On
the Susquehanna river in the State of Pennsylvania, USA, 16 km from
Harrisburg, the State capital, a town with 90,000 inhabitants
the
unit 2 of the plant was a pressurized water reactor built by Babcock
and Wilcox. The principle was the same as that of the French 900
and 1300 MWe reactors, built under license from Westinghouse (Starting
with the N4 series, French reactors are no longer built under license
to Westinghouse). The main difference relates to the steam generators
which have straight tubes in the Babcock and Wilcox design and U-tubes
in the Westinghouse design. The Babcock and Wilcox steam generators
contain less water and have less inertia in certain transients.
The accident
occurred when the reactor was at power, approximately three months
after initial startup.
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28/03/79
04:00
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Operating incident:
loss of normal feedwater supply to steam generators.
|
Normal automatic action:
opening of pressurizer relief valve,tripping of turbine,
startup signal to auxiliary feed water supply pumps,
automatic reactor scram
|
Opening of the pressurizer relief
valve was normal in this type of reactor in view of the low
thermal inertia of the steam generators |
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28/03/79
04:00 12s
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Automatic signal for reclosing of pressurizer relief valve:
the valve remains stuck open,
however, the "valve closed" signal appears in the
control room
|
the primary coolant system empties
into the pressurizer relief tank |
: the valve
stuck in the open position.
the control
room indication corresponds to sending of the command and
not the position of the valve
|
|
28/03/79
04:00 30s
|
Startup of steam generator auxiliary
feedwater supply pumps. The valves were left closed after maintenance
work. |
The water cannot enter the steam
generators. The steam generators dry out in between 2 and 3
min, preventing any cooling of the primary coolant system. |
: after a maintenance
operation.
Note: the valves were opened by the operators 8 min later
|
|
28/03/79
04:02
|
Primary pressure drops to 110 bar. |
Automatic activation of the safety
injection system: injection of cold water into the primary coolant
system |
The primary coolant system continues
to empty through the pressurizer relief valve which is stuck
open. |
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28/03/79
04:06
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The operator notices that the pressurizer
level is rising |
|
The rise in the pressurizer level
is due to the pressure of the steam bubble formed over the core:
this is a normal phenomenon in this type of accident |
| |
The operator assumes that the primary
coolant system is completely full of water and stops safety
injection. |
The primary coolant is no longer
supplied with water. |
the operator did not possess suitable procedures
for this type of accident. |
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28/03/79
04:15
|
The rupture disc of the pressurizer
relief tank bursts. |
The primary coolant water escapes
into the reactor building. |
The primary coolant water escaping
from the pressuriser relief valve overflowed from the tank. |
|
28/03/79
05:40
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The reactor coolant pumps begin to
vibrate, the operators shut them down to avoid damaging them |
There is no longer any circulation
of water between the core and the steam generators |
It is the mixture of water and steam
that causes the reactor coolant pumps to vibrate. |
| |
The water continues to boil in the
core |
The core is uncovered. The temperature
of the cladding rises to a point where a chemical reaction is
initiated between the metal of the cladding (zirconium) and
the steam, with the release of heat and hydrogen. |
|
| |
The cladding temperature continues
to rise |
Radioactive substances
are released into the reactor coolant system, and hence into
the reactor building. |
|
|
28/03/79
06:14
|
Radioactivity alert in the reactor building
The operator closes the relief line isolation valve.
|
The primary coolant system leak is
isolated, but no heat is removed. The core overheats and the
primary pressure begins to rise again. |
The alarm is due to the release of radioactive substances.
The operator knew that the relief valve was leaking before
the accident and assumed that the alarm was due to the leakage.
|
| |
Reactor coolant pump restarted by
operator |
Water cooled in steam generators
sent into the extremely hot core, resulting in dispersal of
the radioactive substances. |
|
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28/03/79
07:12
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The primary pressure increases considerably.
The operator opens the relief line to depressurize |
Radioactive substances are transferred
from the primary coolant system to the reactor building |
Increase in pressure is due to vaporization
of the primary water by the fuel. |
| |
The reactor building sump water is
automatically pumped out into a non-leaktight auxiliary building. |
Release of radioactive substances outside the plant.
|
:
No provision was made for isolating the pump discharge line
in the event of an accident.
|
| |
Isolation of containment, resumption
of safety injection, hydrogen explosion in reactor building. |
Release of radioactive substances
to the exterior ceases. Cooling of core by safety injection
system water. |
|
|
28/03/79
20:00
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End of accident |
|
|
The containment
played its role perfectly. Only a transfer from the reactor building
sump resulted in the release of radioactivity. The release remained
limited. It is estimated that an individual permanently at the site
boundary downwind would have received a maximum dose of 1 mSv. Six
years later, it was discovered using a camera introduced into the
pressure vessel that a significant part of the fuel had melted
but had not passed through the bottom of the pressure vessel.
