Reactor protection system

Short description: Nuclear safety and security components in a nuclear power plant

A reactor protection system (RPS) is a set of nuclear safety and security components in a nuclear power plant designed to safely shut down the reactor and prevent the release of radioactive materials. The system can "trip" automatically (initiating a scram), or it can be tripped by the operators. Trips occur when the parameters meet or exceed the limit setpoint. A trip of the RPS results in full insertion (by gravity in pressurized water reactors or high-speed injection in boiling water reactors) of all control rods and shutdown of the reactor.

## Contents

* 1 Safety role
* 2 Operational design
* 2.1 Software system
* 2.2 Cybersecurity
* 3 Implementation
* 3.1 Pressurized water reactors
* 3.2 Boiling water reactors
* 3.3 Advanced reactors
* 4 See also
* 5 References

## Safety role

The RPS provides a first line of automatic protective response to both anticipated operational occurrences and postulated accident conditions, by detecting abnormal conditions and initiating reactor trip and other protective actions that help preserve core and pressure-boundary integrity. [1] In advanced reactor I&C frameworks, the RPS may also provide additional post-trip functions to support continued heat removal after shutdown. [2]

Because protective actions depend on timely and reliable detection and actuation, the RPS is tightly coupled to plant sensors and actuators and is treated as among the most safety-significant subsystems in safety analyses. [3]

## Operational design

RPS actuation logic is configured around measured process variables (trip variables) and setpoints derived from safety analysis, including fuel and thermal-hydraulic design limits. [2] In order to ensure reliability, the RPS often employs multi-channel redundancy combined with voting logic to reduce spurious trips. One common configuration is four channels with 2-out-of-4 trip logic. [4]

Independence among redundant channels is emphasized as a design principle to mitigate random single failures and certain classes of common-cause vulnerabilities. However, while redundancy and independence can mitigate some failures, they may not address all systematic failures such as during natural disasters. [1]

### Software system

Digitization of safety I&C has been pursued to address obsolescence and to realize functional advantages such as improved diagnostics, but it also introduces new challenges, especially software failure and increased system complexity. As functionality now depends on extensive software, strict verification and validation (V&V) practices for safety-critical code are emphasized. [5] In a typical RPS software workflow, protection software is specified, designed in function-block or ladder-logic representations, translated into C code, and compiled for programmable logic controllers (PLCs). [6]

A variety of reliability modeling approaches are used for RPS software evaluation. Dynamic fault tree (DFT) modeling has been proposed to better capture changes in effective k-out-of-n logic caused by periodic testing and maintenance, addressing limitations of static fault trees in representing time-dependent configurations in RPS safety analysis. [7] Markov-based methods remain common for but may face scalability challenges. [8]

### Cybersecurity

Increasing digitization of RPS and other safety I&C has elevated the importance of cybersecurity controls for systems whose compromise could affect protective functions. Nuclear power plant cybersecurity measures emphasize a defense-in-depth model, including identification of critical digital assets, risk assessment, threat modeling, and establishment of layered protections. [4]

## Implementation

### Pressurized water reactors

Some of the measured parameters for US pressurized water plants would include:

* "High power", auctioneered between high nuclear power and high differential temperature (delta T) between the inlet and outlet of the reactor vessel (a measure of the thermal power for a given RCS flowrate).
* "High startup rate" (active below 10-4 percent power) at low power levels.
* "High pressurizer pressure"
* "Low reactor coolant flow"
* "Thermal margin / low pressure" (reactor power versus RCS pressure)
* "High containment pressure"
* "Low steam generator level"
* "Low steam generator pressure"
* "Loss of load" (main turbine trip)

Each parameter is measured by independent channels such that actuation of any two channels would result in an automatic SCRAM or reactor shutdown. The system also allows manual actuation by the operator.[9]

### Boiling water reactors

### Advanced reactors

While much of historical RPS development is rooted in large light-water reactor practice, RPS concepts and design requirements also appear in advanced and smaller reactors. For example, digital RPS development has also been documented for high-temperature gas-cooled reactors (HTGRs). [10]

## See also

* Nuclear power
* Nuclear safety and security
* Generation III reactor (evolutionary improvements of existing designs 1996–present)
* Generation IV reactor (technologies still under development unknown start date, possibly 2030)[11]

## References

1. ↑ 1.0 1.1 "Work Management to Optimise Occupational Radiological Protection at Nuclear Power Plants" (in en). doi:10.1787/9789264106857-en. https://www.oecd-nea.org/jcms/pl_14422/work-management-to-optimise-occupational-radiological-protection-at-nuclear-power-plants?details=true.
2. ↑ 2.0 2.1 OECD; Agency, Nuclear Energy (2005-10-30). "Occupational Exposure Management at Nuclear Power Plants: Fourth ISOE European Symposium - Lyon, France 24-26 March 2004" (in en). Radiation Protection. doi:10.1787/9789264010376-en. https://www.oecd.org/en/publications/occupational-exposure-management-at-nuclear-power-plants_9789264010376-en.html.
3. ↑ Ouni, Bassem; Aussagues, Christophe; Dhouib, Saadia; Mraidha, Chokri (2021-07-29). "Model-Driven Architectural Framework towards Safe and Secure Nuclear Power Reactors" (in en). Sensors 21 (15): 5136. doi:10.3390/s21155136. ISSN 1424-8220. PMID 34372378. PMC 8348382. https://www.mdpi.com/1424-8220/21/15/5136.
4. ↑ 4.0 4.1 Chowdhury, Nabin (2021-11-04). "CS Measures for Nuclear Power Plant Protection: A Systematic Literature Review" (in en). Signals 2 (4): 803–819. doi:10.3390/signals2040046. ISSN 2624-6120. https://www.mdpi.com/2624-6120/2/4/46.
5. ↑ Park, JaeKwan; Park, JeYun; Kim, YoungKi (2014-07-01). "System lifecycle processes for cyber security in a research reactor facility" (in en). Science China Information Sciences 57 (7): 1–12. doi:10.1007/s11432-013-4792-y. ISSN 1869-1919. https://doi.org/10.1007/s11432-013-4792-y.
6. ↑ Lee, Jong-Hoon; Kim, Eui-Sub; Yoo, Junbeom; Lee, Jang-Soo (2013-01-01). "A Preliminary Report on Static Analysis of C Code for Nuclear Reactor Protection System". IFAC Proceedings Volumes. 7th IFAC Conference on Manufacturing Modelling, Management, and Control 46 (9): 2134–2139. doi:10.3182/20130619-3-RU-3018.00259. ISSN 1474-6670. https://www.sciencedirect.com/science/article/pii/S1474667016346122.
7. ↑ Fristikawati, Yani (2022-02-14). "Legal analysis regarding nuclear power plant and its relation to the protection of environment and society" (in en). International Journal of Research in Business and Social Science (2147- 4478) 11 (1): 290–297. doi:10.20525/ijrbs.v11i1.1611. ISSN 2147-4478. https://www.ssbfnet.com/ojs/index.php/ijrbs/article/view/1611.
8. ↑ Ouni, Bassem; Aussagues, Christophe; Dhouib, Saadia; Mraidha, Chokri (2021-07-29). "Model-Driven Architectural Framework towards Safe and Secure Nuclear Power Reactors" (in en). Sensors 21 (15): 5136. doi:10.3390/s21155136. ISSN 1424-8220. PMID 34372378. PMC 8348382. https://www.mdpi.com/1424-8220/21/15/5136.
9. ↑ "NRC: Westinghouse (W) Reactor Protection System (RPS)". https://nrcoe.inl.gov/resultsdb/SysStudy/W.aspx.
10. ↑ OECD; Agency, Nuclear Energy (2005-10-30). "Evolution of the System of Radiological Protection: Second Asian Regional Conference - Tokyo, Japan 28-29 July 2004" (in en). Radiation Protection. doi:10.1787/9789264013636-en. https://www.oecd.org/en/publications/evolution-of-the-system-of-radiological-protection_9789264013636-en.html.
11. ↑ "Generation IV Nuclear Reactors". World Nuclear Association. http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/generation-iv-nuclear-reactors.aspx.

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