Re: Could the three mile island accident have been prevented? How?

I think to answer this question I first have to explain what caused the TMI 
accident in the first place.  The accident began about 4:00 a.m. on March 
28, 1979, when the plant experienced a failure in the secondary, 
non-nuclear section of the plant. The main feedwater pumps stopped running, 
caused by either a mechanical or electrical failure, which prevented the 
steam generators from removing heat. First the turbine, then the reactor 
automatically shut down as they were supposed to.

From the Nuclear Regulatory Commission web site:
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Immediately, the pressure in the primary system (the nuclear portion of the
plant) began to increase. In order to prevent that pressure from becoming
excessive, the pressurizer relief valve (a valve located at the top of the
pressurizer) opened. (Again this was as the system was supposed to 
respond.)

The valve should have closed when the pressure decreased by a certain 
amount, but it did not. Signals available to the operator failed to show 
that the valve was still open. As a result, the stuck-open valve caused the 
pressure to continue to decrease in the system.

Meanwhile, another problem appeared elsewhere in the plant. The emergency
feedwater system (backup to main feedwater) was tested 42 hours prior to 
the accident. As part of the test, a valve is closed and then reopened at 
the end of the test. But this time, through either an administrative or 
human error, the valve was not reopened - - preventing the emergency 
feedwater system from functioning. The valve was discovered closed about 
eight minutes into the accident. Once it was reopened, the emergency 
feedwater system began to work correctly, allowing cooling water to flow 
into the steam generators. 

As the system pressure in the primary system continued to decrease, voids
(areas where no water is present) began to form in portions of the system
other than the pressurizer. Because of these voids, the water in the system
was redistributed and the pressurizer became full of water. The level 
indicator, which tells the operator the amount of coolant capable of heat 
removal, incorrectly indicated the system was full of water. Thus, the 
operator stopped adding water. He was unaware that, because of the stuck 
valve, the indicator can, and in this instance did, provide false readings.

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     For a more detailed version of the TMI event and great 
     information on nuclear power, I recommend:

     http://www.cannon.net/~gonyeau/nuclear/

What were the causes that allowed the accident to proceed this far?  
Primarily, poor operator training and instrumentation design. 
Improvements in these areas would have allowed the operator to recognize 
and properly correct the problems in the plant before they proceeded to the 
point of damage to the nuclear reactor core.  It's my opinion that these 
are the fundamental causes of the accident along with the maintenance error 
that left the valve in the emergency feedwater system shut. 

Could the accident have been avoided?  This is harder to answer.  As 
outlined at the above web address, several changes to nuclear operations 
have been made since the accident:

All electric utilities expanded significantly the training conducted for 
personnel who work at and support nuclear plant operations. This included 
establishing the National Nuclear Academy which accredits the plant 
training programs in 10 areas. Also, all utilities purchased simulators for 
training personnel who work in the main control room. Equipment changes 
included monitoring instrumentation capable of withstanding severe
accidents and hydrogen recombiners. Personnel-related changes involved 
upgrading of training and qualification requirements and a requirement to 
have a degreed shift technical advisor assigned to each shift to evaluate 
abnormal conditions. 

The purpose of these changes was to avoid similar problems in the future.  
If these changes had been made earlier the TMI accident could have likely 
been avoided.   If operators had correctly interpreted the indicators, had 
improved indicators, or had the maintenance error been avoided through 
better procedures and supervision, then the accident would have been 
avaoided.  

But, I am not sure if the need for these changes would have been recognized 
if the accident had not happened.  Since nobody was hurt by the accident 
many people in the industry regard it as an expensive (since the power 
plant was a financial loss) lesson that has lead to great improvements in 
nuclear power plant operations.  

See the web address above or its links to get more information on those 
improvements.


Michael Baker, Ph.D., P.E.
Nuclear Engineer