Maple – a multipurpose reactor

29 November 2000

Two 10MWt Maple reactors have been built at AECL’s Chalk River Laboratories in Canada. The reactors have been licensed by the Canadian Nuclear Safety Commission, and will begin commercial operation in 2001.

Research reactors provide a wide range of applications, including acting as neutron sources for the production of medical and industrial isotopes. Every day, about 50,000 medical procedures using radioactive isotopes are performed around the world. That is equivalent to over 18 million procedures a year, ranging from medical diagnoses through to therapy. About 70% of the global demand for these radioisotopes is currently produced by the NRU research reactor, which was designed, constructed and is operated by AECL. AECL has designed, developed and constructed 17 research reactors around the world.

From this experience, AECL developed the Maple reactor. The Maple reactor requires about half the staff that is needed to operate the NRU reactor. In addition to this, the Maple reactor is designed to carry out multi-purpose functions, and it can be configured to suit specific applications. The Maple reactor is more compact than the NRU reactor, with a smaller core.

The Maple core design technology was first incorporated into the Korea Atomic Energy Research Institute’s (KAERI’s) 30MWt Hanaro multi-purpose research reactor, which has been in service since 1995.

In August 1996, MDS Nordion awarded a contract to AECL to construct two 10Wt Maple reactors and an associated processing facility, at AECL’s Chalk River Laboratories in Canada. The new facilities will be dedicated to medical radioisotope production and will replace most of the radioisotope production at the ageing NRU reactor. The reactors, which will be owned by MDS Nordion, have been licensed by the Canadian Nuclear Safety Commission and will be operated by AECL for MDS Nordion. It is intended that one reactor will be in operation, with the other on standby. This is to ensure a continuous supply of medical isotopes, many of which have a short life. Isotope production will begin in 2001.

Maple design and technology

Maple is a low-pressure, low-temperature, open tank–in–pool type research reactor that uses low-enriched uranium fuel with the option to use advantageous high density LEU fuels in the future. It can be designed for power levels of 10– 40MWt. The core is located at the bottom of a pool. It is compact, and is both cooled and moderated by light water. Surrounding the light water core is a heavy water reflector tank, which maximises the available neutron fluxes.

Access to the Maple core is through an open chimney. Vertical in-core and out-of-core irradiation sites can be provided for radioisotope production, neutron activaton analysis, and silicon doping. These are directly accessible from the pool top. Horizontal beam tubes, for neutron scattering and other applications, can also be provided, as can loops for testing nuclear fuels and components.

Maple has passive safety features from its tank-in-pool design, as well as other safety features. Depending on customer requirements, these may include: a plant display system that works in concert with the digital control system to enhance plant-state monitoring and diagnostic capabilities; two independent and diverse safety shutdown systems; two parallel and independent cooling loops; and a building design that incorporates containment and confinement features.

The Maple reactors built for MDS Nordion have cores that approximate a hexagonal shape of about 41cm flat-to-flat with a fuelled height of 60cm. The core is surrounded by a tank containing heavy water with outer dimensions of 160cm diameter by 90cm height. The reactor assembly is located at the bottom of a rectangualar pool, 3x3.5 x9.5m (water depth).

A Maple reactor can be designed with several options that include the following possibilities: a cold neutron source, a hot neutron source, thermal and cold neutron guides, irradiation rigs, epithermal columns for boron neutron capture therapy, fuellable sites in-core or in- reflector, and test loops for nuclear fuels and materials.

MDS Nordion

MDS Nordion is the world’s leading supplier of medical radioisotopes to over 60 countries, supplying 70% of the world’s medical radioisotopes. It provides much of the isotope needs of over 3000 hospitals in North America alone. MDS Nordion’s new facility in Chalk River will produce molybdenum-99, which decays into technetium-99 – the isotope most widely used in hospitals and clinics to diagnose many illnesses including cancer and heart disease. The facilities will also produce xenon-133, iodine-131, and iodine-125, which are used for diagnosis and therapy.

The new CNF Maple research reactor proposal

AECL has recently completed pre-project activities for a new Maple research reactor project called the Canadian Neutron Facility (CNF) for Materials Research. The 40MWt CNF is proposed by AECL and the National Research Council of Canada to replace the ageing NRU research reactor – as a source for advanced materials research, neutron beam applications, and Candu reactor development. The CNF design incorporates up-to-date neutron beam technology, and will feature a state-of-the-art cold source and a configuration that will allow testing of full-size Candu fuel bundles.`

Maple reactor fuel details

The fuel for the Maple reactors is uranium silicide, dispersed in aluminium, with an enrichment of 19.7% by weight U-235. The fuel is in the form of rods with finned aluminium cladding. The individual fuel elements are fabricated using an extrusion process and clad using a co-extrusion process. These fuel elements are assembled into 36-element hexagonal bundles and 18-element circular bundles. The 18-element fuel bundles are used in the control and shutoff rod sites, along with the cylindrical control and shutoff absorbers. The 36-element fuel bundles contain about 426g U-235 and the 18-element bundles contain about 213g U-235. The typical discharge burnup for the 36-element fuel bundles is about 180 MWD, or about 50% of the fissile atoms consumed. The typical discharge burnup for the 18-element fuel bundles is about 100 MWD. At a total core power level of 10MW, the peak thermal neutron flux levels in the core are about 1.8 x1014 n/cm2/s and the peak thermal neutron flux levels in the heavy water reflector are about 1.4 x1014 n/cm2/s. The ratio of the peak thermal neutron flux in the heavy water reactor to the total core power is 1.4 x1014 n/cm2/s/MW.

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