The results of neutron transport calculations of the He formation based on the JENDL gas-production cross section file are discussed for some metals and alloys, namely ²⁷Al, Ti, ⁵¹V, Cr, ⁵⁵Mn, Fe, Ni, Zr, Mo, austenitic stainless steel (Ti modified 316 SS : PCA), Ni-base alloy (Inconel 625), ferritic steel (Fe-11Cr-1Mo : HT-9), Ti-base alloy (Ti-6AI-4V) and V-base alloy (V-5Cr-5Ti). Impacts of the two shields having the steel-rich and the H₂O-rich compositions and the two blankets having the Li₂O/Be-base and the liquid Li/Be-base compositions on the He formation rate in the above-mentioned metals and alloys are discussed. The relation between the He formation rate and the fast neutron flux (14.1 MeV>E>0.1 MeV) is investigated. The decrease of He formation at any distance Δ from the first wall more than Δ_as, the distance where the shape of neutron spectrum reaches its asymptotic form, is modelled by the simple formula based on the exponential dependence, as those reported so far for the fast neutron flux and the displacement damage rate.

View full abstract

A reactivity effect due to spatial variation of nuclear fuel concentration is an important problem for nuclear criticality safety in a reprocessing plant. As a method theoretically estimating this reactivity effect, the Goertzel's necessary condition is well known. To investigate this theoretical prediction, we have performed systematically a series of measurements of reactivity effect due to nonuniformity in the fuel distribution using a solid moderated core of the Kyoto University Critical Assembly. Thermal neutron flux distributions have been also measured using the activation method with gold wire. A nonuniform assembly was found to have an excess reactivity of 0.2%Δk/k when it had the same uranium mass as the uniform assembly was exactly critical. The fuel concentration of this nonuniform assembly was 40% higher in the center region than the uniform one. Moreover, the spatial distribution of reaction rate of gold wire due to thermal neutrons was flatter in this nonuniform assembly than in the uniform one, as expected by the Goertzel's condition.

View full abstract

A new homogenization method of control rod assemblies has been developed to accurately calculate control rod worth in fast reactors. In this method, the homogenized cross sections are defined so as to preserve the reaction rate ratio between a control rod region and a surrounding fuel region in a supercell model before and after homogenization. This new method has an advantage that the cross section of adjacent fuel assemblies is not necessary to change. From supercell calculation, this method produced k∞ values which agreed with the reference heterogeneous calculation within 0.2%Δk. The flux weighting method underestimated k∞ up to 1.4% Δk and the reaction rate preservation method did not produced a converged cross section.
In the verification test of core calculation in RZ geometry, we confirmed that the reaction rate ratio preservation method is more accurate than the flux weighting method for control rod worth calculations.
It was found from core calculations that the control rod worth obtained by the flux weighting method is overestimated compared to that by the reaction rate ratio preservation method by about 3%. This overestimation is remarkable for a safety rod with 90 w t % enriched 10B than for a shim rod with 40 wt% enriched 10B.

View full abstract

In high temperature gas-cooled reactors (HTGRs), some amounts of fission products (FPs) are released mainly from fuel with failed coatings and are transported in the primary cooling system with the primary coolant during normal operation. In that case, condensable FPs plateout on the inner surface of components in the primary cooling system. On the other hand, since the HTGRs use helium gas as primary coolant, the primary coolant is not activated itself and very small amount of corrosion products is generated. Then, γ-ray emitted from the FPs becomes main source in shielding design of the HTGRs, and not only release amount from fuel but also plateout distributions of the FPs should be properly evaluated. Therefore, prediction of plateout behavior in the primary cooling system of HTGRs was carried out based on the calculation result of plateout distribution in High Temperature Engineering Test Reactor. Before the calculation, analytical model was verified by comparison with experimentally obtained plateout distributions and the applicability of the model to predict the plateout distributions in the primary cooling system of HTGR was certified.
This report describes the predicted result of plateout distribution in the primary cooling system of HTGR together with the verification result of the analytical model.

View full abstract

In order to evaluate the breached pin behavior in FBR, the breached pin performance analysis code SAFFRON was developed. Based on the results of run-beyond-cladding-breach test in EBR- II as a collaborative program between PNC and U.S. DOE, the following behaviors were taken into cosideration ; fuel sodium reaction product (FSRP) formation, resultant fuel expansion, breach extension of cladding and release of delayed neutron precursors into the coolant. Using 3-dimensional elastic analyses by finite element method, breached pin diameter increase is adequately predicted with the reduced Young's modulus of the breached fuel. The delayed neutron signal response in on-line diagnosis was evaluated in relation to the growth of FSRP and breached area enlargement.

View full abstract

A refabricated commercial PWR fuel rod (active length : 0.12 m) base-irradiated up to burn-up of 39 MWd/kg U and an unirradiated PWR fuel rod were prepared. After loading each test fuel rod into an atmospheric irradiation capsule at the Nuclear Safety Research Reactor (NSRR) in the Japan Atomic Energy Research Institute (JAERI), the rods were irradiated with rapid increase of linear heat rating from 4 to 32 kW/m with ramp rate of 3 kW/ms.
The results obtained are summarized as follows :
(1) A magnitude of pellet-cladding mechanical interaction (PCMI) evaluated by rod axial strain was 0.07% for base-irradiated PWR fuel and 0.08% for unirradiated PWR one. The prevention of fuel rods from failure was partially due to relative low terminal power level (32 kW/m) against defected fuel rod in TRANS RAMP II (60 kW/m) and partially due to the strong PCMI relaxation occurred in UO₂ fuel in the course of rapid power increase. The PCMI decrease by relaxation which occurred during rapid power increase is quite new phenomenon, and may be useful for PWR fuel rods from view point of PCMI failure pre- vention.
(2) Fission gas release (FGR) of the tested PWR fuel rod was 0.17% for base-irradiation and 0.63% for rapid power transient. The principal mechanism for the former may be the recoil/knock out and that for latter may be the micro haircracking in UO₂ fuel. Within this experimental scope, the observed magnitude of FGR was insufficient to induce stress corrosion cracking on the tested fuel rods.

View full abstract

Neptunium mononitride with carbon and oxygen contents of less than 1, 000 ppm was prepared by the carbothermic reduction of Np_O2 in flowing N₂ gas at 1, 823 K, followed by the removal of excess carbon in a N2-H2 mixed gas stream at 1, 723 K. It is suggested from the present result and also the consideration on the chemical form of residual carbon in actinide mononitrides synthesized by carbothermic reduction that the removability of carbon by hydrogen might strongly depend on the solubility of carbon in the nitrides. It was also confirmed that sintering behavior of NpN is similar to that of UN and PuN.

View full abstract

A water chemistry simulator has been developed for quantitative evaluation as one function in a water chemistry diagnosis system. The primary cooling system is divided into 18 regions. The model can handle 29 species. The simulator makes it possible not only to evaluate the short term effect of an anomalous event on water quality, but also to evaluate the long term effect on dose rate around primary cooling pipings. Cation resin intrusion was evaluated as an application of the water chemistry simulator. The decomposition rate constant for the cation resin was experimentally evaluated. When cation resin intrusion of 10 ppb from the condensate demineralizer was assumed, reactor water pH decreased from 7.01 to 5.75. Lowered pH during the following 5, 000 EFPH operation without' corrective actions led to about a 50% increase in the ⁶⁰Co amount on PLR piping as a long term effect.

View full abstract

A comparison between nitrogen and tripled Nd-YAG laser source for Time-Resolved Laser-Induced Fluorescence (TRLIF) of uranium in nitric acid solutions in the framework of on-line feasibility has been performed. Hence, for uranium on-line monitoring with fiber optics, nitrogen laser (337 nm), most usual source in TRLIF has several drawback ; poor beam quality and non negligible nitric acid absorption at 337 nm. Tripled Nd-YAG laser (355 nm) despite lower uranium absorption coefficient has several advantages : no influence of nitric acid absorption, better beam quality, better fiber transmission and solid state technology. No significant difference for fluorescence spectra and lifetimes are observed for both lasers. Limit of detection for direct determination of uranium in nitric acid by TRLIF is in the fig/1 (10^-8M) range for both lasers.

View full abstract

A photochemically-induced valency adjustment method has been studied to remove Np from the mixed nitric acid solutions of Pu and Np in connection with the Purex reprocessing. The valencies of Pu and Np ions were adjusted to be Pu(III) and Np(V) under the initial conditions and their concentrations were 1 × 10^-4 and 1× 10^-3' mol•dm^-3, respectively. The experiments were carried out under the various conditions changing the irradiation intensities of the Hg lamp in the various concentrations of HNO₃. It was found that the rates of the redox reactions of the Pu ions were significantly affected by the irradiated light as well as the acid strength. Under the irradiation of the 0.015 W Hg lamp in 3 HNO₃ solution containing a tenfold excess of a hydroxylamine and hydrazine, more than 95% Pu(III) was oxidized rapidly to Pu(IV) within 10 min irradiation and it remained at the same valency even after the continuous further irradiation.
On the other hand, the irradiation did not change the valency of Np(V) under the conditions studied. These valency conditions, i.e. Pu(IV) and Np( V), are appropriate for separating Np from Pu by the solvent extraction with TBP-n-dodecane.
The present results lead to the conclusion that the photochemical method has a high potential for removing Np from the mixed solution of Pu and Np. The photochemical redox reaction mechanisms of Pu and Np in the nitric acid solution were discussed from the stand-points of the thermodynamic and kinetic considerations related to the variation in their standard electrode potentials of the photo-excited ion species by the light irradiation.

View full abstract

The distribution coefficient (K_d) of Cs on ferrierite decreased with increasing concentration of the coexisting inorganic cations in the order of Na⁺>H⁺>K⁺>NH₄⁺. A linear relationship with a slope of about -1 was obtained between log K_d and log[M⁺] above 0.1 M (=mol/dm³). The retention volume (V_R) of Cs in chromatography also decreased in a similar manner to Kd, depending on the kind of inorganic cations. The V_R value can be predicted from the K_d value based on the linear relation. The column efficiency was improved with fine particles of ferrie-rite, yielding the elution percentage above 95%.

View full abstract

Monte Carlo calculations were performed to estimate the effective doses E for external photons based on the recent proposals in the ICRP 1990 Recommendations. Twelve photon energies between 17 keV and 8.5 MeV and eleven irradiation geometries were selected to be applicable to many exposure conditions encountered in the working and living environment. A MIRD-based unisex phantom was used in conjunction with the Monte Carlo transport code MORSE-CG. The results were presented as a form of dose conversion coefficients transforming the air kerma or fluence in free air to the effective dose. These coefficients were given in graphical and tabular forms. Analyses of organs' fractional contribution to E showed that the gonadal exposure has generally a great contribution for all the geometries except the incidence from above. A comparison of conversion coefficients with other results gave a reasonably satisfactory agreement. Finally, the ambient dose equivalent H^*(10), one of the operational quantities proposed by the ICRU, was found to give conservative estimates of E for most of the irradiation geometries considered, but it gives a considerable overestimate of E at the incidences from above and below.

View full abstract

In some postulated accidents with coincident loss of emergency coolant injection of the Advanced Thermal Reactor (ATR), the rate of heat transfer to the heavy water moderator that acts as heat sink for the decay heat depends on the contact conductance between cladding and pressure tube, and the same between pressure and calandria tubes. Experiments were per-formed to assess these contact conductances for clean plates and plates with simulated crud of Fe₂O₃ powder that is the main ingredient of the crud, and the applicable correlations were also studied. Test specimens were cut from actual pressure tube made of Zr-2.5%Nb and calandria tube made of Zircaloy-2 and flattened to sizes. The artificial waviness of various kinds of height and wave length of 10 mm was machined on the surface of the pressure tube specimen. The ranges of contact pressure, roughness, specimen temperature and gas pressure were from 0.5 to 7 MPa, 4.8 to 100 pm, 400 to 840 K and 0.001 Torr to atmospheric respectively. The experimental results without crud compare favorably with Tachibana's correlation. The experi-mental contact conductances with crud are found to be lowered. Since Tachibana's correlation was not applicable to the present situation, the empirical correlation was proposed. The correlation fitted well with the experimental values.

View full abstract

A water wall type passive containment cooling system with an outer pool surrounding the suppression pool is one passive containment cooling system. In the system, a baffle plate in the suppression pool mitigates thermal stratification formed at the vent tube outlet level and enlarges the heat transfer area. The effectiveness of the baffle plate in mitigating thermal stratification was experimentally confirmed; the heat transferred to the outer pool increased about 50% due to a larger high temperature region and a longer effective heat transfer length. The experimental analysis was performed using a three-dimensional thermal-hydraulic analysis program. In the analysis, a laminar flow model and slip conditions on structural walls were used, and the calculated temperature profiles and natural circulation flow rates along the baffle plate agreed with measurements. The model was then judged as a valid and practical tool to evaluate global natural circulation and temperature distributions in a large pool. And it was analytically comfirmed that the thermal resistance of the PCV wall and the heat flux to the outer pool affected the performance of the baffle plate.

View full abstract