Reactor Design

Because MNR was constructed in the 1950s – a time when fossil fuels were inexpensive and concerns about environmental pollution were much less than they are today – there are no systems in place to recover and use the energy created at MNR. Moreover, the McMaster Nuclear Reactor was designed to be used for research purposes, not for power generation. With its end use in mind, an “open-pool” reactor design was implemented as it allows easy access to the reactor core for experimental purposes such as the insertion of samples for neutron irradiation.

Workers move a fuel bundle from the core

The “open-pool” design of MNR allows easy access to the reactor core.

While the majority of nuclear reactors that are intended for research purposes possess some variation on the open-pool design, nuclear reactors that are intended for energy production are quite different in appearance, though they function on the same operating principles. Rather than being immersed in a pool of moderator/coolant that is open to the atmosphere, the core of a power reactor is contained within a sealed vessel called a pressure vessel or calandria. The energy released by nuclear fission in the reactor core heats the (usually) light water moderator/coolant within the reactor vessel, and the hot coolant is circulated through a heat exchanger where the thermal energy (heat) of the coolant is used to generate steam. While the now-cooled coolant/moderator is recirculated into the reactor vessel, the steam is directed to a turbine that is connected to an electric generator: as the steam causes the turbine blades to turn, electricity is generated and routed into the local power grid.

A pressure vessel

A pressure vessel for use in a power reactor; the protruding black-capped rods are control rods. Image courtesy of www.nrc.gov

The different design features of power generating reactors as opposed to research reactors is function of their differing end goals: while convenient access to the reactor core is the preferred characteristic of a research reactor, the focus in a power generating station is to efficiently capture the energy released by nuclear fission and convert it into useable (electrical) energy.

Containment Structures

Because nuclear power reactors generate large amounts of both energy and radioactivity, they are housed within leak-tight concrete "containment structures" that are designed to protect the surrounding area from exposure to radiation or radioactive materials. Each of these buildings - the distinctive concrete domes present at nuclear power plants - is engineered to withstand the maximum credible accident scenario for its particular reactor design and power, with the natural hazards of the region (e.g. earthquakes, floods, etc.) also taken into account. In the unlikely event that a severe accident scenario is realized, the containment structure will prevent nuclear material and radiation from being released into the atmosphere.

Reactor Containment Structure

The containment building of the McMaster Nuclear Reactor

(photo credit: Science Media Lab).

In contrast, nuclear research reactors are generally housed in buildings constructed of more standard materials such as wood or aluminum siding. This is due to the difference in scale between power reactors (several hundred megawatts of energy) and research reactors (less than ten megawatts): the much smaller research reactors do not require full concrete containment structures in order for the facility to be considered safe. When plans were initially made to have a nuclear reactor installed at McMaster, the facility was designed with maximum safety in mind, as the reactor was to be located in the middle of a densely populated university campus. The decision was made to include a full concrete containment structure for the MNR, though for a medium-flux (5 MW) reactor, a less robust structure would have been sufficient from a safety standpoint. The containment building at the McMaster Nuclear Reactor – a 15-sided concrete polyhedron – is one of the more recognizable structures on the McMaster University campus. Designed with maximum safety in mind, the foundation of the building is a 1.5 m thick reinforced concrete pad: the minor earthquakes that occur in the Hamilton region cannot be felt from within the containment building. The walls (70 cm thick concrete with reinforcing rods) and roof (minimum thickness of 30 cm) are of similarly sturdy construction.