Chalk River Nuclear
Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole
And I’m Brian. And we’re both from Calgary, AB.
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This week in engineering news, eye imaging technology to help robots and cars “see” better.
Engineers are using LiDAR to improve the vision, if you could call it that, of autonomous systems such as driverless cars or robotics.
The system kind of works like radar, but instead of using broad radio waves, it uses short pulses of light from lasers.
LiDAR can be overwhelmed by ambient sunlight and can take a long time to scan a large area. So researchers have been experimenting with a form of LiDAR called frequency-modulated continuous wave or FMCW LiDAR. Which works the same as optical coherence tomography (OCT). This technology has been developed for biomedical engineering fields since the early 90s; they just need to trade in high resolution for distance and speed.
The FMCW LiDAR sends out a laser beam that shifts between different frequencies and measures how long it takes to detect each of the frequencies. Not only can this be used in any light source, but it also measures unimpeded beams.
Previous LiDAR systems work on a mechanical mirror that rotates to scan the landscape, but the speed of the mirror can limit the laser. The new tech uses a diffraction grating that works like a prism to spread the frequencies out as they travel away from the source.
Now on to this week’s engineering failure; the Chalk River Nuclear Accident.
The Chalk River Laboratories site is located along the Ottawa River, 180km north-west of Ottawa, just on the Ontario side of the Quebec border and east of a town called Chalk River
The facility opened in 1944 on a 36 square kilometre campus. When it first opened it was referred to as Petawawa Works.
The site specialised in the major research, development and advancement of nuclear technology, particularly CANDU technology; having expertise in physics, metallurgy, chemistry, biology, and engineering.
There have been two Nobel laureates who have worked at Chalk River labs.
Bertram Brockhouse received the 1994 Nobel Prize in Physics for his work on the neuron spectroscopy while at Chalk River from 1950 to 1962.
Sir John Cockcroft shared the 1951 Nobel Prize in Physics with Ernest Walton for splitting the atomic nucleus. He was also instrumental in the development of nuclear power and served as director of the Montreal Laboratory in 1944 and creation of Chalk River Labs.
The lab is still open today and is owned by Canadian Nuclear Laboratories, subsidiary of Atomic Energy of Canada Ltd. and is operated under contract by the Canadian National Energy Alliance, a private sector group led by SNC-Lavalin.
The site housed the first nuclear reactor outside of the US called ZEEP, which stood for Zero Energy Experimental Pile. ZEEP was one of the world’s first heavy water reactors and was designed to use natural or unenriched uranium. Its development led to the creation of the NRX and NRU reactors as well as the CANDU reactors. ZEEP was decommissioned in 1973.
At one time Chalk River produced a third of the world’s medical isotopes and about half of North America’s. Despite their claim to peaceful use, the Chalk River facility sold 254 kg of spent reactor fuel in the form of Plutonium to the US from 1955-1985 for the production of nuclear weapons.
There have been at least 10 reactors in operation at Chalk River throughout 1945-2018. They were all different styles and capacities. We’re going to talk about two of them today, both of which suffered accidents in the 1950s. Shocking that we would pick those two to talk about on this podcast. /s
1952 NRX Accident
The NRX reactor or National Research Experimental was a 8m diameter by 3m tall heavy water moderated and light water cooled nuclear reactor and created 30 MW of power.
Heavy water meaning that rather than having two hydrogen elements, the hydrogen is replaced with two deuterium atoms. Instead of a chemical formula of H2O, heavy water is D2O. The heavy water slows down the neutrons better than light water so they react with the fuel, which in this case is uranium.
Light water is essentially drinking water, equivalent (give or take) to water that comes out of the tap.
Now the NRX reactor is different from Chernobyl and Three Mile Island, but they all work the same conceptually. They use the heat generated from a nuclear reaction to create steam, the steam turns a turbine and generates power. A moderator, which in this case is heavy water, controls the reaction rate. Control rods are also used to control the reaction. Inserting the control rods or removing the moderator will speed up the reaction. (the moderator slows the neutrons, causing a chain reaction that is less likely with fast moving neutrons)
The Chernobyl (ep 13) reactor used graphite as the moderator, and three mile island (ep 41) used light water as the moderator. As we mentioned, the NRX reactor used heavy water, so it was more like three mile island, but with a better moderator.
On December 12, 1952 the NRX reactor suffered a partial meltdown due to operator error and mechanical problems in the shut off systems. During a test, some of the tubes were disconnected from the high pressure water cooling system and connected to a temporary cooling system by hoses. One of the tubes was cooled by airflow.
During the test on low power, with low coolant through the core, the reactor supervisor noticed several control rods being pulled from the core which would increase the reaction.
The supervisor went to the basement to check and found an operator opening pneumatic valves. After putting the valves back into the correct position, some of the control rods didn’t reenter the core. The power level exponentially increased, doubling every 2 seconds and tripped the reactor.
There were four emergency control rods but due to mechanical defects three were not inserted into the core and the fourth took 90 seconds to insert, which in reactor meltdown time is an eternity.
After ten seconds, the power levels reached 17 MW, which was a little over half of the intended output of the reactor.
The cooling water boiled and some of the temporary tubes ruptured and the power continued to rise.
14 seconds later the heavy water was drained from the reactor.
The power peaked at 80 MW, almost three times the intended capacity, then decreased as the heavy water was drained and was back down to 0 MW 25 seconds later.
Some of the fuel elements melted and the reactor cylinder, called a calandria, was pierced in several places.
The cooling system kept running to remove decay heat, but leaked 4000 cubic metres of contaminated coolant water into the basement of the reactor building over the next few days.
The cleanup took several months and had some help from the US navy, including future US president Jimmy Carter.
They removed and buried the reactor core and calandria and replaced them. The refurbished reactor was back up and running in 2 years and ran until 1992.
Even though there were operating errors, they were not outside of the normal range of human error. The design and management was intended to be safe even with human error. However, the combination of mechanical issues and human error which led to the failure suggests that some improvement is necessary.
The Atomic Energy of Canada Ltd report called for a better system of review and inspection to be established to relate the design considerations to current practice.
So this is pretty early days as far as nuclear reactors and what we know about how they work. But it's the same problem as Chernobyl and Three Mile Island, different cause, but same problems with a runaway reactor that they couldn’t get under control. Luckily, due to the small size of NRX the damage was minimal and they were able to rebuild it.
1958 NRU Accident
The second Chalk River nuclear accident we want to talk about is the NRU (which stands for National Research Universal reactor) accident that occured on Saturday May 24, 1958
The NRU reactor went live on November 3, 1957, a decade after the NRX, and was initially designed as a 200 MW reactor fueled with natural uranium. It was later converted to 60MW in 1964 with high enriched uranium and to 135 MW in 1991 with low enriched uranium.
But in 1958, several metallic uranium fuel rods overheated and ruptured in the reactor core. One rod caught fire and was torn in two. It was being removed from the core with a robotic crane at the time. The large half of the rod fell into the containment vessel, still on fire.
Scientists and maintenance men were able to extinguish the fire with wet sand.
But by the time the fire was extinguished, a significant amount of radioactive combustible products had contaminated the interior of the reactor building and the adjacent laboratory site.
The ventilation valves were in the open position, also contaminating some of the area outside the building as well.
We had a real challenge trying to figure out why the rods overheated and caught fire. If you know, please reach out to us.
The cleanup took three months and the reactor was operating again by August.
Staff involved in the cleanup took efforts to mitigate exposure and were monitored over the following decades.
So there you have it, the Chalk River Nuclear Reactor Accidents. Two nuclear incidents in the 1950s when nuclear engineering was in its infancy. Luckily both accidents were not catastrophic and the reactors came back online after the cleanup. The NRX and NRU accidents taught us a lot about reactor safety and operations; although apparently not enough because Three Mile Island followed in 1979 and Chernobyl in 1986.
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Bye everyone, talk soon!