Accident Possibilities at Gentilly-2 and Other CANDU Reactors

Canadian Coalition
for Nuclear

Regroupement pour
la surveillance
du nucléaire

Accident Possibilities at Gentilly-2
and Other CANDU Reactors


[ pour la version française ]

by Gordon Edwards, Ph.D., President,
Canadian Coalition for Nuclear Responsibility

  1. Gentilly-2 and Other CANDU reactors

  2. Emergency Electrical Power

  3. Steam Line Accidents

  4. Pressure Tube Ruptures

  5. Pressure Relief Valves

  6. The Containment System

  7. Citations from Official Documents

  8. References

1. Gentilly-2 and Other CANDU Reactors

Gentilly-2 is a 600 megawatt nuclear reactor fueled with natural uranium. In order to generate 600 megawatts of electrical energy, the core of the reactor must produce almost three times as much heat energy: that is, 1800 megawatts of heat. The heat, which is removed from the core of the reactor by heavy water driven by powerful pumps, is used to boil ordinary water, and the resulting steam turns the blades of a turbine to generate electricity.

The necessary heat is produced by the splitting, or "fissioning", of uranium atoms, which releases atomic energy. However, the broken pieces of uranium atoms are new materials, which are intensely radioactive and highly toxic, called "fission products".

There are hundreds of different fission products -- radioactive poisons such as iodine-131, strontium-90, cesium-137, krypton-85, et cetera. Because of the buildup of these fission products in the fuel, a used fuel bundle is millions of times more radioactive than a fresh fuel bundle, containing only uranium. Indeed, a used nuclear fuel bundle is the most lethal object on earth.

A nuclear catastrophe is an accident resulting in the release of a significant fraction of these fission products to the environment. The Chernobyl accident, for example, released about 3 percent of the radioactive materials from the core of the reactor into the environment. In principal, a similarly catastrophic accident can happen at any nuclear reactor, including Gentilly-2. Such a catastrophe could cause tens of billions of dollars worth of damage due to radioactive contamination of buildings, soil and water supplies. For this reason, insurance companies will not provide coverage to any of its clients in the event of radioactive contamination from a nuclear catastrophe. Such an accident could also kill thousands of people due to the delayed biological effects of radiation -- mainly cancer and damage to unborn children.

Gentilly-2, like all nuclear reactors, is equipped with a number of safety systems designed to prevent any serious accidents from happening. However, as we all know, accidents will happen -- and the worst accidents are often those that are totally unanticipated. In the last few years, a number of previously unanticipated safety problems have occurred at Gentilly-2, all of them requiring expensive corrective action costing millions of dollars each.

2. Emergency Electrical Power

In the last 15 years, the frequency of blackouts in Quebec has greatly increased, When a prolonged blackout occurs, the Gentilly-2 reactor must shut down -- it can't keep running, because it has nowhere to send its electricity. But the heat generation in the core of the reactor cannot be shut off. Even after the reactor is shut down, heat continues to be produced at about 7 percent of the "full power" level, because of the intense radioactivity of the fission products. Seven percent of 1800 megawatts is a lot of heat -- about 125 megawatts! If that heat is not removed, the core will melt and a catastrophe may result.

Thus, even during a blackout, Gentilly-2 must have electrical power to keep the huge pumps running so that heat can be removed from the core of the reactor. For this reason, there are several diesel generators at Gentilly-2 which can be started up during an emergency to give enough power to run the pumps, the control room, and other vital equipment. But these emergency generators are not as reliable as one might hope, and the experience with them has not always been good.

To provide adequate safety to the population, the AECB required Hydro-Quebec to ensure that there was a safe, reliable, electrical supply for the Gentilly-2 reactor in the case of a prolonged blackout. That safety requirement was one of the main reasons given by Hydro-Quebec for the construction of the 300 megawatt Gas Turbine Plant at Bécancour, inside the exclusion zone of the Gentilly-2 nuclear reactor. The TAG cost several hundred million dollars, and it produces peak electricity at more than 50 cents for each kilowatt-hour. Gentilly-2 is the only reactor in the world that has another large electrical generating plant, using fossil fuel, built right beside it. Keeping nuclear reactors safe is not always easy.

3. Steam Line Accidents

When Gentilly-2 was built, nobody thought about the possibility that an accident could kill everybody in the control room of the nuclear reactor, possibly resulting in a catastrophic accident. (Even if the reactor is shut down, the heat still has to be removed from the core to prevent a catastrophe -- and that cannot be guaranteed if the operators are dead.) A few years ago, AECB discovered that a sudden break in one of the steam pipes passing over the roof of the control room could, in fact, kill everyone inside and make the control room unusable. Obviously, this improbable situation could be extremely hazardous for the population at large.

At first, AECB wanted Hydro-Quebec to relocate the steam pipes, but Hydro-Quebec argued that this would be too expensive. Instead they offered to make some substantial alterations to the interior design of the building so as to minimize the effects of such a break in the steam pipes, and to carefully monitor the pipes so as to detect any weakening which might (or might not) occur before such a break would happen. Those corrective actions are still being carried out. Whether they are adequate to eliminate the possibility of a catastrophic accident caused by a steam pipe rupture is a matter of judgment.

4. Pressure Tube Ruptures

One of the most serious kinds of accidents in any nuclear reactor is a Loss of Coolant Accident, or LOCA. It is caused by a pipe break or an open valve, allowing the water normally used to remove the heat from the core of the reactor to escape. Without adequate cooling, the core of the reactor will overheat and fission products will be released from the damaged fuel, many of them in the form of radioactive gases and vapours. If these materials escape into the environment in large enough quantities, a nuclear catastrophe will result.

Every CANDU reactor has an Emergency Core Cooling System (ECCS) which is supposed to flood the core of the reactor with ordinary water in case of a large LOCA to prevent it from overheating. Sometimes, however, the ECCS is not available. The AECB allows the ECCS to be unavailable for up to eight hours in any given year, and in some cases it is unavailable for a much longer period of time. If a large LOCA were to happen at such a time, serious core damage could occur and a nuclear catastrophe could result.

The first LOCA in a Canadian nuclear generating station was at Pickering, just outside of Toronto, in 1983, when a pressure tube burst without warning in the core of the reactor. A few years earlier, nuclear experts had insisted that a pressure tube could not burst suddenly, because it would begin to leak long before it would break, giving the operators enough time to shut the reactor down and correct the situation. But the experts were wrong.

Fortunately, it was not a very large LOCA, and the ECCS was not needed to keep the fuel from overheating. Most of the fuel remained undamaged and so not much radioactivity was released. However, repairs to the core of the Pickering reactor took four years and cost more than 500 million dollars. All of the pressure tubes had to be replaced, since many of them were showing signs of serious deterioration and some were developing blisters. If several pressure tubes had burst at the same time, a much more frightening situation would have to be dealt with.

The Core of One of the Four Darlington Reactors
-- Before Startup --

The core of a CANDU reactor lies in a cylindrical vessel called a calandria. Running from one end of the calandria to the other are several hundred pressure tubes containing uranium fuel bundles. Heavy water coolant flows through these tubes .

The superheated coolant is used to boil ordinary water, which produces steam, which spins a turbine, which generates electricity.

Once the reactor has operated for a while, the irradiated fuel becomes so radioactive that a man standing in front of the reactor face (as the man in the picture is) would receive a lethal dose of radiation in a very short time, even if the reactor were completely shut down.

photo by Robert Del Tredici
At Work In The Fields Of The Bomb
(Harper and Row, 1987)

All CANDU reactors have pressure tubes, and all of them deteriorate with time. But the cost of replacing all the pressure tubes is so great that Ontario Hydro has decided to shut down some of its reactors permanently rather than spend the money needed to repair them. At Gentilly-2, until recently, Hydro-Quebec was also planning to replace all the pressure tubes; but now, to save money, they have decided to try something less expensive -- to re-adjust the "garter springs" which support the pressure tubes inside the core of the reactor, so they will not sag as much and, hopefully, will not develop the same kind of blisters that were seen in the pressure tubes at Pickering. This approach does not halt or reverse the deterioration of the pressure tubes, but it will perhaps slow it down.

5. Pressure Relief Valves

In December, 1994, a second LOCA occurred at a Canadian nuclear reactor. This time, it was not a pressure tube, but a relief valve that broke at Pickering.As the superheated mixture of water and steam escaped through the broken valve, a powerful vibration shook the pipe so hard that it broke, resulting in a large LOCA that required -- for the first time in Canadian history -- the use of the ECCS to prevent damage to the nuclear fuel. Similarly accidents involving broken relief valves in CANDU reactors have also occurred in South Korea, and -- in May 1995 -- at one of the Bruce reactors in Ontario. In these situations, however, the vibrations did not result in broken pipes.

In June 1995, the AECB made public its analysis of these accidents. They have concluded that a similar accident could occur at any time at Gentilly-2, and that the resulting vibrations would very likely result in either broken pipes or damage to other valves, thus resulting in a serious LOCA. Hydro-Quebec has made no statement about the situation, and may not agree with the AECB as to the possibility of such an accident or the necessity of undertaking the necessary repair work which would involve redesigning that portion of the pressure relief system.

6. The Containment System

If the nuclear fuel is damaged, large quantities of radioactive materials can escape into the reactor building. However, the reactor building has an elaborate containment system which is designed to prevent most of this radioactive material from escaping into the environment. If the containment system works perfectly, then a catastrophe can be avoided even when the core of the reactor melts, as happened at Three Mile Island in 1979.

The problem is, however, that the containment system itself is not always available. Like the ECCS, it can legally be unavailable for up to eight hours a year, and may sometimes be unavailable for much longer periods of time. At Gentilly-2, the estimated unavailability of the containment system over the last several years has been as indicated below:

	1989	38 hours		1990	34 hours
	1991	31 hours		1992	32 hours
	1993	34 hours		1994	29 hours

As AECB stated in its Évaluation de l'exploitation de la centrale Gentilly-2 en 1994,
"En 1994, la performance des systèmes spéciaux de sûreté a été adéquate. Hydro-Québec a respecté nos exigences quant à l'indisponibilité prévue de trois des quatres systèmes spéciaux de sûreté; celle du système du confinement demeure élevée."

[In 1994, the performance of the special safety systems was satisfactory. Hydro-Québec has met the AECB requirements regarding the predicted unavailability of three of the four special safety systems; however, the predicted unavailability of the containment system remains high.]

Citations from Official Documents

Nuclear proponents often say that catastrophic nuclear accidents are not really possible. That's what they said about the Three Mile Island reactor before the 1979 accident. That's what they said about the Chernobyl reactor before the 1983 accident. That's what they are saying about the Gentilly-2 reactor today.

Every nuclear reactor contains an enormous inventory of radioactive poisons which are created inside the reactor as inevitable by-products of routine operation. If even a rather small percentage of these materials escape into the environment, the results can be catastrophic. At Chernobyl, between 3 and 5 percent of the total radioactive inventory escaped into the environment.

It is certainly true that great care has been taken to build these reactors so that the population is protected from the consequences of a catastrophic accident. Nevertheless, technology is not perfect; these accidents can happen.

In 1974, the U.S. Nuclear Regulatory Commission published a report commonly called the Reactor Safety Study (WASH-1400), authored by Norman Rasmussen of M I T. This report studied the probability and the consequences of catastrophic nuclear reactor accidents in the U.S.A. In Canada, there has never been a comparable study of the safety of CANDU nuclear reactors.

After the Three Mile Island accident in 1979, the U.S. President set up a commission of inquiry into the causes of the accident. The President's Commission found that one of the most important factors contributing to the accident was the unwarranted belief on the part of workers within the industry that nuclear reactors are basically safe. The Commission warned that nuclear energy should not be regarded as a technology which is inherently safe, but rather as one which is fundamentally dangerous.

On the following pages [linked below as "Citations on CANDU Safety"] you will find several verbatim excerpts taken from government documents published in Canada on the possibility of catastrophic accidents in a Canadian CANDU reactor such as Gentilly-2.

  1. A Race Against Time. Ontario Royal Commission on Electric Power Planning. Toronto, 1978.
  2. Submission to the Treasury Board of Canada. Atomic Energy Control Board. Ottawa, 1989.
  3. Nuclear Policy Review: Background Papers. Department of Energy Mines and Resources. Ottawa, 1982.
  4. The Safety of Ontario's Nuclear Reactors: Final Report. Select Committee on Ontario Hydro Affairs. Toronto, 1980.
~ finis ~

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