The average number of elastic collisions necessary for the slowing down of neutrons was derived from the moment generating function. The exact expression obtained by an algebraic operation program REDUCE 3.3 was complex for media with A>1, mass number A, and average logarithmic energy loss ξ. When neutron energy was reduced from E0 to E, an approximate expression was <N>=_0.7+ξ-1In(E0/E).

View full abstract

A prototype high energy X-ray CT (computed tomography) system has been developed which employs a linear accelerator as the X-ray source (max. photon energy : 12 MeV).
One problem encountered in development of this CT system was to reduce the scattered photons from adjacent detectors, i.e. crosstalk, due to high energy X-rays. This crosstalk was reduced to 2% by means of detector shields using tungsten spacers. Spatial resolution was not affected by such small crosstalk as confirmed by numerical simulations. A second problem was to reduce the scattered photons from the test object. This was done using collimators. A third concern was to realize a wide dynamic range data processing which would allow applications to large and dense objects. This problem was solved by using a sample and hold data acquisition method to reduce the dark current of the photo detectors. The dynamic range of this system was experimentally confirmed over 60 dB. It was demonstrated that slits (width : 2 mm) in an iron object (diameter : 25 cm) could be imaged by this prototype CT system.

View full abstract

A new unfolding theory has been established on the basis of the maximum entropy principle and the maximum likelihood method. This theory correctly embodies the Poisson statistics of neutron detection, and always brings a positive solution over the whole energy range. Moreover, the theory unifies both problems of overdetermined and of underdetermined. For the latter, the ambiguity in assigning a prior probability, i.e. the initial guess in the Bayesian sense, has become extinct by virtue of the principle. An approximate expression of the covariance matrix for the resultant spectra is also presented. An efficient algorithm to solve the nonlinear system, which appears in the present study, has been established. Results of computer simulation showed the effectiveness of the present theory.

View full abstract

Numerical model calculations and evaluation of plant experience with chemical behaviors of radioactive nitrogen (¹³N and ¹⁶N) in BWR primary systems have suggested the followings :
(1) Calculated results showed that the major species of radioactive nitrogen released to the turbine system was nitrogen mono-oxide.
(2) Based on ¹³N measurement in the main steam of a BWR, 85% of ¹³N was found to be anion. This result supported the prediction above.
(3) Calculated results showed that chemical behaviors of ¹³N and ¹⁶N were not the same because of the difference in half-lives of ¹³N and ¹⁶N; ¹³N was easily accumulated in the core by-pass channels where the residence time of water was several times longer than in fuel channels, but the main source of ¹⁶N was the boiling channels in the reactor core.
(4) Nitrogen-16 released to the main steam line could be decreased by lowering the hydrogen atom concentration in the water ; typically to around 1/10 of the normal level by the addition of nitrous acid of 1×10^-6M at the reactor core inlet.

View full abstract

Measurement of the distribution ratios of Pu(IV), U(VI) and HNO₃ at low temperatures and its treatment with DIST code revealed that a high U(VI)-loading of 30% TBP in n-dodecane splits Pu(IV) down to the aqueous phase more strongly than do at 25°C. Based on these findings, flowsheet conditions to separate Pu (IV) from U(VI) were investigated with EXTRA. M code including the distribution equations obtained above. And tentative flowsheets for non-reductive Pu-splitting process at a temperature of 5°C were proposed for fuel reprocessing mainly based on the effects of U(VI)-loading in the solvent and temperature on distribution ratios of Pu(IV) and U(VI). Distribution ratios of the fission products, Zr, Nb, Ru and Ce were also measured to assess their decontamination from U or Pu products in the above process. Finally behavior of Np in the proposed partitioning process was discussed by analysis with EXTRA. M code and a redox reaction model.

View full abstract

The sorption behavior of Cs and Sr ions on natural clay minerals such as sand, bentonite and loam, and mixtures of bentonite-sand and -loam was studied under static condition.
It was found that the distribution ratio of Sr ion on bentonite and loam are influenced by H ion concentration of solution, along with the concentration of ion of interest. The obtained results revealed that the distribution ratio of ion on the clay mixture can be expressed in terms of distribution ratio of each constituent of the mixed sorbent, provided that the corrections for pH and sorbed ion concentration of solution are made.

View full abstract

Potentials of inert metals immersed in nitric acid solutions, which are used in the reprocessing plants of spent nuclear fuel, have been considered to be useful as an index of redox properties of nitric acid solutions in which the various ions change their valency. We measured the immersion potentials of gold and platinum electrodes in (111)×10³ mol• m³ nitric acid solutions at 25 and 75°C with and without the Ce⁴⁺/Ce³⁺ ion couple. Linear sweep voltammetry was carried out for these electrodes. The obtained potentials of the gold electrode were lower by about 0.15 V than the platinum potential for the corresponding conditions. The immersion potential of platinum was increased when the Ce⁴⁺/Ce³⁺ was added to the solution. The immersion potential of gold stayed constant when the Ce⁴⁺ concentration was less than 10 mol• m^-3, while it increased when Ce⁴⁺ concentration was increased above that value. The voltammetry results suggested that there were surface species formation and water oxidation, including an adsorption process on the metal, occurring when gold and platinum were immersed, respectively. We concluded the immersion potentials were a mixed potential of these reactions and the redox reaction of ion couples in nitric acid solution.

View full abstract

The long-lived radioactivities of structural materials induced by recoil protons in BWR were estimated for land disposal of low level waste after reactor decommissioning. Reaction products of interest are ⁵³Mn, ⁹¹Nb, ⁹⁴Nb, ⁹⁷Tc, ¹²⁵Sb, ¹⁷³Lu and ¹⁷⁴Lu. A method of calculating the proton spectrum in materials was presented. The program PEGASUS-P was developed by modifying the PEGASUS, a preequilibrium and multistep evaporation theory code, to calculate proton reaction cross sections. The proton-induced activities in stainless steel, Inconel and Zircaloy-2 were calculated under typical irradiation conditions in an operating BWR. It was shown that even the most dominant activity due to ⁹¹Nb from Zircaloy-2 did not exceed 1/1, 000 of that of a typical neutron induced activity of ⁶³Ni for cooling up to 1, 000 yr after the irradiation of 40 yr. The results are believed to hold for the case of PWR as well.

View full abstract

Measurements of certain integral parameters such as transmitted biological dose, secondary r-ray buildup are essential for determining the effectiveness of shield materials. Data on such parameters for gadolinium as an additive in concrete is very sparse in literature. The present work deals with integral measurements as a function of shield thickness for gadolinium loaded concretes and polypropylene. The biological dose was measured using a rem counter. The dose transmission factor reduces as much as 30% in heavy concretes with 1% gadolinium by weight which shows its effectiveness as an additive in concretes. The results also indicate that even in regions where secondary neutrons have not attained complete spectral equilibrium, the integral parameters have nearly exponential attenuation.

View full abstract

Cement-glass, which is a mixture of sodium silicate (kNa₂O•mSiO₂•nH₂O), silicon phosphate (P₂O₅•2SiO₂) and cement, was developed to solidify radioactive waste pellets in containers. The optimum molar ratio of Si(OH)₄, NaOH and P₂O₅•2SiO₂ was found to be 2:2:1, based on compressive strength measurement of solidified samples. The compressive strength of solidified sodium silicate with silicon phosphate was increased when the amount of solidified SiO₂ per unit volume was increased by reducing the water content. Cement-glass had a higher Cs distribution factor than ordinary Portland cement.

View full abstract