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Comprehensive Analysis of Burnup-Driven Fission Product Evolution and Radiological Releases in VVER-1200 in Bangladesh’s First Nuclear Power Plant

This study analyzes the burnup-dependent evolution of radionuclide inventories in the VVER-1200 reactor at Bangladesh's Rooppur Nuclear Power Plant up to 19.808 MWD/kg, revealing that higher burnup significantly increases key fission product and actinide concentrations, thereby providing critical data for safety analysis and emergency preparedness despite a 15–50% underestimation compared to NUREG-1940 reference values.

Original authors: Md Rosaidul Mawla, Radmim Ryean, Anisur Rahman, Abdus Sattar Mollah, Md. Shafiqul Islam

Published 2026-06-24

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Md Rosaidul Mawla, Radmim Ryean, Anisur Rahman, Abdus Sattar Mollah, Md. Shafiqul Islam

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A "Recipe" for Radioactive Ingredients

Imagine the nuclear reactor at Bangladesh's new power plant (the Rooppur NPP) as a giant, high-tech kitchen. Inside this kitchen, they are cooking uranium fuel to generate electricity. But just like cooking a complex stew, the longer you cook it, the more the ingredients change.

This paper is a detailed inventory list. The researchers wanted to answer a simple question: "If we cook this fuel for a specific amount of time (called 'burnup'), what exactly is inside the pot, and how dangerous would it be if the pot broke?"

They used a computer program called ORIGEN (think of it as a super-accurate digital scale and calculator) to simulate the reactor running for about 18 months. They tracked how the "ingredients" (radionuclides) grew, shrank, or stayed the same as the fuel got "older."

The Cast of Characters: Who's in the Pot?

The researchers didn't just look at one ingredient; they sorted the radioactive "stew" into seven main groups, each with its own personality:

The "Gas Gang" (Noble Gases like Xenon and Krypton):
- Analogy: These are like bubbles in a soda. They are light, floaty, and don't stick to anything.
- Behavior: Some of them (like Xenon-133) build up steadily like bubbles forming in a shaken bottle. Others appear quickly and then vanish (pop) as the fuel gets older.
- Risk: Because they are gases, if the reactor leaks, they are the first to escape into the air.
The "Sticky Halogens" (Iodine and Bromine):
- Analogy: These are like Velcro. They are very reactive and love to stick to things (like your thyroid gland if inhaled).
- Behavior: Some Iodine isotopes are like a short-lived party—they show up early, have a big peak, and then fade away. Others (like Iodine-129) are like a slow-growing weed that just keeps getting bigger the longer you wait.
The "Heavy Hitters" (Alkali Metals like Cesium):
- Analogy: Think of these as the heavy bricks in the soup. They are dense and stay put.
- Behavior: Cesium-137 is the "star" of this group. It accumulates steadily over time. The longer the fuel burns, the more Cesium you have. It's a major concern for long-term contamination because it stays radioactive for decades.
The "Solid Metals" (Tellurium, Barium, Strontium):
- Analogy: These are the chunks of meat or vegetables. Some are tough and last a long time (like Strontium-90, which acts like a long-term contaminant), while others are soft and break down quickly.
- Behavior: The "Tellurium group" is a mix. Some isotopes build up continuously, while others hit a peak early and then start to decline.
The "Noble Metals" (Ruthenium, Rhodium, etc.):
- Analogy: These are the precious metals in the mix. They are generally stubborn and don't react easily.
- Behavior: Most of these just keep growing in number as the fuel burns, acting like a slow-accumulating savings account of radioactivity.
The "Rare Earths" (Lanthanides):
- Analogy: These are the spices. They are everywhere in the mix but in smaller, specific amounts.
- Behavior: They tend to hit a peak activity in the middle of the cooking process and then level off or drop slightly.
The "Transuranics" (Plutonium, Neptunium, etc.):
- Analogy: These are the "new creations" formed during cooking. They weren't there at the start; they are made when the uranium fuel gets hit by neutrons.
- Behavior: These are the "long-term residents." They keep growing and growing. Neptunium-239 was found to be the single most active ingredient in the entire pot by the end of the cycle.

The "Burnup" Effect: The Longer You Cook, The More You Have

The main discovery of the paper is that time matters.

Short cooking time: You mostly have short-lived, volatile ingredients that appear and disappear quickly.
Long cooking time (High Burnup): You end up with a massive pile of long-lived ingredients (like Cesium-137 and Strontium-90).

The paper concludes that as the fuel burns longer, the total amount of radioactive material increases. This means that if an accident happened later in the fuel cycle, there would be more radioactive material available to be released than if it happened early on.

The "Safety Check": How Does This Compare to Real Disasters?

The researchers compared their "perfect kitchen" simulation to two famous real-world disasters: Chernobyl and Fukushima.

Chernobyl: This was like a kitchen explosion where the roof blew off. Almost everything inside the pot (50-60% of the iodine and 20-40% of the cesium) flew out into the atmosphere because there was no lid (containment).
Fukushima: This was like a kitchen fire where the lid was cracked. Some stuff got out, but the containment structure held back a lot more than Chernobyl.
The Rooppur (VVER-1200) Simulation: The paper suggests that if an accident happened here, the "lid" (containment) would be much stronger. The amount of radioactive material released would likely be lower than Chernobyl and comparable to or lower than Fukushima. The design of this new reactor is built to keep the "soup" inside the pot.

The "Underestimation" Note

The researchers noticed something interesting: Their computer calculations predicted slightly less radioactivity than some standard reference books (NUREG-1940) suggest.

Analogy: It's like their digital scale said the cake weighed 2 pounds, but the recipe book said it should be 2.5 pounds.
Why? This doesn't mean their math is wrong; it likely means they used slightly different ingredients (nuclear data) or cooking assumptions. However, the pattern of how the ingredients changed over time was consistent.

The Bottom Line

This paper is a safety manual for the future. It tells us that:

Time is a factor: The longer the fuel stays in the reactor, the more dangerous the "inventory" becomes.
Different elements behave differently: Some escape easily (gases), some stick to the ground (cesium/strontium), and some are created over time (plutonium/neptunium).
Safety first: The new reactor in Bangladesh has a design that should keep these radioactive ingredients contained much better than older reactors did during past accidents.

This information helps the people in charge of the plant prepare for emergencies, ensuring that if something goes wrong, they know exactly what kind of "soup" might spill and how to stop it.

Technical Summary: Comprehensive Analysis of Burnup-Driven Fission Product Evolution and Radiological Releases in VVER-1200 in Bangladesh's First Nuclear Power Plant

Problem Statement
The quantitative evaluation of radionuclide composition in nuclear materials is critical for assessing environmental risks during accidents, evaluating reactor performance, and managing radioactive waste. While international research has extensively investigated source terms for various reactor types, there is a scarcity of studies focusing specifically on the burnup-dependent behavior of fission products and the resulting radiological source terms for the VVER-1200/V392M reactor design, which is the technology selected for the Rooppur Nuclear Power Plant (NPP) in Bangladesh. Existing generic data often fails to account for the specific operating conditions, fuel enrichment, and burnup levels relevant to this facility. Consequently, there is a need to establish a reliable, burnup-dependent source term model to support safety assessments, emergency preparedness, and regulatory decision-making for Bangladesh's nuclear energy sector.

Methodology
This study employs a computational approach using the ORIGEN 2.2 depletion code to model the evolution of radionuclide inventories in a generic VVER-1200 reactor core representative of the Rooppur NPP.

Reactor Model: The analysis utilizes design parameters for the VVER-1200 (V-392M), including a thermal power of 3200 MWth, a hexagonal fuel assembly configuration with 163 assemblies, and fuel enriched up to 4.95% $^{235}$ U. The fuel cycle involves a mix of fresh, once-burned, and twice-burned assemblies, with a focus on a burnup range up to 19.808 MWd/kg (representing approximately 18 months of operation at 70% power).
Calculation Approach: The study calculates the time-dependent behavior of radionuclides by accounting for generation via fission, depletion via radioactive decay, and transmutation via neutron capture. The core inventory ( $I_{ACTUAL}$ ) is benchmarked against reference values from NUREG-1940, adjusted for the specific burnup of the VVER-1200.
Source Term Estimation: Radiological source terms are estimated using the relation $R_i = A_i \times DF_i \times RF_i \times LF_i$ , where $A_i$ is the inventory, and $DF_i$ , $RF_i$ , and $LF_i$ represent fuel damage, release, and containment leak fractions, respectively. Release fractions are derived from NRC Regulatory Guide 1.183 (2000) based on chemical groupings (noble gases, halogens, alkali metals, etc.).
Scope: The analysis covers seven primary radionuclide groups: noble gases, halogens, alkali metals, tellurium-group isotopes, noble metals, lanthanides, and cerium-associated actinides.

Key Contributions and Results
The study provides a detailed characterization of how fuel burnup influences the inventory and potential release of specific radionuclides:

Burnup-Dependent Evolution Patterns: The research identifies distinct behavioral patterns for different isotopic groups as burnup increases from 0 to 19.808 MWd/kg:
- Continuous Accumulation: Long-lived isotopes such as $^{85}$ Kr, $^{129}$ I, $^{134}$ Cs, $^{137}$ Cs, $^{90}$ Sr, and transuranics like $^{239}$ Pu, $^{241}$ Pu, and $^{239}$ Np show monotonic growth throughout the irradiation period.
- Early Peak and Depletion: Short-lived or metastable isotopes (e.g., $^{89}$ Kr, $^{135}$ Xe, $^{138}$ Cs, $^{133}$ I, $^{134}$ I) reach maximum activities at very low burnup levels (often < 0.6 MWd/kg) and subsequently decline due to radioactive decay outpacing production.
- Growth Followed by Stabilization: Intermediate-lived isotopes (e.g., $^{131}$ I, $^{132}$ I, $^{95}$ Zr, $^{95}$ Nb) exhibit an initial increase followed by near-equilibrium stabilization or slight decline at higher burnups.
Inventory Quantification:
- The total core inventory at the end of the cycle is calculated at approximately $2.008 \times 10^{20}$ Bq.
- Dominant Contributors: The inventory is dominated by fission products. The top contributors include $^{239}$ Np ( $3.883 \times 10^{19}$ Bq), followed by iodine isotopes ( $^{134}$ I, $^{133}$ I) and noble gases ( $^{133}$ Xe, $^{137}$ Xe).
- Group Contributions: Noble gases and halogens contribute significantly to the total activity ( $2.045 \times 10^{19}$ Bq and $2.283 \times 10^{19}$ Bq, respectively), while lanthanides and cerium-associated actinides also play major roles.
Radiological Release Potential:
- Under accident conditions, noble gases exhibit the highest release fraction (60.40%), followed by halogens (25.41%) and alkali metals (9.97%).
- The estimated total radioactive release into containment is approximately $3.386 \times 10^{19}$ Bq.
- Volatile isotopes such as $^{131}$ I, $^{133}$ Xe, and $^{137}$ Cs are identified as the primary drivers of off-site radiological impact.
Comparison with Reference Data (NUREG-1940):
- The study finds that calculated inventories are generally underestimated compared to NUREG-1940 reference values, with deviations ranging from approximately -15% to -50%.
- Noble gases showed underestimations of 15–44%, halogens around 30%, and alkali metals between -20% and -37%.
- Metastable isotopes (e.g., $^{131m}$ Xe, $^{129m}$ Te) exhibited the largest discrepancies (up to -52%), attributed to uncertainties in nuclear data libraries, isomeric formation ratios, and decay schemes.
- A rare positive deviation (+15.75%) was observed for $^{93}$ Y.

Significance and Claims
The authors assert that this study establishes a direct relationship between fuel burnup, radionuclide inventory evolution, and potential radiological releases for the VVER-1200 reactor. The findings are presented as a useful resource for:

Safety Analysis: Providing burnup-dependent inventories essential for Design Basis Accident (DBA) and severe accident consequence assessments.
Emergency Preparedness: Informing emergency planning zones and mitigation strategies by identifying the specific isotopes likely to be released (e.g., noble gases and iodine) and their quantities.
Regulatory Support: Offering data to support regulatory decision-making and the development of safety cases for the Rooppur NPP.
Comparative Safety: The study claims that the estimated source term for the VVER-1200 is generally lower than the releases observed in the Chernobyl disaster and comparable to or lower than those from the Fukushima Daiichi accident. This is attributed to the improved safety features and robust containment design of the VVER-1200, suggesting a lower potential for uncontrolled environmental release under the assumed conditions.

The paper concludes that while the VVER-1200 design offers enhanced safety, the presence of substantial inventories of volatile and long-lived radionuclides necessitates continued rigorous safety assessment, effective emergency planning, and severe accident management protocols tailored to the specific burnup-dependent characteristics of the fuel.