Nuclear criticality control

American Nuclear Society American National Standard, Nuclear Criticality Control and Safety of Homogeneous Plutonium-Uranium Fuel Mixtures Outside Reactors, Report ANSI/ANS-8.12-1978, La Grange Park, lU. 

AR502 3.47 (Withdrawn see Regulatory Guide 3.71, ML003739492, 08/1998) (Nuclear criticality control... 

AMERICAN NUCLEAR SOCIETY, American National Standard for Nuclear Criticality Control of Special Actinide Elements, ANSI/ANS-8.15-1981 (reaffirmed 1987), ANS, New York (1981). 

In order to comply with the general requirements for nuclear criticality control and radiation protection, limits are set for the maximum TI, the maximum CSI and the maximum external surface radiation level for packages and overpacks (see also paras 531 and 532). In the case of transport under exclusive use, these limits may be exceeded because of the additional operational controls (see also paras 221.1-221.6). 

The requirement to maintain a spacing of 6 m is necessary for nuclear criticality control. Where two storage areas are divided by a wall, floor or similar boundary, storage of the packages on opposite sides of the separating physical boundary has still to meet the requirement for 6 m segregation. 

For packages containing fissile material, the programme should also take appropriate account of the need for nuclear criticality control. 

The application of fault tree analysis to nuclear criticality control was discussed in an invited paper of that name by one of the present authors at the 1977 Winter Meeting of the American Nuclear Society. The present tutorial session expands this material into a full special session. 

The main future. effort in this standard development will be to develop the specific appendixes containing detailed checklists applicable to design standards for an AFR, a dry storage facility, and other fuel cycle components. With these appendixes, the standard will become a very valuable complement to the more generic standards pertaining to nuclear criticality control It will provide a much needed information base to support design efforts and fuel handling procedure development in the nuclear fuel cycle. 

Nuclear criticality control and safety of plutonium-uranium fuel mixtures outside reactors... 

Use of soluble neutron absorbers in nuclear facilities outside reactors Nuclear criticality control of special actinide elements... 

The energy produced in a nuclear reactor vessel is the result of a nuclear fission (atom splitting) process that occurs when sufficient nuclear material is brought together (critical mass). Under these circumstances, a chain reaction occurs and an external supply of neutrons is not required. A nuclear fuel control rod system raises or lowers the nuclear fuel (which is contained within fuel rods) inside the reactor vessel. 

Reviews of designs from the point of view of their nuclear criticality safety, reviews of technical documentation of nuclear danger areas, plants, installation, and equipment if necessary, reviews of technical documentation for methods, tools, and the system of control over nuclear safety parameters final reports and conclusions based on the review results ... 

Operational controls, derived from a CSA [Mitchell and Romeo, 1999], are shown in Table 6.4-2. The referenced CSA addresses all aspects of HCF operations dealing with the chemical extraction/processing of isotopes from reactor-irradiated targets. The operational controls are implemented in accordance with the Nuclear Criticality Safety supplement to the SNL ES H Manual. 

A CSA is required for operations that involve total amounts of fissile material in excess of the threshold amount for non-aqueous forms (700 g U) where formal criticality safety evaluations and controls are required [Philbin, 1998, Criticality Safety Supplement to the SNL ES H Manual, Applicability Section]. The CSA requires that process designs incorporate a double-contingency principle so that at least two unlikely, independent, and concurrent changes in process conditions must occur before an accidental nuclear criticality is possible. Table 6.4-3 is a qualitative summary of the double contingency analysis from the CSA for isotope processing [Mitchell and Romero, 1999 (draft). Section 5]. 

Inventories of less than the threshold limits defined in the Applicability Section of the Criticality Safety Supplement to the SNL ES H Manual (which is consistent with ANSi/ANS-8.1) do not require criticality controls. However, if operations are planned to exceed those limits, appropriate nuclear criticality safety control are required. The following controls are applied to work at SNL geometry controls, criticality index (Cl) control, administrative controls, mass controls, and other nuclear crifa cality safety controls (density controls, neutron absorbers, and moderation controls). Preference of the control method depends upon whether the operation is temporary (e.g., an experiment) or permanent (e.g., a long-term storage facility). The first... 

The nuclear criticality safety program at SRS is ddineated in Procedure Manual WSRC>IM 93> 13, Rev. 1, Nuclear Criticahtv Safety Manu (Re 6-2). Nudear criticality safeQr is maintained by exercising control over the quantity and configurations of fissile materials and of assodated materials that may affect reactivity. 

Performance criteria established by regulatory statutes to ensure that the shipment is safely subcritical must also be met. Nuclear criticality in Irradiated fUel shipments can be successfully controlled through the use of l-in. boral plates to reduce the excess reactivity of the fuel element array such that kw, the infinite-medium neutron multiplication factor, is less than unity. This is a requisite for Fissile Class I shipments. 

E. R. WCX COCK, Fundamentals of Criticality Control," Criticality Control in Chemical and MetaUurgUial Plants, Karlsruhe Symposium, pp. 15-46, Organization for Ecmomic Co-Operation and Development, European Nuclear Energy Agency (1961). 

During operation of the plant, deviations from procedures and changes in process conditions that affect nuclear criticality safety must be investigated and acted upon promptly. Maximum use should be made of instrumentation for early detection of process changes so that adjustments can be made before the process control limits are approached. Before process or equipment changes are made they must be evaluated as to their effect oh the criticality safety of the plant. The depth of this review must be the same as that required for the initial evaluation. 

In conclusion, the manager with the responsibility of ( >erating the plant must ensure that the operations wUl be canied out within the criticality control specifications. Engineered safeguards for criticality control should minimize reliance on administrative control. The ANSI N16.1 Standard On nuclear criticality safety provides the basic guidelines necessary to accomplish these jobs. 

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