Neutron absorbers criticality

CP-1 was assembled in an approximately spherical shape with the purest graphite in the center. About 6 tons of luanium metal fuel was used, in addition to approximately 40.5 tons of uranium oxide fuel. The lowest point of the reactor rested on the floor and the periphery was supported on a wooden structure. The whole pile was surrounded by a tent of mbberized balloon fabric so that neutron absorbing air could be evacuated. About 75 layers of 10.48-cm (4.125-in.) graphite bricks would have been required to complete the 790-cm diameter sphere. However, criticality was achieved at layer 56 without the need to evacuate the air, and assembly was discontinued at layer 57. The core then had an ellipsoidal cross section, with a polar radius of 209 cm and an equatorial radius of309 cm [20]. CP-1 was operated at low power (0.5 W) for several days. Fortuitously, it was found that the nuclear chain reaction could be controlled with cadmium strips which were inserted into the reactor to absorb neutrons and hence reduce the value of k to considerably less than 1. The pile was then disassembled and rebuilt at what is now the site of Argonne National Laboratory, U.S.A, with a concrete biological shield. Designated CP-2, the pile eventually reached a power level of 100 kW [22]. 

Plutonium is the most important transuranium element. Its two isotopes Pu-238 and Pu-239 have the widest applications among all plutonium isotopes. Plutonium-239 is the fuel for nuclear weapons. The detonation power of 1 kg of plutonium-239 is about 20,000 tons of chemical explosive. The critical mass for its fission is only a few pounds for a solid block depending on the shape of the mass and its proximity to neutron absorbing or reflecting substances. This critical mass is much lower for plutonium in aqueous solution. Also, it is used in nuclear power reactors to generate electricity. The energy output of 1 kg of plutonium is about 22 million kilowatt hours. Plutonium-238 has been used to generate power to run seismic and other lunar surface equipment. It also is used in radionuclide batteries for pacemakers and in various thermoelectric devices. 

The dissolver solution is treated with chemicals to adjust the acidity, valence, and concentrations of the species involved. The HNO3 concentrations are 2-3 M, the U02(N03)2 concentrations are 1 -2 M, and the Pu is stabilized as Pu(IV) using N2O4 or hydroxylamine. In these and subsequent manipulations of these solutions, attention must be given to criticality control. This is done by regulating the solution geometry, the concentrations of fissile materials, and by the addition of neutron absorbers such as Gd. 

The fuel is transported from the reactor pools to Sellafield in MEBs contained within heavily shielded, high integrity, transport flasks. The MEBs are cylindrical stainless steel vessels containing stainless steel clad "Boral" or boronated stainless steel dividers between the fuel assemblies to prevent criticality. "Boral" consists of boron carbide particles in an aluminimn matrix clad with pure aluminimn and is widely used as a neutron absorber. MEB s are used to contain mobile contamination from crud or spalling smface layers of the fuel pins. 

Use of europium oxide as a neutron absorber in the control rods avoids gas generation under irradiation and gives a slower loss of reactivity with neutron exposure than boron carbide (10%). The main problems are obtaining adequate critical nuclear... 

The changes in fuel composition just described cause the reactivity of the fuel to decrease with increasing bumup. The reactivity is defined as the difference between the rate of neutron production by fuel and the rate of neutron consumption, divided by the rate of neutron production. If the reactivity is zero, the reactor will be just critical without insertion of control poisons if the reactivity is negative, the reactor power will die out if the reactivity is positive, the reactor can be brought to a steady power level by insertion of sufficient neutron-absorbing control poison to reduce its reactivity to zero. Figure 3.4 shows how the reactivity of a PWR whose fuel composition is spatially uniform decreases with bumup. Lines are plotted for four different initial fuel compositions 2.8, 3.2, 3.6, and 4.0 w/o To a rough approximation, reactivity decreases linearly with bumup and increases linearly with w/o in fuel at the start of irradiation. 

Soluble neutron absorbers. The preceding limits on critical concentration or dimensions can be greatly relaxed when soluble neutron absorbers, such as boric acid or gadolinium nitrate, are... 

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The plant starts up by heat entering from the primary pump and the system temperature rises to 350°C from the cold shutdown state. Under this condition, all parts of the system, including the recirculation line in the water system, are uniformly heated. Then, a neutron absorber at the center of the core is withdrawn. At temperatures below 350°C, the neutron absorber cannot be withdrawn by the self-connected mechanism using the thermal expansion difference between the stainless steel and Cr-Mo steel (Fig. 14). After withdrawal of the neutron absorber, the reflector is lifted up by the hydraulic system to reach critical condition at 350 C. A ficzy control system is employed for this approach and a fully automatic operation circuit is provided because no malfunction causes severe reactivity insertion as described previously. 

Contains about 20 wt% of neutron absorbers, Hf02 and Gd203, which are used for criticality control ... 

Only the coohmt has to be circulate, and the same tope free from fission fragments. A schedule of replace-circulation cools the reaction and absorption zones. No 65 ment can readily be worked out to keep the reactor gas is evolved from the reaction zone and poisoning fac- operating with minimum shutdown times, in accordance tors, due to retention of fission product neutron absorb- with the power output and resultant use rate of the ers in the reaction zone, do not become critical because plutonium. 

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 First Shutdown System (FSS) is designed to shut down the core, when abnormal or deviated from normal situations occur, and to maintain the core sub-critical during all shutdown states. This function is achieved by dropping the neutron-absorbing elements into the core by the action of gravity when the water flow in the CRD mechanism is interrupted, so malfunction of any powered part of the hydraulic circuit will cause the immediate shutdown of the reactor. Six out of twenty-five absorbing elements are part of the Fast Extinction... 

Neutron absorbers are sometimes employed in the packaging to reduce the effect of moderation and the contribution to the neutron multiplication resulting from interaction among packages (see para. 501.8). Typical neutron absorbing materials used for criticality control are most effective when a neutron moderator is present... 

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