carolina

ICONE24-61053

1 Copyright © 2016 by ASME

2000) designed a passively cooled containment for a high-rating

pressurized water reactor.

In this paper, we discussed the method in modifying the

actuation conditions of a passive safety system designed for

CPR1000. Due to the application of the passive safety system,

the standard assumptions of the CPR1000 during the accidents

were not proper any more. Thus, the accidental assumptions

should be adjusted so that the accident could result in the most

dangerous accidental transient phenomenon. With the most

dangerous assumptions, some sensitivity analyses were needed

to determine the proper actuation conditions of the safety

system to make sure that it mitigates the accident and keeps the

reactor’s safety. In this paper, the process of actuation

conditions’ modification during SGTR was presented. SGTR

was considered as one of the design basis accidents having a

significant impact on safety in a viewpoint of radiological

release (Jiang et al., 2010; Park et al., 2013). The reason of

choosing this accident was that the application of PEFSs greatly

affected the transient phenomenon during SGTR, which

attracted much research interest (Kang et al., 2006; Kang et al.,

2012; Lee et al., 2011).

2. DESCRIPTION OF THE PASSIVE SAFETY SYSTEM The components of the passive safety system are as

follows: (1) the RMTs (reactor makeup tank); (2) the A-ACCs

(advanced accumulator); (3) the IRWST (in-containment

refueling water storage tank); (4) the PEFS (passive emergency

feed water system), which are installed on the secondary side of

SGs; (5) the PDS (passive depressurization system).

Figure 1 Passive Safety Systems on the Primary Loop

Figure 2 Schematic of PEFS

Figure 1 and Figure 2 show the proposed passive safety

system. As shown in Figure 1, each loop equips one RMT, an A-

ACC and a PEFS. The RMT, filled with boron water, is located

above the RCS loops. When triggered, the RMTs inject boron

water into the reactor vessel through the DVI lines. The

advanced accumulator includes a flow rate control device in the

lower portion of the tank to create a small injection flow after

the large injection flow phase. The PEFS (Figure 2) is

connected to the SG secondary side and includes a HX, a PEFS

water tank, pipes and valves. It removes the core decay heat and

primary loop sensible heat by natural circulation.

The IRWST, which is shown in Figure 1, provides long-

term injection water after the depressurization of the RCS. This

large water tank locates above the vessel, and is isolated from

the RCS by check valves.

The PDS system, which is shown in Figure 1, consists of

four-stage depressurization valves that open sequentially. Each

stage is arranged into two identical flow paths. The stages (1-3)

of PDS connect the top of pressurizer with a common discharge

line to the IRWST. And the stage4 (4A and 4B) of PDS

connects the RCS hot legs to the reactor containment.

3. RELAP5 MODELING The best-estimate transient simulation code

RELAP5/MOD3.3 is utilized to carry out the calculation

presented here. Figure 3 shows the nodalization of the

CPR1000 and the passive system. The components (reactor

vessel, core, pressurizer, coolant pumps, steam generators,

RMTs, A-ACCs, IRWST and PDS) are modeled specifically.

The core of the reactor is divided into three parts, the average

channel, the hot channel and the bypass channel. Other systems,

such as the steam turbine, the main feed water system, are

modeled by time-dependent volumes (TMDPVOL) and time-

dependent junctions (TMDPJUN).

The modeling for the PEFS is shown on Figure 4. The heat

exchanger (HX), the external cooling water tank(CWT) and the

corresponding pipes are simulated.


Figure 3 RELAP5 nodalization of the passive system

Figure 4 RELAP5 nodalization of the PEFS

4. SGTR ANALYSIS AND DISCUSSIONS

4.1 ASSUMPTIONS Based on the accidental assumptions of CPR1000 and

some advanced PWRs, the following conservative assumptions

were adopted in the SGTR.

1. One steam generator tube was totally ruptured, which

resulted in the maximum leakage of coolant.

2. The reactor scram signal was triggered by high water level

in SGs.

3. The maximum power of PRZ heaters and the maximum

injection rate of the RCV system were assumed to promote

the system pressure so that the reactor scram signal was

delayed.

4. The low-low system pressure signal stopped PRZ heaters

and RCV pumps.

5. The low SG pressure (3.69MPa) signal closed the main

steam isolation valves, with 7s delay.

6. When the low-low system pressure signal triggered, the

relief valve of the broken SG open. This valve was then

closed by the low SG pressure signal. This assumption was

adopted to induce a maximum leakage of steam into the

environment.

4.2 STEADY STATE AND ACTUATION CONDITIONS The initial state conditions of the reactor are given in Table 1.


Table 1 Initial state conditions

Parameter Value

Reactor power (MWth) 2895.0

Coolant average temperature (oC) 310.0

Primary loop pressure (MPa) 15.5

Pump flow rate (kg/s) 4750

SG pressure (MPa) 6.56

Main feed water temperature (℃) 226.0

Main feed water flow rate (kg/s) 537.0

The actuation conditions, which were used in the accident

analysis, are given in Table 2.

Table 2 Actuation conditions(SMNPC, 2012)

Actions Conditions

Reactor scram 1. high water level(85% of

narrow range) in the SGs

Coolant pump trip

Main feed water

pump trip

Turbine trip

1. loss of the off-site power

RCV pump trip

PRZ heater trip

1. low-low system pressure

(11.6MPa)

Main steam

isolation valve

closed

1. low SG pressure(3.69MPa,

with 7s delay)

RMT valves open 1. low water level in the

PRZ(27.7%)

2. low-low system pressure

(11.6MPa)

A-ACC valves open 1. low primary loop

pressure(4.5MPa)

PEFS valves open

1. low-low system

pressure(11.6MPa)

2. low water level in the

SGs(15% of narrow range)

3. high PRZ water level(76%)

PDS stage1 valves

open

1. low water level in the

RMTs(67.5%)

(other valves are going to open in

the sequence of stage number)

IRWST valves open 1. low water level of the

loops(20%)

2. PDS stage4 valves open

4.3 SGTR BENCHMARK ANALYSIS The characteristics of the reactor are analyzed through

Figure 5 to Figure 8. The SGTR accident is assumed to occur at

0s. The accident sequence of the SGTR is on Table 3.

Table 3 SGTR accident sequence

EVENT TIME

SGTR occurred 0h(0s)

RMT triggered 0.1139h(410s)

Reactor scram 0.1375h(495s)

Loss of off-site power 0.1375h(495s)

PRZ heater stopped 0.8236h(2965s)

RCV pump stopped 0.8236h(2965s)

SG relief valve fail open 0.8236h(2965s)

PEFS triggered 0.8236h(2965s)

MSIV isolated 1.1833h(4260s)

SG relief valve isolated 1.1944h(4300s)

PDS triggered 1.2986h(4675s)

A-ACC triggered 1.3625h-1.5319h

(4905s-5515s)

IRWST triggered 1.5h(5400s)

Figure 5 Flow rate through the break

Figure 6 System pressure

Figure 7 Water level of RMTs


Figure 8 Flow rate of the PDS valve (stage 1)

As shown in Figure 5 and Figure 6, after the SGTR

accident, the coolant went into the SG secondary side, which

led to a decrease in system pressure and PRZ water level. The

low PRZ water level triggered the RMTs. The water injected by

the RMTs contributed to the system pressure. At 0.13h after the

accident, the high water level in the SGs triggered the reactor

scram signal. The reactor lost its offsite power immediately. The

coolant pumps, the main feed water pumps and the turbine

stopped. The system pressure decreased again. The low-low

system pressure triggered the PEFSs at 0.82h. This signal also

stopped PRZ heaters and RCV pumps.

According to accident assumptions, when the low-low

system pressure signal triggered, the relief valve of the broken

SG open. This valve was then closed by the low SG pressure

signal. During this SG depressurization period, the pressure in

both primary side and secondary side decreased drastically.

As the system pressure decreased, the hot water in the

RMTs began to vaporize. The water level in the RMTs began to

drop (shown in Figure 7). The low water level triggered the

PDS (shown in Figure 8) which will depressurized the system.

The pressure in the SGs became higher than that in the primary

loop, resulting in a reverse flow through the break. Afterwards,

the advanced accumulators and the IRWST began to inject

water into the system.

As discussed above, the benchmark accident analysis

showed (1) the RMTs and PEFSs caused a continuous leakage

of coolant through the break; (2) the PDS valves open after the

accident, which was not allowed in SGTR due to safety

requirements. Thus, the actuation conditions of the passive

safety system must be re-considered.

4.4 SENSITIVITY ANALYSIS Some sensitivity analyses were carried out to determine the

feasible actuation conditions of the passive safety system during

the SGTR accident. The trigger signals of RMT and PEFS were

analyzed because they are main reasons those affect the

transient phenomenon.

Three conditions were compared:

1. Original actuation conditions, used as a benchmark;

2. Original actuation conditions, with RMT valves closed

when the SG water level is high (85% narrow range);

3. Original actuation conditions, with RMT valves closed

when the SG water level is high (85% narrow range) and with

the PEFS, which was placed on the broken SG, closed due to a

high radiation level in the broken SG.

The accident sequences are listed in Table 4.

Table 4 SGTR accident sequences

EVENT

RMT open

RMT

closed

RMT&PEFS

closed

SGTR

occurred

0h

(0s)

0h

(0s)

0h

(0s)

RMT triggered 0.114h

(410s)

0.114h

(410s)

0.114h

(410s)

Reactor scram 0.138h

(495s)

0.138h

(495s)

0.138h

(495s)

Loss of off-

site power

0.138h

(495s)

0.138h

(495s)

0.138h

(495s)

RMT closed - 0.138h

(495s)

0.138h

(495s)

PRZ heater

stopped 0.824h

(2965s)

0.247h

(890s)

0.247h

(890s)

RCV pump

stopped

0.824h

(2965s)

0.247h

(890s)

0.247h

(890s)

SG relief

valve

open(misuse)

0.824h

(2965s)

0.247h

(890s)

0.247h

(890s)

PEFS

triggered

0.824h

(2965s)

0.247h

(890s)

0.247h

(890s)

MSIV isolated 1.183h

(4260s)

0.355h

(1285s)

0.368h

(1325s)

SG relief

valve isolated

1.194h

(4300s)

0.368h

(1325s)

0.379h

(1365s)

PDS triggered 1.299h

(1.2986h)

- -

A-ACC

triggered

1.362h-

1.532h

(4905s-

5515s)

0.689h-

4.944h

(2480s-

17798s)

-

IRWST

triggered

1.5h

(5400s)

- -


Figure 9 Flow rate through the break

Figure 10 Primary side pressure

Figure 11 pressure in broken SG

Figure 12 Flow rate of the SG relief valves

Figure 9 illustrates the flow rate through the break. If the

RMTs were open, the primary loop pressure was promoted so

that the coolant leakage rate was higher than that of the other

conditions. However, after the PDS triggered, the system was

depressurized, leading to a reverse flow through the break. If

the RMTs were isolated due to the high water level in the SGs,

the PDS was not triggered. Yet, the vapor would still be

condensed by the PEFSs, the coolant still flowed continuously

into the broken SG. With RMTs and the PEFS (on the broken

SG) isolated after the accident, the minimum leakage of the

coolant was acquired.

The variation of the system pressure is presented on Figure

10. With RMTs open, the system pressure was relatively higher

before the PDSs depressurized the system at 1.3h after the

accident. With RMTs closed, the system pressure decreased to

11.6MPa (the low-low system pressure signal) quickly. Then,

due to the mis-open of the SG relief valve, the system pressure

decreased greatly. If the PEFSs were all open, the system

pressure decreased to 2.0MPa, which triggered the advanced

accumulator and interrupted the pressure balance between the

primary side and the secondary side. If the PEFS on the broken

SG was closed, the system pressure rose after the main steam

isolation valves closed, the pressure was finally kept around

6.0MPa. The advanced accumulators were not initiated.

The variation of the broken SG’s pressure is presented on

Figure 11. With RMTs open, the pressure decreased as the

primary side pressure decreased. With RMTs closed, the

pressure maintained around 2.0MPa. With RMTs and the PEFS

closed, the pressure was the highest among the three conditions.

Figure 12 illustrates the flow rate through the SG relief

valves. Before the PEFSs were triggered, the SG relief valves

were open to depressurize the SGs. Due to accident

assumptions, when the low-low system pressure signal was

created, the relief valve of the broken SG kept open until the

low SG pressure signal was created. This assumption caused a

drastic decrease of the SGs’ pressure. If the RMTs were open

after the accident, the system pressure was higher than that of

the other conditions so that the mis-open of the relief valve was

postponed.

Figure 13 Heat transfer rate of the broken SG


Figure 14 Heat transfer rate of the intact SGs

Figure 15 Water level of the broken SG

Figure 13 and Figure 14 illustrate the variation of the heat

transfer rate of the SGs. With RMTs open, the PEFSs were

triggered at 2965s after the accident. The PEFSs transferred

heat from the primary side till the depressurization of the

system. With RMTs closed, the PEFSs were triggered at 890s

after the accident. If the PEFS on the broken SG was open, the

broken SG still transferred heat from the primary side until the

SG was over-filled. If the PEFS on the broken SG was closed,

only the intact SGs transferred heat.

Figure 15 presents the variation of the water level of the

broken SG. With RMTs open, the water level decreased because

of the depressurization of the system. With RMTs closed and

PEFSs open, the coolant injected into the SG contributed to the

water level. The broken SG was finally overfilled. With the

PEFS on the broken SG closed, the water level in the SG was

relatively stable.

If the RMTs were closed after the accident, the PDSs were

not triggered, the residual heat was removed by the PEFSs. If

the PEFS on the broken SG kept working after the accident, the

advanced accumulators were triggered and the broken SG was

over-filled. It can be concluded that the RMTs and the PEFS on

the broken SG should be isolated in the SGTR accident.

CONCLUSION To improve the safety of the CPR1000 NPP, a set of

passive safety system is applied on the plant as the backup for

the standard safety system. Even though the system is based on

the passive technology of some advanced reactors, the structural

parameters and trigger signals must be adjusted for CPR1000.

In this paper, the modification of the actuation conditions of the

passive safety system was discussed and the SGTR accident

was chosen as an example. The choice of SGTR was to present

the impact induced by the PEFSs. Several conclusions can be

drawn.

(1) During SGTR, the original actuation conditions of the

passive safety system should be modified to meet the safety

requirement.

(2) The isolation of the RMTs (when the water level was

high in the SGs) prevented the depressurization of the system

caused by the PDS.

(3) The isolation of the PEFS (which was placed on the

broken SG) shorten the time took for pressure balance between

primary and secondary sides.

In conclusion, the RMT valves should be closed when the

water level in the SG was high, and the PEFS (on the broken

SG) should be isolated when the radiation level in the SG

exceeds the limit.

NOMENCLATURE RMT: reactor makeup tank

A-ACC: advanced accumulators

IRWST: in-containment refueling water storage tank

PEFS: passive emergency feed water system

PDS: passive depressurization system

SGTR: steam generator tube rupture

PRZ: pressurizer

RCV: Chemistry and volume control system

RCS: reactor coolant system

MSIV: main steam isolation valve

ACKNOWLEDGMENTS The authors would like to express their appreciation to the

National High-tech R&D Program of China (Grant No.

2012AA050905).

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Influence Study of Actuation Conditions of Passive Safety