Severe Accident Management Guidelines (SAMG) — Entry, Diagnosis, Mitigation Strategies, Exit and Negative-Impact Evaluation

Principle and mechanism

How it was developed — from SECY-89-012 to generic owners-group SAMGs

After TMI-2 the United States set out the five elements of accident management in SECY-89-012 (strategies, decision-making framework, instrumentation and equipment, guidelines and calculational aids, training), and the owners groups for each reactor type produced generic SAMGs (WOG Rev. 0, June 1994; CEOG and B&WOG in the 1990s). Since Fukushima, SAMGs have been extended into an integrated SAMG that adds shutdown/low-power and spent fuel pool (SFP) strategies.

Structure of a WOG-type SAMG

The original WOG version separates the DFC from the severe challenge status tree (SCST), and the mitigation guidelines (SAG-1 to 8) from the severe challenge guidelines (SCG-1 to 4). Some implementations merge the SCST conditions into the DFC and absorb the SCGs into the SAGs, adding a step inside each SAG that checks whether a severe challenge setpoint has been exceeded. The DFC judges at two levels, "potential challenge" and "ongoing severe challenge", with the severe challenge taking precedence. The table and figures show such a merged variant.

Entry — core-exit temperature 649 °C

At power, the plant transfers to SAMG when the core-exit temperature (CET) rises to 649 °C (1,200 °F) or more and the EOP actions to restore core cooling have failed. In the Westinghouse ERGs, the transfer is made from FR-C.1. 649 °C is not a melting temperature but a sign that the core is uncovered and overheating (the UO₂ melting point is about 2,830 °C). At shutdown or low power, if CET cannot be used, core damage is judged from the radiation level in the upper containment. An SFP-only accident enters the SFP strategy directly on a low pool-level condition.

Seven mitigation strategies and their negative impacts

The numbers are the DFC's diagnostic priority in this variant; they do not mean every strategy is executed in turn. The order is SG injection → RCS depressurization → RCS injection → containment injection → fission product release control → containment conditions control → containment hydrogen control. APR1400-type plants put RCS depressurization ahead of SG injection to make external reactor vessel cooling (IVR-ERVC) succeed.

Each mitigation guideline reaches a decision in this order: ① confirm the means → ② identify negative impacts → ③ evaluate actions that mitigate those impacts → ④ compare with the consequences of not acting → ⑤ if acceptable, direct the action → ⑥ monitor the effect and long-term concerns. For example, containment spray lowers pressure, but condensing the steam raises the hydrogen mole fraction and can end the steam-inerted condition, so the hydrogen state is checked at the same time.

RCS depressurization

• Purpose: prevent high-pressure melt ejection (HPME) and direct containment heating (DCH) from a vessel failure at high pressure; prevent creep failure of the hot leg or SG tubes by high-pressure natural circulation (induced SGTR); make low-pressure injection sources usable (shutdown cooling and spray pumps, safety injection tanks, mobile pumps).
• Target (APR1400 example): 250 psia (1.72 MPa) or less before vessel failure.
• Means: APR1400 rapid depressurization through the four POSRVs; the OPR1000 safety depressurization system (SDS); the pressurizer PORVs in Westinghouse plants; indirect secondary-side depressurization through SG feed plus the atmospheric dump valves; pressurizer auxiliary spray.
• Negative impacts: in a high-pressure accident, manual depressurization amounts to a deliberate LOCA. Without an injection source, the loss of inventory speeds up core uncovery. Hydrogen, steam and fission products are discharged into containment, causing local hydrogen accumulation and pressurization. Hydrogen can surge when the safety injection tanks inject and the core is reflooded. Depressurizing the secondary side raises the differential pressure across the SG tubes.

What it means in a plant

Decision-making in the Technical Support Center (TSC)

When a radiological emergency is declared, the main control room first carries out the initial response strategies. Once the TSC is staffed, its evaluation team diagnoses with the DFC and decides on strategies, and the control room implements those decisions.

Exit

When all fission product boundaries are under control and the key variables are stable or decreasing within the exit criteria, the SAEG ends SAMG and hands over to the recovery organization.

Expansion after Fukushima and the PWROG SAMG (2016)

After Fukushima, shutdown/low-power and SFP SAMG were added, together with guidelines for mobile equipment and extensive damage (in the United States, the FLEX support guidelines (FSG) and EDMG).

In February 2016 the PWROG issued PWROG-15015-P. It merges the DFC, SCST, long-term monitoring and exit into a single diagnostic process guideline (DPG). Variable worksheets are flagged red, orange, yellow and green, and red is dealt with first. The SAGs and SCGs are combined into TSC SAG-3 to 10, and new technical support guidelines TSG-1 to 5 (instrumentation, decision makers, site capabilities, benefit and consequence information, calculational aids) were added. The US version turned the guidance into procedures, simplifying the comparison of gains and losses. Hydrogen reduction was placed ahead of containment pressure control, and the entry variable for the RCS injection strategy changed from core temperature to injection flow.

Regulatory and design criteria

SAMG is mostly framed by industry guidance and international standards; for example:

The United States included mandatory SAMG in its 2015 proposed rule and then dropped it from the final rule.

Easily confused pairs

Both belong to the wider accident management programme described in IAEA SSG-54: EOPs cover the preventive domain and SAMG the mitigative domain. The SAMG DFC carries the idea of the CSF status trees over into the period after core damage.

Key points at a glance

1. Definition: judgement-based guidelines for stopping damage progression, maintaining containment and minimizing releases after core damage.
2. Goals: end releases, maintain the fission product boundaries (containment), reach a controlled stable state.
3. Entry: CET ≥ 649 °C (1,200 °F) + failure of EOP core-cooling recovery.
4. Structure: SACRG-1 and 2 → TSC DFC (SCST merged in some variants) → SAG-01 to 07 (+ SFP) → LTMG and SAEG, CA.
5. Strategies: SG injection, RCS depressurization, RCS injection, containment injection, FP release control, containment conditions control, hydrogen control.
6. Decision: means → negative impacts → compare with the result of not acting → execute and monitor.
7. RCS depressurization: prevents HPME, DCH and induced SGTR; 250 psia (APR1400 example).
8. Exit: boundaries stable, variables stable or decreasing (SAEG).
9. Basis: SECY-89-012 and NEI 91-04 (US, voluntary); IAEA SSG-54 (2019).
10. Trends: integrated SAMG (LPSD, SFP); PWROG 2016 DPG and TSGs.