Reactor shutdown

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Red SCRAM button (right) on the control panel of the TRIGA reactor at the Atomic Institute of the TU Vienna, Austria

The reactor shutdown ( RESA or Scram for short , English for "get away", "pull the leash"), also called a reactor trip , is a safety measure in nuclear reactors . In the event of a malfunction, the RESA can be triggered manually by the operating personnel or automatically by a reactor protection system when certain limit values ​​are exceeded. In order to avoid false alarms as far as possible, the reactor protection system determines measured values redundantly several times before it triggers the shutdown .


The RESA makes the reactor very subcritical , i.e. it ends the nuclear fission chain reaction. It is not used for the usual load adjustment , but shuts down the reactor immediately. This is generally done by collapsing or completely retracting (“shooting in” by means of gas pressure) of the control rods and - in typical pressurized water reactors - feeding boric acid into the cooling circuit. In boiling water reactors , the boric acid feed (technically called boronization ) can be used as an additional, redundant and diverse shutdown measure . These measures prevent the nuclear chain reaction and the performance of the reactor is reduced to the decay heat .

In the Chernobyl nuclear disaster occurred due to constructive peculiarities of RBMK -Steuerstäbe after triggering the RESA switch ( Russian Аварийная Защита 5-й категории (АЗ-5) , Awarijnaja Saschtschita 5-j kategorii (AZ-5) , " emergency protection of the 5 . Category ”) nevertheless led to a short-term increase in performance , which promptly made the reactor overly critical and thus triggered its destruction with very serious consequences.


Already in 1942 Enrico Fermi had provided a quick shutdown option for the decisive experiment on the first nuclear reactor, the Chicago Pile . A specially trained person should cut a cord with an ax in an emergency. A cadmium rod hung on the cord , which would have stopped the chain reaction immediately when it entered the reactor.

The English word to scram means something like “run away quickly”, “run away”, “evaporate”; Fermi himself explained it - obviously jokingly - as the acronym for Safety Cut Rope Ax Man , something like: "Safety man who cuts the rope with an ax". The term scram for the reactor shutdown is still common in English today.

As ATWS (Anticipated Transient without Scram) an event is called, in which due to technical failure a necessary fast shutdown does not occur.

Individual evidence

  1. ↑ A nuclear power plant stress test useful against residual risk? , Swissinfo, May 19, 2011.
  2. a b NRC Glossary: ​​Scram. Nuclear Regulatory Commission , August 2, 2010, accessed March 14, 2011 .
  3. David Baurac: Tales of Enrico Fermi. Argonne National Laboratory , accessed March 14, 2011 .
  4. Edwin Blackburn: Scram! - Reactor veteran recalls account of the birth of a key word in the nuclear vernacular. Oak Ridge National Laboratory , September 2000, accessed March 14, 2011 .