Conference Proceedings Paper
Dose Reduction in Reactor Coolant
9th International Conference on CANDU® Maintenance - 2011 December 04-06
N. Prasad Gubbala (AECL Nuclear Laboratories)
The reduction of radiation dose exposure to operating and maintenance personnel is an important objective for an operating nuclear reactor. The National Research Universal (NRU) is a heavy-water cooled and moderated research reactor that uses aluminum clad fuel in an aluminum reactor vessel. The activated corrosion products of Al-28 and tramp uranium, recently characterized as non-ionic particulates, provide the largest contributions to radiation fields during reactor operation.
The heavy water coolant purification system consists of an evaporator, mechanical filters, and mixed bed resin ion exchange columns. The evaporator alone provides a purification half-life of 12 days. However, the existing mechanical filters and the mixed bed ion exchange columns are not effective at removing the non‑ionic particulates and reducing the radiation fields.
Improvements are being made at NRU to reduce the concentration of suspended particulates, and therefore the activity, circulating in the reactor water. The existing 50 µm stainless steel filters are being replaced with 1 µm stainless steel filters in a cross flow configuration (to avoid pressure drop issues) and the top portion of the mixed bed ion exchange column is being replaced with strong cation resin.
Operation with the modified ion exchange column in parallel with the evaporator has resulted in a reduction in the concentration of uranium from 10.0 to 1.5µg/kg n 4.5 months. Over the same time period, the modified ion exchange column has reduced the radiation field to 2.0 R/h, one half of the historical background radiation field in absence of fuel failures.
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