The ZEEP manually-jammed pump button overpower

By Dr. Nick Touran, Ph.D., P.E., 2024-02-25 , Reading time: 2 minutes

The ZEEP reactor building, with NRX under construction in the background (AECL, 1945)

What happened?

This is an old story from Canada’s first nuclear reactor, ZEEP.

To pump water into the reactor tank one had to push a button at the control desk to start the pump. However, the pump ran only for a fraction of a minute at a time, and then stopped. So an operator had to repeatedly push the button to keep the pump running. Since this was rather tedious, one operator made a block of wood that could be used to jam the pump button so the pump would run continuously.

One day, a couple of researchers were on the top of the reactor inserting detectors, and an operator was at the control desk pumping up the heavy water, with the pump button jammed. Suddenly, the phone rang at the other side of the building and the operator left the control desk to answer it, leaving the pump running. The call took longer than expected and the next thing the researchers heard was the shutoff rods dropping into the reactor. The reactor had tripped on overpower. No one knows how much radiation the researchers received since they had left their film badges in their coat pockets on the floor below! However, it couldn’t have been too much since one of the researchers’ wives later had a healthy baby. One might deduce from this that “a little neutron flux never hurt anyone”. This incident was never reported to senior management.


  • Don’t make extremely tedious processes. Staff will find ways to work around them in ways that might bypass whatever safety you were trying to enhance.
  • Don’t work on an unshielded reactor when it is being brought to power
  • If you are working on a reactor, wear your dosimetry badge, even if it is offline.
  • Report all overpower incidents and radiation accidents to senior management

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