Teachin g Notes

Operationa l Reacto r Safet y Course

Lecture : 1 9 – Thre e Mil e Islan d Accident

Objective :

Having now spent tim e in two reacto r sim u la tors, this lectu r e should provide a deeper appreciation of what happened at Three Mile Island and what the operators had to deal with to understand and m a nage the acciden t . Th e objective of this le cture is to identify what happened and why with a focus on the therm a l hydraulics of the accident which drove the partial m e ltdown. The failure of system s and lack of operator knowledge and inform ation is to be stressed.

Ke y Point s t o Brin g Out:

Slid e num be r Point s

2-9 This ser i es o f slides in tro du ces the Babcock and W ilcox PWR design and describes a precursor ev en t at th e Davis Besse nuclear plant. Em phasis the difference in steam generator design and the am ount of water (therm a l inertia) in each. Describe briefly the integrated control system of BW designs m eant to im prove operations but resulted in a sensitive plant design. Point out the lack of industry inform ation shar ing about this event that could have helped TMI operators be m o re aware of the possible problem s.

10-23 Review TMI accid e nt sequence - key points are the acceptan ce of a leak in the P O RV which m a sked the failur e to c l o se PORV which led to the accident. Point out how fast things happen in a PWR. Go over the tim elines and what operators thought was going on versus what was going on. Explain the graphs from the standpoint of plant response and signals to hav e the studen t s appreciate the role of syste m s in the plant.

24 Go over the hydrogen bubble scenar io. Discuss why such an obvious technical question that coul d have been addressed with the consultation of any chem ical engineer prolonged the public confusion and anxiety. Point out that the hydrogen in the vessel was never in any danger of exploding and why.

Sho w Histor y Channe l technica l segm en t o n TM I t o gai n an appreciatio n o f ho w thing s unfolde d i n term s o f NR C an d industry actions . P l eas e se e dis k m a rke d TMI.

25-26 The photos of the extent of core m elting are aim e d at showing the students the enorm ous heat generate d in the co re that cou l d cause fuel m e lting which up to this point in the history of nuclear power was viewed as not a c r ed ible even t. Hopef u lly these im ages will m a ke a lasting im pression for those working in the industry.

27-28 Review public health consequences - point is that containm ents work.

29-31 Review lessons learned - good - im proved understanding of core m e lt acciden t s dispelling m a ny of the past held b e liefs about vessel failure, steam explosions and the im portance of water in the vessel.

The not so good lessons are m ore num erous pointing out the lack of adequate instrum e ntation, operator training, upgrading of em ergency response plans and em ergency operating procedures. Brought back the im portance of risk assessm ents in better understanding of safety of nuc lear plants as opposed to hypothetical acciden t s used in li censing (Large B r eak LOCA).

32 Go over the bottom lines of the TMI acciden t - importance o f the role of precursors and industry inform ation sharing am ong nuclear operators. T r aining on nuclear fundam e ntals is important in addition to following procedures.

MIT OpenCourseWare http://ocw.mit.edu

22.091 / 22.903 Nuclear Reactor Safety

Spring 200 8

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