Nuclear Safety and Security Commission

The NSSC Approved Criticality of Kori Unit 4 During Periodic

Inspection and to Conduct Power Ascension and Other Remaining Tests

On November 26, the Nuclear Safety and Security Commission (Chairperson Uhm

Jaesik, hereinafter referred to as “NSSC”) approved the criticality of the Kori Unit 4

which had been undergoing a periodic inspection since July 21.

* Criticality means to make nuclear fissions in the reactor occur continuously, resulting in the state

where the generated neutrons are equal to the extinct neutrons and therefore, the number of neutrons

is in equilibrium.

※ If criticality is approved, 9 remaining tests, including a core physics test, are conducted during the

process of or after reaching the criticality of reactors.

The NSSC checked the result of 80 test items to be carried out before the reactor

reaches its criticality, and confirmed that the reactor can safely reach its criticality.

During this periodic inspection, a follow up inspection on the Containment Liner

Plates (CLP*) was carried out visual inspection on 12 plates, and ultrasonic

thickness measurement of a total of 97,594 spots on 75 plates and as a result, it was

confirmed that there is no abnormality.

* Containment Liner Plates prevent radioactive materials from leaking when an accident happens.

In addition, 56 spots, which were confirmed to have abnormalities through close

visual inspections on the CLPs of the co ntainment building dome, went through

thickness measurement, and among the measured spots, 4 spots needing

maintenance have been repaired.

Also, it was confirmed that the non destructive inspection of the tubes of the steam

generators had been properly performed, and as a result of the foreign substance

inspection, a total of 8 foreign substances, including metal pieces, were found and

removed.

And the follow up measures for the vulnerability of the off site power system to salt

damage, which was revealed by the typhoon (Maisak on September 20, 2020) that

had an impact on the Kori nuclear power plant last year were intensively checked.

To prepare for typhoons, the Korea Hydro and Nuclear Power improved the

overhead line leading to the switch yard, and it was confirmed that the facility

improvements satisfy the relevant technical standards.

The NSSC has approved the criticality of the Kori Unit 4 based on the result

of the periodic inspection so far and will finally confirm safety of the unit through 9 follow

up tests such as power uprate test.

The NSSC Approve d Resumption of Shinkori Unit 1

On November 23, the Nuclear Safety and Security Commission (Chairperson Uhm

Jaesik, hereinafter referred to as “NSSC”) approved the resumption of the Shinkori

Unit 1 which had been manuall y shut down on November 11 as the NSSC finalized

the investigation and confirmation of safety.

The Shinkori Unit 1 had a turbine/generator trip during normal operation on

November 8 as a ration differential relay on the phase B was activated and its reactor

power was reduced from 100% to 45%. The Korea Hydro and Nuclear Power

manually shut down the reactor to replace the facilities that were confirmed to have

been damaged.

※ Off site power to the power plant was stably supplied through a Standby Auxiliary Transformer

(SAT), and the reacto r maintained a stable state. There were no radiation related issues.

* The main transformer measures the current differences between the transmission lines within the

switch yard circuit breaker section and when the differences exceed the set value, turbine trip signals

are generated.

As a result of the investigation, it was confirmed that the high pressure bushing* of

the gas insulated bus (GIB) on the phase B of the Shinkori Unit 1 was damaged.

* High pressure bushing is located between GIB and a pylon and connects the conductors between

the two facilities.

It was identified that the cause of damage was overheating and melting due to poor

contact between the internal connection of the conductors and contactors inside the

high pressure bushings.

※ No other signs of damage or foreign substances inside were found.

The NSSC confirmed that measures such as replacement of damaged

high pressure bushings, inspection of the facilities on the phases , and improvement of maintenance

procedures and standards had been completed.

The NSSC Approved Criticality of Kori Unit 3 During Periodic

Inspection and to Conduct Power Asce nsion and Other Remaining

Tests

On November 19, the Nuclear Safety and Security Commission (Chairperson Uhm Jaesik, hereinafter referred to as “NSSC”) 

approved the criticality of the Kori Unit 3

which had been undergoing a periodic inspection since July 14.

* Criticality means to make nuclear fissions in the reactor occur continuously, resulting in the state

where the generated neutrons are equal to the extinct neutrons and therefore, the number of neutrons

is in equilibrium.

※ If criticality is approved, 9 remaining tests, including a core physics test, are conducted during the

process of or after reaching the criticality of reactors.

The NSSC checked the result of 87 test items to be carried out before the reactor

reaches its criticality, and confirmed that the reactor can safely reach its criticalit y.

During this periodic inspection, a follow up inspection on the Containment Liner

Plates (CLP*) were carried out visual inspection on 1 2 plates, ultrasonic thickness

measurement of a total of 43,054 spots on 69 plates and as a result, it was

confirmed that there is no abnormality.

* Containment Liner Plates prevent radioactive materials from leaking when an accident happens.

In addition, 11 spots which were confirmed to have abnormalities through close

visual inspections on the CLPs of th e containment building dome, went through

thickness measurement, and among the measured spots, 4 spots needing

maintenance have been repaired.

Also, it was confirmed that the nondestructive inspection of the tubes of the steam

generators had been properly performed, and as a result of the foreign substance

inspection, a total of 3 foreign substances, including wires, were found and removed.

The focus of the inspection was intensively placed on the follow up measures for the

vulnerability of the off site power system to salt damage, which was revealed by the

typhoon (Maisak on September 20, 2020) that had an impact on the Kori nuclear

power plant last year.

To prepare for typhoons, the Korea Hydro and Nuclear Power improved the

overhead line leading to the swit ch yard, and it was confirmed that the facility

improvements satisfy the relevant technical standards.

In particular, measures, includ ing re placement of the internal stem* of the Main

Steam Isolation Valves (MSIV* MSIV***) that were confirmed to be damaged by the reactor

trip that occurred during the power cutoff for the overhaul of the Kori Unit 3 on July

12 have been completed.

* Stems transmit driving force for the opening/closing of MSIVs.

* Main Steam Isolation Valve s isolate the main steam pipe s from the steam generator s in case of an

accident

The KHNP will continuously check the implementation status of the design

improvement and inspection reinforcement of the weak parts of the stem,

improvement of re placement and maintenance, and the MSIV vibration management

for each type of reactors as a measure to prevent the recurrence of the case.

The NSSC has approved the criticality of the Kori Unit 3 based on the result of the

periodic inspection so far and will finally confirm safety of the unit through 9 follow

up tests such as power ascension test.

NSSC Held the 149th Meeting of the Commission

■ The 「 bill to modify the operating license of nuclear facilities 」 for the Kori Units 3 and 4 was

deliberated and passed.

On November 12 2021, the Nuclear Safety and Security Commission (Uhm Jaesik, hereinafter referred to as

the “NSSC”) held the 14 9 th meeting of the Commission where the commissioners delib erated and passed 1

bill and reported on 1 bill.

The commissioners deliberated and passed the 「 bill to modify the operating license of nuclear facilities 」 ,

applied by the Korea Hydro and Nuclear Power , to reflect the information on suppliers of safety grade

equipment and verification documents for Kori Units 3 and 4 in the final safety analysis report.

※ The commissioners decided to discuss the bill no.1 for deliberation and resolution, the bill on 「 administrative actions on

p ower r eactor i nstallers and o perators 」 , which is about the violation of the Nuclear Safety Ac t by the Korea Hydro and Nuclear

Power, at a next meeting.

In addition, the commissioners were briefed from the Korea Institute of Nuclear Safety on its review plan for

a construction permit of the Advanced Reactor for multi purpose Applications ( applied by the Korea

Atomic Energy Research Institute.

NSSC Said Shinkori Unit 1 Was Manually Shut Down and to Be Repaired

The Nuclear Safety and Security Commission (Chairperson Uhm Jae sik, hereinafter

referred to as the NSSC) announced that as a result of an on site inspection of the

Shinkori Unit 1*, of which the turbine had tripped due to the activated ratio differential 

relay** during normal operation around 21:03 on November 8, a failure

in the facilities related to transmission was confirmed and the Korea Hydro and

Nuclear Power would manually shut dow n the reactor for maintenance.

* Reactor power has been maintained at 45% since the turbine trip

* It measures the current difference between the generator, switch yard breaker and Unit Auxiliary

Transformer (UAT) and generates a turbine trip signal when the set value is exceeded

In addition, the NSSC plans to form an investigation team composed of experts from

the Kori Site Office and the Korea Institute of Nuclear Safety (KINS) to check on the

progress of each maintenance measure by conducting on site inspection of the

maintenance work, maintenance results review and so forth.

NSSC Conducted 2021 Unified Radiological Emergency Response Exercise

Implemented comprehensive training in response to nuclear accidents assuming a severe accident at the Hanul Nuclear Power Plant

The Nuclear Safety and Security Commission (Chairperson Uhm Jae sik hereinafter

referred to as the NSSC held the 2021 Unified Radiological Emergency Response

Exercise at the Hanul Nuclear Power Plant (hereinafter referred to as the Unified

Exercise)' on the assumption that an accident involving radiation leak occurred at the

Hanul Unit 1 in Uljin gun, Gyeongbuk Province, on October 27.

* (Central governments) 18 ministries including the NSSC, the Ministry of the Interior and Safety

(Municipalities) Gyeongbuk Province (regional), Gyeongbuk Uljingun, Gyeongbuk Bonghwa gun,

Samcheok City (basic municipality)

(Licensee) Korea Hydro & Nuclear Power Co., Ltd.

(Specialized institutions) Korea Institute of Nuclear Safety, Korea Institute of Radiological Medical

Sciences

(Related organizations) local military, police, coast guard, firefighters, etc.

This Unified Exercise assumed a situation in which two earthquakes (magnitude 6.2

and 9.0) occurred in the waters near the Hanul Nuclear Power Plant, and a strong

tsunami intruded the Hanul Nuclear Power Plant, together lea ding to a severe

accident condition.

The integrated response system for radiological emergency w as checked through the

establishment and operation of the National Emergency Management Committee

(NEMC), the Local Emergency Management Center (LEMC), the Off site Emergency

Management Center (OEMC) OEMC), on site training of licensees and specialized

institutions, table top exercise of municipalities

* (Table Top Exercise) In consideration of the COVID19 pandemic, local residents did not participate .

Instead, a table top exercise was conducted.

This year's Unified Exercise was conducted in connection with the 「 Emergency

Response Safe Korea Exercise 」 hosted by the Ministry of the Interior and Safety,

and about 320 related people including government ministries, local governments

licensees and specialized institutions participated.

Drills for each agency include:

The NSSC Headquarters launched and operated the National Emergency

Management Committee (NEMC), which oversees emergency response, and

conducted decision making training on declaring radioactive disasters and reviewing

declarations of special disaster areas.

In addition, the Uljin Emergency Center under the NSSC launched and operated the

Off site Emergency Management Center (OEMC) and conducted training on

decision making related to resident protection measures (evacuation of residents,

indoor evacuation, instructions for taking thyroid protection drugs, etc.).

Korea Hydro and Nuclear Power (KHNP) focused on disseminating radiological

emergency announ cements, training for emergency power supply and external

injection of emergency coolant.

The four metropolitan and basic local governments around the Hanul Nuclear Power

Plant launched and operated the Local Emergency Management Center (LEMC) to

protect residents promptly, and checked the implementation of resident protection

measures.

To prevent the spread of infectious diseases, the local residents did not participate

in the training . promotional materials for action tips in case of a

radiological emergency were produced and distributed to boost the interest of the

local residents

The Korea Institute of Nuclear Safety conducted field training to search for

radioactivity using radiation detecting equipment and to provide information

necessary for decision making on measures to protect residents on the basis of the

collected radioactivity data

The Korea Institute of Radiological Medical Sciences launched an off

site clinic near the Uljin Sports Complex in cooperation with four radiological emergency medical

centers, including Uljin gun Medical Center and Dongguk University Gyeongju

Hospital, and focused on treatment for radioactively injured people,

decontamination and evacuation training.

Chairperson Uhm said, “In the event of a radiological disaster, we will thoroughly

check the cooperation and response system between related organizations and the

NSSC for early responses to such accident and prompt resident protection, and

continue to conduct practical radiological emergency exercises assuming the worst

case scenario.”