CANDU reactor control and safety

A number of mechanisms provide for reactivity control during normal operation and in postulated accidents in the CANDU reactor. Most of the reactivity control devices are vertically oriented and are located between fuel channels in the unpressurized, low-temperature moderator. (Liquid poison injection nozzles in the second shutdown system are located horizontally between fuel channels.) This removes high pressure as a possible driving force in accidently ejecting control rods from the reactor, and significantly reduces the environmental degradation of the mechanical components of the control and safety systems.

On-power refuelling maintains the core reactivity close to zero and compensates for the reactivity loss due to fuel burnup. In a CANDU 6 reactor, liquid zone controllers provide fine reactivity control. These controllers comprise vertical tubes containing light water, which is an absorber in a CANDU reactor core. The light water level in the tubes can be varied to provide bulk reactivity control and power shaping over 14 regions of the core, thus compensating for reactivity and local power changes as burnup proceeds and refuelling takes place, while also ensuring that bundle and channel powers are kept below specified limits.

The CANDU reactor employs a system of absorber rods (called adjuster rods), which provides a coarser range of reactivity control to compensate for the change in absorption in saturating fission products (primarily xenon) during power changes, such as during load following or reactor restart after a shutdown. The adjuster rods are made of stainless steel or cobalt and are normally in the core. Their withdrawal after an unanticipated shutdown, to compensate for build-up of xenon, provides a certain ‘decision and action’ time for restarting the reactor. The adjuster rods also shape the radial and axial power distributions with natural uranium fuel. In the CANDU 6 reactor, there are 21 vertical adjuster rods arranged axially in three rows.

Four mechanical control absorbers are parked above the CANDU 6 reactor core, and supplement the negative reactivity worth of the zone control system. They are inserted to achieve reductions in power at pre-determined rates, and compensate for the reactivity increase due to a reduction in fuel temperature at low power.

Soluble neutron absorbers (boron or gadolinium) can also be added to (and removed from) the moderator for additional negative reactivity.

CANDU reactors have two different, fast acting, independent shutdown systems, which are separate from the control system. The first shutdown system consists of spring-driven vertical shutoff rods, which can be dropped into the low — pressure moderator. The second shutdown system consists of a dissolved neutron absorber, which can be quickly injected into the moderator through horizontal nozzles located between the fuel channels that run the width of the core. Both systems act independently and both are capable of meeting safety requirements in shutting down the reactor. Coolant void reactivity is positive in a CANDU reactor with natural uranium fuel. Hence, a postulated large loss of coolant accident (LOCA) largely determines the requirements of the shutdown system and the emergency core cooling system. If a LOCA were to occur, the neutronic pulse would provide a fast trip signal, which would trigger the shutdown system and quickly turn over the power pulse, meeting safety requirements. In these circumstances, safety is aided by a large prompt-neutron lifetime (about 9 ms with natural uranium fuel, about 40 times longer than in a LWR), which slows down the reactivity (and power) transient. All reactors have postulated accidents that can insert positive reactivity, often without the mitigation of a long prompt — neutron lifetime (Meneley and Mujumdar, 2009; Mujumdar and Meneley, 2009) and normal practice is to utilize a combination of inherent features and engineered systems to cope with these.

The two other main safety systems in a CANDU reactor are emergency core cooling, in the event of the loss of the normal primary heat transport system cooling, and the containment building.

There are large volumes of water present in CANDU reactors, which would provide an ultimate heat sink in the case of a severe accident — heavy-water moderator surrounding the fuel channels in the calandria and light water in the shielding tank, which surrounds the calandria.

The CANDU reactor has, for a long time, included sophisticated digital computer control of the reactor, and a system of in-core, self-powered neutron flux detectors that provide an accurate and fast measure of the flux distribution in the reactor core. Different types of detectors are used in the reactor control and shutdown systems. Many other system parameters are also monitored in the control and safety systems.