CANDU

Niobium is also important in nonferrous metallurgy. Addition of niobium to tirconium reduces the corrosion resistance somewhat but increases the mechanical strength. Because niobium has a low thermal-neutron cross section, it can be alloyed with tirconium for use in the cladding of nuclear fuel rods. A Zr—l%Nb [11107-78-1] alloy has been used as primary cladding in the countries of the former USSR and in Canada. A Zr—2.5 wt % Nb alloy has been used to replace Zircaloy-2 as the cladding in Candu-PHW (pressurized hot water) reactors and has resulted in a 20% reduction in wall thickness of cladding (63) (see Nuclear reactors). 

Canadian reactors (CANDU) are fueled using natural uranium. The discharged fuel contains small amounts of plutonium, but the fissile uranium content is below that of natural uranium. Therefore, the irradiated fuel is not normally considered a candidate for economic reprocessing. 

The GANDU Reactors. The Canadian deuterium uranium (CANDU) reactors are unique among power reactors in several respects. Heavy water is used as moderator natural uranium having 235u... 

Early research and development is described in a symposium proceedings (54). The status of the CANDU program as of 1975 is given in Reference 55 and a brief history maybe found in a more recent pubHcation of the American Nuclear Society (37). 

Figure 11.2-1 shows the layout of a CANDU. The uranium fuel is supported horizontally by uniformly spaced tubes in the calandria (5) which... 

The Pickering A Risk Assessment (PARA) (Ontario Hydro, 1995) is also a level 3 PSA for 1 of the 4 units at Pickering. A difference between PARA and DPSE is that sequences beyond the design basis were modeled using the MAAP-CANDU codes with best estimate assumptions. Other parts of the analysis used licensing-type conservative assumptions. 

In addition to the PSAs that the utility has prepared, the vendor, AECL, has prepared a CANDU-6 PSA (level 2- Allen, 1990), CANDU-3 PSA (level 1 - AECL, 1989), Wolsong 2/3/4 PSA (level 2 - AECL, 1995), and CANDU-9 PSA (level 1 - AECL, 1996, Jaitly, 1995). 

CANDU-9 is the next generation of CANDUs. It is a larger version of the CANDU producing 870 MWe, the CANDU-9 to complement the mid-size CANDU-6 with updated proven technology, a modified CANDU-6 station layout, improved construction methods and operational features. Standardization, a feature of CANDU reactors, is emphasized in CANDU-9 in the key components (steam generators, coolant pumps, pressure tubes, etc.) being the same design as those proven in service at CANDU power stations. 

A key part of the CANDU 9 design program was the review by the AECB that explored and resolved issues during design. PSA in the design process ensures that safety related requirements are addressed early to reduce retrofits as well as its role in safety assessment and licensing. 

Lessons learned from earlier PSAs incorporated in the CANDU 9 design are ... 

Completion of the PSA to higher levels depends on the sale of a CANDU-9. The plant is expected to meet the following PSA acceptance criteria. 

AECL, 1989, CANDU-3 Conceptual Probabilistic Safety Assessment. ... 

Allen, P. J. et al., 1990, Summary of CANDU-6 Probabilistic Safety Assessment Study Results, Nuclear Safety 31, 2, pp 202-214. 

Jaitly, R K., 1995, The CANDU-9 Probabilistic Safety Assessment Program, PSA 95, Seoul, Korea, pp 33-38, November 26-30. 

King, F. K. et al., 1987 The Darlington Probabiiisitic Safety Evaluation - A CANDU Risk Assessment, Canadian Nuclear Society 8th Annual Conference, Proceedings, pp 15 i -178. 

Snell, V. G., et al., 1990, CANDU Safety under Severe Accidents, Nuclear Safety, 21,1, pp 20-36, January-March. 

Some nuclear fission reactors are designed to use natural uranium having 0.7 percent and 99.3 percent CANDU reactors, manufactured in... 

Deuterium, 8 456—485 13 759. See also Canadian Deuterium Uranium (CANDU) reactors analytical methods, 8 467-468 economic aspects, 8 467 health and safety factors, 8 461-462 physical properties, 8 457-462, 459t production of heavy water, 8 459t, 462-467... 

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