Fellow CNS and Nuclear Listserv members, and others:
In this year of 50th anniversaries let us not forget one more of national and international significance:
50 years ago today, on December 12, 1952 (at 3:07 PM), a combination of mechanical failure and human error lead to a now-famous power excursion and fuel failure in the NRX reactor at Chalk River. At the time NRX was one of the most significant research reactors in the world, in its sixth year of operation.
This accident is historically important, not only because it was the first of its type and magnitude, but also because of its legacy to Canadian and international practice in reactor safety and design.
Nobody was killed or hurt in the incident, but a massive clean-up operation was required, and the reactor core itself was rendered unusable for an extended period. Environmental effects outside the plant were negligible, as was radiation exposure to members of the public.
Several of today's fundamental safety principles of reactor design and operation stem from the lessons learned at this formative stage of Canada's nuclear program, making Canada an early leader in this field.
Among these were:
- the need for an independent, reliable, fast-acting shutdown system, separate from routine reactor control;
- the need for shutdown capability even in a reactor that is already shutdown (i.e., the safest reactor configuration may not be one with all neutron absorbers in the core);
- the need for a reactor trip on "rate of change" in power, in addition to a high power threshold;
- the importance of written and thoroughly reviewed procedures for every operational and experimental activity;
- the importance of an efficient human-machine interface in the control room;
- the need to balance thorough safety coverage with simplicity that does not interfere unduly with operations.
The accident also demonstrated that, due to a combination of redundant safety features, emergency procedures, and a level of inherent "forgiveness" (or robustness) in the technology, a major fuel-melt accident in a nuclear reactor can occur without significant environmental effects and radiation exposure to the surrounding population.
The NRX core was completely rebuilt, improved, and restarted within 14 months following the accident (the first time something like this was attempted), and the reactor continued to perform for another four decades before being retired.
As with the analysis of the accident itself, the clean-up and repair of the NRX reactor shed light on several new concepts of reactor operation and design. A major example of these is the complete rehabilitation of a large reactor core, which contributed to the unique long-term maintenance philosophy of not only research-reactors at Chalk River Laboratories, and but also CANDU power reactors.
A well-known anecdote has Admiral Rickover, of the US nuclear navy, taking advantage of the clean-up operation to train his men in the field. One of the young officers sent to Chalk River was Lt. Jimmy Carter, a nuclear engineer who became the 39th president of the United States in 1977, and a Nobel laureate in 2002.
For those interested, a list of historical references specific to this event can be found on "The Canadian Nuclear FAQ" website at www.ncf.ca/~cz725/cnf_sectionD.htm#x.