Progress in reactivity-initiated accident research

NSRR reactor core  at the instant of pulse irradiation (Image NSSR,JAEA)

A reactivity-initiated accident (RIA) is a nuclear reactor accident that involves an unwanted increase in fission rate and reactor power. RIA research is an important focus area for the Nuclear Energy Agency (NEA) Committee on the Safety of Nuclear Installations (CSNI) Working Group on Fuel Safety (WGFS); two new reports from the WGFS highlight the latest developments in RIA research, providing a knowledge base for RIA safety analyses and identifying gaps where additional collaborative research efforts are required.

The primary mission of the WGFS is to advance the current understanding and address cross-cutting issues related to fuel behaviour in accident conditions, including work on associated aspects of thermal-hydraulics, oxidation, chemistry, mechanical behaviour and reactor physics.

Their work has informed the updated report State-of-the-art Report on Nuclear Fuel Behaviour Under Reactivity-initiated Accident Conditions (RIA SOAR) – first published in 2010 – and presents advanced knowledge on nuclear fuel behaviour under RIA conditions for light-water reactors (LWRs) and conventional fuels.

This report examines scenarios for RIAs in major types of reactors and provides a description of key phenomena in fuel rod behaviour under RIAs, including burn-up effects. This updated publication also presents the tests and experiments that have been conducted to study RIAs. In the area of fuel safety analyses, it explores the transferability of test results to LWR RIA conditions and state-of-the-art methods for analysing postulated scenarios for RIAs in LWRs. Finally, there is a review of the most recent regulatory acceptance criteria for RIAs applied in twelve NEA member countries.

The ‘State-of-the-art’ report found that additional RIA simulation tests are needed; specifically tests that:

  1. Better represent conditions expected in the Light Water Reactor (LWR) operating fleet;
  2. Have extended instrumentation to measure key properties on-line; and
  3. Investigate the performance of new fuel types and mixed oxide fuels.

The NEA Committee on the Safety of Nuclear Installations (CSNI) oversees the NEA programmes and activities that support the maintenance and advancement of the scientific and technical knowledge base for the safety of nuclear installations. Their work informed Reactivity-Initiated Accident Fuel Rod Codes Benchmark Phases I-III: Synthesis Report, which included data from 21 organisations in 14 countries.

The report provides a synthesis of the three benchmark phases conducted by the WGFS between 2010 and 2019 with codes for calculating fuel behaviour in RIA. There was a broad spectrum of computer codes used, with:

  • Base irradiation codes;
  • 10 transient analysis codes; and
  • 12 statistical analysis tools (codes and in-house procedure).

One of the key findings of the report are the recommendations for code enhancements for RIA safety analysis.

Identifying knowledge gaps and the research that will be required to fill these gaps was an essential outcome of both of these new reports. A number of recommendations for specific research work are currently being addressed in various NEA joint safety research projects. The CABRI International Project (CIP) includes tests on new fuel types and mixed oxide fuels (NEA News 35.2). The High Burnup Experiments in Reactivity Initiated Accident (HERA) project under FIDES-II will better represent conditions expected in LWRs with tests in the TREAT reactor at INL, USA. 

In addition, a major recommendation from the latest research was to “…build a comprehensive and robust database consisting of results from both separate effect tests and integral RIA simulation tests,” a recommendation which is currently being addressed by a task group created within the WGFS.

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