301. Paragraph 188.8.131.52 of INFCIRC/225/Rev.3 concisely states that the rationale for the categorization of nuclear material is to provide a basis to establish "an appropriate relationship between the material concerned and the protective measures." The State should define the categorization of nuclear material to ensure that appropriate protection measures are implemented. Since implementation of physical protection measures requires extensive resources, State regulators need a coherent basis for the measures being required of facility operators. However, paragraph 184.108.40.206 and other portions of INFCIRC/225/Rev.3 provide minimal explanation of how the recommendations are to be carried out. This section provides additional guidance for understanding the intended process of material categorization.
302. INFCIRC/225/Rev.3 notes that "physical protection measures should be implemented for nuclear facilities which may be subject to sabotage regardless of the categorization of nuclear materials" [220.127.116.11]. No relationship is intended for the categorization levels in the table of INFCIRC/225/Rev.3 and the facilities and activities that may be subject to sabotage. The table is related only to the theft scenario. However, a regulatory organization can utilize all or portions of the requirements provided in Section 5 of INFCIRC/225/Rev.3 as a basis for physical protection measures against sabotage for such facilities or activities (see paragraph 429 below).
303. In using the Table: Categorization of Nuclear Material, the original fissile content of material is the approach which is used by many States to determine the appropriate level of physical protection prior to irradiation of the material. For instance, 15kg of research reactor fuel comprising uranium enriched to 20% = 3kg U-235 fissile weight for categorisation purposes. Similarly, 10kg of uranium fuel enriched to 90% = 9kg U-235 fissile weight.
304. The table indicates that "fuel which by virtue of its original fissile material content is classified as Category I or II before irradiation may be reduced one category level while the radiation level from the fuel exceeds 1 Gy/hr (100 rads/hr) at one meter unshielded." The method of making such measurements has not been provided in INFCIRC/225/Rev.3. A determination of the radiation level can be made by measurement in air or water or by calculational means. It is preferable to estimate the absorbed dose rate on the basis of direct measurements in water (spent fuel pond) and then calculate the radiation level at one meter unshielded. Where possible, measurements should be made on individual fuel elements. A constraint on this type of measurement is the necessity of moving or lifting elements in order to isolate individual elements from neighbouring spent fuel. However, if that is not possible, measurements of a number of fuel elements would be satisfactory. In all cases, measurements should be made by means of underwater probe (e.g. shielded G-M counter) at a certain distance from the vertical axis of the item and at the mid point at several positions around a fuel element or a fuel assembly. Then recalculation for one meter distance and air absorber should be performed.
305. Where calculations are performed instead of actual measurements (e.g. by means of Monte Carlo code), the fuel structure, isotopic composition, initial fissile content, burn-up and cooling time should be taken into account to calculate the absorbed dose rate.
306. Nuclear material in a form that is no longer usable for any nuclear activity, minimizes environmental dispersal and is practicably irrecoverable (e.g. encapsulated, immobilised or vitrified wastes) can be protected with prudent management practices, i.e., practices which require no special physical protection measures except for normal industrial security or safety practices based on the materials level of concern, attractiveness and radiological consequences. [18.104.22.168].
307. In some facilities nuclear material can be distributed in several buildings. When this is the case, it is suggested that if there are several buildings within a small area, the total amount of nuclear material should be added together to determine the protective arrangements which should be applied to these buildings as required by the categorisation table. [4.2.2].