Neutron poisons

Large quantities of fissile isotopes, U and U, should be handled and stored appropriately to avoid a criticahty hazard. Clear and relatively simple precautions, such as dividing quantities so that the minimum critical mass is avoided, following adniinisttative controls, using neutron poisons, and avoiding critical configurations (or shapes), must be followed to prevent an extremely treacherous explosion (246). 

BWRs do not operate with dissolved boron like a PWR but use pure, demineralized water with a continuous water quality control system. The reactivity is controlled by the large number of control rods (>100) containing burnable neutron poisons, and by varying the flow rate through the reactor for normal, fine control. Two recirculation loops using variable speed recirculation pumps inject water into the jet pumps inside of the reactor vessel to increase the flow rate by several times over that in the recirculation loops. The steam bubble formation reduces the moderator density and... 

Table 6.1 shows some other best-fit parameters to Solar-System s-process abundances. The seed nucleus is basically 56Fe light nuclei have low cross-sections (but can act as neutron poisons , e.g. 14N for the 13C(a, n) neutron source), whereas heavier nuclei are not abundant enough to have a major influence. Certain nuclidic ratios, e.g. 37Cl/36Ar and 41K/40Ca, indicate that under 1 per cent of Solar-System material has been s-processed. 

Fig. 8.35. According to their calculations, the efficiency of the s-process increases sharply as the metallicity decreases to about 0.1 solar, but thereafter decreases rapidly at still lower metallicities because of shortage of iron seeds and a relatively high amount of neutron poisons such as C, N and O. Thus below [Fe/H] = — 1 or so, heavy nuclei like Ba are predominantly due to the r-process and are assumed to track europium.

The fission neutrons at birth have energies of approximately 1 to 2 MeV In a thermal reactor the neutron energy is rapidly reduced through collisions with light nuclei to thermal (—.02 to 1 eV), to promote for more efficient capture. Besides the nuclear fuel, there are many other materials in the reactor core also competing for the neutrons, including the moderator (the material used to slow down or thermalize the neutrons), fertile nuclides that produce additional fissile material (discussed in a later section), neutron poisons present in control rods, the coolant, fuel element cladding, and other structural materials. 

Since 233Pa is a neutron poison, (3) a significant waiting period may be necessary before the reprocessed fuel would be returned to the reactor. Separation of the 233Pa would allow a quick return to the reactor, but would lead to a pure 233U product, and thus increase the proliferation risk. 

Another important objective is to follow the changes in reactivity that take place as fissile nuclides are depleted or formed from fertile nuclides, and as neutron poisons are formed through buildup of fission products or burned out through reaction with neutrons. 

Height of mixer-settler limited to 7.6 cm. Not amenable to efficient neutron poisoning for criticaUty control. 

Built of stainless steel containing a neutron poison such as gadolinium or boron. 

The main part of the HLLW is aqueous raffinate from the Purex cycle. It contains 99.9% of the nonvolatile FPs, <0.5% of the uranium, <0.2% of the plutonium, and some corrosion products. For each ton of uranium reprocessed about 5 m of HLLW is produced. This is usually concentrated to 0.5-1 m for interim tank storage specific activity is in the range 10 GBq m. The amounts of various elements in the waste and their concentration in 0.5 m solution is shown in Table 21.9. The HNO3 concentration may vary within a factor of 2 depending on the concentration procedure. The metal salt concentration is 0.5 M it is not possible to keep the salt in solution except at high acidity. The amounts of corrosion products, phosphate, and gadolinium (or other neutron poison added) also may vary considerably. Wastes from the HTGR and FBR cycles are expected to be rather similar. 

Third, Pa (protactinium), which occurs in the transmutation chain for the conversion of thorium to acts as a power history dependent neutron poison in a thorium-fueled nuclear reactor. There is no isotope with comparable properties present in a fuel stem. 

The design has three principal negative characteristics. The first one is that the reactor power tends to strongly increase when the cooling water inventory in the reactor decreases the cooling water is a neutron poison . 

Once the scram button was pressed, the reactor power started to increase strongly because of the aforementioned characteristic of positive scram and because of the progressive increase of the amount of steam in the reactor (caused by the increase of thermal power) and because of the corresponding decrease of water (the water in this reactor is a neutron poison). 

On the other hand, the core of a reactor may die from only two illnesses only the lack of water and the lack of neutron poisons for the shutdown of the chain reaction. The first case has happened in TMI-2. 

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