Chemical analysis of VVER reactor fluids can provide much information about the core and reactor systems under accident conditions. This was of great importance after the Fukushima Daiichi NPP disaster in March, 2011. It is water coolant which carries most complete information about accident reactor systems, core fuel and spent fuel pool conditions and hence presents a great interest for better understanding of accident progression. A fuller picture can be provided only by multichannel repeated sampling from different points of NPU including a drain sump and a fuel pond.
APAS as a component of reactor accident management system is in compliance with the requirements of IAEA Safety Standard No.SSR-2/1 (2012) concerning the necessity to implement Post-Accident Sampling Systems (PASS) at the existing NPPs and NPPs under design and provides:
- Biological radiation shielding of operating personnel due to isolation of an initial sample (including radionuclides in gas) from the environment and remote-controlled sampling and dilution
- Reduction of initial sample radioactivity to the values acceptable for laboratory chemical and radiochemical analysis
APAS module is intended to take samples and dilute them with high-purity water at the ratio of 1:10, 1:100, and 1:1000. The following representative samples can be taken:
- water coolant from reactor vessel (two sampling points);
- water from borated water storage tank (two sampling points);
- water from spent fuel pool (three sampling points).
The APAS module is designed to be easily incorporated into accident management systems of the existing NPPs and NPPs under design. The sampling and dispensing line is designed to prevent the emission of aerosols and sample-dissolved gases (including volatile radionuclides) into environment and to ensure precipitation of suspended particles from water for further particle-size, chemical, and isotope analysis. The sample representativity is controlled by preliminarily pouring sampled water into the controlled leakage collection system or a collection basin and continuously measuring dosage rate till it is stabilized in a spiral-form sample loop. When the required degree of sample dilution is achieved, the corresponding signal is generated and necessary switching in a dispensing line is actuated without the intervention of an operator. At blackout accident all the components of the APAS module can be connected to a reliable power supply unit that is a component of an automatic remote control unit.
Cabinets for sample preparation and dispensing, a collector, a passive heat-exchanger are low-level hardware components that should be located in non-serviceable access-controlled rooms. A dosage rate sensor installed in a sample dispensing cabinet generates a signal that is displayed at the operator’s panel and informs the operator that dispensing is completed and the sample can be transported to the laboratory. Under normal operating conditions the APAS functionality and stability of dilution coefficients are checked by calibrating the dispensing loops using nonradioactive reference salt solutions or by dilution of working fluid samples (including water coolant from reactor vessel) and further comparison of their total radioactivity and radionuclide composition with reference values.
The operating modes of the APAS module are as follows:
- main operating modes:
- automated sampling and dispensing
- automatic sampling and dispensing
- manual sampling and dispensing control
- emergency work termination in case of abnormal situations occurring during operation in automatic control modes
- auxiliary process modes:
- deactivation mode
- flushing mode
- dilution check mode
- adjustment mode
| Technical parameters | |
| Max temperature of sampled fluid, °С | 365 |
| Max pressure of sampled fluid, MPa | 17,6 |
| Min linear rate of water flow in sampling line, m/s | 1 |
| Max length of sampling lines upstream of APAS module inlet, m | 100 |
| Thermal margin of passive heat exchanger without water exchange | at least 8 liquid samples |
| Max temperature of discharge water, °С | 40 |
| Max volume of diluted liquid sample, dm3 | 1 |
| Dosage rate threshold value for sampling process to be automatically blocked and for operator to make decision on sample dilution initialization moment based on radiation monitoring data, mSv/h | 3,0 |
When compared to the analogues, the advantage of the engineering solutions implemented in the APAS module and protected by the RF Patent No. 2699141 dated 04.03.2019 is that sampling and dispensing are carried out without direct contact of operators with highly radioactive fluids.
Information about system conditions, operator actions and process equipment parameters is recorded in an internal information carrier and available for further analysis. For the module to operate in auxiliary process modes the emergency sampling system should be extended with containers for preparation and storage of decontamination fluids and high-purity water.
Testing of APAS experimental model in the large-scale test facility that simulates working thermal and hydraulic conditions at VVER NPPs has shown the results convincing enough to put the APAS module into repetitive production accounting for design-oriented safety class and location conditions.