Hydrogenous moderators

Individual sources of 252Cf are currently available in quantities up to 10—50mgs of the element. Such sources, when contained in a suitable hydrogenous moderator, can provide thermal neutron outputs adequate for the activation analysis of many elements in the fraction of a percent wt range... 

Transition metal complexes with chiral phosphorous and nitrogen ligands have also been used for promoting asymmetric transfer hydrogenation. Moderate to good results have been obtained.114... 

Fast neutron detection sometimes uses a hydrogenous moderator to slow down the neutrons and then employs a low-energy neutron detector as described above. One common fast neutron detector is a Bonner sphere. In this detector, a scintillator is placed in the center of a polyethylene sphere. Radiation transport calculations are used to produce efficiency curves that depend on the energy of the incident neutron. Another common fast neutron detector is a long counter. This detector uses a slow neutron detector (originally a BF3 chamber) at the center of a cylindrical moderator designed so that the detector is only sensitive to neutrons incident from one side. 

Decomposes with water, lower alcohols, and carboxylic acids to form hydrogen moderately powerful condensing agent with ketones and acid esters more powerful reducing agent toward metal oxides than lithium or sodium hydrides.1... 

The foregoing facts fairly summarize the information which was available to us when we undertook the investigations to be reported below. Earlier or simultaneous work on resonance absorption includes, first of all, Bethe s calculation of the absorption of uniformly distributed material in a hydrogeneous moderator. Bethe s results were adapted to the particular case of resonance absorption by uniformly distributed (i.e., not lumped) uranium and generalized to other moderators in a classified report by J. A. Wheeler (A-88). We aid not learn about this report until some time in 1943. In addition, both J. Fisk and W. Shockley, and also Eckart, treated the resonance absorption of uranium lumps embedded in a moderator. We still know very little about Fisk s and Shockley s work. Eckart s work... 

Abstract. The resonance escape probability, p, in a lattice is expressed in terms of the resonance absorption integral (f era dEjE)efi and the spatial distribution of resonance neutrons. It is shown that the dependence on the spatial distribution is very nearly the same whether the slowing down in the moderator is Gaussian or exponential. The dependence on (f a a dEjE)ef( is rigorous only for a hydrogen moderator for a heavy moderator the formula overestimates the resonance escape. 

The safety considerations underlying the three exceptions in para. 672(a) are based upon the assumption of hydrogenous moderation and reflection thus a restriction on the presence of the potentially more effective elements beryllium and deuterium is applied. 

D. Hydrogenous moderators. Hydrogen may be used as a moderator in the form of water, organic liquids, plastics such as polyethylene, or metallic hydrides. In considering the thermalization of neutrons each of these chemical forms must be treated separately. The only two forms which have been studied in any detail are water and zirconium hydride [21]. Since these two cases represent extreme limits in the effect of chemical binding on the neutron spectrum, they will be discussed in this paper. The case of water will be treated in this section and zirconium hydride in the following section. 

G. Goertzel, Criticality of hydrogen moderated reactors, TAB-53, July, 1950. 

D. S. Selengut, Critical mass calculations for hare hydrogen moderated recuitors by means of transport theory, APEX-121, September, 1952. 

The semi-log plot of P as a function of thickness d is Knear both for graphite and for hydrogenous moderators, that is, the following simple equation... 

J.J. Lynn, J. T. Mihalczo, W. C. Connelly, Homogeneous Hydrogen Moderated Critical Assemblies with 2 Per Cent U Enriched Uranium, ORNL-2609 (1958). 

BIDINGER, G. H., et al., "Nuclear Safety Experiments on Plutonium and Enriched Uranium Hydrogen Moderated Assemblies Containing Boron, RFP-201 (Oct. 13, 1960). 

A series of experiments with hydrogen-moderated plutonium systems has been performed in the Physical Constants Testing Reactor (PCTR). The boron concentration required to reduce the infinite multiplication factor, k , to unity was measured for various Pu-240 concentrations, fuel plate thicknesses, and moderator-to-fuel ratios. The measured values of the boron concentra-Uon were compared to theoretical predictions to check the accural of calculational methods and parameters. The principal components for these experiments were Pu-Al (M wt% Pu), pure polyethylene, and borated lyetlqrlene (0.90 wt% natural boron). All three materials were in the form of disks, 1.960-in. in diam and 0.020-in. thick. 

A series of experiments and complementary calculations using the KENO program on boOi bare and thick-polyethylene-reflected, graphite-moderated cubic arrays of 8 nominally 15.7 kg 17(93.2) metal cylinders, has been started. Data from these experiments supplement that on unmoderated and hydrogenously moderated arrays of the same metal cylinders, and so will be valuable in the analysis of the effects of array moderation and reflection. Also, the data provide valuable check points lor validation of calculational techniques, such as the KENO program, that are used in nuclear criticality safety analysis. This summary presents data calculational results on the first several ejqperlmentsiin the series. 

These results extend the recent calculational results of Thomas on arrays of 93.2 wt% enrichment uranium cylinders. The conclusion reached by Thomas, that as little as 0.125 in. of steel surrounding the individual units of a reflected array is sufficient to maintain subcriticality independent of the amount and distribution of added interspersed hydrogenous moderator, appears valid for subcritical arrays of 3.5 wt% UO2 pellets as well. 

Note that the inconsistent treatment of the metal cube data suggests the critical mass value should not be the unreflected cube but that for the condition with associated reflector. The solution data reported by Odegaarden and Stevenson appear to associate well with the data for U(93) metal spheres. The sphere data also represent the 44 different fissile materials given In the ANSI Standard N16.5. It is clear that an area of applicability can be defined. However, the effect on the criticality of these systems of Introducing interstitial low-density hydrogenous moderation needs to be examined before firm limits are proposed. 

In -the moderation-control method for S% enriched UFii, criticality safety is assured by control of two parameters- the U enrichment and the hydrogen moderation. Topical subjects to be reviewed in method development include UF , purity control in liquefaction operations the Influence of container size on internal and external moderntion effects decoupling properties of the cylinder wall In water-moderated UFa arrays special neutron multiplication experiments with large UFo assemblies derivation of moderation control limits large cylinder cleaning procedures and protective packaging. The administrative requirements for method application are al.so discussed briefly. 

E. F. PLECHATY, Monte Carlo Calculations of Hydrogen Moderated Uranium Systems Using the Probability Table Method and Multigroup Cross Section Representations, UCID-17455, Lawrence Livermore Lab. (1977). 

T yo tanks were added in 1964, for use with Pu solu-dohs. They were separated by 0.2 m of hydrogenous moderator and Cd sheets. They were placed in the experimental cell against a thick concrete wall, from which they were separated by. 0,1 m of moderator. No moderator was placed against the large face of the room-side tank. 

Figure 1 The neutron flux distribution for three different moderators at the ILL reactor and for the liquid hydrogen moderator at the ISIS accelerator. The accelerator flux distribution is adjusted by a fector 10 to represent the increased efficiency for time-of-flight experiments due to the pulse structure. Modified with permission from Price DL and Skdid K (1986) Introduction to neutron scattering in Neutron Scattering, Part A, Skdid K and Price DL (eds). Orlando Academic Press.

Residence times of up to 1 h are typical, with the continuous addition of monomer, catalyst, and fresh diluent. Productivity can exceed more than 10,000 lb of polymer per poimd of catalyst. The use of low concentrations of catalyst allows the grade of the product to be changed within a few hours. The average molecular weight of the product can be controlled by the type of catalyst used, the activation procedure, operating conditions, and by changing the partial pres-sitre of the hydrogen moderator. 

Y. Oka, T. Jevremovic and S. Koshizuka, Negative Void Reactivity in Large FBRs with a Hydrogeneous Moderator, Transactions of American Nuclear Society, Vol. 68, Part A, 301-303 (1993)... 

---