Positive Void Coefficient of Reactivity and CANDUs

A well-understood technical aspect

August 28, 2015

The void coefficient of reactivity – not exactly your dinner table discussion topic, and chances are you’ve never heard of it. What’s important to know is that it is a well-known characteristic that is understood by the Canadian Nuclear Safety Commission.

In nuclear engineering, all nuclear reactors have a void coefficient of reactivity – which is the increase or decrease in the rate of fission that would occur, and heat generation, following a loss of coolant.

In some reactors, such as pressurized water reactors and boiling water reactors, this coefficient is negative. This means that the level of energy and heat produced by a reactor following a loss of coolant would naturally decrease before shutdown.

In CANDU reactors, the void coefficient of reactivity is positive.

How do CANDU reactors ensure safety?

CANDU positive void coefficient of reactivity is a well-understood design characteristic and specific measures have been implemented in the design to ensure safety.

Video: Void coefficient of reactivity and CANDU reactors

Transcript (click to expand)

Title:

CNSC: Canada’s Nuclear Regulator
Void Coefficient of Reactivity and CANDU Reactors

Gerry Frappier (Director General of Assessment and Analysis): Positive void reactivity, very simply, is that in the unlikely event of a loss of coolant accident or a big pipe break, where there is gas introduced or water removed from the pipes, in a CANDU design, the power will increase. In other designs, the power will decrease. This is a very known feature; it’s been known since the very beginning. And there’s engineering controls for that.

In particular, the CANDU system has two very, very fast independent shut-down systems, which will shut down the reactor within 2 seconds and completely eliminate that problem. We’re very confident that it’s a controlled item. Should it not function as designed, there still would not be a serious consequence in the sense of a public danger because the reactor geometry is very, very precise for this phenomena to occur. So as soon as any kind of damage starts happening to the reactor itself, the reactor will shut itself down even without the control systems. That aspect is very well understood, there’s lots of engineering controls around it, and we do not see that as a safety concern.

Title: We will never compromise on safety.

Every plant in Canada must have two independent, fast-acting and automated emergency shutdown systems to stop the reactor in case of an emergency:

• The first consists of spring-assisted, gravity-driven, neutron-absorbing shutoff rods, which drop into the core.
• The second is an injection of a neutron-absorbing liquid, containing gadolinium, into the moderator.

In addition to those emergency systems, there are several layers of protection, within and outside the reactor, to contain any release of radioactive material in the unlikely event that it occurs.

Nuclear systems and equipment in Canada are designed, built, operated and maintained for maximum plant safety. Sophisticated, redundant control systems are installed in each reactor unit.

For instance, the CNSC requires all reactor designs in Canada to include backup computer systems, electrical generators and tanks of coolant and air, to reduce the risk of potential malfunctions of the main systems.

• Learn more about nuclear power plant safety systems

Since CANDU reactors and the type of reactor involved in the 1986 Chernobyl incident both have a positive void coefficient of reactivity, some people mistakenly think they also share the same safety risks.

For instance, the Chernobyl reactor had only partial containment, one shutdown system, and used graphite as a moderator, which can burn under certain accident conditions.

CANDU reactors have operated safely in Canada, and around the world, for more than 35 years and do not pose a significant risk to the health and safety of Canadians, or to the environment.

The CNSC has site offices at each of Canada’s nuclear power plants, where CNSC experts regularly inspect and analyze operations to ensure they are safe.

The CNSC welcomes questions and comments from all its stakeholders. It is committed to its mandate to regulate the Canadian nuclear industry to protect Canadians and the environment, which includes disseminating scientific information to the public about how it carries out its work.