Inadvertent Criticality

Nuclear Safety means those aspects of safety that encompass activities and systems that present the potential for uncontrolled releases of fission products or other radioactive materials to the environment or for inadvertent criticality. 

Technical Safety Requirements shall define the operating limits and surveillance requirements, the basis thereof, safety boundaries, and management or administrative controls necessary to protect the health and safety of the public and to minimize the potential risk to workers from the uncontrolled release of radioactive or other hazardous materials and from radiation exposure due to inadvertent criticality. 

The TSRs are not based upon maintaining worker exposures below some acceptable level following an uncontrolled release of hazardous material or inadvertent criticality rather the risk to workers is reduced through the reduction of the likelihood and potential impact of such events. This is accomplished by the development of safety requirements in the TSR for those systems, components, and equipment that (a) are barriers preventing the uncontrolled release of radioactive and other hazardous materials (b) mitigate such releases ... 

The HCF has several areas in which the potential for an inadvertent criticality exists. These are the SCBs (Zone 1), the entrance to the SCBs (Zone 2), Zone 2A, Rooms 108, 109, 111, 112, 113, 113A, 114, B6596, B6597, and the monorail storage holes. Chapter 2 describes each of these areas and the processes accomplished in each area. 

A CSA (Mitchell and Romero, 1999) has been prepared and approved in accordance with the ES H Manual Supplement to evaluate the criticality concerns and document the basis, for operating limitations for planned uses of fissile materials in the HCF. Both engineered features and administrative limits are used to prevent inadvertent criticality in the HCF. These features and limits are based on physical principals of mass, geometry, and neutron moderation to provide high confidence that criticality cannot occur in the HCF under all normal, abnormal, and accident conditions. They provide the basis for an assessment of the application of the double-contingency principle implemented in the HCF. 

Hazards to personnel who work with Pu in the laboratory arise primarily from two causes. First, Pu is extremely poisonous because of its hi specific alpha activity, long biological baU-life, and tendency to concentrate in the bone. Second, inadvertent criticality may occur. A full discussion of these hazards is beyond the scope of this review, but some general comments concerning precautions and techniques can be made. 

Fuel handling and storage facilities are designed to prevent inadvertent criticality and to maintain shielding and cooling of spent fixel as necessary to meet operating and off-site dose constraints. 

A safety concern for the five main areas that comprise K-Reactor in Cold Standby and the Disassembly Basins of L- and P-Reactors is inadvertent criticality. Ttus concern is described below for each area. 

The primary safety concern in the K-Reactor Assembly Area is inadvertent criticality. Hangers (which protnde separation between foel assemblies) and borated concrete ston e racks are used to prevent criticaltty during movement and storage of Mark 22 fiid assemblies. Calculations and mtHtipUcation measurements of the fulfy loaded borated concrete storage racks confirmed subcriticality by a substantial amount (Ref 6-13, Section 9.1.1.2.2). 

Disassembly Basin Draindown Disassembly Basin Overfill Inadvertent Criticality Criticality in Basin Sludge Criticality in the Settlor Tank Criticality in the Sand Alters Criticality in the Ddlonizers Loss of Disassembly Ba Cooliqg Ifezardous Chemical SpiU Fire... 

Four of the scenarios assodated with irradiated foel/taiget storage in the K-, L-, and P-Reactor Disassembly Basms 0Disassembly Basin draindown. Disassembly Basin overfill, and inadvertent criticality) were combined in a probabilistic manner in the analysis discussed m the PHA (Ref 8-17) Since each of these events contains numerous end-states (depending upon automatic and/or operator actions, fiiult-tree probabilities, and the pt cs of each event), a Monte-Carlo type method (Ref. 8-17) was used vdiich integrates the fiidt-tree end states of all four events (and thdr respective sub-events) into one consequence versus fiequency curve. It is... 

Unlike K-Reactor Disass nbly Basin, most of the assemblies in L- and P-Reactor Disassembfy Basins are in the HBBS or transfer bay. This means that these assemblies do not need to be handled or disassembled in the VTS or MB, thus lowering the possibility of operator errors leading to inadvertent criticality. 

Because only a few assemblies reside in the L- and P-Reactor VTSs, their inadvertent criticality probabilities are bounded by the K-Reactor VTS probabUidest... 

Because fewer assemblies are to be removed from L- and P-Reactors VTSs and stored in the HBBS and fewer assemblies as a whole (those presenter in HBBS and those to be placed alter procesdng in VTS) are to be removed from the L- and P-Reactoir HBBS, the inadvertent criticality potential in the HBBS is bounded by that of K-Reactor HBBS. 

Because fewer assemblies are to be ronoved from L- and P-Reactor VTSs, MBs, HBBSs, and transfer bays and placed in shipping casks than in K>Reactor, the inadvertent criticality potential during slupping cask loading is bounded by the K>Reach anatysis. 

This scenario has a frequency of 4.5 x 10" /yr (Ref 8-43). The total frequency of occurrence of an inadvertent criticality during shipping cask loading in the K-Reactor transfer bay from all... 

Conseouences/Damage Associated with Inadvertent Criticality... 

The consequences for an inadvertent criticality in P- and L-Reactor Disassembly Basins are the same as the corresponding inadvertent criticality for K-Reactor Disassembly Batin. 

The primaiy confinement Mure (fiid assembly mechanical damage) is boimded by the inadvertent criticality event (Section 83.23), which rdeases M more radionuclides eluding noble gases) than mechanical damage to any particular fiid assenably or (less likely) any group of assemblies. 

The natural and process acddent analyses that credit this system are those that involve a loss of Disassembly Basin water level control, such as DBE, draindown, overfill, and in vertent criticality. The filtered water makoip system is credited in these events to provide the correct amount of water to the Disass nbly Basin to compensate for any losses, thus minimizing the consequences of these events. If the st is not credited, Welding is reduced and the - -radiological consequences to the co-located worker and the public are increased for the draindown, DBE, and inadvertent criticality scenarios. If the filter wato makeup is not or can not be stopped tty the opmtor due to operator error or equipment Muie an overfill event may occur which could results in the release of contaminated basin water to uncontrolled areas and the process sewer. 

The VTS rows and anchor supports, hangers, trolleys, and Disassembly Basin monorail system are credited in the inadvertent criticality event for preventing a criticality by providing adequate spadng between fiiel assemblies. Also, the structural integrity of this equipment was edited for preventing a credible criticality firom the feilure of the equipment. 

The machine basin cutting saw stop is credited in the inadvertent criticality event for preventing a criticality by ensuring that cuts are not made into the fiiel r on of assemblies thereby eliminating the pototdal for pieces of fiiel assemblies from being inadvertently assembled. 

These monitors are credited by the inadvatent criticality event in order to reduce file dumces of fuel forming a critical configuration. If the monitors were not operable or not used, the chances of inadvertent criticality would increase. 

The criticality bars are aedited in the analysis of the inadvertent criticality event for redudng the possibility of a critical configuration of fiid fix>m bdng assembled. 

The shipping cask is oredited in events involving the transportation aiKl loading of the shipping cask (dropped shipping cask inadvertent criticality within the shipping cask). If the cadmium plates or blanks are not used, the chances for inadvertent criticality increase. 

TSR means the conditions, safe boundaries, and the management, or administrative controls (ACs) necessary to ensure the safe operation of a nuclear facility and to reduce the potential risk to the public and facility workers from uncontrolled releases of radioactive materials or from radiation exposures due to inadvertent criticality. A TSR consists of safety limits, operating limits, surveillance requirements, ACs, use and application instructions, and the basis thereof. 

The procedures and safeguards which have been successful In preventing the attainment of Inadvertent critical loasses with enriched fuel are based on the following criteria. 

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