4.133. The containment structure and its internals provide the first mechanisms for the removal of airborne radioactive material, since they present a large surface area for deposition. The plate-out and desorption factors ascribed to the containment structure should be conservatively based on the best available knowledge of deposition of radionuclides on surfaces. The surfaces of the containment and its internal structures should be decontami — nable to the greatest extent possible.

Containment spray system

4.134. The radionuclide management function of the containment spray system is intended to reduce amounts of airborne radioactive substances by removing them from the containment atmosphere and retaining them in the water of the containment sump or the suppression pool. This serves to limit any radiological consequences resulting from leakage of radioactive material from the containment to the atmosphere in postulated accident conditions.

4.135. Important parameters and factors that should be considered in the design of the containment spray system include spray coverage, spray drop size, drop residence time and the chemical composition of the spray medium. Chemicals should typically be added to the spray water to enhance the removal of radionuclides from the atmosphere. Radioiodine is of particular importance, because of its potential consequences in terms of high specific doses. The chemical additive system should be designed to maximize the dissolution of radioiodine and to maintain the sump chemistry or the suppression pool chemistry such that radioiodine will not be released from solution in the long term following an accident.

4.136. Any chemicals added should be non-corrosive for the materials present in the containment, both in the short term and in the long term after an accident. Corrosion might not only reduce the strength of vital structural components and impair the operation of safety systems but might also generate combustible gases and other undesirable compounds.

4.137. The design of the containment spray systems should be such as to ensure that the probability of spurious actuation is low.