Chalk River Unidentified Deposit Research Articles

Experimental investigation on distribution of boric acid in a vertical 1 × 2 rod array channel

During the reactor operation, the mixing mass flow of boron acid caused by diversion cross-flow, turbulent mixing and void drift changes the boron concentration distribution in the coolant and affects the amount of boron in the Chalk River Unidentified Deposit (CRUD). Accumulation of boron within the porous crud layer on the fuel rods in the cores of pressured water reactors (PWR) results in the crud-induced power shift (CIPS), threatening the operation safety of reactors. In order to obtain the mixing mass flow of boric acid between adjacent channels, a vertical test channel containing two subchannels simulating a PWR fuel rod bundle is constructed. Using this channel, flow distributions and mixing mass flow of boric acid are measured for single-phase and two-phase air–water flows. The measurements show that the boric acid distribution is affected by turbulent mixing for single-phase flow and the mixing mass flow of boric acid increases with the Reynolds number. For two-phase flow, the bubble-induced turbulence enhances the turbulence intensity of the liquid flow, the mixing mass flow of boric acid increases with the increase of void drift and reaches the maximum in the slug flow. The experimental data is used to support the development and validation of a new transverse source term model of boron mixing. The comparison result shows the significant improvement of the code prediction.

파울링된 피복관 표면의 가압 경수로 운전환경에서의 미포화 비등 열전달 성능평가를 위한 실험장치 구축 연구

The CRUD (Chalk River Unidentified Deposit or Corrosion Related Unidentified Deposit) is a fouling deposit of metal or metal oxide on the nuclear fuel cladding under PWR (Pressurized Water Reactor) operating conditions and causing the several operational and safety-related problems. The major concerns related to the CRUD are AOA (Axial Offset Anomaly) caused by the accumulation of the boron species inside the porous CRUD from the primary reactor coolant and CILC (CRUD Induced Localized Corrosion) caused by locally increased cladding surface temperature due to the additional thermal resistance of the CRUD layer. The growth of CRUD and the CRUD-related problems are known to be governed by the sub-cooled nucleate boiling phenomena inside the upper part of the reactor core, especially on the hot fuel assembly and hot fuel pin. In the current study, the experimental facility called DISNY (crud DeposItion Simulator for Nuclear energY) was proposed as a testbed to simulate the CRUD growth and to investigate the sub-cooled nucleate boiling heat transfer performance of the CRUD deposited cladding surface under prototypical PWR operating conditions.

Research of Thermal Hydraulic Conditions Effect on PWR CIPS Risk

In order to reveal the effect of thermal hydraulic conditions on the PWR CIPS risk, the evaluation of a PWR CIPS risk in the first cycle under different core flow rates, average primary temperatures, power levels, and primary pressures was conducted by combining thermal hydraulic codes LINDEN and CRUD (Chalk River unidentified deposit) analysis software CAMPSIS. The research result illustrating the essential effect of thermal hydraulic conditions on CIPS is changing the SNB (subcooled nucleate boiling) level of the fuel assembly’s surface; thus, boron precipitation and local power distribution will be affected. Theoretical evidence and statistical support of the effect of thermal hydraulic conditions on the PWR CIPS risk could be obtained via this research.

Decontamination of radioactive metal wastes using underwater microwave plasma

As nuclear power plants (NPPs) operate, the levels of radioactively contaminated oxide films are increased at the surface of the metal components of these plants. The Chalk River Unidentified Deposit (CRUD) is one such radioactive waste. Decontamination of CRUD is an essential task because radionuclides such as cobalt-60 (60Co; γ-emitter) can be deposited in the CRUD and react with the inverse spinel nickel ferrite. Hence, decontaminating the CRUD is an essential yet challenging process. Compared with conventional techniques, underwater microwave plasma was developed to provide high reliability and low environmental impact because the plasma system does not require external additives. In addition, the formation of highly reactive species such as radicals available to remove the deposited radioactive isotopes and underwater conditions of microwave plasma decontamination systems can reduce the risk of radioactive exposure during the decommissioning and decontamination of NPPs. This study demonstrated that the removal efficiency of cobalt of the microwave plasma system is 96.9%, which can be a new decontamination technique associated with the underwater cutting of metal wastes during the NPPs decommissioning process.

Assessment of the Predictive Capability of VERA—CS for CASL Challenge Problems

Abstract This paper describes the process for assessing the predictive capability of the Consortium for the advanced simulation of light-water reactors (CASL) virtual environment for reactor applications code suite (VERA—CS) for different challenge problems. The assessment process is guided by the two qualitative frameworks, i.e., phenomena identification and ranking table (PIRT) and predictive capability maturity model (PCMM). The capability and credibility of VERA codes (individual and coupled simulation codes) are evaluated. Capability refers to evidence of required functionality for capturing phenomena of interest while credibility refers to the evidence that provides confidence in the calculated results. For this assessment, each challenge problem defines a set of phenomenological requirements (based on PIRT) against which the VERA software is evaluated. This approach, in turn, enables the focused assessment of only those capabilities that are relevant to the challenge problem. The credibility assessment using PCMM is based on different decision attributes that encompass verification, validation, and uncertainty quantification (VVUQ) of the CASL codes. For each attribute, a maturity score from zero to three is assigned to ascertain the acquired maturity level of the VERA codes with respect to the challenge problem. Credibility in the assessment is established by mapping relevant evidence obtained from VVUQ of codes to the corresponding PCMM attribute. The illustration of the proposed approach is presented using one of the CASL challenge problems called chalk river unidentified deposit (CRUD) induced power shift (CIPS). The assessment framework described in this paper can be considered applicable to other M & S code development efforts.

BOTANI: High-fidelity multiphysics model for boron chemistry in CRUD deposits

We develop a new high-fidelity multiphysics model to simulate boron chemistry in the porous Chalk River Unidentified Deposit (CRUD) deposits. Heat transfer, capillary flow, solute transport, and chemical reactions are fully coupled. The evaporation of coolant in the deposits is included in governing equations modified by the volume-averaged assumption of wick boiling. The axial offset anomaly (AOA) of the Seabrook nuclear power plant is simulated. The new model reasonably predicts the distributions of temperature, pressure, velocity, volumetric boiling heat density, and chemical concentrations. In the thicker CRUD regions, 60% of the total heat is removed by evaporative heat transfer, causing boron species accumulation. The new model successfully shows the quantitative effect of coolant evaporation on the local distributions of boron. The total amount of boron in the CRUD layer increases by a factor of 1.21 when an evaporation-driven increase of soluble and precipitated boron concentrations is reflected. In addition, the concentrations of B(OH)3 and LiBO2 are estimated according to various conditions such as different CRUD thickness and porosity. At the end of the cycle in the AOA case, the total mass of boron incorporated in CRUD deposits of a reference single fuel rod is estimated to be about 0.5 mg.

Sampling and Analysis of PWR Primary Cooling System Corrosion Products Using an EPMA

Chalk River Unidentified Deposit (CRUD) specimens were sampled and analyzed using an electron probe micro analyzer (EPMA; JEOL JXA-8230R) with a bundle of spent nuclear fuel (actual burnup 49 655 MWd/tonne U) from a Korean nuclear power plant. CRUD collection in the reactor refueling cavity was carried out using the following two methods. The first method used an Al2O3 scraper to scrape a specific area of the fuel cladding and then a paper filter to draw the suspended pieces around it. The second method removed CRUD deposited outside the cladding by vibrating the fuel bundle with an ultrasonic wave. In the first method, scraping some parts of the fuel rods with the Al2O3 scraper made it difficult to obtain an analytical sample because of the very small sample collection. However, the results of the analysis of the identified CRUD samples accurately confirmed the shape and composition of the CRUD in the specific area of the fuel rods desired by the user. In contrast, the second method using the ultrasonic cleaner made it very difficult to analyze samples because of the excessive amount of collected CRUD. In this study, we will discuss two methods for collecting corrosion products existing in the primary cooling system of a pressurized water reactor and the results of an analysis using a shielded EPMA. In addition, we will confirm the value of the Ni/Fe ratio.

Comparison of Analysis Results on Three Methods for Sampling Crud, a Radioactive Corrosion Product with Zinc-Injected Spent PWR Nuclear Fuel Rods

A radioactive corrosion product, Chalk River unidentified deposit (crud) was sampled and analyzed using an electron probe micro-analyzer with zinc-injected spent nuclear fuel rods (HU Unit 1, actual burnup 49 655 MWd/tonne U). Hot-cell facilities, a space for handling highly radioactive materials, were used as a way to collect crud deposited in the fuel rod cladding tube at a specific location of the spent fuel rod. A soft collection method for collecting crud using rubbings or adhesive tape was used to collect a sample, and a sample was collected with hard collection using a steel knife from the cladding tube of the fuel rod. The spent fuel rods were used for two cycles burned after zinc was injected into the primary coolant, which is known to inhibit the generation of crud. To compare the analysis results of the soft and hard collection methods for sampling crud, the results of the crud collected using an ultrasonic wave system were analyzed. The crud used in this study used burned fuel rods for two cycles after zinc ions were injected into the primary coolant. Based on the results, the Ni/Fe ratio can be estimated to be about 1.18. The Ni/Fe ratio value of 1.18 derived from this study is not much different from the Ni/Fe ratio values derived from nuclear power plants operating around the world.

CRUD removal via hydrodynamic cavitation in micro-orifices

Chalk river unidentified deposit (CRUD) is an acronym used to describe the corrosion deposits that form in nuclear power plants. Such deposits are often associated to reduction of heat transfer in fuel rods and to blockage of channels in regions of fast hydrodynamic flow. Given the significance of the operational consequences associated to CRUD deposition, the development of methodologies to remove it is of scientific and technical interest. In this study, a microscale flow loop setup, replicating conditions relevant to plant operations, was used to explore the potential of exploiting hydrodynamic cavitation for CRUD removal. Tests were performed on discs with micro-orifices that were previously exposed to high temperature water flow to induce CRUD deposition. CRUD removal from within the micro-orifices was performed by inducing cavitation, and monitored via periodic microstructural examinations. The results indicate that a cavitating regime can reduce the volume of CRUD deposit by 80–90% within minutes, with no detectable damage to the metal surface.

Modeling film boiling within chimney-structured porous media and heat pipes

A porous medium with separated paths for liquid and vapor flows does not fail even after part of the porous medium is dried out. Instead, a vapor film resides within the porous medium, and it grows very slowly. This heat transfer regime was named as “confined film-boiling regime” in this study, and a heat transfer model for this regime was suggested in this paper. Especially, this paper mainly focuses on heat transfer of a CRUD (Chalk River Unidentified Deposit), which is a naturally occurring porous medium found in nuclear reactors. In the present model, the balance between capillary pressure and pressure drops of liquid and vapor flows determined thickness of the vapor film. In addition, we assumed that capillary pressure was changed with applied heat flux. This assumption was validated with a planar heat pipe case: the root-mean-square-error (RMSE) was 16% for the model with the heat flux dependent capillary pressure, while one for a model with the constant capillary pressure was 790%. Furthermore, this approach also turned out to be valid for the case of the CRUD: the model predicted the wall superheat during the film boiling of the CRUD, and its relative error was only within 20% when it was compared with the measured wall superheats. Finally, sensitivity analysis for CRUD parameters found that the heat transfer performance of the CRUD is largely sensitive to chimney density and pore size.

Development of a microfluidic setup to study the corrosion product deposition in accelerated flow regions

CRUD (Chalk River Unidentified Deposit) forms in the water circuits of nuclear reactors due to corrosion of structural materials and the consequent release of species into the coolant. The deposition of CRUD is known to occur preferentially in regions of the primary circuit of pressurised water reactors (PWRs) where the water flow accelerates. In order to investigate this phenomenon, a micro-fluidic system, recreating plant conditions while using a simplified experimental set-up, was realised. A flow cell, comprising a stainless steel disc with a central micro-orifice, was used to create accelerated flow under representative operating conditions. By monitoring the pressure drop across the cell, the build-up rate (BUR) of CRUD within the micro-orifice was monitored in real time. By this setup, the conditions inducing deposition of CRUD under PWR conditions were emulated and CRUD deposition was induced in the accelerated flow region. Further effects associated with the presence of lithium hydroxide were investigated in real-time.

Removal of Chalk River unidentified deposit (CRUD) radioactive waste by enhanced electrokinetic process

Decontamination techniques proposed and used to remove Chalk River unidentified deposit (CRUD) in radioactive waste management. In cases of huge volumes of metal or radionuclides contaminated by CRUD, removal of CRUD by mechanical or chemical decontamination is difficult. An advanced electrokinetic process combined with chemical decontamination was applied to remove CRUD and experimentally evaluated. Oxalic acid was used for CRUD removal, and cobalt (Co) released from the CRUD was transferred to the cathode in an electrokinetic reactor. Results indicate that the combined system is efficient for CRUD removal with enhanced, efficiency by use of the cation exchange membrane and zeolite.

Investigation into the effect of water chemistry on corrosion product formation in areas of accelerated flow

The deposition of CRUD (Chalk River Unidentified Deposit) in the primary circuit of a Pressurised Water Reactor (PWR) is known to preferentially occur in regions of the circuit where flow acceleration of coolant occurs. A micro-fluidic flow cell was used to recreate accelerated flow under simulated PWR conditions, by flowing water through a disc with a central micro-orifice. CRUD deposition was reproduced on the disc, and CRUD Build-Up Rates (BUR) in various regions of the disc were analysed. The effect of the local environment on BUR was investigated. In particular, the effect of flow velocity, specimen material and Fe concentration were considered. The morphology and composition of the deposits were analysed with respect to experimental conditions. The BUR of CRUD was found to be sensitive to flow velocity and Fe concentration, suggesting that mass transfer is an important factor. The morphology of the deposit was affected by the specimen material indicating a dependence on surface/particle electrostatics meaning surface chemistry plays an important role in deposition. The preferential deposition of CRUD in accelerated flow regions due to electrokinetic effects was observed and it was shown that higher Fe concentrations in solution increased BURs within the orifice whereas increased flow velocity reduced BURs.

EELS and electron diffraction studies on possible bonaccordite crystals in pressurized water reactor fuel CRUD and in oxide films of alloy 600 material

Abstract Experimental verification of boron species in fuel CRUD (Chalk River Unidentified Deposit) would provide essential and important information about the root cause of CRUD-induced power shifts (CIPS). To date, only bonaccordite and elemental boron were reported to exist in fuel CRUD in CIPS-troubled pressurized water reactor (PWR) cores and lithium tetraborate to exist in simulated PWR fuel CRUD from some autoclave tests. We have reevaluated previous analysis of similar threadlike crystals along with examining some similar threadlike crystals from CRUD samples collected from a PWR cycle that had no indications of CIPS. These threadlike crystals have a typical [Ni]/[Fe] atomic ratio of ∼2 and similar crystal morphology as the one (bonaccordite) reported previously. In addition to electron diffraction study, we have applied electron energy loss spectroscopy to determine boron content in such a crystal and found a good agreement with that of bonaccordite. Surprisingly, such crystals seem to appear also on corroded surfaces of Alloy 600 that was exposed to simulated PWR primary water with a dissolved hydrogen level of 5 mL H 2 /kg H 2 O, but absent when exposed under 75 mL H 2 /kg H 2 O condition. It remains to be verified as to what extent and in which chemical environment this phase would be formed in PWR primary systems.

Modeling heat transfer through chimney-structured porous deposit formed in pressurized water reactors

The model was developed to predict the heat transfer within the Chalk River Unidentified Deposit (CRUD), which is a kind of fouling found on the fuel rods of pressurized water reactors (PWR). The CRUD tends to develop steam chimneys that separate the liquid from the vapor phase. Therefore, the model describes the CRUD as a porous medium with steam chimneys. Unlike the previous approaches that assume that the evaporation takes place at the lateral surfaces of the chimneys, in the present study it is postulated that the vapor is generated by the bubble nucleation at the CRUD–clad interface, as observed via the visualization study for the chimney-structured porous medium. The generated bubble escapes through the steam chimney. The heat transfer in the CRUD can be described by three mechanisms of heat removal, which are nucleate boiling, liquid convection in the CRUD, and forced convective heat removal from the surface of the CRUD. The predicted CRUD–clad interface temperatures and overall heat transfer coefficients were compared to the experimental results, which were produced under the simulated PWR conditions (approximately 15MPa, 300°C). The prediction data presented better agreement with the experimental data; the normalized Root Mean Square Error (RMSE) of the present model is 18.6% in contrast with the 42.4% obtained with the Cohen model. After the validation with the experimental data, the prediction of temperature in the model was used to investigate how the heat transfer characteristics tended to change within the CRUD. Furthermore, the parametric study regarding the CRUD properties revealed that the effect of permeability on heat transfer is not significant in the nucleate boiling regime.

Hydrodynamic cavitation characteristics of an orifice system and its effects on CRUD-like SiC deposition

In a nuclear power plant, chalk river unidentified deposit (CRUD) is known as a deposit that is composed of corrosion and oxidation materials. It has a porous structure, which combines with boron that is injected into the coolant for controlling power levels. The buildup of corrosion products on the fuel cladding surface has proven to be particularly significant for both BWRs and PWRs. The high temperature of the cladding surface attracts impurities and chemical additives in the reactor coolant that deposit on the fuel rod surface in a process. The deposits on a fuel rod, known as CRUD, can be tenacious, insulative compounds capable of increasing the local clad temperature and accelerating clad corrosion—sometimes to the point of fuel failure.The deposition of CRUD on fuel cladding surfaces causes uneven heating of the reactor core. The situation is exacerbated by boron, which is added to the coolant to control power levels. However, boron becomes concentrated and is deposited within thick CRUD deposits. Ultrasonic mechanisms were developed but they have limitations for decontamination. In this experiment, a decontamination test was conducted using a sample sheet that was composed of SiC/water nanofluids. In addition, it was exposed to swirl flow and common flow for checking enhanced cavitation. It is measured by a pressure film, as shock pressure is associated with cavitation number. As a pressure film is wetted easily in water, it was injected into a holder. In the experiment, the maximum shock pressure was obtained during swirl flow at a low cavitation number. This indicates that pressure was concentrated on the pressure film. Consequently, cavitation can get rid of CRUD layers partially.

Effects of Al2O3 nanoparticles deposition on critical heat flux of R-123 in flow boiling heat transfer

Abstract In this study, R-123 flow boiling experiments were carried out to investigate the effects of nanoparticle deposition on heater surfaces on flow critical heat flux (CHF) and boiling heat transfer. It is known that CHF enhancement by nanoparticles results from porous structures that are very similar to layers of Chalk River unidentified deposit formed on nuclear fuel rod surfaces during the reactor operation period. Although previous studies have investigated the surface effects through surface modifications, most studies are limited to pool boiling conditions, and therefore, the effects of porous surfaces on flow boiling heat transfer are still unclear. In addition, there have been only few reports on suppression of wetting for decoupled approaches of reasoning. In this study, bare and Al 2 O 3 nanoparticle-coated surfaces were prepared for the study experiments. The CHF of each surface was measured with different mass fluxes of 1,600 kg/m 2 s, 1,800 kg/m 2 s, 2,100 kg/m 2 s, 2,400 kg/m 2 s, and 2,600 kg/m 2 s. The nanoparticle-coated tube showed CHF enhancement up to 17% at a mass flux of 2,400 kg/m 2 s compared with the bare tube. The factors for CHF enhancement are related to the enhanced rewetting process derived from capillary action through porous structures built-up by nanoparticles while suppressing relative wettability effects between two sample surfaces as a highly wettable R-123 refrigerant was used as a working fluid.

Chemical and radiochemical analysis of corrosion products on BWR fuel surfaces

This paper aimed at the characterization of the metallic composition and surface analysis of fuel rod Chalk River Unidentified Deposit (crud) for a BWR-6 unit at a nuclear power plant. In this study, inductively coupled plasma-atomic emission spectroscopy and gamma spectrometry were carried out to analyze the corrosion product distributions and to determine the elemental compositions along the fuel rod under conditions of hydrogen water chemistry (HWC) switched from normal water chemistry (NWC) as the reactor coolant. Most of the crud consisted of flakes and irregular shapes via scanning electron microscope morphology. The loosely adherent oxide layer was mostly composed of hematite (α-Fe2O3) with amorphous iron oxides, as determined using X-ray diffractometer results. The deposited amount of crud was the order of 0.2 mg/cm2, suggesting that the fuel surface of this plant under the HWC environment appeared to have lower crud deposition due to low feedwater iron levels. There was also no significant difference in comparison with the NWC condition.

Characteristics and Behavior of High-Burnup Fuel That May Affect the Source Terms for Cask Accidents

Current risk assessments of spent fuel in storage and transportation casks use the properties of light water reactor fuel below 45 GWd/t U. Fuel is being driven to higher burnups that may influence the source term in cask accidents. To achieve these burnups the manufacturers are introducing new assembly designs and cladding alloys. As a result, at the higher burnups (≥50 GWd/t U) some of the characteristics of the fuel pellets, cladding, and assembly design used in the safety analysis have changed. The fuel pellet has developed a fine-grained, Pu-rich rim zone on its exterior surface. The source term may increase by up to three orders of magnitude over that expected from the particulate size distribution based on the fracture of the body of the pellets. The actual increase will depend on the fracture characteristics of the rim and number of fracture sites in the cladding. The cladding may acquire hydrogen contents up to 700 parts per million by weight during the increased exposure. Embrittlement of the cladding with subsequent loss of ductility may occur, especially if there is hydride reorientation. As a result, there may be a greater propensity for fracture of the rods upon impact, with subsequent release of fuel particulate and gas. Significantly improved source terms can be developed if additional data on fuel rim fracture as a function of impact energy, the dependence of cladding ductility for Zircaloy and the newer cladding alloys as a function of hydride reorientation, and release characteristics for fractured rods are obtained. Chalk River unidentified deposit spallation characteristics only make a significant contribution to the source term if the rods do not fracture in the accident or if a fire-only accident occurs.

Synthesis of simulated cruds for development of decontaminating agents

The Chalk River Unidentified Deposit (cruds) were classified into Types 1 and 2 based on the analyses of actual cruds from target objects such as steam generator (SG) of Korea and Japan Pressurized Water Reactor (PWR). A metal oxide or metal hydroxide was used to synthesize simulated cruds having chemical compositions and crystalline structure similar to the actual one by 12 different methods. CRUD 4 (metal oxides in the autoclave vessel) and CRUD 10 (metal oxides in a crucible after hydrazine pretreatment) were chosen as the best method for Types 1 and 2, respectively. These simulated cruds showed reasonable dissolubilities by dissolution tests using well-known decontaminating agents. As these cruds can be synthesized easily without using any specialized equipment or reagents in a short time and in large quantities, they are expected to stimulate the development of decontaminating agents and processes.