Sodium Fires and Aerosols

Release of ¹³¹I associated with burning of contaminated Na, decay of airborne concentration of the released ¹³¹I and the size distribution of aerosols referred to radioiodine are investigated in experimental runs conducted on laboratory scale. These investigations are carried out in conjunction with similar investigations for the Na matrix. An experimental chamber (5.4 m³) is used for burning a small pool of Na (–50 g) containing spiked ¹³¹I (40–500 μCi). Values of the specific activity ratio, viz. the ratio of the concentration of ¹³¹I in the aerosol Na to that in pool Na lay in the range of 0.9×10^?2—5.7×10^?2 at pool temperatures of about 300°C. The concentration decay half-time and the aerosol size distribution characteristics referred to ¹³¹I remain similar to those applicable to the Na content of the aerosols. Surface concentrations of ¹³¹I in the residues examined differ from the bulk concentrations (calculated) in the pool and are lower by a factor of ?20. The concentrations of ¹³¹I in the aerosols released are further lower than the surface concentrations in the residues by a factor of ?2.     

Behavior of Sodium Oxide Aerosol in a Closed Chamber

The behavior of sodium oxide aerosol in a closed chamber was studied for the safety analysis of a Na-cooled fast reactor. The experimental apparatus and techniques are first described. The aerosol was released during a short time by blowing air onto heated Na in a 1 m³ chamber. The maximum mass concentration of the aerosol in the form of Na₂O ranged of 0.05 ～ 10g/m³. The particle size distribution, the aerosol mass concentration and the mass deposition rates were measured as a function of time.

It was found that the mass median diameter of the aerosol was related to the maximum mass concentration. To determine the character of the behavior of sodium oxide aerosol in the chamber, the density of the aerosol material and the thickness of the boundary layer through which the particles deposit on the chamber wall were observed. The initial half-time of the aerosol mass concentration was compared with the values numerically calculated under certain assumptions.     

Description of Aerosol Release Associated with Sodium Burning

Incipient temperatures of sodium oxidation, sodium oxide aerosol release and sodium ignition were observed in relation with oxygen concentration in supplying gas by a thermobalance method. On this results, a burning rate and an aerosol release fraction were determined by burning about 0.5 g of sodium samples on a crucible in a burn tube under conditions ranging 400 to 600°C in sodium temperature, 0.1 to 1.0 Nl/min in the supplying gas flow rate and 2.5 to 20v/0 in the oxygen concentration.

Particle sizes of sodium aerosols released from the samples made a log-normal distribution, of which mass median diameter ranged from 0.6 to 2.0 μm. The burning rate increased with the increases of the sodium temperature, of the gas flow rate and of the oxygen concentration, and the aerosol release fraction also increased in proportion to the burning rate at fixed oxygen concentrations. It was found that the aerosol release rate defined as a product of the burning rate and the aerosol release fraction describes fairly well not only the present experimental data but also reported ones.     

钠雾化喷射技术研究

钠雾化喷射技术是大量放射性废钠处理的一项关键技术,为探索钠雾化工艺,设计了钠雾化设备,并对钠雾化喷射的动力学性能进行研究。试验在体积为2.4 m³的密封容器内进行。氩气气氛下,采用激光粒度仪测量距喷头150 mm处钠喷射液滴粒径的分布,试验压力分别为0.05、0.1和0.15 MPa。试验结果表明：3种压力下,喷射中位粒径分别为567.2、554.8和544.2 μm,中位粒径随喷射压力的变大而变小;不同压力下,钠喷射液滴均在464.2～735.6 μm区间的粒径分布最多,占总分布的50%以上。通过钠喷射与水喷射的相似性,建立粒径相似性模型,利用已知水喷射液滴粒径空间分布模拟得出钠喷射液滴粒径的空间分布。     

A Method of Transforming the Prompt Decay Constants αi into the Static Reactivity ρs

This report describes an analytical method of transforming the prompt decay constants α_i into the static reactivity ρ_s. It has been developed through the use of the time-dependent, two-group diffusion theory ignoring delayed neutrons under several assumptions. A generalized characteristic equation which relates the prompt decay constants to the effective multiplication constant can in general be derived by expanding the static fluxes into kinetic fluxes containing the fundamental and higher modes, and several approximated formulas for this equation have been obtained with the application of certain conditions. In particular, a formula that takes into account the change in the generation time — ρ_d2 =(β_eff/α_c) {α_c-(G_α/G_αc)α}— proposed as a most useful method for reactivity measurement. Error evaluation has been performed in regard to several approximate formulas, and some problems involved in numerical calculation have been examined.     

某地下建筑内天然放射性气溶胶粒度分布研究

天然放射性气溶胶粒度分布是评价氡暴露所致有效剂量的重要参数。本文采用撞击法对某地下建筑物室内天然放射性气溶胶的粒度分布进行试验研究,58个点位的4次测量表明：该建筑内天然放射性气溶胶粒度分布可采用单峰对数正态分布形式描述,活度中值空气动力学直径（AMAD）范围为0.087～0.427μm,平均值为0.194μm;AMAD主要分布在0.1～0.3μm之间,约占全部测量结果的85%。     

考虑聚并及热泳的微米级气溶胶粒子沉积数值模拟研究

本文基于C++编写的程序对0.5~10 μm气溶胶粒子沉积进行了数值模拟,研究了聚并和热泳对沉积的影响及其耦合作用。结果表明,气溶胶粒子数量浓度小于10¹²m^-3时,聚并可忽略;气溶胶粒子数量浓度为10¹⁴ m^-3时,聚并不可忽略。考虑聚并后,气溶胶粒子的沉积速度显著增加,如在温度梯度为3 000 K/m时,10 μm气溶胶粒子的沉积速度较不考虑聚并时增大了37.2%。不考虑聚并、温度梯度为3 000 K/m时,由于热泳作用,0.5 μm气溶胶粒子沉积速度增大到无温度梯度的5.96倍,考虑聚并后,减小为4.41倍（数量浓度为10¹⁴ m^-3）。聚并和热泳会相互影响,但总体上聚并和热泳均会增强气溶胶粒子的沉积,从而加快气溶胶粒子的损失。     

Formation of Volatile Radioiodine Compounds in Sodium Pool Burning

In sodium pool burning occurring in the case of an LMFBR accident, some radio-iodine in the sodium coolant may be transported into the gas phase and act in common with sodium oxide aerosol. If some iodine is converted to volatile compounds, the radioactivity may remain for many hours in the gas phase of the reactor containment. The present work was carried out in an attempt to throw more light on these circumstances.

Reactor-grade sodium with sodium iodide tagged with ¹³¹I in an amount of about 1 ppm was burned by heating in a closed vessel containing air. Most of the iodine released into the gas phase took the form of aerosol, but some amount remained in vaporous state. It was determined by Maypacks and radio-gaschromatography that the volatile radio-iodine compounds were of organic form. The proportion of organic iodide referred to total airborne iodine in the present experiments falls within the experimental data on what will be formed in a loss of coolant accident involving a light water reactor. It is concluded that volatile iodine formation would present similar aspects under accidental conditions affecting both LWR and FBR, though the mechanisms governing the two cases would be different.     

Sodium Mist Behavior in Cover Gas Space of LMFBR Out-Pile Experiment

Sodium mist behavior in argon cover gas space over sodium pool was investigated experimentally using of a test vessel having cover gas volume of ～100l. Mass concentration and gravitational settling flux of the mist (i.e. sodium aerosol) were determined between pool temperature range of 290～520°C. Apparatuses used for the concentration determination were a He-Ne laser mist concentration meter (LCM) and mist traps, which were developed specifically for the present experiment. The mist gravitational settling flux φ was determined by using of collection plates, which were exposed to the mist entrained cover gas over sodium pool.

The experimental results revealed that the concentration C was very high from 0.15 to 20g/m³. It increased with pool temperature. The mist particle radii, which were determined from the gravitational settling flux φ and decay curve of the concentration, were from 1.5 to 13 μm in the range of the present experiment. The vapor evaporation rate φe from the pool surface, which were also determined from the experimental φ data, were found to increase with saturated vapor pressure P_s at pool surface. This rate did not depend so strongly on experimental geometries and cover gas flow convection pattern over sodium pool. Evaluations of transient C changing and steady state C fluctuation indicated that the mist formation was occurred near the pool surface.     

温度引起的放射性气溶胶连续监测仪峰位漂移修正技术

本文介绍了放射性气溶胶连续监测仪中半导体探测器的温度特性引起的α谱的峰位漂移对测量结果的影响，并通过温度跟踪方法和计算机软件处理技术解决这一问题。其中采取了两种方法：第一种是对峰位进行温度修正，得到修正过的各道计数；第二种是采用跟踪天然放射性核素Po-214峰位的漂移，再根据被测人工放射性核素能量与Po-214能量的相对值来进行修正。     

Effect of Barriers on Sodium Mist Deposition on LMFBR Components

Tests were made to evaluate the effect of two barriers (a convection barrier and a drip receiver) against sodium mist deposition on LMFBR conponents in the cover gas space. Two models of LMFBR rotating shield plugs (1,800 mm in height, 680 and 850 mm in diameter) and a sodium test tank were manufactured for this purpose, and the mist deposition rate on the walls of these models was measured both in the cases with and without a barrier. The sodium pool temperature during the test was 580°C. In parallel with these tests, the relation between the form of deposit and the wall temperature was examined to determine the critical wall temperature above which deposits do not accumulate. This was conducted by exposing a vertical stainless steel cylinder to cover gas entrained with mist over 500°C sodium for 30 to 1,000 h.

The test results revealed that the barriers functioned effectively, and they reduced the local deposition rate near top of the annulus by three orders of magnitude relative to the cases without a barrier. The critical wall temperature to avoid deposit accumulation was found to be 100 to 150°C.     

Features of Dendritic Oxide during Sodium Combustion

Liquid sodium is used as the coolant in fast reactors (FRs). It is a superior thermal medium, but its chemical reactivity with oxygen and water is very high. After the Monju sodium leak accident, a better physical and chemical understanding of the reaction phenomena was required from the viewpoint of the public acceptance of FRs. The purpose of this study is to understand the oxidation behavior of liquid sodium in detail, because it plays an important role in the continuation of the combustion process. The liquid sodium surface is covered with a thin oxide layer at the start of combustion. Generally, the surface oxide layer on the solid metal prevents further oxidation; however, dendritic sodium oxide is generated, and it grows on the oxide layer. This means that sodium can be supplied to the interface of the reaction without being blocked by the reaction products. As a result, the reaction continues spontaneously. Therefore, reaction products such as dendritic oxide have a significant influence on the overall reaction behavior. However, the precise roles of such products on the reaction have not been clarified in previous research. Understanding the role of the dendritic oxide in the reaction can be helpful for controlling sodium combustion phenomena, for example, the extinction process. Therefore, this study is a useful contribution to ensuring FR safety.     

环境气溶胶的核孔膜、扫描电子显微镜和微电子探针研究

用云母核孔膜、扫描电子显微镜和微电子探针对工艺房间空气中悬浮的气溶胶进行了采集 ,并对气溶胶的粒径分布、微粒形态及结构、元素成分和捕集机制进行了研究。     

Using Anemometer for Particle Size Measurement of Sodium Mist

A code “ACTFCI” has been developed for analyses of Fuel Coolant Thermal Interaction in a coolant channel. The code can deal with the dynamic behavior of coolant in r-z geometry, but does not take into account the dynamics of fuel particle two-dimensional movement, the fuel being considered only as heat source for the coolant. The ICED-ALE method was used for the numerical analyses of the three conservation equations and two equations of state for single and two-phase sodium. A sample calculation is given concerning the period up to initiation of sodium boiling, for a geometry representing a wrapper tube of reduced size. Despite this difference of the model from actual size, the calculated results suffice for demonstrating the utility of this method in obtaining stable solutions on single-phase pressure wave propagation in both r and z directions, and on sodium boiling initiation triggered by the rarefaction wave returning to the interaction region from channel extremity.     

α放射性气溶胶沉积对微孔滤膜自吸收的影响研究

模拟基于α能谱法的混合~(222)Rn、~(220)Rn子体测量程序,分别就醋酸纤维素酯微孔滤膜对~(222)Rn、~(220)Rn各α放射性子体的自吸收进行了实验研究。研究结果表明:该滤膜对~(222)Rn、~(220)Rn子体的自吸收以及同种滤膜间的差异较小,可取0.978为α能谱法(或总α法)的滤膜自吸收因子;微孔滤膜模拟α能谱法以120 min为测量周期,重复使用时沉积在滤膜表面的α放射性气溶胶对滤膜自吸收的影响可忽略。     

Development of Fluctuation Monitor Type Sodium Ionization Detector

In order to improve the sensitivity and the reliability of the sodium leak detection system used in the fast breeder reactors, a new type SID (sodium ionization detector) has been developed, in which the monitored signal is only the fluctuating component of the current between the filament and the ion collector. The fluctuating component was extracted by a band pass filter and its root mean square value was calculated as the SID signal. Fluctuation characteristics of the output current were studied by its frequency spectrum. The results revealed that the current spectrum was affected by the particle size distribution of the aerosol and was most clearly distinguished from that of the background current in the frequency region of 0.5–10 Hz.

Output characteristics of the fluctuation monitor type SID (FM-SID) were obtained as a function of sodium concentration in the gas. The FM-SID sensitivity was lowered by impurities in the gas, such as oxygen and water vapor. Finally, in comparisons with the conventional DC-SIDs, the background noise level of the FM-SID was much lower and SIN ratio was greatly improved. The detectable sodium concentration level was ten times lower than that of the DC-SID.     

纳米磁性氧化铁溶液的颗粒度测定

用透射电镜、激光散射、原子力显微镜、穆斯堡尔谱等方法测定了3种不同批号自制磁性氧化铁溶液 样品的颗粒度。结果显示,本工作制备的磁性氧化铁的颗粒度有一个大致分布范围,为5-100 nm,激光散射 扫描仪和原子力显微镜的结果还显示,<20 nm的颗粒是主要成分;各种方法测得颗粒度略有差异。