Page 64 - AERB Annual Report 2020

Events rated at level 4 and above are termed as Two signicant events, one each at RAPS-
'Accidents'. The accidents at Chernobyl NPP in 5&6 and KKNPP-1&2 were not rated on INES as
former USSR (now in Ukraine) in April 1986 and these were related to industrial safety and had no
Fukushima NPPs in Japan in March 2011 were relevance to nuclear and radiological safety.
rated at level 7 on INES. Events rated at levels 1, 2
and 3 are called 'Incidents'. The level 0 or below Table 1.11: INES Rating of Signicant Events
scale means events that have no nuclear and in NPPs Reported during Calendar Year (2020)
radiological safety signicance.
NPPs INES Rating of Events
1.8.1 Signicant Events and INES rating w.r.t.
NPPs INES-0 INES-1
TAPS - 1&2 0 2
All the signicant events reported were TAPS - 3&4 2 0
reviewed in detail by AERB and measures to
RAPS - 1&2 1 0
prevent recurrence of such events were
RAPS - 3&4 1 0
recommended.
RAPS - 5&6 3 0

MAPS - 1&2 4* 0
(i) Signicant Events in NPP under
Construction / Commissioning NAPS - 1&2 1 1
KAPS -1&2 4 0
During 2020, one signicant event was KGS - 1&2 3 0
reported from KAPP-3 during Phase-B KGS - 3&4 1 0
commissioning of KAPP-3. The signicant event KKNPP - 1&2 9 * 0
was rated at Level-0 on INES of IAEA. Total 29 3

v Reactor Manual Shutdown due to internal * One event at MAPS-2 and three events at
seal leak in Primary Circulating Pump-2 at KKNPP-1&2 were assigned provisional rating
KAPP-3 of level 0 as these events are still under investigation.

KAPP-3 reactor was critical and Reactor For the purpose of analysis, the events
power was 0.1% FP and stable. PHT temperature reported during year 2020 were also categorized
was 258.6°C with all four PCPs running. as per the IAEA-IRS coding system. The
Inadvertent intermittent actuation of PCP-2 back classication of systems failed/affected during
up seal while PCP-2 was running led to higher the signicant events is given in Figure-1.2. The
vibration and failure of mechanical seal leading to classication of direct causes and root causes of
PCP internal leak and minor external leak. the signicant events are given in Figure-1.3 & 1.4
Signicant event report was reviewed in detail by respectively.
AERB and corrective measures to prevent
recurrence of such event was recommended. v Signicant Events Rated at Level-1 on
INES
Direct cause for the SER is attributed to PCP
vibration and mechanical seal leak. Root cause of (i) On January 8, 2020, TAPS-1 was shut down
SER is attributed to un-envisaged design aspect. for refueling. During the refueling outage, as
per scheduled In-Service Inspection (ISI)
(ii) Signicant Events in Operating NPPs program, inspection of weld joints of reactor
coolant system piping was carried out.
During the year 2020, a total of 34 signicant Liquid Penetration Test (LPT) revealed a
events were reported from operating NPPs. Out defect /seepage in a circumferential weld
of these, 25 signicant events were rated at level 0 joint (J-16) in the recirculation loop A piping
on INES and three events (one each at NAPS-1, o f m a i n r e a c t o r c o o l a n t s y s t e m .
TAPS-1 and TAPS-2) were rated at level 1 on Subsequently, the weld joint was examined
INES. 4 signicant events (one at MAPS-2 and by ultrasonic method and this revealed a
three at KKNPP-1&2) were assigned provisional circumferential crack in Heat Affected Zone
rating of level 0 since these are still under (HAZ) of the weld joint. The detailed
investigation. investigations revealed that the crack had
occurred due to Inter-Granular Stress
The number of SERs in operating NPPs Corrosion Cracking (IGSCC). The previous
along with their ratings on INES are given in inspections of the affected weld joint was
Table-1.11.
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AERB Annual Report 2020