Policy, Economics & Reactor Types · C-PE-002

APR1400 Design Features — What Changed from the OPR1000 (Power, Design Life, Seismic, Safety Injection, MMIS, Severe Accidents)

How the 1,400 MWe evolutionary PWR moved from the OPR1000: 3,983 MWt, 60 years, SSE 0.3 g, four DVI trains, an in-containment water tank and a fully digital control room.

Concept C-PE-002

Definition

The APR1400 is a 1,400 MWe-class evolutionary pressurized water reactor developed on the basis of OPR1000 operating experience and the US System 80+ design, to a utility requirements document prepared with reference to the US EPRI ALWR requirements. It keeps the basic two-loop configuration and raises power, design life, seismic capacity, safety injection, instrumentation and control, and severe-accident response by one step each.

Development began in 1992; in the United States the design was certified in 2019 (10 CFR Part 52, Appendix F). It is an evolutionary design based on active safety systems, not a passive design.

Principle and mechanism

Requirements drive the design — the utility requirements

The APR1400's design features follow from the utility requirements set early in development. The main values of the utility requirements document are:

Thermal power 4,000 MWt (NSSS), design life 60 years, SSE 0.3 g
Core damage frequency (CDF) below 10⁻⁵/RY, containment failure frequency (CFF) below 10⁻⁶/RY
Core thermal margin of 10 % or more, at least 30 minutes for operator action, SBO coping of 8 hours or more
Availability 90 %, occupational dose below 1 man-Sv/RY, fewer than 0.8 unplanned trips per year

The design PSA results are CDF 2.25×10⁻⁶/RY and CFF 2.84×10⁻⁷/RY, inside the targets (developer's published PSA results). The CDF and CFF targets are utility performance goals and sit at a different level from regulatory safety goals.

Primary system — the same two loops, larger margins

The piping arrangement is the same as the OPR1000 (two loops, each with one hot leg, two cold legs, one SG and two RCPs). What changed is the margin.

Hot-leg temperature 621 °F → 615 °F: more operating margin and less stress corrosion in SG tubes
Larger pressurizer: absorbs transients up to Condition III without a reactor trip
SGs: Alloy 690TT tubes (13,102 per SG), tube-plugging margin 8 → 10 %, more secondary-side inventory for a longer dry-out time
Core of 241 assemblies (16×16 PLUS7), 93 CEAs, cycle of 18 months or more, thermal margin of 10 % or more
The spring safety valves on top of the pressurizer and the separate SDS valves are combined into four POSRVs (set pressure 2,470 psia, 244.9 t/h each).

Safety injection — from two cold-leg trains to four direct vessel injection trains

The OPR1000 injects water from the RWT outside containment into the cold legs with two HPSI and two LPSI pumps. When the RWT empties, a recirculation actuation (RAS) is needed to switch suction to the containment sump. The APR1400 changed this as follows.

Four mechanically independent trains: one SIP and one SIT per train, with the cross-connecting piping (common header) removed. Electrically there are two divisions (two EDGs, each supplying two SIPs).
Direct vessel injection (DVI): water goes straight into the upper downcomer of the reactor vessel, reducing the path by which injection water spills out of a cold-leg break.
SIT fluidic device: while the water level is above the standpipe, flow is high; below it, vortex resistance gives a low flow for a long time (about 200 seconds) → the large LPSI pumps are eliminated.
IRWST: the refueling water is kept in an annular tank inside containment and serves as the single source for safety injection, containment spray and shutdown cooling. Spilled water returns to the IRWST, so there is no recirculation switchover, and the tank also serves as the quench tank for POSRV steam discharge and as the water source for reactor cavity flooding.

The shutdown cooling system was separated from safety injection, and its design pressure was raised to 900 psig against interfacing-system LOCA.

Left: OPR1000 schematic with the refueling water tank outside containment, two HPSI and two LPSI pumps injecting into the cold legs, four SITs, break flow to the sump and a recirculation switch. Right: cutaway of the APR1400 containment with the in-containment refueling water storage tank, one of four safety injection pumps drawing from the IRWST and injecting directly into the reactor vessel upper downcomer, SITs with fluidic devices feeding the same-train DVI line, and break flow draining back to the IRWST so no suction switchover is needed — Safety-injection water paths: OPR1000 (two trains, cold-leg injection, RWT outside containment, recirculation switchover) and APR1400 (four mechanical trains, direct vessel injection, IRWST inside containment, SIT fluidic devices replacing LPSI). Conceptual layout; pipe numbers and locations simplified.

Auxiliary feedwater and power supply

Auxiliary feedwater has two divisions, each with one 100 % motor-driven pump, one 100 % turbine-driven pump and a dedicated auxiliary feedwater storage tank. For emergency power, the original design has two EDGs (8,000 kW in the first units) plus a 7,200 kW AAC-DG per unit, while the US certified design (DC) has four EDGs plus a gas-turbine AAC.

Instrumentation and control — a fully digital MMIS

The main control room consists of a large display panel (LDP), operator workstations, soft controls, computer-based procedures and a safety console for essential safety functions. Safety I&C (PPS, CPCS, ESF-CCS) runs on a four-channel PLC platform, and a DPS on a different platform guards against software common-cause failure (two channels with 2-out-of-2 logic in the original design; four FPGA channels with 2-out-of-4 logic in the US DC).

Severe-accident response

Dedicated features were provided to meet severe-accident requirements for evolutionary LWRs (for example, SECY-93-087 in the United States): rapid depressurization through the POSRVs (to 250 psia or below), 30 PARs plus 10 igniters, the cavity flooding system (IRWST → HVT → cavity), a large reactor cavity (0.02 m²/MWt), and the emergency containment spray backup system (ECSBS).

What it means in a plant

The differences in numbers

Item	OPR1000	APR1400
Core thermal power / electrical output	2,815 MWt / 1,000 MWe class	3,983 MWt (NSSS 4,000) / 1,400 MWe class
Design life	40 years	60 years
Seismic (SSE)	0.2 g	0.3 g
CDF / CFF targets	10⁻⁴ / 10⁻⁵	10⁻⁵ / 10⁻⁶
Safety injection	2 trains (HPSI, LPSI), cold leg, RWT (outside)	4 SIP trains, DVI, IRWST (inside), SIT + FD
Pressurizer overpressure protection	Safety valves + separate SDS	4 POSRVs serving both
Main control room	Mainly analog	Fully digital MMIS
Dedicated severe-accident features	None (added later)	SDVS, PAR, CFS, ECSBS

Dumbbell chart comparing OPR1000 and APR1400: core thermal power 2,815 to 3,983 MWt (x1.41), design life 40 to 60 years (x1.5), SSE 0.2 to 0.3 g (x1.5), safety injection mechanical trains 2 to 4 (x2), CDF target below 10^-4 to below 10^-5 per reactor-year and containment failure frequency target below 10^-5 to below 10^-6 per reactor-year on log scales (target cap 1/10) — OPR1000 → APR1400 in numbers. Each row has its own scale; risk rows are logarithmic. CDF/CFF targets are utility design requirements, not regulatory safety goals; thermal power is on a core basis (NSSS 4,000 MWt includes RCP heat).

Differences within the APR1400 — later-unit improvements

Later APR1400 units added the following design improvements over the first units.

Item	First units	Later units	Purpose
Aircraft impact assessment	Not applied (design predates the requirement)	Containment and auxiliary building walls 4.0 → 4.5 ft	Prevent perforation in a large aircraft impact
Severe-accident depressurization	POSRVs + manual switching of a three-way valve	Dedicated ERDV, two trains, added	Remove manual switching, prevent hydrogen build-up in the IRWST
Reactor cavity concrete	Basaltic + 1 ft of limestone	Limestone (LCS) only	Better coolability in MCCI
Batteries	Existing capacity	24 h design	Extended SBO coping
Flooding protection	—	Watertight doors for EDGs, submersible drain pumps	External flooding

The US DC differs in its four EDGs, gas-turbine AAC, FPGA-based DPS, single-unit layout and cooling towers.

Building layout

The APR1400 places twin units side by side (slide-along) with shared buildings between them. The reactor building and auxiliary building sit on a common basemat, which raises seismic resistance and reduces the walls between buildings. The auxiliary building surrounds the reactor building, and the four SIPs are placed one per quadrant so that fire, flooding or sabotage cannot spread between trains; the two EDGs are also separated in compartments on opposite sides. The turbine building is placed so that the reactor building lies on the extension of the turbine axis, reducing the turbine-missile risk. The equipment hatch (inside diameter 7.8 m) is sized for one-piece SG replacement.

Regulatory and design criteria

For example, the US NRC criteria that apply to evolutionary designs such as the APR1400:

Basis	Key point
10 CFR Part 52, Appendix F	US design certification of the APR1400 (2019)
SECY-93-087, 10 CFR 52.47(a)(23)	Severe-accident design of evolutionary LWRs
10 CFR 50.150	Aircraft impact assessment (new designs after 13 July 2009)

Easily confused pairs

	APR1400	APR+	EU-APR
Nature	Evolutionary, active safety systems	Successor to the APR1400, more passive features such as passive auxiliary feedwater	Version modified for the European requirements (EUR)
Electrical trains	2 (2 EDGs)	4	4
Ex-vessel melt	CFS + large cavity	—	Core catcher

"Four mechanical trains = four EDGs" is not true for the original APR1400 design, which has two EDGs; the US DC has four.

Key points at a glance

Definition: a 1,400 MWe-class evolutionary PWR based on the OPR1000 and System 80+, built to utility requirements referencing the EPRI ALWR requirements (two loops, four RCPs).
Requirements: 60 years, SSE 0.3 g, CDF < 10⁻⁵ and CFF < 10⁻⁶/RY, thermal margin ≥ 10 %, SBO 8 h.
Primary system: 615 °F, larger pressurizer and SGs, Alloy 690TT, four POSRVs (safety valve and depressurization combined).
Safety injection: four mechanical trains with DVI, SIT fluidic device (no LPSI), IRWST (no recirculation switchover; single water source, quench tank for POSRV discharge, cavity-flooding source, protection from external hazards).
Power: two EDGs + AAC-DG (US DC: four EDGs + gas turbine).
I&C: fully digital MMIS (LDP, soft controls, computerized procedures), four-channel PPS + DPS.
Severe accidents: POSRV depressurization, PARs and igniters, CFS, ECSBS.
Layout: common basemat, SI pumps in separate quadrants, turbine-axis orientation.
Later units: aircraft impact walls 4.5 ft, ERDV, LCS cavity, 24 h batteries.

Plot plan: circular reactor building surrounded by the auxiliary building on a common basemat, four safety injection pump rooms SIP-1 to SIP-4 in separate quadrants, fuel handling area and main control room in the auxiliary building, EDG-A and EDG-B on opposite sides, and the turbine building on the extension of the turbine axis with missile directions normal to the axis — Sketch of the APR1400 building layout (concept only, not to scale).