Matriz de problemáticas de la mentoría

During a reactor accident with fuel failure or core disruption, fission product gases (noble gases such as krypton and xenon and volatile fission products such as caesium iodine) are released from the reactor core. These gases may need to be removed to reduce the airborne radiation hazard. In addition, provisions are to be made to capture iodine after the accident to protect workers and the public.

Hydrogen is generated as a result of the radiolysis reaction of steam with zirconium alloy. Trapped hydrogen and hydrogen that continues to be generated needs to be removed to prevent explosions [11].

When the original containment system is intact, controlled release of the gases to the atmosphere can be conducted in compliance with regulations. At National Research Experimental, a research reactor in Chalk River [50], Windscale Pile 1 [51] and A1 NPP [52] the original containment functions were not lost. The radioactivity released to the environment was mainly in the form of gases from the discharge stacks during or following the accident.

FIG. 9. Fukushima Daiichi incineration facility. (Courtesy of TEPCO, Japan.)

FIG. 10. TMI-2 decay heat removal pathway. (Courtesy of EPRI, USA.)

5.1.4.1. TMI-2 gas control

Although the TMI-2 containment was not breached, the hydrogen that accumulated detonated and caused a brief pressure spike. Some damage was observed later, but it was not serious enough to impede operations. Access to the containment at TMI-2 was limited by an inside atmosphere with levels of ⁸⁵Kr that were unacceptably high for any sustained occupation (37 000 Bq/cc measured shortly before venting). To proceed with the cleanup safely and quickly and also to reduce risk of the unpredictable and uncontrollable leaks to the environment, the gas had to be removed. After a long process of preparation and review, the containment was vented of approximately 1700 TBq of ⁸⁵Kr and the first entry was made approximately 16 months after the accident.

Seven months of intensive licensing and legal effort was required to obtain the regulatory approval to vent.

During this period, an environmental assessment was prepared, a citizens’ monitoring programme was established that recruited area residents for monitoring activities during the purge, and the public was involved in a number of other ways.

The public process resulted in at least three schemes as alternatives to venting. One was selective absorption with charcoal, another was a scheme for a balloon-supported sleeve, and a third was for jet assisted boosting of the vent effluent to higher elevations. None of these were seriously considered by the regulatory agency. A fourth option, called the selective absorption process, was conceptualized at a USDOE laboratory. An independent technical evaluation of the process was conducted. It was concluded that purging is preferred in all respects, including feasibility, effectiveness, practicality, health and safety, psychological stress on the nearby population, schedule and cost.

Stress and other psychological impacts led to a consideration by the regulatory agency to allow faster venting than was originally proposed. The venting operation technical specifications allowed the use of real time meteorological data to compute off-site doses. This permitted the project team to take advantage of optimum dispersal conditions by increasing the release rate when meteorological conditions allowed, and thus complete the venting more rapidly while still meeting the requirements for release limitations.

Two existing systems were used for the purge. The hydrogen control system (modified with a higher capacity fan, new controls and interlocks) was used to vent at a rate up to 0.28 m³ per second while the containment atmosphere was rich in ⁸⁵Kr. The containment air purge and purification system was used for rates up to 8.7 m³ per second during later stages when the concentrations were lower. The flow rate was controlled based on the off-site integrated dose criteria. All releases were through the station vent, which contained monitoring instrumentation. An extensive off-site network of monitors and samplers was established for the purge.

Slow rate purges were conducted over an 11 day period followed by 4 days of fast purging. The operation was accomplished without incident. During this period, the ⁸⁵Kr concentration within containment dropped to approximately 2.2 Bq/cc. There were also a number of smaller purges later to vent the ⁸⁵Kr subsequently released from the water in the containment basement.

Following the ⁸⁵Kr venting, two technicians, heavily laden with protective gear and instruments, made the first post-accident entry into the dark and dripping wet containment building. With access, the project team was finally able to evaluate fully the damage to the plant and to work directly on the systems and equipment that had been most affected. More than 2000 entry days were to follow.

5.1.4.2. Chernobyl NPP gas control

At the Chernobyl NPP, the accidental explosion substantially destroyed the reactor building. The sarcophagus of Chernobyl NPP Unit 4 (called the shelter) was equipped with ventilation shafts for convection and ventilation systems.

A stationary dust suppression installation (SPP) was commissioned at the end of 1989 for the purpose of limiting the spread of radioactivity from the shelter to the outside. The SPP was made up of one distribution pipe header with 14 nozzles, covering the central part of the under roof space. The installation was designed to reduce the concentration of radioactive aerosols within the shelter premises and prevent their spread into the environment.

During the first years after its commissioning, the average activity of aerosols inside the shelter premises was reduced by a factor of ten. While in use (from the middle of 1990s), film forming compounds (fixatives) were applied to seal in place small quantities of dry residue.

Based on the results of SPP operation, it was found that the system was not operating efficiently enough because dust suppression was effective only within a limited area of the central hall (approximately one third of the total area). In 2003, the SPP was upgraded by extending the system coverage area to the whole under roof space of the shelter, and by optimizing the applied fixatives and modes of their application. The effectiveness of SPP operations on the radiation situation eventually decreased. However, its shutdown in the long term could result in increases of release and deteriorating radiation conditions both inside and outside the shelter.

Additional upgrades to the dust suppression system were done in 2004. The improvement resulted in the enlargement of the spraying area from 1500 m² to 5200 m². The number of spray nozzles was increased from 14 to 49 and; the network of collectors and pipelines was enhanced.

A full check of the upgraded SPP pilot industrial operation was conducted during 2004–2005, including the operation of nozzles in the shelter under roof space. Optimizing modernized SPP (MSPP) operation modes drastically reduced the number of leakages of fixatives and solutions into the bottom premises of the shelter and helped to decrease the consumption of dust suppression composition.

The MSPP was put into routine operation in 2006 and its additional safety function (protection function) is the reduction of the effective neutron multiplication factor (K_eff) in fuel containing material (FCM) accumulations located in the reactor hall. The reduction of K_eff is achieved by spraying a neutron absorbing solution of gadolinium nitrate onto the surfaces of those accumulations. The solution is sprayed when the safe operation limits are exceeded over the parameters measured by the systems controlling the condition of FCM accumulations. It was further proposed to establish an additional operation mode of MSPP that would allow for radioactive aerosol deposition from the air during emergency situations, including in negative temperatures.

In 2006–2008, the MSPP was used for the preparation of work areas in the shelter under roof space during shelter stabilization activities. Attention was given to the application of a protective polymeric coating on the localized dust forming surfaces to ensure reliable immobilization of dust forming substrates (sand, construction chippings). High immobilization effect was confirmed especially for alpha and beta contaminants. Various dust suppression compositions were tested and the optimal one, marked AK-510 based on siloxane acrylate, was selected before MSPP commissioning. This composition ensured all necessary characteristics, including the following:

— Required thickness of surface up to 200 mm;

— Drying time up to grade 3;

— Necessary adhesion and uniformity of thickness distribution;

— Chemical inertness;

— Water resistance;

— Flame spread rate and smoke developed index;

— Time of protective effect, radiation resistance.

The MSPP can be used for adding neutron absorbing materials (0.1% of gadolinium solution) in accumulations of FCMs, both in addition to and independently from the standard system of adding the gadolinium solution to the MSPP upgrade. The addition of neutron absorbing materials resulted in the following:

— Improved shelter radiation and nuclear safety during the current operation and emergency situations;

— Improved working conditions of personnel;

— A protective polymeric coating (an essential preventive measure of safety in case of an accidental collapse of shelter structures).

The MSPP operations succeeded in reducing the shelter environmental impact by decreasing (more than twice) the release of radioactive aerosols from the shelter and by decreasing (more than four times) the loose surface contamination in the under roof space.

5.1.4.3. Fukushima Daiichi NPP radioactive gas control

At Fukushima Daiichi, for each of Units 1, 2 and 3, which still have fuel materials in the core, a system was installed to control the gas discharge from the containment vessel by keeping the gas at a negative pressure with filtered circulation. It also has a defence in depth function of nitrogen injection to prevent the possibility of

explosion from hydrogen that continues to be released, although in much smaller quantities compared with those during the accident.

Building covers were installed on Units 1, 3 and 4, where the upper parts of the reactor building were destroyed during the accident [9]. The covers consist of frames and panels. The cover for Unit 4, where all the fuel was within the spent fuel pool, used the remaining reactor building as the support. It was completed in 2013; fuel removal started in November of that year, and was completed in December 2014. The covers for Units 1 and 3 are self-supporting. Construction of more reliable containment that enables good working conditions for full scale cleanup activities, such as fuel debris retrieval, will ultimately occur [7].