Initial Criticality

US Food and Drug Administration. Challenge and Opportunity on the Path to New Medical Products, March, 2004, http //www.fda.gov/oc/initiatives/critical-path/whitepaper.html... 

Executive Action - Control process performed to initiate critical instructions or signals to safety devices. 

About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation (erythropoiesis). This storage iron is mostly in the form of ferritin, like bacterioferritin a 24-subunit protein in the form of a spherical protein shell enclosing a cavity within which up to 4500 atoms of iron can be stored, essentially as the mineral ferrihydrite. Despite the water insolubility of ferrihydrite, it is kept in a solution within the protein shell, such that one can easily prepare mammalian ferritin solutions that contain 1 M ferric iron (i.e. 56 mg/ml). Mammalian ferritins, unlike most bacterial and plant ferritins, have the particularity that they are heteropolymers, made up of two subunit types, H and L. Whereas H-subunits have a ferroxidase activity, catalysing the oxidation of two Fe2+ atoms to Fe3+, L-subunits appear to be involved in the nucleation of the mineral iron core once this has formed an initial critical mass, further iron oxidation and deposition in the biomineral takes place on the surface of the ferrihydrite crystallite itself (see a further discussion in Chapter 19). 

Livers from toxin-injected mice showed severe congestion whether or not the mice survived the initial critical 2-hour post injection period. Occasionally, a mouse injected with a nominally lethal dose of toxin-LR survived the critical two hours but remained listless until it died several hours or several days later. The reasons for the delayed death are unknown. Such mice developed focal fatty degeneration of the liver and active regeneration of liver cells. 

However, the contribution of this mechanism to the toughness is mainly to crack initiation critical value (J0.2). the crack propagation resistance dJ/dAa. 

CA 64, 12453(1966) (Equation of State and Derived Shock Initiation Criticality Conditions for Liquid Explosives)... 

The first step in the theoretical procedure is to iteratively solve the reaction equilibrium problem for composition of the equilibrium reaction mixture assuming that the reactant phase is initially critical or supercritical. Critical properties of the reaction mixture and fugacities are then estimated. Operating temperature and pressure are then re-evaluated such that the reaction mixture is constrained to the supercritical region. The iterative process is continued until satisfactory convergence is achieved. 

Partial crystallization, small crystal sizes, impurity and copolymer content, and straining of molecules from the random coil conformation are the major effects to be considered. The changes of heats of transition and temperatures of transition and their use to characterize the non-equilibrium state have been given an initial critical survey (2) ... 

This initial criticism led to an increase in the data for organic molecules in swarm... 

JAEA has developed the HTTR, a 30 MWt engineering test HTGR [ 1]. Since the initial criticality... 

We assume that the velocity and pressure fields are axisymmetric even in the disturbance fields. Although comparison with experiment shows this assumption to be valid for the linear perturbations that are unstable, it eventually breaks down far enough beyond the initial critical point in terms of the cylinder rotation rates. The corresponding linearized [i.e., 0(e)] equations of motion and continuity in dimensional form are... 

At the time of the attack, there were no established measures to deal with chemical weapons, decontamination was not undertaken, and no antidote was administered at the sites of the attack. During the initial critical hours when doctors could have played an important role, none were present at the sites of the attack. At the request of the Tokyo Fire Department, a total of 73 medical staff - 47 doctors, 23 nurses and three clerks - were dispatched to 4 subway stations for triage and emergency treatment (Chigusa, 1995). However, because they reached the stations after 9 00 am, they did not provide emergency medical treatment such as tracheal intubation. 

Covalent binding of the carcinogen to DNA is believed to be the initial critical step in chemical carcinogenesis. Thus, measurea nt of carcinogen-DNA adducts should be a more relevant marker of exposure to carcinogens than measurement of the chemical itself in the environment or body fluids. Such assays take into account individual differences in absorption and metabolism of carcinogens as well as repair of adducts once they are formed. In addition to being a more relevant marker of exposure, it is hoped that such measurements will be useful as markers of individual risk for development of cancer. 

PRE STE REA SSURIZER AM GENERATOR (PRIMARY COOLANT SIDE) CTOR VESSEL PRIOR TO INITIAL CRITICALITY. CTOR VESSEL AFTER START OE LOW POWER HYSICS TEST (t = a +45T) CTOR VESSEL AFTER START OF POWER ANGE OPERATIONS + 40T (10 CFR 50, APPENDIX G ... 

First, a higher fissile material loading requirement exists for initial criticality for a given reactor system and fissile fuel when thorium is used than is the case for an otherwise comparable system using... 

Monju is Japan s prototype fast breeder reactor 280 MWe(714MWt), fueled with mixed oxides of plutonium and uranium, cooled tty liquid sodium. Construction was started in 1985 nd initial criticality was attained in April 1994... 

Fig. 8. Critical current as a function of strain in NbaSn bronze composites manufactured by the bronze-diffusion technique [ ]. Critical current values have been normalized to the initial critical current Iq. Curves 1, 2 (nearly identical), and 3 were obtained at 7 T for a specimen with 3553 filaments, each 3.6jLcm in diameter, NbaSn bronze ratio of 0.2. Curve 4 is from Easton and Schwall [ ] and was obtained at 7 T for a specimen with 24,781 filaments, each 4/xm in diameter, NbaSn bronze ratio of 0.2. Curves 5, 6 and 7 are from Rupp [ ] and were obtained at 4.4 T for specimens with 1615, 61, and 61 filaments, each 1.3,4.0, and 1.0 fim in diameter, and having a NbsSn bronze ratio of 0.22, 0.21, and 0.06, respectively. Curve 8 was obtained at 7T for a specimen containing 24,871 filaments, each 2.6 um in diameter, with a NbsSn bronze ratio of 0.26 [ ].

The experimental fast reactor JOYO at the Japan Nuclear Cycle Development Institute s Oarai Engineering Center attained initial criticality in April 1977 and was the first liquid metal cooled fast reactor in Japan. From 1983 to 2000, JOYO operated with the MK-II core as an irradiation test bed to develop the fuels and materials for future Japanese fast reactors. Thirty-five duty cycle operations and thirteen special tests with the MK-II core were completed by June 2000 without any fuel pin failures or serious plant trouble. The reactor is currently being upgraded to the MK-III core. This paper provides a review of the operational experiences obtained through the JOYO s operation. 

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