Radioactive waste in the Sarcophagus

By fall 1986, the measurements conducted by a number of scientific research institutes, state hydrometeorology services made it possible to estimate the fuel component release. The fuel component release was 3+/- 1.5% of the total pre-accident core fuel. These figures for the fuel component fallout (deposits) were subsequently confirmed by the analysis of thousands of soil samples: 3.5+/- 0.5%.

However, the results for the volatile components were quite different. The ¹³⁷Cs release was estimated to be 20% of the total accumulated activity (which was 2.6 x 10¹⁷ Bq), while the ¹³¹I release was 30% of the total accumulated activity.

Quantitative Estimate of Nuclear Fuel Modifications Inside the Shelter. Studies conducted between 1986 and 1995 demonstrate that the irradiated nuclear fuel in the Shelter (Sarcophagus) exists in four modified forms:

– core fragments

– fuel particles or radioactive dust

– solidified lava-like fuel-containing material

– dissolved forms of certain radionuclides.

Fuel distribution data are presented in Table 2-1. However, currently available information cannot be used to complete a quantitative mass balance for the core. This will require additional research. Only an upper and lower boundary on this value can be determined.

Table 2-1. Distribution of Fuel-Containing Material by Shelter Room/Location

Room/Location (elevation)	FCM Classification and State	Estimate of Fuel Content of FCM (in metric tons of uranium)
Central hall (35.50) other upper unit locations	Core fragments (the majority are saturated with materials ejected during the active stage of the accident; lava-like FCMs may be located underneath the material) In the vicinity of System E - 10 - 36	60–70
Southern spent fuel pool (18.00-35.50)	Spent fuel assemblies	~20
Majority of locations	Fuel dust. Hot fuel particles	~10
Subreactor locations 305/2 (9.00) + 307/2 + System OR + reactor vault	Lava-like FCMs, core fragments	75
Steam distribution corridor (6.00) allowing for FCM in the valves	Lava-like FCMs	25^+/-11
Second level pressure suppression pool (PSP-2)	Lava-like FCMs	8^+/-3
Level 1 pressure suppression pool (PSP-1)	Lava-like FCMs	1.5^+/-0.7
304/3, 304/3, 304/3, 304/3, “Elephant’s Foot”, etc.	Lava-like FCMs	11^+/-5
Reactor unit, reactor building auxiliary system and Turbine Hall locations	Water with dissolved uranium salts	~3,000 m^3 water < 3 kg uranium
Fuel under cascade wall	Core fragments	Not identified
Fuel at site under concrete and crushed rock layer	Core fragments, fuel dust	+0.3 0.6^+0.3/-0.2
FCM = fuel-containing material.

The majority of the core fragments were ejected during the explosion into the upper stages of the unit, specifically, into the central hall. The finely dispersed fuel (dust) is made of hot fuel particles that vary in size from fractions of a micron to hundreds of microns. The fuel dust can be observed in virtually all locations throughout the facility. Solidified lava-like fuel-containing material was formed during the active stage of the accident (26 April–6 May 1986) from the high-temperature interaction between the fuel and structural materials of the unit. The lava-like material spread into the subreactor locations. In 1990, scientists discovered that water located in a number of the lower areas of the facility contained dissolved forms of uranium, plutonium and americium. These soluble compounds were caused by the breakdown of a variety of compounds created from the uranium dioxide fuel under multiple factors, the primary such factor being the water that penetrated the Shelter.

Core Fragments. Significant quantities of isolated core fragments and accumulated fragments were observed around the destroyed Unit 4 reactor immediately following the accident. Fuel assemblies, fuel elements, and isolated parts of these components were ejected by the explosion onto the vent stack, the roofs of the deaerator mezzanine, the turbine hall, and the reactor building auxiliary systems as well as the roof of Unit 3 structures and in the central hall. Some of the fragments were remediated following the accident. However, the majority of the fragments were not remediated. Researchers have performed several studies to determine the location of the fragments.

Several methods were used during accident remediation to deal with core fragments. Some of the core fragments from the site were moved to the debris pile and were buried in the cascade wall. Other core fragments were placed into containers with highly radioactive waste or were buried under the concrete and crushed rock layer surrounding the building. Some of the core fragments located on the roofs of buildings and the vent stack were discarded into the collapsed area of the building. Finally, a small quantity of core fragments remained on the vent stack areas and on the roofs.

The majority of the core fragments are probably in the reactor central hall. However, the material dropped by helicopter to put out the reactor core fire makes it impossible to directly observe significant accumulations of core fragments. Nonetheless, indirect facts such as the nature of the explosion, the large number of fragments ejected onto the roofs and around the reactor building, and the likely trajectories of the core fragments are consistent with this hypothesis. It is also confirmed by the rather significant number of fragments observed in the northwest and western sections of the central hall.

A comparison of the gamma-scanning data to the video and later photo survey data suggests that the majority of the fuel is located in fuel channel remnants on the surface of System E. Approximately 1 metric ton of fuel is located on the walls of the central hall and other structural elements outside System E.

Efforts have also been made to assess the location and states of nuclear fuel in the spent fuel pool and new fuel storage areas. Periscopic observations and video surveillance revealed that the panels containing fuel assemblies in the southern spent fuel pool continued to be suspended in their expected locations after the accident without noticeable damage.

Periscopic and video surveys revealed core fragments in the reactor vault. These core fragments were located on System OR. The number of such fragments is difficult to determine because of the collapse of the graphite masonry and concrete that was added to the shaft following the accident.

Approximately 10 accumulations of core fragments were discovered through visual surveys in the subreactor area. It is noteworthy that intact fuel pellets (uranium dioxide) from this location were identified from samples of the lava-like fuel-containing material indicating that there was a very broad range of temperatures in the area following the accident.

As the core fragments represent the most hazardous form of nuclear material from an occupational and environmental safety viewpoint, a conservative estimate, i.e., the upper limit on the quantity of this fuel modification, should be used. The total pre-accident uranium mass in the core, the southern spent fuel pool, and the central hall is estimated at 212 metric tons (this is a maximum value, neglecting any fuel assemblies removed from the fuel preparation area), while the uranium mass of the lava-like fuel-containing materials and fuel particles is estimated at 80 metric tons (the minimum value). The difference between these values (132 metric tons) after subtracting the ejected fuel (6 tons) is equal to 126 metric tons. This value represents an estimated upper limit on the uranium mass in the form of core fragments.

Fuel Particles or Radioactive Dust. A fairly large number of studies have been devoted to analyzing the physical and chemical composition of the radioactive dust (also called fuel particles or finely dispersed fuel) in various locations throughout the Shelter.

Calculating the total amount of finely dispersed fuel on the surface of the central hall debris and other open surfaces under the roof of the Shelter provides a conservative estimate, resulting in an order‑of‑magnitude value of 34 metric ton. Other efforts to calculate the amount of radioactive dust in other shelter locations have been based on measurements of the dose exposure rates in these locations. These calculations assumed that in those areas lacking noticeable quantities of core fragments or “fuel lava,” the dose exposure rate was determined by the uniform layer of active dust remaining on the walls, floors, and ceiling of the room location.

Soluble Fuel Forms. For some time, it was believed that none of the compounds created from the uranium dioxide fuel during the accident were soluble. However, in September 1990, bright yellow spots and pools were discovered on the surface of solidified lava flows in the steam separation corridor. Analysis of these spots revealed the presence of soluble uranium compounds. X-ray phase and microstructure analysis revealed the following compounds:

– UO₂CO₃ Rutherfordine

– UO₃ ^. 2H₂O Eliantinite

– UO₃ ^. 16CO₂ ^. 1.19H₂O Studitite

– UO₄ ^. 4H₂O

– Na(UO₄)₂(CO₃)₃

The plutonium isotope content in these new formations is hundreds of times lower while the mass concentration of uranium is several times greater than in the lava-like fuel-containing materials.

Lava-Like Fuel-Containing Materials. The high temperatures associated with the accident melted the zirconium fuel cladding and led to an interaction between the molten zirconium and the uranium dioxide, resulting in a uranium-zirconium-oxygen phase. When this phase interacted with structural materials (serpentine, concrete and sand) as well as air, lava-like fuel-containing materials were formed.

Researchers working at Chernobyl NPP Unit 4 encountered this lava-like fuel-containing material for the first time in the fall of 1986. Subreactor location 217/2 was found to contain a large solidified mass, approximately 1 m wide that came to be called the Elephant’s Foot (Figure 2.2-1). Analysis of the Elephant’s Foot revealed that it consists primarily of silicon dioxide with other compounds as impurities, including uranium compounds. The mixture of radionuclides found in samples of the Elephant’s Foot match those found in the irradiated nuclear fuel with an average burnup for Unit 4.

Figure 2-1. Elephant’s Foot, a lava-like fuel-containing materials formation

Lava-like fuel-containing materials were subsequently discovered in many subreactor room locations. This material contains significant amounts of pre-accident uranium from the reactor core and a significant number of reactor radionuclides.

Formation and dissemination of the lava-like fuel-containing materials: the heat in the reactor core during the accident and afterwards may have melted portions of the uranium dioxide fuel and other materials in the core. The precise sequence of events that created the lava-like material is unknown. However, researchers do know the lava remained heterogeneous – much of the structural metal retained its composition while the ceramic mass interacted with structural materials as it flowed.

The melted materials formed into a larger mass. As the mass of the melt increased, it spread along the floor of location 305/2, reached the edges of the steam relief valves, migrated downward through the valves, and passed into the upper and lower portions of the Suppression Chamber, part of the confinement systems designed to isolate steam in a design-basis accident. These locations include the steam distribution corridor and two levels of the pressure suppression chamber, which are located at elevations 6.00, 3.00 and 0.00, respectively.

The melt could also flow horizontally, since a breach (or a burn-through hole––this is not known precisely) formed in the wall between locations 305/2 and 304/3.

Physicochemical properties and radionuclide composition of the lava-like fuel-containing materials: In studying the lava-like materials, several materials are easily identified. The most common materials can be arbitrarily categorized as follows:

– black ceramic with 4-5% uranium concentration by mass

– black ceramic with 7-8% uranium concentration by mass

– brown ceramic with 9-10% uranium concentration by mass

– slag and pumice-like fuel-containing materials.

Figure 2-2 is a histogram of the percentage uranium for the lava‑like fuel-containing material (based on data from 225 samples), while Figure 2.2-3 is a histogram of the fuel burnup for the lava‑like materials.

Figure 2-2. Fuel content (as U) in pure lava

Figure 2-3. Fuel burnup in lava-like fuel-containing material samples

Degradation of lava-like fuel-containing materials: Degradation of the lava-like fuel-containing masses is significant because it represents a transition from a stable to a less stable form of radioactive material, increasing the amount of activity available for release to the environment. The degradation begins with a loss of integrity in the surface layers of the lava-like fuel-containing materials. This can accelerate processes that depend on the fuel-containing material contact area and the interacting medium: dissolution, erosion, etc. Thus, analyses were conducted on the lava-like material decomposition. The analyses have revealed three decomposition processes:

– surface decomposition of the lava-like materials and the formation of radioactive dust on these materials

– leaching of radionuclides from interaction between the “lavas” and water, and the formation of new chemical compounds on their surfaces

– matrix cracking due to interior stresses.

An initial attempt to take a sample of lava-like fuel-containing material from the Elephant’s Foot demonstrated its significant strength; small weapons were required to separate a sample from surface of the formation in 1987. Repeat sampling from the Elephant Foot, beginning in 1990, no longer requires significant effort. Attempts to remove surface contamination from the Elephant’s Foot, by means of a glue-treated wad, easily separated an upper 1-2 cm layer. At present, the lava degradation processes have caused the Elephant’s Foot to nearly lose its initial shape, to slide down and settle. According to observers, the lava-like fuel-containing materials at room locations 304/3, 305/2, etc. have undergone similar changes.

Processes leading to lava-like fuel-containing material embrittlement have not been the focus of special study. The most likely mechanism is associated with cooling of the lava-like materials and their impregnation with water: observations have revealed that the temperature of even small lava-like materials differs only slightly from the ambient air temperature. In winter, freezing water in the pores and microcracks of the lava-like materials may lead to their cracking. Moreover, microscopic analyses have revealed the severe inhomogeneity of the lava-like materials on the microlevel. Interior stresses caused by differences in the coefficients of thermal expansion of the components may arise as the temperature of the lava-like materials decreases substantially below the fuel-containing material solidification temperature. Because of the heterogeneous quality of the lava-like fuel-containing material, the coefficient of thermal expansion and even melting temperatures can vary significantly. Given this situation and the uncontrolled rate at which the lava-like fuel-containing material cooled, there are significant residual natural stresses that can accelerate the degradation of the material.

References

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