Neutron exposure

Fig. 3 Transfer NR with Dy(0.1mm) screen and IP UR type Object ENRWG and US standard fuel rods Neutron exposure 7 min at 4.5 10+5/cm2sec (250 kW)...

Radiation information Radiation survey Alpha/beta/gamma/neutron exposure and contamination ... 

The use of this direct oxide reduction process is replacing fluoride reduction as it eliminates neutron exposure to operating personnel (alpha particles from plutonium decay have sufficient energy to eject neutrons from fluorine by the a,n reaction) and eliminates reduction residues which require subsequent recovery. 

Fast neutron exposure determined by counting individual proton recoil tracks. 

The chemical analysis has revealed that rather low C/O ratios are found in metal-poor extragalactic carbon stars, as found for galactic carbon stars of the solar vicinity. Furthermore, the three analyzed stars show similar s-elements enhancements [ls/Fe]=0.8-1.3 and [hs/Fe]=l.l-1.7. This leads to new constraints for evolutionary models. For instance, the derived C/O and 13C/12C ratios are lower than model predictions at low metallicity. On the contrary, theoretical predictions of neutrons exposures for the production of the s-elements are compatible with observations (see Fig. 1). Finally, from their known distances, we have estimated the luminosities and masses of the three stars. It results that SMC-B30 and Sgr-C3 are most probably intrinsic carbon stars while Sgr-Cl could be extrinsic. 

Fig. 1. Heavy over light s-elements ratio versus metallicity. Lines are theoretical predictions for a 1.5M AGB stars with C/0=1.1 and three different choices for the neutron exposure rate (see Busso et al., 2001, ApJ, 557, 802). Black dots are for galactic carbon stars analyzed in Abia et al. (2002, ApJ 578, 817).

In practice, the Szilard-Chalmers reaction has not been very successful for production of substantial quantities of any isotope although it often works well for tracer or low irradiation levels. When greater levels of neutron irradiation became feasible with development of nuclear reactors, it was found that both enrichment factors and product yields decreased as neutron exposures increased to the values required for significant isotope production. 

In the work reported here, which was directed toward the attainment of an isotopic enrichment of the trivalent actinide and lanthanide elements, the problem was compounded by the fact that these elements do not readily form appropriate compounds, like iodine in ethyl iodide. They do form some stable organic chelates, and, indeed, it is possible to obtain a Szilard-Chalmers reaction with such compounds. However, their radiation damage resistance does not appear adequate to permit useful production of an isotope like 247Cm, which requires a thermal neutron exposure ap-... 

Effect of Prolonged Irradiation. Optimum product enrichment requires irradiation to a thermal neutron exposure in the range of 1020 neutrons cm-2, depending on the nuclear properties of the isotope being used. 

Irradiations with PrX zeolite, Table VI, showed that above an exposure of 1019 thermal neutrons cm-2, which is the range of interest, the 142Pr yield decreased, the 141Pr target content increased, and the enrichment factor decreased from >60 to 4. The fast neutron exposure was approximately one-third the thermal. In each test 10 mg of PrX was irradiated, allowed to decay for several days, and then eluted with 2 ml of 7M LiCl. Experimental conditions during these irradiations were not well defined. 

Time of Flux, neutrons Exposure neutrons % of Isotope in Product ... 

At this time it is not clear whether or not the isotopic enrichment described here can be accomplished at a neutron exposure high enough to yield a really useful product, but indications are that a highly thermalized neutron source may yield such products. An alternative development, which would yield a higher enrichment factor or comparable enrichment at lower neutron exposures, would be some means to decrease target elution to much less than 1%. 

The question of the oxygen isotopes is further complicated by the apparent correlation of the isotope ratios with the neutron exposure parameter (as defined by... 

The effect of reactor irradiation on the thermal decompn of Pb styphnate monohydrate was studied by Flanagan (Ref 61). He observed that a total neutron exposure of 2.2 x 10l8n/cm2 in the Brookhaven National Laboratory reactor enhanced the decompn rate by a factor of three as shown in Fig 3 with the decompn rate increasing monotonically with the amount of exposure. Flanagan also noted that a sample decompd 8 days after irradiation produced a decompn curve almost identical with one stored 46 days after irradiation and then decompd. Subsequently Flanagan (Ref 105) showed that the activation energy was significantly decreased... 

Beryllium (Bulk) High after long-term neutron exposure 800-1200K... 

Noble gas measurements were made on bulk samples of four density fractions (Amari et al., 1995b). In contrast to SiC, a substantial fraction of Ne-E in graphite seems to come from the decay of short-lived (T1/2 = 2.6 yr) Na (Clayton, 1975). This is supported by the low " He/ Ne ratios measured in individual grains (Nichols et al., 2003). Krypton in graphite has two s-process components with apparent different neutron exposures, residing in different density fractions (Amari et al., 1995b). 

AGB stars. The heavy noble gases in graphite (cf. Amari et al., 1995), like those in silicon carbide, are evidently of s-process type, but with a wider range of neutron exposure. 

Neutron activation This is the term used for neutron reactions that result in the formation of radioactive nuclides. This type of reaction is used in solid-state use-once devices containing one or more materials that are activated by neutron radiation. The induced activity of each material can be measured and the neutron exposure can then be calculated from these activities. 

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... 

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