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Published:  2016-02-12 Views:  749
Author:  karelnel
Published in:  Technical/Tegnies
Nuclear Energy - The Shark Syndrome

The Shark Syndrome (Safety of  Nuclear Power reactors)

 

Who of you have heard of the shark syndrome ? Many people will decide not to go on holiday when confronted with the news of a shark attack , but they think nothing of it, to drive  100’s of kilometres by car.  The irony is that the probability of being  killed in a car accident, is  vastly greater than being killed by a shark.. This example shows the irrational way in which people react to risk.

Nuclear Power Reactors  are probably  the most fascinating engines ever built on this earth, despite all the fears that people have about them. The following slide will introduce you very shortly to  the  functioning of a Pressurised Water Reactor. (Slide) Note the following:

1. The reactor core is housed in a Reactor pressure vessel 15m high, 5m diameter and wall thickness  25cm.

2. It is cooled by massive  volumes of water pumped by 3 or 4 Primary pumps, through the core. Pressure is 150 bars and temperature 325 deg Centigrade.

3. Hot water flows through a  steam generator and back to the reactor.

4. Steam generators are 30 meters high, receive water  from feedwater pumps and delivers  steam to the turbines that drives the generators.

5. Steam is condensed in a condenser that is in turn cooled by cooling water from the sea, a river or a cooling tower.

6. The reactor pressure vessel and steam generators are housed in the reactor containment building, which is designed such that it could withstand the pressure, even if the  reactor pressure vessel should burst.

7. It should be noted that the  primary system do not have a direct interface  with the outside environment.

Such a nuclear reactor has only two fundamental safety problems, and engineers have developed many ingenious ways to deal with those. The two fundamental safety issues are:

1. The reactor produce a large inventory of  extremely hazardous radio-active material, and these have to be confined.

2. The reactor continues to produce energy, even when shut down - called the residual energy, and these have to be removed.

The  confinement of radio-active material is done in a fourfold way:

  • Confinement in the matrix of the fuel pellet
  • Confinement in the fuel tube - the part of the  fission products escaping from the pellet
  • Confinement in the primary system - should some of the fission products leak from the fuel tubes.
  • Confinement in the  reactor building - should some contamination escape  from the primary system..

The effectiveness of these measures could be demonstrated by the following  experiment. (Experiment)

  • Ink absorbed in cotton wool (Matrix of fuel pellet)
  • Cotton wool in bottle (Fuel tube)
  • Bottle  within larger bottle (Primary containment)
  • Larger bottle  within even larger bottle (Containment building)

From this simple experiment one can easily deduce  how small the risk is for exposure of  radioactivity to the environment when a reactor  is operated properly.

The  removal of residual heat from the reactor core, after shutdown, or after an accident is done  by systems called : Engineered Safety Systems, those are systems designed in such a way that  they can  guarantee the removal of heat, no matter what the circumstances.

There are a number of these  type of systems :

  • High Pressure  Emergency Core Cooling system : Capable of injecting cooling water at extremely high pressure into the  primary system in case of a small break.
  • Low pressure Emergency Core Cooling System : Capable of  dumping massive volumes of cooling water into the primary system in case of a large break
  • Containment cooling system : Capable of cooling the containment building atmosphere by water spraying after a  break of the primary system

To absolutely ensure that these  systems will be able to perform these  safety functions , the specific systems are duplicated or  even triplicated , with each one system capable of fulfilling the full function. This is called a 200% or a 300% redundancy. These systems are not dependent on normal electric power, but all have separate  emergency power systems, and  separate water supplies that will ensure that at least one of the systems will function correctly at any time.

There is also another principle being used to even further enhance reliability : called Diversity. Redundancy & diversity could be explained by the following example : (Example)

Say it is vital that one should  learn about the news on a daily basis. There may be  various ways to go about it.

1. One could use a radio. For the occasion that the radio is  defect, another radio could be kept, and even a third.

2. The abovementioned plan will however fail when for instance the  radio-broadcasting authority experiences a problem. For that occasion one could  try and  look at the TV

3. Both the abovementioned plans may fail when for instance a power failure may stop radio and TV broadcasts. For such an occasion one could make use of the daily paper.

This illustrates the principle of  diversity. By increasing redundancy alone, reliability cannot  be increased above a certain level, because of a problem called a probable ‘common mode failure’ Therefore the emergency power for the safety systems  may be generated by two diesel generators and a steam turbine.

These  safety functions must be guaranteed under various  very strict conditions, called design basis accidents:

1. The reactor safety systems are designed to withstand an earthquake. The Koeberg Nuclear reactors are for example built on a seismic island, which effectively de-couples the systems from the environment in case of an earthquake.

2. The safety systems  will be able to  withstand the effects of a Tsunami when the reactor is close to the sea

3.  The safety systems will be able to withstand  the crash of  an aircraft as large as a Boeing or as fast as  a F16 fighter-jet.

From the abovementioned you will realise that very few, if any other systems in the world, are designed with such safety precautions in place. Only recently the same type of  safety philosophies were introduced into the building of large dams, that can threaten the lives of  thousands of people.

Despite all these  extreme safety precautions there is a strong bias against nuclear power in the world. This stems  mainly from ignorance about the subject, although it must be admitted that all nuclear reactors are not as safe as the PWR described above.

When one is properly informed about the abovementioned, one could clearly see the  shark-syndrome  in operation amongst many of the so-called ‘Greens’ - protesting about things they know very little about, meanwhile exposing themselves to other risks like motor accidents, smoking, drinking or other hazardous lifestyles- e.g. paragliding.

It reminds  me of the joke  where  the two protesters are   picketing the nuclear power plant, and the one  saying to the other one : If there happens to be an earthquake,   run for cover in the  building for it was designed to be safe during an earthquake!


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