Page 137 - Dynamics and Control of Nuclear Reactors

11.5 Accidents in Generation-II power reactors 133

Without feedwater, the steam generators lost the ability to remove heat from the
primary water. The resulting increase in primary temperature and pressure caused an
automatic reactor scram. Auxiliary feedwater pumps were activated, but a closed
valve blocked flow. This valve closure was a serious violation of procedure.
The increased primary loop pressure caused opening of the pilot-operated relief
valve at the top of the pressurizer. This allowed fluid to flow to the pressurizer relief
tank. Opening the relief valve caused reduction of primary pressure. The pressure
reduction should have caused the relief valve to close, but it was stuck open. Primary
water continued to escape through the open valve. The operators had no indication of
a stuck-open relief valve. The only indication was that a “close valve” signal had
been sent, but there was no instrumentation to reveal the actual valve condition.
As the outflow from the pressurizer continued, the operators acted on the indica-
tion that the pressurizer level was too high. The prescribed response is to reduce pri-
mary loop flow. At this point, coolant was escaping and no replacement coolant was
entering. The result was fuel overheating and melting.
Molten Zirconium reacted with water to produce hydrogen. Since hydrogen is an
explosion hazard, it was crucial to remove it. Careful venting eliminated the hydro-
gen. Eventually, operators closed a block valve in the pressurizer relief piping and
restored coolant flow. These actions resulted in ending the incident.
In summary, four main missteps and design deficiencies caused the accident.
Maintenance personnel performed a procedure that caused disabling of feedwater
flow. Maintenance personnel closed a valve (a violation of plant procedures) that
prevented flow from auxiliary feedwater pumps. A mechanical flaw caused the pres-
surizer relief valve to stick open. Inadequate instrumentation on relief valve position
that left operators unaware of actual valve condition.

11.5.2 Chernobyl [15]
An accident occurred at Unit 4 of the Chernobyl boiling water nuclear power plant
near Pripyat, Ukraine on April 26, 1986. The1000 MWe reactor was a Soviet design
called an RBMK plant. It uses water coolant flowing in vertical pressure tubes
imbedded in a graphite moderator. The fuel is 2% enriched Uranium in UO 2 pellets
enclosed in Zircaloy cladding. The coolant boils and, after passing through steam
separators, the resulting dry, saturated steam flows into the turbine.
Neutrons are slowed down by the graphite moderator and the in-core water mod-
erator/coolant. The reactor is over-moderated. That is, removal of moderator
increases reactivity. In an over-moderated reactor, the increase in reactivity occurs
because the reduction of moderator absorptions is greater than the decrease in reac-
tivity because of reduction in neutron thermalization. Boiling of the water in an
RBMK decreases the quantity of water in the core, thereby reducing the neutron
absorptions in the water. Of course, absorptions and slowing down in the graphite
are unaffected by a change in water density, so the water density effect dominates,
giving the RBMK a positive void coefficient.