diff --git a/index.html b/index.html
index 34ff839..7e47541 100644
--- a/index.html
+++ b/index.html
@@ -146,7 +146,7 @@
         Roadmap:
       </p>
       <ul>
-        <li>Reactor verification and stats</li>
+        <li>Reactor stats</li>
         <li>Casing improvements</li>
         <li>Keyboard shortcuts</li>
         <li>MSR support</li>
diff --git a/scripts/dev-mode.js b/scripts/dev-mode.js
index 0c7a91d..21948f3 100644
--- a/scripts/dev-mode.js
+++ b/scripts/dev-mode.js
@@ -39,7 +39,7 @@ registerLoadCallback(() => {
   });
 
   awaitEvent("config:end").then(() => {
-    void loadExampleFile("neutron test");
+    void loadExampleFile("Pandora's Fission Reactor");
   });
 
   queueMicrotask(() => emitEvent("dev-mode:end"));
diff --git a/scripts/files.js b/scripts/files.js
index 3cb2ca9..8fd080c 100644
--- a/scripts/files.js
+++ b/scripts/files.js
@@ -132,7 +132,7 @@ export async function saveFile(name = undefined) {
                 if (!fullBlock || !fullBlock.modules || !("ncpf:block_recipes" in fullBlock.modules)) return;
 
                 const designIndex = designBlocksAtlasMap[Number(designAsCellData[x][y][z].tile)].designIndex;
-                const solverMetadata = getMetadata(designAsCellData[x][y][z].solverMetadata);
+                const solverMetadata = getMetadata(designAsCellData[x][y][z].metadata);
                 const minimizedBlock = minimizedBlocks[designIndex];
                 let recipeIndex = -1;
 
@@ -156,6 +156,8 @@ export async function saveFile(name = undefined) {
                   }
                 }
 
+                console.log(recipeIndex)
+
                 return recipeIndex;
               })
               .filter(b => b != null)
diff --git a/scripts/overlays.js b/scripts/overlays.js
index b7cb80e..786bb9c 100644
--- a/scripts/overlays.js
+++ b/scripts/overlays.js
@@ -11,6 +11,19 @@ export function processCellOverlays(node, key = null, added = null) {
       overlayNode.classList.add("planner--overlay");
       node.appendChild(overlayNode);
     }
+
+    if (key === "active" && metadata) {
+      const overlayNode = makeAtlasImage(-1);
+      overlayNode.dataset.overlay = key;
+      overlayNode.classList.add("planner--overlay");
+      node.appendChild(overlayNode);
+    }
+    if (key === "active" && !metadata) {
+      const overlayNode = makeAtlasImage(-1);
+      overlayNode.dataset.overlay = "inactive";
+      overlayNode.classList.add("planner--overlay");
+      node.appendChild(overlayNode);
+    }
     // FIXME: handle remove
   }
   // TODO: handle generic update
diff --git a/scripts/planner.js b/scripts/planner.js
index b81bdb8..ed36ae0 100644
--- a/scripts/planner.js
+++ b/scripts/planner.js
@@ -384,6 +384,22 @@ export function* getCellTooltip(cell) {
     const moderator = data.modules["nuclearcraft:overhaul_sfr:moderator"];
     yield `Flux factor: ${moderator.flux}`;
     yield `Efficiency: ${moderator.efficiency}`;
+    yield {
+      type: "compound",
+      items: [
+        "Moderator ",
+        {
+          type: "bold",
+          text: solverMetadata?.active ? "active" : "inactive",
+        },
+        " for heat sinks",
+      ],
+    };
+    if (solverMetadata?.flux) {
+      for (const [axis, flux] of Object.entries(solverMetadata.flux)) {
+        yield `Flux on ${axis}: ${flux}`;
+      }
+    }
   }
 
   if ("nuclearcraft:overhaul_sfr:reflector" in data.modules) {
@@ -394,13 +410,19 @@ export function* getCellTooltip(cell) {
 
   if ("nuclearcraft:overhaul_sfr:heat_sink" in data.modules) {
     const heatSink = data.modules["nuclearcraft:overhaul_sfr:heat_sink"];
-    if (heatSink.rules) {
+    if (heatSink.rules && !solverMetadata.active) {
       for (const rule of heatSink.rules) {
         yield "Rule not met: " + parseRuleAsString(rule, { configurationHint: metadata.configuration });
       }
     }
   }
 
+  if ("nuclearcraft:overhaul_sfr:irradiator" in data.modules) {
+    if (solverMetadata?.flux) {
+      yield `Flux: ${solverMetadata.flux}`;
+    }
+  }
+
   yield {
     type: "italics",
     text: metadata.configurationAddon.name,
diff --git a/scripts/solver.js b/scripts/solver.js
index 03cdb2b..415cc2d 100644
--- a/scripts/solver.js
+++ b/scripts/solver.js
@@ -72,6 +72,18 @@ function *travelUntil(grid, { x, y, z }, {x: dirX = 0, y: dirY = 0, z: dirZ = 0}
     yield grid[x][y][z];
   }
 }
+function neighbor(grid, { x, y, z }, {x: dirX = 0, y: dirY = 0, z: dirZ = 0}) {
+  x = Number(x);
+  y = Number(y);
+  z = Number(z);
+  if (x > 0 && y > 0 && z > 0
+        && x < grid.length - 1 && y < grid[0].length - 1 && z < grid[0][0].length - 1) {
+    x += dirX;
+    y += dirY;
+    z += dirZ;
+    return grid[x][y][z];
+  }
+}
 
 async function debugBlockSolver(data) {
   const planner = getPlannerDom();
@@ -107,13 +119,15 @@ export async function runSolver() {
     // const reflectors = plannerBlocks.filter(b => b.tile.modules && b.tile.modules["nuclearcraft:overhaul_sfr:reflector"]);
     // const shields = plannerBlocks.filter(b => b.tile.modules && b.tile.modules["nuclearcraft:overhaul_sfr:neutron_shield"]);
     // const moderators = plannerBlocks.filter(b => b.tile.modules && b.tile.modules["nuclearcraft:overhaul_sfr:moderator"]);
-    // const coolers = plannerBlocks.filter(b => b.tile.modules && b.tile.modules["nuclearcraft:overhaul_sfr:heat_sink"]);
+    const heatSinks = plannerBlocks.filter(b => b.tile.modules && b.tile.modules["nuclearcraft:overhaul_sfr:heat_sink"]);
     // const irradiators = plannerBlocks.filter(b => b.tile.modules && b.tile.modules["nuclearcraft:overhaul_sfr:irradiator"]);
     // const conductors = plannerBlocks.filter(b => b.tile.modules && b.tile.modules["nuclearcraft:overhaul_sfr:conductor"]);
 
     fuelCells.forEach(cell => {
       const recipeList = cell.tile.modules["ncpf:block_recipes"].recipes;
-      cell.recipe = recipeList.find(r => checkEquivalence(r, cell.metadata.block_recipe));
+      cell.recipe = recipeList.find(r => checkEquivalence(r, cell.metadata?.block_recipe));
+      // TODO: handle blank recipe
+      if (!cell.recipe) return; // FIXME
       cell.axes = Object.values(sixAxes).map(dir => [...travelUntil(
           plannerBlocksGrid,
           cell,
@@ -136,36 +150,172 @@ export async function runSolver() {
       )]);
       if (cell.recipe.modules["nuclearcraft:overhaul_sfr:fuel_stats"].self_priming) {
         cell.primer = cell;
+        cell.active = true;
       } else if (cell.axes.some(a => (a[a.length - 1]?.tile?.modules ?? {})["nuclearcraft:overhaul_sfr:neutron_source"])) {
         const primerAxis = cell.axes.find(a => (a[a.length - 1]?.tile?.modules ?? {})["nuclearcraft:overhaul_sfr:neutron_source"]);
         cell.primer = primerAxis[primerAxis.length - 1];
+        cell.active = true;
       }
       setCellMetadata(cell.dom, "primer", cell.primer);
     });
 
     await debugBlockSolver("primed");
 
-    fuelCells.forEach(cell => {
+    const seenModerators = new Set();
+    const activeCells = fuelCells.filter(c => c.active);
+    let activeCellIndex = 0;
+    while (activeCellIndex < activeCells.length) {
+      const cell = activeCells[activeCellIndex];
+      if (!cell.axes) return; // ignore bad cells
       Object.keys(sixAxes).map((name, i) => {
-        console.log(name, cell.axes[i]);
         let moderatorSum = 0;
+        let moderatorPath = [];
         for (const block of cell.axes[i]) {
+          if (!block) break;
+
           if (block.tile.modules["nuclearcraft:overhaul_sfr:moderator"]) {
             moderatorSum += block.tile.modules["nuclearcraft:overhaul_sfr:moderator"].flux;
+            moderatorPath.push(block);
+            seenModerators.add(block);
+            if (!block.flux) {
+              block.flux = {};
+            }
+            block.flux[name] = moderatorSum;
             continue;
           }
-          console.log(block);
           if (block.tile.modules["nuclearcraft:overhaul_sfr:fuel_cell"] || block.tile.modules["nuclearcraft:overhaul_sfr:irradiator"]) {
             block.flux = (block.flux ?? 0) + moderatorSum;
-            setCellMetadata(block.dom, "flux", block.flux);
           }
           if (block.tile.modules["nuclearcraft:overhaul_sfr:reflector"]) {
-            cell.flux = (cell.flux ?? 0) + moderatorSum * 2;
-            setCellMetadata(cell.dom, "flux", cell.flux);
+            const backAxis = name.replace(/^[+-]/, s => s === "+" ? "-" : "+");
+            for (const blockBack of moderatorPath.toReversed()) {
+              moderatorSum += blockBack.tile.modules["nuclearcraft:overhaul_sfr:moderator"].flux;
+              moderatorPath.push(blockBack);
+              blockBack.flux[backAxis] = moderatorSum;
+            }
+            cell.flux = (cell.flux ?? 0) + moderatorSum;
+          }
+          if (block.tile.modules["nuclearcraft:overhaul_sfr:fuel_cell"] && !block.active) {
+            if (!block.recipe) break; // FIXME
+            const fuelStats = block.recipe.modules["nuclearcraft:overhaul_sfr:fuel_stats"];
+            if (fuelStats.criticality <= block.flux) {
+              block.active = true;
+              activeCells.push(block);
+            }
           }
           break;
         }
+        if (moderatorPath.length > 0) {
+          moderatorPath[0].active = true;
+        }
       });
+
+      activeCellIndex++;
+    }
+    seenModerators.forEach(moderator => {
+      setCellMetadata(moderator.dom, "flux", moderator.flux);
+      if (moderator.active) {
+        setCellMetadata(moderator.dom, "active", moderator.active);
+      }
     });
+    fuelCells.forEach(cell => {
+      if (cell.flux) {
+        setCellMetadata(cell.dom, "flux", cell.flux);
+      }
+      setCellMetadata(cell.dom, "active", cell.active ?? false);
+    });
+
+    await debugBlockSolver("moderators");
+
+    heatSinks.forEach(hs => {
+      hs.neighbors = {};
+      Object.entries(sixAxes).forEach(([name, dir]) => {
+        hs.neighbors[name] = neighbor(plannerBlocksGrid, hs, dir);
+      });
+      hs.rules = hs.tile.modules["nuclearcraft:overhaul_sfr:heat_sink"].rules;
+    });
+    let heatSinkProcessed = false;
+    const unprocessedHeatSinks = new Set(heatSinks);
+    do {
+      heatSinkProcessed = false;
+      unprocessedHeatSinks.forEach(hs => {
+        if (hs.active) {
+          console.log("active hs in queue", hs);
+          return;
+        }
+        const activeNeighbors = Object.entries(hs.neighbors).filter(([_, n]) => {
+          if (!n) return false;
+          if (n.active) return true;
+          if (n.tile.modules) {
+            return n.tile.modules["nuclearcraft:overhaul_sfr:reflector"]
+                || n.tile.modules["nuclearcraft:overhaul_sfr:irradiator"]
+                || n.tile.modules["nuclearcraft:overhaul_sfr:casing"];
+          }
+          return false;
+        });
+        if (activeNeighbors.length === 0) return;
+
+        let valid = false;
+        for (const rule of hs.rules) {
+          if (checkHeatSinkRule(hs, rule, activeNeighbors)) {
+            valid = true;
+            break;
+          }
+        }
+        if (valid) {
+          heatSinkProcessed = true;
+          unprocessedHeatSinks.delete(hs);
+          hs.active = true;
+          setCellMetadata(hs.dom, "active", true);
+        }
+      });
+    } while (heatSinkProcessed);
+    unprocessedHeatSinks.forEach(hs => {
+      setCellMetadata(hs.dom, "active", false);
+    });
+    console.log(unprocessedHeatSinks);
+
+    await debugBlockSolver("heatsinks");
+  }
+}
+
+function checkHeatSinkRule(hs, rule, activeNeighbors) {
+  switch (rule.type) {
+    case "and":
+      return rule.rules.every(r => checkHeatSinkRule(hs, r, activeNeighbors));
+
+    case "or":
+      return rule.rules.some(r => checkHeatSinkRule(hs, r, activeNeighbors));
+
+    case "between": {
+      const blocks = activeNeighbors
+          .map(([,b]) => b.tile)
+          .filter(b => {
+            if (rule.block.type === "module") {
+              return b.modules && rule.block.name in b.modules;
+            }
+            return checkEquivalence(rule.block, b);
+          });
+      return rule.min <= blocks.length && blocks.length <= rule.max;
+    }
+    case "axial": {
+      const axialPairs = activeNeighbors
+          .filter(([,b]) => {
+            if (rule.block.type === "module") {
+              return b.tile.modules && rule.block.name in b.tile.modules;
+            }
+            return checkEquivalence(rule.block, b.tile);
+          })
+          .reduce((acc, [dir, block]) => {
+            acc[dir[1]] = acc[dir[1]] ?? [];
+            acc[dir[1]].push(block);
+            return acc;
+          }, {});
+      const count = Object.values(axialPairs).filter(p => p.length === 2).length;
+      return rule.min <= count && count <= rule.max;
+    }
+    default:
+      console.warn("unknown rule: %o", rule);
+      return false;
   }
 }
diff --git a/style.css b/style.css
index 999a47e..d31436d 100644
--- a/style.css
+++ b/style.css
@@ -271,6 +271,14 @@ input:not([type="checkbox"]):not([type="radio"]):not([type="file"]) {
 .planner--cell > .planner--overlay[data-overlay="block_recipe"] {
     transform: scale(var(--recipe-scale-factor, 0.8));
 }
+#planner .planner--cell > .planner--overlay[data-overlay="active"] {
+    outline-offset: calc(var(--scale-factor) * -1px);
+    outline: calc(var(--scale-factor) * 1px) solid var(--cell-active-outline, #0f08);
+}
+#planner .planner--cell > .planner--overlay[data-overlay="inactive"] {
+    outline-offset: calc(var(--scale-factor) * -1px);
+    outline: calc(var(--scale-factor) * 1px) solid var(--cell-inactive-outline, #f008);
+}
 
 .planner--resize {
     --resize-width: var(--width);