Clean Energy Prof: The Fermi Chronicles - Part 16: Nuclear Events

This incident occurred at the Chalk River nuclear facility on December 12, 1952. The Canadian government built the Chalk River facility in cooperation with the U.S. and Britain back during WWII. It was an experimental reactor (weren't they alll back then?) called the NRX. This accident is an eerie forshadowing of the Chernobyl disaster in that a combination of mechanical and human errors led to the accident. From the Canadian Nuclear FAQ:

On December 12, 1952 a combination of mechanical failure and human error
led to a now-famous power excursion and fuel failure in the NRX reactor at AECL Chalk River Laboratories. At the time NRX was one of the most significant
research reactors in the world (rated at that time for 30 MW operation), in its
sixth year of operation.

During preparations for a reactor-physics experiment at low power, a defect in the NRX shut-off rod mechanism combined with a number of operator errors to cause a temporary loss of control over reactor power. Power surged ultimately to somewhere between 60 and 90 MW over a period of about a minute (the total energy surge is estimated to be approximately 4000 MW-seconds). This energy load would normally not have been a problem, but several experimental fuel rods that were at that moment receiving inadequate cooling for high power operation ruptured and melted. About 10,000 Curies of fission products were carried by about a million gallons of cooling water into the basement of the reactor building. This water was subsequently pumped to Chalk River laboratories' waste management facility, where the long-term ground water outflow was monitored thereafter to ensure adherence to the drinking water standard. The core of the reactor was left severely damaged.

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 that involved hundreds of AECL staff, as well as Canadian and American military personnel, and employees of an external construction company working at the site. In addition the reactor core itself was rendered unusable for an extended period.

More detail on the human error is present over at Hyperphysics:

First, four valves which kept air pressure from raising the control rods were opened in error by an operator. The supervisor noted warning lights and rushed to the basement to close the valves. Once he had closed them, he assumed that the rods had dropped back, but they hadn't dropped fully - they had dropped only far enough to shut off the warning lights.

The supervisor, realizing that the reaction was still on, called the control room to order the operator to push buttons 4 and 3 to stop the reactor, but mistakenly said 4 and 1! The operator rushed off to do it before he could correct his mistake. Button 1 raised 4 banks of control rods, causing the reaction rate to double every 2 seconds. This buildup was noted after about 20 seconds and the reactor was scrammed. Because of the air pressure problem, the control rods didn't go all the way down. After about 44 seconds, the plant physicist dumped the heavy water to kill the moderation and stop the reaction. This dumped tons of radioactive water into the basement.

More detail on the reactor itself can be found over at a site run by Peter Jedicke for his students:

NRX was contained in an aluminum cylinder, called the calandria, with a diameter of about 8 m, a height of about 3 m and its axis oriented vertically. The calandria was fitted with about 175 vertical calandria tubes arrayed in a hexagonal lattice, was filled with up to 14 000 litres of heavy water (D2O) and helium gas and sealed. The purpose of the helium gas was to prevent chemical reactions with air. The helium was held at a constant pressure of about 3 kPa above atmospheric, even as the fluid level in the calandria changed, by connection to an external helium gas holder with a capacity of approximately 40 m3. As helium was moved back and forth from the gas holder to keep the pressure constant in the calandria, the volume of the gas holder changed by means of a domed cylinder that slid up and down on sealed tracks. Thus the changing height of this dome gave an indication of gas flow to and from the calandria.

The NRX calandria tubes were sleeves, 6 cm in diameter, into which various rods could be inserted from above into the reactor. A few calandria tubes were sometimes used to insert other materials into the reactor to be irradiated as part of various experiments. Most of the tubes held uranium fuel rods, 3.1 m long, 3.1 cm in diameter, with a mass of about 55 kg. The fuel rods were originally clad in aluminum, which was chosen because it absorbed less neutrons than steel or other materials available at the time. There was actually a double-walled jacket of aluminum around the fuel rod, with ordinary water flowing between the walls as a coolant. This also left a ring of air between the outer aluminum sheath and the calandria tube. Up to 250 litres of water taken from the Ottawa river passed through the coolant sheaths of the fuel rods each second; and an airflow of about 8 kg per second was maintained through the calandria tubes. These coolant systems included considerable plumbing outside the calandria which could be altered as part of the experiments. In particular, there were storage tanks for the water and a 61 m stack for the air.

More detailed analysis of the accident at the above link. As with other nuclear incidents, a whole lot was learned. Redundant safety systems were implemented. Here's what was learned from the accident, as detailed by the Canadian Nuclear FAQ:

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.

So a lot of good came from this. I should note that no one was injured or killed in this event. Also, as a bit of trivia, one of the officers of the U.S. nuclear navy that was involved in the cleanup was one Lt. Jimmy Carter, who later would become the 39th President and, unfortunately, would be largely responsible for holding back nuclear power in the U.S. with policies some of which are still in effect to this day. Another bit of trivia - the current reactor at Chalk River, in addition to one in the Netherlands, produce about 80% of material for nuclear medicine - in the world!

UPDATE: 7/24/2009 - From the New York Times via Instapundit: Radioactive Drug for Tests Is in Short Supply. Both of the above reactors that produce nuclear material for medicine are currently shut down, leading to a shortage. Maybe they just need to dedicate a few more reactors for that purpose?

A global shortage of a radioactive drug crucial to tests for cardiac disease, cancer and kidney function in children is emerging because two aging nuclear reactors that provide most of the world’s supply are shut for repairs.

The 51-year-old reactor in Ontario, Canada, that produces most of this drug, a radioisotope, has been shut since May 14 because of safety problems, and it will stay shut through the end of the year, at least.

Some experts fear it will never reopen. The isotope, technetium-99m, is used in more than 40,000 medical procedures a day in the United States.

Loss of the Ontario reactor created a shortage over the last few weeks. But last Saturday a Dutch reactor that is the other major supplier also closed for a month.
The last of the material it produced is now reaching hospitals and doctors’ offices. The Dutch reactor, at Petten, is 47 years old, and even if it reopens on schedule, it will have to be shut for several months in 2010 for repairs, its operators say.

Tech-99m, as it is abbreviated, emits a gamma ray that makes its presence obvious. It has a half-life of six hours, meaning that it loses half its strength in that period. Thus it does its job quickly, without lingering to give the patient a big dose. But it also means the isotope must be produced and used faster than most other drugs.

Tech-99m is the product of another isotope, molybdenum-99, which also has a short half-life, 66 hours. Thus a week after it is made, less than a quarter of the molybdenum-99 remains. Stockpiling is not practical.

Molybdenum-99 is made when uranium-235 is split, but only about 6 percent of the fission fragments are molybdenum. Purification has to be done in a heavily shielded “hot cell.”

The common method is to put a uranium target into the stream of neutrons produced in the reactor as uranium is split. But the preferred material is a high-purity uranium-235, which is also bomb fuel.

The reactors’ poor condition has been obvious for a while. In 2007, Canadian safety regulators said the Ontario reactor should not restart, but the Canadian Parliament overruled them.

In 1996, the company that purifies the molybdenum from the Ontario reactor, MDS Nordion, contracted with Atomic Energy of Canada Ltd., which owns the reactor, to build two new ones. MDS Nordion paid more than $350 million for them.

But when the new reactors were started up, both showed a problem: as the power level increased, the reactors had a tendency to run faster and faster, a condition called positive coefficient of reactivity. That is a highly undesirable characteristic in a reactor, one that contributed heavily to the Chernobyl disaster in 1986. So Atomic Energy of Canada Ltd., which is owned by the Canadian government, said it would not open them.

Clean Energy Prof

Friday, June 26, 2009

The Fermi Chronicles - Part 16: Nuclear Events - Chalk River, CAN, 1952

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